earth’s exchange: ground source heat pumps
Passionate advocates of ground source heat pump systems hold forth the technology as something of a panacea for the world's energy and environmental woes. A recent study commissioned by Natural Resour...
Passionate advocates of ground source heat pump systems hold forth the technology as something of a panacea for the world’s energy and environmental woes. A recent study commissioned by Natural Resources Canada, for example, suggests that “There is unlikely to be a potentially larger mitigating effect on greenhouse gas emissions and the resulting global warming impact of buildings from any other current, market-available single technology, than from ground-source heat pumps.” Yet, despite numerous studies and programs by government and other organizations, ground source heat pump technology is viewed with more suspicion than ever by the mainstream building design industry and consulting engineers.
Ground source heat pump systems (GSHPs) began making inroads into Canadian commercial and institutional buildings over two decades ago. Estimates vary with the source, but the general agreement is that there have been hundreds of commercial or institutional applications in Canada over the last 20 years. During that time, the technology has experienced successes, failures, and the emergence and passing of various equipment manufacturers and suppliers.
What catapulted the technology into the spotlight in the 1980s was the installation of dozens of ground source heat pump systems in schools in Manitoba and Ontario. At about the same time, however, the catastrophic failure of hundreds of these systems in residential demonstration projects in Ontario set the industry back several years.
Does the technology have a role to play in Canada’s pursuit of energy conservation and environmental sustainability? Certainly. Is it the perfect, or even most appropriate technology for all buildings? Certainly not.
Much of the problem is that despite all the studies, the technology of ground source heat pumps is still not well understood. There exists a great deal of hype surrounding the technology, but comparatively little objective and unbiased information, particularly in the Canadian context.
Like any other heating, ventilating and air-conditioning (HVAC) system, ground source heat pump systems have advantages and disadvantages. A clear and levelheaded look at the issues puts their somewhat checkered past into perspective, and suggests directions for the future.
Sounds simple — is complex
Simply stated, ground source heat pumps are refrigeration devices, usually electric, which extract “free” low-temperature energy from the ground for heating, and/or reject high-temperature energy to the ground for cooling. Most systems are designed to do both. The magic of a ground source heat pump, or any heat pump for that matter, is that a relatively small amount of energy is required to power the pumping process compared to the amount of free energy which undergoes the low-to-high or high-to-low temperature transfer. The ratio of total output energy to the input energy is called the Coefficient of Performance, or COP. Ground source heat pump systems typically display whole-system heating COPs ranging from 1.5 to over 4.0.
The equipment which does the heat transfer to and from the ground is the ground heat exchanger. Closed loop systems use piping circuits buried underground (in either a horizontal or vertical configuration). These circulate an aqueous solution that is usually freeze-protected. Closed loops can also be submerged in streams, lakes, and ponds. Open Loop systems rely on ground or surface water for the energy source or sink. Most systems return the water to the source after extracting or adding energy.
This all sounds very straightforward, but in practice ground source systems are probably the most complex HVAC technology we have at our disposal. There are myriad ways in which they can be configured and designed, and herein lies much of the confusion, uncertainty, and challenge.
Competing with the utility
At the most basic level the economics of a ground source heat pump system can be reduced to three considerations. First is the capital cost of the system, which can be higher or lower than a conventional alternative. Second is the life-cycle maintenance and replacement cost, which again can be higher or lower than a conventional baseline. Third is the energy operating cost, which tends to be the focus of interest, and which can also be higher or lower than a conventional alternative. To presume that ground source systems save energy compared to conventional HVAC systems is defensible in most cases. However, this is not the same as saving energy cost.
To reiterate, a COP of 4.0 means that the system produces four units of energy for every unit of input energy. If the competing conventional energy source is electricity (e.g. electric resistance), then the energy cost saving of a GSHP system with a COP of 4.0 is approximately 75 per cent, presuming that the heat pump is electrically driven and equivalent electrical rate structures apply.
