Exhaust air from buildings is a significant source of recoverable energy that can be used to reduce energy consumption, lower GHG emissions, and improve indoor air quality.
How does energy recovery work?
A device is used to recover heat captured from exhaust air to preheat incoming air in cold weather, or to remove heat in hot weather to reduce cooling needs. Depending on the system, it may recover only heat (sensible heat) or both heat and moisture (latent heat).
Different methods
There are several energy recovery methods, each offering a different level of efficiency. The choice of method depends on the application, budget, and operational constraints. Let’s look at the available options:
- 1
A heat recovery ventilator (HRV)
includes a heat exchange core in which stale air and fresh air flow in opposite directions through separate ducts, allowing heat or cold to be transferred without direct contact. It combines ventilation mechanics and heat recovery in one piece of equipment.
- Applications: Residential buildings, small retail spaces, offices; ventilation with integrated heat recovery. Livestock buildings.
- Efficiency rate: 40 to 70%.
- $
- 2
A plate exchanger
consists of multiple thin plates, spaced slightly apart, through which hot and cold fluids circulate in adjacent channels, enabling heat transfer across the plate surfaces.
- Applications: Industrial processes, air handling units, heat recovery from hot water or glycol systems. External fans or pumps are required to circulate the fluids.
- Efficiency rate: 70 to 80%.
- $$
- 3
A thermal wheel
is a cylindrical heat exchanger in which half of the surface is in contact with exhaust air and the other half with fresh air to be heated. This technology can transfer moisture in addition to heat using a desiccant coating such as silica gel (in which case it is called an enthalpy wheel). When both heat and moisture (latent heat) must be recovered, the interior of the wheel includes an additional coating, typically a silica gel-based polymer.
- Applications: Large air handling units (AHUs), commercial buildings; sensible and/or enthalpy recovery; very effective for high airflows
- Efficiency rate: 70 to 85%.
- $$
- 4
A coil (or run-around) system
consists of finned coils located in the fresh and exhaust air streams of a ventilation system. The coils are connected in a closed loop by piping, through which a heat transfer fluid, such as an antifreeze glycol solution and/or water, circulates. A pump circulates the fluid between the two coils. The exhaust air heats the fluid in the first coil, which then preheats the incoming air through the second. It enables heat recovery even when the two air streams are far apart (unlike a plate heat exchanger, which requires proximity) and where strict hygiene constraints apply (hospitals).
- Applications: AHUs where air streams are far apart; hospitals, schools, industrial facilities, specialized air exhaust; no cross-contamination.
- Efficiency rate: 45 to 65%.
- $$$
- 5
A regenerative heat exchanger of the cassette type
consists of blocks (cassettes) housing a series of spaced aluminum plates. The cassettes are alternately exposed to exhaust air and then to fresh air flowing in the opposite direction. In each cycle, the material first absorbs heat from the exhaust air and then releases it to the fresh air as it flows in the opposite direction.
- Applications: Compact residential and tertiary buildings; sensible and/or enthalpy recovery; high efficiency without a wheel.
- Efficiency rate: 60 to 80% (up to 85%).
- $$
- 6
A heat pipe heat exchanger
is made of sealed tubes containing a refrigerant, with one end located in the exhaust air circuit and the other in the supply air circuit. The refrigerant vaporizes at the high-temperature end, then flows to the cooler end, where it condenses and releases its heat. It is well suited to applications requiring no air mixing and reduced mechanical complexity.
- Applications: Situations where air mixing must be avoided and mechanical complexity reduced; AHUs with strict air stream separation (for hygiene); compact installations; laboratories.
- Efficiency rate: 60 to 75%.
- $$
Sensible heat and latent heat: What’s the difference?
While sensible heat causes a change in the temperature of a substance (air or water, for example), latent heat changes its physical state (from liquid to gas, for example). Many energy recovery systems capture only sensible heat. Some, however, such as energy recovery ventilators, can also capture part of the latent heat in the form of moisture and transfer it to incoming air to maintain appropriate humidity levels in a building.
“The air quality is much better, the birds are under less stress, and they don’t need to eat as much to warm up. This is a revolution compared to how we used to do things.”
Pondeuses JL
With heat recovery ventilators, poultry company Pondeuses JL saves more than 950,000 m3 of natural gas.
Good to know
Since 2012, all new buildings must be equipped with a heat recovery ventilator under section 11 of the Quebec Construction Code.
