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AbstractApplication limitations for solar heating are overcome by greatly improving heat storage. Adequate heat storage is achieved by: reducing time dependent thermal losses, reducing storage volume, and allowing easy adjustment of storage geometries.To this purpose much theoretical and practical work has been done at Empa, including a laboratory scale proof of concept of a closed sorption heat storage. This work has provided valuable insight into the potential as well as the challenges and limitations of this technology for the application as heat storage.The closed sorption heat storage concept is based on a continuous, but not full cycle, liquid state absorption heat pump. Heat is not directly stored; instead the potential to regain heat at a desired temperature from a low temperature thermal input is stored. The significant benefit in this is undoubtedly the time independent energy losses. Losses are encountered in the conversion processes during charging as well as discharging but not during storage time. For this reason there is great potential in the application of the closed sorption heat storage as a long term solar heat storage.In the scope of the EU funded project COMTES a prototype system based on the working pair sodium hydroxide and water is under construction. The system is dimensioned to cover space heating as well as domestic hot water in a single family house built to passive energy standards and stationed in Zurich. The two major challenges in the system design are: keeping the system volume favorable and keeping the parasitic electric energy consumption at a minimum.

AbstractClosed sorption heat storage opens up a way to achieve seasonal thermal storage without loss during storage. Thermal energy is not stored as sensible heat or latent heat, but by the separation of substances. In this manner it is possible to store heat, or better said, the potential to regain heat.Such a system functioning as an absorption heat pump with intermediate storage is well suited for long term thermal energy storage. Nevertheless, the conversion losses in both the regenerating and heating process make it inferior to sensible thermal storage for short storage cycles. For this reason a hybrid system including a water tank as sensible thermal storage to cover the thermal demands of several days, and a closed sorption heat storage to cover longer periods of insufficient solar thermal input is proposed.The storage system energy density is directly proportional to the sorbate mass difference between regenerated and diluted sorbent. In order to reach high utilization, a large dilution in the sorbent must be reached during heating mode. Thus, the operating temperature parameters must be adjusted and regulated accordingly.In the scope of the EU funded project COMTES, a prototype system based on the working pair sodium hydroxide and water is under construction. The system is dimensioned to cover space heating as well as domestic hot water in a single family house in Zurich, built to passive energy standards. The prototype is starting operation in spring 2014 whereby the system is regenerated to be ready to cover the heating demands the following winter.

AbstractThis paper focuses on the development of a reaction zone dedicated to an absorption/desorption seasonal thermal energy storage. The modelling of the tube bundle constituting the reaction zone is described as well as the boundary conditions in worst working conditions and some modelling results are presented for the desorber/absorber. In parallel to this sizing work, investigations were lead on the tube bundle optimisation by studying the wetting and fluid distribution. A specific developed experimental set up based on imaging enabled to quantify the influence of tube texturing and to improve the manifold design. This work will lead to the reaction zone construction for an aqueous sodium hydroxide seasonal thermal energy storage prototype.

AbstractNitrate molten salts are extensively used for sensible heat storage in Concentrated Solar Power (CSP) plants and thermal energy storage (TES) systems. They are the most promising materials for latent heat storage applications. By combining classical molecular dynamics and differential scanning calorimetry experiments, we present a systematic study of all thermostatic, high temperature properties of pure KNO3and NaNO3salts and their eutectic and ”solar salt” mixtures, technologically relevant. We first study, in solid and liquid regimes, their mass densities, enthalpies, thermal expansion coefficients and isothermal compressibilities. We then analyze thecPandcVspecific heats of the pure salts and of the liquid phase of the mixtures. Our theoretical results allow to resolve a long-standing experimental uncertainty about thecP(T) thermal behaviour of these systems. In particular, they revisit empirical laws on thecP(T) behaviour, extensively used at industrial level in the design of TES components employing the ”solar salt” as main storage material. Our findings, numerically precise and internally consistent, can be used as a reference for the development of innovative nanomaterials based on nitrate molten salts, crucial in technologies as CSP, waste heat recovery, and advanced adiabatic compressed air energy storage.

