Unit 5 Solar Energy Photo Voltaic

8/9/2019 Unit 5 Solar Energy Photo Voltaic

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Introduction to Alternative EnergiesIntroduction to Alternative EnergiesUnit 5Unit 5 Solar Energy (Photovoltaic)Solar Energy (Photovoltaic)

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Is There Enough Solar Energy in Ohio? Ohio receives 1.4 megawatt hours per square

meter (MWh/m2) in an average year

Annually residents of Ohio use 4.5 MWh for

electricity and 7.6 MWh for heat

Covered in the previous unit Solar Energy forheat

This unit now covers the systems used for solar

electricity or solar photovoltaic

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After completing this unit you will

Be able to explain what a photovoltaic

cell is and how it converts sunlight to

electrical energy

Be aware of some of the different typesand variations of photovoltaic cells

Be able to determine the amount ofenergy achieved from a photovoltaic cell

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What is a

photovoltaic celland how do they

into electric

energy

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Lets start with

Photovoltaic

Photo = light Voltaic = electricity

Light-electricity or electricity from light

A solar photovoltaic cell produces electricity

from sunlight

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A simple way to explainphotovoltaics

Is the reverse of an LED

(light emitting diode)

When electricity is applied

to an LED, ig t is emitte Photovoltaics work just

the opposite, when

exposed to light (sunlight) electricity is

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Electricity is the flow ofelectrons electrons are negatively charged particles

Certain materials, called semi-conductors, can

e a apte to re ease e ectrons w en t ey areexposed to light

One of the more common materials is silicon

Silicon is the main material in 98% of photovoltaic

cells made today

http://www.co-operativebank.co.uk/

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The main parts of a photovoltaic cell aretwo layers of a modified (doped) semi-

conductive material

The N-Layeris blended with

phosphorous, making it rich in electrons

The P-Layeris blended with boron,

making it electron-poor leaving

positively-chargedholes whereelectrons can fit

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At the junction of the N and P-layers is anelectrically neutral barrier

This barrier makes it difficult for the extra

electrons of the N-layer to jump over to fill the

holes in the P-layer

On t e outsi e o t e N an P- ayers isconductive material, which is connected

(wired) to an electrical load

This creates a closed circuit and a current pathfor the electrons to flow from the N-layer to

the P-layer9


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A photovoltaic cell

Protective glass

Anti-reflective coat

Photons from the sun

P-type silicon

Conductive backing

Conductive mesh

N/P junction

N-type siliconLoad

-- -- --

-- -- --

--

--

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The conductive material on the N-layer side isa mesh allowing sunlight to pass through

When energy is added to a semi-conductive

material, such as silicone, it can cause

electrons to break free from their atoms

In the case of a photovoltaic cell, energy

from the sun hits the electron-rich N-layer in the form of light photons and

causes electrons to break free

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If they are close enough to the P-layer, theseelectrons can jump across the electrical field

and fill the P-layer's holes

The resulting electrical imbalance encourages

these electrons to flow back to the N-layer

,

The greater the intensity of the light,

the greater the flow of electricity

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Theoretical Efficiency

100% efficiency is

achieved when exposedto monochromic light

,

typically not the case

Typically they are

exposed to the fullspectrum of light (right

side of figure)

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When exposed to the full spectrum, there aretwo mechanisms that will limit efficiency

Photons that do not have enough energy to create

the electron-holes and just pass through creatingonly heat

Photons that have too much ener creatin

electron-holes too fast and are quickly dissipatedbefore generating any electric energy

The efficiency is then the ratio of the load

power to the input power of the radiation

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What are some

different types orvariations of

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Spectrally Selective Beam Splitting

As noted, depending on the material, only a

portion of the solar spectrum is used

Therefore, when manufacturing the

, -

spectrum must be split into appropriate

sections with corresponding frequencies

This can be accomplished in several differentways including cascaded cells, filtered cells,

and holographic concentrators

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Cascaded cells are basically two photodiodeswith different band-gaps superimposed

As the solar energy passes through the top

photodiode, the relevant band-gap is used

The remaining spectrum passes through to

t e next p oto io e w ere a i erentband-gap is used

Conceptually, very simplified

a solar spectrum sieve

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Filtered cells work in one of two differentmethods

Absorption, where sunlight passes through a

particular substance such as cobalt sulfate

which absorbs certain bands of the spectrum

Interference, where different materials are

used to refract different parts of the spectrum

allowing only the specific band of thespectrum needed

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Holographic concentrators are probably themost attractive of the different devises

These devises take the sunlight and split it

into separate bands and refocuses the

specific sections of the spectrum

T e p oto io es can t en e mounte insuch a manner that the various photodiode

materials can align with the appropriate

section of the spectrum

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Thermo-photovoltaic Cells

Another method of increasing

the efficiency of photovoltaicconverters is to recirculate

for electric generation for

heat

This can be accomplishedseveral different ways

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One method includes placing the photodiodebetween a thermal radiator and a mirror

