First tests on a solid sorption prototype for seasonal solar thermal storage. Wim van Helden 1, Waldemar Wagner 1, Verena Schubert 1, Christof Krampe-Zadler 2, Henner Kerskes 3, Florian Bertsch 3, Barbara Mette 3, Jochen Jänchen 4 1 AEE INTEC, Feldgasse 19, Gleisdorf, Austria, email: w.vanhelden@aee.at, 2 Vaillant, Germany, 3 ITW Stuttgart, Germany, 4 TH Wildau, Germany. Introduction For most regions in the world, solar thermal technology has the potential to provide 100% of the heating and DHW demand in buildings, provided that heat can be stored over the season. A large part of the potential can be filled in with already existing sensible heat storage, mostly in combination with heat pumps. These water-based systems are more profitable when they are very large, supplying storage for a large number of houses or for a district. For a more decentralised demand of heat, compact solutions need to be found, needing less volume for energy storage and thermal insulation. Since a few years, several larger R&D projects are working on compact solutions. In the EU-funded R&D project COMTES, three technologies for the seasonal storage of solar thermal energy are being developed. The aim of the project is to arrive at storage systems that need less volume than comparable systems working with water as the sensible storage material. In one development line, the Swiss research institutes EMPA and SPF work together with the UK company Kingspan Renewables on a system based on the liquid sorption technology, using sodium hydroxide as storage material. The principle of this technology is that with charging the storage, heat is used to change the diluted lye into a concentrated lye. The challenges with liquid sorption are maintaining a low pressure, that is needed to have effective evaporation of the water from the lye at reduced temperatures, and maximising the concentration of the lye, in order to have the highest storage density, without crystal formation. A second technology, based on the supercooling effect of sodium acetate trihydrate, is developed by DTU and the Nilan company from Denmark, the Austrian TU of Graz and Kingspan Renewables. Here, the principle of storage is based on supercooled phase change material. In effect, the storage material can cool down to temperatures below the solidification temperature without solidifying. Therefore, the material can be stored for very long periods of time without heat losses. When the supercooled liquid is triggered, solidification starts and heat becomes available. The challenges with this technology are maintaining a stable supercooling and finding techniques to have a controlled solidification. In the third development line, AEE INTEC from Austria, ITW, TH Wildau and the company Vaillant from Germany collaborate on the development of a solid sorption thermal storage. In an earlier stage, a comparison was made between an open and a closed sorption system [Bertsch2013]. The choice has been made for a closed system, evacuated to work at low water vapour pressures with a number of storage modules that consist of a vessel with a fixed bed of sorption material and an embedded heat exchanger for charging and discharging. See Figure 1 for the principle of closed sorption and Figure 4 for a schematic of the large test vessel.
The aim is to have a test system ready in summer 2014 that will be monitored for a period of one year. In the preparation of this system, several prototype modules were built and tested at ITW and at AEE INTEC. The goal of these tests is to study and understand the dynamic behaviour of a fixed bed of sorption material, in order to optimise the modules used for the test system. Charging High temperature heat Desorption Water vapor Condensation Low temperature heat Storage Dry silica gel Liquid water Discharging Adsorption Evaporation High temperature heat Water vapor Figure 1: Working principle of a closed-cycle adsorption heat store Low temperature heat Materials and method The active material chosen for the storage is a faujastic type of zeolite, 13XBF. It was chosen to use material shapes in the form of small beads, instead of material composed of microscopic powder particles, as the latter have lower permeability and hence a limited water vapour transport in the powder bulk was expected. A series of different zeolite types have been tested by the University of Applied Sciences Wildau. A comparison was made between the normal zeolites and a new class, the so-called binderless zeolites that have a higher storage density due to the absence of a (passive) binder material. Thermogravimetric and calorimetric experiments were performed to determine the isotherms, and cycling experiments performed to determine the material long-term stability. The material 13XBF was chosen because of the somewhat higher water uptake, a higher adsorption enthalpy and consequently a higher energy storage density. Figure 2 below illustrates the water isotherms of 13XBF for adsorption and desorption, at three different temperatures. See also [Jänchen2012], [Jänchen2013] and [Mette2014]. The determined zeolite properties are used as input for a numerical model at ITW with which the heat and mass transfer dynamics in a sorption vessel are to be simulated. This model will also serve as template for a more simplified model that will be built into the larger system model. The earlier system simulation models use an idealised sorption storage that is sufficient for the initial aim of the model: system dimensioning and material choice. The system simulation model contains the following main elements (see Figure 3): 1: the heating and hot tap water demand of a single family house. This heat demand pattern can be generated for a number of different houses at different geographical positions, according to the method developed in IEA SHC Task32 [Heimrath2007]. 2: A solar collector field, consisting of either vacuum flat plate or vacuum tube collectors. 3: The thermal storage, consisting of several unit vessels. 4: A water storage vessel, to contain the condensed water produced when charging the thermal storage units in summer. And 5: auxiliary equipment: tubes, valves, pumps and vacuum parts. With the system simulation model, the target dimensions of the main element were determined by calculating the optimal solar fraction of such a system for the meteorological data of Zurich, Switzerland. These values will be used in the design of the laboratory test system that will be built before summer 2014 and then tested for one complete year.
