Solar Energy Research Paper, June 2013: The Cost-Efficiency Dynamic of Solar Technology

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Running Head: THE COST-EFFICIENCY DYNAMIC OF SOLAR TECHNOLOGY 1

The Cost-Efficiency Dynamic of Solar Technology:

The Prevalent Tradeoff Dilemma in the Solar Industry

Aditya Srinivasan

American School of Bombay

June 2013



THE COST-EFFICIENCY DYNAMIC OF SOLAR TECHNOLOGY 2

Abstract

This report examines the solar power industry and its status as a growing and increasingly

favorable source of energy. The applications of solar energy are discussed ranging from the

typical commercial functions to the exclusive utility usage. In addition, a detailed overview of

the various solar technologies and a further examination of the two most paramount in the

current solar field – solar thermal power and photovoltaic – are included. The two

aforementioned technologies are studied and considered from both a technical and a utilitarian

perspective, investigating both the science and viability of the technologies. The prior

investigation leads to an evaluation of the merits and limitations of modern solar technologies

with an emphasis on two cardinal disadvantages: high cost and low efficiency. The remainder of

the research paper offers an exploration of advancements in the field by companies developing

methods to lower the cost and/or improve the efficiency of solar technologies. The cost-

efficiency dilemma is stressed, wherein companies who increase the efficiency of technologies

inevitably do so at the expense of increasing costs due to more expensive materials, or,

conversely, companies lower the cost of technologies at the expense of lower efficiencies due to

lower quality materials. The current research by companies who have developed methods to

avoid this dilemma are subsequently mentioned. Finally, a prediction of the future of solar

energy is presented with supporting statistics and data.

Keywords: solar energy, solar thermal, photovoltaic, cost, efficiency




Acknowledgements

I would like to thank Mrs. Sudha Kannan for her help and guidance throughout this

project, Mr. Vishal Agarwal for his assistance in developing the structure for this report, Mr.

Vivek Srivastava for providing me with the background and history of Aditya Birla‟s

involvement in the solar industry. I would additionally like to acknowledge the Aditya Birla

Science and Technology Company for permitting me to use its facilities in order to complete this

report.




Table of Contents

Abstract ...........................................................................................................................................2

Acknowledgements ........................................................................................................................3

Introduction to Solar Energy ........................................................................................................6

Solar Radiation...............................................................................................................................6

Solar Technology ..........................................................................................................................15

Passive Solar Technology.......................................................................................................16

Active Solar Technology ........................................................................................................24

Solar Thermal Power ...................................................................................................................26

Types of Solar Thermal Power ...............................................................................................27

Evaluation of Solar Thermal Power Technologies .................................................................32

Photovoltaic Solar ........................................................................................................................35

Evaluation of Single-Junction Photovoltaic Cells ..................................................................44

Cost-Efficiency Dynamic .............................................................................................................47

Limits on Cost ........................................................................................................................47

Limits on Efficiency ...............................................................................................................48

Improvements in Cost and Efficiency ....................................................................................52

Multijunction Photovoltaic Devices ...................................................................................57




Quantum Dots ....................................................................................................................62

Concentrated Photovoltaics ...............................................................................................64

Thin-film Photovoltaics ......................................................................................................66

Storage of Solar Energy .....................................................................................................66

The Aditya Birla Group ..............................................................................................................68

Suggestions for Future Action ................................................................................................69

Global Action ................................................................................................................................71

References .....................................................................................................................................74




The Cost-Efficiency Dynamic of Solar Technology:

The Prevalent Tradeoff Dilemma in the Solar Industry

Introduction to Solar Energy

Solar energy is a renewable form of energy that has been harvested by human beings

since as early as the 7th century B.C. (“The History of Solar”, n.d., p. 1). Although only a small

fraction – one part in two billion (“Photovoltaics – Student Guide”, n.d., p. 3) – of the Sun‟s

radiance falls as incident light on the Earth, the solar energy received each year is commensurate

to 15,000 times the annual energy consumption of the planet‟s population (Anderson & Ahmed,

1995, p. 1). A more fathomable (but no less spectacular) statistic is that in just one hour the Sun

blankets the Earth with enough energy to satisfy the population‟s energy consumption needs for

a year (Robertson, 2013). Solar technologies first emerged in primitive forms such as magnifying

glasses that intensified the Sun‟s energy to make fires and burn ants (“The History of Solar”,

n.d., p. 1). Over the course of the last two hundred years, solar technologies have become more

elegant and the industry shows no signs of decelerating progress.

Solar Radiation

Solar radiation is the physical mechanism that drives solar technologies. All solar

technology derives power from the Sun‟s solar energy which is transmitted through solar

radiation. It thereby follows that a thorough understanding of solar radiation is crucial in

designing and perfecting solar technologies – a fact that all solar energy harvesters have

acknowledged and embraced. This chapter provides an understanding of the physics underlying

solar radiation and serves to assist readers in fully understanding the functionality of solar

technologies that are present today.




Thermonuclear Fusion

The Sun is an enormous hot sphere comprised of various gases – primarily hydrogen and

helium. At the core of the Sun, a multistep process called thermonuclear fusion occurs wherein

the nuclei of 1H atoms fuse at incredibly high temperatures of 15,000,000 K (Sagan, 1980) to

produce 4He atoms (Johann, John & Greg, 1996). The multistep process occurs as follows

(Johann et al., 1996):

Initially two protons thermally collide to produce a deuteron, a positron and a neutrino.

The produced positron encounters a free electron and electron-positron annihilation occurs

wherein two gamma ray photons are engendered. The deuteron produced in the first reaction

reacts thermally with a proton to produce a 3He nucleus and a gamma ray photon. Finally, two

3He nuclei react to form a 4He alpha particle (two protons and two neutrons in the nucleus) and

two protons. The overall reaction can be simplified by eliminating intermediates and expressed

as a thermal reaction between four protons and two electrons to produce an alpha particle, two

neutrinos and six gamma rays (Johann et al., 1996):

In the above reaction, there is less mass in the product – the alpha particle, two neutrinos

and six gamma ray photons – than there is in the reactants – the four protons and two electrons.

Namely, there is a 0.71% loss in mass (“Astronomy 1002”, 2002, p. 4). By Einstein‟s mass-




energy equivalence equation, , and the conservation laws of mass and energy, it is

understood that the decrease in mass results in energy produced (“Thermonuclear fusion”, 2009,

p. 2) which is equivalent to approximately 26.7 MeV (Johann et al., 1996). The energy produced

radiates outward through the multiple layers of the Sun including the radiative zone, convective

zone and photosphere until it is finally emitted from the surface and propagated through space

(Coffey, 2010).

Thermal Radiation

All bodies with nonzero Kelvin temperatures emit energy in the form of radiation. This is

due to the mechanism wherein bodies with temperatures above absolute zero contain atoms and

molecules with kinetic energies. These kinetic energies produce oscillating charged particles

which emit energy known as electromagnetic radiation (Finley, 2003). The energy transferred

from the Sun to the Earth occurs by means of thermal electromagnetic radiation and is

transferred in the form of heat since the surface temperature of the Sun (6000 K) are much higher

than that of the Earth (Giancoli, 2005, p. 399). The radiation by the Sun consists primarily of

electromagnetic waves. In addition to emitting radiation, objects absorb radiation from

surroundings (Finley, 2003).

Electromagnetic Spectrum. The electromagnetic waves that are emitted by heated

bodies form a continuous electromagnetic spectrum. The electromagnetic spectrum consists of

all wavelengths of light ranging from long-wavelength infrared to medium-wavelength visible

light to short-wavelength ultraviolet light, as portrayed in Figure 1. The wavelength is also

inversely proportional to the frequency and energy of a particular electromagnetic wave.




Electromagnetic Radiation. Any object with a temperature above absolute zero emits

electromagnetic waves mostly in the infrared region of the spectrum with wavelengths of about

10 microns, or 10,000 nm (“Cool Cosmos”, n.d.). Infrared waves are imperceptible by the naked

eye and thus there is no visual evidence that bodies at room temperature emit any radiation;

instead, infrared radiation is felt as radiant heat and thermal energy. When the temperature of an

object increases to approximately 800 K – the Draper point (Lienhard, 2010) – it begins emitting

electromagnetic waves in the visible light region of the spectrum with wavelengths ranging from

390 nm to 700 nm, a property commonly known as incandescence. An example of

incandescence is when metals glow when heated to high temperatures. It is important to note that

in addition to the orange-red visible light being radiated, there is still infrared radiation in the

form of thermal energy being emitted. When an object approaches temperatures above 3000 K, it

begins emitting electromagnetic waves in the ultraviolet region of the region with wavelengths

ranging from 10 nm to 390 nm. The most common source of ultraviolet radiation is the Sun,

whose surface temperatures approach 6000 K (Giancoli, 2005, p.399). The majority of the

Figure 1. The electromagnetic spectrum. Retrieved June 18, 2013, from:http://imagine.gsfc.nasa.gov/Images/science/EM_spectrum_compare_level1_lg.jpg

http://imagine.gsfc.nasa.gov/Images/science/EM_spectrum_compare_level1_lg.jpg






electromagnetic radiation of the Sun resides in the visible region with infrared waves being the

second most abundant and ultraviolet waves being the least (Giancoli, 2005, p. 757).

Blackbody Radiation. A blackbody is a body that absorbs all the radiation falling on it

(Giancoli, 2005, p. 757). Its name comes from the fact that if self-luminescence was eradicated,

the body would appear black due to its complete absorption of all wavelengths. Additionally, a

blackbody emits radiation based only on its temperature, with no other factors affecting its

radiance. While no perfect blackbody exists, the Sun is a close candidate. Thus, the radiation of

the Sun is often considered blackbody radiation. Three radiation laws are used to understand the

radiation of blackbodies: the Stefan-Boltzmann equation, Planck‟s law and Wien‟s law.

The Stefan-Bol tzmann equation. Austrian physicists Joseph Stefan and Ludwig

Boltzmann studied the properties of radiating objects and developed the Stefan-Boltzmann

equation relating the total power output of an emitting object to the surface area and Kelvin

temperature:

In the equation, ϵ is the emissivity of an object – a number between 0 and 1 that is

characteristic of the surface of the radiating material (a perfect blackbody would have an

emissivity constant of unity). The σ is a universal constant called the Stefan-Boltzmann constant

which is equal to 5.67 × 10-8 W/m2·K 4. A represents the surface area of the emitting object and T

represents its surface temperature in Kelvin (Giancoli, 2005, p. 399).

