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Solar Photovoltaic and Thermal

In his widely-read recent book, The Solar Economy, Hermann Scheer argues that the US currently functions on a “fossil economy.” He contends that steps should be made for a full scale transition to a “solar economy” but instead the US is becoming more fossil-oriented than ever before [1]. To Scheer, “solar resources” include photovoltaic cells, solar thermal, wind power, geothermal, biomass, wave, and tidal power due to the fact that these sources are all ultimately powered by the sun.

Photovoltaics and solar thermal directly harness the sun’s power; wind is created from differential heating of land and water areas by the sun; geothermal is residual heat of the earth, created billions of years ago during the formation of the solar system; biomass gets energy from the sun and converts it through photosynthesis; waves results from wind (which are powered by the sun); and tides are caused by the tidal forces of the moon and the sun. A utilization of all of these sources, claims Scheer, would provide energy several magnitudes greater than total global energy consumption for generations if not indefinitely.

Although Scheer’s utopian portrayal of a solar economy may one day exist, for the time being, producing power directly from solar energy is not currently a significant factor in electricity generation [2].

When most people refer to “solar power” they mean photovoltaic (PV) cells. PV cells convert sunlight directly into electricity [3]. PV cells are made of semiconductors such as silicon, which when struck by light, absorbs the light into the material. The energy is transferred to the semiconductor, spurring the flow of electrons and then is converted to current. The current can then be transferred off of the cell by placing metal contacts on the top and bottom of the PV cell [4]. For a detailed explanation on how solar cells work, see How Solar Cells Work.

The first patented PV cells were produced in 1954 by Bell Laboratory and used silicon-based technology. Today, PVs are widely used in calculators, sidewalk lighting systems, and in remote transportation infrastructure. Since around the 1970’s PVs have been applied to large-scale electricity generation. These operations, which consist of many PV arrays working together, have proven useful to electric utilities.

Benefits of large-scale PV plants include the fact that the modularity of PV technology allows appropriately sized PV systems to be collocated with power needs for a wide range of applications; PV systems have demonstrated high reliability; and PV systems can operate over a wide range of direct or diffuse solar radiation levels [5]. Furthermore, PV arrays can be constructed much faster and sited much easier than traditional power plants, and unlike conventional power plants, PV plants can be expanded incrementally as demand increases. Lastly, PV power plants consume no fuel and produce no air or water pollution while they silently generate electricity [6].

“Solar thermal” is a term used to describe the process of directly utilizing the warmth of the sun to heat water for domestic hot water systems, or to use the sun’s light to heat water temperatures to make steam and electricity [7]. Solar thermal power plants typically use curved mirrored troughs that concentrate sunlight. The sun heats a liquid that creates steam to turn a traditional turbine, creating electricity [7].

Despite the recognized benefits of PV systems, PV power plants generate only a few tenths of a percent of the nation’s electricity supply. Worldwide, solar electric generation technologies contribute only about 2,000 MW of electricity – less than a tenth of the world’s global electricity supply [7]. The development of PV power has been limited by its high cost relative to conventional and other renewable resources and thus development has only occurred in niche markets such as residential hot water supply, remote power generation, consumer electronics, and limited grid-tied applications to offset utility peak power requirements [8]. Using current utility accounting practices, PV-generated electricity costs more than electricity generated by conventional plants in most places, and regulatory agencies require most utilities to supply the lowest-cost electricity [6]. Furthermore, there are limitations in the fact that PV systems produce power only during daylight hours, their output can vary with the weather, and electricity storage methods (such as high-capacity batteries) are too expensive to deploy for practical use [6].

In recent years, PV technology has attracted the attention of electric utilities and industry as an alternative for future energy production [2]. Indeed, PV technology has progressed remarkably in terms of both performance and cost. Thousands of systems are successfully operating, serving applications that range from marine buoys and water pumps to communications systems, residential power systems and utility power plants [5].

The Department of Energy, the Electric Power Research Institute, and several utilities have formed a joint venture called the Photovoltaics for Utility-Scale Applications (PVUSA), which currently operates three pilot test stations to test utility-scale PV systems. These projects allow utilities to experiment with newly developing PV technologies with little financial risk – an incentive which may spur future technology growth [6].

Future growth in solar power may also arise from the inherent incentive in the short energy supply chain of solar technology. PV cells can be located where they are needed most, and this short energy supply chain allows solar power to have the lowest distribution costs of all generation technologies [1]. This advantage is of particular interest in remote locations of the United States and in non-grid-connected areas of the developing world.

Although the use of PV technology in the US has a bright future, the bottom line is that the future of solar power is dependent on the cost of the technology. Significant research and development must be conducted to reduce the cost and to diversify the applications of the technology (such as application to production of hydrogen) [3]. Through projects such as the aforementioned PVUSA, investors may gain confidence in solar technology and being producing large-scale systems, thus realizing economies of scale and lowering prices. Scheer argues that fossil fuel supply chains do not consider the true cost of extraction, transportation, and distribution and this results in making fossil fuels look artificially inexpensive. If fossil fuels reflected the real costs of their full chain, solar energy would become competitive with traditional energy sources [1].