Realistic Domestic Hot Water Load Profiles for District Heating Networks
we are happy to present to you the second edition of our heatbeat Research Newsletter. In our first issue we looked into the integration of waste heat and solar thermal energy into district heating networks. In this issue we shift our focus to the demand side, where heat demands for domestic hot water (DHW) play an increasingly important role. The paper we selected for this issue explores methods to generate realistic DHW load profiles investigates the influence of different substation types on DHW demands, losses, and return temperatures.
The short version
We dedicate our second issue to the paper District heating load profiles for domestic hot water preparation with realistic simultaneity using DHWcalc and TRNSYS by Hagen Braas et al. from the Department of Solar and Systems Engineering of the Institute of Thermal Engineering at the University of Kassel. It presents methods to model the heat demand for domestic hot water preparation in district heating networks with realistic load profiles and simultaneities. In addition, it examines different substation types and their influence on the return temperatures for the district heating network.
On the one hand, the results show that domestic hot water profiles with realistic simultaneity factors can be generated with the free software DHWcalc. On the other hand, simulations of the substations show that a direct preparation of domestic hot water can achieve lower average return temperatures than systems using a DHW storage in the building. At the same time, as expected, there are significantly higher peak loads without a storage. The paper also shows the relation between heat demand and distribution and storage losses for all investigated variants.
From our point of view, the researchers are addressing an important topic in the analysis of district heating networks. In many cases, static load values are not sufficient for the interpretation and the evaluation of the network operation. The paper offers solutions for this by generating individual stochastic tap profiles for each building. In addition, the paper analyzes the DHW heat demands and heat losses as well as the simultaneity factors in detail and compares the results with other sources. The work thus provides good reference values for further applications.
Our full summary
The paper District heating load profiles for domestic hot water preparation with realistic simultaneity using DHWcalc and TRNSYS by Hagen Braas et al. addresses important questions about the influence of the domestic hot water demand in investigations and evaluations of district heating networks. Especially when considering the decreasing space heating demand due to higher building insulation standards and milder winters, the DHW demand is playing an increasingly important role. In this research paper, a special focus is placed on three aspects:
- Heat demands and heat losses: For different building sizes and different substation designs, the demands for DHW are determined and compared to the distribution and storage losses
- Return temperatures:Simulations of different substations designs investigate their impact on the return temperatures for the district heating network
- Simultaneity: The paper presents methods to generate stochastic load profiles for DHW with realistic simultaneity factors. This means that significantly better results can be achieved than with static standard profiles.
As the title of the paper already indicates, the researchers work with the two software solutions DHWcalc and TRNSYS. DHWcalc is free software that is developed at the Department of Solar and Systems Engineering of the Institute of Thermal Engineering at University of Kassel and is offered as a free download. This software has already been described to be a well-suited solution for the generation of domestic hot water profiles in several scientific publications. With these profiles, natural variations of the tap water discharge can be modelled realistically.
With the simulation software TRNSYS the authors model different substation designs. These models take the tap profiles created with DHWcalc as input data and simulate the operation of the substations and their interactions with the district heating network. Of particular interest are the return temperatures that can be achieved at given supply temperatures with different substation designs.
The authors model and simulate 4 different substation designs for heat transfer between the network and buildings. A distinction is made between substations for single-family houses and for multi-family houses. These models differ mainly in the single-family house being connected to the district heating network with a single heat exchanger, while the substations for the multi-family house have 2 heat exchangers. By dividing the heat exchange into a pre-heater and an after-heater, the network return temperature can be lowered when the building system is in circulation mode in order to achieve greater efficiency. One variant with and one without DHW storage is analyzed for both the single-family house and the multi-family house. For all 4 variants, a constant supply temperature from the district heating network of 70 °C is used.
With this setup, the paper yields interesting results for the various investigated variants: From the simulations, the researchers determine specific DHW heat demands between 8.5 and 11 kWh/m²a for different single-family houses. The values for the considered multi-family houses lie between 11.7 and 12.5 kWh/m²a. In relation to this, the simulations determine distribution losses in the building of 8 - 13 kWh/m²a. In the variants with DHW storage, the storage losses vary significantly, with values up to 14 kWh/m²a for small single-family houses ranging to below 1 kWh/m²a for larger multi-family houses.
In addition to these values for demands and losses, the paper also analyzes the return temperatures that can be achieved for the various substation designs. The paper shows that the substations without a DHW storage can achieve annual averages for the return temperatures that are around 8 - 9 K lower than for the substations with a DHW storage. As further means to achieve lower return temperatures, the researchers mention stronger pipe insulation in the building and avoiding the circulating of hot water, which leads to high return temperatures during reheating.
Another important aspect of the paper is the simultaneity of DHW demands in a district and the influence of different substation designs on the peak loads. For a small single-family house without a DHW storage, the main DHW heat demand is determined to cover less than 300 hours in the annual duration curve, while the circulation mode extends over about 6000 hours. While, as already mentioned, lower return temperatures can be achieved for the system without DHW storage, a peak load of 42 kW is specified for this variant. In contrast, a peak load of 3 kW is found to be sufficient when the storage is charged under ideal conditions in the model.
With regard to simultaneity, the paper shows a comparison between its own results and different values from the relevant literature for single-family houses with and without DHW storage. Consistent with the values from the relevant literature, it can be seen that the simultaneity factor with storage decreases from the initial value of 100 % for one building to about 40 - 50 % for 20 buildings and to about 30 - 45 % for 100 buildings. Without storage, the effects are more pronounced with a factor of around 15 - 20 % for 20 buildings and simultaneities of around 10 % for 100 buildings. In addition, the investigation shows that on hourly resolutions the results change significantly compared to the original resolution of 3 minute intervals and thus the selected frequency has a strong influence on the results.
From our point of view, the paper shows a good overview and many interesting aspects for the evaluations of DHW demands in the context of district heating networks. Even though the demand for space heating is not considered in this paper, it offers good orientation and many reference values for a holistic view of thermal network operation in a district. And since, as mentioned at the beginning, higher building insulations standards and warmer winters lead to a stronger influence of DHW demands on the network operation, DHW is playing an increasingly important role in such systems. Therefore, these results are of particular importance for the future. Our project experience at heatbeat, in which the influence of the DHW demands in the dynamic simulation and holistic evaluation of district heating networks is increasingly evident, confirms this trend. In our projects, we place a high importance on considering buildings as an integral part of district heating networks. This enables us to consider the interactions between DHW demands, return temperatures and network efficiency in detail.
Further information
The original article can be found at https://doi.org/10.1016/j.energy.2020.117552. Further information as well as the free download of DHWcalc and the associated manual can be found on the homepage of the Department of Solar and Systems Engineering. A newer version including features used in the paper is currently in development. For interested parties, the authors offer to provide a pre-release version when contacted via email.
Furthermore, the authors of the presented paper have published another paper titled Systematic investigation of building energy efficiency standard and hot water preparation systems’ influence on the heat load profile of districts from Best et al. It addresses the influence of building insulatinos standards and the type of DHW generation on the heat demand profile of districts in detail.
Feedback on the second issue
At the end of this second edition, we would like to invite you again to give us feedback so that the newsletter can offer you the greatest possible value. Do you have any ideas on how we can make the newsletter more valuable to you? Then contact us at newsletter@heatbeat.de
The next issue of our newsletter will be released on January 6, 2021.
