District Heating Research Trends 2021

for our 6th newsletter we are excited to share a special issue with you. Usually, we focus each issue of this newsletter around one outstanding research paper from the field of district heating. For this special issue, we instead reviewed all research works published in 2020 to better understand the state of district heating research in 2021 and analyze current trends. In the following, we will quickly describe our methodology before presenting our analysis in more detail.

Methodology

To get a full picture of as many research publications in the field of district heating as possible, we started our process with a Google Scholar search for the term "district heating" limited to publication in 2020. This search returned 476 results. After removing duplicates, wrong results, non-english-language results and unavailable documents, we were left with 366 valid search results. For each of these research works, we analyzed the title, abstract, and keywords to understand the main focus of the study. Based on this, we classified the studies into several categories from which we derived the following analysis. This allowed us to understand how many of the considered papers addressed certain topics and applied certain methods for their research.

Results

For this special issue, we reviewed all district heating research published in 2020 with the goal of understanding current trends and see which research topics and methods have been of most interest to the community in the past year. The 366 research papers considered in our analysis included mainly Journal articles (248 papers) but also conference proceedings (70) as well as Master (27) and PhD (10) Thesis, and research reports (10).

From our analysis, we learned that most of the district heating research focused their studies on the scale of one or more individual networks (308 papers). In contrast, only a minority of studies investigated topics at national (38) or international (18) scale. Of these larger-scale studies, a larger share focused on e.g. policy and tariff topics while the network-focused studies often investigated the efficiency, costs, and emissions of district heating networks. For the network scale, we also saw that around 26 % of studies further limited their scope to look mainly at the supply plant(s) while around 16 % focused exclusively on the buildings and their substations connected to the network.

Many of the papers we analyzed aimed to improve one or more key metrics about district heating systems. By classifying which metrics were of most interest to the authors we were able to quantify that around 35 % of all considered research papers used economic indicators like the heat production costs as a key metric. By comparison, greenhouse gas emissions were used by around 16 % of all papers as a key metric. Furthermore, we saw that around 9 % considered both economics and emissions in their key metrics for a more holistic evaluation of their research results.

Regarding the applied methods, our analysis clearly showed that a large share (56 %) of all papers used thermodynamic modeling or analysis for their research. In contrast, the share of data driven methods was significantly lower (14 %). Within this subset of papers, we were especially interested in how much the current global trends towards machine learning and artificial intelligence had already reached the field of district heating. In this context, we found 21 studies (around 6 %) which applied some form of machine learning and artificial intelligence. We think it will be interesting to see how improved methods for thermodynamic modeling on the one hand and more available data from smart meters on the other hands will shape these numbers for the coming years.

In addition to methods and topics, we also saw the potential in our dataset to investigate how certain terms and definitions are used within the research community. In this regard, we saw that while 50 papers (around 14 %) investigated similar topics regarding modern district heating networks with dynamic operation and lower temperatures, there was no consensus on how to name such networks. In our dataset, 23 papers used the terms "Low Temperature" and "Ultra-Low-Temperature" for such networks, while 27 papers relied on the common distinction of different "generations" of district heating. From this subset, we found 7 papers which went beyond the term "4th generation district heating" and studied "5th generation district heating" and cooling networks. Again, we will be looking forward to see how these terms will continue to shape the discussions in the coming years.

Beyond these results, we were able to identify other topics with varying levels of interest of the research community. A clear topic that stood out was the integration of renewable heat sources into district heating, which was a focus of an impressive number of 124 studies (34 %). To a large share this included studies on waste heat, solar thermal and geothermal energy. Furthermore, we found that 75 papers (20 %) study topics related to flexibility and improved control strategies for the operation of district heating networks. In relation to both the topics of flexibility and control as well as renewables, we saw 27 studies with a focus on thermal storage technologies. Many of these deal with the challenges of seasonal thermal storage. In addition we found it interesting to see that the analysis of the entire life cycle of district heating systems was studied in 12 papers.

Outlook

Based on this overview of the state of district heating research in 2021 we will make sure to present some of the most outstanding individual studies of 2020 in the upcoming issues of this newsletter. Furthermore, we are looking forward to repeating this review in the coming year to gain further insights into how these trends develop over time. Please contact us if you are interested in further analysis of the topics and methods which are of most interest to you. In the meantime we will also continue to expand our analysis and we're looking forward to sharing more of our insights with you.

The next regular issue of our newsletter will be published on May 5, 2021.