Complications arise when the competing energy source is fossil fuel or some other non- electric source. The analysis becomes even more challenging when complex electrical rate structures such as demand-plus-consumption come into play. In fact, neglecting to separate utility electrical consumption and demand charges in the feasibility analysis (instead using an assumed “blended” per-kWh cost) is by far the most common error in the industry. The result is that the installation under-performs relative to its engineering savings projections, sometimes in a spectacular fashion. On the other hand, a well considered and properly designed ground source heat pump system can generate equally spectacular HVAC energy cost savings.
Centralized or decentralized?
Insofar as the equipment installed in the building aboveground is concerned, ground source heat pump systems have evolved into two general categories: decentralized and centralized. Decentralized systems were first to be widely used and they remain by far the dominant design. Decentralized GSHPs are a direct derivative of the conventional commercial integrated water loop heat pump (IWLHP) system, using multiple small capacity heat pumps distributed throughout a building and connected by a common water loop which acts as the heat source or sink. Hot or cold air from the heat pumps provides the space conditioning.
By extending the operating range of conventional IWLHP heat pumps and using a ground source heat exchanger, the decentralized ground source heat pump system was developed. A less sophisticated version of this configuration foregoes the integrated water loop and instead connects each heat pump to its own ground source heat exchanger, which is usually vertical.
Decentralized systems have become popular due to the availability of packaged equipment, relative ease of design, and aggressive marketing by equipment manufacturers and other industry interests. In the proper application decentralized systems can offer a cost-effective and simple solution. However, the decentralized configuration is not without disadvantages, most of which are shared by its IWLHP progenitor. The equipment of these systems has a comparatively limited service life, has to be maintained over a distributed area, and takes up a relatively large ceiling space. They can also be noisy and generally have a higher capital cost. Perhaps most important, decentralized systems are vulnerable to demand-based utility electricity rates.
Centralized GSHP configurations evolved largely to circumvent the disadvantages of decentralized designs. One or more large capacity ground source heat pumps replaces the multiple machines of the decentralized configuration. The centralized unit provides hot and chilled water to a conventional secondary HVAC distribution system, usually a four-pipe fan coil or radiant system. The heat pumps are generally extended-range conventional chillers.
An advantage of the centralized approach is that it can be “hybridized” with conventional fossil fuel heating equipment. This ability allows the heat pump to be optimally sized for base heating and cooling loads, and means it can be operated cost effectively under demand
-rate and other complex electrical rate structures.
The centralized system can also be used with open loop ground or surface water to provide direct cooling i.e. by-passing the heat pump entirely if the water is sufficiently cold. Groundwater cooling systems were actually the precursor to ground source heat pump systems, with hundreds of applications in Canada and the U.S. taking place over the last 50 years. Many of these systems have cooling capacities of several hundred tons. Direct open loop cooling cannot be practically accomplished with decentralized systems.
The primary disadvantages of centralized systems are that they require dedicated mechanical space. As well, they require intensive analysis and engineering if they are to be properly designed and operated.
So, which system is better? There is no simple answer. Each application must be considered in the context of factors such as the site and ground conditions, geographic location and climate, building loads and thermodynamic profiles, energy costs and utility rates. One must also consider the occupancy type, how the building is to function, and what the client expects of its economic performance.
When the heat/cold cycle breaks down
The ground heat exchanger is what sets GSHP systems apart from conventional HVAC counterparts, and catastrophic failures are most often related to this part of the system.
Vertical closed loop exchangers consist of multiple vertical boreholes, typically 45.7 m to 76 metres (150 ft. to 250 ft.) deep, encasing a grouted “u” pipe that circulates the heat exchange fluid. Horizontal systems consist of multiple piping circuits buried horizontally from 1.2 m to 2.4 m deep.
Vertical exchangers are popular because they minimize the footprint of the field, and are often installed under the building itself. However, they require drilling and thus can have a high installation cost (several hundred boreholes can be required for a large system). They also can be vulnerable to anomalous stratified soil freeze conditions (which can constrict and even pinch closed the piping in the boreholes).
