Energy Recovery Ventilation - Rotary Air-to-air Enthalpy Wheel

Rotary Air-to-air Enthalpy Wheel

The rotating wheel heat exchanger is composed of a rotating cylinder filled with an air permeable material resulting in a large surface area. The surface area is the medium for the sensible energy transfer. As the wheel rotates between the ventilation and exhaust air streams it picks up heat energy and releases it into the colder air stream. The driving force behind the exchange is the difference in temperatures between the opposing air streams which is also called the thermal gradient. Typical media used consists of polymer, aluminum, and synthetic fiber.

The Enthalpy Exchange is accomplished through the use of desiccants. 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. Typical desiccants consist of Silica Gel, and molecular sieves.

Though very effective in its energy recovery, rotary enthalpy wheels have the common characteristic of high static pressures and poor durability. Therefore they are not as practical for energy savings purposes, and should only be considered for a cheaper alternative - in comparison to other ERVs - for situations where increased fresh outdoor ventilation is required. High static pressures result in increased fan power lowering the net energy savings of an installation. As for durability, rotary enthalpy wheels are normally guaranteed for no longer than 1 year, and the characteristic lifetime is about 5 years. Some companies, like the 1983-started Finnish Enervent Oy, provide warranties for two years which is extendable up to five years for ERVs with integrated heat pump.

There are additional disadvantages to the use of the heat wheel. Initial costs are higher due to the power needed for the fan to overcome its resistance. The system demands that the two air streams be adjacent to one another, they consistently be maintained, and they have filtration. Weekly maintenance is focused on the required rotating mechanism. Further up-keeping of the fill medium is also required. Colder climates may call for an increase in services. Caution must be taken when providing all upkeep, specifically if additional services are required, because cross-contamination (of air streams) can occur.