Commercial geothermal heating and cooling system design
Geothermal heating and cooling systems, also known as geoexchange or ground source heat pump (GSHP) systems, provide heating and cooling for buildings. They also may provide domestic water heating to either supplement or replace existing more conventional water heaters, and may even be used for humidity control.
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Geothermal heating and cooling systems, also known as geoexchange or ground source heat pump (GSHP) systems, provide heating and cooling for buildings. They also may provide domestic water heating to either supplement or replace existing more conventional water heaters, and may even be used for humidity control. GSHP systems work by moving heat, rather than by converting chemical energy to heat as in a combustion process.
We all live with heat pumps, the most widely recognized of which is the refrigerator. A refrigerator “pumps” heat from the cold storage area to the room air via a set of heat exchange coils (the condenser coil cage in the back or bottom of the refrigerator). A geothermal heat pump works much in the same way, only its operation is reversible—it can pump heat out of a building to cool it or pump heat into a building to warm it.
GSHP technology is not new. Though GSHP systems have existed for decades and the technology has been around for more than 50 years, many improvements have been made to the types of materials used, design and installation methods, and the efficiencies of compressors, pumps, and other equipment. GSHP systems are among the most efficient heating and cooling systems in existence today, and are a powerful tool in the engineer’s arsenal for application in green buildings due to their energy benefit.
There are thousands of GSHP system installations around North America, and they generally provide a high degree of satisfaction. When considering a GSHP system, first cost is often the biggest deterrent for owners and a major factor in the slow advance of the technology. Maintenance and long-term performance are also common issues.
Primary system types
One system approach that has successfully been used in practice is shown in Figure 1a. It is a distributed or decentralized system of water-to-air heat pumps located to serve each zone. Each heat pump is served by a common building piping loop. The building piping loop serves as a heat source (heating mode) or sink (cooling mode) for each heat pump and is connected to the ground loop in a primary-secondary piping arrangement. The pumps serving the building and ground loops can be controlled by variable speed drives to reduce power consumption during non-peak periods. A ventilation mode can operate during mild ambient conditions where the building loop recirculates through the building without a need to run the ground loop.
For example, this ventilation mode may run when building supply water temperature is between 40 and 70 F and the return water temperature is between 42 and 68 F, and the ground loop bypass valve is closed. This ventilation mode is shown in Figure 1a with two-position mode valve V-1. Large spaces that cannot be served by one heat pump can use multiple water-to-air heat pumps; alternatively, they can use one or more water-to-water heat pumps serving multispeed air handlers. A variation of this system is in use at the 46,000-sq-ft Salud Health Center in Denver.
Figure 1b depicts a piping variation to 1a, where the building loop and ground loop are tied together as a single primary loop. The ventilation mode also exists in this variation and is controlled by valve V-1. Switching to ventilation mode in this scenario will bypass the ground loop, reducing pipe friction loss and pumping energy.
Another scenario has been employed in smaller commercial facilities as shown in Figure 2. Water-to-water heat pumps are arranged in a centralized system, and hot and chilled water is fed to buffer tanks. This is termed the “charging loop.” The buffer tanks serve to decouple the primary loop from the distribution loop and accommodate the differing flow rates in the tank charging loop and distribution loop. These systems also are a useful design option when the facility has a mix of heating systems, such as radiant and forced air, and to reduce or prevent short cycling of the central plant equipment/heat pump unit(s) in a system with small water volumes and variable loads. This system is in use at the 8,000-sq-ft Morningstar Learning Center in Big Sky, Mont .
Outdoor air for either scenario can be effectively introduced through a dedicated outdoor air system. The outdoor air can be tempered to a neutral temperature of roughly 65 F using any number of sources including a fossil fuel heating system, air conditioning, or heat pump system. Greatest efficiency is gained when the outdoor air is fed into a heat or energy recovery device whereby the exhaust air energy is transferred to the incoming outdoor air as shown in Figure 1a.
The ground piping loop, or loopfield, comes in two main forms, open or closed. The three most common geothermal systems are all closed-loop, and include the horizontal (including slinky approach), pond/lake installation, or vertical borehole installation. This article will focus on the vertical as it represents the largest market share of all systems installed, is the predominant choice in commercial construction, and often provides the most efficient use of space.
