RETScreen - Ground-Source Heat Pump Project Analysis - Speaker's notes
SLIDE 1: Ground-Source Heat Pump Project Analysis
This is the Ground-Source Heat Pump Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here we discuss systems that provide heat by extracting it from the ground or a body of water and provide cooling by reversing this process. These systems are also called geothermal heat pumps, earth energy systems, or GeoExchange systems. This building, located in Pennsylvania, USA, is heated and cooled by such a system.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of ground-source heat pump (GSHP), systems; second, to illustrate key considerations in ground-source heat pump project analysis; and third, to introduce the RETScreen International Ground-Source Heat Pump Project Model.
SLIDE 3: What do GSHP systems provide?
In winter, ground-source heat pumps systems take heat from the ground or a body of water and, using an electrically powered device called a heat pump, raise its temperature to a level appropriate for building heating. In summer, this is reversed, and heat from a building is rejected to the ground or body of water, thus providing building cooling. Ground-source heat pumps can also provide hot water, for domestic use for example, with minimal additional power input. In arctic areas, groundsource heat pumps can even be used to maintain the permafrost that supports building foundations, by extracting heat that would otherwise melt the permafrost. The same system will typically provide 20 to 50% of the building's space heating requirements.
Ground-source heat pumps provide benefits beyond heating and cooling. First, compared with other heating and cooling systems, significant energy savings can be achieved through the use of ground-source heat pumps. This is due to their use of a free resource - heat stored in the ground - and their high efficiency.
Second, while most ground-source heat pump systems are initially more expensive than conventional building heating and cooling systems, their maintenance costs are generally lower than those of conventional systems, and their very high efficiency results in low operating costs. As a result, they can be the least cost heating and cooling system on a life-cycle cost basis.
Third, ground-source heat pumps require less space than conventional heating and cooling systems. Only one unit is needed for both heating and cooling. Furthermore, no equipment is located on the roof or in exposed locations outside, reducing the risk of damage by vandalism. In a large building, where a conventional system would require voluminous air ducts to transport heating and cooling from a central plant to the extremities of the building, a more compact liquid loop can be used to transport heat between the ground and multiple, smaller heat pumps scattered around the building.
Fourth, a ground-source heat pump's capacity, or the maximum heating or cooling load that can be met by the system, is less affected by extreme heat or extreme cold than an air conditioner or air-source heat pump is. Heating and cooling plants are sized on the basis of their capacity in worst-case conditions, so a smaller ground-source heat pump can achieve a desired level of plant capacity.
Fifth, the ground-source heat pump generally provides a more comfortable interior environment and better air quality than conventional heating and cooling systems. There are a number of reasons for this. The temperature of the air heated by a GSHP tends to be lower than that of a combustion system, and the volume of heated air higher. In addition, in cooling mode, better air quality results from increased removal of humidity. The cooling surfaces of a ground-source heat pump are often kept at a lower temperature than those of air-source heat pumps and conventional air conditioners; more water vapour condenses on these colder surfaces, reducing humidity levels. Furthermore, multiple, small heat pumps scattered around a large building permit the occupants of the area serviced by each heat pump to control their environment directly, rather than having to try to control the output of a central plant servicing the entire building.
A final advantage of ground-source heat pumps is reduced peak electricity consumption during summer time. Peak loads during the summer generally coincide with periods of high cooling loads, so ground-source heat pumps, which are more efficient than conventional systems, can lower peak electricity load charges levied on commercial or industrial buildings and can reduce strain on the electric network.
The photo on the top left shows an advanced house in Massachusetts, USA, that makes use of not just a ground-source heat pump but also a photovoltaic system, a solar thermal system, and passive solar heating and cooling. The residence was commissioned by the Boston Edison utility in the 1990's as a demonstration of design techniques and technologies that may become commonplace in the early decades of the 21st century. A typical residential ground-source heat pump is shown on the bottom right.
SLIDE 4: Components of GSHP Systems
A ground-source heat pump system has three major components: the earth connection, a heat pump, and the interior heating or cooling distribution system.
The earth connection transfers heat into or out of the ground or water body. It often takes the form of an outdoor heat exchanger. This is a coil or pipe carrying water, an antifreeze mixture, or another heat transfer fluid. It may be buried in the ground, in which case it is called a ground-coupled system, or submerged in a lake or pond, in which case it is called a surface water system. When the temperature of the fluid in the heat exchanger is higher than the temperature of the ground or water body, heat flows out of the system and building cooling can be performed. When the temperature of the fluid in the heat exchanger is lower than its surroundings, heat flows into the system.
These are closed-loop systems: the heat transfer fluid flows from the heat pump, located inside the building, around the outdoor heat exchanger, and back to the heat pump. In contrast, in some systems the groundwater drawn from a well is used directly as the heat transfer fluid. Groundwater is fed to the heat pump and then flows back into the ground in these open loop systems.
The second major component is a liquid-source heat pump. This an apparatus that uses compression and expansion of a refrigerant to drive heat flows between the inside of the building and the outdoor heat exchanger loop. Normally heat will flow only from hotter to colder matter, but a heat pump will draw heat from the ground at, say, 5ºC and use it to warm a building to 21ºC. The device "pumps" heat against its natural tendency, just like a water pump makes water flow uphill.
We say "liquid source" heat pump in contradistinction to air-source heat pumps. The former transfers heat to and from a liquid, which is, in a ground-source heat pump system, the heat transfer fluid used for the earth connection. Air-source heat pumps, which include simple air conditioning units such as those mounted in windows, transfer heat directly to and from the outside air.
The third major component is the system for distributing heating and cooling inside the building. GSHP systems typically use conventional ductwork to distribute hot or cold air and to provide humidity control. For larger commercial buildings there are usually multiple heat pumps (perhaps one for each zone) attached to the earth connection through a building loop. This provides greater control of the conditions of each zone, and even heat exchange between zones. For exemple, sunny rooms can extract excess heat and redistribute it to cooler areas of the building.
SLIDE 5: Liquid-Source Heat Pumps
The heart of the ground-source heat pump system is the liquid source heat pump. Let's examine it in more detail.
The liquid-source heat pump operates according to the same principle as conventional vapour compression heat pumps, such as water-source heat pumps, common in commercial buildings, and air-source units, found in residential applications. It transports heat between the earth connection and the distribution system; this transfer requires forcing heat to flow from colder matter to warmer matter. It does this in much the same way that a refrigerator works.
In a heating mode, the heat pump works as follows: heat from the earth connection arrives at a heat exchanger, called the evaporator. On the other side of the heat exchanger is cold refrigerant in a mostly liquid state. The refrigerant is even colder than the temperature of the heat transfer fluid from the earth connection, so heat flows into the refrigerant. This heat causes the liquid refrigerant to evaporate; its temperature does not change much, if at all.
This gaseous, low pressure and low temperature refrigerant then passes into an electrically-driven compressor. This drastically raises the refrigerant's pressure and, as a consequence, its temperature.
The high temperature, high pressure, gaseous output of the compressor is fed into a second heat exchanger, called the condenser. In most heat pumps, air to be heated is blown by a fan through this heat exchanger; in some systems, however, water or another heat transfer fluid will take the place of air for heat distribution. Since the refrigerant is hotter than the air or heat transfer fluid, it transfers heat to it. As it loses heat, the refrigerant's temperature drops somewhat and it condenses.
This high temperature liquid refrigerant then passes through an expansion valve. The valve reduces the pressure of the refrigerant, and consequently, its temperature drops significantly. This low temperature liquid is then fed into the evaporator, and the cycle continues. In this way, the heat from the water or other heat transfer fluid in the earth connection is transferred to the air in the building; hence, the name "water-to-air heat pump".
