for high performance buildings
The Earth Rangers Centre uses earth tubes and a double foundation wall. The energy recovery in winter and summer is sufficient to temper the air about 5º C on average and up to 10º C on cold days.by Richard Lay .
It has become increasingly common to provide heat recovery on ventilation systems in buildings. In fact, it is difficult to achieve the minimum energy performance requirements for a LEED or other high performance building without it.
The technologies are mature, with a variety of products available including plate heat exchangers, sensible and enthalpy wheels, heat pipes and runaround coils, all intended to pre-condition incoming ventilation air by using the energy in the building exhaust air.
But there are additional applications for heat exchangers in buildings that will recover “free” heat from the ground, from drain water, from engine generators and from chiller condensers. The recovered heat is used to heat the building ventilation air, water for hydronic heating or water for domestic hot water.
This article surveys the established ventilation heat recovery systems and the less common ground heat exchangers, drain water heat exchangers and equipment heat recovery systems.
Ventilation air heat recovery
In the 1980s, Canadian companies pioneered the development of packaged heat recovery ventilation [HRV] systems which became a cornerstone for R2000 and similar energy efficient houses. Building scientists had shown that houses, and other buildings, needed adequate ventilation for healthy indoor air quality. Recovering heat from building exhaust air to pre-warm the fresh incoming air significantly reduced the energy cost of this ventilation.
Innovative manufacturers assembled a neat, ready-to-install package consisting of an efficient plate heat exchanger core, balanced supply and exhaust fans, a defrost system and simple remote controls. Builders then only had to connect it to exhaust grilles in washrooms, kitchen and laundry rooms, and supply diffusers in bedrooms and living rooms, and then run two ducts to outside wall hoods. The result was a simple system that controlled humidity, odours and other indoor air contaminants, and which did not de-pressurize the building or cost much to run. That was over 25 years ago – the rest is a history of continuous innovation in products and application.
The early HRVs used plate heat exchangers, which were very effective in removing excess moisture from houses in winter, even too effective. They solved the high humidity problems of condensation on windows, for example, but now many houses became too dry from the continuous ventilation. What to do? Materials were developed that allowed moisture to transfer from the exhaust air to the incoming air, and so re-introduced the moisture into the building and helped maintain the indoor humidity.
Heat exchanger cores were also built into wheels that rotated between the exhaust and fresh air stream, using materials that were either permeable to moisture or were coated with a desiccant that successively absorbed then released moisture. This process recovered more of the energy of the exhaust air and so energy or enthalpy recovery ventilators [ERVs] became available offering total heat recovery efficiency and winter time humidification.
While the small HRVs and ERVs in the 100 – 200 cfm range found a place in the energy efficient houses across Canada and the US, large models with 300 – 500 cfm capacity were soon developed for use in light commercial and small industrial applications that would benefit from continuous ventilation – swimming pools, health clubs, vet clinics, offices, workshops, garages, etc. When properly designed and installed, the HRVs and ERVs could dramatically improve the problems these buildings had with high humidity, odours, fumes and smoke. The idea of delivering continuous exhaust and an equal amount of conditioned makeup air was not new; what was new was the ability to do this with off-the-shelf equipment in one package.
Now, several manufactures offer packaged HRVs and ERVs in capacities above 1000 cfm designed for larger buildings. Energy recovery efficiencies of 75% are not uncommon. Large capacity units can also be supplied as add-on upgrades to other roof-top equipment, although there can be complications when trying to interface and optimize the equipment from two different manufacturers.
The heat recovery technology goes beyond cross-flow plate and rotary wheel heat exchangers, however. Heat pipe exchangers are built with pipe loops containing refrigerant and extending into the exhaust and fresh air ducts. The heat in the exhaust air is transferred to the fresh air by the action of the refrigerant evaporating and condensing at each end of the pipes. Heat pipes offer the benefit of a more compact installation for large airflow systems and simple defrost control.
When it is not possible to arrange the main exhaust duct beside the building fresh air duct, glycol runaround coils have often been installed to transfer heat between the two airstreams. A dedicated pump and piping system circulate a glycol solution between coils in each of the two main ducts. Heat recovery efficiency is less than other technologies and is generally limited by thermodynamics and coil capacity to about 45%. While all heat recovery devices suffer a fan power penalty because of their airstream resistance, runaround coil systems incur an additional energy penalty because of the glycol pump.
