5GDHC in short

The two-pipe grid is not configured like for a 4th generation heating grid, which has a ‘supply line’ to deliver heat, and a ‘return line’ back to the heat source where it gets heated up again. In a 5th generation grid, there is a ‘warm pipe’ and a ‘cold pipe’. The exact temperature inside these pipes is not fixed, but is allowed to drift up and down within a certain range. Energystations needing heat will take water from the ‘warm pipe’, extract heat, and return cold water to the ‘cold pipe’. Cooling is provided the other way round. The pumps in the energystations along the grid will move water from the warm to the cold pipe, or vice versa. That means that both the size and direction of the waterflow in the two pipes is not fixed, but simply the sum of all the flows demanded by all the energystations. The energystations always increase the temperature differential between the two pipes of the grid. The grid transports energy and the needs for heating and cooling power will automatically cancel each other within the local grid. At the level of single buildings, it is possible to reduce energy needs by various measures. At the level of a district, the grid adds the new opportunity to reduce energy consumption: by exchanging energy between customers who need either heating or cooling. A demand-driven grid facilitates such exchange for low temperature heat, and higher temperature cooling. The added value of the grid is that the flow of thermal energy to the heatpumps results in a high enough systemic seasonal COP to make the investment worthwhile.

A network like this would be able to function, if the demands for heat and cold would always exactly cancel each other. At any point in time, this may happen, but it is unlikely. The next step is to add sufficiently large thermal storage reservoirs in which surplus heat and cold can be stored and kept for a later time when it can be utilized. At the point where thermal storage is connected to the grid, it can balance demand flows at a heat exchanger where any surplus from the ‘cold’ side is converted to ‘warm’, or vice versa. Effectively, storage acts like a pipe that takes surplus energy from the present to exchange it with the future.

Storage can be large and centralized, but also smaller and distributed over locations nearer to the points of demand. Different storage facilities serve the grid over different time scales. The domestic hot water boilers are an example of very short term storage that helps to guarantee a sufficient capacity in moments of large demand, like when many people want to take a shower. The grid itself and buildings also have a significant capacity to store heat and stabilize the temperature. Aquifer Thermal Energy Storage (ATES) can store heat and cold over medium time scales, but also seasonally. At Mijnwater, the energystations are combined with buffertanks to store heat and cold, providing sufficient capacity to cope with moments of high demand. The Mijnwater grid in Heerlen has the advantage of having a huge thermal storage volume consisting of the disused coal mines under the town, filled with water. Both the mine water, and aquifer thermal storage have the additional advantage that they get replenished over time with geothermal energy.

Heating and cooling will never be without any loss of energy, and it is not always possible to balance the demand for heat and cooling power, even when averaging over the season. It can however be assumed that most towns currently have sufficient quantities of waste heat and cooling power, while in addition there is also a large potential for renewable low temperature solar heat. Such sources of heat will be needed to balance and replenish the long term thermal storage. But these sources will never need to play the role of past centralized thermal plants that had a high capacity to cover for all possible peaks in demand.

It is not necessary, but probably useful. In the specific implementation of Mijnwater, the source wells connecting to the warm and cold sides of the mine are about 8 km apart, and these had to be connected with the original two-pipe ‘backbone’. The design of Mijnwater grid is hierarchical: the ‘backbone’ is connected to 4 ‘cluster grids’ via large heat exchangers in a so-called ‘cluster-installation’. As necessary, energy from the mine water is used to balance the demand for heat and cold in each cluster grid. The ‘backbone’ has two functions. On the one hand it is the direct interface to the mine water reservoir, and it is pressurized to prevent minerals in the mine water from depositing and clogging up the system. On the other hand it exchanges any surplus energy between the clusters before the final surplus of heat or cold gets stored in the mine.

 Other 5GDHC grids without a mine water reservoir may function well without a hierarchy between a ‘backbone’ and ‘clustergrids’. One thing is clear: new grid topologies may be quite different from the traditional tree structure starting with a very wide pipe at the central heat source, splitting up into ever narrower pipes from which finally separate dwellings are connected. The pipes might all have the same diameter. The topology can be linear, with a few branches, possibly contain a closed circle, and even evolve to a mesh topology if that provides the right connectivity for optimal local energy flows.

Energystations with heatpumps and thermal storage tanks will take up more space than the traditional substations with no more than heat exchangers and metering equipment. The consequence is that it makes sense to for an energystation to serve a large building, a mall, or a small area grid of dwellings. The Mijnwater grid in Heerlen is currently deploying the first such small area grids, using the term of ‘sectorgrid’. It is powered from a prefab ‘sectorbasement’, containing a prefab skid with an energystation large enough to serve a large apartment block, or a street block with 100-200 homes. At this local area scale, the decision was made for a more conventional 4-pipe area grid for heating and cooling. Domestic hot water is still produced at each dwelling with a separate booster heatpump and buffertank.