Energy recovery ventilation

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 (precondition) the incoming outdoor ventilation air in residential and commercial HVAC systems. During the warmer seasons, the system pre-cools and dehumidifies while humidifying and pre-heating in the cooler seasons.[1] 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.

This technology has not only demonstrated an effective means of reducing energy cost and heating and cooling loads, but has allowed for the scaling down of equipment. Additionally, this system will allow for the indoor environment to maintain a relative humidity of 40% to 50%. This range can be maintained under essentially all conditions. The only energy penalty is the power needed for the blower to overcome the pressure drop in the system.[2]

Importance

Nearly half of global energy is used in buildings,[3] and half of heating/cooling cost is caused by ventilation when it is done by the "open window" method according to the regulations. Secondly, energy generation and grid is made to meet the peak demand of power. To use proper ventilation; recovery is a cost-efficient, sustainable and quick way to reduce global energy consumption and give better indoor air quality (IAQ) and protect buildings (Sick Building Syndrome, SBS) and environment.

Methods of transfer

An energy recovery ventilator (also abbreviated ERV) is a type of air-to-air heat exchanger that not only transfers sensible heat but also latent heat. Because both temperature and moisture are transferred, ERVs can be considered total enthalpic devices. On the other hand, a heat recovery ventilator (HRV) can only transfer sensible heat. HRVs can be considered sensible only devices because they only exchange sensible heat. In other words, whereas all ERVs are HRVs, not all HRVs are ERVs, but many people use the terms HRV, AAHX (air-to-air heat exchanger), and ERV interchangeably.[4]

Throughout the cooling season, the system works to cool and dehumidify the incoming, outside air. This is accomplished by the system taking the rejected heat and sending it into the exhaust airstream. Subsequently, this air cools the condenser coil at a lower temperature than if the rejected heat had not entered the exhaust airstream. During the heating seasons, the system works in reverse. Instead of discharging the heat into the exhaust airstream, the system draws heat from the exhaust airstream in order to pre-heat the incoming air. At this stage, the air passes through a primary unit and then into a space. With this type of system, it is normal, during the cooling seasons, for the exhaust air to be cooler than the ventilation air and, during the heating seasons, warmer than the ventilation air. It is for this reason the system works very efficiently and effectively. The Coefficient of Performance (COP) will increase as the conditions become more extreme (i.e., more hot and humid for cooling and colder for heating).[5]

Efficiency

The efficiency of an ERV system is the ratio of energy transferred between the two air streams compared with the total energy transported through the heat exchanger.[6][7]

With the variety of products on the market, efficiency is unquestionably going to vary from product to product. Some of these systems have been known to have heat exchange efficiencies as high as 70-80% while others have as low as 50%. Even though this lower figure is preferable to the basic HVAC system, it is not up to par with the rest of its class. Studies are being done to increase the heat transfer efficiency to 90%.[8]

The use of modern low-cost gas-phase heat exchanger technology will allow for significant improvements in efficiency. The use of high conductivity porous material is believed to produce an exchange effectiveness in excess of 90%. By exceeding a 90% effective rate, an improvement of up to 5 factors in energy loss can be seen.[9]

The Home Ventilation Institute (HVI) has developed a standard test for any and all units manufactured within the United States. Regardless, not all have been tested. It is imperative to investigate efficiency claims, comparing data produced by HVI as well as that produced by the manufacturer. (Note: all units sold in Canada are placed through the R-2000 program, a standard test synonymous to the HVI test).[10]

Types of energy recovery devices

Energy Recovery Devices Type of Transfer
Rotary Enthalpy Wheel Total & Sensible
Fixed Plate Total** & Sensible
Heat Pipe Sensible
Run around coil Sensible
Thermosiphon Sensible
Twin Towers Sensible

**Total Energy Exchange only available on Hygroscopic units and Condensate Return units

Rotary air-to-air enthalpy wheel

Main article: Thermal 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.

