Development of a High Energy Density Sorption Storage System Günter Gartler, Dagmar Jähnig, Gottfried Purkarthofer, Waldemar Wagner AEE-INTEC, A-82 Gleisdorf, Feldgasse 19, Austria Phone: +43/3112/5886/64, Fax: +43/3112/5886/18, Email: g.gartler@aee.at Long-term heat storage enables a major technical break-through for an effective year round use of solar thermal energy. Thermo-chemical processes as used in sorption storage systems give a new chance to store the heat with a high energy density and for extended periods. The project MODESTORE (Modular High Energy Density Sorption Heat Storage) is being supported by the European Commission since April 23. The major objectives of this project are the monitoring of a first generation system installed in the past and the development of a second generation prototype. The main improvement is the integration of key components (evaporator/condenser and the reactor) into one single container. A modular design enables to operate in a wide variety of applications in the near future. Test runs of the installed first generation system have been done under controlled conditions to gain operation experience and to assure a detailed characterisation of the complete system. A control program was written in order to operate the storage in a reliable and automatic way. Detailed experimental data of all tests were obtained, analysed and evaluated. Furthermore a simulation model was developed. The validation of the simulation model was done comparing simulated and measured charging and discharging cycles. The model represents well the basic performance of the adsorption heat store. The newly developed second generation prototype will be engineered and extensively tested, analysed and evaluated under practical conditions to allow the finalisation of the product development and identification of the most attractive market. Previous Projects The Fraunhofer Institute for Solar Energy Systems (Freiburg/Breisgau, Germany) and the company UFE Solar GmbH (Berlin, Germany) started with preliminary tests and first investigations concerning seasonal heat storage based on the adsorption process in 1995. Research work was continued with financial aid of the European Commission and the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) in the project HYDES (High Energy Density Sorption Heat Storage for Solar Space Heating). In this project, the principle technical feasibility of the sorption storage system was proved. The experience gained during this project will be of use in the frame of the new project MODESTORE which is also funded by the European Commission and on the Austrian national level by BMVIT. The work in this project started in April 23 and will be continued for three years. Basic Principles of an Adsorption Heat Storage System In sensible and latent heat storage devices heat is stored together with its corresponding amount of entropy. In these so-called direct heat storage media, heat
i.e. energy is transferred directly to the storage medium. The achievable energy density is limited by the entropy storage capacity of the material. Otherwise the adsorption process is a reversible physico-chemical reaction suitable to store heat in an indirect way. This kind of thermal storage allows to separate energy and entropy flow. The storage capacity is not limited to the maximum of entropy intake. The energy density can be much higher if entropy is not stored directly in the medium. Therefore a heat source and a heat sink is involved both during the charging and discharging process to withdraw or collect the necessary entropy. The storage works like a heat transformer on the principle of a chemical heat pump. During adsorption of water vapour, a phase chance takes place between vapour and liquid phase on the surface of the in this project used silica gel. The released adsorption enthalpy consists of the evaporation enthalpy of the working fluid and the binding energy of the adsorption pair. The working principle involves several different phases illustrated in figure 1 and described below: 1. Charging process (desorption, drying of silicagel): heat from a high temperature source is fed into the device, heats the silica gel and vapour is desorbed from the porous solid. The desorbed vapour is led to the condenser and condensed at a lower temperature level. The heat of condensation has to be withdrawn to the environment. 