The major disadvantage of the system results from the characteristics of
the heat transfer fluid. These fluids are hard to contain, and most heat transfer
fluids are flammable. Decomposition, when the fluids are exposed to air,
can greatly reduce ignition point temperatures and leaks into certain types of
insulation can cause combustion at temperatures that are considerably lower
than measured self-ignition temperatures. Heat transfer fluids are also relatively
expensive and present a potential pollution problem that makes them
unsuitable for food industry applications (Murphy and Keneth, 1982). Heat
transfer fluids have much poorer heat transfer characteristics than water. They
are more viscous at ambient temperatures, are less dense, and have lower specific
heats and thermal conductivities than water. These characteristics mean
that higher flow rates, higher collector differential temperatures, and greater
pumping power are required to obtain the equivalent quantity of energy transport
when compared to a system using water. In addition, heat transfer coefficients
are lower, so there is a larger temperature differential between the
receiver tube and the collector fluid. Higher temperatures are also necessary to
achieve cost-effective heat exchange. These effects result in reduced collector
efficiency.
It should be noted that, for every application, the suitable system has to be
selected by taking into consideration all these factors and constraints