Differences between conventional district heating and 5GDHC networks
5GDHC networks play a central role in decarbonizing the heat supply of districts. However, they differ in many aspects from conventional heating networks.
What are the technical differences?
The main difference between conventaional district heating networks and 5GDHC networks is the operating temperature of the network: While conventaional district heating networks operate at flow temperatures of around 70 °C and more, the temperatures of 5GDHC networks are around 5-35 °C. This means that the temperature level in 5GDHC networks is not sufficient to transfer the heat from the network through heat exchangers to the buildings. Therefore, in 5GDHC networks, a heat pump is installed in each building to raise the temperature level of the heat. The concepts are compared in Figure 1.
Which type of heat network is suitable for which district?
The choice of the right heat network type depends on several factors: If the consumer buildings are to be supplied with heat and cold, a 5GDHC network is suitable because it can be operated at such low temperatures that cooling demands can also be met. A fundamental advantage of 5GDHC networks is that in the event of simultaneous heating and cooling loads in the district, these demands can be partially balanced by the network. This can be particularly lucrative for operators, as they can sell both cooling to cooling consumers and at the same time the resulting waste heat to heat-consuming buildings. Another important aspect in selecting the right type of heat network is the temperature level of the heat source. If only heat sources at ambient temperature (< 30 °C) are available, a 5GDHC network can be advantageous. Conventional district heating networks, on the other hand, are clearly preferable if heat sources with a high temperature level are available (> 50 °C), since in this case no further temperature increase (in decentralized or central heat pumps) is necessary. In case of heat sources at ambient temperature, in principle both types of district heating networks can be useful: The advantage of conventional district heating networks is that one large heat pump can be installed centrally, which is cheaper than many small heat pumps in buildings. The advantage of 5GDHC networks, however, is that there is hardly any heat loss in the pipe network due to the low temperatures (in some cases, heat gains can even be achieved if the surrounding soil has a higher temperature than the district heating network). In addition, low-cost (possibly even uninsulated) plastic pipes can be used for 5GDHC networks. This compensates for the additional costs for larger pipe diameters, which are necessary due to higher volume flows in 5GDHC networks. The high volume flows result from the low temperature spread between the warm and cold pipe, which in turn are the result of a low temperature difference across the evaporator of the decentralized heat pumps.
What do these differences mean for planning?
The calculation of cold district heating networks is more complex than the calculation of ordinary heating networks. On the one hand, the decentralized heat pumps in the buildings must be taken into account. On the other hand, the balancing effects of heating and cooling demands in buildings and in the district heating networks must be accounted for. In particular, it is therefore no longer sufficient for 5GDHC networks to consider only peak loads and annual energy quantities. Rather, it is necessary to use hourly annual profiles for the calculation in order to correctly represent the balancing effects.
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