What causes the most concern, however, is that the vertical exchanger can be sensitive to the heat extraction/rejection balance. Larger vertical borehole fields act much like thermal storage devices and have limited thermal interaction with the surrounding ground. Over the years, significant differences between the extraction and rejection of heat can lead to the gradual thermal degradation of the field. This effect can ultimately result in catastrophic failure as the ground becomes too hot or too cold.
Horizontal exchanger fields are far less sensitive to this dynamic since considerable thermal interaction occurs between the field and the immediate environment. However, horizontal systems have a large footprint, so they need a large site that is mostly unobstructed above grade. For this reason they are commonly used in schools since the playing field provides a convenient property for the exchanger.
Open loop systems, particularly groundwater, avoid many of the problems of closed loops. Their installation costs are also comparable to, or even lower than, the most cost-effective horizontal systems. Obviously a suitable groundwater resource is required (a sustainable flow of 2.5 to 3.0 USGPM per ton of heat extraction capacity is a good rule of thumb). Beyond that, a designer must pay attention to hydrogeological factors such as the aquifer background flow, the location and design of supply and recharge wells, and avoiding recirculation between the supply and return wells — a problem which is the open loop equivalent of closed loop thermal degradation.
The big picture
By this point is should be abundantly apparent that designing a ground source heat pump system is a greater challenge than a conventional HVAC design. To start with, you have to assess whether such a system is even feasible.
In essence, conventional HVAC design tends to be concerned more with static or worst-case design conditions. Catastrophic failure, either in the functional or economic sense, is seldom a significant issue. The design of ground source heat pump systems is far more complex, with the functional and economic success of the system dependent on carefully designing to dynamic, real-time conditions. Utility rate interactions, the building energy balance, and the ground exchanger thermodynamic response are only some of the broad issues that must be considered and quantified. These tasks require design methods and tools of high sophistication.
A wide, but unfortunately fragmented, variety of resources is available, ranging from design manuals and standards (ASHRAE, CSA, and others), to computer simulation programs that calculate energy use by the hour. There are very few reliable “cookie-cutter” solutions with ground source heat pump systems. The technology demands engineering in the purest sense, using thoroughness, resourcefulness, diligence and creativity.
It should also be abundantly clear by this point that ground source heat pump systems are not for the faint of heart. It is foolish to ignore or deny the fact that while it is widely touted as “proven,” the technology carries a much higher level of risk than conventional HVAC systems. The design engineer has an obligation not only to determine the risk/reward ratio of a potential application, but also to convey this information clearly and objectively to the client. While as engineers we have an obligation to further the cause of environmental sustainability and stewardship, the risk/reward decision ultimately rests with the client.
Taking a step back, recent developments in building design indicate that no single technology is going to be the magic solution to our environmental problems. Rather we have to take a wider view, looking to the methods and approaches we use to design buildings as much as to the equipment we put into them.
The application of holistic, integrated building design processes has resulted in quantum leaps in building energy and environmental performance. The CANMET C-2000 Program for Advanced Commercial Buildings has repeatedly demonstrated that building energy use can be halved through improvements in the design process such as by taking a multi-disciplinary team approach to the design. This method has a negligible impact in capital cost compared to conventional design solutions. The C-2000 buildings seldom employ advanced technologies, instead relying on system integration and synergies to boost performance from both environmental and functional perspectives. In fact, none of the C-2000 buildings has used a ground source heat pump system so far.
Does this mean that building owners and designers should “pass” on ground source heat pump technology? No. In the right application, these systems can raise a building’s energy performance to superlative levels. The point is, geothermal systems must be applied in a thoughtful and well considered way, ideally as part of a cohesive and integrated building performance strategy. Installing an ultra-efficient HVAC system of any kind in a less than efficient building is at best a dubious approach to improving building environmental performance. To view geothermal heating and cooling systems as a solution in isolation is to win a battle, but lose the war.CCE
Gordon Shymko, P.Eng. is president of G.F. Symko & Associates specializing in sustainable design and based in Calgary.