In Europe, heating and cooling represent 50% of the total energy consumption for buildings and the industry. 5th generation district heating and cooling (5GDHC), a highly optimised, low-temperature grid, is a promising solution to decarbonise our building stock. The D2Grids project aims at accelerating the roll-out of 5GDHC grids in Europe, through an industrialisation of the system, the creation of solid business plans, and the development of 5 pilot sites. The D2Grids project team has recently reached an important deliverable, which is the definition of the generic technology model of 5GDHC. To ensure a flexible and resilient energy network to meet current and future needs, a 5th generation heating and cooling system (5GDHC) is established on the following five principles. This article will help you understand the 5 main aspects of a 5GDHC grid.
Closing the energy loop
The first principle of 5th generation DHC, is to prevent energy waste within the system. A 5th generation network is indeed distinguished from other DHC networks by its capacity to exchange energy to other connected consumers/customers: it is a circular thermal network.
The network is conceived as a closed loop, which reuses energy as much as possible: heated buildings deliver cold to the DHC system, while cooled buildings will deliver excess heat to the network.
The key factor in the 5GDHC approach is the optimal re-use of the return flows at differing spatial and time scales. Energy storage is used to manage temporal imbalance between offer and demand within the system, on a daily, but also on a seasonal basis.
For instance, we see often heat pumps being applied in green building concepts to generate the demanded supply temperatures. Those heat pumps will take their input from outside air, surface water or soil. On the one hand this needs investments in source facilities, which also need space and might induce hindrance. On the other hand, the outcoming energy is lost to surrounding environment. Within the 5GDHC concept the backside of those heat pumps is connected to the grid and as such the return flow of energy conserved in the system.
This fist principle of 5GDHC is implemented in a bottom-up approach, by first exchanging energy within a building/complex, second on cluster/neighbourhood level, and finally at city/district level. Further optimization is also possible through efficient spatial planning.
Using low-grade sources for low-grade demand
Energy sources can be classified according to their application opportunities. Foundational to 5GDHC is looking at the necessary heating and cooling demands of the network, and only bringing energy into the system at the quality level that it is needed. A 5GDHC system does not need many high-grade (high exergy) sources to meet most of its heating and cooling demand. This means that the available flows of low-grade energy sources (like shallow geothermal, industrial waste flows, conversion waste, waste from cooling processes, sewage etc.) get prioritized for bringing any additional heating or cooling not met by exchange within the system.
To optimize the 5GDHC goals, a ranking by preference is proposed for the types of input energy to be used in a 5GDHC heating and cooling network. The first and highest priority is given to thermal energy that is exchanged between users. Following at position 2 are ambient thermal sources, but also renewable thermal sources with temperatures higher than the typical ‘warm’ temperature of the grid. Last energy source in the energy ranking comes from grey electricity, produced by fossil fuels.
The second principle of 5th generation district heating is thus about matching those available low-grade sources to the low-grade demand. The reduced demand in high-grade energy can then be covered 100% with renewable sources like deep geothermal, windmills, solar, hydropower, and biomass.
Decentralized & demand-driven energy supply
Traditional energy systems are centralized and circulate a lot of energy which is in fact never used.
On the contrary, 5GDHC systems are “demand-driven”, which means that they start generating and circulating energy only if a demand occurs. No energy wasted: it is produced only when and where needed. This means that temperature raises are only done at the needed time and location.
The system can simultaneously deliver heating and cooling services at different temperatures to different customers, exactly as demanded.
In practice, 5GHDC operates a shift from large centralised (often monopolized) plants to a cloud of small end-user appliances which co-operate in a smart intelligent network. In thermal heating and cooling world, it is a technology shift comparable to the evolution in the IT sector
An integrated approach of energy flows
Many energy systems contain split incentives, which means that they do not optimize on the integral need across systems and sectors. For instance, a building owner powered by electricity inducing high peaks on the regional power grid, might want to heat up in the morning after night setback. Utilising thermal mass and buffers might ask extra investments for the building but achieve major savings on the power grid. The objective of 5GDHC is to maximize optimum efficiency of energy delivery and use. This is made possible through an integrated approach to all other energy flows in a given area, like power grid, transport, industry, agriculture, etc.
Indeed, huge losses within some sectors can be deployed to serve others. Large power plants which waste a large amount of low grade cooling energy, could cover the building heating demand. The transport sector vehicles also have low efficiencies, losing a lot of heat and new technologies, like hydrogen conversion, have high thermal losses. Integrating those different sectors can bring large savings on total energy balance.
In addition to enabling energy savings, integration to the electricity grid of a 5GDHC network will help balancing the electricity grid and increase its flexibility. This leads to smaller, more efficient energy infrastructure, with less embedded energy in materials and operation. The system, by operating with smaller peak capacities, will also need lower overall investments in infrastructures (pipelines, pumps, heat pumps, etc.) and reduce the demanded capacity of the electrical power connection.
Local sources as a priority
Too often, energy plans are made at a large scale and considering big plans with distant energy sources. This final principle of 5th generation district heating and cooling is one of common-sense: prioritizing what can be provided locally above more distant sources.
Local sources limit transport losses and prevent energy waste. But the benefit is also about costs. Indeed, big plans are often made on large scale with distant energy sources, without taking into account societal costs, which are finally costs for the end-users. Local sources mean localized energy and investments to benefit the local area of the network.
The perspective of filling up the Sahara with solar farms and filling the seas with windmills, might probably be an unavoidable strategy. However, in the ranking of options 5GDHC is about prioritizing local waste sources, before considering investing in distant (and often foreign) sources with all the challenges of needed infrastructure, transport, dependencies, spatial imbedding, etc.