2. Storage period: the dry adsorbent is separated from the liquid working fluid (the connecting valve is closed). As long as these components stay separate heat storage without losses is possible if the sensible heat involved is neglected. 3. Discharging process (adsorption, loading of silica gel with water vapour): the valve between the evaporator and the adsorber is opened. The liquid working fluid evaporates in the evaporator taking up heat at a low temperature level. The vapour is adsorbed and releases the adsorption heat at a higher temperature level. This is the useful heat. Figure 1: The working principle of an adsorption heat storage. There are several quantities and process parameters important when the potential energy density of a sorption pair for heat storage applications is evaluated. The main ones are:
1. Temperature lift: it depends on the current loading level of the sorbent and is a material property. 2. Adsorption enthalpy: it consists of the evaporation enthalpy of the working fluid and the binding energy of the adsorption pair. A high specific evaporation enthalpy is a must for high energy densities, therefore water is one of the primary candidates. 3. Sensible heat and process management: an intelligent system design and process management along with good insulation is essential. 4. Energy density: the energy per unit volume is the quantity of primary interest. It is the product of specific energy (energy per mass of sorbent) and the bulk density ρ s. After due consideration, the process of thermo-chemical heat storage with the adsorption pair silica gel and water was selected. Silica gel is a very porous and vitreous substance. The material is made up mainly of SiO 2 and is extracted from aqueous silicic acid. The equipment installed in the laboratory of AEE-INTEC in Gleisdorf/Austria is filled with commercial silica gel GRACE 127 B. This silica gel consists of spherical particles with a diameter of two to three millimetres. Its bulk density is 79 kg/m 3, the interior surface is 65 m 2 /g. The high energy density, the quantity of primary interest, is achieved by a high evaporation enthalpy, the polarity of water and the large interior surface of silica gel. Additional components like heat exchangers reduce the energy density if the whole system is considered. The system is evacuated to enable water vapour transport without use of mechanical energy. The vapour pressure add up to 1 to 5 mbar in the system. The Equipment The first generation prototype system (illustrated in figure 2) was installed in the laboratory of AEE-INTEC in Gleisdorf/Austria. System simulations showed that for a long-term heat storage application (storage periods of several months) a system consisting of several adsorbers and one evaporator/condenser would yield the best performance. This was the reason to install a system assembled of two adsorbers. All containers have a nominal volume of 1.25 m 3, filled with 1.1 m 3 of silica gel, and are equipped with internal heat exchangers. For the adsorber heat exchangers, a spiral plate heat exchanger was chosen. The evaporator/condenser contains two plate fin heat exchangers of different size located in the upper (condenser) and lower (evaporator) part of the container. The hydraulic system consists of two distribution manifolds. There are two heat sources (the solar plant and the electrical flow heater) and three heat sinks (low temperature heat delivery system in a test apartment, hot water boiler in the test facility and a rain water cistern) available. A solar thermal plant with an aperture area of 2.4 m 2 is on-hand for the test plant as the primary source of energy, the electrical flow heater is used as additional energy source. Figure 3 shows the four individual components in schematic terms as well as the heat flows between components.
Hydraulic Steam pipe EV / CO tank Adsorber 1 Adsorber 2 DHW storage tank Figure 2: Sorption storage tank plant at the AEE INTEC in Gleisdorf/Austria (EV/CO: evaporator/condenser, DHW: domestic hot water). Figure 3: Schematic presentation of hydraulic circuits.
