Table of Contents
Introduction
Geothermal heat pumps use the almost constant temperature of the shallow ground to provide heating, cooling, and sometimes hot water for buildings. Unlike high temperature geothermal power plants, which use hot water or steam from deep underground, geothermal heat pumps usually work with ground temperatures between about 5 and 20 °C. They are sometimes called ground source heat pumps or ground coupled heat pumps.
Basic Working Principle
A geothermal heat pump is a special kind of heat pump that exchanges heat with the ground instead of the outside air. The key idea is that the ground temperature a few meters below the surface changes much less over the year than the air temperature. In winter, the ground is usually warmer than the air, so the system absorbs heat from the ground and moves it indoors. In summer, the ground is often cooler than the air, so the system rejects heat from the building into the ground and provides cooling.
Inside the system, a refrigerant circulates in a closed loop through the heat pump unit. In heating mode, the refrigerant evaporates at a low temperature, picking up heat from the ground loop. A compressor then raises the pressure and temperature of the refrigerant. The hot refrigerant transfers heat to the building’s heating system, usually through a heat exchanger connected to air ducts or a hydronic (water based) system, and then expands back to a low pressure, low temperature state to start the cycle again.
In cooling mode, the cycle is reversed. The refrigerant absorbs heat from indoors, the compressor raises its temperature, and the system transfers that heat to the ground loop, where it is carried into the soil or rock around the buried pipes.
Components Of A Geothermal Heat Pump System
A typical geothermal heat pump system has three main parts. The ground loop is the buried or submerged piping system that exchanges heat with the earth. It carries a heat transfer fluid, often water or a mixture of water and antifreeze, which picks up or releases heat to the surrounding soil or rock. The heat pump unit is usually located indoors. It contains the compressor, heat exchangers, expansion device, and circulating pumps or fans. The distribution system delivers heating or cooling to the building, using ducts for forced air systems or pipes and radiators, fan coils, or underfloor loops for hydronic systems.
Some systems also have an integrated desuperheater. This small heat exchanger can reclaim some of the heat from the refrigerant when the system is running and use it to preheat domestic hot water, which improves overall efficiency.
Types Of Ground Loop Configurations
The design of the ground loop is a key feature that is specific to geothermal heat pumps. There are several common configurations that adapt the system to different site conditions.
Horizontal closed loop systems use trenches dug a few meters deep. Plastic pipes are laid in the trenches in straight lines, slinky coils, or multiple layers, then backfilled with soil. Horizontal loops require more land area but are often cheaper to install where space is available and soil conditions are suitable.
Vertical closed loop systems use a series of boreholes drilled typically 50 to 200 meters deep. U shaped plastic pipes go down and back up each borehole, and the remaining space is filled with grout to improve thermal contact with the surrounding ground. Vertical systems are preferred where land area is limited, in urban locations, or where the upper soil layer is shallow or unsuitable. They usually have higher installation costs but need less surface space.
Pond or lake loop systems are a special case of horizontal loops. Coils of pipe are placed at the bottom of a suitable water body that meets depth, volume, and quality requirements. The water provides a stable temperature environment and good heat transfer. This can reduce drilling or trenching costs, but only sites with appropriate water resources can use this option, and environmental rules apply.
Open loop systems use groundwater directly rather than a separate closed loop fluid. Water is pumped from a well, passes through a heat exchanger in the heat pump, and is then discharged back into another well or into a surface water body if regulations allow. Open loops can be efficient because flowing groundwater often has a stable temperature, but they depend on sufficient water quantity and quality and may need filtration or treatment to avoid scaling or fouling.
Performance And Efficiency
The performance of geothermal heat pumps is usually expressed with the coefficient of performance, or COP, in heating mode, and with an energy efficiency ratio, or EER, or seasonal indices in cooling mode.
The coefficient of performance is defined as the useful heating output divided by the electrical energy input to the heat pump:
$$
\text{COP} = \frac{Q_{\text{out}}}{W_{\text{in}}}
$$
Here, $Q_{\text{out}}$ is the heat delivered to the building, and $W_{\text{in}}$ is the electrical work supplied to the compressor and other components.
A higher COP means higher efficiency, because the system delivers more heat per unit of electricity consumed. Typical geothermal heat pumps often reach COP values of 3 to 5 under standard conditions, which means 3 to 5 units of heat output for each unit of electrical input.
In cooling mode, the system’s efficiency can be described by similar ratios. Seasonal performance factors average the efficiency over an entire heating or cooling season, which takes into account variations in operating conditions. Geothermal systems often have better seasonal performance than air source heat pumps in cold climates, because the ground temperature remains relatively mild when the air is very cold.
The real performance of a system depends on many local factors. The ground temperature and thermal properties of the soil or rock, the design and size of the ground loop, the quality of installation, and the building’s insulation and distribution system all influence the COP and seasonal performance.
Site Conditions And Design Considerations
Geothermal heat pump design is very sensitive to the characteristics of the site. The thermal conductivity, moisture content, and temperature of the soil or rock determine how easily heat can move to and from the ground loop. Wet, dense soils or rock with good thermal conductivity are favorable, because they allow shorter loops for the same heating or cooling capacity. Dry, loose, or very sandy soils usually need longer loops to achieve similar performance.
