Parabolic trough collectors are frequently employed for solar steam generation because relatively high temperatures can be obtained without serious degradation in the collector efficiency. Low-temperature steam can be used in industrial applications, in sterilization, and for powering desalination evaporators.
Three methods have been employed to generate steam using parabolic trough collectors (Kalogirou et al., 1997):
1. The steam-flash concept, in which pressurized water is heated in the collector and flashed to steam in a separate vessel.
2. The direct or in situ concept, in which two-phase flow is allowed in the collector receiver so that steam is generated directly.
3. The unfired boiler concept, in which a heat transfer fluid is circulated through the collector and steam is generated via heat exchange in an unfired boiler.
All three steam generation systems have advantages and disadvantages. These are examined in the following section.
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