Planck’s Law. Max Planck, a German physicist, described the spectral radiance of a

blackbody which was the electromagnetic radiance that a blackbody emitted within a given solid




angle measured in watts per steradian per square meter of the emitting surface per frequency

(W/sr·m-2·Hz). His spectral radiance formula, also known as Planck‟s Law, related the intensity

of the spectral radiance of a blackbody radiator at a fixed solid angle as a function of wavelength

for a fixed temperature (“Radiation Laws”, 2007):

( )

In the equation, h is Planck‟s constant which is equal to 6.625 × 10-34 J·s, c is the speed of

light which is equal to 3 × 1010 cm/s, and k is a universal constant called Boltzmann‟s constant

which is equal to 1.38 × 10-23 J/K. Additionally, T refers to the aforementioned fixed temperature

(“Radiation Laws”, 2007). Using Planck‟s Law and the knowledge of wavelengths and energies

of various electromagnetic waves, it is possible to construct a table demonstrating the

electromagnetic radiation characteristic for blackbodies of various temperatures, similar to the

discussion in the previous Electromagnetic Spectrum section – however, the data pertains

specifically to theoretically perfect blackbodies and thus cannot be compared to the data

mentioned in the aforementioned section which pertained to non-blackbody radiators such as

human beings and metals. Table 1 portrays the relationship expressed by Planck‟s Law




Table 1

Blackbody temperatures and their electromagnetic radiation regions

Region Wavelength (μm) Energy (eV) Blackbody

Temperature (K)

Radio > 10 < 10- < 0.03

Microwave 10 – 10 10- – 0.01 0.03 – 30

Infrared 10 - 0.7 0.01 – 2 30 – 4100

Visible 0.7 – 0.4 2 – 3 4100 – 7300

Ultraviolet 0.4 – 0.001 3 – 10 7300 – 3 × 10

X-Rays 0.001 – 10- 10 – 10 3 × 10 – 3 × 10

Gamma Rays < 10- > 10 > 3 × 10

Note. Adapted from “Radiation Laws” by University of Tennessee, Knoxville from

http://csep10.phys.utk.edu/astr162/lect/light/radiation.html

Planck’s quantum hypothesis. Furthermore, Planck proposed another theory regarding the

photons emitted by a blackbody. He suggested that the oscillating charges that provided the

energy could not possess any value of energy; instead, each photon had an energy that was a

multiple of some minimum value related to the oscillation frequency given by his equation:

In Planck‟s equation, h is the Planck constant, E is the energy of the photon and f is the

frequency of the oscillation. Thus, Planck further suggested that other than this minimum energy

value, a photon could only be a whole number multiple of this value:







In this second similar equation, n is merely a natural number – also known as a quantum

number. Planck‟s hypothesis was not utilized until later when the details of photoelectric effect

were scrutinized.

Wien’s Displacement Law. The final radiation law is Wien‟s Displacement Law,

developed by German physicist Wilhelm Wien, which demonstrates the relation between the

Kelvin temperature of a blackbody radiator and the peak wavelength, or the wavelength at which

the blackbody‟s emission is at a maximum (“Wien‟s Displacement Law”, n.d., p. 1). Wien‟s

Displacement Law establishes an inverse proportionality between the temperature and peak

wavelength of a blackbody radiator:

In the equation, λmax signifies the peak wavelength, or the wavelength at which maximum

emission occurs. T represents the temperature of the blackbody radiator in Kelvin. Finally, b is a

constant of proportionality called Wien’s displacement constant which is approximately equal to

2.898 × 10-3 m·K (CODATA, 2010).

Irradiance. In contrast to spectral radiance, which was the measure of the total

electromagnetic radiance that a blackbody emits per unit area at a given solid angle, irradiance is

the measure of the electromagnetic radiance incident on an object from blackbody radiation. The

units for irradiance are watts per square meter (W/m

2

). Irradiance is sometimes called radiant

emittance or power density. The solar radiance at the Sun‟s surface is approximately 6.33 × 107

W/m2 (“Power from the Sun”, n.d.). However, the solar irradiance at the Earth‟s surface is

approximately 1440 W/m2 (Brooks & Tracy, 1957). This is due to the distance between the




Sun‟s surface and that of the Earth. Over this distance, the power density decreases. On planets

closer to the Sun, such as Mercury, irradiance levels reach as high as 9116 W/m2. Conversely,

planets further away, such as Neptune, have extremely low irradiance levels of approximately

1.51 W/m2 (“Solar Radiation in Space”, n.d.).

Blackbody radiation curve. With the three equations and knowledge of power density, a

spectral radiance vs. wavelength graph can be plotted demonstrating the relationship between

temperature, peak wavelength and irradiance of a blackbody. Such a figure is illustrated in

Figure 2.

Figure 2. A blackbody radiation curve for blackbodies with various temperatures.Retrieved June 19, 2013 fromhttp://www.azimuthproject.org/azimuth/files/blackbody_radiation.jpg

http://www.azimuthproject.org/azimuth/files/blackbody_radiation.jpg






The integration of each curve in the above graph yields the irradiance of each blackbody.

Thus, it can clearly be seen that blackbodies with high temperatures (5800 K) have a much

greater irradiance than blackbodies with lower temperatures (50 K). Additionally, it can be seen

that blackbodies with temperatures lower than 800 K – the Draper point – exhibit almost no

emission in the visible region of the electromagnetic spectrum. This type of emission occurs

beyond this point, as mentioned before. The Sun is represented in this graph by the 5800 K

blackbody curve. As shown, the majority of the Sun‟s emission occurs in the visible region with

infrared radiation being the second most emitted and ultraviolet being the least. The dashed line

represents the linear characteristic of the peak wavelength for various blackbody temperatures.

The figure also clearly highlights another aspect of Wien‟s Displacement Law which states that

all blackbodies represent the shape curve shape on a spectral radiance vs. wavelength graph

although their temperature affects their displacement along the graph.

This acquired knowledge of solar radiation will be applied in later chapters wherein the

various solar technologies will be explored in greater depth.

Solar Technology

Solar technology has evolved incredibly since its dawn in the 7th century B.C. (“History

of Solar). Advances in the fields of physics and chemistry and indefatigable innovation have

engendered increasingly sophisticated forms of technology that will one day resolve the pressing

issue of depleting resources. While solar energy can be harnessed by various methods and

devices, there are two predominant classifications of solar technology; passive and active. The

implications of passive solar technology are different from those of active solar technology;




passive solar is a demand-side technology which decreases the demand for conventional

resources of energy whereas active solar is a supply-side technology which increases the supply

of energy.

Passive Solar Technology

Passive solar technology is the classification of solar technology that employs specific

architecture and the strategic positioning of buildings in order to best utilize the Sun‟s radiation.

Additionally, passive solar makes use of specific heat capacities of materials in order to adjust

the heat absorption characteristics of certain surfaces which in turn adjusts the temperature of the

surrounding areas. As mentioned in the previous section, passive solar assumes the role of a

demand-side technology by reducing the demand for alternative forms of technology such as

ventilation devices – air-conditioners and ceiling fans – and lighting technologies such as light

bulbs. Passive solar is predominant in architecture, urban planning, agriculture and horticulture.

In comparison to active solar technology, passive solar has a fairly limited scope with regard to

its applications and uses.

Architecture. Passive solar technology is most commonly seen in specifically designed

architectural structures. The passive architectural design serves to provide a less expensive and

less environmentally damaging alternative to conventional electricity powered by fossil fuel

consumption. “Passive Heating and Cooling” (2003) states that:

Americans spend about $54 billion each year to heat and cool their homes. The fossil fuel

consumed to accomplish this, in turn, is responsible for one-fifth of this nation's emission

of carbon dioxide. But by incorporating passive solar design techniques, homeowners can

reduce energy bills by more than 80% while greatly increasing comfort levels.




The purpose of passive solar in this field is twofold: to provide interiors of buildings with

adequate lighting and to regulate a comfortable temperature for its denizens. In order to serve

this purpose, architects consider several factors when designing passive-solar structures

including the orientation of the building and the heat capacities of various materials.

Basic archi tectural elements. Passive technology is sometimes very simple, as is the case

in the field of architecture. Simple design elements such as windows and eaves help to regulate

light and ventilation. Windows serve the purpose of allowing interiors to be illuminated without

the need for light bulbs or other electrical devices. Additionally, open windows can regulate

temperature by allowing a house to adjust to ambient environmental temperatures. Eaves play the

role of regulating temperature by blocking the penetration of solar radiation during the summer

when the Sun follows a higher path relative to the winter Sun, as depicted by Figure 3.

Figure 3. Passive solar architecture. Retrieved June 13, 2013 from

http://www.tankonyvtar.hu/hu/tartalom/tamop425/0032_kornyezettechnologia_en/images/risk/44.jpg







Furthermore, the mere positioning and orientation of the building is an important

consideration for passive-solar structure designers. In order to ensure exposure to the Sun during

maximum points in its orbital path, architects deliberately ascertain that any windows and open

walls are facing the south (Kachadorian, 2006, p. 17). Table 2 highlights the exponentially

deleterious effect of deviating from “true South”:

Table 2

Solar benefit at different rotations relative to due South

Orientation Solar Benefit

True South 100%

22.5o rotation off true south towards west or east 92%

45o rotation off true south towards west or east 70%

67.5o rotation off true south towards west or east 36%

Note. Adapted from “The Passive Solar Concept”, by J. Kachadorian, 2006, Passive Solar

House: Using Solar Design to Cool and Heat Your Home, 2nd

Edition, p. 17, Copyright 2006 by

James Kachadorian.

Trombe wall . While windows

and eaves provide direct regulation and

illumination during the daytime, their

existence is redundant when heat is

needed during the nighttime. In the

1950s, French inventor Felix Trombe

developed a heat storage and delivery Figure 4. An unvented (solid-body) Trombe wall system.Retrieved June 20, 2013 from http://ltgovernors.com/wp-content/uploads/2009/08/trombe-wall.png

http://ltgovernors.com/wp-content/uploads/2009/08/trombe-wall.png









system called the Trombe wall (Torcellini & Pless, 2004, p. 3). The Trombe wall comes in two

permutations: vented and solid-body. Both versions of the Trombe Wall consist of a “south-

facing masonry wall with a dark, heat-absorbing material on the exterior surface and faced with a

single or double layer of glass” approximately 1 to 2 inches apart (Torcellini & Pless, 2004, p.

3). As the sun‟s energy passes through the glass pane, it is trapped by the dark concrete wall

(Kachadorian, 2006, p. 15). With the case of the solid-body Trombe wall, shown in Figure 4, the

heat that is trapped slowly radiates through the concrete into the interior of the building. With the

thickness of Trombe wall‟s typically being between 4 to 16 inches, the time taken for heat to

radiate is approximately 8 to 10 hours (Torcellini & Pless, 2004, p. 3). This allows the heat to

radiate into interiors long after the Sun has set, which resolves the limitations of basic

architectural elements (Torcellini & Pless, 2004, p. 3).

A vented Trombe wall is

designed slightly differently; it

contains two rectangular horizontal

vents along the top and bottom of

the masonry wall (Kachadorian,

2006, p. 15). As shown in the

schematic diagram in Figure 5, the

dark concrete wall is placed

approximately 1 to 2 inches away

from the glass pane (Torcellini &

Pless, 2004, p. 3). Kachadorian

Figure 5. A vented Trombe wall system. Adapted from “ThePassive Solar Concept”, by J. Kachadorian, 2006, Passive Solar

House: Using Solar Design to Cool and Heat Your Home, 2nd

Edition, p. 16, Copyright 2006 by James Kachadorian.




(2006) explains how the system functions:

As the concrete warms, air rises in the spaces between the glass and the blackened

concrete wall. Rectangular openings at the bottom and top of the Trombe wall allow this

warm air to flow to and from the living space. This movement of air is called

thermosiphoning.

While the ventilation system creates a pocket of warm air during the daytime for

thermosiphoning to occur productively, the effects are reversed during the nighttime. Once the

Sun sets, the glass becomes cold and the air space becomes relatively cooler than the now warm

living room. As a result, reverse thermosiphoning occurs wherein warm air cycles out of the

interior into the air space and cool air cycles into the interior from the air space (Kachadorian,

2006, p. 15). A solution for this issue is to install mechanical shutters which can be opened

during the daytime and shut during the nighttime so as to prevent heat loss from interior living

spaces (Kachadorian, 2006, p. 15).