The ground loop can be a single common loop for all heat pumps in the system or can serve each heat pump or small group of heat pumps with its/their own bore(s). The latter is appropriate in single-story buildings with small core areas or when retrofitting an existing building. The former is used for multistory buildings or large core areas to take advantage of load diversity.
Hybrid systems (see Figures 1a and 2) use the heat pump system to meet a portion of the design load, and a majority of the annual load, thereby offering an opportunity to minimize initial costs and borefield requirements.
Sizing the ground heat exchanger for the heating requirements of the building and adding a fluid cooler to supplement the heat rejection can minimize first cost of the system. This helps preserve the economics of the GSHP system. The use of a fluid cooler adds to the complexity of system controls and adds a piece of outdoor equipment, along with water treatment considerations. However, the potential cost reduction of the ground heat exchanger may justify this option in projects with cooling dominant loads. A hybrid system is generally attractive in installations where ground loop costs per ton are high, and where the heating loop length requirement is low (roughly 40%) relative to the cooling loop length requirement. Similarly, if the heating load greatly exceeds the cooling load, it is often more economical to meet a portion of the heating load with a boiler and size the loop for a smaller peak heating load.
GSHP system benefits are greatest in buildings with similarly sized annual heating and cooling loads and those desiring independent climate control of many rooms with the potential for heating and cooling different zones simultaneously. Other good applications for the technology are facilities with relatively high occupancy, fluctuating usage schedules, and widely varying heating and cooling requirements within individual zones, such as offices and classrooms.
GSHP system benefits are also greatest when a new building is being planned, or when considering the replacement of an existing system that no longer meets the needs of the building or has reached the end of its useful life.
The design process
Detailed design information is available from ASHRAE
Ensure that the building is efficient as possible. Harvest passive energy sources, using high-quality insulations, glazing, and other envelope features to minimize loads and reduce air infiltration. This can be accomplished through retrofits or more cost-effectively during new building design. Mechanical engineers should work with the design team to stress the importance of reducing building load and evaluate the load reduction’s effect on overall mechanical system size and cost.
Calculate the building’s heating and cooling design loads. These calculations can be done manually or with a computer loads program. It is important to use accurate input data for internal gains, weather conditions, and building data to ensure that heating and cooling loads are not overestimated, which can result in oversized systems and drive up cost. Ensure the sizing takes into account the peak diversified or block loads to avoid oversizing and keep away from relying on rule-of-thumb sizing. As building system designs improve, old rule-of-thumb sizing becomes less accurate.
Estimate the building’s monthly loads. This process is typically completed using a dynamic (hour-by-hour) simulation software package for large commercial buildings; however, the bin calculation method can be used for smaller buildings. Consult the ASHRAE Handbook of Fundamentals for more information on these calculation methods. Monthly loads take into account the heating and cooling loads; the type and size of equipment selected; and the thermal characteristics of building, climate, and soil. Matching heating and cooling loads will ensure that the system is used to its full potential and also enable more complete charging and discharging of the earth around the loopfield. Installations with unbalanced annual loads have, over time, degraded performance of GSHP systems.
Select equipment. Once the cooling and heating loads are known, it is time to select equipment. When selecting water-to-air heat pumps, the rating conditions for the heat pumps should use 77 F entering liquid temperature (ELT) and 80.6 F entering air temperature (EAT) in cooling mode, and 32 F ELT and 68 F EAT in heating mode. With ELT approaching 32 F, consider a glycol/water mix to prevent the possibility of freezing the liquid. Evaluate whether the equipment size should be based on the heating load or the cooling load. In climates where heating operation predominates and cooling operation is minimal or non-existent, optimum heat pump size may be 65% to 75% of the design heating load. It may be advantageous to specify two-speed or variable speed systems where heating requirements are much greater than cooling. This would accommodate sizing to or oversizing on the heating side, while allowing desirable operation for cooling. Or, as discussed, hybrid systems may be chosen. Provide careful matching of heat pump and ground exchanger capacity to building load. Water flow through each unit should be designed to simplify water balancing; keep unit water pressure drops as close to each other as possible. Ensure that the geoexchange pump and piping accessories are all bronze or stainless steel. The high-density polyethylene piping used for geoexchange systems is not an oxygen barrier material, and the fluid will always have a certain amount of dissolved oxygen that will corrode any unprotected ferrous material in the piping system.
