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Other STE Systems
Direct Steam Generation (DSG)

In the Shuman solar trough plant design (built in Egypt in 1913), water ciculated through the collectors. The collectors converted (boiled) the water into steam which powered a steam engine. After powering the steam engine, the steam is lower pressure and is cooled in a condenser (e.g., a cooling tower) to become water again (hot water instead of steam) which is circulated through the collectors again (to become higher pressure steam again). This type of system, in which the steam is created in the collectors, is called direct steam generation (DSG).

For small and medium scale electricity production, a variation of the original DSG method can be used, with a small turbine instead of a steam engine. Such a system in now operating in Thailand, generating electricity with a 5 MW turbine (Figure 1).

Figure 1. DSG solar trough plant in Thailand (5 MW). [ DLR ]

Larger trough collector systems, including all of the SEGS plants, circulate synthetic oil through the collectors. The synthetic oil has greater heat capacity than water, and creates steam (transfers heat to water) in a heat exchanger. That is more efficient for large scale electricity production.

Power Tower

Another type of dispersed (small and medium scale) solar thermal electricity generation is the power tower. That type of system uses a field of mirrors (called heliostats) to reflect sunlight to the top of a tower (Figures 2 and 3).


Figure 2. Power tower under construction. Two tanks at the base of the tower are for molten salt storage, to store heat energy for generating electricity at night. NREL ]


Figure 3. The same power tower, in operation. NREL ]

Power towers can use molten salt as the heat transfer medium. A power tower produces more electricity than the DSG trough system described above, but less electricity than a solar trough plant.

Power towers have been proposed for isolated loads. For example, a power tower could power an isolated water pumping station, or a remote desalination plant, or a small town, etc.

Power towers use higher temperatures, causing material fatigue that shortens the life of the system (material degradation is also a problem for photovoltaics, which like power towers will experience waves of replacement costs in the future).

Another problem that precludes use of power towers is that power towers direct glare in all directions, posing risk of eye damage for aircraft pilots, and also harming birds. Pilots cannot simply glance away from the antipodal direction of the sun, as with other collectors, and birds cannot overshoot the glare.

Linear Strips (Fresnel)

Linear strip collectors are like trough collectors, but with the linear receiver stationary, and with the trough split into long horizontally-adjacent strips that are individually rotated to reflect sunlight to the receiver (Figure 4).


Figure 4. Linear strip (Fresnel) collector system. NREL ]

Strips that are narrow may be flat. Wider strips are slightly concave to prevent the reflected light from being wider than the receiver.

The primary purpose of this design is to reduce wind load on the collector. However, this changes the solar geometry of the system.

Overall collectors can be spaced closer together without shading each other. But linear strips of a collector shade other linear strips of the collector early and late in the day. To correct for this problem, linear strips are separated with gaps that allow some of the sunlight to pass through and not be collected.

Linear strip collectors require less manufacturing and installation infrastructure.

References:

 1.  Jan Fabian Feldhoff, Kai Schmitz, Markus Eck, Lars Schnatbaum-Laumann, Doerte Laing, Francisco Ortiz-Vives, Jan Schulte-Fischedick, “Comparative system analysis of direct steam generation and synthetic oil parabolic trough power plants with integrated thermal storage”, Solar Energy, 86 (2012) 520–530 [ Abstract ]

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