One manufacturer offers a reversing flow HRV in which exhaust and supply air are alternately routed through two thermal storage cassettes. For a short interval, a damper routes air one way through the cassette heat exchange media; then the damper repositions and redirects the air through the opposite cassettes. Heat recovery is reported to be very high and operation is mechanically simple.
Standards writing organizations for the HVAC industry – CSA and ARI – have published standards for the design and performance of HRVs and ERVs and offer a reliable third party basis of evaluating most manufactured products.
Ground heat recovery
Heat can be recovered from the ground to help heat buildings, using either a greater or lesser amount of energy to retrieve it. Ground source heat pumps rely on electrically powered refrigeration compressors to pump heat from the relatively low temperature shallow ground. They can recover about two to four units of energy from the ground with about one unit of electricity. However, with much less energy input, ventilation tunnels and double foundation walls can also recover usable heat to precondition ventilation air.
Originally pioneered in Europe to reduce the air conditioning energy, ventilation tunnels serve as the main air inlet for the building ventilation system. In the cooling season, ventilation air is pre-cooled a few degrees from outdoor temperature before entering the main air handling units. In winter, ventilation air is pre-warmed a few degrees.
The tunnels are made of precast concrete sewer pipe and run 2-3 m below grade from the air inlet structure to the building foundation wall and air intake plenum.
Alternatively or additionally, the building foundation can be constructed with an inner wall to form a perimeter corridor serving as a fresh air plenum. Air is admitted to the plenum at one end of the foundation and extracted by the air handlers at the other. The un insulated outer wall serves as a heat transfer surface between the ground and the incoming ventilation air. The Earth Rangers Centre [earthrangers.ca] near Toronto uses tunnels [earth tubes] and a double foundation wall. The energy recovery in winter and summer is sufficient to temper the air about 5º C on average and up to 10º C on cold days. The energy cost is a small amount of additional fan energy.
The ground can also be used to store solar heat that is abundant in the summer, to be extracted months later when needed for winter space heating. The Drake Landing Solar Community in Okotoks Alberta [www.dlsc.ca] uses a borehole thermal energy storage field of 144 boreholes and heat transfer piping to reduce the purchased heat requirements of 50 houses by about 90%.
Drain water heat recovery
When we take a shower, we send a lot of hot water down the drain that could be used for heating the cold water going to that very same shower water. There are now Canadian products designed to do just that. Manufacturers estimate that the heat exchanger can reduce water heating energy by 25 to 80%. The heat exchanger is a central section of copper drain pipe, surrounded by a spiral of copper water pipe. The hot drain water goes down; the cold makeup water goes up – and comes out hot. A great example of useful heat exchange - no gas or electricity required.
Engine generator heat recovery (cogeneration)
Many buildings, particularly in remote sites far from the utility grid, generate their own electricity with on-site generators. A gas or diesel engine generator is not very efficient at producing electricity – only about 35% of the fuel usually ends up as electricity. But it is very effective producing heat – that’s where the other 65% goes. So if your building is in a cold climate, like Canada, most of the annual energy requirement is for heating, and much of that heat can be captured from the engine coolant and from the engine exhaust using special heat exchangers designed for cogeneration [equals combined heat and power]. When both electricity and heat are utilized, the overall efficiency of the system can reach 75%.
The key to developing a more efficient electrical generation system on a community wide basis is to build electrical generators that are small enough to be able to efficiently distribute and use the heat within a short distance of the generator. Currently, electrical generators operated by the electrical utilities are too big – they generate too much heat to be used economically within the neighbourhood. In the cogeneration world, small is beautiful.
»»» Related Information:….
Suppliers of heat recovery equipment:
Ventilation air heat recovery
- Plate heat exchangers [Venmar, vanEE/CES, Fantech]
- Heat wheels – sensible and enthalpy [Venmar, Fantech, Semco, RotorSource, Greenheck, Spinnaker]
- Heat pipes [DesChamps]
- Runaround coils [coil mftrs – Madok, Aerofin, Trane, Heatcraft]
- Reversing flow [BKM Reverse Flow]
Engine generator heat recovery
- Jacket/ coolant heat recovery: DTE Energy
- Toromont Industries
- Stack heat recovery: Cain Industries
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