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The Solar Collector
Solar
energy collectors are heat exchangers that convert incoming solar
energy to internal energy of the transport medium, in this case
seawater. The function of the solar collector within the
desalination unit is to heat the incoming salt water to a determined
temperature that will allow for operation of the evaporator unit and
the separation of evaporated fresh water from denser salt-rich
water.
The
novel design of the solar energy collector is a form of flat-plate,
a collector that has the same area for intercepting and absorbing
solar radiation. Side,
top and overall schematics of the design can be found in Appendix A.
A
transparent glass box encloses the absorber plate of the collector.
The spectral emissitivity of glass is such that it is transparent to
short-wave radiation emitted by the sun but nearly opaque to
long-wave radiation emitted by the interior of the collector. The
glass box therefore reduces radiation losses by trapping the
radiated heat between the collector and the glass. Unlike
conventional flat-plate collectors, which include a layer of
stagnant air, the area between the glass and the absorber plate will
be kept at a low vacuum. Under such conditions, convection losses
from the absorber will be negligible. In commercial collectors, only
the tops of the collectors are covered by glass, while the sides are
made from an insulating material. Choosing instead to enclose the
absorber in a glass box, it is possible to also capture incoming
solar radiation (insolation) hitting the collector from lower angles
than possible with opaque side-walls.
A reflective surface covering the bottom of the box maximizes
this added radiation effect. A
more efficient usage of available insolation is thus possible and
the effect of changing solar-angles becomes less of a problem.
The box itself should be made of white glass with a low iron
oxide content in order to reduce absorption losses.
While regular glass absorbs up to 5-10% of incoming solar
light, low-iron glass absorbs less than 1%. The addition of a
surface film with a refraction index between that of air (1.00) and
glass (1.526) reduces reflection losses of the incoming light. For
example, the glass can be dipped in a solution to create a porous
silica film,
reducing reflection losses of light at normal incidence to less than
1%.
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