Dalarna University's logo and link to the university's website

Large-scale solar district heating has increased fast recently and is a commercial industry in Denmark with over 100 systems. Pit stores with high solar fraction exist in several of these systems. Some economic factors in Sweden are not as good as in Denmark. However, recent events have forced up the price of biomass in Sweden and other competing uses for the forest resource point to greater competition and thus higher prices, resulting in better viability for solar. The project has used geographic and geological data from e.g. SGU to identify which areas in Sweden that are potentially suitable for pit stores. The results from the screening process show that the vast majority of district heating areas have potentially suitable areas for pit storage within a reasonable distance from the network. Only 86 district heating areas do not. As there is no reliable data for the whole country for ground water level and flow, both important factors for economic viability, the results are optimistic. As to be expected, the results show large variations over the country, but in general there are more suitable areas in the south than north. The project also estimated how much heat solar and pit storage systems could potentially deliver to the district heating areas with suitable areas. If all of these identified district heating areas installed solar and pit storage systems covering 20% of their demand, 15% of Sweden’s total district heating demand would be supplied by solar. If all networks with potential areas for pit storage installed systems covering 40% of the local demand, the equivalent figure is 36%. Pre-feasibility studies for Råneå, Härnösand and Söderhamn show that the heat cost for solar heating systems is slightly higher than the current production heat cost in these networks, given the current interest rate. As the heat cost for solar is mostly dependent on the investment cost, it varies over time in principle only with the interest rate. A lower interest rate or small annual increases in fuel costs would make the studied solar heating systems economically viable. Costs, as for many other technologies, are lower for larger systems. Integration of a heat pump in the system is cost effective if the site of the pit store is in the periphery of the network and the district heating system has exhaust gas condensation. In practice there are many additional factors that can hinder building a pit store in the sites identified as suitable in the screening. The results of the screening are theoretical and are based on the assumption that none of these negative factors exist. Detailed on-site geological measurements are needed if one wants to take the next step in actually building a pit store.

This paper is the second of three accounts which describes a Swedish study aiming to derive a first order assessment of the national potential for large-scale solar thermal heat production with pit thermal energy storage’s (PTES) connected to existing district heating systems (DHS). Whereas the first paper presented project objectives, outset parameters, and an updated Swedish district heating database – and the third is planned to report on the final project results and conclusions – this paper focuses on the assembled study data and the associated selection criteria applied to these data categories under the objective to distinguish suitable (and non-suitable) land areas within cost-efficient heat transmission distances from the existing DHS. The approach centres around a principal spatial analysis with superposition of study data and elimination of non-suitable land areas according to the used selections criteria but also entails a wide periphery of related activities, such as literature reviews, gathering of technology preferences, meetings with sector experts, data management etc. Apart from technical specifications for solar heat production and seasonal storage, key data categories for the spatial analysis consist of geological data (e.g. soil types, soil depth, bedrock etc.), hydrological data (lakes, rivers, wells, soil moisture, ground water levels etc.), geographical data (elevation, built-up areas, administrative units etc.), and thematic data (energy statistics, building heat demands, district heat deliveries etc.). Selection criteria for the relevant data categories have been definediteratively during e.g. expert consultancy, for example minimum soil depth, preferred soil types, maximum feasible transmission distance to existing DHS etc. By application of the selection criteria, raw input data are converted to processed data extracts to be used in the final analysis. Study data categories are illustrated and summarised (raw and processed) together with a listing and discussion of the used selection criteria.

Sweden was among the first countries to install solar thermal plants for district heating (DH) as early as in 1970s. However, in recent years, the focus on solar DH installations has shifted primarily to Denmark and Germany, with only one recent installation reported in Sweden. Nonetheless, due to changes in the overall heating market, the use of large-scale storage (both with and without solar heat) is becoming increasingly important. Despite significant advancements in adopting DH systems, the combination of solar DH with PTES is not well studied from Swedish context. The economic and geological prerequisites for the deployment of PTES remain largely unexplored. This paper explores the integration of large-scale solar thermal systems into DH networks in Sweden, particularly highlighting the feasibility and potential of pit thermal energy storage (PTES) systems. Through findings from a national project, this paper assesses the techno-economic-geological viability of PTES alongside solar thermal collectors, providing insights into the project’s methodological approach and initial findings.

