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Energy Recovery Ventilation (ERV) is the energy recovery process of exchanging the energy contained in normally exhausted building or space air and using it to treat the incoming outdoor ventilation air in residential and commercial HVAC systems. The benefit of using energy recovery is the ability to meet the ASHRAE ventilation & energy standards, while improving indoor air quality, and reducing total HVAC equipment capacity.
Methods of transfer Edit
An Energy Recovery Ventilator (ERV) is a type of air-to-air heat exchanger that not only can transfer sensible heat but also latent heat. Since both temperature and moisture is transferred, ERVs can be considered total enthalpic devices. On the other hand, a Heat Recovery Ventilator (HRV) is limited to only transferring sensible heat. HRVs can be considered sensible only devices because they only exchange sensible heat.
Types of energy recovery devices Edit
|Energy Recovery Devices||Type of Transfer|
|Rotary Enthalpy Wheel||Total & Sensible|
|Fixed Plate||Total** & Sensible|
|Run Around Loop||Sensible|
**Total Energy Exchange only available on Hygroscopic units and Condensate Return units
Rotary air-to-air enthalpy wheel Edit
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.
Plate heat exchanger Edit
Fixed plate heat exchangers have no moving parts. Plates consist of alternating layers of plates that are separated and sealed. Typical flow is cross current and since the majority of plates are solid and non permeable, sensible only transfer is the result.
Enthalpy plates were introduced 2006 by Paul, a special company for ventilation systems for passive houses. A crosscurrent countercurrent air to air heat exchanger built with a humidity permeable material. Polymer fixed-plate countercurrent energy recovery ventilators were introduced in 1998 by Building Performance Equipment (BPE), a residential, commercial, and industrial air-to-air energy recovery manufacturer. These heat exchangers can be both introduced as a retrofit for increased energy savings and fresh air as well as an alternative to new construction. In new construction situations, energy recovery will effectively reduce the required heating/cooling capacity of the system. The percentage of the total energy saved will depend on the efficiency of the device (up to 90%) and the latitude of the building. A result of an ERV is that the HVAC install's initial cost is lower and the overall energy consumed by the building is lower as well.
- Designing Dedicated Outdoor Air Systems, Stanley A. Mumma
- http://www.lowkwh.com, Energy Recovery methods and publications