Urban Planning. In the planning of urban landscapes, passive solar concepts are

important. There are several factors that cause the temperature to rise in urban areas: pollution,

various materials, domestic heating and industrial processes (Codrington, 2005, p. 385-386).

Pollutants such as carbon monoxide, nitrogen oxides, sulfur oxides, hydrocarbons and

particulates are all engendered through various means such as car engines, industrial processes

and other mechanisms requiring fossil fuel combustion. As a result, these pollutants fill the air

and trap the incoming solar radiation acting as greenhouse gases. Additionally, concrete

buildings, dark roads, dark rooftops and other heat-absorbing materials all trap the heat during

the daytime and release it into the air during the nighttime (Codrington, 2005, p. 385); a function




similar to that of the Trombe wall. However, in this case, high temperatures are unfavorable as

they create many problems that will be discussed shortly. This urban area with temperatures

relatively higher than its surroundings is named an urban heat island . During the daytime, the

temperature difference is relatively less significant – approximately 1o C to 3o C higher than the

surrounding environment. However, when the heat absorbed by the materials with high heat

capacities release their acquired thermal energy during the nighttime, the temperature differences

are much greater – approximately 4o C to 12o C (“Heat Island Effect”, 2013). Figure 6 illustrates

an arbitrary urban heat island in which temperatures rise significantly relative to the surrounding

environments.

The implications of urban heat islands are manifold and all detrimental to society. From

an economic perspective, urban heat islands necessitate greater consumption of cooling devices

such as air-conditioners and ceiling fans. Thus, the increased demand for energy implies greater

monetary costs to society and governments which can create issues economically. Furthermore,

Figure 6. Temperature gradient across urban environment. Retrieved June 13, 2013 from

http://www.monument-info-search.co.uk/site/wp-content/uploads/2011/08/blocks_image_5_1.png







the profuse consumption of these cooling devices requires electricity generated from the

combustion of fossil fuels. To appease this excess demand, power plants and energy providers

must increase their energy production rates. As a result, the pollution is exacerbated and the

temperatures only rise further. The issue of urban heat islands can also be considered from a

health perspective: the rise in temperatures have been shown to directly correlate with heat-

related illnesses and deaths (“Heat Island Impacts”, 2013). Additionally, urban heat islands

adversely affect aquatic ecosystems. The heated pavements and rooftops as a result of urban

heating have been found to increase the initial temperature of rainwater from 21o C to over 35o C

(“Heat Island Impacts”, 2013). When the rainwater is drained into lakes and oceans, they cause

an overall increase in temperature of these bodies of water. As a result, aquatic life suffers since

the metabolic and reproductive rates of marine life are negatively impacted.

The mitigation strategies employed today revolve primarily around passive solar

concepts. One method that is used to reduce the impacts of urban heat islands is to cover urban

surfaces with reflective surfaces. Cool rooftops are created by painting rooftops with a white or

light color. This increases the solar reflectance – or albedo – of the surface enabling it to absorb

Figure 7. Albedo characteristic of white-painted rooftop (right) compared to unpainted rooftop (left).Retrieved June 20, 2013 from http://user.cloudfront.goodinc.com/community/etling/white-roofs-graphic.jpg

http://user.cloudfront.goodinc.com/community/etling/white-roofs-graphic.jpg







less heat and lower temperatures of urban areas by approximately 28o C to 33o C (“Cool Roofs”,

2013). Similarly, pavements are painted with a white or light color creating cool pavements.

Alternatively or in combination, the pavements can be paved with whitened asphalt and covered

with a light concrete cover called “whitetopping” (“The Urban Heat Island (UHI) Effect”, 2011)

Agriculture and horticulture. The fields of agriculture and horticulture regard the

growth and cultivation of crops for human usage. The utilization of solar radiation for promoting

the celerity of crop growth rates is based on passive solar concepts. Passive solar structures

exploit the Sun‟s power and enhance its nurturing effects thereby accelerating plant growth.

There are two common passive solar methods to encourage crop growth via solar radiation:

thermal masses and greenhouses.

Thermal masses. Functionally, thermal masses functionally analogously to Trombe

walls. They are constructed from a blackened heat-absorbing material and are built with the

intent of providing surrounding crops with sustained warmth even during the nighttime. These

thermal masses are typically constructed from materials with high specific heat capacities such

as concrete, clay, brick, stone and mud. Additionally, thermal masses can be designed such that a

hollow interior is filled with water, a material that has the highest volumetric heat capacity. Since

heat is essential for successful growth, thermal masses are essential to maintain a healthy

environment. Additionally, the utilization of thermal masses negates the necessity for additional

heating devices powered by the combustion of fossil fuels that pollute the environment and foster

global warming.

Greenhouses. While thermal masses may be effective within small areas, a preferable

alternative for large-scale crop growth is the greenhouse. The greenhouse creates an area in




which temperatures are increased allowing for crops and plants within the greenhouse to be

warmed perpetually. Constructed from glass, the structure allows solar radiation to enter the

greenhouse from all sides. On the interior, insulation is applied so that heat loss is minimized,

thus enhancing the greenhouse „oven‟ effect. Additionally, thermal masses – exactly the same as

the aforementioned structures – can be implemented allowing for sustained heat during

nighttime.

Active Solar Technology

As mentioned before, active solar technology impacts the energy industry by increasing the

supply of solar energy as opposed to its counterpart, passive solar technology, which decreases

the demand for other energies. While the term „passive solar technology‟ could only be used with

Figure 8. The greenhouse process. Retrieved June 13, 2013 fromhttp://www.thediygreenhouse.com/wp-content/uploads/2011/06/Howitworks.png

http://www.thediygreenhouse.com/wp-content/uploads/2011/06/Howitworks.png






very loose emphasis on the word „technology‟, active solar technology meets all the criteria for

the definition of the word. This classification deals with greater scientific and engineering

thinking and comes in many shapes and forms. It is the type of solar technology that sees more

attention from firms investing in solar research and development. Perpetual innovation occurs in

this field with the objective of establishing solar energy as a competitive resource of energy – a

dream that is steadily becoming realized. The applications of active solar technology are much

broader due to the fact that it creates electricity as opposed to passive solar technology which

merely the warming abilities of sunlight. Active solar can be applied to all of the aforementioned

fields by supplementing the passive solar technologies. For example, in the field of architecture,

heat pumps or fans can be implemented, powered by active solar technology. In fact, any

electrical device can be used and powered by active-solar means. This is what makes the scope

of active solar technology so broad.

The two predominant active solar technologies, solar thermal power and photovoltaic

cells will be given their own chapters due to the sheer magnitude of the information pertaining to

each technology. While both function similarly by converting solar radiation into electricity,

solar thermal power technologies do so indirectly while photovoltaic devices do so directly. A

thorough scientific understanding of each will be provided in addition to explanations of

advances in the technology. Additionally, an evaluation of the merits and disadvantages of the

technologies will be offered. Furthermore, an emphasis will be put on photovoltaic cells and the

titular cost-efficiency dynamic will be addressed with respect to said technology.




Solar Thermal Power

Solar thermal power is the first of the two branches of active solar technologies. This

particular type of active solar technology converts solar radiation into electricity through indirect

means. All solar thermal power devices consist of two chief components: a reflector and a

receiver. The variations between solar thermal power technologies lie primarily in the shape and

structure of reflectors. The fundamental mechanism of all solar thermal power devices is similar

and will be discussed in detail in the following text.

Solar Thermal Power Process

Solar thermal power devices function by a multistep process that generates electricity

indirectly. The process begins with a reflector that concentrates and directs the incident solar

radiation towards a receiver. This receiver is generally a tube through which a heat carrying

liquid , such as oil or water,

passes. The reflector

intensifies the power of the

Sun to heat the liquid

flowing through the tubes.

The liquid then either

vaporizes itself or is heated

to high temperatures (while

maintaining its liquid state)

in order to turn another

body of liquid into steam. Once the steam is generated through either method, it continues to

Figure 9. A steam-turbine generator. Retrieved June 23, 2013 from:http://turbinegenerator.org/wp-content/uploads/2012/01/Generator.Front1_1.jpg

http://turbinegenerator.org/wp-content/uploads/2012/01/Generator.Front1_1.jpg








rotate steam-powered turbine. This turbine is rotated through the mechanical energy of the steam

which in turn powers a generator. This can occur by several ways, one of which is by the rotation

of a loop of copper wire in a magnetic field. The turbine has a loop of copper wire (indicated by

the numeral 3 in the diagram above) attached to the turbine shaft called the armature (2). On

either side of the copper wire is a north and south side of a dipole magnet (4) between which a

magnetic field is generated (“How a Generator Works”, n.d.). From prior knowledge of

electromagnetism, it is known that when a wire is placed in a changing magnetic field an

electromotive force is induced producing electricity. Thus, this rotation of the copper wire (5)

induces voltage and current which is where the electricity is generated from (6). Additionally,

from Faraday‟s Law of Induction, the rate of change of magnetic flux (i.e., how quickly the

magnetic field changes within the loop) is proportional to the electromotive force induced.

Therefore, the more sunlight concentrated, the more steam generated and the greater the voltage

produced. An illustration in Figure 9 shows a basic diagram of a steam-turbine generator.

Types of Solar Thermal Power

In this chapter, the four main variations of solar thermal power technology are also

referred to as concentrated solar power technology due to the fact that they concentrate the

Sun‟s radiation towards the liquid being used. These four technologies are: parabolic troughs,

Fresnel reflectors, dish Sterling and solar power towers.

Parabolic trough. The parabolic trough system is one of the most widely implemented

systems in the solar industry due to its low cost and infrequent maintenance requirements. The

system consists of a parabolic mirror oriented to face the Sun. The reflective surfaces direct the

sunlight towards a receiver through which a liquid passes. This liquid is heated to produce steam




which in turn powers a turbine-driven generator to produce electricity. This system is illustrated

in Figure 10.

Parabolic tr acking system. Some parabolic troughs are enhanced by installing a tracking

system to allow the mirrors to

follow the path of the Sun. This

improves the efficiency since the

mirror more accurately redirects

the solar radiation for a larger

proportion of the day. Figure 11

shows how a parabolic trough

system can be made to

track the Sun along a

Figure 10. Parabolic trough system. Retrieved June 13, 2013 from:

http://whatwow.org/wp-content/uploads/2011/02/drawing_parabolic-

through-600x411.jpg

Figure 11. Parabolic trough tracking system. Retrieved June 13, 2013 from:

http://aalborgcsp.com/media/49803/CSP_parabolic_trough_sun_path_540x3

71.jpg

http://whatwow.org/wp-content/uploads/2011/02/drawing_parabolic-through-600x411.jpg



http://aalborgcsp.com/media/49803/CSP_parabolic_trough_sun_path_540x371.jpg









single axis during the course of a day.

Fresnel reflectors. The Fresnel reflector system, also called the linear Fresnel reflector

system is similar to the parabolic trough system with the one difference being the shape of the

reflecting mirror. As the name suggests, the system consists of several rows of linear mirrors

placed at varying angles to ensure reflection towards the receiver. These linear mirrors are

typically flat along the x-axis and do not track the sun. The remainder of the system is identical

with a liquid passing through the heated receiver in order to power a turbine-driven generator.

Figure 12 shows the device – note the slight variations in the inclinations of the linear mirrors.

