Solar Photovoltaic Overview

Photovoltaic (PV) cells, flat solid-state panels that directly convert sunlight into electricity, are in many ways the most attractive solar energy technology. Not only do they require little maintenance, have no moving parts, and essentially no environmental impact, they also can be installed in small modules, can be installed quickly anywhere there is sunlight, and offer the long-term prospect for mass production and application in most parts of the world. Great strides have been taken over the past few years in decreasing photovoltaic (PV) technology costs, increasing efficiency, and extending cell lifetimes. Many new materials are being tested to reduce costs and to automate manufacturing. Flat plate modules, ready for installation, can achieve a conversion efficiency of 15%. Efficiencies of over 30% have been achieved in the laboratory.

Grid-connected photovoltaics can be configured in a variety of ways, including (1) units greater than 1 MW, (2) smaller units less than 1 MW, and (3) end-user units, less than 20 kW. There are many off-grid applications as well, particularly for remote locations. With the many different applications of solar PV, it is difficult to estimate how many megawatts are currently installed. According to the Renewable Electric Plant Information System (REPiS), at least 28 MW of grid-connected PV capacity was in use in 2000, up from 15 MW of PV capacity in 1999.

The largest use of cells today, about half, is for supplying power far from utility grids: powering remote communications and navigation equipment. The Coast Guard has installed more than 11,000 systems to power navigational aids and telecommunications equipment. In the United States and Europe about 15,000 vacation homes are equipped with PV systems.

According to the Solar Energy Industries Association (SEIA), the potential US market for PVs is approximately 9000 MW. Compared to the number of megawatts currently installed, great potential exists to expand the PV market in the US.

Outside of the US, the quantity of solar cells manufactured each year worldwide continues to increase rapidly. Since 1976, PV shipments have increased from 0.5 MW to 201 MW, peak, with the cumulative production between 1986 and 1999 being about 1,000 MW, peak. Of the 1999 world production, approximately 65 MW, or 32 %, were manufactured in the United States. The annual growth has varied between 20 and 40 percent over the past three years.

With continued interest in reducing greenhouse gas emissions and competitive electricity markets, it is highly likely that the rate of production and installation of PVs will increase. If costs continue to follow the curve shown in Figure 4, which is by no means guaranteed, PV could become competitive for large scale application with another tenfold increase in production.

Solar Resources

The amount of electricity that can be produced by solar cells depends directly on the amount of sunlight available, and, of course, on the efficiency of the solar cells. The total amount of sunlight, in turn, depends on the latitude of the site as well as the average cloudiness and weather. The amount of sunlight reaching the earth in the southwest United States is at least 50% greater than in the northern states.

Accounting for the variability among states and the fact that the sun doesn't shine at night, an average 4.8 kW-hr per square meter of sunlight is estimated to fall on the United States each day. Multiplying this average by the area of the United States, the annual sunlight falling on the country is approximately 6.7 x 1019 Btus. 6.7 x 1019 BTUs is more than 700 times total US annual primary fuels energy consumption. Clearly, the solar resource is enormous. Capturing just 15% of this sunlight as electricity with solar cells would yield over 100 times the present total US energy consumption.

Technology and Siting

The southwest area of the US, as mentioned above, is a prime location for PV installations due to the solar resource in that area. However, the amount of solar resource in a given area is not the only consideration to be made in siting PV systems. Effective load carrying capacity (ELCC) is the ability of the power generator to contribute to the system output in a way that effectively meets the load requirements. Certain areas of the country that do not have particularly notable solar resources have electricity demands that align well with solar PV output. For on-site installation, however, the solar resource would be the most important consideration.

In recent years, the popularity of building-integrated photovoltaics (BIPV's) has grown considerably. In this application, PV devices are are designed into building materials (i.e. roofs and siding) to both produce electricity and enhance building architecture. Some of a BIPV system's costs are offset by replacing the costs of normal construction materials. Other advantages include insulation and protection of roofing structures, better aesthetics than mounted solar panels, and life expectancies in excess of 30 years. There are more than 3,000 BIPV systems in Germany and Japan has a program that will build 70,000 BIPV buildings. Costs of the PV material itself is currently in the range of $1,200/m2. This cost is expected to drop significantly as demand increases.

