Solar Thermal Energy Systems

Thermal energy generation from solar energy can be accomplished with a wide range of conversion devices, from passive and active space and water heating to high-pressure steam generation. Hot water is commonly generated using traditional solar plates, while high-pressure steam is raised in specially designed solar furnaces. Both systems operate by concentrating heat and transferring it to a fluid. A solar collector differs in several respects from more conventional heat exchangers. The latter usually accomplish a fluid-to-fluid exchange with high heat transfer rates and with radiation as an unimportant factor. In the solar collector, radiation is the primary mode of heat transfer. This radiant energy is then transferred to a fluid such as water or a water-glycol mixture.

Solar thermal collectors can be divided into three categories: • Low-temperature collectors provide low-grade heat, less than 110°F (43°C), through either metallic or non-metallic absorbers. These are used for applica-

Fig. 7-37 Large Capacity Biomass-Fired Power Plant. Source: Warren Gretz, DoE/NREL
Fig. 7-38 Three Generic Coal Gasification Reactors and their Temperature Profiles. Source: Babcock and Wilcox (Courtesy of the Electric Power Research Institute)

Harvesting

Processing

Chopping

Briquetting

Dehydration

Drying

Briquetting

Dehydration

Gas Cleaning

Turbine

Boiler

Fig. 7-40 Biomass (Bagasse) Fuel Gasifier and Fuel Supply. Source: Warren Gretz, DoE/NREL

Fig. 7-39 Five Stages of Biomass Gasification Process.

Fig. 7-40 Biomass (Bagasse) Fuel Gasifier and Fuel Supply. Source: Warren Gretz, DoE/NREL

tions such as swimming pool heating and low-grade water and space heating.

• Medium-temperature collectors provide heat at temperatures ranging from 110 to 180°F (43 to 82°C), either through glazed flat-plate collectors using air or liquid as the heat transfer medium, or through concentrator collectors that concentrate the heat to elevated levels. These typically feature evacuated tube collectors and are most commonly used for domestic water heating or a variety of low-to-medium temperature process, space, and domestic water heating applications.

• High-temperature systems feature heliostat, parabolic dish, or trough type collectors that intensely concentrate solar energy, allowing for generation of extremely high fluid temperatures sufficient to produce high-pressure steam.

Low- and medium-temperature passive and active systems are the predominate solar thermal energy technology applied throughout the world for residential and CI&I facilities. Other than in passive construction design, the major applications are in solar water heating and building space heating. The basic components of a typical flat-plate solar collector are: the solar energy-absorbing surface, with means for transferring the absorbed energy to a fluid; and the envelopes, or enclosures that are transparent to solar radiation, but reduce convection and radiation losses to the surroundings. As shown in Figure 7-41, flat-plate collectors are usually mounted in a stationary position with an orientation (tilt and azimuth angle) optimized for the particular location and time of year in which the device is intended to operate. A complete system would also include a mechanism for circulating the heat transfer fluid, either by mechanical pumping or natural convection, a storage tank, and controls.

High-temperature systems are most typically applied in large-scale independent power production (IPP)

Figure 7-41 Typical Solar Flat Plate Collector. Source: Roch A. Ducey, DoE/NREL

Receiver

Concentrator

Figure 7-41 Typical Solar Flat Plate Collector. Source: Roch A. Ducey, DoE/NREL

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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