Figure 12. Fresnel reflector system. Retrieved June 13 from :http://us.arevablog.com/wp-content/uploads/AREVA-SSG4.jpg

http://us.arevablog.com/wp-content/uploads/AREVA-SSG4.jpg






Compact linear F resnel refl ector. The compact linear Fresnel reflector is a variation on

the typical design. This modified system implements multiple receivers (also called absorbers) in

order to improve the efficiency by

providing greater electricity

generation in the same area (this is

why it is called a compact linear

Fresnel reflector). The mirrors are

also oriented in an alternating

fashion in order to avoid any

shadowing that may occur

by adjacent mirrors in a

conventional Fresnel reflector device.

Dish Sterling. The dish Sterling

design functions most similarly to the

parabolic trough system with the one

difference being – again – the shape of

the mirror. The dish Sterling system is

(as the name suggests) a large dish

comprised of reflective surfaces such as

mirrors. The dish is constructed so that

the solar radiation is redirected at every

point on the dish towards a

Figure 13. Compact linear Fresnel reflector. Retrieved June 22, 2013 from:http://upload.wikimedia.org/wikipedia/commons/b/b2/CLFR_Alternating_Inclination.JPG

Figure 14. Dish Sterling system. Retrieved June 13, 2013 from:

http://upload.wikimedia.org/wikipedia/commons/f/f2/Dish-stirling-at-

odeillo.jpg

http://upload.wikimedia.org/wikipedia/commons/b/b2/CLFR_Alternating_Inclination.JPG



http://upload.wikimedia.org/wikipedia/commons/f/f2/Dish-stirling-at-odeillo.jpg










receiver. Once more, the liquid is heated to power a turbine-driven generator to produce

electricity.

Dish Ster li ng tracking system. As opposed to the single-axis tracking system in the

parabolic trough design, dish Sterling

systems are implemented with a dual-

axis tracking system; the dish can

rotate along both the x-axis and the y-

axis as shown in Figure 15. Again, the

tracking system enables the dish to be

exposed to the Sun for a longer period

of time thus increasing the efficiency of

the system.

Solar power tower. The final

variation of solar thermal power technology is

the solar power tower. Once more, the system

is similar to that of the other systems with the

difference being in the way that solar radiation

is reflected. In this system, a central tower is

positioned surrounded by concentric circles of

angled mirrors. The mirrors are angled to

reflect the sunlight towards a

central receiver positioned atop

Figure 15. Dual-axis tracking system. Retrieved June22, 2013 from:http://literature.rockwellautomation.com/idc/groups/liter ature/documents/wp/oem-wp009_-en-p.pdf (p. 2)

Figure 16. Solar power tower. Retrieved June 13, 2013 from:

http://www.scientificamerican.com/media/inline/is-the-sun-setting-

on-solar-power-in-spain_1.jpg

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/oem-wp009_-en-p.pdf


http://www.scientificamerican.com/media/inline/is-the-sun-setting-on-solar-power-in-spain_1.jpg










the tower. This receiver contains a liquid which is heated to power a turbine-driven generator to

produce electricity. This system occupies the largest area relative to the other systems. A picture

of a solar power tower is shown in Figure 16.

Evaluation of Solar Thermal Power Technologies

Users often evaluate the advantages and disadvantages of a certain technology in order to

understand the costs that they will need to cover and the benefits that they will reap. A

comprehensive assessment of solar thermal power technologies provides a deep understanding of

the technology‟s assets and drawbacks with regard to many aspects. This helps prospective users

discern whether installing the product would be beneficial or whether a more favorable

alternative should be sought.

Advantages. There are several advantages that make solar thermal power devices

favorable. They will be explained in detail in the following text.

Renewability . One of the most obvious yet important merits of any solar technology is

renewability. Since the energy is harvested from incident solar radiation, the systems are all

generating electricity from renewable sources – unless oil is used as the heat carrying liquid. This

renewable characteristic of solar energy greatly lowers costs for users since the resource is free

and the only investment that must be made is that of installment.

Non-polluting. With the exception of the pollution created from the manufacturing and

production of the product, solar thermal energy produces no toxic and harmful byproducts and

greenhouse gases such as carbon monoxide, oxides of nitrogen and sulfur, hydrocarbons and

particulates. In these times, environmentally friendly approaches are gaining popularity in




contrast to the more harmful energy production methods such as fossil fuel combustion which

emit a large amount of pollutants.

Low maintenance. Solar thermal power technologies have high life expectancies.

Following the installation, infrequent maintenance is required due to the fact that solar thermal

systems function in a simpler manner relative to other complex natural gas or fossil fuel energy

production systems.

High eff iciencies. The solar thermal power systems have higher solar to electrical energy

conversion efficiency rates compared to photovoltaic cells (more on this later). The dish Sterling

system has been found to have the highest efficiency rates of 31.25% (“Stirling Energy Systems

set new world record”, 2008) – that is, 31.25% of the solar energy incident on the Sterling dish is

converted into useful electrical energy. Following dish Sterling systems is the parabolic trough

design which have reached efficiencies of around 20%, still higher than the 15% efficiencies of

conventional photovoltaic panels.

High output. Solar thermal power systems are also capable of producing large amounts

of power. The AndaSol-1 Power Plant in Spain is a prime exemplar of such a system. The

parabolic trough system, although only possessing efficiency rates of 16%, spans a vast area of

approximately 200 hectares (“Andasol-1”, 2013). Due to its expanse, the power station is able to

generate up to 50 MW of power (“Andasol-1”, 2013).

Disadvantages. While the solar thermal power systems have advantages, they are also

flawed in certain aspects. These disadvantages are highlighted in the following.

Expensive. The foremost inconvenience with solar thermal power systems is the price.

Albeit there are low maintenance costs, the initial capital cost of installation is extremely high.




As a result, solar thermal systems are primarily exclusive for utility purposes as opposed to

residential and commercial usage. Additionally, the high relative cost makes the systems

unfavorable compared to cheaper conventional energy production means such as fossil fuels or

natural gases.

I nconsistent. Although the Sun provides an abundant source of energy, this radiation is

only available during certain periods. During cloudy days or nighttime, the energy production

capabilities of solar technology are diminished or halted completely. Thus, the energy production

process is intermittent and fairly unreliable.

Space required. All solar thermal power systems require a large amount of space

dedicated to the reflective constructions themselves and the turbine-driven generator. Again, the

vast area needed to support these systems makes them candidates for utility purposes as opposed

to residential or commercial usages.

Storage. As the solar energy is collected, it must be used immediately since the process

of heat transfer cannot be delayed naturally. As a result, storage is an issue with solar thermal

systems thus causing trouble during cloudy days or nighttime. In order to surpass this problem,

methods have been developed to store some of the heat collected for later use; however, these

processes are expensive and all of the heat cannot be completely stored which reduces the

efficiency.

Overview of Solar Thermal Power. With the advantages and disadvantages of the solar

thermal systems in mind, it‟s possible to construct an encompassing evaluation of the

technology. In the United States, as of 2010, the average cost of energy from natural gases and

coal was approximately 7¢/kWh - 10¢/kWh (U.S. Energy Information Administration, 2013).




The average cost of solar thermal energy, however, was much higher at approximately 25¢/kWh

(U.S. Energy Information Administration, 2013). These costs arise from the space required in

addition to the high capital cost. The fact that these costs are so much greater means that it is

difficult for solar energy to compete in the economy with conventional energy sources. In order

to compensate for this high cost, many governments provide incentives and subsidies to solar

energy producers in order to offset the high price. This evaluation narrows the scope of

prospective users of solar thermal to mainly those with utility based purposes. Only when the

cost of solar thermal energy can be reduced to a more feasible value of approximately 10¢/kWh

will this form of energy surpass typical forms of energy production in the market.

Photovoltaic Solar

Photovoltaic solar technology is a relatively new field in the energy industry but has been

studied for a number of years. As a result, the field of study is still in its infancy and the

technologies are quite simple and have not matured as much as other energy production

processes have. Nevertheless, the field of photovoltaic solar has seen remarkable progress since

its discovery as a source of energy. There have been rapid advancements improving many

aspects of the technology providing a promising future for this form of solar energy.

The Photoelectric Effect

In the early 1900s, there was still controversy over the behavior of light – while some

argued light was best characterized as a wave, others claimed it was better described as a

particle. In 1887, a physical phenomenon called the photoelectric effect was discovered by

German physicist Heinrich Hertz. While he was testing Maxwell‟s equations, Hertz

experimented with a coil of wire to detect electromagnetic radiation. When Hertz could not see




the spark that was being produced, he placed the apparatus in a dark environment. He noticed

that the sparks were shorter and less intense in this dark environment. Additionally, when he

removed the equipment from the box and exposed it to ultraviolet radiation, he discovered the

sparks increased in size and length (“Lesson 33: The Photoelectric Effect”, 2012, p. 1). A

primitive understanding of the photoelectric effect – the emission of an electron from a metal due

to the absorption of light – was achieved.

In 1905, Einstein aimed to resolve the dispute over the behavior of light by investigating

the photoelectric effect. He realized that the electrons emitted were proportional to the energy of

the electromagnetic radiation incident on a metal. However, the question still remained: was the

radiation a wave or a particle? Einstein examined quantitative results of the photoelectric effect

in order to discern the truth (Giancoli, 2005, p. 759).

First he considered the wave theory and the two most important properties of light: the

intensity and the frequency. Giancoli (2005) states that according to the wave theory:

1. If the light intensity is increased, the number of electrons ejected and their maximum

kinetic energy should be increased because the higher intensity means a greater

electric field amplitude, and the greater electric field should eject electrons with

higher speed.

2. The frequency of the light should not affect the kinetic energy of the ejected

electrons. Only the intensity should affect the maximum kinetic energy.

Einstein continued by considering the contradicting particle theory of light which

suggests light exists in packets called photons. For this, Einstein drew from the work of Max

Planck and his quantum hypothesis in which he stated photons can contain energy values in




discrete quanta ( E = hf). Furthermore, Einstein expanded by establishing the work function of

metals. Since the electrons in metals were attracted by molecular forces, a certain amount of

energy was required to eject an electron from the surface. Einstein developed the work function:

In this equation, W o is the minimum energy required for a photon to eject an electron, f o is

the minimum oscillation frequency and h is Planck‟s constant. Each metal has its own work

function and thus, the minimum energy requirements of a photon required to eject an electron

can be calculated.

Using the work function, three possible scenarios were imagined. In the first, a photon

with insufficient energy – i.e., energy less than the work function (hf < hf o) – would be absorbed

by the metal without ejecting an electron. In the second scenario, a photon with energy greater

than work function (hf > hf o) would cause the emittance of an electron with a kinetic energy

equal to the surplus energy of the photon. In the third rare case, a photon with energy equal to the

work function (hf = hf o) would cause an electron to be emitted without any kinetic energy.

Considering all these, Einstein found that the photon theory made the following

predictions, according to Giancoli (2005):

1. An increase in intensity of the light beam means more photons are incident, so more

electrons will be ejected; but since the energy of each photon is not changed, the

maximum kinetic energy of electrons is not changed by an increase in intensity.

2. If the frequency of the light is increased, the maximum kinetic energy of the electrons

increases linearly.




3. If the frequency is less than the minimum frequency f o , no electrons will be ejected,

no matter how great the intensity of the light.

Upon examining his results and comparing them to both the wave and particle theory of

light, Einstein concluded that the photon theory was valid.

