Cold district heating
What is cold district heating?
An ordinary heat supply usually has a flow temperature of over 70°C (newer innovative heat networks 45-60°C). Cold district heating networks, on the other hand, get by with low transmission temperatures of between about 5-25°C. Thus, they can provide both heating and cooling. To raise the operating temperatures for hot water and heating production to the required level, each consumer needs a heat pump.
Conversely, this pump can also produce cooling. The resulting waste heat flows back into the heating network. This energy feed-in turns consumers into so-called prosumers, who can both consume and produce heat.
Cold district heating networks are particularly efficient. Because of the low temperature difference to the surrounding ground, insulation of the pipes is not necessary. To prevent the pipes from freezing in winter, a so-called brine, for example a water-glycol mixture, usually flows through the pipelines.
Sources for cold district heating networks - renewable energies
Cold district heating networks can be fed entirely from renewable energy sources. Therefore, they offer a good opportunity for a sustainable, potentially CO2- and emission-free heat supply - and thus for climate protection and the achievement of climate protection targets.
Because cold district heating networks are modular, they can be gradually expanded and fed from different sources. A combination of sources is therefore not only possible, but makes perfect sense. Suitable energy suppliers include earth, water, solar thermal energy, ambient air, and also commercial and industrial waste heat.
- Geothermal energy: collectors and probes close to the surface collect the energy, which is fed to the consumers in the cold district heating network and only then brought individually to the desired flow temperature by heat pumps.
- Aquathermy uses the thermal energy contained in water to heat or cool. Depending on availability, groundwater, rivers, lakes or even wastewater serve as heat sources.
- Solar thermal energy is particularly suitable for regenerating geothermal sources, charging thermal storage tanks, and generally loading storage tanks.
- Environmental heat or waste heat is also a possible source, for example via the waste heat from the cooling elements of supermarkets or offices. In cities, data centers should also be considered here, whose waste heat is usually (if air-cooled) just under 30°C warm. Here, feeding into cold district heating networks makes particular sense. The return line of conventional district heating networks can also serve as a further heat source.
- The source wastewater is permanently available - especially in urban areas. In addition, there are no negative effects if the water temperature fluctuates due to the use of the heat.
- If the operating temperature of the cold heat network is lower than the ground temperature, the network itself can also absorb heat from the surrounding ground. In this case, the network acts as a kind of geothermal collector.
Storage as a component of cold district heating networks
Unlike air-conditioning systems, which simply release their waste heat unused into the environment, the waste heat in the cold district heating network can be put to good use and thus save a lot of energy. Heat storage systems offer the possibility to store, for example, this waste heat and thus compensate for seasonal fluctuations in heat production. Such heat storage is particularly useful where heating and cooling requirements are not balanced or where there is no sufficient heat source throughout the year. Excess heat from the summer half-year is stored and the temperature of the ground is thus increased. The process is then reversed during the heating season. One possibility for storage is an ice store.
Laying and connection of a cold district heating network
Because insulation is not required, the piping of cold district heating networks is simpler and less expensive. However, compared to conventional district heating pipes, pipes with a larger diameter must be used to transport the same amount of heat. Due to this larger volume, the heat pump consumes more energy for its pumping power.
In the cold district heating network system, each consumer requires its own heat pump and hot water storage tank. The heat pump raises the temperature to the level needed to heat the home and produces the hot water. It can also cool the house.
Importance of cold district heating networks
Cold district heating networks can make a decisive contribution to the decarbonization of heating and cooling supply in the context of the energy transition and climate protection. If renewable energies are used, the primary energy requirement is low - in principle, cold district heating networks can be fed 100% from renewable energies. This also means that there are no CO2 and pollutant emissions on site.
An important aspect of the use of cold district heating networks is the possible sector coupling. Power-to-heat technologies can use electrical surpluses from renewable energies to provide heat - with the help of the heat pump. If these surpluses are stored, this technology makes an important contribution to security of supply.
goodmen energy: Simulation of cold district heating networks
As mentioned before, cold district heating networks absorb heat energy from the ground and thus act like a ground collector themselves. We were interested in how high this contribution is.
For this purpose, we were guided by a feasibility study that we conducted for an urban district. The cold network is to provide an output of up to 530 kW. This is roughly equivalent to 100 modern single-family homes. The required mass flow and pipe diameters result from the power.
To make a simplified estimate of the thermal yield of the cold network, we have simulatively buried an 880 m long pipeline at a depth of one meter and flowed water through it (in reality, a water-glycol mixture would be used). For the water temperature, hourly resolved values for one year are given, which we had previously determined from the simulation of the energy system (borehole heat exchangers + consumers).
For the simulation we made the following simplifications:
- Water instead of glycol as heat transfer medium
- Constant mass flow over entire pipe length
- Mains inlet temperature = geothermal probe outlet temperature
- Laminar flow
- No mutual influence of supply and return flow
The illustration shows the thermal contribution of the cold network (KNW) compared to the contribution of the borehole heat exchanger field (EWS). In summer, the values are negative. Here, an energy release to the environment takes place. On an annual average, about 20% of the required anergy is obtained from the cold district heating network. In January it is about 16%. A considerable amount of energy, which is given to us for free. In reality, we expect even higher rates, since the network temperature in the return flow from the heat pumps to the probe field is about three degrees lower. This results in even higher thermal yields.
Advantages and disadvantages of anergy networks at a glance
Advantages:
- Renewable energy sources available free of charge and in unlimited supply.
- Possibility of CO2-free, local supply without dependence on fossil fuels
- No insulation of the pipelines necessary
- Efficiency due to only low heat losses in the pipe network
- Ideally, heat is absorbed from the ground
- Reduced operating costs
Disadvantages:
- Higher investment costs for heat pump and connection to district heating network compared to connection to a conventional district heating network
- Due to large volume flows and larger pipe diameters, there is a higher power requirement for the pumps
Promotion for cold district heating in Germany
The German Federal Ministry for Economic Affairs and Energy (BMWi) has been funding feasibility studies and the realization of heat network systems 4.0 since 01.07.2017. A feasibility study must demonstrate that network temperatures below 20°C will save costs, energy or CO2 emissions. The feasibility study "Heat Networks 4.0" is the first funding program to support not only individual technologies and components, but also complete systems. Read more here.