Global InformationThermal wheel information
A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, energy recovery wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of air-handling units or rooftop units or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.
Rotary thermal wheels are a mechanical means of heat recovery. A rotating porous metallic wheel transfers thermal energy from one air stream to another by passing through each fluid alternately. The system operates by working as a thermal storage mass whereby the heat from the air is temporarily stored within the wheel matrix until it is transferred to the cooler air stream.[1]
Two types of rotary thermal wheels exist: heat wheels and enthalpy (desiccant) wheels. Though there is a geometrical similarity between heat and enthalpy wheels, there are differences that affect the operation of each design. In a system using a desiccant wheel, the moisture in the air stream with the highest relative humidity is transferred to the opposite air stream after flowing through the wheel. This can work in both directions of incoming air to exhaust air and exhaust air to incoming air. The supply air can then be used directly or employed to further cool the air. This is an energy-intensive process.[2][need quotation to verify][why?]
The rotary air-to-air enthalpy wheel heat exchanger is a rotating cylinder filled with an air permeable material, typically polymer, aluminum, or synthetic fiber, providing the large surface area required for the sensible enthalpy transfer (enthalpy is a measure of heat). As the wheel rotates between the supply 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 (the thermal gradient).
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.
Enthalpy wheels are the most effective devices to transfer both latent and sensible heat energy. Choice of construction materials for the rotor, most commonly polymer, aluminum, or fiberglass, determines durability.
When using rotary energy recovery devices the two air streams must be adjacent to one another to allow for the local transfer of energy. Also, there should be special considerations paid in colder climates to avoid wheel frosting. Systems can avoid frosting by modulating wheel speed, preheating the air, or stop/jogging the system.
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. The simulation was validated with a scale model experiment in a closed-loop subsonic wind tunnel. The data obtained from both tests were compared in order to analyze the flow rates. Although the flow rates were reduced compared to a wind tower which did not include a rotary thermal wheel, the guideline ventilation rates for occupants in a school or office building were met above an external wind speed of 3 m/s, which is lower than the average wind speed of the UK (4–5 m/s).
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. However, despite the air flow rate decrease within the building after the introduction of the rotary thermal wheel, the reduction was not large enough to prevent the ventilation guideline rates from being met. Sufficient research has not yet been conducted to determine the suitability of rotary thermal wheels in natural ventilation, ventilation supply rates can be met but the thermal capabilities of the rotary thermal wheel have not yet been investigated. Further work would beneficial to increase understanding of the system.[4]
- ^ Mardiana-Idayu, A.; Riffat, S.B. (February 2012). "Review on heat recovery technologies for building applications". Renewable and Sustainable Energy Reviews. 16 (2): 1241–1255. doi:10.1016/j.rser.2011.09.026. ISSN 1364-0321. S2CID 108291190.
- ^ Fehrm, Mats; Reiners, Wilhelm; Ungemach, Matthias (June 2002). "Exhaust air heat recovery in buildings". International Journal of Refrigeration. 25 (4): 439–449. doi:10.1016/s0140-7007(01)00035-4. ISSN 0140-7007.
- ^ O’Connor, Dominic; Calautit, John Kaiser; Hughes, Ben Richard (October 2014). "A study of passive ventilation integrated with heat recovery" (PDF). Energy and Buildings. 82: 799–811. doi:10.1016/j.enbuild.2014.05.050. ISSN 0378-7788.
- ^ O’Connor, Dominic; Calautit, John Kaiser S.; Hughes, Ben Richard (February 2016). "A review of heat recovery technology for passive ventilation applications" (PDF). Renewable and Sustainable Energy Reviews. 54: 1481–1493. doi:10.1016/j.rser.2015.10.039. ISSN 1364-0321.