Band Theory of Solids

The photoelectric effect can be further clarified from a chemistry perspective. Electrons

occupy electron orbitals and are all attracted towards the nucleus of an atom. Depending on

which orbital level an electron occupies, the strength of attraction between the electron and the

nucleus differs – the lower the orbital level, the greater the nuclear attraction and vice versa. An

electron can be excited and ascend orbitals if energy is absorbed by the atom. Conversely,

Figure 17. The photoelectric effect. . Retrieved June 24, 2013 from:

http://www.sciencetech.technomuses.ca/english/whatson/pdf/sno/img/sno_page_17b.jpg







electrons can emit energy and descend orbitals. Electrons on the valence shell – the occupied

orbital furthest from the nucleus – are attracted with the least force since they are the greatest

distance from the nucleus. In a given atom, these electrons make up the valence band. In contrast

to the valence band is the conduction band. Electrons in the conduction band have been ejected

from the atom and are not specific to any atom in particular; they are free to travel. The energy

difference between the valence band and the conduction band is often referred to as the bandgap.

Moreover, the bandgap energy is the energy required to excite an electron from the valence band

to the conduction band. This bandgap energy is similar to the concept of the minimum oscillation

frequency f o in the work function. A photon can possess energy greater than, equal to or less than

the bandgap energy with the outcomes of each case being similar to those discussed previously.

For different types of solids, the bands are arranged differently. In conductors, the

valence band and

conduction band are

“close” to each other

with a very small

bandgap. This gives

conductors their

„conducting‟ property

since the energy

required to excite an

electron to the conduction state is very low. For insulators, the valence band and conduction

band ar e “far apart” from each other with a very large bandgap. This gives insulators an

„insulating‟ property since there is a very large amount of energy required to excite an electron to

Figure 18. Band theory. Retrieved June 24, 2013 from:

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/imgsol/band2.gif







the conduction state. Semiconductors are the third type of solid whose band properties are

between those of conductors and insulators. For this reason, semiconductors are used in the

production of photovoltaic solar cells.

An important concept is the Fermi level. At absolute zero, the highest orbital level that an

electron can occupy is called the Fermi level and these topmost electrons possess Fermi energy.

As the temperature increases, the fraction of electrons existing above the level increases.

Additionally, the Fermi level can be intentionally adjusted in order to manipulate energy

requirements.

The Photovoltaic Process

The photovoltaic process is heavily based on the photoelectric effect. It is the process by

which all photovoltaic devices function. By understanding the photoelectric effect, scientists

have been able to improve photovoltaic devices since the two are very closely related.

In a nutshell, the photovoltaic process functions by forming a circuit so as to channel the

electrons emitted. When a photon with energy greater than or equal to the bandgap energy is

absorbed, the photovoltaic circuit functions like a conventional electric circuit.

However, oppositely charged poles must be created to establish a potential difference

gradient for the electrons to travel along. The reality is that the photovoltaic process is

complicated and must be manufactured.

P-N junction. In order to create the aforementioned voltage difference, a semiconductor

wafer must be specially treated in order for electrons to travel along a potential gradient. Thus, a

p-n junction must be created. This is done through a process called doping . In this process, two




sides of a prepared semiconductor crystal wafer are treated to create oppositely charged poles.

To understand doping, one must first

understand the crystal lattice

structure of a semiconductor.

Semiconductors typically contain

four valence electrons. When a

semiconductor crystal is formed,

each atom forms four bonds with

surrounding semiconductor atoms.

The resulting structure is a strong

macromolecular covalent network of

bonds with no delocalized electrons. As a result, there is poor conduction which is why doping is

required.

Doping. There are two types of doping that occur: n-type doping (also called n-doping)

and p-type doping (also called p-

doping ). Each type of doping creates a

negatively charged side and a

positively charged side respectively.

The first type, n-doping, is done

by bonding the semiconductor with a

Group V element. For example, silicon

is typically n-doped by phosphorus.

Figure 19. Semiconductor structure. Retrieved June 24,2013 from:http://www.asdn.net/asdn/physics/images/BOND-SI.gif

Figure 20. N-doped Silicon. Retrieved June 24, 2013from: http://www.plexoft.com/SBF/images/tokuyasu-mirror/phosdope.gif

http://www.asdn.net/asdn/physics/images/BOND-SI.gif


http://www.plexoft.com/SBF/images/tokuyasu-mirror/phosdope.gif










Phosphorus is an element with five valence electrons. This means that when phosphorus is

bonded with silicon, one electron is free to conduct. By bonding many phosphorus atoms in a

silicon crystal structure, many conducting electrons can be created thus creating an abundance of

electrons on this n-doped side.

The opposite process, p-doping, is done by bonding the semiconductor with a Group III

element. For example, silicon is

typically p-doped by boron. Boron is

an element with three valence

electrons which means that when it

bonds with silicon, there is one

electron less than required. This

creates an absence of an electron,

commonly called holes. Holes are not

positively charged particles per se;

instead, they represent an area that an

electron could fill. By bonding many boron atoms in a silicon crystal structure, many holes can

be created thus creating a p-doped side.

Both conducting electrons and holes are similar in that they are both mobile. On the n-

doped side, the majority charge carrier is the more abundant electron and the minority charge

carrier is the less abundant hole. The opposite holds true for the p-doped side. The proportion of

minority carriers to majority carriers is analogous to the proportion of one human being to the

entire population of the Earth.

Figure 21. P-doped Silicon. Retrieved June 24, 2013from:http://carmaux.cs.gsu.edu/~mweeks/csc4250/silicon3.gif

http://carmaux.cs.gsu.edu/~mweeks/csc4250/silicon3.gif

http://carmaux.cs.gsu.edu/~mweeks/csc4250/silicon3.gif




Depletion zone . When the n-doped region is brought into contact with the p-doped

region, a p-n junction is formed. This creates a concentration gradient which in turn creates

chemical potential energy. Because of this potential, diffusion takes place. The electrons from

the n-doped region diffuse to the p-doped side. Over there, they recombine with holes forming

negative ions. Conversely, mobile holes diffuse from the p-doped side to the n-doped side to

recombine with electrons forming positive ions. This movement of mobile charge carriers creates

a depletion region which forms an electric field.

The electric field (going from right to left in the diagram above) opposes the diffusion

and eventually equilibrium is achieved wherein the forces of the electric field and chemical

diffusion are balanced at which point the flow of charge carriers ceases.

When a photon with sufficient energy strikes either side of the p-n junction, an electron-

hole pair is created. While the creation of this pair has no effect on the concentration of majority

charge carriers, it has a significant impact on the concentration of minority charge carriers – this

Figure 22. Depletion region formed by diffusion of mobile charge carriers. Retrieved andadapted June 24, 2013 from: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/imgsol/pn2.gif

electric field

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/imgsol/pn2.gif







is because, as stated before, the number of minority carriers is much smaller than that of majority

carriers. As a result, a minority carrier created by the absorption of an energized photon will

upset the equilibrium and will diffuse into the depletion zone. There, the minority carrier is

repelled to the opposite side where they become a majority carrier. This movement of charges,

specifically electrons, is what drives the process. When a photon strikes the p-doped side, an

electron-hole pair is created. This minority electron diffuses into the depletion zone and it

propelled to the n-doped side. On either region wires are connected to allow the flow of

electrons. A load is connected to this circuit and the electricity is provided from the flow of

electrons. Figure 23 shows the movement of charge carriers when a photon strikes either side of

the junction.

Evaluation of Single-Junction Photovoltaic Cells

As with solar thermal power, photovoltaic cells are evaluated on their assets and

limitations. This provides prospective users with the information necessary to determine whether

the technology would be a favorable investment or not. In addition, manufacturers and scientists

share a role in the evaluation process; by reviewing the drawbacks, many improvements and

Figure 23. Diagram of a photovoltaic cell circuit. Retrieved June 13, 2013 from:

http://www.solinvictus.co.uk/Images/PWwaferdiag.png







modifications are made to ensure that these flaws are either minimized or rectified altogether.

This is the driving force of research in the field of photovoltaic solar which has made significant

progress over the last few decades and shows no indication of ceasing.

Advantages. There are many aspects of photovoltaic technology that make it appealing

to consumers. These reasons will be explained in the following text.

Renewability . As was the case with solar thermal power, photovoltaic solar energy is

renewable; in fact, any solar energy is renewable. This provides the aforementioned merits

wherein running costs are much lower due to the fact that the source of the energy is the Sun.

Non-polluting . Once again this is a reiteration of an advantage of solar thermal power.

Energy produced via solar harvesting offers no pollutants or harmful toxic byproducts like other

energy production systems. As a result photovoltaic solar, among other solar technologies, is an

environmentally friendly option.

Low maintenance. Once the device is installed, there is very little maintenance required.

Modern photovoltaic cells are covered with protective surfaces to ensure that the technology is

not damaged by flying particles or rainwater. This reduces any running costs from the user‟s end.

Disadvantages. While the advantages offer certain clear benefits that other energy

sources may not be able to provide, the technology inevitably possesses flaws that render its

scope fairly limited.

Low eff iciency . One of the cardinal drawbacks of photovoltaic technology is the low

efficiency. Single-junction cells have been reported to reach maximum efficiencies of 20.4%

(“Top 10 World‟s Most Efficient Solar PV Modules”, n.d.). Typical commercial panels possess




efficiencies of around 15-17%. Low efficiency an area of great concern for many users who fear

solar technology will not be able to provide sufficient power.

High cost. The second fundamental constraint of photovoltaic technology is the high

cost. This cost is derived from the high capital cost of installation (grids to support the panels and

inverters to convert the DC current to AC current), the high land cost (the area required to install

the panels) and the high labor cost (the workforce required to install the panels). Although there

are low maintenance costs, the price per watt for photovoltaic solar compared to that for the

other competitive energy sources in the market such as fossil fuels, coal and natural gases is

extremely expensive, although prices are falling (see page 52. Nevertheless, many users refrain

from investing in a photovoltaic setup and instead invest in the cheaper alternatives.

I nconsistent. Once more, the solar energy can only be produced when incident sunlight

strikes the solar panels. On cloudy days or during the nighttime, solar energy cannot be

harvested. Additionally, energy storage is an issue with many photovoltaic devices being unable

to store the energy that they collect during the daytime. As a result, in the event of a cloudy day,

nighttime or an excess demand for energy, there is often no stored energy available for access.

This makes the technology somewhat unreliable in unpredictable climate zones.

Overview. While it will not be determined in this paper whether the disadvantages

outweigh the advantages or vice versa, it will clearly be stated that the success of the field of

photovoltaic solar technology is purely dependent on the two key impediments: high cost and

low efficiency. These two themes will be the subject of a large portion of the remaining paper

and the titular cost-efficiency dynamic with reference to photovoltaic technology will be

addressed in the following chapter.




Cost-Efficiency Dynamic

The high capital and low efficiency dynamic of photovoltaic cells is the restraining factor

in allowing the technology to become competitive in the energy market. Additionally, there is a

tradeoff dilemma. When companies seek to make their devices more affordable to appeal to the

low-income portion of the consumer population, the most obvious solution is to lower the factors

of production, i.e., the materials and means by which the product is produced. However, doing so

inevitably decreases the efficiency since the materials are of a lower quality. Alternatively,

companies may seek to make their devices more efficient to appeal to a specific group of

consumers by increasing efficiencies. This can be done by using higher quality materials or even

covering more land with photovoltaic panels. However, better materials and additional land

would unfortunately drive up the costs. Thus, there is an innate tendency for both cost and

efficiency to move in the same direction. The tradeoff dilemma is that companies have to make a

compromise between the efficiency and the cost, not easily being able to achieve ideal situations

for both.

