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Chapter 3

Direct Solar Energy

Executive Summary

Solar energy is abundant and offers significant potential for near-term (2020) and long-term (2050) climate change mitiga- tion. There are a wide variety of solar technologies of varying maturities that can, in most regions of the world, contribute to a suite of energy services. Even though solar energy generation still only represents a small fraction of total energy con- sumption, markets for solar technologies are growing rapidly. Much of the desirability of solar technology is its inherently smaller environmental burden and the opportunity it offers for positive social impacts. The cost of solar technologies has been reduced significantly over the past 30 years and technical advances and supportive public policies continue to offer the potential for additional cost reductions. Potential deployment scenarios range widely—from a marginal role of direct solar energy in 2050 to one of the major sources of energy supply. The actual deployment achieved will depend on the degree of continued innovation, cost reductions and supportive public policies.

Solar energy is the most abundant of all energy resources. Indeed, the rate at which solar energy is intercepted by the Earth is about 10,000 times greater than the rate at which humankind consumes energy. Although not all countries are equally endowed with solar energy, a significant contribution to the energy mix from direct solar energy is possible for almost every country. Currently, there is no evidence indicating a substantial impact of climate change on regional solar resources.

Solar energy conversion consists of a large family of different technologies capable of meeting a variety of energy service needs. Solar technologies can deliver heat, cooling, natural lighting, electricity, and fuels for a host of applications. Conversion of solar energy to heat (i.e., thermal conversion) is comparatively straightforward, because any material object placed in the sun will absorb thermal energy. However, maximizing that absorbed energy and stopping it from escaping to the surroundings can take specialized techniques and devices such as evacuated spaces, optical coatings and mirrors. Which technique is used depends on the application and temperature at which the heat is to be delivered. This can range from 25°C (e.g., for swimming pool heating) to 1,000°C (e.g., for dish/Stirling concentrating solar power), and even up to 3,000°C in solar furnaces.

Passive solar heating is a technique for maintaining comfortable conditions in buildings by exploiting the solar irradi- ance incident on the buildings through the use of glazing (windows, sun spaces, conservatories) and other transparent materials and managing heat gain and loss in the structure without the dominant use of pumps or fans. Solar cooling for buildings can also be achieved, for example, by using solar-derived heat to drive thermodynamic refrigeration absorption or adsorption cycles. Solar energy for lighting actually requires no conversion since solar lighting occurs naturally in build- ings through windows. However, maximizing the effect requires specialized engineering and architectural design.

Generation of electricity can be achieved in two ways. In the first, solar energy is converted directly into electricity in a device called a photovoltaic (PV) cell. In the second, solar thermal energy is used in a concentrating solar power (CSP) plant to produce high-temperature heat, which is then converted to electricity via a heat engine and generator. Both approaches are currently in use. Furthermore, solar driven systems can deliver process heat and cooling, and other solar technologies are being developed that will deliver energy carriers such as hydrogen or hydrocarbon fuels—known as solar fuels.

The various solar technologies have differing maturities, and their applicability depends on local conditions and government policies to support their adoption. Some technologies are already competitive with market prices in certain locations, and in general, the overall viability of solar technologies is improving. Solar thermal can be used for a wide variety of applications, such as for domestic hot water, comfort heating of buildings, and industrial process heat. This is significant, as many countries spend up to one-third of their annual energy usage for heat. Service hot water heating for domestic and commercial buildings is now a mature technology growing at a rate of about 16% per year and employed in most countries of the world. The world installed capacity of solar thermal systems at the end of 2009 has been estimated to be 180 GWth.


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