4th or 5th generation district heating - That is the Question

new and innovative concepts are currently being developed for existing and new district heating and cooling networks. The goal is to integrate the highest possible share of renewable energy sources or waste heat into thermal networks. It is noticeable that often similar or even same concepts are named differently, and new terms appear frequently. Very commonly used terms are "4th generation district heating" or "5th generation district heating and cooling".

But what exactly are the differences between 4th and 5th generation district heating? How can concepts and future developments be characterized consistently? Exactly these questions are discussed by M. Sulzer et al. and H. Lund et al. in recently published papers, which we would like to present in our 9th heatbeat Research Newsletter.

The short version

This time we present two articles:

• Vocabulary for the fourth generation of district heating and cooling by Matthias Sulzer et al. (EMPA – Swiss Federal Laboratories for Materials Science and Technology, Switzerland)
• Perspectives on fourth and fifth generation district heating by Henrik Lund et al. (Department of Planning, Aalborg University, Denmark)

District heating networks are classified into different generations. Until now, the generations 1 - 4 existed in the literature. In recent publications, the term 5th generation district heating and cooling appears more often.

The presented articles discuss the classification of new concepts into 4th and 5th generation district heating. Both authors conclude that the term 5th generation district heating and cooling can be misleading, since according to common definitions the requirements for a new generation are not fulfilled. Furthermore, in contrast to 4th generation district heating there is no uniform definition yet.

Sulzer et al. argue that there are no changes in the objectives (i.e. in particular decarbonization of heat supply) compared to the fourth generation and new technologies should be developed under the name 4th generation district heating and cooling.

Lund et al. highlight the advantage that the term 5th generation district heating and cooling has already been established in many scientific publications and projects. Thus, higher attention, especially in countries with low market penetration of heating networks, can be achieved. However, Lund et al. refer to 5th generation heating and cooling networks as a parallel development and not a sequential replacement.

Both papers open the important discussion on how to characterize and label new concepts and technologies for thermal networks consistently. This is a crucial step for a common understanding of new technologies and further development of thermal networks.

Our full summary

To achieve the goals of decarbonizing of heating and cooling supply, many new technologies and concepts for thermal networks are currently being developed. We have already shown this trend in a cross-analysis of scientific publications in the 6th issue of our newsletter in April. (https://heatbeat.de/newsletter/6/)

This broad development is necessary to meet different local requirements and to make optimal use of local conditions. In addition to new technologies, many new terms appear. A very common term is 5th generation heating and cooling networks. The papers Vocabulary for the fourth generation of district heating and cooling by Matthias Sulzer et al. (EMPA - Swiss Federal Laboratories for Materials Science and Technology, Switzerland) and Perspectives on fourth and fifth generation district heating by Henrik Lund et al. (Department of Planning, Aalborg University, Denmark) discuss the labelling of different concepts for thermal networks.

District heating networks are historically divided into different generations (1st - 4th generation). The requirements for a new generation are described by Lund et al. as follows:

To make a classification of new concepts into different generations, it is first necessary to consider how the terms 4th generation district heating and 5th generation district heating and cooling are defined and which requirements must be fulfilled to classify a technology into a new generation.

Lund et al. summarize the main characteristics of 4th generation district heating (4GDH). In contrast to generations 1 - 3, 4GDH aims to keep the temperature in the system as close as possible to the end user's temperature requirements, with the maximum temperature not exceeding 60 - 70 °C. This not only promotes the integration of renewable energy sources (e.g. solar thermal) and waste heat ("heat recycling") into the system, but also enables more efficient use of heat pumps and combined heat and power plants. For 4th generation district heating, it is essential that efficiency improvements can be achieved by coupling sectors. For example, heating and cooling networks can be coupled by means of a heat pump and combined with storages. Also, unlike previous generations, buildings can consume energy from the network while other buildings feed heat into it. Sulzer et al. discuss this and conclude that the terms "producer" and "consumer" cannot always be clearly assigned in new concepts. In addition to the technical aspects, Lund et al. list the possibility of adapting to new "smart energy systems" through appropriate planning, cost, and incentive structures.

As shown in the previous section, 4GDH are not characterized by their temperature level alone, but have a variety of attributes that distinguish them from 3rd generation district heating. These definitions are well established and accepted. Sulzer et al. use identical definitions for 4th generation district heating. Both contributions place the overarching goal of fossil fuel substitution and decarbonization of heat supply, respectively, as a key characteristic of 4GDH.

Both authors point out that, in contrast to 4GDH, there is not yet a uniform definition of the term 5th generation district heating and cooling (5GDHC). Both publications acknowledge that the lowest common denominator is the simultaneous use of heating and cooling using one infrastructure. This requires very low network temperatures, which are increased to the required temperature level in the buildings using heat pumps close to the building (heating mode) or, if necessary, additionally cooled (cooling mode) (see also heatbeat Research Newsletter Issue 3 - January: https://heatbeat.de/newsletter/3/).

Sulzer et al. cite several references that classify thermal networks in 5GDHC based on network temperature. In addition to the synergies between heating and cooling and the network temperature, Lund et al. identify the integration of low-cost heat sources at low temperature levels and the possibility to expand the network flexibly, since a central supply is no longer crucial, as further essential new ideas of the 5th generation district heating and cooling.

But can these new developments really be classified into a new generation, which would also mean that the fourth generation will be replaced in the foreseeable future? Both Lund et al. and Sulzer et al. conclude that the term 5th generation district heating and cooling does not fit into the existing classification of different generations of district heating networks. In particular, Lund et al. points out that a significant increase in efficiency (with respect to heating) is not given (Lund et al.). Sulzer et al. discuss that the common overarching goal of decarbonization (Sulzer et al.) speak against the introduction of a new generation.

With their publication, Sulzer et al. mainly argue for a common vocabulary for district heating research and engineering. They propose to push new developments under the term 4th generation district heating and cooling. According to Sulzer et al., a new generation is only given when the overarching goals for the development of new technologies and concepts change. However, Lund et al. see the term 5th generation district heating and cooling as an opportunity to increase efforts for research and development of new concepts, especially in those countries with few district heating networks so far. They emphasize, however, that 5th generation district heating and cooling is not a sequential development to 4th generation district heating. Rather, it is a parallel development and will never replace 4GDH.

Both articles provide good arguments for or against the use of the term 5th generation district heating and cooling. Thus, the articles open the important discussion on how to characterize and name new concepts and technologies for thermal networks in a uniform way. This is a crucial step for a common understanding of new technologies and further development of thermal networks. At heatbeat, the labels of different technologies are a constant source of debate, both internally and externally. We therefore very much welcome the international efforts to develop a uniform understanding and standardized terms.

Like the authors, we believe that diverse and creative concepts for thermal networks are necessary to achieve climate goals. However, we also realize that the diversity of new concepts and their terms is often a challenge. We are happy to support you in developing and planning the best energy system solution for your project. Please contact us!

Further Information

The article by Sulzer et al. is freely available at https://doi.org/10.1016/j.segy.2021.100003, and the article by Lund et al. can also be obtained free of charge at https://doi.org/10.1016/j.energy.2021.120520. We recommend reading both articles in full.

M. Sulzer et al. list several examples of commonly used but not uniformly defined terms related to new thermal network concepts. They thereby demonstrate the need for a uniform vocabulary for thermal networks. Lund et al. use several literature examples to show the ambiguous definition of 5th generation district heating and cooling, supporting the call for a unified definition.

The next issue of our newsletter will be published on August 4, 2021.