Solar energy as a source of electricity: A comparison between photovoltaic and solar thermal systems

Toni Menninger, University of Arkansas, April 2009

Introduction: overview over solar energy uses

Almost all organisms on Earth depend on solar energy for their survival. Plants and algae

capture photons emitted by the sun and transform their radiation energy into chemical energy, which in turn powers the metabolism of all plants and animals, including humans.

Solar energy is also at the origin of wind and wave power, hydro power, and biomass fuels, and even the energy embodied in the fossil fuels coal, oil, and natural gas is nothing

but solar energy captured millions of years ago and conserved in geological formations.

Almost all energy used by human civilization is therefore ultimately of solar origin. Solar

energy is an abundant and renewable energy source and there are many ways of technically

taking advantage of this source of power. Some of the most interesting applications are small

scale and decentralized and don't require much capital or knowhow. Examples are solar

water disinfection and solar cookers. Solar Water Disinfection is a low-tech process

developed by the Swiss research institute EAWAG that consists of filling potentially

contaminated water into transparent bottles and exposing them to six hours of sunlight.

Laboratory tests confirmed that the microbiological quality of the water is significantly

improved and pathogens causing diarrhea are killed by UV radiation and heat (EAWAG

2009).

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Solar cookers are devices made at very low cost of cardboard and aluminum that allow poor

families in developing countries to cook water and prepare meals without the need for burning

fuels (JWW 2009).

Widely used in regions where sunlight is abundant are solar water heating systems consisting of an often very simple and inexpensive solar collector. Israel and Spain currently

require the installation of solar water heating systems in all new buildings (REN21 2006:2).

Photovoltaic (PV) cells are becoming widespread in many micro scale applications such as

pocket calculators, thus eliminating the need for batteries, and in small scale uses such as

road signs, street lights, or remote telecommunications. On the household scale, PV cells

supply electricity to off the grid households in remote areas, often in combination with wind

power (Gipe, 2004:229). Grid connected PV systems, however, have become “the fastest

growing energy technology in the world, with 50 percent annual increases in cumulative

installed capacity in both 2006 and 2007, to an estimated 7.7 GW. This translates into 1.5

million homes with rooftop solar PV feeding into the grid worldwide”. (REN21 2007:6)

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Figure 1: Solar PV world capacity. From REN21 (2007:11)

Solar energy, as we have seen, is used in countless small scale applications all over the

world. But can solar energy also be a large scale source of electricity to power our industrial

economy? This paper will discuss and compare two technical systems for large scale

electricity generation based on sunshine: photovoltaic power plants, and solar thermal plants.

Photovoltaic power plants

Photovoltaic cells convert sunlight immediately into electricity. The conversion efficiency of

current systems is in the 10-20% range and the output directly depends on the amount of

sunlight that reached the cell. For grid-tied systems, the PV output, which is direct current,

must be converted to alternating current, resulting in some loss of energy.

In large scale commercial power plants, tens of thousands of PV panels are arranged on

hundreds of acres of land, typically on agriculturally marginal or desert areas. Often, these

panels are automatically adjusted to track the diurnal and seasonal cycle of the sun's

apparent movement, thereby maximizing the energy yield.

Most PV energy systems are only a few kilowatts (kW) in size but more large-scale

installations with capacities of hundreds or more kW have been developed in recent years.

According to REN21 (2007:12), there were, as of 2007, more than 800 plants exceeding 200

kW in capacity and at least 9 plants larger than 10 MW—in Germany, Portugal, Spain, and

the United States. The world's two largest PV power plants (20 MW each) were in Spain

(REN21, 2007; pvresources.com, 2008).

However, the number of large-scale PV installations has grown dramatically since 2007.

Pvresources.com (2009) currently lists 66 PV installations with 10 MW or more in capacity,

most of which have been constructed in 2008, mostly in Spain where these plants have been

supported by generous subsidies. In the USA, only two plants > 10 MW are listed, both in

Nevada. The largest photovoltaic system in North America is in Nellis Air Force Base in Clark

County, Nevada. Inaugurated in December 2007, it occupies 140 acres and has a capacity of

about 14 MW.

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Solar thermal power plants

Solar thermal power generation uses the solar radiation to heat a thermal fluid (also known as

a collector or receiver). This collected heat is then used to generate electricity e. g. with a

steam turbine. This is in principle the same process as in any thermal power plant (the heat is

first transformed into mechanical energy and then into electricity). Due to the second law of

Thermodynamics, the efficiency of converting heat into electricity increases with the

temperature. The efficiency of any thermal machine cannot exceed the Carnot efficiency,

which is given by =1−T CT H

where T H is the temperature (on the Kelvin scale) of the

incoming heat, and T C is the ambient temperature. It follows that, as with any thermal

power plant, it is advantageous to reach as high a temperature as possible.

In solar thermal power plants, therefore, the solar radiation is concentrated by means of

reflectors to achieve a high temperature, a process known as Concentrating Solar Power

(CSP) (DOE 2007). The requirement to reach a high temperature limits the conditions under

which CSP can work. Whereas PV systems can generate power even on cloudy days and

under diffuse light conditions, CSP systems require direct, strong sunlight (CNN 2008).

