• Energy Research

Thermochemical energy storage is an attractive option for seasonal thermal energy storage, particularly in building applications. However, several research gaps in the field of sorption storage systems such as restricted focus on specific reactor concepts or sorption couples or lack of systematic performance studies hinder their practical implementation. This study addresses these gaps by evaluating the performance and cost-effectiveness of a solar thermal space heating system integrated with liquid sorption storage across various building types (single and multi-family homes with different envelope qualities) and climates (Zurich, Switzerland; Harbin, China; Helsinki, Finland). The study systematically investigates the impact of different sizes of individual system components (number of ground heat exchangers, solar collector area, sorption reactor capacity, size and distribution of thermal buffers) on the overall system performance using a previously presented greybox sorption reactor model based on a lab-scale prototype. The simulation results demonstrate that high solar fractions above 80 % can be achieved with long-term sorption storage. To reach this, substantial storage volumes of around 0.8––1 m3 per m2 of solar collector area are needed for the multi-family home cases in Zurich climate despite the increased volumetric energy density of sorption storage when compared to classical water storage. This emphasizes the significance of building envelope quality, available roof area, and careful system component sizing for enhancing solar fractions and cost-effective renewable heat generation. The findings provide valuable insights into optimizing sorption storage systems, fostering the practical implementation of renewable energy solutions for space heating in buildings. Highlights: • Building integrated sorption storage combined with solar thermal collectors. • High solar fractions above 80% can be achieved. • Dynamic building simulations for optimal component sizing and system performance. • Simulations are based on a measured laboratory prototype.

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.

AbstractIn order to overcome limitations of solar heating, great improvements in seasonal heat storage are required. Adequate heat storage is achieved by: reducing time dependent thermal losses, reducing storage volume, and allowing easy adjustment of storage geometries to enable building retrofitting.To this purpose much theoretical and practical work has been done at Empa, including a laboratory scale proof of concept of an aqueous sodium hydroxide based seasonal thermal storage.This 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 main benefit is that there are no capacity losses during storage time. For this reason there is great potential in the application of the closed sorption heat storage system for long term solar heat storage. Due to the losses encountered during charging/discharging (efficiency of the heat and mass exchanger), the concept is less suitable for short term heat storage. Therefore a hybrid system is proposed, consisting of hot water tanks for short term storage and closed sorption heat storage for seasonal storage.In the scope of the EU funded project COMTES a prototype aqueous sodium hydroxide seasonal thermal storage system 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 located in the region of Zurich.

This paper presents a practical study towards the development of a heat and mass exchanger fitting toliquid absorption heat storage for building application. Results of a lab scale setup are shown. To reachhigh heat capacity in absorption storage, a maximum temperature gain and concentration difference ismandatory. A conventional spiral fined tube heat exchanger is employed as heat and mass exchanger,whereby the tube is installed vertically and the absorbent flows slowly along the fin from top to bottomdue to gravitational force. Sufficient time is given for absorption and heat release. Operating with sodiumhydroxide as absorbent, a temperature lift of 35 K measured between maximum absorbent temperatureand absorbate temperature as well as dilution from 50 wt% to 27 wt% in one continuous process step isattained in absorption. During desorption, a concentration lift from 25 wt% to 53 wt% at a temperaturespread of 44 K between desorber and condenser is reached. In relation to the concentration difference,a theoretical energy density of 435 kW h/m3 in respect to the discharged absorbent is reached. This development enables compact, lossless, long term heat storage suitable for space heating and domestic hotwater. + ID der Publikation: hslu_101504 + Art des Beitrages: Wissenschaftliche Medien + Sprache: Englisch + Letzte Aktualisierung: 2024-07-03 14:49:14

AbstractExtensive work is undertaken in search of new materials suitable for thermal sorption storage. High energy capacity is the all sought after goal. In most cases this translates to a high maximum water vapor uptake. While this is notably important, in the system development and operation additional factors become strong contributors to the success or failure of a seasonal thermal storage system. Included are, the required system charging temperature. In domestic applications temperatures below 100°C are most fitting to the existing building solar collector infrastructure. Further charging limitations can result from possible material characteristics such as crystallization. Just as critical as charging is discharging. It is precisely at this point where much can be gained or lost. In discharging the temperature difference between the minimum absorber temperature and the minimum evaporator temperature is critical. A low temperature difference between these two temperatures permits low resulting sorbent concentrations and thus a high accessible capacity. In a system application, these temperature levels are not freely chosen. These considerations lead to highly varying operation results in both output temperature and concentration. In this paper insight is given in respect to a sorption demonstrator plant based on sodium hydroxide as sorbent and water as sorbate.

Compact interseasonal thermal storage is a key enabler for renewable heating. A promising approach is the liquid absorption process, extended with absorbent and absorbate storage. In sorption heat storage, the conventional parameters—temperature gain and power density, governing the sorption heat pump process—are extended by the parameter energy density. This opens up new challenges for heat and mass exchanger design, demanding a detailed understanding of the fundamental mass transport process under technically relevant constraints. Toward this objective, investigation in the water mass transport in a static aqueous sodium hydroxide thin film at application‐specific temperature and pressure using temporally and spatially resolved Raman spectroscopy is performed. Based on the measured concentration gradient in the film, it is determined that the mass transport in the film and not the liquid–gas interface is limiting.

This paper describes a thermochemical seasonal storage with emphasis on the development of a reaction zone for an absorption/desorption unit. The heat and mass exchange is modelled and the design of a suitable reaction zone is explained. A tube bundle concept is presented for the heat and mass exchangers and the most demanding working conditions they should fulfil are modelled and discussed. To estimate the performance of such a reaction zone and to design it, numerical models were developed and are described in this paper. Several parameters influencing these models were tested such as the sensitivity of the models to the correlation used to calculate the heat and mass exchanges, the tube diameter and the tube pitch influence. The final contribution of the tube bundle modelling is to size and design the heat and mass exchanger constituting the reaction zone. This work will be used as a basis for the reaction zone construction of an aqueous sodium hydroxide seasonal thermal energy storage prototype. + ID der Publikation: hslu_101510 + Art des Beitrages: Wissenschaftliche Medien + Sprache: Englisch + Letzte Aktualisierung: 2024-07-03 14:49:55