Ground Source Heat Pump
A ground source heat pump collects solar heat stored in soil, rock or water. In the deeper parts of the borehole, heat is derived more substantially from fission energy from the Earth’s core in the rock and from warm groundwater flows. Geothermal heat is well suited as a year-round heating system for both new and old buildings.
Ground source heat typically has the highest initial investment of the different heat pump options, but the lowest running costs. The profitability of geothermal improves with increasing building size and annual energy consumption. The efficiency of heat pumps is at its best in underfloor heating applications or other low-temperature heating systems.
When renovating the heating system, the building’s previous water heating system can be directly replaced by a geothermal heat pump. The majority of heat collection circuits in geothermal solutions are implemented through boreholes drilled into the ground. Other methods of installing a heat recovery circuit include horizontal installation in the soil surface layer at a depth of about one metre or installation in a body of water by submerging the circuit in the bottom of the water using weights.
Purchase of a ground source heat pump
The installation of geothermal piping requires a permit from the municipality. Permits are subject to factors such as possible underground structures in urban areas, groundwater areas and protective distances from buildings, property lines and other heating wells. If the pipeline is planned to be installed in a body of water, permission must also be obtained from the owner of the body of water.
Before making a purchase decision, you should check with the grid company responsible for your house’s electricity supply whether, for example, you need to increase the size of the main outlet, purchase a soft starter to limit the maximum current during the start-up phase or purchase a DC-controlled (inverter) geothermal heat pump model. Especially in sparsely populated areas, the compressor of a geothermal heat pump can cause, among other things, flickering of lights in your own and/or neighbouring houses.
Ground source heat pump models
Some geothermal heat pumps are alternately charging models, where the domestic hot water is made in the outer jacket of the double-coil heat exchanger – alternately heating the water for heating and alternately for domestic hot water. Domestic hot water is heated at a lower coefficient of performance than space heating, because the condensing temperature for domestic hot water is usually higher at the heat distribution network. Wherever possible, domestic hot water is heated first. The rechargeable model is suitable for both underfloor and radiator heating applications.
Some of the ground source heat pumps are so-called “forced draught” heat pumps. A Tulip heat pump has a separate heat exchanger between the compressor and the condenser, where heat is transferred from the hottest refrigerant vapour to the domestic hot water. Tulip type ground source heat pumps are usually equipped with a domestic hot water coil. Usually the hot water energy is recovered for domestic hot water reheating.
There are also dual-compressor models with their own refrigerant circuits, temperature ranges and different refrigerants. Even these models can reach temperatures above 70°C with compressor output.
Heat well as a heat source for a geothermal heat pump
Today, by far the majority of geothermal applications are implemented with heat wells. This is a borehole with an outer diameter of 115-165 mm, in which a pipe system is installed in which the heat transfer fluid circulates. The freezing point of a 30% bioethanol mixture is around -17 degrees Celsius. A geothermal system using a heat well can usually be installed on a small plot of land, but is usually the most expensive heat collection option.
Horizontal piping as a heat collection circuit for geothermal heat
A ground-source heat pump can also use the heat energy stored in the soil surface layer from solar radiation. The heat energy is collected by a heat collection pipe installed in the ground, which is installed horizontally, depending on the climate zone, at a depth of about one metre, deeper in northern Finland. The horizontal distance to the adjacent pipe should be at least 1.5 m, preferably more. Horizontal piping is generally the most cost-effective method of geothermal heat collection.
Heat collection loop in a water body
A small proportion of geothermal heat collectors are installed in lakes, the sea or even high flow ditches. The heat collection pipes are anchored to the bottom of the water body by weights at intervals of about 3 to 5 metres.
Piping in the water body can provide higher power and energy than the corresponding piping in the ground, because the heat transfer properties of water are better than those of soil. However, the design must ensure that the water temperature around the pipe does not fall below +1°C, even in winter. If the temperature falls below this, there is a risk of ice accumulating on the surface of the pipes. Accumulated ice can cause a high lift in the piping system, which can cause the piping to rise to the surface.
The depth of the pipework should therefore be more than two metres to allow water to move freely around the pipework, even in winter. Especially in flowing water, the water temperature must be checked, as the temperature of the flowing water can be very low (so-called subcooled water). In practice, the cost of installing a water system is slightly lower for a small house than for a heating well. For larger sites with a higher energy demand, the water system is more cost-effective than for smaller sites, as the installation of the collection pipe requires some special preparation and equipment.
Power rating of a ground source heat pump
It may not be worth sizing a ground source heat pump to meet the peak power demand of a building, due to the higher cost-effectiveness of the system and the longer life of the compressor. A geothermal heat pump can be sized at around 60-80% of the building’s design capacity, providing around 95-99% of the annual energy demand. The remaining 1-5% is produced by the geothermal heat pump’s back-up/supplementary heating resistor.
For small houses, a geothermal heat pump can be sized to full capacity, where slightly higher investment costs can be reasonably justified. Full capacity reduces the peak power consumption of the building, which is becoming increasingly important. At full capacity, inverter control of the ground source heat pump reduces the heat pump’s duty cycle and extends the compressor’s lifetime.
Sizing of the heat collection circuit
The heat-collection circuit should be sized according to the annual energy required for space heating and domestic hot water. A fairly sized heat-collection loop will pay for itself in the form of a slightly better thermal coefficient in the long term. Undersizing the heat well will also lead to the heat source cooling down gradually over the years.
The power rating of the heat pump has little effect on the sizing of the heat recovery circuit. In the soil, the amount of moisture in the soil has a significant impact on the energy efficiency of the horizontal pipe heat exchanger. Water flow improves the efficiency of the heat collector in the heat well and even in small bodies of water.