AbstractThe European funded cooperative research and demonstration project PITAGORAS is focused on efficient integration of city districts with industrial parks through smart thermal grids. The overall objective of the project is to demonstrate a highly replicable, cost-effective and greatly energy efficient large-scale energy generation system for sustainable urban planning of low energy city districts. As part of the project two demonstration plants namely in Brescia, Italy and Kremsmünster, Austria are being designed and will be built and tested during the project. The demonstration plant in Austria focuses on installing a large-scale solar thermal system of 10,000 m2; including seasonal storages with a total capacity of 60,000 m3 in order to supply the local district with heat and to reduce the gas consumption of a large combined heat and power (CHP) plant nearby. Additionally, to the demonstration plant in Austria the collector efficiency of several different collector types of different collector manufacturers is being tested under real outside conditions, which may give valuable insights on these collectors in order to choose the most efficient collector for the demonstration plant and ultimately on the future development of solar collectors.

AbstractExperimental and theoretical investigations are carried out to study the heating of a 302 x 302 x 55mm test box of steel containing a sodium acetate water mixture. A thermostatic bath has been set up to control the charging and discharging of the steel box. The charging and discharging has been investigated experimentally by measuring surface temperatures of the box as well as the internal temperature of the sodium acetate water mixture through a probe located in the center of the steel box. The temperature developments on the outer surfaces of the steel box are used as input parameters for a Computational Fluid Dynamics (CFD) model. The CFD calculated temperatures are compared to measured temperatures internally in the box to validate the CFD model. Four cases are investigated; heating the test module with the sodium acetate water mixture in solid phase from ambient temperature to 52oC; heating the module starting with the salt water mixture in liquid phase from 72oC to 95oC; heating up the module from ambient temperature with the salt water mixture in solid phase, going through melting, ending in liquid phase at 78oC/82oC; and discharging the test module from liquid phase at 82oC, going through the crystallization, ending at ambient temperature with the sodium acetate water mixture in solid phase. Comparisons have shown reasonable good agreement between experimental measurements and theoretical simulation results for the investigated scenarios.

Utilizing solar energy for heat supply can reduce CO2 emissions and mitigate global climate change. In the Nordic region (e.g., Iceland and Finland), a tremendous seasonal mismatch exists between the availability of solar radiation and building heating demand. This paper proposes a local hybrid energy system based on solar energy for a residential district. It applies a borehole thermal energy storage to store solar energy in non-heating seasons, and uses stored energy for part of total heating demand in a residential neighbourhood in heating seasons. Photovoltaic panels are used to generate electricity for heat pump operation. To find out cost-optimal and ecofriendly solutions, the local energy system was first modelled and simulated in TRNSYS. Then, genetic algorithms were applied to optimize the system performance and costs. In optimal solutions, 38%-58% of total heating demand could be covered by on-site heat energy with the levelized cost of energy of 110-184 euro/MWh. On this basis, importing additional electricity from grid to increase the utilization rate of air-to-water heat pumps can further increase the on-site heat energy fraction to 41%-88% with the levelized cost of energy of 108-201 euro/MWh. Compared with the situation of fully district heating input, the proposed system can annually reduce CO2 emissions by 102-217 tons with the rate of 31-66%. Although the initial cost of the studied system is higher than that of district heating, the local hybrid energy system is worth further developing considering decentralizing heat energy production and reducing CO2 emissions. Peer reviewed

EU targets require nearly zero energy buildings (NZEB) by 2020. However few monitored examples exist of how NZEB has been achieved in practise in individual residential buildings. This paper provides an example of how a low-energy building (built in 2006), has achieved nearly zero energy heating through the addition of a solar domestic hot water and space heating system (“combi system”) with a Seasonal Thermal Energy Store (STES). The paper also presents a cumulative life cycle energy and cumulative life cycle carbon analysis for the installation based on the recorded DHW and space heating demand in addition to energy payback periods and net energy ratios. In addition, the carbon and energy analysis is carried out for four other heating system scenarios including hybrid solar thermal/PV systems in order to obtain the optimal system from a carbon efficiency perspective.