One significant difference between a ground-source heat pump and a refrigerator is that the ground-source heat pump is meant to run in both directions. When in cooling mode, the heat exchanger between the earth connection and the refrigerant becomes the condenser, and the air-to-refrigerant heat exchanger becomes the evaporator. This is accomplished through a reversing valve inside the heat pump.
The heat pump requires electricity to run the compressor, fans, circulation pumps, and controls, but the heating or cooling energy provided by the system is generally two to four times the electrical energy consumed. This 200 to 400% efficient operation compares favourably with electrical resistance heating, which cannot exceed 100% efficiency, and reduces electricity consumption compared to conventional air conditioners and air-source heat pumps by 30 to 70% for heating and 20 to 50% for cooling. When compared to even the most efficient gas technologies, ground-source heat pumps can save significant quantities of energy.
All the components of the heat pump are typically housed in a single enclosure. This includes the earth connection-to-refrigerant heat exchanger, the compressor, controls, the fan, an air filter, an air handler, and refrigerant-to-air heat exchanger. The unit will typically have a thermal capacity of 3.5 to 35 kW. This is sufficient for a small building, and for larger buildings, multiple units can use the same earth connection.
Where there is a use for hot water, such as in many residential applications, a desuperheater can be included in the heat pump. This is a small auxiliary water-to-refrigerant heat exchanger at the outlet of the compressor. It bleeds heat from the refrigerant and transfers it to the house's hot water tank. It generates most hot water
when the cooling load is high, furnishes some hot water during winter, but provides nothing when the compressor is not operating, such as during spring and fall. Some new heat pumps include a heat exchanger and controls dedicated to water heating that permit the system to fully meet the hot water requirement, even when space heating or cooling is not required.
SLIDE 6: Types of Earth Connection
As mentioned earlier, the earth connection can take a number of different forms. Let's examine some of these here.
There are two common configurations for ground-coupled heat pump systems: the vertical and the horizontal heat exchanger configurations. In the vertical configuration, heat transfer occurs in a series of vertical boreholes drilled 45 to 150 m into the ground. The heat exchanger pipe runs down to the bottom of the hole and then back to the surface. Horizontal subsurface supply and return headers connect the boreholes in parallel. Once the pipe has been laid in the holes, the holes are back-filled and grouted. Grouting is the filling of the borehole with a special material that prevents surface water from contaminating groundwater and one aquifer from flowing into another. Grouting materials usually have poorer heat transfer characteristics and higher costs than common backfill material. Where local regulations permit, grouting only the upper 6 to 10 m of a borehole will often provide adequate protection from surface water seepage, increase the heat exchanger efficiency, and lower its cost.
In the horizontal heat exchanger configuration, the pipe is buried, usually between 1 and 2 m below the surface, in one or more horizontal trenches. Supply and return headers connect the trenches in parallel. The pipe may be laid in the trench in a variety of ways: there may be a single pipe per trench, the supply and return pipe may be side-by-side or stacked one on top each other, though always separated by ½ m or so, four pipes may be laid in a trench, or the pipe may be a stretched coil, called a "slinky" or spiral. This last configuration provides a large surface area for heat exchange and may therefore be advantageous when land area is limited, although more piping is used and this increases costs. In typical horizontal heat exchangers about 35 to 55 m of pipe are installed per kW of heating and cooling capacity. The trench is backfilled once the pipe has been laid in it.
The vertical heat exchanger makes most sense when the ground is rocky near the surface and therefore trenching would be difficult, where land area is limited, or where disruption of the landscape must be minimized. It is commonly used for large buildings. Where ground conditions facilitate trenching, the horizontal heat exchanger is often the least costly earth connection to install. It requires the largest land area, however, and is usually best suited to smaller applications such as residential and small commercial buildings.
The efficiency of the heat pump is related to the temperature of the heat transfer fluid arriving from the earth connection. In the vertical configuration, the heat exchanger is buried deep in the ground, where, as we will see, temperature is stable year-round. In contrast, the heat exchanger of the horizontal configuration is near the surface and is therefore influenced by seasonal variations in the ground temperature. As a result, the vertical exchanger permits the heat pump to operate more efficiently and permits a shorter pipe length than the horizontal configuration.
In contrast to the closed loop vertical and horizontal heat exchangers, the groundwater earth connection pumps water from a supply well leading to an aquifer, uses it as a heat source or sink, and then returns the water to the same aquifer through a second well, called an injection well. The water from the well is pumped either directly to the heat pump's water-to-refrigerant heat exchanger or, commonly, to an intermediate heat exchanger connected to a building loop. This intermediate heat exchanger protects the heat pump from the fouling, abrasive, or corrosive action of the well water.
The groundwater earth connection was the first to appear on the market, is the simplest to install, and has been used successfully for decades. Environmental regulations and insufficient water availability may limit its use in some areas, however.
Ground-source heat pumps can also use surface water, such as ponds or lakes, as a source or sink of heat. The heat exchanger consists of a closed loop of submerged piping, as in the ground-coupled heat pump systems. Piping and excavation requirements are low. Yet another type of earth connection is the standing column well. This approach makes use of groundwater but requires only one well for supply and return. The well is typically 15 cm in diameter and may be as deep as 450 m. Water from the bottom of the well is pumped to the building's heat exchanger but is returned to the top of the same well.
SLIDE 7: GSHP Resource: Ground Temperatures
Clearly the ground is the heat sink and source for a ground-source heat pump. But where does the heat that is extracted from the ground come from? And what happens to the heat that is rejected to the ground?
Ultimately, the heat extracted from the ground by a ground-source heat pump comes from either the sun or from heat injected into the ground during the cooling season. The ground absorbs about half of the solar radiation that is incident upon the earth. The earth's crust stores this energy over long periods of time. In contrast, the quantity of heat coming from the core of the earth is not significant.
The ground also loses a great deal of energy, for example, by radiating it to space and transferring it to the atmosphere. Heat added to the ground by the system will eventually contribute to these losses.
A heat balance determines the temperature of the ground: it is the temperature at which all forms of losses match all forms of energy gains. At the surface, the gains and losses vary with the seasons: in winter the losses are higher and the gains lower than in summer. As a consequence, the surface temperature follows the air temperature. But below the surface, the ground stores energy, and reduces this variation. This dampening effect increases with greater depth, until, at a depth of about 15 m, the seasonal variation in ground temperature is negligible. Rather, the temperature will be at a constant level, somewhat above the annual average air temperature above ground. The actual difference between the annual average air temperature and the ground temperature will
depend on such factors as climate, ground cover and vegetation, snow cover, slope, and soil properties.
This is illustrated in the diagram on this slide, showing ground temperature in Ottawa, Ontario, Canada, at three depths below the surface. Thirty centimeters below the surface, the ground reaches a minimum of -1ºC during the winter months, and a maximum of about 20ºC in late summer. The amplitude of this cycle is less than half as great 2 m below the surface, and is barely discernable at 5 m. Note, too, that the phase of the cycle of temperature variation shifts with depth: with increasing depth, ground temperature lags the moving average of air temperature more and more. At 5 m below the surface, they are completely out of phase: the maximum ground temperature occurs during winter, and the minimum during summer. This is due to the long time required for heat to be transported through the ground.
The ground's dampening effect on temperature variation is a key advantage of groundsource heat pumps compared with air-source heat pumps. The efficiency and capacity of all heat pumps are closely related to the difference in temperature between the inside of the building and the source or sink of heat. The smaller this difference, the more efficiently the heat pump can operate. Because the ground temperature stays closer to
the interior building temperature than does the outside air temperature, the groundsource heat pump operates more efficiently than conventional air conditioners and airsource heat pumps and its capacity varies less.
SLIDE 8: Example of GSHP System Costs
The financial viability of ground-source heat pump systems varies from location to location. Here we examine two residential applications, one in Southern Finland and the other in Connecticut, USA. The ground-source heat pump is compared with the leading competing conventional technology at each of these locations.