Scientists and researchers, however, have made much progress since the dawn of

photovoltaic technology. They have developed methods to isolate the two and adjust one without

affecting the other. In some cases, efficiencies have been increased while lowering costs. Each

new invention serves to mitigate the common flaws in photovoltaic technology.

Limits on Cost

A factor that negatively affects the popularity of photovoltaic technology is the limits on

cost and efficiency. These limits arise from a number of factors. Limits on cost are relatively less

concrete and are based on factors such as profitability and factors of production. These limits are




based on the cost of materials needed to produce the panels including the workforce and the

capital costs of machinery. It has been said that the “magic” price of solar energy is

approximately $1 per watt, or 10¢/kWh. At this price, it has been predicted that solar energy will

become a competitive and viable form of energy.

Limits on Efficiency

The limits on efficiency are less of an area of speculation. Efficiency is given by the

proportion of the power of the Sun turned into useful power, or:

Scientific properties and physical phenomenon govern and restrict the maximum

theoretical efficiency of photovoltaic devices. There are natural losses that occur that limit the

conversion rates of photovoltaic cells. The maximum theoretical efficiency of a single-junction

photovoltaic device has been calculated to be 33.7%.

Shockley-Queisser Limit. Scientists William Shockley and Hans Queisser established

this maximum theoretical efficiency limit in 1961 (Shockley & Queisser, 1960). They explained

that the maximum solar conversion efficiency of a single p-n junction solar cell is 33.7%

(Shockley & Queisser, 1960). That is, with the approximate irradiance on the Earth‟s surface

from the Sun being 1000 W/m2, approximately 337 W/m2 would be produced as useful power.

The Shockley-Queisser Limit identifies a number of factors that impede the efficiency including

blackbody radiation. Additionally, the limit makes certain assumptions.

Firstly, Shockley and Queisser assumed that the solar panel would receive solar radiation

from one sun as opposed to multiple suns from some concentrated optics device. Secondly, it is




assumed that the Sun and cell are both

blackbodies that emit radiation with

temperatures of 6000 K and 300 K

respectively. Thirdly, they assumed that

the absorption of photons was a step

function. In Figure 24, E2 represents the

minimum energy required for a photon to

eject an electron from a metal. Photons

with energies less than this minimum

energy all have absorption coefficients of

0 in that they are not absorbed by the metal and are reflected. However, once an electron

achieves this minimum energy the absorption coefficient increases like a step function and it is

absorbed by the metal. Finally, Shockley and Queisser assumed that the only recombination

mechanism was that of radiative recombination – a process in which electrons and holes

recombine to produce a photon with similar bandgap energy – as opposed to Auger

recombination or Shockley-Read-Hall recombination processes in which an extra electron is

produced from recombination and electrons are trapped in between bandgaps respectively.

Shockley and Queisser defined the efficiency of a solar cell as the generated photon

energy divided by the input power from the solar radiation:

In the above equation, hv represents the energy of a photon from Planck‟s quantum

hypothesis ( E=hv). Additionally, Q s represents the number of photons absorbed as given by the

Figure 24. Absorption step function of photons.Retrieved June 25, 2013 from: http://www.epjpv.org/articles/epjpv/full_html/2012/01/pv110021/pv110021-fig2.jpg

http://www.epj-pv.org/articles/epjpv/full_html/2012/01/pv110021/pv110021-fig2.jpg











step function for a given metal with a specific absorption coefficient. Finally, P s represents the

power radiated from the Sun.

The values of Q s and P s are both calculated from Planck‟s Law that had been explained

in detail previously. As we know, Planck‟s Law gives the value of the power contained in a

spectrum as a function of the frequency and the temperature of a blackbody. In the case of Q s,

the Planck‟s Law equation is integrated over an interval of the minimum required frequency to

infinity and in the case of P s , the equation is integrated over an interval of zero to infinity:

( )

( ) ∫

()

∫

∫

() ∫

The P s function is also similar to the Stefan-Boltzmann equation mentioned previously.

To simplify calculations, Shockley and Queisser considered the cell to have a

temperature of 0 K instead of 300 K. Once these numbers are calculated they can be substituted

into the maximum efficiency formula at varying frequencies to provide an ultimate efficiency vs.

energy (bandgap) graph as shown in Figure 25. The graph suggests that the maximum theoretical

efficiency is approximately 44% at bandgap energy of 1.08. This bandgap energy is most similar

to that of silicon. However, the predicted efficiency is 44% whereas the maximum theoretical




efficiency they finally deduced was

33.7%. This is because 44% conversion

efficiency corresponds to an idealized

body at absolute temperature. This ideal

efficiency also pertains to a model in

which solar radiation is absorbed from

all angles and in which no load is

attached providing resistance. To model

a more realistic scenario,

Shockley and Queisser took

into account the incident angle of sunlight, blackbody radiation losses and resistance from an

attached load.

Since the only loss

mechanism assumed is that of

radiative recombination, the

radiation from the blackbody

solar cell would be equal to

the energy emitted as photons

from recombination. The two

developed an equation

relating the recombination

rate to the voltage of the

Figure 25. Efficiency vs. energy gap graph. Retrieved June 25, 2013from: http://web.mit.edu/bolin/www/Shockley-Quisser-limit.pdf

Figure 26. Efficiency vs. energy curve for different blackbodytemperatures.

http://web.mit.edu/bolin/www/Shockley-Quisser-limit.pdf







circuit and the temperature of the cell. They plotted an efficiency vs. bandgap curve for varying

temperatures and deduced their true maximum efficiency value at a bandgap of approximately

1.1 eV was approximately 33.7%. Furthermore, the maximum efficiency decreases as the

temperature increases because blackbody radiation losses increase with higher temperatures.

Other Loss Mechanisms. While the Shockley-Queisser Limit places the maximum

theoretical efficiency at 33.7%, single junction solar cells have only achieved efficiencies of

approximately 20% thus far. This is the result of other losses. One of these losses is the reflection

of photons; not all photons are absorbed by the semiconductor due to the reflective surfaces on

solar cells or lower-than-required energy characteristics of photons. Since a certain proportion of

the photons are reflected back, their energy cannot be used to create electricity which signifies a

loss in efficiency. Additionally, photons that possess higher-than-required energy represent

losses in efficiency; when a photon with excess energy is absorbed, the amount of energy greater

than the bandgap is merely dissipated as heat and cannot be used to generate electricity. These

ranges at which a single junction device can operate is called the quantum efficiency and will be

referred to as such in later chapters. Other factors such as dust accumulation on the surface of

solar cells can inhibit the absorbance of solar radiation which decreases efficiency.

Improvements in Costs and Efficiency

Although imposed with these limits of cost and efficiencies, researchers have developed

new and improved photovoltaic technologies that are steadily approaching the maximum

theoretical efficiency mark. Additionally, some designs are aimed to lower costs to make the

device more affordable in rural areas or for low-income workers. Furthermore, a few of these

innovations can improve one aspect of the technology without adversely impacting the other;




some devices even ameliorate both of the fundamental limitations. Additionally, increasing the

efficiencies of systems means that less space is required to produce the same power, thereby

reducing costs.

According to research, the price of utility scale solar devices (solar devices used at a large

scale to power a utility) has fallen 45.8% in the course of two years from 2010 to September of

2012 to $2.60 per watt (Hoium, 2012). Additionally, the price of residential solar devices fell

21.8% over the same time to $5.46 per watt (Hoium, 2012). Figure 27 shows the falling trend of

prices over time.

Figure 27. Price trends of photovoltaic panels. Retrieved June 25, 2013 from:http://www.seia.org/sites/default/files/Fig2.6.jpg

http://www.seia.org/sites/default/files/Fig2.6.jpg






One company named V3Solar has developed a new form of photovoltaic technology that

has announced prices of merely 8¢/kWh. This price is two-thirds the price of retail electricity and

almost three times cheaper than current solar technology (Roberts, 2013).

Furthermore, the number of photovoltaic installations has grown over time. This is due to

the falling cost of installation which appeals to a larger proportion of the market. Additionally,

governments offer incentives for the installation of photovoltaic devices. The Indian government

launched the Jawaharlal Nehru National Solar Mission initiative as a motivation for companies

to establish solar energy stations. The growth in photovoltaic installations in the United States is

shown in Figure 28.

Figure 28. Growth of photovoltaic installations in the U.S. market. Retrieved June 25, 2013 from:http://www.seia.org/sites/default/files/Fig2.1.jpg







In addition, the prices of other sources of energy such as oil have dramatically increased

in the recent decade as shown in Figure 29.

The impact of the falling prices of solar energy and rising prices of other forms of energy

is significant; it marks a movement of the energy industry to a more solar-oriented business.

Work in solar energy may continue to bring about changes that will cause the solar industry to

overtake the most prominent of industries such as coal and natural gas.

Apart from costs, device efficiencies have also seen major improvement over the course

of history. Naturally, new scientific findings and innovations have engendered new creations

Figure 29. Oil prices from 1973 to 2012. Retrieved June 25, 2013 from:http://www.bp.com/en/global/corporate/about-bp/statistical-review-of-world-energy-2013/energy-charting-tool-.html

http://www.bp.com/en/global/corporate/about-bp/statistical-review-of-world-energy-2013/energy-charting-tool-.html








with unprecedented conversion efficiencies. The rapid rate at which efficiencies are increasing is

shown in Figure 30.

Figure 30 is a comprehensive diagram depicting the various types of photovoltaic

technology (that will further be discussed starting on page 57) including further innovations in

each field. The graph shows an overall positive trend in efficiencies. From the dawn of

photovoltaic technology to present day, it can be seen that every type of photovoltaic technology

has seen major improvements in efficiencies. The orange trend line represents the emerging

Figure 30. Timeline of improving efficiencies of solar cells. Retrieved June 13, 2013 from:

http://upload.wikimedia.org/wikipedia/commons/4/45/PVeff%28rev130528%29.jpg







photovoltaic technologies (which will also be discussed). While these technologies currently

hold the lowest efficiencies, they are merely in their infancy only being discovered in the early

2000s. If the graph shows anything, it‟s that there is always room for improvement – until the

maximum theoretical efficiency limit, of course.

