Cold district heating
Cold district heating is a technical variant of a district heating network that operates at low transmission temperatures well below those of conventional district heating systems and can provide both space heating and cooling. Transmission temperatures in the range of approx. are common, allowing different consumers to heat and cool simultaneously and independently of each other. Hot water is produced and the building heated by water heat pumps, which obtain their thermal energy from the heating network, while cooling can be provided either directly via the cold heat network or, if necessary, indirectly via chillers. Cold local heating is sometimes also referred to as an anergy network. The collective term for such systems in scientific terminology is 5th generation district heating and cooling. Due to the possibility of being operated entirely by renewable energies and at the same time contributing to balancing the fluctuating production of wind turbines and photovoltaic systems, cold local heating networks are considered a promising option for a sustainable, potentially greenhouse gas and emission-free heat supply.
Terms
As of 2019, the fifth generation heating networks described here have not yet been given a uniform name, and there are also various definitions for the general technical concept. In the English language technical literature the terms Low temperature District Heating and Cooling, Low temperature networks, Cold District Heating and Anergy networks or Anergy grid are used. In addition, some publications have definitional conflicts in the delimitation to "warm" district heating networks, because certain authors consider Low temperature District Heating and Cooling as well as Ultra-low temperature District Heating as subforms of 4th generation district heating. In addition, the definition of so-called low-ex networks allows to classify them as both fourth and fifth generation.History
The first cold district heating network is the heating network in Arzberg in Upper Franconia, Germany. In the Arzberg power station there, which has since been shut down, uncooled cooling water was taken from between the turbine condenser and the cooling tower and piped to various buildings, where it was then used as a heat source for heat pumps. This was used to heat the school and the swimming pool in addition to various residential buildings and commercial enterprises.Another very early plant was put into operation in Wulfen in 1979. There, 71 buildings were supplied with heat energy, which was taken from the groundwater. Finally, in 1994, the first cold heating network was opened, using waste heat from an industrial company, a textile company. Also in 1994 a cold local heating network was built in the Swiss village Oberwald, which is operated with seepage water from the Furka base tunnel.
As of January 2018, a total of 40 schemes were in operation in Europe, 15 each in Germany and Switzerland. Most of the projects were pilot plants with a heat output of several 100 kWth up to the single-digit MW range, the largest plant had an output of approx. 10 MWth. In the 2010s about three plants per year were added.
Concept
Cold heat networks are heat networks that are operated at very low temperatures. They can be fed from a variety of frequently regenerative heat sources and allow the simultaneous production of heat and cold. Since the operating temperatures are not sufficient for the production of hot water and heating heat, the temperature at the consumer is raised to the required level by means of heat pumps. In the same way, cold can be produced and the waste heat can be fed back into the heating network. In this way, connected consumers are not only customers, but can also act as prosumers, who can either consume or produce heat depending on the circumstances.The concept of cold local heating networks is derived from groundwater heat pumps as well as open-loop heat pumps. While the former are mainly used to supply individual houses, the latter are often found in commercial buildings which have both heating and cooling needs and have to meet these needs in parallel. Cold local heating extends this concept to individual residential areas or districts. Like ordinary geothermal heat pumps, cold local heating networks have the advantage over air heat pumps of operating more efficiently due to the lower temperature difference between the heat source and the heating temperature. However, compared to geothermal heat pumps, cold local heating networks have the additional advantage that even in urban areas, where space problems often prevent the use of geothermal heat pumps, heat can be stored seasonally via central heat storage, and in addition, the different load profiles of different buildings may allow a balance between heating and cooling requirements.
Cold district heating is particularly suitable where there are different types of buildings and therefore there is a demand for both heating and cooling, enabling energy balancing over short or long periods of time. Alternatively, seasonal heat storage systems allow for a balance of energy supply and demand. By using different heat sources and combining heat sources and heat sinks, synergies can also be created and the heat supply can be further developed in the direction of a circular economy. In addition, the low operating temperature of the cold-heating networks makes it possible to feed otherwise hardly usable low-temperature waste heat into the network in an uncomplicated manner. At the same time, the low operating temperature significantly reduces the heat losses of the heating network, which limits the energy losses, especially in summer, when there is little demand for heat. The annual performance factor of heat pumps is also relatively high, especially compared to air-sourced heat pumps. A study of 40 systems commissioned up to 2018 showed that the heat pumps achieved an seasonal COP of at least 4 for the majority of the systems studied; the highest seasonal COP values were about 6.
Technologically, cold heat networks are part of the concept of smart heat networks.
Components
Heat sources
Various heat sources can be used as energy suppliers for the cold heating network, in particular renewable sources such as the ground, water, commercial and industrial waste heat, solar thermal energy and ambient air, which can be used individually or in combination. Due to the generally modular design of cold local heating networks, new heat sources can be gradually developed as the network is further expanded, so that larger heating networks can be fed from a variety of different sources.In practice almost inexhaustible sources are e.g. sea water, rivers, lakes or groundwater. Of the 40 cold heating networks in operation in Europe as of January 2018, 17 used water bodies or groundwater as a heat source. The second most important heat source was geothermal energy. This is usually accessed via geothermal boreholes using vertical borehole heat exchangers. However, it is also possible to use surface collectors such as agrothermal collectors. In this case, horizontal collectors are ploughed into agricultural land at a depth of 1.5 to 2 m, i.e. below the working depth of agricultural machines, which can extract heat from the soil as required. This concept, which allows further agricultural use, has been realized, for example, in a cold heat network in the German town Wüstenrot.
In addition, there are cold-heating networks that extract geothermal energy from tunnels and abandoned coal mines. Waste heat from industrial and commercial enterprises can also be used. For example, two cold-heating networks in Aurich and Herford use waste heat from dairies and another plant in Switzerland uses waste heat from a biomass power plant, while another cold-heating network uses waste heat from a textile company. Other possible heat sources include solar thermal energy, large heat pumps that use environmental heat, the sewage system, combined heat and power plants and biomass- or fossil-fired peak load boilers to support other heat sources. The low operating temperatures of cold-heating networks are particularly favourable to solar thermal systems, CHP units and waste heat recovery, as these can operate at maximum efficiency under these conditions. At the same time, cold heating networks enable industrial and commercial companies with waste heat potential, such as supermarkets and data centres, to feed thermal energy into the grid without any major financial investment risk, since at the temperature level of cold heating networks, direct heat feed is possible without a heat pump.
Another heat source can also be the return line of conventional district heating networks. If the operating temperature of the cold heating network is lower than the soil temperature, the network itself can also absorb heat from the surrounding soil. In this case the network then acts as a kind of geothermal collector.