Exergy: Energy, Environment, and Sustainable Development

Exergy analyses are performed in this chapter of several renewable energy systems including solar photovoltaic systems,
solar ponds, wind turbines and geothermal district heating systems and power plants. These and other renewable energy
systems are likely to play increasingly important roles in societies in the future.

Solar photovoltaic (PV) technology converts sunlight directly into electrical energy. Direct current electricity is produced, which can be used in that form, converted to alternating current or stored for later use. Solar PV systems operate in an environmentally benign manner, have no moving components, and have no parts that wear out if the device is correctly protected from the environment. By operating on sunlight, PV devices are usable and acceptable to almost all inhabitants of our planet. PV systems can be sized over a wide range, so their electrical power output can be engineered for virtually any application, from low-power consumer uses like wristwatches, calculators and battery chargers to significantly energy-intensive applications such as generating power at central electric utility stations. PV systems are modular, so various incremental power capacity additions are easily accommodated, unlike for fossil or nuclear fuel plants, which require multi-megawatt plants to be economically feasible.

The solar PV cell is one of the most significant and rapidly developing renewable-energy technologies, and its potential future uses are notable. By using solar radiation, a clean energy source, PV systems are relatively benign environmentally. During the last decade, PV applications have increased in many countries and are observed throughout the residential, commercial, institutional and industrial sectors. The clean, renewable and in some instances economic features of PV systems have attracted attention from political and business decision makers and individuals. Advances in PV technology have also driven the trend to increased usage.

A PV cell is a type of photochemical energy conversion device. Others include photoelectric devices and biological photosynthesis. Such systems operate by collecting a fraction of the radiation within some range of wavelengths. In PV devices, photon energies greater than the cutoff (or band-gap) energy are dissipated as heat, while photons with wavelengths longer than the cutoff wavelength are not used.

The energy conversion factor of a solar PV system sometimes is described as the efficiency, but this usage can lead to difficulties. The efficiency of a solar PV cell can be considered as the ratio of the electricity generated to the total, or global, solar irradiation. In this definition only the electricity generated by a solar PV cell is considered. Other properties of PV systems, which may affect efficiency, such as ambient temperature, cell temperature and chemical components of the solar cell, are not directly taken into account.

The higher performance, lower cost and better reliability demonstrated by today's PV systems are leading many potential users to consider the value of these systems for particular applications. Together, these applications will likely lead industry to build larger and more cost-effective production facilities, leading to lower PV costs. Public demand for environmentally benign sources of electricity will almost certainly hasten adoption of PV. The rate of adoption will be greatly affected by the economic viability of PV with respect to competing options. Many analysts and researchers believe that it is no longer a question of if, but when and in what quantity, PV systems will gain adoption. Since direct solar radiation is intermittent at most locations, fossil fuel-based electricity generation often must supplement PV systems. Many studies have addressed this need.

This section describes solar PV systems and their components and discusses the use of exergy analysis to assess and improve solar PV systems. Exergy methods provide a physical basis for understanding, refining and predicting the

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