Multijunction Photovoltaic Devices. The multijunction photovoltaic device is one of the

most revolutionary breakthroughs in the photovoltaic industry. In a conventional single junction

device, the semiconductor is made out of one specific material, typically silicon. Each material

has a specific bandgap energy characteristic – a certain amount of energy required for the

electrons in each atom of the material to be ejected. Silicon has bandgap energy of 1.1 eV which

corresponds closely to the maximum theoretically efficient bandgap energy. Additionally, silicon

is relatively cheap compared to other materials and so it is often used in the production of

photovoltaic cells. However, one

major drawback of any single junction

device is the fact that only one material

semiconductor is used. This means that

only a small range of photons that

strike the cell are absorbed; those that

have insufficient energy are reflected

and those that have excess energy are

partially wasted as heat. As a result,

the multijunction photovoltaic device

was devised. The design of the

device intended to broaden the range of the electromagnetic spectrum that could be absorbed by

Figure 31. Multijunction photovoltaic device. RetrievedJune 13, 2013 from:http://www.nrel.gov/continuum/spectrum/images/graphic_news5_02_large.jpg

http://www.nrel.gov/continuum/spectrum/images/graphic_news5_02_large.jpg








broadening the range of photons that could be absorbed. By doing so, the device would allow for

much greater absorption and thus much greater efficiency rates. The design of the device is

shown in Figure 31. The top cell (Junction 1) is typically made from a semiconducting material

with a high bandgap energy. This first junction absorbs high energy photons with energy greater

than or equal to 1.9 eV. The anti-reflection device allows the lower-energy photons to be

transmitted through the top cell to the middle cell. This cell has a slightly lower bandgap energy

which allows it to absorb some of the photons that were transmitted from the top cell. The

process repeats with the low-energy photons being transmitted through the middle cell towards

the last junction. This bottom cell absorbs the low energy photons easily with the lowest bandgap

energy of the three cells (“The Basic Physics and Design of III-V Multijunction Solar Cells”,

n.d., p. 13). This mechanism ensures that a larger proportion of the electromagnetic spectrum is

absorbed and thus maximizes the efficiency of solar conversion. It has been calculated that an

infinite-junction device could achieve a maximum theoretical efficiency of approximately 68.2%

under the concentration of one sun (Yastrebova, 2007, p. 14).

One extremely important limitation of multijunction devices is its expense; the materials

used in the construction of these solar cells are not easily available and are quite expensive. As a

result, the uses of the device are largely limited to utility scale projects as opposed to residential

and commercial usage.

When assembling a multijunction device, several key factors must be taken into

consideration to maximize the power output from the device. These considerations are the

quantum efficiencies of each material used in addition to the lattice constants.




Quantum eff iciency. As

mentioned in previous chapters, the

quantum efficiency is the range at

which a single semiconductor material

can absorb photons efficiently. Varying

materials have varying bandgap which

causes varying quantum

efficiencies. In order to

maximize the efficiency of an

object, materials must be selected that possess quantum efficiencies that complement each other.

This is depicted by Figure 32 which shows the quantum efficiencies of the three materials used

in a popular triple junction device GaInP/GaAs/Ge (“The Basic Physics and Design of III-V

Multijunction Solar Cells”, n.d., p. 16). These quantum efficiencies are related to the bandgap

energy of each junction. GaInP possesses bandgap energy of 1.85 eV, GaAs 1.42 eV and Ge 0.67

eV (“The Basic Physics and Design of III-V Multijunction Solar Cells”, n.d., p. 16). Thus each

junction serves to absorb a certain range of the spectrum and there are minimal quantum losses

due to reflection.

Lattice Matching . Another important consideration in devising multijunction devices is

the lattice constant of each material. The crystallized semiconductor possesses a lattice constant

which describes the distance between atoms in the crystal lattice. Thus lattice matching is a

process in which materials are chosen with similar lattice constants. Lattice matching is

important since it ensures a smooth transmission and maximum conductivity of electrons

throughout the device (“The Basic Physics and Design of III-V Multijunction Solar Cells”, n.d.,

Figure 32. Quantum efficiencies of various semiconductors.Retrieved June 25, 2013 from:http://science.gov.tm/projects/soltme/images/database/35_nrel.pdf

http://science.gov.tm/projects/soltme/images/database/35_nrel.pdf






p. 13). When lattices are mismatched, there are dislocations in the lattice structure which serve as

recombination centers wherein recombination of electrons and holes can occur. As mentioned

before, recombinations represent losses in efficiency due to blackbody radiative recombination.

Thus, lattice matching is an important process and is highly favorable for many designers.

Table 3

Lattice Constants of Semiconductors

Element or Compound Lattice Constant at 300 K

Carbon (Diamond) 3.56683

Germanium 5.64613

Silicon 5.43095

Grey Tin 6.48920

Aluminum arsenide 5.6605

Gallium arsenide 5.6533

Gallium phosphide 5.4512

Indium arsenide 6.0584

Cadmium sulfide 5.8320

Zinc oxide 4.580

Zinc sulfide 5.420

Lead sulfide 5.9362

Lead telluride 6.4620

Note. Adapted from “Lattice Constants”, 2006 on June 25, 2013 from:

http://www.siliconfareast.com/lattice_constants.htm







The table above shows the lattice constants of many semiconductors. While the lattice

constants are important in lattice matching, the bandgap energy of each semiconductor also is

important. For example, aluminum arsenide may seem like a favorable candidate to replace

gallium arsenide in the triple junction device explored earlier. However, the bandgap energy of

aluminum arsenide does not maximize the quantum efficiency matching of the system.

Research in Mul tij unction Devices. The field of multijunction solar technology is of

great interest to many researchers. Since the technology is relatively new, there are plenty of

opportunities to improve the effectiveness of the system.

One group of researchers has optimized the bandgap harmony of three semiconductor

materials: InAlAs, InGaAsP and InGaAs with bandgap energies of 1.93 eV, 1.39 eV and 0.94 eV

respectively (Zyga, 2013). Researchers at the California Institute of Technology in Pasadena, the

National Institute of Standards and Technology in Gaithersburg, the University of Maryland and

Boeing-Spectrolab Inc. in California collaborated to execute this device (Zyga, 2013).

Simulations of the triple junction cell have suggested efficiency rates of 51.8% under 100 sun

concentration, higher than any other efficiency recorded before (Zyga, 2013). The researchers

took many factors into consideration when determining the subcell materials including material

composition, lattice constants, dielectric constants, electron affinities, bandgap energies,

conduction and valence band densities, recombination rates and the number of photons each

subcell would absorb (Zyga, 2013).

Another group of researchers interested in multijunction devices is Harry Atwater and his

team of researchers from Caltech. The group has looked at various forms of multijunction

devices and devised different ways to split the spectrum of incident solar radiation (Orcutt,




2013). One of these designs includes a transparent structure coated with six to eight different

semiconductor materials with varying bandgaps. The interior of the structure contains optical

filters that redirect the spectrum towards different materials thus serving the same purpose as the

device shown in Figure 31 in a new form (Orcutt, 2013). The group has also investigated the use

of holographic redirection in which a hologram is used to divert the incoming radiation as

opposed to optical filters. While Atwater and his team are not sure about which will engender the

highest efficiencies, their research will surely deduce some fascinating results.

Quantum Dots. A new technology called quantum dots have emerged that have allowed

the efficiencies of photovoltaic devices to be increased. Quantum dots are semiconductor

nanoparticles with a unique property; their bandgap energies can be „tuned‟ to the desired value

(Sargent, 2012, p. 134). The diameter of the particles is directly correlated to its bandgap energy

and so, altering the size of the particles allows the bandgap to be easily adjusted. As a result,

quantum dots allow for quantum efficiencies to be matched ideally, thereby minimizing the

losses due to reflection and transmission. It has been deduced that the ideal combination of

bandgap energies in a triple junction cell is 1.95 eV, 1.2 eV and 0.7 eV (Pancholi, 2008, p. 7).

While the bandgaps of GaInP/GaAs/Ge are similar, they are not quite the same. Quantum dots

allow the fine-tuning of bandgap energies of a material to maximize quantum efficiency.

Another unique property that quantum dot devices possess is that of multiple exciton

generation. In this phenomenon, an energized photon strikes the cell and produces multiple

excitons, or electron-hole pairs. As a result, more electricity is created from the same amount of

input power which increases the conversion rates of the system.




It has been calculated that the maximum theoretical efficiency of a single junction

quantum dot device can be increased to approximately 42% due to factors such as ideal bandgap

synchronization and multiple exciton generation.

This new technology is still in its infancy. The Schottky cell was the first quantum dot

device developed with an efficiency of 1%. However, several years later quantum dot devices

have reached efficiencies of approximately 6% and progress is not abating (Sargent, 2012, p.

134).

Quantum dot devices are comprised of thin films of the nanoparticles used. The process

of creating the thin films is relatively and so the process also provides an economical advantage.

Nanotechnology Research. While quantum dots are an economic solution, some

researchers remained unconvinced regarding the efficiency of the device. As a result, teams of

scientists have developed ways to improve the efficiency of quantum dot devices using

nanotechnology.

One group of MIT researchers has utilized nanowires to maximize the absorption of

photons. A common tradeoff in the manufacturing of photovoltaic cells is involved with the

thickness of the wafer; thickness is required to absorb the photons but thinness is required for

electrons to traverse the layers. As a result, compromises have to be made where either

absorption or electron mobility are sacrificed. Using vertical nanowires, however, researchers

have eliminated this compromise. The wires penetrate the quantum dot layer allowing electrons

to be conducted along a given path. Additionally, the nanowires efficiently absorb photons by

acting as a trap (Bullis, 2013). Nanowires have been thought to increase efficiencies of devices

and progress in the field of nanoscale solar technology is not abating.




Concentrated photovoltaics. Another approach to improving the efficiencies of

photovoltaic devices is by concentrating and amplifying the intensity of the incident solar

radiation. An optic lens is used to magnify the power of incident sunlight onto a photovoltaic

cell. The effect that this has is that the performance of the cell is increased dramatically and the

power output is increased (Yastrebova, 2007). Additionally, the theoretical maximum

efficiencies are increased for all photovoltaic devices as depicted in Table 4.

Table 4

Maximum theoretical conversion efficiency with and without concentrated solar radiation

Number of junctions Efficiency under 1 sun (%) Efficiency under 500 suns (%)

1 junction 30.8% 40.8%

2 junction 42.9% 55.7%

3 junction 49.3% 63.8%

∞ junctions 68.2% 86.8%

Note. Adapted from “High-efficiency multi-junction solar cells” by Natalya V. Yastrebova, 2007

from http://sunlab.site.uottawa.ca/pdf/whitepapers/HiEfficMjSc-CurrStatus&FuturePotential.pdf

Presently, concentrator photovoltaic devices are quite costly since the optic lenses,

tracking systems and racking layouts require additional investment. As a result, concentrated

multijunction devices are restricted to utility scale usage whereas some concentrated single

junction devices are sold commercially (Yastrebova, 2007, p. 15).

Research in Concentrated Photovoltaics. Concentrated photovoltaic solar is a field that

has shown potential in developing efficient and cost-effective technology.

http://sunlab.site.uottawa.ca/pdf/whitepapers/HiEfficMjSc-CurrStatus&FuturePotential.pdf







One company named V3Solar has developed a technology named the Spin Cell which

has claimed record-breaking prices of 8¢/kWh. This astoundingly low price puts solar energy in

a highly competitive position and its commercialization will reform the landscape of the solar

industry. The technology functions by resolving two problems of typical concentrated

photovoltaic cells. Firstly, the shape of the device is a cone which means that it can receive

incident sunlight regardless of the position of the Sun in its path (Orcutt, 2013). Secondly, many

typical concentrated photovoltaic cells reach extremely high temperatures due to the

concentrated sunlight focused on the cells. As a result, these typical cells require specialized

heat-resistant materials and maintenance in order to ensure no damage is done to the device. The

Spin Cell resolves this issue by spinning; the mechanism cools the cell down sufficiently so that

no expensive material is required (Orcutt, 2013). While the feasibility of the device cannot be

evaluated yet (it has not been released publicly) it is clear that companies are relentlessly

devising new and improved ways to solve prevalent issues.

