As was seen in Chapter 3, Section 3.2.3, dish systems use dish-shaped parabolic mirrors as reflectors to concentrate and focus the sun's rays onto a receiver, which is mounted above the dish at the dish focal point. The receiver absorbs the energy and converts it into thermal energy. This can be used directly as heat or can support chemical processes, but its most common application is in power generation. The thermal energy can be either transported to a central generator for conversion or converted directly into electricity at a local generator coupled to the receiver.
A dish-engine system is a stand-alone unit composed primarily of a collector, a receiver, and an engine, as shown in Figure 10.8. It works by collecting and concentrating the sun's energy with a dish-shaped surface onto a receiver that absorbs the energy and transfers it to the engine. The heat is then converted in the engine to mechanical power, in a manner similar to conventional engines, by compressing the working fluid when it is cold, heating the compressed working fluid, and expanding it through a turbine or with a piston to produce mechanical power. An electric generator converts the mechanical power into electrical power.
Dish-engine systems use a dual-axis tracking system to follow the sun and so are the most efficient collector systems because they are always pointing at the sun. Concentration ratios usually range from 600 to 2000, and they can achieve temperatures in excess of 1500°C. While Rankine cycle engines, Brayton cycle engines, and sodium-heat engines have all been considered for systems using dish-mounted engines, greatest attention has been paid to Stirling-engine systems (Schwarzbozl et al., 2000; Chavez et al., 1993).
The ideal concentrator shape is parabolic, created either by a single reflective surface (as shown in Figure 3.20b) or multiple reflectors or facets (as shown in Figure 10.8). Each dish produces 5 to 25 kW of electricity and can be used independently or linked together to increase generating capacity. A 650 kW plant composed of twenty-five 25-kW dish-engine systems requires about a hectare of land.
The focus of current developments in the United States and Europe is on 10 kWe systems for remote applications. Three dish-Stirling systems are demonstrated at Plataforma Solar de Almeria in Spain. Within the European project EURODISH, a cost-effective 10 kW dish-Stirling engine for decentralized electric power generation was developed by a European consortium with partners from industry and academia.
Systems that employ small generators at the focal point of each dish provide energy in the form of electricity rather than heated fluid. The power conversion unit includes the thermal receiver and the heat engine. The thermal receiver absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the heat engine. A thermal receiver can be a bank of tubes with a cooling fluid circulating through it. The heat transfer medium usually employed as the working fluid for an engine is hydrogen or helium. Alternate thermal receivers are heat pipes wherein the boiling and condensing of an intermediate fluid is used to transfer the heat to the engine.
The heat engine system uses the heat from the thermal receiver to produce electricity. The engine-generators include basically the following components:
• A receiver to absorb the concentrated sunlight to heat the working fluid of the engine, which then converts the thermal energy into mechanical work.
• A generator attached to the engine to convert the work into electricity.
• A waste heat exhaust system to vent excess heat to the atmosphere.
• A control system to match the engine's operation to the available solar energy.
This distributed parabolic dish system lacks thermal storage capabilities but can be hybridized to run on fossil fuel during periods without sunshine. The Stirling engine is the most common type of heat engine used in dish-engine systems. Other possible power conversion unit technologies that are evaluated for future applications are microturbines and concentrating photovoltaics (Pitz-Paal, 2002).
Solar dish systems are the most efficient solar energy systems. They provide economical power for utility line support and distributed and remote applications and are capable of fully autonomous operation. Their size typically ranges from 5 to 15 m in diameter or 5 to 25 kW per dish. Because of their size, they are particularly well suited for decentralized power supply and remote, standalone power systems, such as water pumping or village power applications, or grouped to form megawatt-scale power plants. Like all concentrating systems, they can additionally be powered by fossil fuel or biomass, providing constant capacity at any time.
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