Experimental Results of the Test Runs The test runs were carried out under controlled conditions to assure a detailed characterisation of the complete system including storage module and heat sources under steady state and dynamic operation conditions. The objectives were to run specific test cycles and gain operation experience of the storage. The data were analysed and evaluated regarding energy flows, storage capacity and temperature level achieved. A major objective was to find suitable operation modes, control strategies and recommendations for a redesign of the system for a broader use. The gained experience will now be used as a feedback for the development of the second generation prototype. The development of an optimised control cycle which will be able to operate the storage in a reliable and automatic way is an important goal of the new project. For this reason, a control program was written. The pressure in the various components was used as one of the main control parameters. The following cycle operation was implemented: Measurement of the pressure in the condensate storage module Measurement of the pressure in the adsorption reactor The measured pressures are compared and the valves for charging or discharging of the storage are opened according to the predefined program A fourth step is reserved for the relaxation of the system First tests showed that long-term heat storage using solid sorption processes with water vapour as working fluid is technically feasible. Tests of the Adsorption Process Evaluation of tests of adsorption cycles showed an energy output of exactly 115 kwh/(m 3 silica gel) in the adsorber 1 and 123 kwh/(m 3 silica gel) in the adsorber 2 with a flow temperature out of the adsorber of 32 C minimum. The power values and energies during an adsorption cycle are presented as an example in figure 4. The high power peaks in the evaporator heat exchanger result from the high flow temperature in the collector loop. The average power of the adsorber heat exchanger during the test, which could be supplied to a load in a real application, equaled 2.87 kw with a maximum value of 3.9 kw. In this test, the energy input (evaporator) was 24.3 kwh and the energy output (adsorber 2) 27.4 kwh. Simulations have shown that a storage tank density with a maximum of about 15 kwh/(m 3 silica gel) can be expected with the type of silica gel used. With a density of 79 kg/m 3 and a silica gel mass of 88 kg (dry substance) in adsorber 2, a storage tank density of 123 kwh/m 3 was achieved experimentally. The theoretically calculated value of around 15 kwh/(m 3 silica gel) could be reached with an optimized operating strategy. Nevertheless, it must be realized that energy densities achieved with commercial silica gels under technical conditions are rather insufficient. The energy density that has been reached experimentally, is only in the range of latent heat storage with about 1.8 to 2.2 times the value of water. This means that the storage volume is cut
in half compared to a system with water. Modified sorption materials were tested by the Fraunhofer Institute for Solar Energy Systems, which achieve an energy density which is 5 times higher than water under technical conditions. But up to now, these materials showed insufficient stability under operating conditions. The development of stable modified sorption materials will be an important task and a great challenge. 13 3 12 11 25 power [kw], charging level [%] 1 9 8 7 6 5 4 3 2 power adsorber 2 power evaporator charging level energy output adsorber 2 energy input evaporator 2 15 1 5 energy [kwh] 1 6: 8:24 1:48 13:12 15:36 18: 2:24 22:48 time Figure 4: Power and energy curves of adsorber 2 during an adsorption process. Tests of the Desorption Process The power values and energies during a desorption cycle are presented as an example in figure 5. The power peaks in the adsorber heat exchanger show the heating phase of silica gel from 25 to 75 C between 8: a.m. and 12: a.m. Because the system was already comparably dry, desorption started only on 75 C. The temperature of the adsorber rose continuously to 82 C and was maintained at this temperature level for 24 hours. The condensation temperature in the condensate heat exchanger was around 23 C. The reduction in power at the condenser heat exchanger shows that there is a thermodynamic equilibrium for these temperatures in the adsorber and condenser at a charging level of 4%. Further desorption is now only possible when the condensation temperature is lowered, which was done in this test and can be seen in form of the second power peak of the condenser heat exchanger. Another possibility is to raise the temperature in the adsorber. A rise in temperature was not possible since the maximum temperature of the electrical flow heater is around 9 C. The average power of the adsorber heat exchanger during the test was 1.7 kwh with a maximum value of 1.1 kw, the average power of the condenser heat exchanger was in.67 kw with a maximum value of 4.85 kw.
power [kw], charging level [%] 12 1 8 6 4 2 8:16 1:4 13:4 15:28 17:52 2:16 22:4 1:4 3:28 5:52 8:16 1:4 13:4 Time 5 45 4 35 3 25 2 15 1 5 energy [kwh] power adsorber 1 power condenser charging state energy output condenser energy input adsorber 1 Figure 5: Power and energy curves of adsorber 1 during a desorption process. Development and Validation of a Simulation Model To be able to perform annual performance predictions of the sorption heat store, two new TRNSYS modules were developed. The first one calculates the current temperature and the state of charging of the adsorption material in the storage tank, the second module calculates the temperature and liquid level in the condensate storage tank. These modules were then used in a system simulation that includes a space heating system of a typical single-family low-energy house (defined in IEA- SHC Task 26). The design temperatures of the space heating loop were 35 C flow and 3 C return temperature. Figure 6 shows the modular layout of the TRNSYS model including the numbering of the different components used in TRNSYS. The model includes a standard flat-plate collector array that can be controlled to charge either the adsorption store or the condensate storage tank. To validate the models, simulated and measured charging and discharging cycles were compared. In figure 7, a desorption (charging) process is shown. The measured solar energy gain which was used to charge the adsorption store (Q_Koll_a_o) and the measured energy drawn from the condensate storage tank for condensation were used in the simulation model as inputs. The model then calculates the temperature of the adsorbent (T_Ads) and the state of charging (x) of the adsorption store. The bold lines in the figure represent the measured data, the thin lines the calculated values. A very good agreement of the state of charging values can be observed whereas there are still differences of a few Kelvin between the measured and the calculated adsorbent temperature. Therefore small adaptations to the models will still be necessary to improve the agreement of the model with measured values.
