Solar Power Paris Expositions

As was seen in Chapter 1, Section 1.5, solar thermal power systems were among the very first applications of solar energy. During the 18th century, solar furnaces capable of melting iron, copper, and other metals were constructed of polished iron, glass lenses, and mirrors. The furnaces were in use throughout Europe and the Middle East. The most notable examples are the solar furnace built by the well-known French chemist Lavoisier in 1774, various concentrators built by the French naturalist Bouffon (1747-1748), and a steam-powered printing press exhibited at the Paris Exposition by Mouchot in 1872. This last application utilized a concentrating collector to supply steam to a heat engine.

Many of the early applications of solar thermal-mechanical systems were for small-scale applications, such as water pumping, with output ranging up to 100 kW. During the last 40 years, several large-scale experimental power systems have been constructed and operated, which led to the commercialization of some types of systems, and plants of 30-80 MW electric generating capacity, are now in operation for many years.

Although the thermal processes for conversion of solar to mechanical and electrical energy operate at higher temperatures than those treated in earlier chapters, these are fundamentally similar to other solar thermal processes.

As was discussed Chapter 9, the direct conversion of solar to electrical energy can be done with photovoltaics, which are solid-state devices. Electricity can also be produced with geothermal energy and wind power. However, with concentrating solar power systems, there are no complicated silicon manufacturing processes, as in the case of PVs; no deep holes to drill, as in the case of geothermal systems; and no turbine housings that need to be kept greased at high elevations from the ground, as in wind power systems. This chapter deals with the generation of mechanical and subsequently electrical energy from solar energy by heat engines powered with concentrating solar collectors. 521

Solar Thermal Turbine Diagram
FIGURE 10.1 Schematic diagram of a solar-thermal energy conversion system.

The use of solar ponds for power production is also examined. The cost of thermal power systems is much lower than that for photovoltaics, but most of them are suitable only for large-scale systems. Concentrating solar power plants use mirrors to generate high-temperature heat that drives steam turbines traditionally powered from conventional fossil fuels.

The basic schematic of conversion of solar to mechanical energy is shown in Figure 10.1. In these systems, solar thermal energy, usually collected by concentrating solar collectors, is used to operate a heat engine. Some of these systems also incorporate heat storage, which allows them to operate during cloudy weather and nighttime. The main challenge in designing these systems is to select the correct operating temperature. This is because the efficiency of the heat engine rises as its operating temperature rises, whereas the efficiency of the solar collector reduces as its operating temperature rises. Concentrating solar collectors are used exclusively for such applications because the maximum operating temperature for flat-plate collectors is low relative to the desirable input temperature for heat engines, and therefore system efficiencies would be very low.

Five system architectures have been used for such applications. The first four are high-temperature systems: the parabolic trough collector system, the linear Fresnel reflector, the power tower system, and the dish system. The last one is the solar pond, which is a low-temperature system. These, except the linear Fresnel reflector system, which has not yet reached industrial maturity, are analyzed in this chapter, together with models of heat engines derived from basic thermodynamic principles.

In concentrating solar power (CSP) systems, sunlight is concentrated using mirrors to create heat, then the heat is used to create steam, which is used to drive turbines and generators, just like in a conventional power station. Such plants have been operating successfully in California since the mid-1980s and currently provide power for about 100,000 homes. Recently, a CSP plant, called Nevada Solar I, started operating in Nevada, and another one called PS10 started operating in Spain, and more CSP plants are under construction in several other countries of the world. Apparently, the Spanish government has realized the huge potential of the CSP industry and is subsidizing the electricity produced with a feed-in tariff scheme. When PS20, currently built, becomes fully operational, it—together with PS10—will provide electricity for 200,000 homes.

Because of the large area required for the CSP plants, these are usually located on non-fertile ground, such as deserts. According to the Trans-Mediterranean Renewable Energy Corporation (TREN), each square kilometer of the desert receives solar energy equivalent to 1.5 million barrels of oil. It has also been estimated that, if an area of desert measuring 65,000 km2, which is less than 1% of the Sahara Desert, were covered with CSP plants, it could produce electricity equal to the year 2000 world electricity consumption (Geyer and Quaschning, 2000). One fifth of this area could produce the current electricity consumption of the European Union. Similar studies in the United States predict that the solar resource in southwestern states could produce about 7000 GW with CSP, which is about seven times the current total U.S. electric capacity (Wolff et al., 2008).

The main technologies used in CSP plants are the parabolic trough collectors, power towers, and dish/Stirling-engine systems. Mainly due to the plants operating in California for more than 20 years, parabolic troughs are the most proven technology, and today they produce electricity at about US$0.10/kWh. The success and durability of these plants demonstrates the robustness and reliability of the parabolic trough technology. An interesting feature of parabolic troughs and power tower systems is that it is possible to store heat, which enables them to continue producing electricity during the night or cloudy days. For this purpose, concrete, molten salts, ceramics, or phase-change media can be used, and this method is currently much cheaper than storing electricity in batteries. Fossil and renewable fuels such as oil, gas, coal, and biomass can be used for backup energy in these plants. The flexibility of heat storage combined with backup fuel operation enables the plants to provide both base load power and peak power, which can be used to cover the air-conditioning load usually occurring in midday during summer, when the plants produce higher output.

Table 10.1 gives an overview of some of the performance characteristics of the concentrating solar power concepts (Muller-Steinhagen and Trieb, 2004). Parabolic troughs, linear Fresnel reflectors, and power towers can be coupled to steam cycles of 10-200 MW electric capacity, with thermal cycle efficiencies of 30-40%. The same efficiency range applies for Stirling engines coupled to dish systems. The conversion efficiency of the power block remains essentially the same as in fuel-fired power plants. Overall solar-electric efficiencies, defined as the net power generation over incident beam radiation, are lower than the conversion efficiencies of conventional steam or combined cycles, because they include the conversion of solar radiative energy to heat within the collector and the conversion of the heat to electricity in the power block.

Due to the higher levels of concentration, dish systems usually achieve higher efficiencies than the parabolic trough system and are better suited for stand-alone, small power-producing systems; however, for higher outputs, many dish systems could be used.

Table 10.1 Performance Characteristics of Various CSP Technologies

(MW)

Concentration

Peak solar efficiency (%)

Solar-electric efficiency (%)

Land use (m2/MWh-a)

Parabolic

10-200

70-80

21

10-15

6-8

trough

Fresnel

10-200

25-100

20

9-11

4-6

reflector

Power tower

10-150

300-1000

20

8-10

8-12

Dish-Stirling

0.01-0.4

1000-3000

29

16-18

8-12

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