Heat Pumps for heating and cooling buildings
Building owners and designers wanting to reduce the greenhouse gas emissions and energy use of their buildings are confronted by an array of technologies for heating and cooling, all claiming superior performance and even promising guaranteed LEED credits. This article briefly discusses how heat pumps can contribute to building performance, and some of their limitations.
Heat pumps are classified by their source of heat. As a device that mechanically transfers heat from a source at low temperature to a load at a higher temperature, a heat pump can extract heat from the air, from the ground or from a circulating water loop within a building.
Water-source Heat Pumps
A water-source heat pump [WSHP] system connects a heat pump in each building zone to a common recirculating piping system. Using a refrigeration compressor, the heat pump heats or cools its individual zone by extracting or adding heat to the circulating water, which is maintained within a moderate temperature range by a boiler and cooling tower.
Different heat pumps can operate simultaneously in heating and cooling mode, thereby transferring and utilizing energy which otherwise would be wasted in conventional HVAC systems. If most heat pumps are in heating mode, then heat must be added to the piping system by the boiler. Likewise, if most heat pumps are in cooling mode, heat must be rejected by the cooling tower or fluid cooler.
Each heat pump would normally be installed above the ceiling or in a mechanical closet close to the conditioned space and connected to the duct work service only at that space. The units are typically 1 to 5 ton cooling capacity, with heating capacities about the same as the cooling capacity [at 3.51 kW / ton].
Ventilation is typically provided by a separate ventilation duct from a central ventilation system.
The main advantage of this system is its ability to provide heating and cooling on a zone by zone basis with simple temperature controls. In addition, the piping is simpler than in two-pipe or four-pipe fan coil systems.
The designer must take care to limit the risk of nuisance noise in the occupied space coming from the heat pump fans and compressors, and provide convenient access for service. Some building operators prefer to have mechanical equipment located in service rooms, rather than outside of the building.
There is some energy efficiency at times of simultaneous heating and cooling; however during heating season, energy use may be higher than a reference building since both the boiler [natural gas] and heat pump systems [electricity] will be operating. The boiler must be a high-efficiency condensing boiler in order to minimize gas use. Electrical demand during the winter will be higher than in a building with gas heat alone. Energy cost can be estimated using Natural Resource Canada’s EE4 program.
Ground-source Heat Pumps
Ground-source heat pumps [GSHP] are installed in the building in the same way as water-source heat pumps, and provide similarly simple zoning and control for heating and cooling. The difference is that no boiler or cooling tower is required. Instead, a ground heat exchanger outside the building provides the source of heat in winter or heat rejection in summer. This significantly reduces the space required in the building for central mechanical equipment, and reduces the energy required compared to conventional water-source heat pumps.
The ground heat exchanger consists of a network of polyethylene tubing in either horizontal loops or in vertical loops in boreholes. The amount of tubing and configuration of the field depends on local soil conditions and must be calculated by computer; however, it can be roughly estimated that about 60 - 100 square metres of horizontal field is required per ton of cooling capacity, and about 20 - 35 square metres per ton for vertical fields.
The piping system for recirculating heat transfer fluid between the ground loops and the heat pumps must also be carefully designed to minimize pump energy, which can be significant - up to one third of the total HVAC energy in some GSHP systems.
As for other comparisons between water-source and ground source heat pumps GSHPs now have higher efficiency ratings [COP in heating, EER in cooling] than water-source heat pumps. The piping for GSHPs requires insulation, and the recirculating water usually needs anti-freeze chemical. Both systems are best suited for buildings with a significant air-conditioning requirement - if there is little air conditioning load, the premium cost of heat pumps is harder to justify.
In many parts of Canada, cooling energy is only 6% or less of total building energy, while heating may be 45% or more. So the HVAC system should be optimized for heating energy efficiency - this still favours natural gas where gas is available and where coal or oil is used to generate electricity.
Energy efficient buildings in Canada also have lower design cooling loads than heating loads, and yet a given heat pump will have a greater cooling capacity than heating capacity. So the HVAC designer is faced with a dilemma: to meet all of the heating load with heat pumps, the system will be over-sized for cooling. Or, if the heat pump system is designed to just meet the cooling load, then it will be under-sized for heating and a supplementary source of heating will be required.
The ideal application for a GSHP system could be:
• Significant cooling requirements, even during winter
• Significant heating requirements, even during summer [heat pumps can heat domestic water at the same time they are heating and cooling air]
• No natural gas service available
• Limited space available for mechanical equipment, either in the building or on the roof
Natural Resources Canada’s RETScreen program is an excellent tool to evaluate the feasibility of a GSHP system.
Air-source Heat Pumps
Recent advances to air-source heat pumps have improved their efficiency during cold weather and their applicability to large buildings. For example, there are now heat pumps that can operate when it is well below freezing outside, when the older technology heat pumps used to require backup electric heating.
And there are a number of “multi-split” heat pumps in which a single outdoor unit can serve a large number of indoor units, operating as air conditioners in cooling mode, as heat pumps in heating mode, or both simultaneously.
These systems use variable refrigerant flow technology to achieve much higher cooling efficiencies than was previously possible. So they offer the same benefit as other heat pump systems - namely a single HVAC system with simple thermostat controls That can provide both heat and cooling to spaces with diversified loads.
Air source heat pumps have also been used to recover waste heat from building exhaust air to heat domestic water.
Example Buildings
A few LEED buildings in Canada which use heat pumps are:
• Toronto Region Conservation Authority Restoration Services Centre
• Stratus Winery, Ontario
• Gulf Islands National Park Operations Centre
• Port Hawkesbury Civic Centre, NS
• 740 Bel-Air, Montreal
As well, the University of Ontario Institute of Technology in Oshawa has one of the largest borehole heat pump installations in Canada.
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