Solar Cell Technology

A discussion on photovoltaics and their viability as a form of renewable energy

Tushar Nandwana
April 1, 2011

Executive Summary

Globally, renewable energy is gaining popularity as source of energy, relative to some of the more conventional sources used today, like fossil fuels and nuclear power. In 2009, 26% of the 4.73 trillion watts of total global energy generation was from renewable sources, with 66% from fossil fuels and just 8% from nuclear power1. Renewable sources of energy included: hydroelectric (80%), wind (13%), biomass (4%), solar power (via grid-connected photovoltaics) (1.7%), geothermal (1%), concentrated solar thermal and ocean2.

While some people may be familiar with most of these types of renewables, solar power through photovoltaics (PV) is an area where there is often a lack of knowledge and widespread misconception about the capabilities available. And although PV only accounted for a small percentage of renewable energy, the past decade has seen explosive growth in the field—in the United States alone, the PV industry shipped 30% more gigawatts of cells/modules in 2009 than in 2008.3

As PV technology improves, the costs associated with solar energy production, and the ultimate cost per watt, will decrease as a result of increasing generation efficiencies. This, along with government policy and incentives for renewables, has and should continue lead to increasing growth in the use of photovoltaics as a viable form of renewable energy.

This paper focuses on photovoltaics and provides an overview on what they are, the various types present in the field, power efficiencies, and the potential for growth.

What are Photovoltaics?

Photovoltaics (PV), or solar cells, are made of a type of semiconductor material that absorbs sunlight and generates a DC electric current. When one of the cells is struck by sunlight, some of the energy of the absorbed light knocks electrons loose, which then flow freely. The electrons are drawn in by metal contacts on the PV material, resulting in the flow of a DC current at a specific voltage. This determines how much power (wattage) the cell produces.

Multiple cells are assembled and connected together to form a solar module or panel, which are then grouped together to form arrays. In order to produce usable power, the arrays are linked to inverters, which convert the direct current (DC) to alternating current (AC), and then passed through a transformer to manage the voltage level before the current is fed into a power grid or other application.

Types of Photovoltaic Systems

There are several types of PVs in use today: Crystalline Silicon (cSi), Thin Film, and Concentrated PV (CPV). cSi is by far the most popular type and accounted for 68% of PV modules shipped within the United States from 2007 to 2009. During the same time period, Thin Film and CPV accounted for 30% and 2% respectively4.

cSi is the most popular type of solar cell, likely because it has been around for the longest. They are formed from silicon, and while pure silicon itself is a poor conductor of electricity, it becomes a semiconductor once it is infused or doped with certain elements, like phosphorus or boron. cSi comes in two varieties–single and polycrystalline—that differ in the solar cell’s molecular structure that results from unique manufacturing processes. Single crystal PVs are more efficient at producing energy but tend to cost more than polycrystalline PVs.

Thin film PVs, commonly known in the industry as CIGS, are the second most popular type, and are made from vaporizing and depositing layers of various semiconductor materials such as amorphous silicon (a-Si), cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). These are deposited onto a glass substrate or flexible film type material, such as a stainless steel band. Thin films, while less expensive to produce than cSi, are also less efficient. But that does not mean that these cells are not as useful — a likely use is in BIPV (Building Integrated Photo Voltaic) which is the integration of the thin film into building products, such as roof and non structural wall panels.

CPV, although not as well known as other, consists of a cSi cell/module that is paired with a reflective (mirror) or refractive (lens) device that gathers, focuses and concentrates the sunlight directly onto the cell. Concentration factors can range from 2 to 500x and industry refers to these as “suns.” These systems tend to be larger and bulkier due to additional hardware required, and they need to face and track the sun at all times to operate at peak efficiency. These tend to be pedestal mounted and as a result, they are geared primarily for use in solar farms.

Power Generation Efficiencies

To measure the efficiency of the various types of photovoltaic cells, a comparison is conducted of how much of the sunlight striking the cell is converted into electrical energy, versus being dissipated as heat. The more light that is turned into energy, the more efficient the cell. In 2009, CPV was measured as the most efficient type of cell, with an average power generation efficiency of 25-38%, followed by 20% for single crystalline cSi, 13-14% for polycrystalline cSi, 8-12% for thin film5.

However, technological advances are promising greater efficiencies, such as Sharp Corporation’s use of a tri-layer increasing cSi efficiencies to 35.8%6. This was done with the use of indium-gallium-arsenide layers to convert light energy at multiple wavelengths into electrical current. CPV-based systems have also seen enhancements: Boeing’s Spectralab has developed tri-layer cells with efficiencies of 41.6%7.

Growth Potential & Applications

Photovoltaic technology has been present since the 1950’s but the use of this technology in grid-connected, non residential power generation systems has become more prevalent in the past 10 years. Globally, from 2004-2009, grid connected PV capacity increased at an annual rate of 60% to nearly 21 gigawatts8. In the US alone, grid connected PV installations increased from about 20,000 in 2004, to 104,000 in 2009.9 Additionally, shipments of PV systems in the US, as measured by peak kilowatts, have increased from 88.2 megawatts in 2000 to nearly 1.3 gigawatts in 2010 – a 1300% increase10.

There has been clear, steady growth in the adoption of solar technologies in the past decade or so, and it is anticipated that such growth will continue. In the US in 2009, by market segment, 47% was for commercial, 36% was for residential, 9% for electrical power, 7% for industrial and 1% for transportation.11 Electricity generation accounted for 99%12 of this use.

Conclusion

Renewable energy, as a percentage of our overall global energy resources, grew from 18% in 2008 to 26% in 200913. Although photovoltaics account for only 1.7% of the total renewable energy mix, we are likely to see an increase in this segment over time. Factors such as pro-solar government policies and tax incentives, ongoing performance increases in efficiency and price drops due to market pressures should result in further decreases in the overall cost per watt, making solar PV systems more cost effective. The growth in this segment is evident by the continued increase in shipments and generating capacity over the past several years.

Additional Information

Manufactures expect their PV systems to be in use for 20-25 years. Numerous studies have been conducted on the long term reliability of these systems. For a technical analysis on this topic, refer to the following paper on "Will Photovoltaics stand the test of Time?"

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References:
1 Renewable Energy Policy Network for the 21st Century (REN 21). Renewables 2010 Global Status Report, revised September 2010, p. 53
2 Ibid p. 53
3 U.S. Energy Information Administration, Solar Photovoltaic Cell/Module Manufacturing Activities 2009, January 2011, p 1
4 Ibid 3, p 12
5 S. Kurtz, Opportunity & Challenges for Development of a Mature Concentrating Photovoltaic Power Industry, Technical Report NREL/TP 250-43208, Revised November 2009, p 6
6 Sharp Corporation news release as of October 2009.
7 Boeing Spectralabs website FAQ, accessed January 2010
8 Renewable Energy Policy Network for the 21st Century (REN 21). Renewables 2010 Global Status Report, revised September 2010, p. 19
9 Larry Sherwood, U. S. Solar Market Trends 2009, IREC – Interstate Renewable Energy Council, released July 2010, p 8
10 Ibid 3, p 8
11 Ibid 3, p 6 & 14
12 Ibid 3, p 14
13 Ibid 1, p 15, 53