The Department of Energy (DOE)'s National Center for Photovoltaics (NCPV) has developed a program for estimating costs associated with siting PV systems. Siting information is available for the entire US and its territories at http://rredc.nrel.gov/solar/codes_algs/PVWATTS/. In addition to the siting considerations of solar PV systems, progress has been made in the technology of PV systems. Cell lifetimes, efficiency, and costs have all improved because of technology. Lifetimes have been extended to 20 years, efficiencies have been improved from 9.8 percent to about 12-15 percent, and costs have dropped from $1140 per square meter to $480.

W: Data withheld to avoid disclosure of proprietary company data.
Source: Energy Information Administration, Renewable Energy Annual 2002; National Center for Photovoltaics, 2000.

Many new materials are being tested to reduce costs and automate manufacturing. Flat plate modules, ready for installation, can achieve an efficiency of 15 percent. One approach to reducing costs is through the use of amorphous (literally, without form) silicon rather than crystalline silicon. The cells are only 1 to 2 micrometers thick as opposed to the 100-300 micrometers for single-crystal silicon. The silicon can be deposited on glass or metal and offers the promise of cheap mass production.

A disadvantage of amorphous thin-film cells is the relatively low efficiency, 12% in the laboratory and 7% for commercial modules. The cell efficiencies have also tended to degrade by up to 10% during their lifetime. Other promising materials include GaAs (gallium arsenide), CuInSe2 (copper indium diselenide), and CdTe (cadmium teluride). Boeing has reported achieving an efficiency of 37% using a multi-layered cell with concentrated light. In the past, a delay of 5 to 10 years between efficiency of laboratory results and commercial cells has not been uncommon.

As cell manufacturing becomes more efficient, the amount of energy needed to produce solar cells declines. Even a decade ago the energy payback period was only 1 to 2 years, a period which has undoubtedly shortened over the ensuing time period.

Environmental Impacts

For the most part, PV cells are environmentally benign: no toxic releases to the air or waterways, no radioactive releases, no catastrophic accidents. Some PV cells, however, do contain toxic substances such as cadmium and these could be released to the environment in the event of a fire. However, the great bulk of PV cells are made from silicon which is not especially toxic.

There is also a possible visual impact from solar arrays on roof tops or on land. These, though, could be mitigated through a design that makes the cells less visually conspicuous. Cells with toxic substances in them should be carefully recycled when they are replaced to minimize health or safety risks.

Barriers to Solar Photovoltaics

Using solar cells with 15% efficiency, it would take less than 1 percent of U.S. land area to meet total present US energy demand, a square area less than 200 miles on a side. One of the primary reasons that solar PV capacity has increased rather slowly is due to high costs. Capital costs are often in the range of $4000-$6000/kW, with electricity generation costs between $0.20-$0.30 per kWh ($500/1752 kWh), plus additional operation and maintenance costs. To decrease this cost, it will be necessary to reduce the cost of modules, reduce the balance of system costs, have less expensive capital (tax credits, etc.), and erect system arrays in areas with high levels of solar radiation. Today the market is driven primarily driven by state incentives.

Bibliography

1. Robert A. Ristinen, J.J. Kraushaar "Energy and the Environment," John Wiley son, Inc. NY 1999, pp 92-96.

2. "Photovoltaics Can Add Capacity to the Utility Grid." September 1996.

3. "Renewable Energy, Power for a Sustainable Future," 1996, Oxford Un. Press, Ed. By Godfdrey Boyle, P.131

4. "Impacts of the Kyoto Protocol on US Energy Markets and Economic Activity."

5. "Solar: Rapidly Growing Markets" Solar Energy Industries Association

6. PV NEWS, Volume 19, No.2 Feb.2000, page 2.

7. "Renewable Energy, Power for a Sustainable Future," 1996, Oxford UN Press, Ed. By Godfdrey Boyle, p. 129 ff.

Solar Photovoltaic

Corporate Case Studies:
Staples Solar Services

Johnson & Johnson

Other Solar Photovoltaic Resources:
DoE EREN Photovoltaics

Solar Electric Power Association