Abstract
The air heater studied in this paper is made of an external transparent plastic cover, such as plexiglas, the role of which is to protect the collector from accidentally thrown stones and an internal transparent glass cover which produces the necessary greenhouse effect for heating the absorber. An analysis of the unsteady state heat exchanges in such a collector is presented. It is shown that at quasi-steady state the energy balance equations of the components of the heater cascade into a single first order differential equation, which is able to predict the thermal behaviour of the collector. The solution of this differential equation is written down as an explicit expression of the local temperature of the fluid flowing in the collector in terms of the time-dependent incident solar intensity. The effect of various parameters such as the inlet fluid temperature, the mass flow rate and the depth of the air channel on the thermal performances of the combined plastic-glass air collector is also studied.
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