In Finland, heating is the main concern and cooling is viewed as a pleasant luxury. The GSHP system is consequently compared to an electric resistance heating system without air conditioner. Both the GSHP and the electrical resistance heating system use hydronic heat distribution; that is, heat is distributed around the house by water pipes embedded in the floor. The GSHP system, at $13,000 for this 150 m² house, initially costs $5,000 more than the electric heating, but uses one third as much electric energy and saves $450 on an annual basis. While the simple payback is over ten years, ground-source heat pumps have about 10% of the residential market in Finland: people recognize that by reducing their energy consumption they protect themselves from the rising energy costs they expect to see over the lifetime of their heating system. Furthermore, they see the GSHP as an environmentally friendly technology.
In Connecticut, the GSHP system must compete with a conventional system consisting of an oil-fired furnace and air conditioner. At $20,000, the GSHP system for this 275 m² house is initially about one third more expensive than the conventional system but uses only 40% as much energy on an annual basis. Heating costs are reduced by a quarter and cooling costs, which are more significant at this location, by a third. The local utility offers a significant subsidy - and reduced electricity tariffs - for customers with houses that are built to certain minimum standards of insulation and who install a GSHP. This makes the GSHP financially attractive to the home-owner.
These examples illustrate the high initial costs of GSHP systems. In general, these are almost double those of conventional central systems in residential applications; 20% to 40% more than constant volume, single zone rooftop units; and up to 20% more than multizone or central two-pipe chilled water systems. These examples also illustrate the significant energy savings achieved by the GSHP systems, but give no indication of other benefits like reduced maintenance costs.
SLIDE 9: Ground-Source Heat Pump Project Considerations
The financial viability of a ground-source heat pump system varies from project to project. Here are a few considerations to look for when assessing whether a ground-source heat pump project may be cost-effective.
First, a project requiring both heating and cooling is preferable to one that requires just one or the other. While the same GSHP system can provide both heating and cooling, two different conventional systems will be required, increasing the cost of the competing conventional technology. Furthermore, since it is operating year-round, the GSHP system can generate larger energy savings.
Second, large seasonal variations in temperature will favour the GSHP system over airsource heat pumps, whose capacity and efficiency decrease at temperature extremes, and ensure that there is a significant energy demand on which the GSHP can generate savings.
Third, if there is already a useable heating and cooling system installed, the purchase and installation of a GSHP is rarely justified on the basis of its energy benefits. Thus, the GSHP is most cost-effective in new construction, especially since this facilitates trenching and drilling, or when an existing heating and cooling system has reached the end of its life and must be replaced.
If heating is the dominant energy requirement, then low electricity prices and high gas or oil prices will make the GSHP more attractive than combustion systems. If cooling is dominant, then high electricity prices will favour ground-source heat pumps over conventional air conditioning, which is less efficient. If both heating and cooling requirements are high, then low electricity prices year-round with high peak load charges during summer are ideal for the GSHP.
Installation of the earth connection is a critical step in any GSHP project. For groundcoupled systems, soil conditions will influence not only the feasibility of drilling and trenching, but also the performance of the earth connection. Similarly, groundwater availability, and regulations concerning its use, will determine the feasibility of a groundwater earth connection. Investigation of the soil or groundwater conditions is an important part of a feasibility study; despite this, the actual cost of the earth connection installation will remain a matter of considerable uncertainty and is the part of the system most likely to cause cost overruns. Costs will be reduced if trenching or drilling equipment is readily available on or near the site. Since earth connections in a GSHP system are usually very difficult to reach after installation, the materials and workmanship must be of the highest quality.
When building heating and cooling loads differ greatly, sizing the earth connection for the lower of the two loads can reduce its cost. This will require supplemental heating or heat rejection capacity, usually provided by low-cost conventional heating systems and cooling towers, respectively.
Ultimately, the financial viability of a ground-source heat pump system will depend on the customer's criteria for cost-effectiveness. A system may be difficult to justify if very short simple payback periods are demanded, but may also be an obvious choice from the point of view of a home-owner who must live with the operation and maintenance costs of the system for twenty or twenty-five years.
SLIDE 10: Residential Building Systems - Examples: Australia, Germany, and Switzerland
The three photos on this slide show residential buildings that make use of ground-source heat pumps. The single-family residence in Switzerland as well as the 320-apartment complex in Adelaide, Australia, both use a vertical outdoor heat exchanger, while the German house uses a groundwater earth connection. These are typical of the higher-end residential construction that tends to make use of ground-source heat pumps. The high initial costs of the GSHP do not constitute an especially large fraction of these more expensive homes, and the home-owners generally view the GSHP system as a long-term investment in their home. Furthermore, they are swayed by the environmental benefits and the improvements in comfort and air quality associated with the GSHP.
Electric utilities often subsidize the installation of ground-source heat pumps. Private utilities benefit through increases to their base load and reductions in their peak load. Public utilities also recognize that the system's environmental benefits accrue to society as a whole, and therefore the initial costs should not be borne by the system's owner alone. Regardless, the subsidy can be a major consideration in the home-owner's decision to install a ground-source heat pump.
SLIDE 11: Commercial Building Systems - Examples: UK and USA
The viability of ground-source heat pump systems for commercial buildings can be impeded by demands for short simple paybacks, generally of less than five years, and limited availability of land for the large earth connections required. Nevertheless, there are many such installations, illustrated by the three examples on this slide. This reflects the many advantages of the ground-source heat pump in these applications. Since the heat pump is physically smaller than conventional heating and cooling plants, and since heat distribution in a large building can be achieved with a compact liquid loop rather than
voluminous air ducts, the ground-source heat pump can free building space for commercial uses. The use of multiple heat pumps distributed around a large building also simplifies control of the interior environment. The elimination of roof-top units, cooling towers, and chimneys reduces opportunities for vandalism. And being more efficient than conventional air conditioners, the ground-source heat pump reduces summertime peak load charges.
The top photo shows a building Axa Sunlife commissioned in Croydon, the United Kingdom. It was the country's first large commercial building using a closed loop groundsource heat pump for heating and cooling, and it won an environmental award from the British Engineering Council.
The cluster of buildings on the left of the photo at the bottom left is the Galt House East Hotel and Waterfront office buildings, in Louisville, Kentucky. This building complex utilizes the world's largest GSHP system, consisting of 1,200 heat pumps with a combined cooling capacity of 16 MW or 4,500 tons. It uses a groundwater earth connection and water storage as a thermal reservoir.
The filling station in Prairie Village, Kansas, seen in the photo at the bottom right, uses a ground-source heat pump system to provide ice melting for the driveway, warm water for the car wash, radiant heat for the car wash bays, and cooling for the store's refrigerators, walk-in freezers, and icemakers all reportedly with a two-year simple payback.
SLIDE 12: Institutional Building Systems - Examples: Canada and USA
Ground-source heat pumps can be very well suited to institutional buildings. Often, the building owners and operators are willing to accept longer paybacks than those required in the commercial sector, and are more open to innovative designs and technologies like GSHPs. Many institutional buildings have a simultaneous need for both heating and cooling, which the building loop of a ground-source heat pump system can take advantage of. This is the case, for example, in arenas, which need to heat the bleachers at the same time that they need to cool the ice.
The photo at the bottom of the slide shows the Saint-Hyacinthe Vocational School, located just east of Montreal, Quebec, Canada. The building has a lot of agricultural land around it and therefore was able to use a very inexpensive horizontal heat exchanger. In addition, the building uses a Solarwall solar air heating system to preheat ventilation air.
