Planning tool for buildings & districts

Storages in 5GDHC networks

To match heat generation and heat demands, heat storages in 5GDHC networks can be a useful addition. For this purpose, large central water-filled storage tanks, seasonal storage tanks or decentralized storage tanks in buildings are used.

What storage is available for 5GDHC networks?

In contrast to conventional district heating networks, the selection of an appropriate storage concept strongly depends on the given boundary conditions of the 5GDHC network, including the type of heat source. Basically, a distinction can be made between large, centralized storage units and smaller, decentralized storage units. In addition, seasonal storage units and medium-sized central heat storage units can be considered.

Central heat storages for 5GDHC networks

Medium-sized, water-filled storage tanks, which usually have volumes of several hundred cubic meters, are a special form of heat storage. They increase the balance of heating and cooling demands in the districts. These storage tanks are usually hydraulically connected directly - or alternatively indirectly via heat exchangers - to the 5GDHC network. The storage tank is layered and has a warm zone in the upper part, which contains water at a temperature of the warm pipe. The zone in the lower part of the accumulator is at the temperature level of the cold pipe. On the upper side, the accumulator is connected to the warm pipe and on the lower side to the cold pipe of the district heating network.

Storages 5GDHC networks, district heating
Figure 1: Charging and discharging of a central storage tank for 5GDHC networks

When heat demands predominate in a district, water from the cold pipe enters the storage tank at the bottom. At the same time, warm water leaves the top of the storage tank and enters the warm pipe of the 5GDHC network. The central storage tank can thus balance the net heat demands of the districts. When the storage tank is fully discharged, this means, completely at the temperature of the cold pipe, the mass flow of water through the storage tank is stopped so that no water overflows from the cold pipe into the warm pipe. As soon as cooling demands in the district predominate, the mass flow of the network at the energy hub flows again through the storage tank and warm water from the warm pipe enters the storage tank at the top. The warm water displaces the cold water flowing into the cold pipe. This type of central heat storage is particularly well suited for districts that have frequently changing net heating and net cooling demands. For example, a frequent change can be observed in the summer when both, domestic hot water demands and cooling demands, occur at different times throughout the day. The more often net heating and cooling demands alternate, the higher the storage tank's utilization. A central storage tank that is hydraulically coupled directly to the 5GDHC network can also function to maintain the pressure in the network pipes.

In the nPro tool, the model combi storage can be used to size and simulate a central storage for 5GDHC networks.

Geothermal fields as storage for 5GDHC networks

Geothermal fields are also suitable for seasonal heat storage. Depending on geological conditions, aquifer storage may also be possible. Geothermal probes provide heat in winter and can feed excess heat into the ground in summer. Geothermal fields are very well suited to supply 5GDHC networks, because these networks can inject surplus waste heat from cooling applications into the ground in summer and thus thermally regenerate the geothermal field. Regeneration in summer causes geothermal fields to cool less over the course of several years of operation, providing higher temperatures to the district heating network in winter. Geothermal fields thus act as a kind of seasonal heat storage.

Decentral heat storages

As a supplement to central storage units, decentralized heat storage units can be installed in buildings which exceed the common buffer storage volumes. The heat accumulators are usually installed between the heat pump and the building energy system. In this way, they increase the operation flexibility of the heat pump. If, for example, electricity from photovoltaic systems is also to be used to operate the heat pump, the building's degree of self-sufficiency can be increased by decentralized heat storage units, since the heat pump can run and charge the heat storage unit primarily during the midday hours. During the rest of the day and at night, domestic hot water and space heating demands can then be met with the heat storage tank.

Sources

  1. Buffa et al.: 5th generation district heating and cooling systems: A review of existing cases in Europe, Renewable and Sustainable Energy Reviews, 104:504-522, 2019.
  2. I. Franzén et al.: Environmental Comparison of Energy Solutions for Heating and Cooling, Sustainability, 11, 7051, 2019.

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