Most of the district heating systems today use higher operating temperatures than those in new-built systems, possibly limiting compatibility with solar energy. This study evaluates the cost-effectiveness in terms of unit heat cost of integrating solar heating into an existing district heating system compared to not using solar energy, under changing economic boundary conditions such as collector and fuel cost, in addition to discount rate. This is investigated for both a scenario where the solar heating and a boiler replacement is done concurrently, as well as a scenario where solar heating is added to an existing system without replacing the boiler. A theoretical district heating supply of 3 MW is modelled and simulated based on a real system and load profile. The heat supply is varied to include storage with or without solar heating. Results for a 3 % discount rate indicate that; Replacing a 3 MW boiler with a slightly smaller boiler of 2.5 MW and adding a storage is cost effective and yields a unit heat cost of 58.0 EUR/MWh (16.1 EUR/TJ) which is a reduction of about 6 %. Installing solar heating together with the boiler replacement yields a unit heat cost as low as 55.7 EUR/MWh (15.4 EUR/GJ) which is a reduction of about 8 %. When replacing the boiler, all system configurations have similar unit heat costs compared to a boiler-only system, so factors such as emission reductions due to solar heating are relevant when considering alternatives. Furthermore, adding solar flat plate collectors corresponding to a 13 % solar fraction without replacing the boiler can reduce the unit heat cost as low as 34.8 EUR/MWh (9.7 EUR/TJ), which is 32 % lower than without solar. Evacuated tube collectors can increase this solar fraction to 17 % with similar system size, although at a higher cost. At a discount rate of 5 % solar heating is cost-competitive when fuel cost is above 26 EUR/MWh (7.2 EUR/TJ) and at 7 % competitive when fuel cost is above 32 EUR/MWh (8.9 EUR/TJ). Increasing solar heating system size reduces the backup-boiler fuel use during summer maintenance and makes fuel type less relevant for the overall unit heat cost. © 2024 The Authors

The deployment of electric vehicles (EVs) is growing significantly in recent years. The increasing EV charging loads pose great stress on power grids in Sweden, as many existing power grids are not designed to host such large shares of new electric loads. Hence, studies investigating the impact of EV charging are needed. This study conducts a case study based on an existing Swedish residential power grid using real-life EV charging data to estimate the local grid hosting capacity (HC) for EVs. A combined time-series and stochastic HC assessment method is used with voltage deviation, cable loading and transformer loading as the performance indices. Uncertainty in EV charging locations and individual charging behaviour have been considered via Monte Carlo simulations. The power grid HC is analysed and compared under three charging strategies and four EV penetration levels. Study results show that a charging strategy based on low electricity prices gave lower HC due to simultaneous EV loads compared to the other two strategies: charging directly after plugging in the EV and an even charging load through the plug-in session. This implies the need for coordinated charging controls of EV fleets or diversified power tariffs to balance power on a large scale. © 2023 The Authors

In recent years, the widespread adoption of photovoltaic (PV) installations across various sectors has created a growing demand for accurate PV design tools. In this pursuit, the latest version of IDA Indoor Climate and Energy (IDA ICE 5 beta) emerges as a contender, offering advanced PV modeling capabilities, together with the advanced building simulation capabilities it already has. This study evaluates the accuracy of PV modeling within IDA ICE by comparing predicted power outputs to real-world data from three existing PV systems located at the Research Institute of Sweden (RISE) in Borås. To achieve this, weather files are created using historical weather and radiation data. As the measured radiation data is only available as total irradiation on a tilted plane, it was deconstructed into direct and diffuse components on the horizontal plane using a modified version of the model by Erbs et al.. Additional known parameters are the geometry of the PV array, and the characteristics of PV panel and inverter, based on their product data sheets. The accuracy of the PV design tool in IDA ICE is evaluated by comparing the power output of unshaded arrays against the measured data from RISE. The calculated power output is compared to the measured power output and analyzed through ASHRAE 14-2014 guidelines for performance evaluation. It was found that the software gives an accurate prediction of both panel temperature and PV power production. A study on shading effects is an open problem to improve the generality of the results in this study.