SLIDE 13: RETScreen Ground-Source Heat Pump Project Model
The RETScreen Ground-source Heat Pump Project Model is a simple but very useful tool for the preliminary investigation of the technical and financial feasibility of ground-source heat pump projects. For an installation anywhere in the world, it can provide an analysis of the energy production, life-cycle costs, and greenhouse gas emissions reductions. The model can be applied to residential, commercial, institutional, and industrial buildings that are ground-coupled by a horizontal or a vertical outdoor heat exchanger or that use groundwater in an open loop system. A minimum of input data is required of the user.
While the RETScreen model can be applied to most ground-source heat pump applications, it has several limitations. It does not model surface water earth connections, nor does it account for long-term thermal imbalances in the ground that can occur when heating loads are much greater than cooling loads, or vice versa. It cannot be used to study buildings with simultaneous heating and cooling loads, but rather deals with block loads - that is, it treats the building as a single space all at the same temperature. Finally, it does not provide any output concerning water heating by GSHPs.
SLIDE 14: RETScreen GSHP Energy Calculation
The RETScreen Ground-source Heat Pump Energy calculation determines the performance of the GSHP system over the period of a year. The calculation is based on a bin method rather than simulation; that is, instead of progressing through the year, day-by-day or hour-by-hour and calculating the building load and GSHP performance, RETScreen estimates the number of hours of the year during which the outdoor temperature will fall into a "bin", or specified range of temperatures. Then, RETScreen calculates the performance of the system for each bin. The approach assumes that the system is operating at a steady-state; that is, that the heat pump system's performance is determined solely by the conditions at a given point in time, not the conditions or state in previous hours or days. An iterative solution is required for this method. Here, we provide an overview of this calculation; for more information, see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
RETScreen requires that the user enter a few key parameters describing the ground conditions, the maximum and minimum air temperatures at the site, and the type of earth connection and heat pump that will be used. The user must also enter either the building's peak thermal load and energy demand for heating and cooling, or a few characteristics of the building so that RETScreen can estimate these loads and demands.
RETScreen uses the heating and cooling design temperatures entered by the user to synthesize the frequency distribution of outdoor air temperature. A series of temperature bins spanning the range of -50 to +50ºC is constructed, and the number of hours in the year that fall in each temperature bin is estimated. It also uses the mean ground temperature, the annual swing in the ground temperature, and the characteristics of the soil type specified by the user to calculate the minimum and maximum ground temperatures at the depth of the earth connection.
If the user has provided data describing the building, RETScreen uses this to determine an equation for the building heating or cooling load as a function of outdoor air temperature. This equation takes into account, in a rudimentary way, losses and gains by transmission through the envelope, solar gains, internal gains, and latent and sensible gains and losses due to fresh air intake and occupants.
If, on the other hand, the user has provided building design loads and annual energy consumption, RETScreen must use this data to estimate the equation for heating or cooling load as a function of outdoor temperature. This involves an iterative process which finds the equation that accurately predicts both the building design load when the temperature is equal to the design temperature and the annual energy use when the equation is applied to all the outdoor air temperature bins.
The equation is applied to each temperature bin to find the building thermal load for the bin; if the user has supplied descriptive data for the building, the equation is also used to calculate the annual energy use (by summing over all bins) and the building design heating and cooling load (by applying it to the design temperatures). In addition, the equation determines the building balance points, those temperatures above and below which the building does not require heating and cooling, respectively.
Next, RETScreen estimates the required heat pump capacity. The heat pump must be able to supply the design cooling load or, if it is larger and the user specifies a heating design criterion, the design heating load. The heat pump capacity is affected by the temperature of the heat transfer fluid entering the pump from the ground loop; it is assumed that the design cooling load must be met when the temperature of the heat transfer fluid is at its maximum and the design heating load must be met when the temperature of the heat transfer fluid is a minimum. The heat pump capacity under standard conditions - that is, the conditions for which heat pumps are rated - is calculated based on a quadratic formula that adjusts the heat pump capacity according to the temperature of the heat transfer fluid entering the pump.
RETScreen must still estimate the size of the ground heat exchanger or the magnitude of the groundwater flow. The length of the heat exchanger is found from the design heating or cooling load, the coefficient of performance of the heat pump, the soil conditions, and the difference between the temperature of the ground and the temperature of the heat transfer fluid entering the heat pump under design conditions. For groundwater heat pumps, the required groundwater flow is that which has the thermal capacity to satisfy the maximum of the design heating and cooling loads. The density and specific heat capacity of water are known, so the critical step in this calculation is the determination of the temperature of the water entering and leaving the intermediate heat exchanger that couples the heat pump to the groundwater flow.
Then, for each temperature bin, RETScreen calculates the heat pump performance. First, it must calculate the actual capacity and coefficient of performance - or COP - of the heat pump for the entering water temperature associated with the bin. Then, based on this capacity, it estimates what fraction of the time the heat pump will have to operate in order to satisfy the load associated with the bin. It adjusts this fraction to account for inefficiencies occurring during heat pump start-up and shutdown. Finally, RETScreen calculates the electric energy used by the heat pump by multiplying the number of hours in the bin, the adjusted fraction of the time that the heat pump will operate, and the quotient of the standard heat pump capacity and its actual COP. Electricity used to operate the building loop pump and the ground loop pump is added in.
If the user specifies that the system be sized based on the design cooling load but the design heating load is actually larger, or vice versa, supplemental heating or cooling rejection capacity will be needed. For every bin, RETScreen determines how much electricity will be used by this supplemental capacity, assuming that it has the same operating characteristics as the specified base case heating ventilation and air conditioning system. This electricity is added to that used by the GSHP system, and the result is summed over all bins to find the total annual energy use.
Slide 15: Example Validation of the RETScreen GSHP Project Model
The RETScreen software has been validated in a number of ways. For example, the algorithm RETScreen uses to synthesize temperature bins, based on heating and cooling design temperatures, was compared to monitored data. In this comparison, the annual energy use of a typical air-source heat pump was estimated based on the monitored data as well as the binned temperature data generated by RETScreen. For all five Canadian cities examined, the results agreed to within 3%.
In a second comparison, RETScreen was validated against a number of GSHP sizing programs and a detailed simulation program. The point of comparison was the required ground heat exchanger length for two residential systems and one commercial system, all in the USA. This comparison was done twice, once using design load and energy demand data directly, and a second time using values for design load and energy demand as estimated by RETScreen from the building characteristics.
In all cases, RETScreen was within 20% of the average heat exchanger length recommended by the other software packages. This was true regardless of whether the sizing was based on performance in a single year or over a period of ten years, when the long-term effects of the heat pump on the ground temperature have appeared. The heat exchanger lengths in the built systems were often considerably different from those recommended by the sizing software, showing that RETScreen was more accurate than the simple rules of thumb probably used by the installers of these systems. This suggests that RETScreen is sufficiently accurate for pre-feasibility purposes.
SLIDE 16: Conclusions
Ground-source heat pumps constitute a cost-effective technology for the provision of space heating, cooling, and hot water. Seasonal and diurnal temperature variations are dampened by the heat capacity and limited conductivity of the ground, and as a result, the GSHP operates more efficiently than comparable air-source heat pumps or air conditioners. Compared to conventional heating, ventilation, and air conditioning systems, GHSP technology has high initial costs but low operating and maintenance costs. The technology is most cost-effective where both heating and cooling is needed over the course of the year; this permits the GSHP system to generate benefits during both summer and winter.
The RETScreen software estimates the frequency distribution of exterior temperature over the course of the year based on heating and cooling design temperatures. Grouped in bins, these temperature data are used in combination with a model for the building load, expressed as a function of outdoor air temperature, to determine the annual space heating and cooling energy benefits of the ground-source heat pump system. RETScreen also determines the size of heat pump, and the length of the ground heat exchanger or flow of groundwater required. Thus, using a few parameters describing climate and soil conditions, RETScreen can provide accuracy comparable to detailed sizing and simulation software, and thereby significantly reduce the cost of conducting preliminary feasibility studies of ground-source heat pump projects.
