Cold local heating
Cold local heat or cold district heating is a technical variant of a heat supply network which works with low transmission temperatures close to the ambient temperature and therefore both heat and cold can provide. Transfer temperatures in the range of approx. 10-25 ° C are common, as a result of which these systems operate at temperatures well below conventional district or local heating systems. This means that different consumers can heat and cool independently of one another at the same time. In contrast to conventional heating networks, hot water generation and building heating do not take place directly via heat exchangers , but via water heat pumps , which obtain their thermal energy from the heating network. The cooling can either take place directly via the cold heating network or, if necessary, indirectly via the heat pumps.
Cold local heating networks are also sometimes referred to as anergy networks . The collective term in the scientific terminology for such systems is English 5th generation district heating and cooling , district heating and cooling of the fifth generation ' . Due to the possibility of being operated entirely using renewable energies and at the same time making a contribution to compensating for the fluctuating production of wind power 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, no uniform designation has emerged for the fifth generation heating networks described here, and there are also various definitions for the general technical concept. The terms low temperature district heating and cooling (LTDHC), low temperature networks (LTN), cold district heating (CHD) and anergy networks or anergy grid are used in the English-language specialist literature . In addition, in some publications there are definitional conflicts in the delimitation of "warm" district heating networks, since certain authors consider low temperature district heating and cooling and ultra-low temperature district heating to be sub-forms of the fourth generation of district heating systems. In addition, the definition of so-called low-ex networks allows them to be classified as both the fourth and fifth generation. In German-speaking countries a. Cold local heating and anergy network in use.
history
The first cold district heating network is the one in Arzberg in Upper Franconia . In the Arzberg power plant there, which has now been closed, uncooled cooling water was taken between the turbine condenser and the cooling tower and piped to various buildings, where it then served as a heat source for heat pumps. In addition to various residential buildings and businesses, the school and the swimming pool were heated with it.
Another very early plant was put into operation in 1979 in Wulfen . There, 71 buildings were supplied, with the thermal energy being taken from the groundwater. In 1994 the first cold heating network was finally opened, which used waste heat from an industrial company, a textile company. Also in 1994 (according to Pellegrini and Bianchini as early as 1991), a cold local heating network was built in the Swiss Oberwald , which is operated with seepage water from the Furka Base Tunnel .
As of January 2018, a total of 40 systems were in operation in Europe, 15 of them each in Germany and Switzerland. Most of the projects involved pilot plants with a heat output of a few 100 kW th up to the single-digit MW range; the largest plant had an output of approx. 10 MW th . In the 2010s, around three systems were added per year.
concept
Cold heating networks are heating networks that are operated at very low temperatures close to the ambient heat (around 5–35 ° C, mostly between 10 and 25 ° C). They can be fed by a large number of often 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, the temperature at the customer is raised to the required level by means of heat pumps. In the same way, cold can also be produced and the waste heat fed back into the heating network. In this way, affiliates are not just customers, but can function as prosumers who, depending on the circumstances, can either consume or produce heat.
The concept of cold local heating networks comes 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 that have both heating and cooling requirements and have to cover them in parallel. Cold local heating extends this concept to individual residential areas or city districts. Like conventional geothermal heat pumps, cold local heating networks have the advantage over air heat pumps that they work more efficiently due to the lower temperature delta between the heat source and the heating temperature. Compared to geothermal heat pumps, however, cold local heating networks have the additional advantage that even in urban areas, where space problems often prevent the use of geothermal heat pumps, seasonal heat can be stored via central heat storage , and in addition the different load profiles of different buildings, if necessary, a balance between heat and Enable cooling demand. While nowadays the cooling requirement is often covered by refrigeration machines, which release their waste heat unused into the environment, in the future the released waste heat could be used meaningfully, which means there is considerable potential for energy savings . The use of this waste heat is also considered important because a significant increase in the cooling requirement is expected in the future.
They are particularly suitable for use where there are different types of buildings (residential buildings, businesses, supermarkets, etc.) and there is thus a demand for both heating and cooling, which enables energy to be balanced over short or long periods of time. Alternatively, seasonal heat storage systems enable the energy supply and demand to be balanced. Through the use of various (waste) heat sources and the combination of 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 network enables otherwise hardly usable low-temperature waste heat to be fed into the network in an uncomplicated manner. At the same time, the low operating temperature significantly reduces the heat losses in the heating network, which in particular limits energy losses in summer, when there is only little heat demand.
The annual coefficient of performance of heat pumps is relatively high compared to air heat pumps. An investigation of 40 systems commissioned by 2018 showed that the heat pumps achieved an annual coefficient of performance of at least 4 in the majority of the systems examined; the highest values were 6.
In terms of technology, cold heating networks are part of the intelligent heating network concept. They follow the general trend of lowering the transfer temperatures of heating networks ever further.
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 structure of cold local heating networks, if the network is further expanded, new heat sources can gradually be developed so that larger heating networks can be fed from a variety of different sources.
