What is heating and cooling
“A heating system in buildings must reliably ensure that the temperature in the rooms always corresponds to the needs of the user. Until a few years ago, the task of the heating system was limited to heating, i.e. supplying the building with sufficient heating energy. With the increasing establishment of the heat pump as a heating system, however, another function is increasingly moving into the focus of users: This system can also take over the cooling of the rooms on warm and hot days! A heat pump can either transport environmental heat into the building or excess room heat out of the building, depending on requirements. Different types of use are possible.
On the one hand there are the brine / water heat pumps, whose ground heat source becomes a "cold source" in summer. On the other hand, there are reversible air-to-water heat pumps which, thanks to the possibility of reversing the cooling circuit, can cool the building in the summer like an air conditioning system. When cooling buildings with heat pumps, a distinction is made between passive and active cooling. The main difference between the two systems is the operation with (active) or without (passive) compressor of the heat pump.
Passive cooling with the brine / water heat pump
The ground, which the brine / water heat pump serves as a heat source, has a temperature of around 9 to 10 ° C all year round at depths of more than eight meters. This means that it is not only an excellent heat source during the heating season, but also an excellent "cold reservoir" in summer, so that the heat source system can be used in summer to cool the building to be heated in winter. The heat extracted from the rooms via the heating or cooling surfaces is simply transferred to the brine circuit by means of an additional plate heat exchanger. The brine then transfers the heat to the ground via the geothermal probe. The temperature of the brine that leaves the ground and enters the heat pump is around 15 ° C. This enables cold water temperatures of 17 ° C or more to be achieved, with which the water from the heating or cooling circuit is then fed to the room cooling surfaces. The heat pump's compressor is not in operation. The cooling takes place passively. In order to be able to cool, the heat pump only needs to be equipped with an additional plate heat exchanger and a brine-side switching valve (see Fig. 4).
With a brine / water heat pump, passive cooling, i.e. cooling without the compressor running, is often sufficient due to the low ground temperature.
Of course, the prerequisites for professional drilling in connection with a brine / water heat pump must be created. In the meantime, some companies offer the complete production of heat sources from a single source at a fixed price. The complete range of services extends from submitting the water rights application to drilling in all soil formations, grouting the drilling with optimized backfill material, creating the documentation for the drilling and connecting the heat pump to the heat source system. This connection includes the laying of the horizontal connection lines, a core drill hole to lead the lines into the house, the connection of the pipelines to the heat pump and the diffusion-tight thermal insulation of all supply lines from the heat source in the room where the heat pump is installed. Finally, the heat source system and heat pump are touched with the heat transfer medium.
Active cooling with the air / water heat pump
Air / water heat pumps use the outside air as a natural heat source. Since the heat source temperature is above the required cooling temperature in summer, only active cooling is possible. In order to be able to implement active cooling, the heat pump must have a reversible / reversible cooling circuit.
In the case of cooling, the cooling circuit of the heat pump is reversed in such a way that the heat source and heat sink are exchanged. According to the same principle of an air conditioning system, heat is extracted from the building by means of a working cooling circuit. The room air is passed through a heat exchanger in which the working medium circulates. The boiling temperature of the working medium is lower than the temperature of the room. It begins to boil and changes from the liquid to the vaporous state of aggregation. The necessary heat of evaporation is withdrawn from the room. The room air cools down. The heat pump's compressor is in operation with active cooling. It sucks in the vaporous working medium and compresses it. The pressure and temperature of the working medium increase. The working medium, which is under high pressure and high temperature, reaches the second heat exchanger of the heat pump. Here the working fluid releases the heat extracted from the room and the electrical power consumption of the compressor to the environment. The physical state changes from vapor to liquid. The high pressure of the now liquid working medium is then reduced with the help of the expansion valve and the cycle begins again.
The right cooling surfaces
Fan convectors, underfloor heating / cooling or wall heating / cooling can be used as room cooling surfaces for both brine / water heat pumps and reversible air / water heat pumps. If fan convectors are used, it should be noted that these must be designed for higher cold water temperatures and lower heating water temperatures than usual. Otherwise the installation of cooling with fan convectors is not a problem. Due to the existing condensation drainage, falling below the dew point temperature of the air is not a problem, so that the cold water temperature does not have to be limited below.
The common feature of cooling via the wall or floor surface is that the room air must avoid falling below the dew point, as otherwise water could condense on the cooling surface. To avoid the formation of condensation on the cooling surface, it is advisable to use a room temperature and humidity sensor that is connected to the heat pump controller. In this way, the controller can calculate the minimum permissible cold water temperature. Of course, it is not possible to dehumidify the room air when using cooling surfaces. You also have to be aware that under certain, rarely occurring conditions, the cooling capacity can be limited by the risk of falling below the dew point.
If underfloor heating / cooling is used to dissipate heat from the room, a cooling output of around 7 W / m²K temperature difference between the floor and room temperature can be transferred. For example, a floor temperature of 22 ° C and a room temperature of 27 ° C results in a cooling output of 35 W / m². Due to the relatively high floor temperature in cooling mode, the thermal comfort is good; the dreaded problem of "cold feet" does not arise with correct interpretation. Compared to cooling with fan convectors, cooling via the floor surface is completely draft-free and quiet. In addition, there is the architectural advantage of free room design.
Wall heating / cooling is a little more suitable than underfloor heating, especially for room cooling. This is due to the greater proportion of radiation during heat transfer, which is a result of the fact that the proportion of the body surface that is in radiation exchange with the cooling surface is greater on the wall than on the floor.
When looking at costs, there is also a lot to be said for a heat pump: Up until now, buildings were almost exclusively cooled with split air conditioning units, chillers or VRF systems (Variable Refrigerant Flow, see the article on types of air conditioning). The heat pump additional benefit of cooling or temperature control is attractive due to the low additional costs for the cooling system with very low refrigeration costs at the same time. With the passive cooling of an office building, up to 80% of the annual cooling costs can be saved compared to cooling with room air conditioners. With active cooling, there are still 20% savings. The simultaneous use for cooling and heating also increases the efficiency of the heat pump with geothermal probes in heating mode, since the geothermal probe regenerates significantly better in summer. "
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