Why is passive geothermal cooling heating 1

Heating and cooling with geothermal energy

Average reading time: 4 minutes

In heating technology, the demand for low-emission and energy-efficient heat generation is increasing. Not least the national and global climate policy decisions of the last few years are driving this trend forward. One possibility of resource-saving heat generation is the use of near-surface geothermal energy or geothermal energy.

What is near-surface geothermal energy?

Thermal energy is stored in the subsurface. The upper soil layers to a depth of about 10 m are subject to strong seasonal temperature fluctuations. In the area below, up to a depth of around 100 m, however, temperatures of around 10 ° C are relatively constant all year round. In even deeper layers, the temperature then rises linearly by around 3 ° C per 100 m. The heat stored in the ground is used as primary energy in geothermal systems.

Image: geothermal plus

A distinction is made between two ways of making the subsurface temperatures usable: the open and the closed systems. The former use the groundwater in the form of well bores. Since the groundwater cannot be used at all locations, closed systems are an alternative. Here, the heat from the surrounding rock is used via a closed, underground fluid circuit. A heat transfer medium (brine from water and antifreeze) circulates through the system, which increases in temperature due to the heat transferred from the rock.

What is the job of the heat pump?

Naturally, a body can only transfer heat to its surroundings if it has a higher temperature level than its surroundings. However, in the case of geothermal use, the desired room temperature is higher than the subsurface temperature obtained. For this reason, you need a heat pump that reverses the natural flow, i.e. generates a warmer one from a cold medium. An energy input in the form of electrical current is required as a drive for this temperature increase.

In order to be able to make a statement about the efficiency of a heat pump, the performance figure - also known as the Coefficient of Performance (COP) - is decisive. It describes how much heat can be obtained from the drive energy of the compressor. The lower the temperature difference between the source temperature of the geothermal system and the flow temperature of the heating circuit, the higher the COP and thus the efficiency of the system. For this reason, large-area heating systems such as underfloor or ceiling heating with low flow temperatures are ideal. In the case of the annual performance factor (JAZ), the heat losses and the auxiliary energy consumption for control equipment and pumps are also included in the balance. It therefore provides information on the total useful heat generated annually in relation to the total energy consumption required for this.

How can the near-surface geothermal energy be used?

One way of using it isGeothermal probes. Here, plastic pipes are installed in around 30 to 100 m deep boreholes through which the brine circulates. The deeper the hole, the higher the temperatures. However, a permit under water law is required, in which the maximum permissible borehole depth at the site is specified. In addition, the type of rock has a major influence on the extraction performance. For large projects, an investigation of the thermal subsurface behavior in so-called thermal response tests is necessary. The installation costs are relatively high due to the complex drilling.

 

Image: Federal Association of Heat Pumps BWP e. V., Berlin

Alternatively, they can be laid horizontally Geothermal collectors can be used. These are meandering pipes laid around 0.2 m below the frost line. At this depth there are still noticeable seasonal fluctuations in the subsurface temperature, which has a negative effect on the heat pump efficiency. They also require a lot of space (around twice the heated living space). This area must not be built over or sealed later. So-called are somewhat more space-saving Energy baskets, which are installed in excavations from about 2.5 to 4 m depth. The installation costs for collectors and baskets are significantly cheaper than for probes, as there is no need for drilling.

Image: Federal Association of Heat Pumps BWP e. V., Berlin

Also the thermal activation of concrete parts in contact with the groundis an option. If a pile foundation is required for large buildings, these can be used as energy piles. The pipelines for the brine are integrated directly into the pile so that additional costs for drilling are eliminated. Furthermore, floor panels can be activated thermally. Here it must be ensured that the floor slab to the basement room is sufficiently insulated to prevent a thermal short circuit. However, because of the shallower depth, this option is less efficient.

Active and passive cooling with geothermal energy

With geothermal heating, the thermal energy of the subsoil is transferred to the building by means of a heat pump. But geothermal energy offers another great advantage. This is because the building can be cooled in the summer months with little additional effort. A distinction is made between passive and active cooling. However, both systems are more suitable for vertical systems such as probes and energy piles, since here the seasonal temperature fluctuations in the subsurface are negligible.

The way it works is very simple. The low temperature level in the brine circuit of the geothermal system is now not raised by the heat pump, but used directly to cool the water circulating in the heating circuit via a heat exchanger. In order to air-condition the rooms with it, however, large-area heating systems such as floor, wall or ceiling heating are absolutely necessary. The thermal energy that the heating circuit transfers to the brine circuit is used to regenerate the subsurface. The more the rock is heated in summer, the more efficiently the heat pump works in the following winter.

In order to obtain a higher cooling capacity, it is necessary to use active cooling. Here, the heat pump process is reversed and the system is used as a refrigeration machine. Heat is extracted from the fluid in the heating circuit and evaporated in the heat pump. The resulting evaporation energy is transferred to the brine circuit, which heats the underground rock. The principle is therefore similar to passive cooling, only the heat pump significantly increases the cooling effect. Compared to passive cooling, however, this technology has a significantly higher consumption of auxiliary energy in the form of electricity and is particularly profitable in buildings with high cooling energy requirements.

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