The 'Heat Motor'

Overview

A "heat-motor" is a device that converts thermal energy to mechanical energy. Typically, a heat-motor is filled with a fusible material such as wax. When wax is heated to its melting point it expands and this is exploited to provide useful mechanical force.

Operation

The single heat-motor is widely used in greenhouses to operate ventilators, this application demonstrates the simplicity of the underlying technology. The following is a brief description of a greenhouse ventilator's operating behaviour. The desirable thermal environment inside a warm greenhouse is when the temperature is a minimum of 13 °C and a maximum of 30 °C^[1]. The temperature inside the greenhouse can be regulated between these thresholds by adjusting the rate of air-changes through the free-area of opening windows. Typically these are located at the base in the lower walls and either side of the roof-ridge.

When the heat-motor has absorbed sufficient heat energy to raise its sensible temperature above the melting-point of the wax then it opens the ventilator, this allows the buoyancy of heated air to rise and escape out of the ridge and drive a 'stack-effect' that draws in cooler air from outside at base level. Air movement around the heat-motor affects its balance of heat-energy, if the air is sufficiently cool to lower the sensible temperature of the wax to below its melting-point then it solidifies and the heat-motor is deactivated and this closes the ventilator.

This is a very simplified description because it ignores the effects of absorbing solar irradiation and the radiative exchange of black-body emission between the heat-motor and the inside surfaces of the greenhouse. The effects of solar irradiation are sufficient to melt the wax inside the heat-motor when the temperature of the ambient air is actually below the wax's melting-point. Clearly, both of these heat-transfers processes affect the balance of heat-energy and therefore the temperature of the wax.

References