SLIDE 17: Questions?
This is the end of the Ground-Source Heat Pump Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
This is the Ground-Source Heat Pump Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here we discuss systems that provide heat by extracting it from the ground or a body of water and provide cooling by reversing this process. These systems are also called geothermal heat pumps, earth energy systems, or GeoExchange systems. This building, located in Pennsylvania, USA, is heated and cooled by such a system.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of ground-source heat pump (GSHP), systems; second, to illustrate key considerations in ground-source heat pump project analysis; and third, to introduce the RETScreen International Ground-Source Heat Pump Project Model.
SLIDE 3: What do GSHP systems provide?
In winter, ground-source heat pumps systems take heat from the ground or a body of water and, using an electrically powered device called a heat pump, raise its temperature to a level appropriate for building heating. In summer, this is reversed, and heat from a building is rejected to the ground or body of water, thus providing building cooling. Ground-source heat pumps can also provide hot water, for domestic use for example, with minimal additional power input. In arctic areas, groundsource heat pumps can even be used to maintain the permafrost that supports building foundations, by extracting heat that would otherwise melt the permafrost. The same system will typically provide 20 to 50% of the building's space heating requirements.
Ground-source heat pumps provide benefits beyond heating and cooling. First, compared with other heating and cooling systems, significant energy savings can be achieved through the use of ground-source heat pumps. This is due to their use of a free resource - heat stored in the ground - and their high efficiency.
Second, while most ground-source heat pump systems are initially more expensive than conventional building heating and cooling systems, their maintenance costs are generally lower than those of conventional systems, and their very high efficiency results in low operating costs. As a result, they can be the least cost heating and cooling system on a life-cycle cost basis.
Third, ground-source heat pumps require less space than conventional heating and cooling systems. Only one unit is needed for both heating and cooling. Furthermore, no equipment is located on the roof or in exposed locations outside, reducing the risk of damage by vandalism. In a large building, where a conventional system would require voluminous air ducts to transport heating and cooling from a central plant to the extremities of the building, a more compact liquid loop can be used to transport heat between the ground and multiple, smaller heat pumps scattered around the building.
Fourth, a ground-source heat pump's capacity, or the maximum heating or cooling load that can be met by the system, is less affected by extreme heat or extreme cold than an air conditioner or air-source heat pump is. Heating and cooling plants are sized on the basis of their capacity in worst-case conditions, so a smaller ground-source heat pump can achieve a desired level of plant capacity.
Fifth, the ground-source heat pump generally provides a more comfortable interior environment and better air quality than conventional heating and cooling systems. There are a number of reasons for this. The temperature of the air heated by a GSHP tends to be lower than that of a combustion system, and the volume of heated air higher. In addition, in cooling mode, better air quality results from increased removal of humidity. The cooling surfaces of a ground-source heat pump are often kept at a lower temperature than those of air-source heat pumps and conventional air conditioners; more water vapour condenses on these colder surfaces, reducing humidity levels. Furthermore, multiple, small heat pumps scattered around a large building permit the occupants of the area serviced by each heat pump to control their environment directly, rather than having to try to control the output of a central plant servicing the entire building.
A final advantage of ground-source heat pumps is reduced peak electricity consumption during summer time. Peak loads during the summer generally coincide with periods of high cooling loads, so ground-source heat pumps, which are more efficient than conventional systems, can lower peak electricity load charges levied on commercial or industrial buildings and can reduce strain on the electric network.
The photo on the top left shows an advanced house in Massachusetts, USA, that makes use of not just a ground-source heat pump but also a photovoltaic system, a solar thermal system, and passive solar heating and cooling. The residence was commissioned by the Boston Edison utility in the 1990's as a demonstration of design techniques and technologies that may become commonplace in the early decades of the 21st century. A typical residential ground-source heat pump is shown on the bottom right.
SLIDE 4: Components of GSHP Systems
A ground-source heat pump system has three major components: the earth connection, a heat pump, and the interior heating or cooling distribution system.
The earth connection transfers heat into or out of the ground or water body. It often takes the form of an outdoor heat exchanger. This is a coil or pipe carrying water, an antifreeze mixture, or another heat transfer fluid. It may be buried in the ground, in which case it is called a ground-coupled system, or submerged in a lake or pond, in which case it is called a surface water system. When the temperature of the fluid in the heat exchanger is higher than the temperature of the ground or water body, heat flows out of the system and building cooling can be performed. When the temperature of the fluid in the heat exchanger is lower than its surroundings, heat flows into the system.
These are closed-loop systems: the heat transfer fluid flows from the heat pump, located inside the building, around the outdoor heat exchanger, and back to the heat pump. In contrast, in some systems the groundwater drawn from a well is used directly as the heat transfer fluid. Groundwater is fed to the heat pump and then flows back into the ground in these open loop systems.
The second major component is a liquid-source heat pump. This an apparatus that uses compression and expansion of a refrigerant to drive heat flows between the inside of the building and the outdoor heat exchanger loop. Normally heat will flow only from hotter to colder matter, but a heat pump will draw heat from the ground at, say, 5ºC and use it to warm a building to 21ºC. The device "pumps" heat against its natural tendency, just like a water pump makes water flow uphill.
We say "liquid source" heat pump in contradistinction to air-source heat pumps. The former transfers heat to and from a liquid, which is, in a ground-source heat pump system, the heat transfer fluid used for the earth connection. Air-source heat pumps, which include simple air conditioning units such as those mounted in windows, transfer heat directly to and from the outside air.
The third major component is the system for distributing heating and cooling inside the building. GSHP systems typically use conventional ductwork to distribute hot or cold air and to provide humidity control. For larger commercial buildings there are usually multiple heat pumps (perhaps one for each zone) attached to the earth connection through a building loop. This provides greater control of the conditions of each zone, and even heat exchange between zones. For exemple, sunny rooms can extract excess heat and redistribute it to cooler areas of the building.
SLIDE 5: Liquid-Source Heat Pumps
The heart of the ground-source heat pump system is the liquid source heat pump. Let's examine it in more detail.
The liquid-source heat pump operates according to the same principle as conventional vapour compression heat pumps, such as water-source heat pumps, common in commercial buildings, and air-source units, found in residential applications. It transports heat between the earth connection and the distribution system; this transfer requires forcing heat to flow from colder matter to warmer matter. It does this in much the same way that a refrigerator works.
In a heating mode, the heat pump works as follows: heat from the earth connection arrives at a heat exchanger, called the evaporator. On the other side of the heat exchanger is cold refrigerant in a mostly liquid state. The refrigerant is even colder than the temperature of the heat transfer fluid from the earth connection, so heat flows into the refrigerant. This heat causes the liquid refrigerant to evaporate; its temperature does not change much, if at all.
This gaseous, low pressure and low temperature refrigerant then passes into an electrically-driven compressor. This drastically raises the refrigerant's pressure and, as a consequence, its temperature.
The high temperature, high pressure, gaseous output of the compressor is fed into a second heat exchanger, called the condenser. In most heat pumps, air to be heated is blown by a fan through this heat exchanger; in some systems, however, water or another heat transfer fluid will take the place of air for heat distribution. Since the refrigerant is hotter than the air or heat transfer fluid, it transfers heat to it. As it loses heat, the refrigerant's temperature drops somewhat and it condenses.
This high temperature liquid refrigerant then passes through an expansion valve. The valve reduces the pressure of the refrigerant, and consequently, its temperature drops significantly. This low temperature liquid is then fed into the evaporator, and the cycle continues. In this way, the heat from the water or other heat transfer fluid in the earth connection is transferred to the air in the building; hence, the name "water-to-air heat pump".
One significant difference between a ground-source heat pump and a refrigerator is that the ground-source heat pump is meant to run in both directions. When in cooling mode, the heat exchanger between the earth connection and the refrigerant becomes the condenser, and the air-to-refrigerant heat exchanger becomes the evaporator. This is accomplished through a reversing valve inside the heat pump.