Enthalpy wheels are the most effective devices to transfer both latent and sensible energy but there are many different types of construction that dictate the wheel's durability. The most common materials used in the construction of the rotor are Polymer, Aluminum and Fiberglass.

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.

Plate heat exchanger

Main article: Recuperator

Fixed plate heat exchangers have no moving parts, and 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.

The tempering of incoming fresh air is done by a heat or energy recovery core. In this case, the core is made of aluminum or plastic plates. Humidity levels are adjusted through the transferring of water vapor. This is done with a rotating wheel either containing a desiccant material or permeable plates.[11]

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% sensible) and the latitude of the building.

Due to the need to use multiple sections, fixed plate energy exchangers are often associated with high pressure drop and larger footprints. Due to their inability to offer a high amount of latent energy transfer these systems also have a high chance for frosting in colder climates.

The technology patented by Finnish company RecyclingEnergy Int. Corp.[12] is based on a regenerative plate heat exchanger taking advantage of humidity of air by cyclical condensation and evaporation, e.g. latent heat, enabling not only high annual thermal efficiency but also microbe-free plates due to self-cleaning/washing method. Therefore the unit is called an enthalpy recovery ventilator rather than heat or energy recovery ventilator. Company´s patented LatentHeatPump is based on its enthalpy recovery ventilator having COP of 33 in the summer and 15 in the winter.

References

  1. ↑ Dieckmann, John. "Improving Humidity Control with Energy Recovery Ventilation." ASHRAE Journal. 50, no. 8, (2008)
  2. ↑ Dieckmann, John. "Improving Humidity Control with Energy Recovery Ventilation." ASHRAE Journal. 50, no. 8, (2008)
  3. ↑ http://www.interacademycouncil.net/CMS/Reports/11840/11914/11920.aspx
  4. ↑ The Healthy House Institute. Staff. "ERV". Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems. June 4, 2009. December 9, 2009. <http://www.healthyhouseinstitute.com/hhip_493-ERV>
  5. ↑ Braun, James E, Kevin B Mercer. "Symposium Papers - OR-05-11 - Energy Recovery Ventilation: Energy, Humidity, and Economic Implications - Evaluation of a Ventilation Heat Pump for Small Commercial Buildings." ASHRAE Transactions. 111, no. 1, (2005)
  6. ↑ Pulsifer, J. E., A. R. Raffray, and M. S. Tillack. "Improved Performance of Energy Recovery Ventilators Using Advanced Porous Heat Transfer Media." UCSD-ENG-089. December 2001.
  7. ↑ Christensen, Bill. “Sustainable Building Sourcebook.” City of Austin’s Green Building Program. Guidelines 3.0. 1994. <http://www.p2pays.org/ref/20/sourcebook/www.greenbuilder.com/sourcebook/EnergyRecoveryVent.html#EFFICIENCY>
  8. ↑ Pulsifer, J. E., A. R. Raffray, and M. S. Tillack. "Improved Performance of Energy Recovery Ventilators Using Advanced Porous Heat Transfer Media." UCSD-ENG-089. December 2001.
  9. ↑ Pulsifer, J. E., A. R. Raffray, and M. S. Tillack. "Improved Performance of Energy Recovery Ventilators Using Advanced Porous Heat Transfer Media." UCSD-ENG-089. December 2001.
  10. ↑ Christensen, Bill. “Sustainable Building Sourcebook.” City of Austin’s Green Building Program. Guidelines 3.0. 1994. <http://www.p2pays.org/ref/20/sourcebook/www.greenbuilder.com/sourcebook/EnergyRecoveryVent.html#EFFICIENCY>
  11. ↑ Huelman, Pat, Wanda Olson. "Common Questions about Heating and Energy Recovery Ventilators." University of Minnesota Extension. 1999. 2010. <http://www.extension.umn.edu/distribution/housingandclothing/dk7284.html>
  12. ↑

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