Dear Reader,

for issue 42 of our heatbeat Research Newsletter we picked papers on two topics with high relevance for future district heating solutions. Whenever solar energy is considered as an option to be a heat source for district heating, a careful evaluation of whether to use solar thermal or PV in combination with a heat pump is necessary. And as heat pumps introduce a strong sector coupling with the electric grid, considering the flexibility potential of heat pumps is also an important topic in many discussions on optimal system designs. And in addition to these two topics, we included some further recommendations for papers on fault detection and diagnostics as well as a paper on a heat transformation plan for the entire city of Munich.

Solar thermal vs. PV + Heat Pump as Heat Source For Seasonal Storage

In the paper "Solar thermal vs. PV with a heat pump: A comparison of different charging technologies for seasonal storage systems in district heating networks" authors Sporleder et al. aim to determine the optimal solution for utilizing solar energy in combination with a seasonal storage. For the seasonal storage, the authors consider a Pit Thermal Energy Storage (PTES) in order to harvest the solar energy during summer and supply a district heating with the stored heat during winter. The investigation presented in the paper is part of a larger effort to find optimal solutions to cost-efficiently decarbonize district heating networks. For this challenge, the authors present an open-source tool called "Heatopia" for designing decarbonized district heating systems.

For the scope of the paper, the authors focus on the common design decision of choosing either solar thermal collectors or a combination of PV with an air sourced heat pump as a heat source. The presented use case is an existing district heating network in eastern Germany, where the solar energy source is meant to charge the PTES seasonal storage. For this application, the authors conclude that in order to maximize the storage capacity of the PTES, it integrates well into networks with temperatures higher than 70 °C. Regarding the comparison of solar thermal and PV, the paper finds that both require large areas on which to build the solar thermal collector field or the PV. Thus, the economic feasibility of both concepts is very sensitive to the costs for the area on which to build. As the authors find that the space requirement is larger for solar thermal, they recommend PV with a heat pump for more densely populated areas and solar thermal for when there is more cheap space available. Yet, for both cases, the authors find acceptable levelized costs of heat with values ranging between 14 - 17 ct/kWh for the use case.

Flexibility Potential of Heat Pumps in District Heating

One important aspect of using heat pumps as a heat source for district heating is that their electricity demands creates a strong sector coupling to the electric grid. And as heat is easier to storage than electricity, while electricity is easier to transport than heat, this sector coupling opens up potentials for a more efficient energy system overall. In this context, the paper "Flexibility potential of heat pumps in Swedish thermal grids: for district heating companies and end users" by Song et al. presents the perspectives from both operators and customers on this important topic. For this study, larger heat pumps that feed into district heating as well as heat pumps in buildings as a hybrid solution combined with district heating are considered. The authors used interviews to analyze the perspective of district heating companies with heat pumps in their network to gain insights on the demand response of heat pumps both responding to dynamic pricing and for dedicated flexibility markets. Furthermore, they ran simulations in order to investigate the end user perspective regarding optimal operation of hybrid heating systems with heat pumps and district heating.

For the network operators perspective, the interviews showed that the main challenge for the use of heat pumps in district heating are still the high investments costs. Yet, when a heat pump is already present in a district heating network, the experience with its control and the advantage of short start and stop times is positive and offers large potential for flexibility. This potential would include reacting to fast fluctuations in electricity prices, but that would require improvements over the traditional dispatch planning with lower time resolutions.

From the end user perspective, the study's simulations show that hybrid solutions of having both a district heating connection and a heat pump offers flexibility to reduce heating costs. Yet, the authors also document the challenges for the district heating operators when a district heating connection is only used for peak load supply of a building. This highlights that demand response in buildings can have a significant effect on the district heating network and its heat generation and that coordination between centralized and decentralized equipment is required to reach a global optimum.

Further Reading

In addition to the two papers presented above, we want to recommend two other papers which were recently published on the topic of fault detection and diagnostics. Zhou et al. published the paper "Multi-fault diagnosis of district heating system based on PCA_BP neural network" in which they present a neural network trained to diagnose multiple faults in district heating networks. Also using a neural network approach, the paper "Data-driven approach for the detection of faults in district heating networks" presents methods to find anomalies in heat and pressure losses based on time series measurement data in real-time.

Furthermore, Kleinertz et al. publisched a paper titled "Analysis of the cost-optimal heat supply strategy for Munich following a clean energy transformation pathway" which they present a heat transformation strategy for the entire city of Munich.

The next issue of our newsletter will be published on May 1, 2024.