The heat pump requires electricity to run the compressor, fans, circulation pumps, and controls, but the heating or cooling energy provided by the system is generally two to four times the electrical energy consumed. This 200 to 400% efficient operation compares favourably with electrical resistance heating, which cannot exceed 100% efficiency, and reduces electricity consumption compared to conventional air conditioners and air-source heat pumps by 30 to 70% for heating and 20 to 50% for cooling. When compared to even the most efficient gas technologies, ground-source heat pumps can save significant quantities of energy.
All the components of the heat pump are typically housed in a single enclosure. This includes the earth connection-to-refrigerant heat exchanger, the compressor, controls, the fan, an air filter, an air handler, and refrigerant-to-air heat exchanger. The unit will typically have a thermal capacity of 3.5 to 35 kW. This is sufficient for a small building, and for larger buildings, multiple units can use the same earth connection.
Where there is a use for hot water, such as in many residential applications, a desuperheater can be included in the heat pump. This is a small auxiliary water-to-refrigerant heat exchanger at the outlet of the compressor. It bleeds heat from the refrigerant and transfers it to the house's hot water tank. It generates most hot water
when the cooling load is high, furnishes some hot water during winter, but provides nothing when the compressor is not operating, such as during spring and fall. Some new heat pumps include a heat exchanger and controls dedicated to water heating that permit the system to fully meet the hot water requirement, even when space heating or cooling is not required.
SLIDE 6: Types of Earth Connection
As mentioned earlier, the earth connection can take a number of different forms. Let's examine some of these here.
There are two common configurations for ground-coupled heat pump systems: the vertical and the horizontal heat exchanger configurations. In the vertical configuration, heat transfer occurs in a series of vertical boreholes drilled 45 to 150 m into the ground. The heat exchanger pipe runs down to the bottom of the hole and then back to the surface. Horizontal subsurface supply and return headers connect the boreholes in parallel. Once the pipe has been laid in the holes, the holes are back-filled and grouted. Grouting is the filling of the borehole with a special material that prevents surface water from contaminating groundwater and one aquifer from flowing into another. Grouting materials usually have poorer heat transfer characteristics and higher costs than common backfill material. Where local regulations permit, grouting only the upper 6 to 10 m of a borehole will often provide adequate protection from surface water seepage, increase the heat exchanger efficiency, and lower its cost.
In the horizontal heat exchanger configuration, the pipe is buried, usually between 1 and 2 m below the surface, in one or more horizontal trenches. Supply and return headers connect the trenches in parallel. The pipe may be laid in the trench in a variety of ways: there may be a single pipe per trench, the supply and return pipe may be side-by-side or stacked one on top each other, though always separated by ½ m or so, four pipes may be laid in a trench, or the pipe may be a stretched coil, called a "slinky" or spiral. This last configuration provides a large surface area for heat exchange and may therefore be advantageous when land area is limited, although more piping is used and this increases costs. In typical horizontal heat exchangers about 35 to 55 m of pipe are installed per kW of heating and cooling capacity. The trench is backfilled once the pipe has been laid in it.
The vertical heat exchanger makes most sense when the ground is rocky near the surface and therefore trenching would be difficult, where land area is limited, or where disruption of the landscape must be minimized. It is commonly used for large buildings. Where ground conditions facilitate trenching, the horizontal heat exchanger is often the least costly earth connection to install. It requires the largest land area, however, and is usually best suited to smaller applications such as residential and small commercial buildings.
The efficiency of the heat pump is related to the temperature of the heat transfer fluid arriving from the earth connection. In the vertical configuration, the heat exchanger is buried deep in the ground, where, as we will see, temperature is stable year-round. In contrast, the heat exchanger of the horizontal configuration is near the surface and is therefore influenced by seasonal variations in the ground temperature. As a result, the vertical exchanger permits the heat pump to operate more efficiently and permits a shorter pipe length than the horizontal configuration.
In contrast to the closed loop vertical and horizontal heat exchangers, the groundwater earth connection pumps water from a supply well leading to an aquifer, uses it as a heat source or sink, and then returns the water to the same aquifer through a second well, called an injection well. The water from the well is pumped either directly to the heat pump's water-to-refrigerant heat exchanger or, commonly, to an intermediate heat exchanger connected to a building loop. This intermediate heat exchanger protects the heat pump from the fouling, abrasive, or corrosive action of the well water.
The groundwater earth connection was the first to appear on the market, is the simplest to install, and has been used successfully for decades. Environmental regulations and insufficient water availability may limit its use in some areas, however.
Ground-source heat pumps can also use surface water, such as ponds or lakes, as a source or sink of heat. The heat exchanger consists of a closed loop of submerged piping, as in the ground-coupled heat pump systems. Piping and excavation requirements are low. Yet another type of earth connection is the standing column well. This approach makes use of groundwater but requires only one well for supply and return. The well is typically 15 cm in diameter and may be as deep as 450 m. Water from the bottom of the well is pumped to the building's heat exchanger but is returned to the top of the same well.
SLIDE 7: GSHP Resource: Ground Temperatures
Clearly the ground is the heat sink and source for a ground-source heat pump. But where does the heat that is extracted from the ground come from? And what happens to the heat that is rejected to the ground?
Ultimately, the heat extracted from the ground by a ground-source heat pump comes from either the sun or from heat injected into the ground during the cooling season. The ground absorbs about half of the solar radiation that is incident upon the earth. The earth's crust stores this energy over long periods of time. In contrast, the quantity of heat coming from the core of the earth is not significant.
The ground also loses a great deal of energy, for example, by radiating it to space and transferring it to the atmosphere. Heat added to the ground by the system will eventually contribute to these losses.
A heat balance determines the temperature of the ground: it is the temperature at which all forms of losses match all forms of energy gains. At the surface, the gains and losses vary with the seasons: in winter the losses are higher and the gains lower than in summer. As a consequence, the surface temperature follows the air temperature. But below the surface, the ground stores energy, and reduces this variation. This dampening effect increases with greater depth, until, at a depth of about 15 m, the seasonal variation in ground temperature is negligible. Rather, the temperature will be at a constant level, somewhat above the annual average air temperature above ground. The actual difference between the annual average air temperature and the ground temperature will
depend on such factors as climate, ground cover and vegetation, snow cover, slope, and soil properties.
This is illustrated in the diagram on this slide, showing ground temperature in Ottawa, Ontario, Canada, at three depths below the surface. Thirty centimeters below the surface, the ground reaches a minimum of -1ºC during the winter months, and a maximum of about 20ºC in late summer. The amplitude of this cycle is less than half as great 2 m below the surface, and is barely discernable at 5 m. Note, too, that the phase of the cycle of temperature variation shifts with depth: with increasing depth, ground temperature lags the moving average of air temperature more and more. At 5 m below the surface, they are completely out of phase: the maximum ground temperature occurs during winter, and the minimum during summer. This is due to the long time required for heat to be transported through the ground.
The ground's dampening effect on temperature variation is a key advantage of groundsource heat pumps compared with air-source heat pumps. The efficiency and capacity of all heat pumps are closely related to the difference in temperature between the inside of the building and the source or sink of heat. The smaller this difference, the more efficiently the heat pump can operate. Because the ground temperature stays closer to
the interior building temperature than does the outside air temperature, the groundsource heat pump operates more efficiently than conventional air conditioners and airsource heat pumps and its capacity varies less.
SLIDE 8: Example of GSHP System Costs
The financial viability of ground-source heat pump systems varies from location to location. Here we examine two residential applications, one in Southern Finland and the other in Connecticut, USA. The ground-source heat pump is compared with the leading competing conventional technology at each of these locations.
