• Energy Research

Abstract The storage of heat in aquifers, also referred to as Aquifer Thermal Energy Storage (ATES), bears a high potential to bridge the seasonal gap between periods of highest thermal energy demand and supply. With storage temperatures higher than 50 °C, High-Temperature (HT) ATES is capable to facilitate the integration of (non-)renewable heat sources into complex energy systems. While the complexity of ATES technology is positively correlated to the required storage temperature, HT-ATES faces multidisciplinary challenges and risks impeding a rapid market uptake worldwide. Therefore, the aim of this study is to provide an overview and analysis of these risks of HT-ATES to facilitate global technology adoption. Risk are identified considering experiences of past HT-ATES projects and analyzed by ATES and geothermal energy experts. An online survey among 38 international experts revealed that technical risks are expected to be less critical than legal, social and organizational risks. This is confirmed by the lessons learned from past HT-ATES projects, where high heat recovery values were achieved, and technical feasibility was demonstrated. Although HT-ATES is less flexible than competing technologies such as pits or buffer tanks, the main problems encountered are attributed to a loss of the heat source and fluctuating or decreasing heating demands. Considering that a HT-ATES system has a lifetime of more than 30 years, it is crucial to develop energy concepts which take into account the conditions both for heat sources and heat sinks. Finally, a site-specific risk analysis for HT-ATES in the city of Hamburg revealed that some risks strongly depend on local boundary conditions. A project-specific risk management is therefore indispensable and should be addressed in future research and project developments.

Thermochemical heat storage is a promising technology towards efficient use of renewable energy resources. Materials based on salts and their hydrates have a high potential for a good energy storage density and the benefit of long-term storage ability. However, the process has not yet been successfully implemented due to limitations in mass and heat transfer. This paper investigates how to improve the less desirable properties of CaCl2 and its hydrates such as low melting points, agglomeration, low cycle stability and low sorption rates. The optimization of CaCl2 properties was achieved by mixing with KCl and impregnation in carrier materials to obtain a composite material. The tests show at first that, with the admixtures of KCl, water uptake during hydration is 2 times higher than that of CaCl2. Water release during dehydration is 1.3 times higher than that of CaCl2. Secondly, the use of compacted expanded natural graphite (ENG) or activated carbon foam (ACF) increases the cycle stability, thermal conductivity and the water sorption performance. Due to their hydrophobic nature those matrices have no influence on the reaction scheme, thus the total amount of water molecules sorbed by the salt-in-matrix is close to the value of CaCl2. The degree of impregnation varies from 31 to 90 wt% depending on the host matrix and the impregnating medium used. The water vapour uptake is up to 0.61 g g−1 and the water released ranges from 0.12 to 0.72 g g−1. The thermal conductivity of CaCl2-in-matrixis is 3 times higher than that of sole CaCl2.

Abstract A lab-scale thermochemical heat storage reactor was developed in the European project “thermal battery” to obtain information on the characteristics of a closed heat storage system, based on thermochemical reactions. The present type of storage is capable of re-using waste heat from cogeneration system to produce useful heat for space heating. The storage material used was SrBr 2 ·6H 2 O. Due to agglomeration or gel-like problems, a structural element was introduced to enhance vapour and heat transfer. Honeycomb heat exchanger was designed and tested. 13 dehydration-hydration cycles were studied under low-temperature conditions (material temperatures 2 ·6H 2 O. Heat transfer fluid recovers heat at a short span of about 43 °C with an average of 22 °C during about 4 h, acceptable temperature for the human comfort (20 °C on day and 16 °C at night). System performances were obtained for a salt bed energy density of 213 kWh·m 3 . The overall heat transfer coefficient of the honeycomb heat exchanger has an average value of 147 W m −2 K −1 . Though promising results have been obtained, ameliorations need to be made, in order to make the closed thermochemical heat storage system competitive for space heating.

Abstract Thermochemical heat storage is highly promising, in particularly with a view to long-term heat storage. For the implementation of heat storage in households, thermochemical reactions in the low temperature range below 120 °C are important. Especially salt hydrates such as MgCl2, CaCl2 or MgSO4 were tested with micro gravimetric methods for their suitability. However, the cycle stability of consecutive charging (dehydration) and discharging (hydration) reactions of these materials was low and could be improved only by control of the water uptake (i.e. discharging time) to prevent overhydration. In contrast, mixtures of these salt hydrates showed significant improvements in cycle stability, mass and enthalpy balances. The experiments also showed that the cycleability of all investigated materials increased if hydration and dehydration reactions were performed under constant vapor pressure of 21 mbar. Contrary to other materials, the mixture of CaCl2 and MgCl2 showed good cycleability under all tested conditions. In addition, the mixture showed superior kinetic properties. Additionally, there is evidence of tachyhydrite (CaMg2Cl6⋅12H2O) formation during cycling of the mixture by the use of XRD after the thermal analysis. Further investigations will be performed to identify further synergies, ideal mixing ratios and formed phases.

Abstract Calcium chloride methanol addition compounds are promising sorbent candidates, which can not only be used for thermal energy storage but also for providing evaporative cooling in industrial applications using low-grade heat. The methanolate dissociation occurs within the working temperature range of low temperature cooling systems. Methanol has a low freezing point and high operating pressure and is less toxic and corrosive than ammonia as refrigerant. In solid-gas reactions the overall specific cooling capacity mainly depends on the sorption rate of the reaction. In general the reaction pattern follows a complex mechanism, in which the formation of intermediate phases and structural changes might occur. In this study a comprehensive micro-scale analysis on the effect of the methanol partial pressure, the thermal history of the calcium chloride, the dissociation temperature and subsequent sorption-desorption cycling on the sorption rate has been carried out. Results show that thermodynamic conditions as well as the thermal history and physicochemical properties of the material have a great influence on the sorption rate, whereas only a marginal dependence between the regeneration temperature and the sorption process was observed.

Abstract Corrosion and clogging are hidden hazards in various engineered systems dealing with water as storage or heat transfer medium. The related efficiency and serviceability losses may entail expensive remedial measures and costs up to the range of millions (€). Especially modern low-exergy installations are affected. They may suffer from oxygen ingress through capillary tubes, higher microbial activity due to moderate temperatures and increased vulnerability of complex components to particulate corrosion products. This field study presents in-situ investigations of chemical and microbial water quality and corrosion in pipes of different non-residential buildings. It identifies processes and recommends measures to prevent corrosion. Causes of corrosion and clogging were found to be i.a. related to operational errors.

Abstract The Leuphana University of Lueneburg changed to renewable energy supply with the first climate-neutral energy balance for heat, electricity, cars and business trips in 2014. The heating network is based on two biomethane-powered combined-heat-and-power (CHP) units of 525 kW el. each. A total of 720 kW p photovoltaics with 95% self-consumption covers > 20% of the electrical demand. We present the campus development and transformation to provide a best-practice example for conversion to exergy-efficient renewable energy systems. The new central building provides a large auditorium, seminar rooms, offices, a cafeteria, machine hall and space for exhibitions and events. It uses low-grade heat at 58 °C for optimized integration of short and long term heat storage installations. The architecture and facade design significantly lower cooling demand (≈ 2.5 kWh/m 2 a), modern lighting systems and user integration allow for superior overall energy efficiency. Exergy efficiency, storage options and emissions of the campus system as well as energy efficiency of the buildings were analysed. A high-temperature aquifer thermal energy storage (HT-ATES) installation perfectly matches the low-exergy heating demands and increases the share of CHP-heat, resulting in an additional surplus of 2.3 GWh/a of renewable electricity and additional savings of 2.424 t CO 2 -eq./a.