As the unused energy passes through the

photodiode, it is reflected back through to thethermal radiator

photodiode and a thermal radiator

As the energy passes through the selective

mirror, only the useable band-gap passes

through to the photodiode and the remaining is

reflected back to the thermal radiator

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The thinness of the cells When the cells are excessively thin, they fail to

react with the available light

Material for the cells is very expensive and theremust be compromise between cost and thickness

And the lifetime of the minority carriers, as

some of the electron holes are created too far

from the potential barrier to survive thelength of time needed to reach it by diffusion

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How do you

determine the

amount of energy

photovoltaic cell

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Economics The obvious benefit of photovoltaic

converters is that the energy (solar) isbasically free

,

available at certain times of the day and

some days may be overcast

The supply is not constant, therefore,the cost associated with these systems is

for the converting and storage of energy

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There are many different ways to look at thecost of photovoltaic converters, which have

been coming down in price over the last few

years due to improvements in technology

Related to cost is efficiency ranging from 5%

the system which affects the cost of the

system

Beyond the cost of the system, there arecosts associated with installation and

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This cost is then compared to the benefit orthe energy obtained

Since the solar energy is not constant, there

are times when there is excess energy

produced and times when no energy is

Depending on the type of system and local

utilities the excess power can be sold back to

local utility companies or used to offset utilitycostsThis type of system would be referred

to as a utility-tie system28


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With a utility-tie systemthere is a utility service inverter tied to the system

used to match the frequency and phase of generated

solar energy to the grid

http://internationalenergysolutions.com/index-2.html 29


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For areas that cannot be tied to a local utility,a system referred to as an off-gridsystem

could be used which incorporates batteries

Hybrids of these two

systems integrated

buildings, building-

integrated

photovoltaic(BIPV)systems are becoming

very popular

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These systems are typically placed on or as theroof of a building creating a large surface area

This system is more appealing to single story

buildings versus a several story complex and,therefore, is more suited to residential

One thing that must be considered is the peak

solar energy available which is dependent on

your location

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Different materials and techniques for buildingthe solar cells also have relevance to the cost

and return, three of which are

1. Silicon

2. Amorphous thin films

3. And organic polymers which are still in theearly stages of development

* The organic polymers may be the best solution

showing promise to be light weight, flexible andrelatively low in cost leading to more applications

and thus better economics

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Energy potentialTo roughly determine the amount of energy

from a photovoltaic system is relatively simple

You will need the ollowin in ormation

a. As with solar collectors, you need the solarpotential or energy production factor for your

area in kilowatt-hours per kilowatt-year

(kWh/kW-year)b. And the size of the system in kilowatts (kW)

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The map below, from the U.S. Department ofEnergy-Energy Efficiency and Renewable

Energys web site,

shows the energypotentialfor

different re ions

of the Unitedstates in

kilowatt-hours

per kilowatt-year(kWh/kW-year)

http://apps1.eere.energy.gov/consumer/your_ho

me/electricity/index.cfm/mytopic=10860

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Looking at the Columbus, Ohio area, asin the previous section, you see the

energy potential is around 1500

(kWh/kW-year)

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A photovoltaic system typically ranges in sizefrom 1 to 5 kW

The table below,from the California Energy Commission's Buying a

Photovoltaic Solar Electric System, Consumers Guide, 2003 edition, showsvarious PV capacity ratings with the related roof area needed

Note: the (5) solar collectors used in the previous

section used a total roof area of 159 ft236


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To roughly determine the availablepowerperyear (kWh/yr) of a photovoltaic system, you

simply multiply the energy production factor by

the system size

kWh/yr= energy prod. x system size

Where :

energy production factor is (kWh/kW-yr)

system size is (kW)

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Using the example from the previous unitLiving in Columbus, Ohio, determine the

annual energy potential for a 2 kW

photovoltaic system

(kWh/yr) = energy prod. x system size= 1500x 2

= 3000 kWh/yr

Note: the (5) solar collectors used in the previoussection produced 7884 kWh/yr

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Comparing a similar sized roof area, 160 ft2

, thehighest efficiency (16%) would be required

and from this you can assume cost to the high side,

$20,000 for a 2 kWh system, which produces less

than half the kWh/yr of the solar collectors

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How about an

interesting

concept

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Solar-Power Satellite As we have learned, the idea of using

photovoltaic converters to generateelectricity is definitely a possibility

utt ey are epen ent on t e a ty o agiven area to collect the solar energy

Therefore, areas that do not receive large

amounts of continuous sunlight will notfind this technology as appealing as others

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The concept of a solar power satellite couldeliminate the concern for continuous sunlight

by placing the solar collectors into space

orbiting to receive constant solar energy

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As the orbiting solar collector absorbs theenergy, it can convert it and then transmit it

to collectors on Earth

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ORWhat if the same concept was

used on some kind of vesselthat traveled the Earthssurface so as to remain inconstant sun g t

This vessel could then convert thesolar energy to electricity and

transmit it via satellites or othermeans to various locations aroundthe rest of the Earth

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Da Rosa, A. V. (2005). Fundamentals of Renewable Energy Processes. Burlington, MA,

USA: Elsevier Inc.

http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm

Work Cited

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Unit 5 Solar Energy Photo Voltaic