Select the type of indoor distribution system. An air system is typical for commercial and institutional applications. A water-to-air system is often considered for larger multizone commercial buildings. Radiant heating and cooling systems are also an option, though much less common.
Size the indoor distribution system. This means calculating air and water flow and selecting diffusers, registers, and grilles and pumping equipment similar to the design of many HVAC systems. Provide careful duct design and installation in which ducts are kept in conditioned spaces and are well sealed. With radiant systems, design for low heating water temperatures and higher cooling water temperatures that exploit the efficiency of the heat pump equipment.
Estimate the ground heat exchanger loads. Consider the monthly loads and the design month’s load.
Size the ground loop. This is the most important element of the geothermal design and is typically performed using third-party software. Collaborating with experienced professionals who size and install ground loops is essential. The installation should follow the prescriptions of the International Ground Source Heat Pump Assn . (IGSHPA). The loop installer should be IGSHPA-certified as well as a member of the Geothermal Heat Pump Consortium (GHPC). One straightforward method for determining pipe length for a two-pipe vertical ground loop is as follows:
L H (ft) = CAPH ((COP H —1)/COP H ) x (R P + R S x F H )
(T L — T MIN )
L H (ft) = Length of ground heat exchanger @ TMIN
CAP H (Btuh) = Heat pump heating capacity @ TMIN (from manufacturer’s data)
COP H (unitless) = Heat pump coefficient of performance @ TMIN (from manufacturer’s data)
R P (h-ft-°F/Btu) = Pipe resistance (from pipe resistance tables)
R S (h-ft-°F/Btu) = Soil resistance (from soil/field resistance tables)
T L (°F) = TM = Mean earth temperature (from tabular geographic data)
T MIN (°F) = Minimum earth temperature = typically 40 F above the coldest outdoor air tempe H rature and not below the minimum EWT specified by the manufacturer’s data
F(%/100) = Design month (i.e., January) heating run fraction = January bin hours x (January monthly load Btuh/heat pump capacity at T MIN )
LC (ft) = CAPC ((EER+3.412)/EER) x (RP + RS x FC)
(TMAX — TH)
L C (ft) = Length of ground heat exchanger @ TMAX
CAP C (Btuh) = Heat pump cooling capacity @ TMAX (from manufacturer’s data)
EER (Btuh/W) = Heat pump energy efficiency ratio @ T MAX (from manufacturer’s data)
T H (°F) = TM = Mean earth temperature (from tabular geographic data)
T MAX (°F) = Maximum earth temperature = typically 10 F below the coldest outdoor air temperature and not above the maximum EWT specified by the manufacturer’s data
F C (%/100) = Design month (i.e., July) cooling run fraction = July bin hours x (July monthly load (Btuh)/heat pump capacity (Btuh) at T MAX )
Run iterations with different values of T MAX and T MIN to select a loop length that minimizes first cost while not compromising system efficiency.
Undersizing the ground heat exchanger can result in poor operating efficiency, reduced comfort, and nuisance heat pump lockouts on safety controls for the life of the system, while an oversized heat exchanger only increases the installation cost of the system. The cost of ground loop installation often will make or break project economics. Because of this, it is natural to assume that there are more instances of undersized heat exchangers. However, the contrary is true, indicating the potential to further close the gap in the cost of the system through right-sized heat exchangers. There also has been past evidence of greatly varying results from different ground loop sizing software with similar input data.
Since local hydrology and geology play a big part in ground loop selection, many design engineering companies opt to contract with local geo-installers and certified loop designers to size the ground loops, as these companies are generally the most knowledgeable in the subject.
If you are designing a ground loop, one helpful source of information is the Natural Resources Conservation Service Soil Survey ( https://soils.usda.gov/survey/ ) available from NRCS offices around the country. It gives a detailed layer-by-layer description of the soil down to a depth of roughly 6 ft, along with any soil types, rock content, density, and available water capacity.