In practice, almost inexhaustible sources are z. B. sea water, rivers, lakes or groundwater. Of the 40 cold heating networks in operation in Europe as of January 2018, 17 used bodies of water or groundwater as a heat source. The second most important heat source was geothermal energy. This is mostly developed via geothermal boreholes using vertical geothermal probes . It is also possible to use surface collectors such as B. Agrothermal collectors . In this case, horizontal collectors are plowed in on agricultural land, approximately at a depth of 1.5 to 2 m and thus below the working depth of agricultural equipment, which can extract heat from the ground if necessary. This concept, which allows further agricultural use, was implemented in a cold heating network in Wüstenrot , for example .
There are also cold heating networks that generate geothermal energy from tunnels and abandoned coal mines. Waste heat from industrial and commercial operations can also be used. For example, two cold heat networks in Aurich and Herford use the waste heat from dairies and another plant in Switzerland uses waste heat from a biomass power plant , while another cold heat network uses waste heat from a textile company. Low-temperature industrial waste heat is an important source of heat: In urban areas, the waste heat potential available can be in the range of 50 and 120% of the total heat demand.
Other possible sources of heat include a. Solar thermal energy (especially for the regeneration of geothermal sources and the loading of storage systems), large heat pumps that use environmental heat, sewer systems , combined heat and power plants and biogenic or fossil-fueled peak load boilers to support other heat sources. The low operating temperatures of cold heating networks are particularly beneficial for solar thermal systems, CHPs and waste heat utilization, as these can work with maximum efficiency under these conditions. At the same time, cold heating networks enable industrial and commercial companies with waste heat potential, such as supermarkets , data centers , etc., to feed in thermal energy in an uncomplicated manner without a major financial investment risk, as direct heat can be fed in without a heat pump at the temperature level of cold heating networks.
Another heat source can also be the return pipe of conventional district heating networks. If the operating temperature of the cold heating network is lower than the ground temperature, the network itself can also absorb heat from the surrounding ground. In this case, the network acts like a kind of geothermal collector .
(Seasonal) heat storage
Heat storage in the form of seasonal storage is a key element of cold local heating systems. To compensate for seasonal fluctuations in heat production and consumption, many cold heating systems are built with seasonal heat storage . This is particularly useful where the customer / prosumer structure does not lead to a largely balanced heating and cooling requirement or where there is sufficient heat source available all year round. Aquifer storage and storage via borehole fields are well suited . These allow excess heat from the summer months, e.g. B. from cooling, but also from other heat sources and thus heat the floor. During the heating season, the process is reversed and heated water is pumped and fed into the cold heating network. However, other types of heat storage are also possible. For example, a cold heating network in Fischerbach uses an ice store .
Heating network
Cold local heating systems allow a variety of network configurations. A rough distinction can be made between open systems, in which water is fed in, channeled through the network, where it is then supplied to the respective consumer, and finally released into the environment, and closed systems, in which a transfer fluid , usually brine , circulates in a circuit. The systems can also be differentiated according to the number of pipes used. Depending on the circumstances, configurations with one to four tubes are possible:
- Single-pipe systems are usually used in open systems that use surface or groundwater as a heat source and release it back into the environment after flowing through the heating network.
- In two-pipe systems, both pipes are operated at different temperatures. In heating mode, the warmer of the two is used as a heat source for the heat pumps of the consumer, the colder one takes up the transmission medium cooled by the heat pump. In cooling mode, the colder one serves as the source, the heat generated by the heat pump is fed into the warmer pipe.
- Three-pipe systems work in a similar way to two-pipe systems, but there is also a third line that is operated with warmer water, so that (at least in the case of heating systems with a low flow temperature, such as underfloor heating systems ) heating can take place without the use of the heat pump. The heat transfer usually takes place via heat exchangers . Depending on the temperature, it is fed back into the warmer or colder pipe after use. Alternatively, the third line can also be used as a cooling line for direct cooling via a heat exchanger.
- Four-pipe systems function like three-pipe systems, except that there is one line each for direct heating and cooling. In this way, energy cascades can be implemented.
In general, it applies to the pipes of cold heating networks that the pipes, in contrast, can be designed in a simpler and cheaper way than with warm / hot district heating systems. Due to the low operating temperatures, there is no thermomechanical stress, which allows the use of ordinary polyethylene pipes without insulation, such as those used in drinking water supply. This allows both quick and inexpensive installation and quick adaptation to different network geometries. This also eliminates the need for expensive X-ray or ultrasound examinations of the tubes, the welding of individual tubes and the time-consuming on-site insulation of connecting pieces. However, compared to conventional district heating pipes, pipes with a larger diameter must be used in order to be able to transport the same amount of heat. The energy requirement of the pumps is also higher due to the larger volume. On the other hand, cold local heating systems can potentially also be set up where the heating requirements of the connected buildings are too low for the operation of a conventional heating network. In 2018, 9 out of 16 systems for which sufficient data were available were below the threshold of 1.2 kW heating capacity / m network length, which is regarded as the lower limit for the economic operation of conventional “warm” local heating systems.