In Finland, heating is the main concern and cooling is viewed as a pleasant luxury. The GSHP system is consequently compared to an electric resistance heating system without air conditioner. Both the GSHP and the electrical resistance heating system use hydronic heat distribution; that is, heat is distributed around the house by water pipes embedded in the floor. The GSHP system, at $13,000 for this 150 m² house, initially costs $5,000 more than the electric heating, but uses one third as much electric energy and saves $450 on an annual basis. While the simple payback is over ten years, ground-source heat pumps have about 10% of the residential market in Finland: people recognize that by reducing their energy consumption they protect themselves from the rising energy costs they expect to see over the lifetime of their heating system. Furthermore, they see the GSHP as an environmentally friendly technology.
In Connecticut, the GSHP system must compete with a conventional system consisting of an oil-fired furnace and air conditioner. At $20,000, the GSHP system for this 275 m² house is initially about one third more expensive than the conventional system but uses only 40% as much energy on an annual basis. Heating costs are reduced by a quarter and cooling costs, which are more significant at this location, by a third. The local utility offers a significant subsidy - and reduced electricity tariffs - for customers with houses that are built to certain minimum standards of insulation and who install a GSHP. This makes the GSHP financially attractive to the home-owner.
These examples illustrate the high initial costs of GSHP systems. In general, these are almost double those of conventional central systems in residential applications; 20% to 40% more than constant volume, single zone rooftop units; and up to 20% more than multizone or central two-pipe chilled water systems. These examples also illustrate the significant energy savings achieved by the GSHP systems, but give no indication of other benefits like reduced maintenance costs.
SLIDE 9: Ground-Source Heat Pump Project Considerations
The financial viability of a ground-source heat pump system varies from project to project. Here are a few considerations to look for when assessing whether a ground-source heat pump project may be cost-effective.
First, a project requiring both heating and cooling is preferable to one that requires just one or the other. While the same GSHP system can provide both heating and cooling, two different conventional systems will be required, increasing the cost of the competing conventional technology. Furthermore, since it is operating year-round, the GSHP system can generate larger energy savings.
Second, large seasonal variations in temperature will favour the GSHP system over airsource heat pumps, whose capacity and efficiency decrease at temperature extremes, and ensure that there is a significant energy demand on which the GSHP can generate savings.
Third, if there is already a useable heating and cooling system installed, the purchase and installation of a GSHP is rarely justified on the basis of its energy benefits. Thus, the GSHP is most cost-effective in new construction, especially since this facilitates trenching and drilling, or when an existing heating and cooling system has reached the end of its life and must be replaced.
If heating is the dominant energy requirement, then low electricity prices and high gas or oil prices will make the GSHP more attractive than combustion systems. If cooling is dominant, then high electricity prices will favour ground-source heat pumps over conventional air conditioning, which is less efficient. If both heating and cooling requirements are high, then low electricity prices year-round with high peak load charges during summer are ideal for the GSHP.
Installation of the earth connection is a critical step in any GSHP project. For groundcoupled systems, soil conditions will influence not only the feasibility of drilling and trenching, but also the performance of the earth connection. Similarly, groundwater availability, and regulations concerning its use, will determine the feasibility of a groundwater earth connection. Investigation of the soil or groundwater conditions is an important part of a feasibility study; despite this, the actual cost of the earth connection installation will remain a matter of considerable uncertainty and is the part of the system most likely to cause cost overruns. Costs will be reduced if trenching or drilling equipment is readily available on or near the site. Since earth connections in a GSHP system are usually very difficult to reach after installation, the materials and workmanship must be of the highest quality.
When building heating and cooling loads differ greatly, sizing the earth connection for the lower of the two loads can reduce its cost. This will require supplemental heating or heat rejection capacity, usually provided by low-cost conventional heating systems and cooling towers, respectively.
Ultimately, the financial viability of a ground-source heat pump system will depend on the customer's criteria for cost-effectiveness. A system may be difficult to justify if very short simple payback periods are demanded, but may also be an obvious choice from the point of view of a home-owner who must live with the operation and maintenance costs of the system for twenty or twenty-five years.
SLIDE 10: Residential Building Systems - Examples: Australia, Germany, and Switzerland
The three photos on this slide show residential buildings that make use of ground-source heat pumps. The single-family residence in Switzerland as well as the 320-apartment complex in Adelaide, Australia, both use a vertical outdoor heat exchanger, while the German house uses a groundwater earth connection. These are typical of the higher-end residential construction that tends to make use of ground-source heat pumps. The high initial costs of the GSHP do not constitute an especially large fraction of these more expensive homes, and the home-owners generally view the GSHP system as a long-term investment in their home. Furthermore, they are swayed by the environmental benefits and the improvements in comfort and air quality associated with the GSHP.
Electric utilities often subsidize the installation of ground-source heat pumps. Private utilities benefit through increases to their base load and reductions in their peak load. Public utilities also recognize that the system's environmental benefits accrue to society as a whole, and therefore the initial costs should not be borne by the system's owner alone. Regardless, the subsidy can be a major consideration in the home-owner's decision to install a ground-source heat pump.
SLIDE 11: Commercial Building Systems - Examples: UK and USA
The viability of ground-source heat pump systems for commercial buildings can be impeded by demands for short simple paybacks, generally of less than five years, and limited availability of land for the large earth connections required. Nevertheless, there are many such installations, illustrated by the three examples on this slide. This reflects the many advantages of the ground-source heat pump in these applications. Since the heat pump is physically smaller than conventional heating and cooling plants, and since heat distribution in a large building can be achieved with a compact liquid loop rather than
voluminous air ducts, the ground-source heat pump can free building space for commercial uses. The use of multiple heat pumps distributed around a large building also simplifies control of the interior environment. The elimination of roof-top units, cooling towers, and chimneys reduces opportunities for vandalism. And being more efficient than conventional air conditioners, the ground-source heat pump reduces summertime peak load charges.
The top photo shows a building Axa Sunlife commissioned in Croydon, the United Kingdom. It was the country's first large commercial building using a closed loop groundsource heat pump for heating and cooling, and it won an environmental award from the British Engineering Council.
The cluster of buildings on the left of the photo at the bottom left is the Galt House East Hotel and Waterfront office buildings, in Louisville, Kentucky. This building complex utilizes the world's largest GSHP system, consisting of 1,200 heat pumps with a combined cooling capacity of 16 MW or 4,500 tons. It uses a groundwater earth connection and water storage as a thermal reservoir.
The filling station in Prairie Village, Kansas, seen in the photo at the bottom right, uses a ground-source heat pump system to provide ice melting for the driveway, warm water for the car wash, radiant heat for the car wash bays, and cooling for the store's refrigerators, walk-in freezers, and icemakers all reportedly with a two-year simple payback.
SLIDE 12: Institutional Building Systems - Examples: Canada and USA
Ground-source heat pumps can be very well suited to institutional buildings. Often, the building owners and operators are willing to accept longer paybacks than those required in the commercial sector, and are more open to innovative designs and technologies like GSHPs. Many institutional buildings have a simultaneous need for both heating and cooling, which the building loop of a ground-source heat pump system can take advantage of. This is the case, for example, in arenas, which need to heat the bleachers at the same time that they need to cool the ice.
The photo at the bottom of the slide shows the Saint-Hyacinthe Vocational School, located just east of Montreal, Quebec, Canada. The building has a lot of agricultural land around it and therefore was able to use a very inexpensive horizontal heat exchanger. In addition, the building uses a Solarwall solar air heating system to preheat ventilation air.
SLIDE 13: RETScreen Ground-Source Heat Pump Project Model
The RETScreen Ground-source Heat Pump Project Model is a simple but very useful tool for the preliminary investigation of the technical and financial feasibility of ground-source heat pump projects. For an installation anywhere in the world, it can provide an analysis of the energy production, life-cycle costs, and greenhouse gas emissions reductions. The model can be applied to residential, commercial, institutional, and industrial buildings that are ground-coupled by a horizontal or a vertical outdoor heat exchanger or that use groundwater in an open loop system. A minimum of input data is required of the user.
