Energy recovery ventilation 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. During the warmer seasons, the system pre-cools and
dehumidifies while humidifying and pre-heating in the cooler seasons. 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.
Importance Nearly half of global energy is used in
buildings, 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 and
protect buildings and environment. Methods of transfer
An energy recovery ventilator 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 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, and ERV interchangeably.
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 will increase as the conditions become more extreme.
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.
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%.
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. The Home Ventilation Institute 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.. Types of energy recovery devices
**Total Energy Exchange only available on Hygroscopic units and Condensate
Return units 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.
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
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.
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, 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 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. 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
External links .com/heat-recovery-efficiency-d_201.html
Designing Dedicated Outdoor Air Systems, Stanley A. Mumma
http:www.lowkwh.com, Energy Recovery methods and publications
http:www.UltimateAir.com Energy and Heat Recovery Ventilators
Heat Recovery Ventilation TANGRA Heat Recovery Ventilaton Recovery