Several technologies are available to concentrate solar radiation:

• Parabolic trough: The sunlight is concentrated by a long rectangular reflector that has

a parabolic profile (figure 2), and the pipe with the collector fluid runs through the focal

point, where the parabola concentrates the sun rays. The trough is adjusted to track

the sun by tilting its angle. The collector fluid reaches up to about 400°C (DOE 2007).

• Heliostat to central tower (also known as power tower) (figure 3): Heliostats are flat

movable mirrors that are arranged in an array around a tower, on top of which the

collector is positioned. All the mirrors must continually track the sun to keep focused on

the tower. The power tower design reaches a higher degree of concentration than the

parabolic trough design and a higher collector temperature (up to 560°C) can be

reached, increasing the power generation efficiency (DOE 2007).

• Dish engines are mirrored dishes shaped like a large satellite dish, concentrating solar

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radiation in the dish's focal point. This design can be very efficient with the receiver

integrated into a Stirling engine (DOE 2007).

Solar thermal power plants are more complex than photovoltaic plants because electricity

generation requires the additional step of thermal power generation whereas PV cells

immediately deliver electricity. This can turn into an advantage, however, because heat can

more easily be stored than electricity. Excess heat can be stored as hot liquid or steam or

transferred to other materials such as molten salt. Solar thermal plants combined with thermal

storage can store heat during the day to deliver power during the night (CNN 2008), and they

can be supplemented with fossil fuels during peak demand periods (DOE 2007). This makes

CSP technology more versatile, more predictable, and more reliable, which makes it much

more useful for large scale utility power generation.

The world's largest solar thermal power plant, and the largest solar energy generating facility

in the world, is Solar Energy Generating Systems (SEGS), a conglomerate of nine

parabolic trough plants in the Mojave desert in California with a combined capacity of 354

MW. Some of the plants have been in operation since 1985 (Flagsol 2008). In recent years, a

resurgence of interest in solar thermal plants has been observed, especially in Spain and the

United States (REN21 2007:12). It has been calculated that a solar thermal plant taking up 92

by 92 miles of desert could satisfy the electricity demand of the entire U.S. (CNN 2008).

In Europe, some experts have called for the development of large solar thermal power plants

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Figure 2: Parabolic troughs Figure 3: A power tower (from

DOE 2007)

in the Sahara desert as a source of clean and sustainable energy for the entire region

including North Africa, the Middle East, and Europe (EU-MENA). Only a small fraction of the

Sahara desert area would be needed for the ambitious project (DESERTEC Foundation

2009:20).

Conclusions

The main differences between photovoltaic (PV) and solar thermal (CSP) power generation

can be summarized as follows:

• The CSP technology is geographically severely limited. The optimal sites are desert

areas in low latitudes, where strong, direct sunlight can be expected most of the time.

Excellent conditions can be found notably in the Southwestern United States,

especially the Mojave desert, in the Sahara, in the Spanish Extremadura, and in

Australia.

• PV systems are more broadly applicable, even in higher latitudes and less sunny

climates, like Central and Northern Europe and the Northeastern United States. They

are also more scalable, from the pocket calculator to the rooftop solar panel to the

large power plant covering thousands of acres. They can be sited close to where the

energy is needed.

• Where the conditions are right, CSP plants have the advantage because they can be

combined with thermal storage and supplemented with fossil fuel. However, the places

most suitable, such as the Sahara, are often far away from consumers. Therefore,

electricity has to be transported over large distances which necessitates significant

infrastructure investments and involves political risks in the case of cross-border

transport.

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References cited

CNN (2008), All about: CSP, last updated March 17, 2008 (http://www.cnn.com/2007/WORLD/asiapcf/11/12/eco.about.csp/index.html#cnnSTCText)

DESERTEC Foundation (2009), Clean Power from Deserts: The DESERTEC Concept for Energy, Water and Climate Security. WhiteBook, 4th Edition (http://www.desertec.org/fileadmin/downloads/DESERTEC-WhiteBook_en_small.pdf)

DOE (2007), Concentrating Solar Power, Last Updated: July 13, 2007 (http://www.eere.energy.gov/de/csp.html)

EAWAG (2009), Solar Water Disinfection (www.sodis.ch)

Flagsol (2008), SEGS - Solar Electricity Generating Systems (http://www.flagsol.com/SEGS_tech.htm)

Gipe, Paul (2004), Wind power: Renewable energy for home, farm, and business. Chelsea Green Publishing Company

JWW (2009), Jewish World Watch: Solar Cooker Project ((http://www.jewishworldwatch.org/refugeerelief/solarcookerproject.html)

Nellis Air Force Base (2007), Nellis activates Nations largest PV Array (http://www.nellis.af.mil/news/story.asp?id=123079933)

pvresources.com (2008), Large-Scale Photovoltaic Power Plants, Annual Report 2007, Revised Edition, April 2008 (http://www.pvresources.com/download/AnnualReport2007.pdf)

pvresources.com (2009), Large-scale photovoltaic power plants range 1 - 50 (http://www.pvresources.com/en/top50pv.php)

REN21 (2006), Renewable Energy Policy Network for the 21st Century: Renewables Global Status Report, 2006 Update (http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_Update.pdf)

REN21 (2007), REN21 Renewables 2007 Global Status Report (http://www.ren21.net/pdf/RE2007_Global_Status_Report.pdf)

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