Figure 2: Water isotherms of zeolite 13XBF, for three different equilibrium temperatures. Filled symbols denote the values determined at desorption. Figure 3: Simplified system layout with the main elements The main element of the thermal storage system is the module or vessel containing the zeolite. The dynamic behaviour of such a module has to be determined in more detail in order to arrive at an optimal design for the modules in the test system. To this end, a series of experiments are performed at AEE INTEC. A module with intermediate size (164 kg of zeolite) was used for these experiments, see Figure 4.
Figure 4: Prototype storage vessel The module is a vacuum tight vessel, as the operating range is between 0.5 and 100 mbar pressure. In the upper part of this figure, the top cross section is shown of the spirally wound plate and tube heat exchanger, that is placed vertically in the cylindrical vessel. This main heat exchanger serves to transfer the heat for charging and discharging the zeolite. The heat transfer fluid in the heat exchanger is thermal oil that can be operated with temperature above 200 C. In the zeolite bulk, a number of thermocouples is positioned in the upper, middle and lower level at different radial distances from the center line. With these, the time-dependent behaviour during charging and discharging can be determined. At the bottom, the old condenser coil is depicted. In the present experiments this was not used and an external condenser/evaporator was used instead. The condenser/evaporator is connected to the top of the module vessel with a water vapour channel (not depicted here). A vacuum valve in this channel is used to control the rate of inflow of water vapour during the adsorption (discharge) phase. The experiments that are performed are both adsorption and desorption tests. With desorption (charging), an external heat source heats up the heat transfer fluid, which is circulated through the spiral heat exchanger in the zeolite. The zeolite heats up and water vapour is driven out of the material, then the vapour flows through the vapour channel, condenses in the condenser/evaporator and the resulting water is stored in a separate vessel. This latter vessel is weighed continuously, giving the mass of water vapour that was driven out of the zeolite. The main control parameters during desorption are the heating temperature and the temperature of the cooling water inflow of the condenser/evaporator. For adsorption experiments, the heat flows are principally reversed. Water from the separate vessel is evaporated and the resulting water vapour transported to the zeolite bed. The water vapour is adsorbed by the zeolite and the resulting heat is transported through the bed and taken up by the heat exchanger and the thermal oil. Again, the determining control parameters are the condenser/evaporator temperature and the heat exchanger inlet temperature.
Results and discussion Adsorptions test The adsorption test is driven with 45 C flow temperature (T_flow) and 40 C return temperature (T_return) from the thermostat unit that represents the heating system of a house. Furthermore, the inlet temperature in the evaporator is set to 10 C and the outlet temperature is set to 12 C. In the sorption storage is 163.8 kg zeolite 13XBF. T_sorp_top T_sorp_bottom T_flow T_return Figure 5: Temperature profiles during the adsorption process The results of the adsorption test can be seen in Figure 5 and Figure 6. The vertical line is marking the start of the adsorption test when the valve between the water reservoir and the storage is opened. The sawtooth shape of almost all measured quantities is caused by the chosen control mechanism: the vapour valve in the vapour channel is opened or closed, depending on the temperature of the outlet (T_return) of the heat exchanger in the vessel. That the adsorption process is working can be seen in the continually decreasing mass of the water reservoir in Figure 6. The starting water load of the sorption material is 15% and the final water load of the sorption material is 28%. The top temperature (T_sorp_top) of the storage initially rises up to 75 C and decreases throughout the adsorption process. This is because the increasing water load changes the ratio between (decreasing) remaining adsorption heat and the (increasing) total heat capacity of the material. The temperature at the top of the sorption storage rises higher and faster than the temperature at the bottom of the sorption storage (T_sorp_bottom), because the water vapour front flows from the top to the bottom through the packed bed. Furthermore, the pressure in the sorption store (p_sorption) increases from 0.4 mbar in the beginning up to 8 mbar in the end and the pressure in the water reservoir (p_water_reservoir) starts with 20 mbar and then decreases to around 13 mbar. A maximum storage performance of 1.08 kw is reached.
