Modeling a Ground Source Heat Pump

Ground source heat pumps (GSHP) are one of the fastest growing applications of renewable energy in the world, with annual increase of 10% in about 30 countries over the past 15 years.  Its main advantage is that it uses normal ground or ground water temperatures to provide heating, cooling and domestic hot water for residential and commercial buildings. GSHP’s are proving to be one of the most reliable and cost-effective heating/cooling systems that are currently available on the market and have the potential of becoming the heating system of choice to many future consumers, because of its capacity for providing a variety of services such as heat generation, hot water, humidity control, and air cooling. Additionally,  they have the potential to reduce primary energy consumption, and subsequently provide lower carbon emissions, as well as operate more quietly and have a longer life span than traditional HVAC systems. The costs associated with GSHP systems are gradually decreasing every year due to successive technological improvements, which makes them more appealing to new consumers.

The basic purpose of a GSHP is to transfer heat from the ground (or a body of water) to the inside of a building. The heat pump’s process can be reversed, in which case it will extract heat from the building and release it into the ground. Thus, the ground is the main heat source and sink. During winter, the ground will provide the heat whereas in the summer it will absorb the heat.

A GSHP comes in two basic configurations: ground-coupled (closed-loop) and groundwater (open loop) systems, which are installed horizontally and vertically, or in wells and lakes. The type chosen depends upon various factors such as the soil and rock type at the installation, the heating and cooling load required, the land available as well as the availability of a water well, or the feasibility of creating one. Figure 1 shows the diagrams of these systems.

Two Basic Configurations
Figure 1. Two Basic Configurations of GSHP Systems. SOURCE: [1]
In the ground-coupled system (Figure 1a), a closed loop of pipe, placed either horizontally (1 to 2 m deep) or vertically (50 to 100 m deep), is placed in the ground and a water-antifreeze solution is circulated through the plastic pipes to either collect heat from the ground in the winter or reject heat to the ground in the summer. The open loop system (Figure 1b), runs groundwater or lake water directly in the heat exchanger and then discharges it into another well, stream, lake, or on the ground depending upon local laws. Between the two, ground-coupled (closed loop) GSHP’s are more popular because they are very adaptable.

Generally, heating and cooling is achieved through three major components/loops: the heat pump itself, the ground loop and the heating/cooling distribution system as shown in Figure 2. Each of them is its own separate loop that is closed (except for the ground loop which can be an open loop type as mentioned earlier), both the ground loop and the heating/cooling distribution system are connected to the heat pump unit, through which they transfer heat.

Schematic Diagram of a GSHP
Figure 2. Schematic Diagram of a GSHP. SOURCE: [2]

GSHP Components/Loops:

  1. Ground Loop – The ground loop is the connection between the GSHP and the ground/water. This loop typically comprises of a network of pipes, introduced either horizontally or vertically into the ground for a closed loop system, or installed in an open loop system to access a water body directly. As mentioned earlier, the type of loop chosen can depend on factors like the soil and rock type at the installation, the heating and cooling load required, available land, etc. The pipes carry an anti-freeze mixture or other kind of suitable type of heat transfer fluid. The ground loop will collect heat from the ground or water body and carry it to the heat pump‘s evaporator or expel the heat into the ground or water body from the heat pump.
  2. Heat Pump – In heating mode, the heat pump receives heat from the ground loop and delivers it to the building. In cooling mode, the process is reversed and heat from the building is transferred to the ground loop for disposal.
  3. Distribution System – The distribution system delivers or removes heat to/from a building. One of the most efficient methods for space heating is to lay pipes in the floor of the building (typically concrete) through which warmed water from the heat pump is circulated. Alternative methods include radiators or forced air systems. The distribution system includes a pump which will circulate the fluid through the network into the building.

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The heat pump is the core element of the ground source heat pump system. It functions in the same way as a standard household refrigerator. The basic principles of a refrigeration cycle are; a liquid absorbs heat as it vaporizes (e.g. boiling water into steam), the compressing  gas will increase in temperature, as the gas expands, it reduces its temperature and finally, as the gas loses heat, it will turn back into a liquid (e.g. steam condensing back to water) to begin the cycle again.

The heat pump will use these principles in the exact same way. It will  circulate a refrigerant through a loop with two heat exchangers (evaporator and condenser), one exchanger to gain heat and another to lose it. A refrigerant is typically a liquid with a very low boiling point, meaning that it can evaporate into a gas and condense back into a liquid at a low temperature.

In a GSHP, the first heat exchanger (evaporator) is placed in the circuit with the ground loop. The second (condenser) is in the circuit within the building (distribution system). The refrigerant can gain heat from the ground loop and then lose the heat while circulating in the building, or can operate in reverse, and cool the building. The closed loop-based configuration of the GSHP cycle  is modeled in the figure below in AxCYCLE™. It consists of a ground loop, the heat pump unit, and distribution system as mentioned earlier.

GSHPs Cycle Configuration in AxCYCLE
Figure 3. GSHP Cycle Configuration in AxCYCLE™

The difference between the vapor-compression refrigeration cycle and a heat pump cycle is that a refrigeration application is only concerned with the low temperature effect produced by the evaporator, while a heat pump may be concerned with both the cooling effect produced by the evaporator as well as the heating effect produced by the condenser.  GSHP systems that are meant to provide both heating and cooling to a building (depending on the user’s current preference) will utilize a reversing valve to ensure the system can do both.

These kinds of ground source heat pump systems that have heating and cooling modes with different configurations or modern refrigeration systems can be modeled and analyzed using AxSTREAM NET™. Designers can evaluate pressure levels, flow rates, and the heat capacity of a heat exchangers by modeling the thermal-fluid network of a GSHP.  An example of one such configuration of ground source heat pump systems is shown in Figure 4.

AxSTREAM NET Heat Pump
Figure 4. A GSHP modeled in AxSTREAM NET™

It’s critical that the compressor for the GSHP is designed and optimized for the specific task of compressing the chosen refrigerant. Luckily, this can also be done easily in the AxSTREAM® platform. By using the boundary conditions established in the GSHP thermodynamic cycle in AxCYCLE™, a conceptual design for a centrifugal compressor can be quickly iterated from scratch to determine if it can feasibly work in this system. After that, a more thorough optimization process can be undertaken to provide a compressor specifically designed to work in this heat pump at maximum efficiency, as seen in figure 5.

Centrifugal Compressor Design
Figure 5. Centrifugal Compressor Designed using AxSTREAM®

Are you interested in learning about how SoftInWay can help you to develop your GSHP system or to design or analyse a compressor using AxSTREAM® platform? Reach out to us at Info@Softinway.com to schedule a demo!

References:

[1] J. Lund, B. Sanner, L. Rybach, R. Curtis, G. Hellstrom, Geothermal (Ground-Source) Heat Pumps, A World Overview, GHC Bulletin, September 2004

[2]https://nzgeothermal.org.nz/heat-pumps-new-zealand-geothermal-association/geothermal-heat-pumps/

[3] https://nzeb.in/knowledge-centre/hvac-2/ground-source-heat-pump/