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Direct Solar Energy



The aim of this chapter is to provide a synopsis of the state-of-the-art and possible future scenarios of the full realization of direct solar ener- gy’s potential for mitigating climate change. It establishes the resource base, describes the many and varied technologies, appraises current market development, outlines some methods for integrating solar into other energy systems, addresses its environmental and social impacts, and finally, evaluates the prospects for future deployment.

Some of the solar energy absorbed by the Earth appears later in the form of wind, wave, ocean thermal, hydropower and excess biomass energies. The scope of this chapter, however, does not include these other indirect forms. Rather, it deals with the direct use of solar energy.

Various books have been written on the history of solar technology (e.g., Butti and Perlin, 1980). This history began when early civilizations dis- covered that buildings with openings facing the Sun were warmer and brighter, even in cold weather. During the late 1800s, solar collectors for heating water and other fluids were invented and put into practical use for domestic water heating and solar industrial applications, for example, large-scale solar desalination. Later, mirrors were used (e.g., by Augustin Mouchot in 1875) to boost the available fluid temperature, so that heat engines driven by the Sun could develop motive power, and thence, elec- trical power. Also, the late 1800s brought the discovery of a device for converting sunlight directly into electricity. Called the photovoltaic (PV) cell, this device bypassed the need for a heat engine. The modern silicon solar cell, attributed to Russell Ohl working at American Telephone and Telegraph’s (AT&T) Bell Labs, was discovered around 1940.

The modern age of solar research began in the 1950s with the estab- lishment of the International Solar Energy Society (ISES) and increased research and development (R&D) efforts in many industries. For example, advances in the solar hot water heater by companies such as Miromit in Israel and the efforts of Harry Tabor at the National Physical Laboratory in Jerusalem helped to make solar energy the standard method for providing hot water for homes in Israel by the early 1960s. At about the same time, national and international networks of solar irradiance measurements were beginning to be established. With the oil crisis of the 1970s, most countries in the world developed programs for solar energy R&D, and this involved efforts in industry, government labs and universities. These policy support efforts, which have, for the most part, continued up to the present, have borne fruit: now one of the fastest- growing renewable energy (RE) technologies, solar energy is poised to play a much larger role on the world energy stage.

Solar energy is an abundant energy resource. Indeed, in just one hour, the solar energy intercepted by the Earth exceeds the world’s energy consumption for the entire year. Solar energy’s potential to mitigate cli- mate change is equally impressive. Except for the modest amount of carbon dioxide (CO2) emissions produced in the manufacture of conver- sion devices (see Section 3.6.1) the direct use of solar energy produces


Chapter 3

very little greenhouse gases, and it has the potential to displace large quantities of non-renewable fuels (Tsilingiridis et al., 2004).

Solar energy conversion is manifest in a family of technologies having a broad range of energy service applications: lighting, comfort heat- ing, hot water for buildings and industry, high-temperature solar heat for electric power and industry, photovoltaic conversion for electrical power, and production of solar fuels, for example, hydrogen or synthesis gas (syngas). This chapter will further detail all of these technologies.

Several solar technologies, such as domestic hot water heating and pool heating, are already competitive and used in locales where they offer the least-cost option.And in jurisdictions where governments have taken steps to actively support solar energy, very large solar electricity (both PV and CSP) installations, approaching 100 MW of power, have been realized, in addition to large numbers of rooftop PV installations. Other applications, such as solar fuels, require additional R&D before achieving significant levels of adoption.

In pursuing any of the solar technologies, there is the need to deal with the variability and the cyclic nature of the Sun. One option is to store excess collected energy until it is needed.This is particularly effective for handling the lack of sunshine at night. For example, a 0.1-m thick slab of concrete in the floor of a home will store much of the solar energy absorbed during the day and release it to the room at night. When totalled over a long period of time such as one year, or over a large geographical area such as a continent, solar energy can offer greater service. The use of both these concepts of time and space, together with energy storage, has enabled designers to produce more effective solar systems. But much more work is needed to capture the full value of solar energy’s contribution.

Because of its inherent variability, solar energy is most useful when inte- grated with another energy source, to be used when solar energy is not available. In the past, that source has generally been a non-renewable one. But there is great potential for integrating direct solar energy with other RE technologies.

The rest of this chapter will include the following topics. Section 3.2 summarizes research that characterizes this solar resource and discusses the global and regional technical potential for direct solar energy as well as the possible impacts of climate change on this resource. Section 3.3 describes the five different technologies and their applications: passive solar heating and lighting for buildings (Section 3.3.1), active solar heat- ing and cooling for buildings and industry (Section 3.3.2), PV electricity generation (Section 3.3.3), CSP electricity generation (Section 3.3.4), and solar fuel production (Section 3.3.5). Section 3.4 reviews the current status of market development, including installed capacity and energy currently being generated (Section 3.4.1), and the industry capacity and supply chain (Section 3.4.2). Following this are sections on the integra- tion of solar technologies into other energy systems (Section 3.5), the environmental and social impacts (Section 3.6), and the prospects for

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