The relatively high pressure difference between water reservoir and storage vessel is caused by the bad performance of the evaporator. A common vertical spiral heat exchanger is used as evaporator/condenser. The high filling level of water in the vertical heat exchanger and the low pressure in the heat exchanger are inducing intense nucleate water boiling. Due to the low pressure in the sorption store, liquid water gets sucked out of the evaporator and evaporates after the heat exchanger. The enthalpy for the evaporation is withdrawn from the surrounding thus the water vapour temperature decreases. The water vapour temperature after the evaporator decreases down to -8 C. p_water_reservoir mass_water_reservoir p_sorption Figure 6: Pressure profiles and mass reduction of the water reservoir during the adsorption process In order to increase the storage performance, the water vapour production efficiency should increase, with higher vapour rates at lower pressure differences. Furthermore, the low temperatures of the water vapour after the heat exchanger should be avoided. To achieve this, the evaporator will be improved. Desorption test The desorption test is driven with a set inlet temperature of the sorption storage (representing the solar collector) of 150 C and a set temperature of the low temperature sink of 15 C. Again, the sorption storage is filled with 163.8 kg zeolite 13XBF. The results of the desorption test can be seen in Figure 7 and Figure 8. The vertical line is marking the start of the desorption test when the valve between the water reservoir and the storage is opened. The desorption of the water out of the sorption material is working. Among others, this can be seen from the continually increasing mass of the water reservoir. The starting water load of the storage sorption material is 28% and the end water load of the storage sorption material is 13%. The sorption temperature rises throughout the desorption process, because of the heat flow from the high temperature source to the storage. Thereby the water gets desorbed from the zeolite according to the water load/pressure/temperature balance. At the end of the desorption process the storage temperature is reaching a maximum temperature that is related to the temperature of
the high temperature source. Furthermore, the pressure of the water reservoir and the sorption storage is around 25 mbar. T_storage_inlet T_storage_outlet T_sorption_top T_sorption_botto m Figure 7: Temperature profiles during the desorption process p_water_reservoir p_sorption mass_water_reservoir Figure 8: Pressure profiles and mass of the water reservoir during the desorption process From the mass difference before and after desorption and the sorption enthalpy of the zeolite the amount of heat that was stored in the zeolite can be calculated. This results in 0.11 kwh/kg zeolite. With a bulk density of the zeolite of 700 kg/m 3, we arrive at a volumetric storage density of 77 kwh/m 3. This is 64% of the theoretical value, mainly due to the relatively high residual water vapour content of the zeolite (13%). It should be stressed that these are already relatively good results from the first experiments. Room for improvement is both on the side of higher temperature charging of the zeolite, as well as a more effective evaporation. With recent series of experiments, these improvements already lead to higher storage densities.
Conclusions The main component of a seasonal solar thermal storage system, based on closed water vapour adsorption in zeolite is the low pressure zeolite vessel. A prototype vessel containing 164 kg of binderless zeolite 13X was used to determine the temperature distribution, pressures and thermal storage density. Several desorption and adsorption experiments were performed with given driving temperatures at the high and the low temperature side of the system. It was found that temperatures in the zeolite can reach up to 75 C during adsorption, making the principle suited for heating tap water. One of main limiting elements in the system is the condenser/ evaporator that needs redesigning in order to limit the pressure and temperature drop at the low temperature (water vapour) side. With these first experiments, a volumetric storage density of the zeolite bulk was found, that is about 64 % of the theoretical value. There are plenty possibilities for improvement of the storage density, by increasing the charging temperature of the zeolite, by improving the efficiency of the heat transfer in evaporator/condenser and the internal heat exchanger. In recent experiments, improvements in these parts already lead to increased storage densities. Acknowledgements The described work is performed in the COMTES project, that receives research funding from the European Union. This article reflects only the author s views. The EU is not liable for any use that may be made of the information contained herein. References [Bertsch2013] Florian Bertsch et al. Comparison of the thermal performance of a solar heating system with open and closed solid sorption storage. IEA SHC Conference, Freiburg, Germany. Energy Procedia [Heimrath2007] Heimrath, R.; Haller, M. (2007): The Reference Heating System, the Template Solar System of Task 32, a Report of IEA Solar Heating and Cooling programme - Task 32 Advanced storage concepts for solar and low energy buildings Report A2 of Subtask A, May 2007 (www.iea-shc.org/task 32) [Jänchen2012] Preparation, Hydrothermal Stability, and Storage Properties of Binderless Zeolite Beads, J. Jänchen, K. Schumann, E. Thrun, A. Brandt, B. Unger, U. Hellwig, International Journal of Low-Carbon Technologies 2012; Vol. 30, p. 289-293, doi: 10.1093/ijct/cts037. [Jänchen2013] Shaping adsorption properties of nano-porous molecular sieves for solar thermal energy storage and heat pump applications, J. Jänchen, H. Stach, Solar Energy, 2013,http://dx.doi.org/10.1016/j.solener.2013.07.018. [Mette2014] Mette B, Kerskes H, Drück H, Müller-Steinhagen H. Experimental and numerical investigations on the water vapor adsorption isotherms and kinetics of binderless zeolite 13X. International Journal of Heat and Mass Transfer 2014;71(0):555 61.