While the RETScreen model can be applied to most ground-source heat pump applications, it has several limitations. It does not model surface water earth connections, nor does it account for long-term thermal imbalances in the ground that can occur when heating loads are much greater than cooling loads, or vice versa. It cannot be used to study buildings with simultaneous heating and cooling loads, but rather deals with block loads - that is, it treats the building as a single space all at the same temperature. Finally, it does not provide any output concerning water heating by GSHPs.
SLIDE 14: RETScreen GSHP Energy Calculation
The RETScreen Ground-source Heat Pump Energy calculation determines the performance of the GSHP system over the period of a year. The calculation is based on a bin method rather than simulation; that is, instead of progressing through the year, day-by-day or hour-by-hour and calculating the building load and GSHP performance, RETScreen estimates the number of hours of the year during which the outdoor temperature will fall into a "bin", or specified range of temperatures. Then, RETScreen calculates the performance of the system for each bin. The approach assumes that the system is operating at a steady-state; that is, that the heat pump system's performance is determined solely by the conditions at a given point in time, not the conditions or state in previous hours or days. An iterative solution is required for this method. Here, we provide an overview of this calculation; for more information, see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
RETScreen requires that the user enter a few key parameters describing the ground conditions, the maximum and minimum air temperatures at the site, and the type of earth connection and heat pump that will be used. The user must also enter either the building's peak thermal load and energy demand for heating and cooling, or a few characteristics of the building so that RETScreen can estimate these loads and demands.
RETScreen uses the heating and cooling design temperatures entered by the user to synthesize the frequency distribution of outdoor air temperature. A series of temperature bins spanning the range of -50 to +50ºC is constructed, and the number of hours in the year that fall in each temperature bin is estimated. It also uses the mean ground temperature, the annual swing in the ground temperature, and the characteristics of the soil type specified by the user to calculate the minimum and maximum ground temperatures at the depth of the earth connection.
If the user has provided data describing the building, RETScreen uses this to determine an equation for the building heating or cooling load as a function of outdoor air temperature. This equation takes into account, in a rudimentary way, losses and gains by transmission through the envelope, solar gains, internal gains, and latent and sensible gains and losses due to fresh air intake and occupants.
If, on the other hand, the user has provided building design loads and annual energy consumption, RETScreen must use this data to estimate the equation for heating or cooling load as a function of outdoor temperature. This involves an iterative process which finds the equation that accurately predicts both the building design load when the temperature is equal to the design temperature and the annual energy use when the equation is applied to all the outdoor air temperature bins.
The equation is applied to each temperature bin to find the building thermal load for the bin; if the user has supplied descriptive data for the building, the equation is also used to calculate the annual energy use (by summing over all bins) and the building design heating and cooling load (by applying it to the design temperatures). In addition, the equation determines the building balance points, those temperatures above and below which the building does not require heating and cooling, respectively.
Next, RETScreen estimates the required heat pump capacity. The heat pump must be able to supply the design cooling load or, if it is larger and the user specifies a heating design criterion, the design heating load. The heat pump capacity is affected by the temperature of the heat transfer fluid entering the pump from the ground loop; it is assumed that the design cooling load must be met when the temperature of the heat transfer fluid is at its maximum and the design heating load must be met when the temperature of the heat transfer fluid is a minimum. The heat pump capacity under standard conditions - that is, the conditions for which heat pumps are rated - is calculated based on a quadratic formula that adjusts the heat pump capacity according to the temperature of the heat transfer fluid entering the pump.
RETScreen must still estimate the size of the ground heat exchanger or the magnitude of the groundwater flow. The length of the heat exchanger is found from the design heating or cooling load, the coefficient of performance of the heat pump, the soil conditions, and the difference between the temperature of the ground and the temperature of the heat transfer fluid entering the heat pump under design conditions. For groundwater heat pumps, the required groundwater flow is that which has the thermal capacity to satisfy the maximum of the design heating and cooling loads. The density and specific heat capacity of water are known, so the critical step in this calculation is the determination of the temperature of the water entering and leaving the intermediate heat exchanger that couples the heat pump to the groundwater flow.
Then, for each temperature bin, RETScreen calculates the heat pump performance. First, it must calculate the actual capacity and coefficient of performance - or COP - of the heat pump for the entering water temperature associated with the bin. Then, based on this capacity, it estimates what fraction of the time the heat pump will have to operate in order to satisfy the load associated with the bin. It adjusts this fraction to account for inefficiencies occurring during heat pump start-up and shutdown. Finally, RETScreen calculates the electric energy used by the heat pump by multiplying the number of hours in the bin, the adjusted fraction of the time that the heat pump will operate, and the quotient of the standard heat pump capacity and its actual COP. Electricity used to operate the building loop pump and the ground loop pump is added in.
If the user specifies that the system be sized based on the design cooling load but the design heating load is actually larger, or vice versa, supplemental heating or cooling rejection capacity will be needed. For every bin, RETScreen determines how much electricity will be used by this supplemental capacity, assuming that it has the same operating characteristics as the specified base case heating ventilation and air conditioning system. This electricity is added to that used by the GSHP system, and the result is summed over all bins to find the total annual energy use.
Slide 15: Example Validation of the RETScreen GSHP Project Model
The RETScreen software has been validated in a number of ways. For example, the algorithm RETScreen uses to synthesize temperature bins, based on heating and cooling design temperatures, was compared to monitored data. In this comparison, the annual energy use of a typical air-source heat pump was estimated based on the monitored data as well as the binned temperature data generated by RETScreen. For all five Canadian cities examined, the results agreed to within 3%.
In a second comparison, RETScreen was validated against a number of GSHP sizing programs and a detailed simulation program. The point of comparison was the required ground heat exchanger length for two residential systems and one commercial system, all in the USA. This comparison was done twice, once using design load and energy demand data directly, and a second time using values for design load and energy demand as estimated by RETScreen from the building characteristics.
In all cases, RETScreen was within 20% of the average heat exchanger length recommended by the other software packages. This was true regardless of whether the sizing was based on performance in a single year or over a period of ten years, when the long-term effects of the heat pump on the ground temperature have appeared. The heat exchanger lengths in the built systems were often considerably different from those recommended by the sizing software, showing that RETScreen was more accurate than the simple rules of thumb probably used by the installers of these systems. This suggests that RETScreen is sufficiently accurate for pre-feasibility purposes.
SLIDE 16: Conclusions
Ground-source heat pumps constitute a cost-effective technology for the provision of space heating, cooling, and hot water. Seasonal and diurnal temperature variations are dampened by the heat capacity and limited conductivity of the ground, and as a result, the GSHP operates more efficiently than comparable air-source heat pumps or air conditioners. Compared to conventional heating, ventilation, and air conditioning systems, GHSP technology has high initial costs but low operating and maintenance costs. The technology is most cost-effective where both heating and cooling is needed over the course of the year; this permits the GSHP system to generate benefits during both summer and winter.
The RETScreen software estimates the frequency distribution of exterior temperature over the course of the year based on heating and cooling design temperatures. Grouped in bins, these temperature data are used in combination with a model for the building load, expressed as a function of outdoor air temperature, to determine the annual space heating and cooling energy benefits of the ground-source heat pump system. RETScreen also determines the size of heat pump, and the length of the ground heat exchanger or flow of groundwater required. Thus, using a few parameters describing climate and soil conditions, RETScreen can provide accuracy comparable to detailed sizing and simulation software, and thereby significantly reduce the cost of conducting preliminary feasibility studies of ground-source heat pump projects.
SLIDE 17: Questions?
This is the end of the Ground-Source Heat Pump Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