Available land area is another critical factor. Horizontal loops require enough open space without many underground obstacles like utilities, foundations, or large tree roots. Vertical loops require access for drilling equipment and must respect minimum distances from buildings, property lines, wells, and septic systems. Local regulations or building codes may impose separation distances and other requirements for environmental protection and safety.
Describing the building’s heating and cooling loads accurately is also essential. The system must be sized to meet most of the demand without being excessively large. Undersized systems can struggle during extreme conditions, while oversized systems cost more and may run less efficiently because of short cycling. Designers use load calculations that consider the building envelope, climate, internal gains, and usage patterns.
In some cases, hybrid systems combine geothermal heat pumps with auxiliary boilers, air source units, or cooling towers. This can reduce the required size of the ground loop if peak loads are covered by the auxiliary system, or if the building has a strong imbalance between annual heating and cooling needs that might otherwise cause long term warming or cooling of the ground around the loop.
Advantages Of Geothermal Heat Pumps
Geothermal heat pumps have several specific advantages compared with conventional heating and cooling systems that use fossil fuels or air source heat pumps. They typically have very high efficiency because the system moves heat rather than creating it by combustion, and because the source temperature is moderate and stable. This high efficiency translates into lower operating energy use and, when electricity is low carbon, significantly reduced greenhouse gas emissions.
The system is almost entirely located indoors or underground, which protects major components from weather and can increase lifespan. The outdoor portion is usually just buried piping, which has no visible units, fans, or noise. This is beneficial in dense neighborhoods, historic areas, and places where aesthetics and noise are important. Geothermal heat pumps can provide both heating and cooling from the same installation, and can also assist with water heating. This allows integrated climate control and can simplify equipment compared to separate boilers and air conditioners.
Because geothermal heat pumps do not burn fuel on site, they avoid direct emissions of pollutants such as nitrogen oxides or particulates in the building. This can improve local air quality and indoor safety. They also reduce the need for fuel deliveries and on site fuel storage.
Limitations And Challenges
Despite these benefits, geothermal heat pumps face specific challenges. The initial installation cost is often higher than traditional heating and cooling systems, mainly due to the need for drilling or excavation for the ground loop. Although operating costs can be lower over time, the payback period depends on local energy prices, incentives, and the building’s heating and cooling needs.
Not all sites are suitable. Very limited land area, difficult ground conditions, or restrictions on drilling or groundwater use can make installations impractical or expensive. In dense urban areas, drilling may be technically possible but logistically complex and subject to tight regulations. Water quality issues such as high mineral content can create scaling or corrosion problems in open loop systems and require treatment or additional maintenance.
System design needs qualified expertise. Poorly sized or installed ground loops can lead to reduced efficiency, excessive energy use, or long term changes in ground temperature that degrade performance. If a building has a very unbalanced annual load, for example much more cooling than heating, designers must consider how to manage the thermal balance of the ground to avoid gradual temperature drift.
Finally, although routine maintenance is modest, it is still necessary to maintain pumps, filters, and controls and to ensure the antifreeze and fluids remain within specifications. Repairing underground loops can be disruptive if leaks occur, although properly installed piping is designed for long life.
Typical Applications And Use Cases
Geothermal heat pumps are widely used in individual houses, small apartment buildings, schools, offices, and other commercial buildings. In new construction, they are often easier to integrate because ground loop installation can be planned together with other site works. Retrofitting existing buildings is also possible, especially when old heating or cooling systems need replacement and there is an opportunity to upgrade insulation and distribution systems.
In cold climates, geothermal heat pumps provide efficient heating even when air temperatures are very low, which is an advantage compared to air source heat pumps that may lose capacity and need backup resistance heating. In warmer climates with significant cooling demand, they can offer efficient air conditioning and can work well with buildings that use hydronic cooling or provide chilled water for fan coil units.
There is also growing interest in shared or community geothermal systems, where a single ground loop field serves multiple buildings. Each building has its own heat pump unit, but the main ground heat exchanger is shared. This approach can improve the use of space, share costs, and smooth out differences in heating and cooling loads among buildings.
Environmental Considerations
The environmental impacts specific to geothermal heat pumps are mainly linked to drilling and excavation, material use, and the choice of working fluids. During construction, soil disturbance and drilling activities must be managed carefully to avoid contamination of groundwater and to protect nearby ecosystems. Grouting materials in vertical boreholes are selected to prevent mixing between different groundwater layers and to ensure good thermal contact.
The heat transfer fluid in ground loops often contains antifreeze, such as glycol or other compounds. These fluids are chosen and handled to minimize environmental risks. Closed loops are sealed systems, and leaks are rare when quality materials and installation practices are followed. In open loop systems, withdrawal and discharge of groundwater must respect local water regulations, and the temperature change of returned water is usually limited to protect aquatic environments.
The choice of refrigerant in the heat pump unit also has environmental relevance. Modern systems tend to use refrigerants with lower global warming potential compared to older ones, and regulations in many regions are pushing for further reductions. Proper installation and servicing are important to avoid refrigerant leakage.
Over the lifetime of the system, the high efficiency of geothermal heat pumps can greatly reduce indirect emissions from electricity generation compared to fossil fuel boilers, especially as the electricity mix becomes cleaner. For beginners in renewable energy and sustainability, geothermal heat pumps are a key example of how using stable, low temperature resources in the shallow subsurface can support comfortable buildings with less energy and lower emissions.