There are, as we have seen, several issues associated with developing large-scale solar power installations: water consumption, impacts on biodiversity, and the fact that it provides only intermittent power. Fortunately, these issues are being addressed.

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The Gemasolar plant in Seville, Spain, has 2,650 sun-tracking mirrors called heliostats, with a total reflective area of over 30 hectares (75 acres) that powers a 17-megawatt turbine. Rather than using oil to store heat, this plant heats salt to its molten state and then stores it in insulated tanks at temperatures up to 565°C (1,049°F), which can be used to produce steam superheated to 540°F (1,004°F). What this means is that the Gemasolar plant can continue generating electricity on demand for 15 hours without additional input of solar energy. In other words, unlike photovoltaic systems without battery storage and concentrating solar power plants without heat storage, Gemasolar can generate electricity day and night. Coupling efficient tracking of solar angle by its heliostats with heat storage, Gemasolar is capable of producing 110,000 megawatt hours (MWh) of electricity annually, enough to power approximately 25,000 homes.

Newer concentrating solar power plants in the southwestern United States use water-saving technology that combines either air-cooling only or water- and air-cooling in a hybrid system. Both hybrid and air-cooling-only systems substantially reduce water consumption by tower-based solar concentrating power plants (Figure 10.38). A company called SolarReserve plans to use these technologies to build three of the world’s largest-capacity solar concentrating power plants in the water-scarce American Southwest. By using molten salt for heat transfer and storage, these three plants will provide round-the-clock electricity, totaling an estimated 450,000 megawatt hours annually—enough to power 140,000 homes during periods of peak power demand.

Another way to reduce water consumption by solar generation is to move from concentrating solar generation to photovoltaic generation. For example, in arid regions, energy developers are building utility-scale photovoltaic power plants. Such plants use less than 20 liters (5 gallons) per megawatt hour, generally just enough to clean the surfaces of the photovoltaic panels. However, because such systems require covering large areas with solar panels, they have a much larger footprint on the land than do concentrating solar power plants.

One proposal to avoid damage to wildlife and valuable agricultural land is to install photovoltaic panels in already disturbed and developed areas (Figure 10.39). For instance, solar panels can be placed on agricultural land that has been damaged by salt buildup. In addition, the potential for generating electrical power on rooftops is enormous. Germany, with more than 43% of the world’s installed photovoltaic power in 2010 (mostly mounted on buildings), is the world leader in photovoltaic power generation (Figure 10.40).

In late 2011 the U.S. Department of Energy provided loan guarantees to support the installation of over 750 megawatts of additional photovoltaic generating capacity on approximately 750 roofs on large commercial buildings in 28 states. The project is intended as a model for further rooftop solar development in the United States. Recently, the city of Los Angeles identified nearly 5,000 hectares (12,000 acres) of suitable roof space in the city with the potential to support 5,500 megawatts (5.5 gigawatts, GW) of photovoltaic generating capacity. Because rooftop solar developments occur in the population centers that consume the energy generated, they also reduce the need for building high-voltage transmission lines.

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As with any new technology, there are often practical, legal, and economic barriers to overcome. With regard to developing photovoltaic (PV) systems, obstacles have included difficulty acquiring permits to connect to the electrical grid, gaining approval for financial assistance to install a PV system, and locating trained licensed contractors to install PV systems. In the United States, such hurdles have been partly overcome through the Renewable Portfolio Standard, or RPS, which requires electrical utility companies to obtain a certain percentage of their power from renewable energy.

For example, the RPS for New York State requires that investor-owned utilities produce approximately 30% of their electricity from renewable energy by 2015. Similarly, California and Colorado each have an RPS requiring approximately 30% of electrical generation from renewables by 2020. To facilitate renewable development, the U.S. federal and state governments offer a variety of incentives, including tax credits for businesses and individuals who install renewable energy systems, as well as financial assistance in the form of grants and loans. In addition, in many areas, consumers who install photo-voltaic systems tied to the grid are generally paid wholesale rates for any electricity generated in excess of their use.