Thermal wheel
A rotating porous metallic wheel transfers thermal energy from one air stream to another by passing through each fluid alternately.
The driving force behind the exchange is the difference in temperatures between the opposing air streams (the thermal gradient).
Desiccants transfer moisture through the process of adsorption which is predominately driven by the difference in the partial pressure of vapor within the opposing air-streams.
Choice of construction materials for the rotor, most commonly polymer, aluminum, or fiberglass, determines durability.
O’Connor et al.[3] studied the effect that a rotary thermal wheel has on the supply air flow rates into a building.
A computational model was created to simulate the effects of a rotary thermal wheel on air flow rates when incorporated into a commercial wind tower system.
No full-scale experimental or field test data was completed in this study, therefore it cannot be conclusively proved that rotary thermal wheels are feasible for integration into a commercial wind tower system.
[4] A thermal wheel consists of a circular honeycomb matrix of heat-absorbing material, which is slowly rotated within the supply and exhaust air streams of an air-handling system.
Heat exchange is most efficient when the streams flow in opposite directions, since this causes a favourable temperature gradient across the thickness of the wheel.
The principle works in reverse, and "cooling" energy can be recovered to the supply air stream if desired and the temperature differential allows.
The heat exchanger is rotated by a small electric motor and belt drive system.
Generally, a thermal wheel will be selected for face velocities between 1.5 and 3.0 metres per second (4.9 and 9.8 ft/s), and with equal air volume flow rates, gross "sensible" efficiencies of 85% can be expected.
Aluminium matrices are also available with an applied hygroscopic coating, and the use of this, or the use of porous synthetic fiber matrices, allows for the adsorption and release of water vapour, at moisture levels much lower than that normally required for condensation and latent heat transfer to occur.
During the automotive industry's interest in gas turbines for vehicle propulsion (around 1965), Chrysler invented a unique type of rotary heat exchanger[5] that consisted of a rotary drum constructed from corrugated metal (similar in appearance to corrugated cardboard).
Such an engine may at some future time be attractive when combined with an electric motor in a hybrid vehicle owing to its robust longevity and an ability to burn a wide variety of liquid fuels.
