“DISTRICT HEATING GENERATIONS” – CLARIFICATION OF THE TERM

In the old days—i.e., before 2014—the world was simple when it came to describing district heating systems (DHS). The systems were based on steam, hot water, or warm water. The pipes were either laid in concrete channels or pre-insulated, and the supply temperature was from 70°C to more than 200°C. District heating was based on relatively few production technologies.

By Dr. Robin Wiltshire, Chair, IEA DHC programme, Dr. Andrej Jentsch, IEA DHC Programme Manager, Senior consultant Lars Gullev, Vice-chair, IEA DHC programme

Published in Hot Cool, edition no. 5/2024 | ISSN 0904 9681 |

It was simple but became challenging when the discussion turned to further developing the DHS so that it could more effectively answer the future’s climate and resource-related challenges.

However, the publication of the article*) “4th Generation District Heating (4GDH) Integrating Smart Thermal Grids into future sustainable energy systems” in March 2014 established a framework for the continued dialogue about the various DHS and the perspectives on future development.

Today, most DHS professionals know Figure 1, which visualizes DHS generations and classifies them by the typical flow and return temperature, production technologies, and the typical period for introducing the systems. Subsequently, the figure has been refined, but the original primary information remains unchanged.

However, precisely the indication of the time of introduction for the systems – or, as stated in Figure 1, “generation”- has given rise to misunderstandings or overinterpretations.

Challenges with the perception of DHS generations

This visualization supports the notion of desirable transitions, with improved technologies such as pre-insulated pipes that typified 3GDH, enabling systems to run at lower temperatures than 2GDH systems. The emergence of 4GDH is being largely driven by the need to transition to post-fossil DHS devised to make best use of renewable sources and available waste heat, which may also require heat pumps to elevate their temperature. In general, a newer generation will likely increase the potential for improving efficiency and decreasing the carbon footprint.

However, it has also given some people the impression that 2GDH is always considered better than 1GDH, that 3GDH is always better than 2GDH, and that all DHS should undergo a development phase to move to the ‘next level’. This is a misunderstanding, and it is important to recognise that there are some vital services that require higher temperatures, such that even 1GDH based on steam can be an optimal starting point for the green transition.

The following two examples illustrate this fact:

Example 1

In Manhattan, New York, the DHS is based on steam. With the very high heat demand density currently present there, a conversion to district heating with a flow temperature of below 1000C would require the establishment of a network of pre-insulated district heating pipes with much larger dimensions than the system in place to cover the current heat demand.

However, this would not be feasible since there is simply not enough space for such large pre-insulated pipes in the “crowded” underground. Therefore, the most sensible goal would likely be to run a 1GDH as effectively as possible and to change the heat source to a carbon neutral one, such as switching from fossil fuel boilers to heat of deep geothermal origin. This would create a carbon neutral and future-proof solution, even though the 1GDH in Figure 1 can be interpreted as belonging to 1890-1930.

Example 2

A DHS system has a large customer base that uses steam for process purposes. In such a situation, parts of the system must be based on steam, but this does not prevent partial areas from converting from steam to hot water if it is technically possible. Thus, the total DHS will consist of both 1GDH, 3GDH, and possibly 4GDH.

The alternative to this solution would be that the customers who need process steam would have to produce the steam themselves – and possibly the production of steam would be based on fossil fuels. Thus, the overall efficiency of the overall energy supply system would possibly decrease at the same time as the environmental impact would increase.

Need for clarification of the term “generations”.

As confusion has thus arisen around the term “generations” and the “generations” are mixed up with the years in which the generations were introduced, IEA DHC – the hub for international DHC research – has found it necessary to prepare a concise definition of the district heating network generations, which can be downloaded here:

https://www.iea-dhc.org/iea-dhc-network-generations

Fig. 1. Illustration of the concept of different generations of district heating systems.

Conclusion

As is evident from Figure 1, the production side of district heating can be complex. Therefore, the focus in the IEA DHC’s definition of the generations is solely on the characteristics of the district heating network (DHN) – i.e., temperature level:

• 1st generation DHNs use steam.
• 2nd generation DHNs use liquid water above 100°C.
• 3rd generation DHNs provide heat between 100°C and 70°C.
• 4th generation DHNs provide heat below 70°C.

As mentioned before decreasing DHN temperatures often provides several benefits. Therefore, a generations terminology based on network temperatures and fluid characteristics makes sense down to 70°C. Below 70°C additional measures need to be introduced to disinfect domestic hot water. These measures usually cause additional resource consumption and greenhouse gas emissions that can outweigh the reductions achieved by further decreasing temperatures. Therefore, from the perspective of IEA DHC it makes little sense to talk about further generations when DHNs are established with supply temperatures lower than 70°C.

Consequently, the IEA DHC ExCo recommendation is not to use the term “5th generation district heating”, as it can be misunderstood as an upgrade in general to 4GDH. Instead, the IEA DHC ExCo suggests to label pipe networks that are mainly used as a source for decentralized technologies as “thermal source networks” (TSN). These TSNs should be considered a subclass of 4GDH networks.

In summary IEA DHC agrees with using the generations terminology in district heating as a general guideline to which exceptions are possible. In the end any new DHS must minimize overall resource consumption and greenhouse gas emissions for the available budget. In many cases, aiming for 4DHNs is a sensible first step to consider but openness must remain towards choosing a district heating network generation that leads to the lowest environmental impact overall.