Another company, IBM, has combined concepts from multijunction cells, concentrated

photovoltaic solar and even solar thermal designs. The research team devised a solar structure

comprised of a mirrored dish (similar to a dish Sterling) that focused and concentrated light

towards a triple junction photovoltaic cell. The dish had the capacity to concentrate the sunlight

by a factor of 2000 onto the multijunction cell (Holloway, 2013). With the combined high

efficiency of the triple junction cell and the intensified sunlight, the structure could potentially

reach extremely high efficiencies. However, the device also suffers from great temperatures

arising from the severe concentration. As a result, IBM has developed a method to make use of

this heat generated – solar thermal power. A receiver similar to those mentioned before is

positioned close to the source of heat: the multijunction cell. When the cell heats to high




temperatures, heat transfer occurs from the cell to the receiver tube thus cooling the cell and

heating the water. The heated water can then be used in a turbine-driven generator system to

produce electricity. This system ensures that the 70% of energy wasted as heat in ordinary

situations is used advantageously (Holloway, 2013). The team claims that their improved

technology would increase efficiency rates to approximately 80% (Holloway, 2013).

Thin-film photovoltaics. The dimensions of a photovoltaic cell are important

considerations when designing the technology. The thickness of a solar cell defines the

proportion of the spectrum absorbed. Each material has its own absorption coefficient and thus

absorbs photons at varying rates. Additionally, photons with different energies are absorbed at

different rates. Thus, the thickness of a solar cell defines the range of photons absorbed and thus

the efficiency of the cell. The thickness of a traditional crystallized solar cell is approximately

350 microns (Harris, n.d., p. 2). Thin-film photovoltaic devices, however, are a mere one micron

in thickness (Harris, n.d., p. 2). Thus, the technology sacrifices efficiency for cost and

convenience. The solar cells are constructed of thin layers of semiconductor material. While the

construction of a thin-film solar cell is less expensive than the crystallization of traditional solar

devices, the efficiency of the thin cell is much less than its counterpart. However, where the cell

lacks in efficiency it makes up for in convenience. The thin-film design allows users to harvest

solar energy without the bulky appearance of typical crystallized panels. Additionally, the thin

sheet-like technology can be easily placed anywhere without laborious construction and

installation of racking systems.

Storage of Solar Energy. While not explicitly related to costs and efficiencies, the

storage of solar energy is a matter of great concern. Without storage, solar energy is an extremely

unreliable and secondary technology that may not be able to be used alone. However, by




introducing storage capabilities, solar energy becomes a form of energy that can be used even

when the Sun is down. Many innovative storage methods have been concocted and are in use by

many companies to store the energy collected from the Sun.

Batter ies and supercapacitors. The energy collected from the Sun can be stored for later

use in batteries and supercapacitors (Panzer, 2013).With the case of batteries, the electric energy

is converted into electrochemical potential (Panzer, 2013). This stored potential can be converted

back to electric energy which can be useful. Alternatively, supercapacitors can store solar energy

by storing the collected energy in an electric field (Panzer, 2013). When this energy is required

again, it can be converted back to useful energy.

Molten salt. Another form of storing solar energy is by storing the thermal heat in molten

salt. These molten salts are used for their characteristic low volatility. As a result, they can reach

extremely high temperatures without changing state. This makes them useful heat storage

devices. When the energy is required for use, the molten salt can be used to heat liquid to

produce steam which can drive a turbine-driven generator system (Panzer, 2013). Large scale

solar plants contain large containers of this molten salt for energy storage purposes.

Fuel cell s. The final common storage method of solar energy is the use of fuel cells.

These devices function by separating elements in gaseous compounds. The recombination of

these elements produces energy which can be used usefully. For example, the energy harnessed

by the Sun can be used to split steam into hydrogen gas and oxygen gas. These gases are held

separately and have great chemical potential. When they recombine, they produce electricity that

can be used. Additionally, the only byproduct of this reaction is water which can also be used

usefully (Panzer, 2013).




The Aditya Birla Group

The solar business group of the Aditya Birla Group and the Aditya Birla Science and

Technology Company are two companies whose involvement in the solar industry was studied.

After a thorough research of solar energy, representatives of the companies were spoken to and

an understanding of the research being done in the solar energy field was acquired. Additionally,

an understanding of the approaches by these companies to resolve certain issues regarding the

costs and efficiencies of solar devices was obtained.

The Aditya Birla Group first began investigating the solar field in 2008. One year was

spent examining various technologies and business models. Additionally, they evaluated the

value of entering the business along with an assessment of the synergies available with their

existing businesses. Additionally, in 2010 the Indian government announced the Jawaharlal

Nehru National Solar Mission, an initiative aimed at achieving 1000 MW of installed solar

power by 2013 and planned further expansion of the Indian solar industry in later years. The

mission proposed an equal allocation of photovoltaic technology and solar thermal power

technology as part of the 1000 MW. The Aditya Birla Group has established a photovoltaic plant

in Rajasthan with the objective of installing 100 MW of capacity by the end of 2013.

The Aditya Birla Science and Technology Company similarly began plans to enter the

solar business in 2008. They began by evaluating the industry and worked with strategists to

identify the merits and limitations of starting a solar business. One important consideration was

the synergies available. Being a multifaceted company, they leveraged their assets by making use

of the products supplied by partner companies such as Hindalco, who produced aluminum. The

company looked into using aluminum reflectors for solar thermal power as opposed to mirrors.




They ran research and development projects and built prototypes to test these aluminum-based

devices. In 2010-11, they completed the project, but did not commercialize their invention until

later. In 2012, the company looked into solar once more, specifically their aluminum-based

structure. They wanted to commercialize this low-cost technology and had their project

approved. The company proceeded to make use of their aluminum device for solar water pumps

aimed at a low cost to increase availability in rural areas. The convention in solar water heaters

was to use copper. However, the copper metal posed issues regarding durability and cost. The

aluminum produced by Hindalco, a group company, was easily available and so by lowering the

costs of the factors of production, the company was able to commercialize their products at

significantly lower prices. The Aditya Birla Science and Technology Company has also

leveraged their assets by powering mobile towers through solar power. This new form of energy

would replace the earlier form – diesel – thereby lowering the company‟s long run costs and

pollution.

It can be seen that each company has established different goals in the field of solar

energy; while the Aditya Birla Group is aiming at increasing installed capacity of solar energy in

order to meet the Jawaharlal Nehru National Solar Mission‟s objectives, the Aditya Birla Science

and Technology Company has focused on providing affordable forms of solar energy in order to

compete with other, more harmful, sources of energy such as coal and natural gases.

Suggestions for Future Action

After extensive research of the technology and approaches by other companies in the field of

solar energy, some ideas were formulated that the Aditya Birla Group and the Aditya Science




and Technology Center may be able to employ to ensure prosperous outcomes in the field of

solar.

Primarily, the high temperatures in the Indian subcontinent present a problem with regard

to photovoltaic devices. The typical crystallized silicon wafer device operates less efficiently at

higher temperatures, as represented by Figure 26. As a result, the high temperatures would mean

that the photovoltaic devices implemented by the Aditya Birla Group in Rajasthan would not

only be inefficient but would additionally require extensive maintenance to ensure damage

caused by heat is dealt with. There are several alternatives that the company could adopt to

improve the economics of their plan. One option is to incorporate thin-film photovoltaic devices

in the Rajasthan power plant. This technology costs less to produce and additionally does not

gain thermal energy like the typical crystallized photovoltaic devices do. While the efficiency of

the thin-film devices are naturally lower, their lack of heat absorption would favorably offset this

disparity and produce similar efficiencies as the typical photovoltaic cells with the additional

benefit of lower maintenance costs. Another option, similar to the work done by IBM, is to make

use of the excess heat generated by running receiver tubes in close proximity to the photovoltaic

devices. The heat source – the photovoltaic cells – would serve to raise the temperature of a heat

carrying liquid passing through the tube which would then run a turbine-driven generator system.

The energy wasted as heat would be transformed into useful energy thereby increasing the

efficiency of the overall system. Finally, the group may be interested in investigating the science

of quantum dot solar cells and the feasibility of developing solar cells made up of the

semiconductor nanoparticles. This technology is a low-cost alternative to the typical photovoltaic

cell and provides an economical advantage. However, given the technology is still in its infancy,

there will be a sacrifice in efficiency rates and so this option is the least advisable. Nevertheless,




R&D projects in the field of nanoscale solar technology may provide fruitful results that would

allow the company to maximize their power production capabilities.

With regard to the Aditya Birla Science and Technology Company, the company could

further exploit the aluminum synergy by taking advantage of the material‟s high heat capacity.

The company could collaborate with the Aditya Birla Group and devise cheap aluminum

protective surfaces to ensure overheating does not occur. Since aluminum has one of the highest

heat capacities of any metal, it is a great candidate for a protective material since its temperature

does not rise significantly relative to other metals. In fact, the aluminum material could be used

in any location where high temperatures pose detriments to efficiency and power output.

Global Action

Solar photovoltaic and solar thermal power are the main constituents of an emerging

form of energy production known as solar energy. However the technologies, being in their

infancy, bear a few limitations – namely their low efficiency conversion rates and relatively high

costs – that undermine their otherwise promising advantages – such as renewability and low

pollution. Nevertheless, companies, scientists, governments and other researchers are

endeavoring to both decrease the costs of the technology and increase the efficiencies and they

are doing so with great success. The research conducted provides sufficient evidence of the

exponential increase in solar installations and the converse correlation for installation prices.

While as of yet the solar industry has not reached the pivotal levelized cost of energy of that will

propel itself into the competitive realm of energy production processes, the price per kilowatt-

hour has dropped significantly over time suggesting that in due time solar technologies will be

the most prominent means of energy production.




In order to accelerate the process, many governments provide incentives and launch

national missions in order to promote the growth of the solar industry. Among the countries with

the most installed solar energy are Germany (17,193 MW), Spain (3,784 MW), Japan (3,662),

Italy (3,494 MW) and the United States (2,528 MW), all of which have secure feed-in tariff

schemes, policies promoting solar growth, national solar missions, lowered taxes on solar energy

output or a combination of the aforementioned (European Photovoltaic Industry Association,

2012). In addition to government-related policies and schemes, these companies share the

common characteristic of a large R&D budget for investigating the potential of solar energy

while other countries are lacking in adequate funding to allow for development in the solar

industry (Blakers et al., 2009). For example, Germany, the leading country in solar installations,

spent a total of approximately US$ 1 billion in R&D efforts in the year of 2006 whereas

Australia, a country less invested in the development of solar energy, spent a mere US$ 23.5

million in R&D efforts (Blakers et al., 2009). Thus, developing countries tend to invest less in

solar energy than developed countries as a result of available R&D budgets. However, by

increasing R&D budgets, countries may generate a deficit in the short-run, but the long-run

merits of solar energy production will compensate the initial costs and provide a more

sustainable future.

The World Energy Vision 2100 forecast developed by the German Advisory Council

predicts that solar energy will make up approximately 20% of the world energy supply by 2050

and above 60% by 2100 (Blakers et al., 2009). The report on research and development states,

however, that for solar energy to achieve prominence, photovoltaic technologies – namely single

cell panels and concentrated photovoltaic devices – need to achieve 25-40% efficiency as

opposed to the current efficiency of approximately 15% (Blakers et al., 2009). In addition to




higher efficiencies, the report states that the target pivotal cost of photovoltaic power generation

should be approximately 7¢/kWh – equivalent to that of conventional electricity generation via

natural gases – by 2030.

With many governments striving to expand their solar industries, the prospect of solar

energy dominating the field of energy production is becoming increasingly likely.




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Documents

Solar Energy Research Paper, June 2013: The Cost-Efficiency Dynamic of Solar Technology