Ambient heat

The term “ambient heat” refers to heat that is present in layers of air close to the ground, in surface bodies of water and in the ground a few metres below the surface, primarily through the storage of solar energy. It is a renewable and natural source of energy, because the heat is produced by solar energy and is thus constantly renewed.

Geothermal energy comes from a different source. Here, heat produced from the decay of naturally present radioactive elements is stored underground. The term “near-surface geothermal energy” refers to the use of heat from a depth of around 500 metres. It concerns the use of heat from groundwater reserves, as well as heat that is transported to heat pumps with the aid of geothermal probes. Thus near-surface geothermal energy has to be distinguished from the use of geothermal energy from much greater depths below ground. The temperatures at these depths can be used directly for heating, as well as for electricity production (depending on the depth).

Heat pumps are used in a variety of sectors today for a broad range of purposes. There is a great deal of potential for the use of ambient heat and near-surface geothermal energy, particularly for room heating and hot water production. Heat pump technology is making a valuable contribution towards the attainment of the declared energy and climate objectives.

The replacement of heating systems within the scope of building renovations is one of the ways in which these objectives can be achieved. By 2050, around 900,000 fossil-fuelled heating systems (status: 2019) will need to be renovated, and these could all be converted to heat pump technology. It is essential to promote the use of this technology and thus exploit the existing enormous potential.

Use of ambient heat in heat pumps

In view of its relatively low temperature, ambient heat can only be converted into useful heat with the aid of heat pumps. Within a heat pump a refrigerant flows through the circuits. When low-temperature ambient heat is absorbed, the refrigerant evaporates and becomes gaseous. The resulting vapour is compressed to a high temperature and high pressure and fed into the condenser. The condensing refrigerant then supplies useful heat to the heating circuit. The refrigerant subsequently liquefies and can now absorb ambient heat again. Thus the cycle is repeated. Depending on the heat source, the quantity of useful energy produced for heating is many times greater than the consumed electrical power, and the efficiency of the heat pump (annual working factor) can be as high as 400 percent (annual working factor 4) or can even reach 500 percent if the source is geothermal energy.

Heat pumps work all the more efficiently when the difference between the source temperature and the heating flow temperature is small.

Operation of a heat pump with air as the heat source

As a heat source, air (or outside air) offers the advantage that, unlike other energy sources, it is permanently available and inexpensive to collect. The disadvantage is that outside air is coldest in winter, precisely when the demand for heat is at its highest. This means that systems have to be appropriately dimensioned.

Operation of a heat pump with the ground as the heat source

The ground stores solar energy in its uppermost layers, but these, too, are subject to seasonal fluctuations. The ground temperature below the surface is stable from a depth of around 15 metres, after which it increases by approximately 3° C per 100 metres, depending on the local geological conditions. Heat pumps utilise the ground heat from near-surface geothermal energy, e.g. with the aid of geothermal probes, which require a licence and can reach to around 500 metres below the surface. Geothermal probes are dimensioned to ensure that an adequate flow of heat is maintained. Below the ground it is only heat that is exchanged – no exchanges of material are involved. The section of the underground that is cooled by the geothermal probe is constantly reheated through natural heat conduction. If over the long term more heat is extracted than is able to flow, the underground has to be regenerated in good time. There are also energy piles, energy cages and ground heat exchangers in use that consist of closed circuits and utilise near-surface geothermal heat from a few metres below the surface.

Geothermal probes can also be used for cooling purposes. The method of geocooling makes use of the fact that, in the summer, temperatures in the underground are lower than those of the outside air. By feeding back the heat from a building into the underground the coldness can be used for refrigeration.

Operation of a heat pump with water as the heat source

As a heat source, water utilises the ambient heat from lakes or rivers, or the heat from the groundwater below the surface. Groundwater has a constant temperature throughout the year and is thus an ideal heat source. The aquiferous rock layer is exploited by means of well drilling. Contrary to the use of geothermal probes, here an exchange of material (water) takes place. The groundwater is fed into a heat pump and the cooled water is fed back into the source system.

Given that Switzerland’s building stock accounts for around 24 percent of greenhouse gas emissions (status: 2018), the utilisation of ambient heat would clearly be beneficial. Depending on the utilised electricity product, heat pumps operate without CO2 emissions. They also require very little operating power, and this means their energy consumption is relatively low. Heat pumps produce 100 percent useful heat from 25 percent operating power and 75 percent ambient heat. Thermal energy from ambient air and the underground is CO2-neutral and renewable. If in addition electricity from renewable energy is used, a heat pump’s environmental impact is even lower.

In 2019, approximately 24,000 heat pumps were sold in Switzerland (around 2,000 more than in 2018). In 2019, heat pumps accounted for 49 percent of all sold heat producing systems. But oil or gas boilers still accounted for 47 percent, and in addition around 10,000 oil/gas burners were replaced. In order for all fossil-fuelled heating systems to be replaced by 2050, around 40,000 heat pumps would have to be installed each year.

Furthermore, heat pumps are increasingly being coupled with photovoltaic systems. Electricity produced on the roof can be used for powering heat pumps. Own consumption is economically viable and ecological, and can ease the burden on the electricity grid. Energy consumption also functions in apartment blocks and throughout entire sites. In the future, heat pumps will exchange data with electricity network operators to an increasing extent and adapt their operation cost-efficiently to the dynamic network load.

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