5GDHC in short
Definition of the
standards of 5GDHC
© Mijnwater B.V, Heerlen & Pascal Moors
Definition of the
standards of 5GDHC
A 5th generation district heat and cold (5GDHC) grid is based on the exchange of thermal energy between buildings with different needs. The principal grid carries a low temperature flow to active and distributed substations which upgrade the temperature to the required level. Distributed thermal storage buffers the fluctuation in supply and demand of heat and cold. This architecture maximizes the share of low-grade renewable and waste energy sources.
Prerequisites for 5GDHC
- A close to ground fluctuating grid temperature to avoid thermal losses
- The ability to address at least two different loads: Heat and cold needs at low temperature difference with the ambient temperature
- Energy sources having low temperature difference with the ambient temperature
- Power grid integration to enable thermal storage of surplus electrical energy generated by renewable sources
- 100% renewable energy target
- Closed energy loops target
- A physical energy source, distribution pipes, pumps, thermal storage, active energy substations with heat pumps and heat exchangers
- An open access network for suppliers
Advantages of 5GDHC
The 5th generation of district heating and cooling (5GDHC) is a fundamentally new concept based on a decentralized network allowing direct energy flows among and within buildings, not requiring a central heat plant. Its key features are: 1) low exergy grid utilizing low temperature heat sources; 2) closed thermal energy loops ensuring hot and cold exchange among clusters of buildings; 3) integration and synergy between thermal and electricity grids. These features enable 5GDHC systems to be solely fuelled by RES.
The decentralized, low-temperature grid with shorter transportation distances reduces distribution losses to 5% compared to 25% typical for conventional heat grids.
The system allows integration of thermal & electricity grids that enables storage of surplus electrical energy generated by RES.
The concept promoted by D2Grids is demand-driven even in the early design phase, as it is configured in such a way that H&C demand and supply profiles of connected buildings are balanced as much as possible. During operation, advanced demand-side management is applied (tackling peak load and demand balancing), which ensures that the system only provides the required temperatures at the right time & place (demand adapted supply) with low grid losses.
5GDHC is the most suitable technological model for increasing the share of renewables in the H&C sector.
Moreover, 5GDHC is a very adaptable system which can be implemented at a small scale first and then be extended, according to the needs for heating and cooling.
The hardware solution: How does a 5GDHC work
The following description of the hardware solution is heavily based on the Mijnwater district heating and cooling grid in Heerlen, with its large combination of a geothermal source and thermal storage in the mine water of the disused coal mine under the town. It is likely that networks in other locations with other local conditions will make somewhat different choices, but very similar solutions should be possible nearly everywhere.
Distributed Heatpumps - no centralized energy source
In 4th generation heating grids, the heat from the grid is delivered to the customer via a ‘substation’ that contains one or more heat exchangers. In a 5th generation grid, these substations have heatpumps, and become ‘energystations’. The heatpumps are there to generate the required temperature, right at the point of demand. Domestic hot water is provided by an additional boiler with a booster heatpump, that forms an integral part of the system. The important consequence is that the grid transports thermal energy, but it does not need to deliver the required temperature. Every building gets exactly what it needs, nothing more. Since these heatpumps are bidirectional, the energystations can deliver both heating and cooling power. The grid and the heatpumps can deliver this service at a very high systemic Coefficient of Performance (COP), which reduces the electricity consumption to a level where it can ultimately be provided from sustainable sources. The heatpumps guarantee a high level of systemic robustness, since they can always deliver more heat, as long as the returned cold water remains well above freezing.
Demand-Driven Two-pipe Grid designed to Exchange Energy
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.
Storage of heat and cold
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.
These three main ingredients are approximately sufficient to define the basic hardware features of 5GDHC, in which the focus if moved away from the energy sources and towards the demand. However, we need to add a few other technical preconditions, that are also essential to make the grid work:
Improvement of the buildings
They are essential for the heating/cooling system. Since the 5GDHC heating and cooling grid must function at ‘ultra low temperature’, it is necessary that buildings adapt their delivery system for heat and cooling to low temperatures. The lower the demanded temperature, the more efficient the heatpumps can be. In addition, buildings can reduce their overall energy demand with high quality insulation and by implementing ventilation with heat recovery. It is important to optimize the balance of investments in the buildings against the investments for the grid.
A hierarchy in the grid topology
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.
Local area networks, the ‘sectorgrid’
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.
The 5GDHC grid has a hydraulic setup that is already hard-wired to share energy between customers, reducing the energy needed. Adding computerized control adds new advantages to the system, both for the capacity of the thermal grid, as to optimize its demands on the electric grid (a 4DHC grid can do this too):
The system becomes much more powerful with a smart computerized control system that optimizes the use of storage at all levels. This allows the system to deal with peaks in the demand for heat or cold much more efficiently. The smart control increases the robustness of the system, makes more efficient use of the pipes in the grid, as well as the heatpumps. This means that more customers can be connected with the same hardware, spreading the costs over a larger number.
System control, delegation, and trust: include the buildings too. The 5GDHC grid contains many elements that can function as thermal storage, but one should also add the buildings themselves, that have a significant heat mass. An optimal systemic use of all these elements together depends critically on smart control mechanisms. This control does not necessarily need to be completely centralized, but in a robust design it is good when control gets delegated as much as sensible to smart components. An important example are buildings, each with their individual building management system, or a smart thermostat. To optimize the system as a whole, the 5GDHC grid management system will need to communicate with buildings to request certain behavior that is good to help optimize the system as a whole. But building owners need the ability to override that request if they have their reasons. If the average building can react to central requests, the whole system will be more efficient, and all building owners will profit from lower prices. Any automated management system that includes the buildings will need to be set up in a way to properly delegate control. Private data can be kept private, and secure. Building owners need to trust such a system, while retaining sufficient control for any special occasions.
Such a smart control system will reduce maximum peak demand on the heatpumps, and will thereby lower the peak electricity usage. But it can do more: it is possible to exploit moments of excess electricity production from sun and wind, storing the energy in the form of heat/cold. If weather predictions foresee a period of scarcity on the electrical grid, it is possible to prepare by preheating buildings themselves, water boilers, the water in the grid itself, and other storage.