InteGradeDH – Large-scale integration of low-grade sources into district heating networks through geothermal seasonal storage and heat pumps
InteGradeDH aims to contribute to the identification of the best strategies to support existing DH (district heating) networks in achieving the transition from fossil fuels to more sustainable and independent/local sources. This will require large-scale installation of RES (renewable energy sources) and/or WH (waste heat). Except for biomass – a sustainable use of which, however, should follow the cascading principle and hence encounters availability limits – all of these sources involve issues related to temperature level, low energy density, space occupation, and fluctuating behavior.
For WH in particular, many sources are at a temperature level below the needed user temperature. Low temperatures can be upgraded through a heat pump (HP), which is however a component with a significant investment cost and hence needs a large number of yearly operation hours to be paid back. Hence, there is a conflict between the target of recovering all the available WH to reach economic sustainability and the absence of demand in summer. Moreover, the available WH/RES sources might not be enough to cover the winter peak load of the network.
These aspects have raised interest in seasonal storages, which would be ideal to absorb the excess heat in summer and reuse it to cover winter peaks. The most prominent options are given by pit thermal energy storages (PTES), borehole thermal energy storages (BTES), and aquifer thermal energy storages (ATES). It has to be remarked that borehole and aquifer applications can also be used as sources, offering an additional degree of flexibility. Moreover, they are advantageous with respect to PTES when open space is limited. Restricting the comparison among these underground solutions, BTES typically has a higher replication potential than ATES, due to the quite high hydrogeological, environmental and legal requirements of the latter. Finally, since the undisturbed ground temperature exhibits a typical vertical gradient of about 0.03 K/m, borehole systems – by varying the drilling depth – can in principle exploit a broad range of temperatures. On the other hand, significant barriers are given by the lack of accurate preliminary information on the lower soil layers and by the higher drilling costs to reach them.
For these reasons, this proposal focuses on medium-deep geothermal (about 1 km depth, undisturbed temperatures of about 40 °C, boreholes with the double role of source and storage) and low- temperature WH (a source with a large potential) combined with HP substations for heat recovery into existing DH networks. Available models will be adapted and enhanced to simulate the behavior of the overall system, assessing its technical performance/efficiency. This will then be included in a full techno-economic model, including investment and operation costs. The impact of the different system factors on key indicators like the levelized cost of heat (LCoH) will be parametrically studied and integrated in a general risk mitigation strategy, with quantitative impacts on business and price models. This framework will then be applied to 2 case studies, one located in Göttingen (Germany) and the other in Bolzano (Italy), where practical examples will be analyzed. The considered WH sources will comprise a data center and a dairy sector company. Given the strong expansion trend of data centers, the rather uniform availability of WH at the considered dairy sector company (common to many other industrial sectors), and the type of local networks (conventional DH), the two selected studies can be considered “critical” cases, i.e., highly representative cases with a large replication potential.
Objectives / goals
Within the main issue of integrating large shares of RES/WH into DH, the proposal tackles the following specific challenges:
(1) Developing a simplified model for medium-deep geothermal systems, acceptably reliable but much faster and easier to use than available detailed numerical models. This is indeed needed for planning purposes, where testing multiple scenarios and carrying out parametric analyses for techno-economic estimates is crucial. To achieve this goal, it is expected to build on existing analytical models, customizing them for an efficient coupling with models of the considered WH sources and HP substations. Numerical models and available literature data will be used for validation. The resulting model will also be able to assess the exchange power and the charge/discharge times in storage mode, which can be an issue in BTES systems.
(2) Optimizing operating temperatures as well as HP configurations and positioning. Here, a significant issue is related to the variable BTES temperatures, which, differently from stratified water storage temperatures, are linked to the state of charge as well as to the charge/discharge rates. Depending on the relative temperature levels (of BTES, WH, and network), different HP connections/positions can be devised, in order to optimize the efficiency with a mix of direct and indirect heat recovery. Another issue is related to the storage energy density, which impacts on its size/costs. A higher energy density can be achieved increasing the charge temperature, which however reduces WH recovery efficiency. It is important to find a suitable compromise between these aspects.
(3) Enhancing flexibility and developing effective business models in relation to substation management, temperature reduction, and mitigation of new risks related to the transition. Here, the main underlying issue is represented by the uncertainty introduced by these new solutions, which has to be reduced in order to attract investors. Price models are part of the problem as well and will be included in the analysis.
Target audience and specific issues
The novel concepts proposed here will be relevant for the entire DH value chain, including applied researchers, utility managers, city planners, engineering offices and plant designers/providers, investors.
Deliverables / Outcomes
- A simplified open-source techno-economic model for the combined integration of deep geothermal systems and low-temperature WH into DH networks through HPs, assessing the overall system efficiency (thermal losses, HP COP) and the resulting LCoH;
- A set of configurations and control schemes for HP-based substations;
- Business models and risk mitigation strategies for WH integration and temperature optimization;
- Practical application to 2 cases studies.
EURAC Research (EURAC)
Institute for Renewable Energy
Viale Druso 1
Dr. Marco Cozzini
Phone: +39 – 0471 – 055675
University of Applied Sciences and Arts Hildesheim/Holzminden/Göttingen (HAWK), Faculty Resource Management, Department NEUTec, Germany
Swedish Environmental Research Institute (IVL), International team, Sweden