Entirely passive solar air heater with water heat store and heat exchanger of automobile radiators

passive-auto-rad-4.GIFThe automobile radiators are protected from night freezing by being enclosed in a heat trap -- an insulated space open only below.

Efficient heat collection requires a significant fall in the temperature of the air as it passes through the radiator, since this temperature fall drives the air flow.  It is important to use the necessary heat losses through the outer glazing to cool only the falling air, as shown. The inner glazing maximizes the chimney effect and eliminates mixing of falling and rising air.  In order for the automobile radiator to cool the air sufficiently for good air flow when the output water from  the radiators is warm, there would have to be a substantial rise in the temperature of the water as it rises through the radiator, so that at least part of the radiator is cool.  This requirement for a substantial temperature rise means that the heat store has to have significant thermal stratification when the top of the heat store is at a high temperature which it is desired to increase efficiently.

The difficulty of balancing these factors means experimentation prior to use would be necessary to determine if this scheme works well. If separate tank or manifold connections to each radiator proved not to produce sufficient temperature rise, connecting radiators in series of pairs might be tried.

This idea was inspired by rumors of Nathan Hurst's work with automobile radiators in active air-water syatems -- work that led to the experiment described here.