Ground thermal properties can be determined from soil and rock manuals including Soil & Rock Classification for the Design of Ground-Coupled Heat Pump Systems, available from the IGSHPA.
Since thermal conductivity (Btu/h-ft-°F) and thermal diffusivity are the most important design variables to consider before attempting to design a commercial geothermal system, an in-situ thermal conductivity test may be warranted for a project. The test runs typically 48 hours and requires a test bore with a unicoil that has been backfilled with grout. The test can typically begin five to seven days after the test bore has been completed and should follow the recommended procedures in the ASHRAE Handbook, HVAC Applications. A detailed drill log must be kept to assist in the proper estimating of drill costs as well as conductivity heat transfer. The test will provide information about drilling conditions, the types of soil present, thermal properties of the soil, and deep-earth temperatures.
Generally, soil conductivity tests are conducted on systems larger than 20 tons, as design variations in loop sizing assumptions do not have a significant impact on installed costs for smaller systems. Typical costs for the test per borehole are $6,000 to $11,000, and the borehole can be reused for the installed system.
Importance of commissioning
A system is only as good as its design and installation, and performance testing must be mandated in any commercial GSHP system. Good commissioning practice will verify proper water and air flow through each heat pump, the heat pump capacity and efficiency, and the ground loop installation.All ground installations must be flushed to remove construction debris, purged to remove air, and pressure tested to 100 psi, or the requirement of the authority having jurisdiction, before backfilling or grouting. Figure 3 shows locations of purge ports required to effectively purge air from a geothermal loop.
Operating efficiencies should be verified using the following equations:
Cooling Efficiency (Imperial Units)
• Heat from electrical energy (Btu) = V x amp x Power Factor x 3.412 Btu/W (single-phase electrical power)
• Heat rejected from loop (Btu) = GPM x Delta T x 485 (water/antifreeze loop)
• Net cooling capacity (Btu) = heat rejected from loop (Btu) — heat from electrical energy (Btu)
• Total cooling capacity (Btu) = heat rejected from loop (Btu) + heat from electrical energy (Btu)
• EER = Net cooling capacity (Btu)/W
Heating Efficiency (Imperial Units)
• Heat from electrical energy (Btu) = V x amp x Power Factor x 3.412 Btu/W (single phase)
• Heat absorbed from loop (Btu) = GPM x Delta T x 485 (water/antifreeze loop)
• Total heating capacity (Btu) = heat absorbed from loop (Btu) + heat from electrical energy (Btu)
• COP = Total heating capacity (Btu)/heat from electrical energy (Btu)
GSHP systems have enjoyed a surge in recent years thanks to developments within the industry and the push toward more energy- and environmentally efficient buildings. However, there is still uncertainty about first cost, lifecycle cost, operation and maintenance, and system long-term reliability.
In order to achieve broader acceptance, mechanical design teams must have a better understanding of not only the economic, environmental, and comfort benefits that these systems can provide, but also the long-term operational, maintenance, and reliability issues that they can expect to face over the life of the system.
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Common parameters: Quick reference for evaluating vertical ground loops
• Bore holes are typically 150 to 450 ft in depth.
• Typical systems require between 150 to 200 ft of bore per ton of peak block load.
• Ground loop heat exchanger costs can vary from $1,200 to $2,000 per ton installed, depending on the drilling conditions and the size of the system.
• At 20 ft bore spacing, a shallow field of 150 ft bores requires approximately 1 acre per 100 tons of peak block load.
For example, for a 50,000-sq-ft office building with a ground loop sized for the peak heating load of 750,000 Btuh (62.5 tons) and 400-ft boreholes:
• Number of boreholes required = (62.5 tons x 200 ft of bore/ton)/400 ft per bore = 31.25 bore holes
• Cost of boreholes = 62.5 tons x $1,600/ton = $100,000
• Acres required for boreholes = 62.5 tons x (1 acre/100 tons) x (150 ft/bore/400 ft/bore) = 0.2325 acres (roughly 100 ft x 100 ft square)
Here is a partial list of resources related to the design of geothermal systems: