Seasonal Thermal Energy Storage Research Articles

Techno-economic optimization and feasibility of PCM-based seasonal thermal energy storage systems for district heating and cooling

Phase change materials (PCM) are an attractive seasonal thermal energy storage solution for load shifting due to relatively high energy density. Nevertheless, the choice of the right design parameters, such as storage size and phase-change temperature, is nontrivial, as these are tightly linked to the expected seasonal operation of the storage. This paper presents a combined design and operation optimization framework for sizing PCM-based seasonal thermal storage, which can capture dynamic behaviour of the storage in sensible and latent phases, and its impact on the efficiency of connected heat pumps and chillers. The proposed method, formulated as a mixed integer quadratically-constrained programming problem, was applied to a case study of heating-dominated district. It was found that the optimal PCM melting temperature equals to the heating demand supply temperature, thus removing the need of a heat pump to discharge the storage. The optimal operation of storage requires a full charging and discharging cycle of both latent and liquid phase. Higher CO2 emission price does not lead to a significant reduction of CO2 emissions, but the storage size does, leading to a higher heating coverage ratio of the storage, and consequently CO2 emissions and cost savings, which can be further enhanced with PV integration. The economic analysis indicates that, unless other benefits such as storage volume reduction are accounted for, PCM cost should drop by a factor 4 to make this solution economically viable for seasonal storage.

Seasonal large-scale thermal energy storage in an evolving district heating system – Long-term modeling of interconnected supply and demand

Given the strong seasonal nature of heating demands, peak heat is important during colder seasons. Instead of peak heat plants, seasonal large-scale thermal energy storage (TES) could be utilized. These can be charged during warmer seasons and discharged when required, decreasing the need for peak heat plants. Systems modelling studies on seasonal TES are lacking. Thus, a long-term local energy system model is applied under different scenarios to investigate the potential roles of seasonal TES in an evolving heating system. The results show that seasonal TES is economically viable for: all future electricity price cases for low TES construction costs, corresponding to repurposing of underground oil storages, and for most electricity price cases for mid- and high construction costs, corresponding to new underground excavations. Seasonal TES mainly decrease the investments in and usage of electric boilers or biogas boilers, while increase the utilization of heat pumps. Other technologies may be affected depending on the future trajectory of electricity price developments. The size of the TES is between 3 and 7% of the annual district heating heat demand, depending on construction cost and electricity price development. The expansion of district heating into new housing is mostly unaffected by the availability of TES.

Seasonal thermal energy storage employing solar heat: A case study of Heilongjiang, China, exploring the transition to clean heating and renewable power integration

Seasonal thermal energy storage (STES) offers an attractive option for decarbonizing heating in the built environment to promote renewable energy and reduce CO2 emissions. A literature review revealed knowledge gaps in evaluating the technical feasibility of replacing district heating (DH) with STES in densely populated areas and its impact on costs, fossil fuel consumption, CO2 emission reduction, and renewable power integration. The effects of large-scale STES applications on the power and DH sectors in a regional energy system were quantified by applying the 2030 and 2050 power structure scenarios. The results indicated that STES reduces fossil fuel consumption and CO2 emissions at an affordable cost. It facilitates the integration of wind and solar power into power grids. With 100 % replacement of DH by STES, fossil fuel consumption and CO2 emissions can be reduced by approximately 40 % and 45 % in 2030 and 2050, respectively, with an annual cost increase of 20 %. The CO2 avoidance costs were predicted to be approximately 60 €/t in 2030 and well below 50 €/t in 2050. STES will reduce renewable power curtailment by 10 % in 2030 and 18 % in 2050. This study investigated STES from an energy-system perspective, supporting the formulation of clean heating transition policies.

Seasonal storage for space heating using solar DHW surplus

Due to the seasonality of solar energy, achieving 100 % of annual solar fraction for domestic hot water (DHW) production is only possible by greatly oversizing the collector area of a solar system, thus creating a significant energy surplus in summer. This simulation study investigates the possibility of using this surplus to promote space heating during winter, in a moderate South European climate, to try achieving a total solar fraction of 100 %. Priority is given to the DHW reservoir, diverting the excess heat to an additional large-capacity seasonal thermal energy storage (STES) reservoir. The best configuration for the number of collectors and STES tank volume was assessed through a parametric study, to reach a compromise between a high solar fraction and a reasonable system efficiency. The results showed that a system with 10 m2 of solar collectors and a 30 m3 STES tank or, alternatively, 20 m2 of collectors and a 20 m3 tank achieved the desired solar fraction and efficiency for the chosen building and local climate conditions. A comparison with the literature shows that this strategy can achieve better results, requiring less collector area and storage volume.

A study of the benefits of including thermal energy stores in district heating networks

A model to simulate district heating networks (DHN) was applied to a hypothetical DHN in Loughborough, UK. The model includes different thermal energy storage (TES) systems: i) short-term water-based stores and phase-change material-based stores and ii) a seasonal thermal energy storage (STES) system. Heat pumps (HP) and evacuated-tube solar thermal collectors (ETSTC) are considered to both provide heat for dwellings and charge the TES systems. The model assumes that the HPs can only be used to charge TES at those times when the electricity is produced by zero-carbon sources. To assess this real CO2 emissions data per kWh of electricity produced in UK was used. The effect of both 1) the STES volume and 2) the half-hourly maximum amount of zero-emissions electricity available to charge TES on a) cost, b) efficiency and c) CO2 emissions was studied. The results showed that CO2 emissions and electricity cost can be reduced to 83.4 % and 12.3 %, respectively, using TES, when comparing with the scenario with no TES. Savings of 41.7 % can be achieved when comparing the cost of electricity used in the proposed DHN with TES with the current cost of the gas.

Development, validation and demonstration of a new Modelica pit thermal energy storage model for system simulation and optimization

Space heating applications account for a high share of global greenhouse gas emissions. To increase the renewable share of heat generation, seasonal thermal energy storage (STES) can be used to make thermal energy from fluctuating renewable sources available in times of high demand. A popular STES technology is pit thermal energy storage (PTES), where heat is stored underground, using water as a storage medium. To evaluate the use of PTES in an energy system, easily adaptable, publicly accessible and tool independent models are needed. In this paper, we improve an existing PTES model developed in the Modelica modeling language. The model is cross-compared with a more detailed and previously validated COMSOL model, considering different amounts of insulation, showing a deviation of 2–13% in the observed annual charged and discharged amount of heat. The results indicate that the presented model is well suited for early design stage and an exemplary case study is performed to demonstrate its applicability in a system context. Dimensions of system components are optimized for the levelized cost of heat (LCOH), both with and without subsidies, highlighting the importance of subsidies for the transition towards climate friendly heating solutions, as the gas boiler use is reduced from 47.6% to 2.7%.

Advanced thermal energy storage made of a ternary CPCM with two phase change temperatures in building walls

This paper introduces a ternary composite phase change material (CPCM) comprising three phase change materials, with a eutectic mixture as the low phase change temperature (PCT) component. The thermal performance and stability of the CPCM are rigorously evaluated through simulations and experiments including step cooling curves, differential scanning calorimetry (DSC), thermal cycling, and Fourier-transform infrared (FTIR) spectroscopy. The ternary CPCM CA/C18-TD (2.0:8.0) exhibits superior latent heat storage—224.53 J/g compared to 143.83 J/g for the binary CA-TD (3.0:7.0)—and maintains stability without supercooling after 300 thermal cycles. It also proves cost-effective compared to its components. In building applications, walls integrated with the CPCM CA/C18-TD (2.0:8.0) exhibited significantly enhanced thermal regulation properties, including longer thermal delay times and reduced attenuation factors across both winter and summer conditions. Specifically, the CPCM wall reduced temperature fluctuations by up to 31.91 % in comparison to ordinary wall and 21.63 % against single PCM wall in winter, 54.51 % and 44.22 % lower than the ordinary and single PCM walls respectively in summer. These walls also showed a remarkable reduction in heat flow density by up to 17.49 W/m2 and 5.41 W/m2 compared to ordinary wall under summer and winter conditions, respectively. This means that the CPCM has superior insulation performance and reduces susceptibility to external environmental changes. These findings confirm CA/C18-TD (2.0:8.0) as an optimal CPCM for seasonal thermal energy storage in energy-efficient PCM wall.

Supercooled erythritol for high-performance seasonal thermal energy storage

Seasonal storage of solar thermal energy through supercooled phase change materials (PCM) offers a promising solution for decarbonizing space and water heating in winter. Despite the high energy density and adaptability, natural PCMs often lack the necessary supercooling for stable, long-term storage. Leveraging erythritol, a sustainable mid-temperature PCM with high latent heat, we introduce a straightforward method to stabilize its supercooling by incorporating carrageenan (CG), a bio-derived food thickener. By improving the solid-liquid interfacial energy with the addition of CG the latent heat of erythritol can be effectively locked at a very low temperature. We show that the composite PCM can sustain an ultrastable supercooled state below −30 °C, which guarantees no accidental loss of the latent heat in severe cold regions on Earth. We further demonstrate that the common ultrasonication method can be used as the key to unlocking the latent heat stored in the CG-thickened erythritol, showing its great potential to serve as a high-performance, eco-friendly PCM for long-term seasonal solar energy storage.

Robust Design of 5th Generation District Heating and Cooling (5GDHC) Systems with Seasonal Thermal Energy Storage via GIS Assessment

This research addresses the need for a method to compare waste heat potential and recovery in 5th Generation District Heating and Cooling (5GDHC) systems. Prosumer buildings, known for enhancing flexibility, are analyzed using Geographic Information Systems (GIS) and image vectorization to evaluate 5GDHC systems. The study predicts the amount of waste heat available with an R2 of 0.96, utilizing Chaikin's algorithm to refine thermal images for automatic recognition of the sources of waste heat. Emphasis is placed on the absence of a method for asset design in 5GDHC, with considerations for annual energy calculations, radial network connections, and energy shares among building types. Recommendations for building connecting in Belgium are proposed, favoring 'B'/'C'-labeled buildings. The research explores the impact of building design on heating and suggests energy savings through regulation strategy changes. Dynamic models for heat pumps aim to reduce errors and emissions. The study connects energy indicators and GIS software, contributing to a top-down design approach in 5GDHC systems. The overall goal is to contribute to decarbonization and reduce CO2 emissions in the energy sector.

An Energy and Ecological Assessment of The Prior and Post-Hybrid Renovation State of Multi-Dwelling Building in the Algerian City of Skikda

In the context of Algerian energy and environmental concerns and its aspirations for energy transition, this article examines the pre- and post-state of a multi-family residential building in the city of Skikda, in northern Algeria, utilizing the UBEM reduced-order approach, alongside QGIS, CEA and METEONORM tools. Primarily focusing on the building's solar exposure, energy aspects, and environmental considerations. Six hybrid renovation scenarios were considered then, and assessed for their contribution for energy savings and ecological improvement. The main results revealed that 87% of annual energy requirements were related to heating and hot water, which are supplied by a natural gas system, a source of various types of greenhouse gases, which suggested the use of triple glazing, with or without two selective low-emissivity coatings only, or be combined by changing hot-water supply system to an electric one, alongside the installation of the hybrid PVT panels on the south façade. This triple glazing was joint in another scenario by integrating the same type of panels on the roof alongside a seasonal thermal energy storage system. This latter scenario being the best in terms of energy and ecology, and making the building highly performant in both aspects.

Analysis of Different Climate-Neutral Heat Supply Concepts for a District Heating System near Munich with Deep Geothermal Heat as the Primary Heat Source

District heating systems are one key element of the transition towards a climate-neutral heat supply since they allow for the use of various renewable energy sources and the increase of renewable heat generation, in particular when combined with seasonal thermal energy storage. Deep geothermal heat is a renewable heat source that can be used to cover the thermal base load of such a district heating system. Also in combination with other renewable heat sources, such as solar thermal or heat pumps, a climate-neutral heat supply for districts or even complete cities can be realised. This paper deals with the analysis of different climate-neutral heat supply concepts that were elaborated for a district heating system currently in the planning phase using deep geothermal heat as the primary heat source, located in the town of Graefelfing near Munich, Germany. The authors present, discuss and assess various options regarding renewable heat sources and including (seasonal) thermal energy storage, to fully cover the thermal loads of a new district heating system in a technically, economically and environmentally favourable way. As a result of this study, a heat supply concept was selected, in which a heat pump using the return flow of the deep geothermal circuit as a heat source maximises the utilisation of the available deep geothermal heat source to cover more than 95 % of the annual heat demand by deep geothermal heat combined with a heat pump.

Experimental parametric evaluation of adsorption characteristics for silica gel - water based open-bed system for seasonal thermal energy storage

This study aims to investigate the potential of using commercial silica gel as an energy storage material in a bulk-scale open bed adsorption-based system to achieve efficient domestic heating using renewable energy sources. Designing an efficient thermal energy storage (TES) system for practical use requires understanding of the sorption properties of the adsorbent and the effects of different operating and physical parameters on the sorption process. One critical parameter that significantly affects the energy storage density when scaling up from a laboratory to a prototype system is the amount of adsorbent in the reactor. Surprisingly, this aspect has been overlooked in previous studies. To address this research gap, a laboratory-scale test rig was designed and constructed. This rig enables the evaluation of the effects of different operating parameters (such as relative humidity, flow rate, and regeneration temperature) and physical parameters (such as the quantity of adsorbent and particle diameter) on the energy storage density of silica gel and the temperature lift in the process. The water adsorption capacity of the silica gel was measured in-house to assist in future theoretical modelling of a prototype TES system. Optimum operating conditions were determined for the system, with a relative humidity of 80 %, an air flow rate of 100 L/min, and material regeneration at 120 °C. The system's performance in terms of material energy storage density and maximum temperature lift was observed for varying amounts of adsorbent, particle diameter, and regeneration temperature at these optimal operating conditions. Finally, the required storage volume to meet domestic space heating demands was estimated based on optimal discharging conditions and compared to other experimental and theoretical studies involving silica gel-based energy storage systems.

Case study on combined heat and water system for district heating in Beijing through recovery of industrial waste heat in Tangshan

With the proposal of China's dual-carbon goal, the recovery of industrial waste heat (IWH) for district heating has gained increasing attention. Recently, some scholars have proposed that combined heat and water (CHW) systems are more economical compared with separate heat and water supplies. Beijing lacks desalinated water (DW) and clean low-carbon heat, whereas Tangshan in Hebei Province is rich in IWH. By combining CHW with seasonal thermal energy storage, a case of long-distance transport of hot DW from Tangshan to Beijing was planned, and the costs, energy consumption, and feasibility were analyzed. As a result, under the optimal heat source temperature and effect number of 70 °C and 10 for seawater desalination, the heat from various IWH sources of the three costal steel plants is recovered maximally to produce about 35000 t/h of 95 °C DW. Then, the hot DW is transported to Beijing with a long-distance heat loss ratio controlled below 3 %. Annually, 271.2 million tons of DW and 91.5 million GJ of heat is supplied. The introduction of IWH can save 2.68 billion Nm3 of natural gas per year and the discounted payback period is about 10 years, which has obvious energy-saving, environmental, and economic benefits.

Multiscale analysis of a seasonal latent thermal energy storage with solar collectors for a single-family building

This paper presents a comprehensive system-to-CFD multiscale analysis of a seasonal thermal energy storage (STES) system based on phase change materials (PCMs) for efficient energy storage and release for space heating. The study investigates the impact of various factors, including the geometry of the individual storage tank and the thermo-fluid dynamic parameters of the heat transfer fluid on the system performance. The work focuses on STES integration with space heating system to meet the thermal energy demands of a typical single-family building located in Naples – South Italy, typical coastline Mediterranean climate. The primary goal is to assess the behaviour of the system-integrated PCM tank and its characteristic parameters, leading to the proposal of sustainable approaches that include renewable energy sources and partially fulfil the building thermal loads. Among the Scenarios, one stands out as it achieves a better operation, allowing storage and solar collectors to cover around 69.2 % of the winter heating demand, i.e., 3.06 MWh. The study also discusses the benefits of modular system operation, as evidenced by Scenario 3, which allows for the parallel discharge of multiple LHTES tanks, leading to an enhancement in efficiency compared to other Scenarios. Results indicate that the total energy discharged by a single STES is 56.30 kWh, with a latent heat fraction released over a time of 111 h and an average temperature difference of 5.65 °C. The tank efficiency reaches a value of 90.4 %, demonstrating the effectiveness of the STES system in meeting heating demands.

Maximizing energy efficiency in wastewater treatment plants: A data-driven approach for waste heat recovery and an economic analysis using Organic Rankine Cycle and thermal energy storage

Maximizing energy efficiency through waste heat recovery (WHR) processes is crucial for sustainable and eco-friendly operations across multiple industries, notably in wastewater treatment plants (WWTPs). This work proposes a comprehensive approach for assessing the WHR feasibility in WWTPs, structured in two main objectives. Firstly, an Artificial Neural Network (ANN) model is developed to accurately predict WHR based on operational data, including biogas temperature, biogas pressure, daily production in kWh, and WHR values in kWhth. The second objective focuses on economically evaluating the WHR feasibility based on the estimated WHR values obtained by the ANN model, and then realistically assessing the economic feasibility of integrating the Organic Rankine Cycle (ORC) and Seasonal Thermal Energy Storage (STES) systems. With an application to the As-Samra WWTP located in Jordan, the developed ANN model demonstrates promising results in the validation phase, with a root mean square error (RMSE) of 2206 kWh/day, a mean absolute error (MAE) of 1674 kWh/day, and an R-squared (R2) value of 68%. On the other hand, the economic analysis reveals that an optimal ORC system of 412.14 kWe capacity yields a Net Present Value (NPV) of 2.09 million US dollars, a Levelized Cost of Energy (LCOE) of 0.0749 USD/kWh, a Payback Period (PBP) of 4.8 years, and annual revenues of 428 kUSD. This work also investigates the techno-economic feasibility of integrating ORC-STES. Results indicate that the LCOE and PBP are highly affected by the ORC's capital cost, and integrating STES increases the LCOE to 0.0824 USD/kWh, rendering its integration with ORC infeasible. This study aims to advance the understanding and application of WHR in WWTPs, paving the way for more efficient and sustainable practices in the field.

Solar thermal vs. PV with a heat pump: A comparison of different charging technologies for seasonal storage systems in district heating networks

The majority of district heating networks (DHNs) in Germany and other European countries are still supplied by fossil fuels. While several possible technical solutions exist to decarbonize DHNs, the problem is the availability of solar energy – thermal and electric – or other renewable electric sources during winter. A solution is to shift the available energy from the summer to the winter by using seasonal thermal energy storage systems. Especially pit thermal energy storage systems (PTES) have shown an economic advantage compared to other storage types. Therefore, we would like further to investigate the design of PTES under different techno-economic conditions.In this study, different charging options for the PTES were examined. Solar thermal energy was compared to photovoltaic (PV), supplying an air-source heat pump (ASHP). We developed an open-source Python package based on mixed-integer linear programming, minimizing the total system cost, i.e., the annuity, including opex and capex. The tool selects, operates, and designs energy converters and storages. Furthermore, it optimizes the network’s storage capacity during the operation. The optimization takes the characteristics of an existing DHN as input and designs the supply systems across different locations (LOCs). The model separates the hydraulic and thermal calculation and builds up a multi-stage process to consider temperatures inside the network and the PTES.The tool was applied to a DHN in Eastern Germany. The results show a low sensitivity to the electricity price in all concepts. Operating the PTES at low temperatures negatively affects the heat capacity, and therefore, it is best suited for high-temperature networks. The concept of PV with an ASHP is better suited for densely populated regions. However, both concepts have a total system cost above 20ct/kWh for an area price of over 120€/m2. For an electricity price of ca. 19ct/kWh, the total system cost is below 17ct/kWh for both concepts.

Long-term operational planning for flexible residential buildings with seasonal storage and capacity-based grid tariffs

When performing operation of the energy system in a building, it is difficult to accurately represent the uncertain long-term objectives in the short-term window, and the correlation between long-term objectives. This work investigates a strategic modelling framework for representing long-term incentives during short-term operational planning for an energy system within flexible buildings. Multi-stage backwards Stochastic Dynamic Programming (SDP) is used to decompose the yearly operational problem into smaller stages, and create long-term cost curves that captures the cost of adjusting long-term price signals, which in this work considers seasonal thermal energy storage and recurring monthly capacity-based grid tariff costs. The proposed method is analyzed for a flexible realistic Norwegian building located in Southern Norway, where the influence both price signals have on long-term operational performance is analyzed. Results show that the framework strategically optimizes the long-term seasonal storage together with the recurring monthly grid tariff, complementing each other by using the seasonal storage to reduce peak costs during winter. With both seasonal storage and monthly demand charge, yearly operational costs are reduced by 4.3% compared to only accounting for the grid tariff. Additionally, the operational performance achieved only a 0.9% higher cost compared to yearly simulation with perfect information.

Seasonal thermal energy storage using natural structures: GIS-based potential assessment for northern China

Seasonal thermal energy storage using natural structures: GIS-based potential assessment for northern China

Seasonal thermal energy storage as a complementary technology: Case study insights from Denmark and The Netherlands

Seasonal thermal energy storage (STES) has potential to act as an enabling technology in the transition to sustainable and low carbon energy systems. It is a relatively mature technology, providing a reliable and large-scale solution to seasonal variations in energy supply and demand where it has been deployed at scale. In practice, however, there remains minimal deployment of STES internationally, with only a small number of countries being exceptions. Here, we analyse STES development in two of these exceptions: the Netherlands and Denmark, and we consider the relevance of the Dutch and Danish STES experiences for other countries, such as the UK, where STES is currently marginal. In explaining the diffusion of STES in leading countries, and its limited uptake elsewhere, we pay attention to energy system context by investigating the complementarities – or misalignments – between different parts of energy systems which have influenced STES deployment. The contribution arising from our mainly qualitative and historical analysis is to demonstrate the importance of contextual factors such as national policy and complementarities between technical and social factors in influencing STES deployment. This approach, we argue, can complement and enrich engineering and economic perspectives on STES.

Drivers to allow widespread adoption of ATES systems: a reflection on 40 years experience in The Netherlands

Drivers to allow widespread adoption of ATES systems: a reflection on 40 years experience in The Netherlands