Figure 6: Modular layout of the simulated system. 6 9 Temperature ( C) and Percent of H2O pick-up of dry siliga gel (kgwater/kgsilica gel) 5 4 3 2 1 T_ads T_ads_m X X_m Q_Koll_a_o Q_des_o Q_Kond_m 7,5 6 4,5 3 1,5 Power (kw) 1 3 5 7 9 11 13 15 Hours Figure 7: Validation of the simulation model.
Development of the Second Generation Prototype Module The basic technical objective of the current project is to develop a modular second generation prototype of an advanced thermal storage system with high energy density based on sorption technology. To achieve this main objective, the existing first generation sorption heat storage unit was monitored and evaluated in the present installation. In order to meet the targets of straight forward installation and a reduction of costs, the main improvement is the integration of key components (evaporator/condenser and the reactor) into one single container as illustrated in figure 8. Then a vacuum connection between the different units is no longer needed, which makes the installation of the modules much easier. Due to the target of having a modular design and reducing the size and weight of the modules, the sorption units are designed much smaller than the first generation prototypes. The heat exchanger design and production of the adsorbers play a dominant role for the system. In the new system the adsorber/desorber heat exchanger will be completely redesigned. For better a heat exchange, U-shaped finned tubes will be used. The silica gel area is surrounded by the evaporation/condensation area. In doing so, the ratio area to volume is greater than it was in the first generation prototype. Since more water vapour per time is able to come in contact with the silica gel, the physico-chemical reaction is faster. The compartments (silica gel area, evaporation/condensation area) are separated from each other by a water vapour permeable membrane and a metal inner surface. The whole weight of the silica gel is supported by a perforated slab. Figure 8: The second generation prototype module.
One goal of this project is to monitor the system under real operation conditions. For this reason, the second generation thermal sorption storage system will be installed in a single-family house with a solar heating system and low temperature heat distribution. The energetic characteristics of the system and the relevant temperatures will be measured for at least one year. The monitoring will give information about the optimum control strategy of the different energy sources, the energy distribution and the general reliability of the system. After the monitoring, the data and information of the field test will be analysed, compared with the simulation and reported. Future Work Future work will put emphasis on the following fields: Continuation of the system monitoring (first and second generation prototype) in order to implement and test the developed control algorithms. Technical redesign of the storage modules. Production of a second generation prototype module. Development of stable modified sorption materials. The newly developed second generation prototype needs to be extensively tested, analysed and evaluated under laboratory and practical conditions as input for the finalisation of the product development and identification of the most attractive market. Acknowledgements We are grateful to our project partners from Fraunhofer Institute for Solar Energy Systems (Freiburg/Breisgau, Germany) and Sortech AG (Freiburg/Breisgau, Germany) for their valuable work which contributed to the success of the project. We thank the European Commission and the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) for financial support of the projects HYDES and MODESTORE. References W. Mittelbach: Final Report HYDES Project. Final Report of a Joule III Project, European Comission 21 Tomas Nunez, Hans-Martin Henning, Walter Mittelbach: High Energy Density Heat Storage System Achievements and Future Work, ISES Solar World Congress 23, Göteborg, Sweden