Transfer station
Compared to conventional “hot” district heating networks, the transfer station for cold local heating systems is more complicated, space-consuming and, accordingly, more expensive. A heat pump and a heat storage tank for hot water must be installed for each connected consumer or prosumer . The heat pump is usually designed as an electrically driven water-to-water heat pump and is also often physically separated from the cold heat network via a heat exchanger. The heat pump raises the temperature to the level required to heat the apartment and generates the hot water. However, it can also be used to cool the house and feed the heat generated there into the heating network, provided that it is not cooled directly without using a heat pump. In addition, a back-up system such as B. be installed a heating element . A heat accumulator can also be installed for the heating system, which enables more flexible operation of the heat pump. Such heat accumulators also help to keep the output of the heat pump low, which in turn reduces installation costs.
Role in the future energy system
Low-temperature heating networks, which include cold local heating systems, are seen as a promising or even a central element for the decarbonization of heating and cooling in the context of the energy transition and climate protection . Local and district heating systems have different advantages compared to individual heating. B. the higher efficiency of the plants, the possibility to use combined heat and power and to exploit previously unused waste heat potential. In addition, they are seen as an important solution to increase the use of renewable energies and to reduce the primary energy requirement and local emissions during heat generation. If there is no combustion technology for feeding into the cold heating network, carbon dioxide emissions and pollutant emissions can be completely avoided on site. Cold heating networks are also seen as an opportunity to build up heating networks in the future that are 100% fed by renewable energies.
The use of cold local heating systems and other heat pump heating systems for sector coupling are also considered to be a promising approach . For example, through power-to-heat technologies, electrical energy is used for heating on the one hand, and on the other hand the heating sector can help provide system services in order to compensate for the fluctuating green electricity generation in the electricity sector. Cold local heating networks can thus contribute to load control via the heat pumps and, together with other storage systems, help to ensure security of supply.
If the roofs of the supplied buildings are equipped with photovoltaic systems, it is also possible to obtain part of the electricity required for the heat pumps from your own roof. For example, 20 energy-plus houses were built in Wüstenrot , all of which are equipped with photovoltaic systems, a solar battery and a heat storage system for the highest possible degree of self-sufficiency through flexible operation of the heat pump.
literature
- Simone Buffa et al .: 5th generation district heating and cooling systems: A review of existing cases in Europe . In: Renewable and Sustainable Energy Reviews . tape 104 , 2019, pp. 504-522 , doi : 10.1016 / j.rser.2018.12.059 .
- Marco Pellegrini, Augusto Bianchini: The Innovative Concept of Cold District Heating Networks: A Literature Review . In: Energies . tape 11 , 2018, p. 236 , doi : 10.3390 / en11010236 .
Web links
- "Cold" local heating network saves 40,000 kg of CO2 per year . Energy Agency NRW. Retrieved March 13, 2017.
- Cold local heating in Dorsten: Pioneering heat pump project has been running for four decades and is still in the running . In: EE-News , November 14, 2019. Retrieved June 28, 2020.
- Dossier on English-language, scientific specialist literature at mwirtz.com/5gdhc_literature.html . Retrieved July 28, 2020.
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s Simone Buffa et al .: 5th generation district heating and cooling systems: A review of existing cases in Europe . In: Renewable and Sustainable Energy Reviews . tape 104 , 2019, pp. 504-522 , doi : 10.1016 / j.rser.2018.12.059 .
- ^ Leonhard Müller: Handbook of the electricity industry: Technical, economic and legal bases . Berlin / Heidelberg 1998, p. 266f.
- ^ A b c d Marco Pellegrini, Augusto Bianchini: The Innovative Concept of Cold District Heating Networks: A Literature Review . In: Energies . tape 11 , 2018, p. 236 , doi : 10.3390 / en11010236 .
- ↑ a b c d Marco Wirtz et al .: Quantifying Demand Balancing in Bidirectional Low Temperature Networks . In: Energy and Buildings . tape 224 , 2020, doi : 10.1016 / j.enbuild.2020.110245 .
- ↑ a b c d Stef Boesten et al .: 5th generation district heating and cooling systems as a solution for renewable urban thermal energy supply . In: Advances in Geoscience . tape 49 , 2019, p. 129-136 , doi : 10.5194 / adgeo-49-129-2019 .
- ↑ a b c Marcus Brennenstuhl et al .: Report on a Plus-Energy District with Low-Temperature DHC Network, Novel Agrothermal Heat Source, and Applied Demand Response . In: Applied Sciences . tape 9 , 2019, doi : 10.3390 / app9235059 .
- ↑ Dietmar Schüwer: Conversion of the heat supply structures . In: Energy industry issues of the day . tape 67 , no. 11 , 2017, p. 21-25 .
- ↑ Laura Romero Rodríguez et al .: Contributions of heat pumps to demand response: A case study of a plus-energy dwelling . In: Applied Energy . tape 214 , 2018, p. 191-204 , doi : 10.1016 / j.apenergy.2018.01.086 .