District heating systems have a long tradition of providing cost-effective heating in Northern Hemisphere cities and are considered a key technology in the shift towards renewable heating and cooling.
However, Australia’s low demand densities have historically made for an unappealing financial proposition at our latitudes. That is set to change with the advent of fifth-generation district heating and cooling, a paradigm-shifting district energy system that promises to disrupt business-as-usual heating and cooling, including, or perhaps especially, in countries like Australia.
The evolution of district heating systems
Typically, a district heating system consists of a centralised plant producing hot water at the desired temperature and distributing it to a cluster of buildings for direct heating, hence the term district heating. District cooling systems follow a similar principle by producing and circulating chilled water to the end users. The first district heating systems were introduced in the US in the 19th Century, though, arguably, they could be traced back to Chaudes-Aigues in 14th Century France or even further back to the geothermal hot water baths of the Roman Empire.
Historical debate aside, what is clear is that, since their inception, district heating systems have seen a remarkable evolution, constantly improving efficiency and reducing thermal losses. Yet, until recently, for a viable system, high demand densities were required to minimise thermal losses in the distribution network. This has limited their diffusion to high demand density areas such as Central and Northern Europe, North America and China for district heating and Middle Eastern countries for district cooling.
Fifth-generation district heating and cooling or 5GDHC
Fifth-generation district heating and cooling systems, known also by the acronym 5GDHC, promise to push the envelope of what district heating means and capture untapped opportunities for efficient and low emissions heating and cooling. While the term fifth-generation implies continuity with previous generations, it understates the extent of the novelty that the latest generation brings to the table. Revolution, rather than evolution.
Starting from the architecture, in contrast to previous systems, in 5GDHC there is no single centralised energy source and the water circulating in the network is at near ambient temperature. The effect of this is twofold; first, it decreases thermal losses making the system viable in low-demand density contexts, and second, it makes it possible to integrate low-grade waste heat and renewable sources helping us reach our climate targets.
However, the water temperature is not suitable for direct heating. Instead, each user’s substation is equipped with reversible water-source heat pumps that raise the water temperature to the required level. “Reversible” is the key word here, as the same heat pump fuelled by the same network can provide cooling as well. This has clear financial implications by significantly lowering capital costs as one system can now provide the service that previously required two distinct systems. Additionally, from an engineering perspective, this means that energy flows in the network are now bidirectional, as opposed to the unidirectional flow of previous systems.
To illustrate this further, it helps to think of heating and cooling as two sides of the same coin.
As a matter of fact, it is the exact same process, to heat our buildings we usually burn natural gas, or if we are already environmentally conscious, we use a heat pump to extract heat from the ambient air and inject it into the building. In summer, we use a chiller to move the heat in the opposite direction from the building towards the air.
If this seems like a Sisyphean task, that’s because it is. 5GDHC systems offer a solution for harvesting, storing, and reusing this otherwise wasted heat, by sharing or trading it with other users through the network or storing it for later use. This changes the dynamics and the hierarchy within the system, as now, a user cooling down the building is producing waste heat making them not just a consumer but a producer as well. A phenomenon referred to as prosumer by the industry.
Currently, Europe is at the forefront of the implementation of 5GDHC with more than 40 systems already active and more planned in the next few years. Among the most prominent examples include:
- ETH Zurich’s Hönggerberg campus in Switzerland, where a 5GDHC system called energy grid, consists of three borehole storage areas and five substations connected through a thermal loop supplying 14 buildings with heating and cooling. The temperature of the warm water loop varies from 8-22 degrees Celsius while that of the cold supply loop ranges between 4-18 degrees Celsius. Depending on the operation, substations exchange thermal energy between them via the loop, making it possible to use waste heat from one cluster as a source for other clusters or store it in the underground storage, if it is not immediately required.
- The Minewater system in Heerlen, the Netherlands, where a flooded decommissioned mine is used as seasonal energy storage and the required heating is provided by multiple decentralised sources like waste heat from the cooling systems of the connected buildings, data centres, supermarket refrigeration systems and general industrial processes. The use of decommissioned mines can be an opportunity for Australia as well, and research is underway to determine its potential.
Potential roadblocks
The prosumer concept is well established in the power sector and there is a clear regulatory framework for selling excess electricity to the grid, and peer-to-peer (P2P) electricity trading is an emerging research topic. However, as is often the case, technological progress outpaces the legislative regulatory process, and in the heating and cooling sector, such a framework is missing. Researchers are, again, looking at technology as a saviour and have proposed various solutions, including blockchain, to securely and transparently trade energy.
For decades, Australia has been a stranger to district heating and cooling systems, perhaps for good reason. However, as there is unlikely to be a silver bullet for achieving our net zero targets, we must be ready to embrace a broad range of solutions to move away from our dependence on fossil fuels.
