7. Geothermal energy (Heat from the subsurface)
Can heat pumps harm the climate?
It depends on the circumstances. Heat pumps need a lot of energy themselves.
A heat pump consists of two so-called heat exchangers. One of the heat exchangers uses the heat supplied from the borehole heat exchanger (or another heat source) to vaporise a cooling agent. The resulting steam is then densified with a compressor so that it heats up rapidly. This requires energy. In the second heat exchanger, the hot steam releases its heat to the heating or the hot-water storage tanks. The website of the Federal Environmental Agency (UBA 2018a) provides a good description of the functional principle of heat pumps (in German language).
If and how much a heat pump will benefit the climate depends on several factors:
Seasonal performance factor: or how much more heat energy becomes usable compared with the energy consumption of the heat pump itself.
A seasonal performance factor of 4 means that a heat pump provides four times more heat than it consumes electricity. However, for many heat pumps the seasonal performance factor is well below 4 if the conditions are not optimal.
The larger the temperature difference between the heat input on one side and the heat required for the heating system on the other side, the more energy the compressor consumes:
- A central heating system with traditional radiators must be operated at a much higher flow temperature of 50 to 70 °C compared to underfloor heating, for which 35 °C is sufficient.
- The poorer the insulation of the building, the more heat is lost and the more often and longer the compressor has to run.
- The warmer the heat source is to start with, the smaller is the temperature difference compared with the heater circuit. A borehole heat exchanger delivers water that already has a temperature of 10 to 15 °C (depending on the depth of the heat exchanger); A near-surface geothermal heat collector will produce significantly less heat in winter, when the ground is cold. The biggest temperature difference has to be bridged if the heat pump is operated with outside air, especially on the coldest days in winter, when you have to heat the most. Outdoor heat pumps usually do not even reach an annual workload of 3.
However, using a combination of solar thermal and geothermal energy, a well-insulated new building with underfloor heating can achieve an annual workload of 5.
Powering the heat pump: how damaging to the climate is the creation of the energy used to operate the heat pump?
- Ideally, a heat pump is operated with climate-neutral green electricity, e.g. from solar power, hydropower and wind power. In this case, heating is almost completely climate-neutral.
- However, heat pumps are often operated with residual electricity, mainly from coal power, that has discount tariffs and does not stem from green-energy sources. In this case the CO<sub>2</sub> emissions, the low efficiency of the generation of power from coal as well as the energy loss in the power transmission have a negative effect. If, on top of this, the seasonal performance factor of the heat pump is well below 3, then the overall system releases more CO<sub>2</sub> than a heating system with a modern gas condensing boiler.
- But there are also gas-powered heat pumps. The gas engine also releases CO<sub>2</sub>, but its efficiency is much higher compared to the detour of consuming energy from coal-burning power plant. Thus, compared to a gas condensing boiler, CO<sub>2</sub> emissions are considerably reduced depending on the seasonal performance factor.
Heat pumps can be very useful if the overall system of insulation, central heating system and energy supply is well though-out and optimized. This works best if you plan a new house accordingly. When renovating an old building, it is highly recommended to consult energy experts, since the investment might not pay off compared to other energy-saving measures.
Literature and Links:
- UBA (2018a): Umgebungswärme und Wärmepumpen. Online: https://www.umweltbundesamt.de/themen/klima-energie/erneuerbare-energien/umgebungswaerme-waermepumpen#umgebungsw%C3%A4rme
- UBA (2018b): Tipps/Wärmepumpen. Online: https://www.umweltbundesamt.de/umwelttipps-fuer-den-alltag/heizen-bauen/waermepumpe
- „Welt der Physik“ Themenseite ‘Geothermie‘ (ohne Jahr): https://www.weltderphysik.de/gebiet/technik/energie/geothermie/
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