Environmental Engineering Reference
In-Depth Information
Salt (hot)
Concentrated
solar radiation
Collector
Heat
exchanger
tubes
Distributor
Salt (cold)
Fig. 5.7
Vertical tube receivers using salt as heat transfer medium
As an alternative to salt tube receivers also salt film receivers may be applied.
During this process either molten salt is directly exposed to concentrated solar
radiation or a flat or cavity absorber is cooled by an interior salt film /5-6/, /5-7/.
Besides molten salts, generally also liquid metals, such as e.g. sodium (Na) can
be applied. However, due to negative experiences made with this heat transfer
medium (fire hazard), this approach has been discontinued.
Open volumetric air receiver.
Concentrated solar radiation is incident on volumet-
ric absorber material consisting of steel wire or porous ceramics. Such volumetric
receivers are characterised by a high ratio of absorbing surface to flow path of the
absorbing heat transfer medium air. Ambient air is sucked in by a blower and
penetrates the radiated absorber material (Fig. 5.8). The air flow absorbs the heat,
so that those absorber areas facing the heliostat field (i.e. illuminated by the solar
radiation reflected by the heliostats) are cooled by the inflowing air. Due to this
cooling effect the absorber areas radiated by solar radiation are cooler than the
interior absorber areas where the heat is transported by the in-flowing air. There-
fore the air leaving the absorber shows also a higher temperature compared to the
temperature of the absorber areas radiated by solar radiation. This is the reason
why this receiver type presents comparatively low thermal losses. Being an open
receiver, such power plants operate with ambient pressure. As air is characterised
by a relatively low heat capacity, large volume flows and absorber surfaces are
required.
Air as heat transfer medium presents the advantages of being non-toxic, non-
corrosive, fire-proof, everywhere available and easy to handle. Its disadvantage is
its comparatively low heat capacity requiring large heat transmission surfaces
which, however, are generally feasible with volumetric receivers. Their lower
thermal masses ensure a smooth start-up of the plant.
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