Solar thermal collectors and applications

Heliostat field collector

For extremely high inputs of radiant energy, a multi­plicity of flat mirrors, or heliostats, using altazimuth mounts,

can be used to reflect their incident direct solar radiation onto a common target as shown in Fig. 12. This is called the heliostat field or central receiver collector. By using slightly concave mirror segments on the heliostats, large amounts of thermal energy can be directed into the cavity of a steam generator to produce steam at high temperature and pressure.

The concentrated heat energy absorbed by the receiver is transferred to a circulating fluid that can be stored and later used to produce power. Central receivers have several advantages:

1. They collect solar energy optically and transfer it to a single receiver, thus minimizing thermal-energy transport require­ments;

2. They typically achieve concentration ratios of 300-1500 and so are highly efficient both in collecting energy and in converting it to electricity;

3. They can conveniently store thermal energy;

4. They are quite large (generally more than 10 MW) and thus benefit from economies of scale.

Each heliostat at a central-receiver facility has from 50 to 150 m2 of reflective surface. The heliostats collect and concentrate sunlight onto the receiver, which absorbs the concentrated sunlight, transferring its energy to a heat - transfer fluid. The heat-transport system, which consists primarily of pipes, pumps, and valves, directs the transfer fluid in a closed loop between the receiver, storage, and power-conversion systems. A thermal-storage system typi­cally stores the collected energy as sensible heat for later delivery to the power-conversion system. The storage system also decouples the collection of solar energy from its conversion to electricity. The power-conversion system consists of a steam generator, turbine generator, and support equipment, which convert the thermal energy into electricity and supply it to the utility grid.

In this case incident sunrays are reflected by large tracking mirrored collectors, which concentrate the energy flux towards radiative/convective heat exchangers, where energy is transferred to a working thermal fluid. After energy collection by the solar system, the conversion of thermal energy to electricity has many similarities with the conventional fossil-fuelled thermal power plants [93].

The average solar flux impinging on the receiver has values between 200 and 1000 kW/m2. This high flux allows working at relatively high temperatures of more than 1500 °C and to integrate thermal energy in more efficient cycles. Central receiver systems can easily integrate in fossil-fuelled plants for hybrid operation in a wide variety of options and have the potential to operate more than half the hours of each year at nominal power using thermal energy storage.

Central receiver systems are considered to have a large potential for mid-term cost reduction of electricity com­pared to parabolic trough technology since they allow many intermediate steps between the integration in a conventional Rankine cycle up to the higher energy cycles using gas turbines at temperatures above 1000 °C, and this sub­sequently leads to higher efficiencies and larger throughputs [94,95]. Another alternative is to use Brayton cycle turbines, which require higher temperature than the ones employed in Rankine cycle.

There are three general configurations for the collector and receiver systems. In the first, heliostats completely surround the receiver tower, and the receiver, which is cylindrical, has an exterior heat-transfer surface. In the second, the heliostats are located north of the receiver tower (in the northern hemisphere), and the receiver has an enclosed heat-transfer surface. In the third, the heliostats are located north of the receiver tower, and the receiver, which is a vertical plane, has a north-facing heat-transfer surface.

In the final analysis, however, it is the selection of the heat-transfer fluid, thermal-storage medium, and power - conversion cycle that defines a central-receiver plant. The heat-transfer fluid may either be water/steam, liquid sodium, or molten nitrate salt (sodium nitrate/potassium nitrate), whereas the thermal-storage medium may be oil mixed with crushed rock, molten nitrate salt, or liquid sodium. All rely on steam-Rankine power-conversion systems, although a more advanced system has been proposed that would use air as the heat-transfer fluid, ceramic bricks for thermal storage, and either a steam-Rankine or open-cycle Brayton power- conversion system.