One challenge in today’s district heating systems is the relatively high distribution heat loss. Lowering distribution temperatures is one way to reduce operational costs resulting from high heat losses, while changing the distribution system from steel pipes to plastic pipes and changing the heat distribution concept can reduce investment costs. The result is that the overall life cycle cost of the district heating system is reduced, leading to the improved cost competitiveness of district heating versus individual heating options. The main aim of this study was to determine the most cost-efficient distribution system for a theoretical solar district heating system, by comparing the marginal life cycle cost of two different distribution systems. A secondary aim was to determine the influence of the employed pipe type and insulation level on the marginal life cycle cost by comparing detailed economic calculations, including differences in pipe installation costs and construction costs, among others. A small solar-assisted district heating system has been modeled in TRNSYS based on a real system, and this “hybrid” model is used as a basis for a second model where a novel distribution system is employed and the heating network operating temperature is changed. Results indicate that a novel distribution concept with lower network temperatures and central domestic hot water preparation is most efficient both from an energy and cost perspective. The total life cycle costs vary less than 2% for a given distribution concept when using different pipe types and insulation classes, indicating that the investment costs are more significant than operational costs in reducing life cycle costs. The largest difference in life cycle cost is observed by changing the distribution concept, the novel concept having approximately 24% lower marginal life cycle cost than the “hybrid” system. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

This study inspects the effect of tapered headers on pressure drop and flow distribution in a U-type polymeric absorber with novel tapered headers and lens-shaped absorber strings using a validated thermo-hydraulic model. The model results are compared with those obtained from the literature to attain credibility in the flow distribution trend for the U-configuration. A good agreement between the developed discrete model and comparison cases is found. Moreover, in order to examine the efficacy of tapered headers in more detail, different scenarios are treated in terms of header configuration by applying cylindrical geometry in one or both inlet/outlet headers. The outcomes exemplify that even a slight cone angle of 1.73 degrees in headers can significantly reduce non-uniformity (phi(max) < 8%) with negligible influence on the total pressured drop. Yet, further reduction in maldistribution (phi(max) < 5%) can be achieved in U-type absorbers if the tapered outlet header is combined with a cylindrical inlet header in the range of AR <= 3.34 and DR <= 0.24. In this case, a compromise between additional pressure drop and flow distribution degree should be found. The present study offers a systematic approach for conducting thermo-hydraulic analysis in flat-plate solar collectors with complex absorber compositions and geometries. (c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

This work presents a discrete model for the thermo-hydraulic analysis of a novel polymeric solar thermal absorber with tapered headers and lens-shaped absorber strings. The numerical model developed is based on empirical correlations for laminar and turbulent flow regimes. The principal aim of this study is to ascertain if tapered headers help improve flow distribution in parallel-flow absorbers under laminar and turbulent flow conditions. In order to verify the model, the simulation results are compared to the measurement for total pressure drop and two studies from the previous literature for flow distribution. Altogether, a good agreement is found in all comparison cases, and the proposed numerical algorithm is proven to be robust and stable for complex thermo-hydraulic analysis. The results are then elaborated by comparing the tapered case to several conventional cases with cylindrical headers but with identical riser configurations to the original model. The results indicate that using tapered headers in compact absorbers with relatively large area ratios can noticeably reduce non-uniformity, especially up to middle range flows, by maintaining higher Reynolds numbers in the headers. The developed model can be used to optimize the hydraulic design of solar collectors with complex geometries and large area ratios. © 2022 The Authors