PCM-module to improve hot water heat stores with stratification: first tests in a complete solar system

Transcription

1 Futurestock th International Conference on Thermal Energy Storage, Warsaw, POLAND PCM-module to improve hot water heat stores with stratification: first tests in a complete solar system *Luisa F. CABEZA 1, Miquel NOGUÉS 1, Joan ROCA 1, Josep ILLA 1, Stefan HIEBLER 2, Harald MEHLING 2 1) Dept. Informàtica i Eng. Industrial, Universitat de Lleida, Jaume II 69, Lleida (Spain), lcabeza@diei.udl.es 2) ZAE Bayern, Division 1, Walther-Meißner-Str. 6, Garching (Germany), mehling@muc.zae-bayern.de KEY-WORDS Phase Change Materials (PCM), Domestic hot water tanks (DHW), Solar Energy ABSTRACT The final goal of this work was to bring the idea to improve hot water heat stores with stratification by inserting PCM-modules at the top closer to application in the market. Therefore, in contrast to previous work, tests were performed under real operating conditions in a complete solar heating system that has been constructed at the University of Lleida, Spain. Furtheron, a new PCM graphite compound with optimized thermal properties was used. The results of this work show that only 3 kg of the PCM compound in a 146 liter storage tank are enough to compensate for a heat loss of 3-4ºC in 32 liters of water in the top of the storage. This is equivalent to the cool down in the storage without PCM module over a period of about 10 hours. Reheating of cold water after a fast cool down (unloading) of the storage was in minutes. This is considered to be fast enough for most applications. 1. INTRODUCTION Thermally stratified storage tanks are an effective heat storage technique that is widely used in energy conservation, for example solar energy systems, and load management applications. If water of different temperatures is contained in a tank, thermal stratification arises because a temperature variation in the water leads to a density variation and warm and cold water are separated by means of gravitational effect, hot on top, cold at the bottom. The stratification phenomena can be employed to improve the efficiency of storage tanks. Heat at an intermediate temperature, not high enough to heat the top layer, can still be used to heat the lower, colder layer. In ordinary hot water heat stores the water mixes and supply of water at an intermediate temperature can lower the maximum temperature to a level not useful any more. In contrast, the temperature change in the top layer of a hot water heat store with stratification is usually small as mixing is avoided, and is held as close as possible at or above the temperature for usage. The temperature change at the bottom however can be large. Latent heat storage with phase change materials (PCM) gives a high heat storage density at small temperature difference, as it is the case in the top layer of a hot water heat store with stratification. This allows a favorable combination of both technologies. The low price and high power by direct discharge of the water as storage medium remain, while small amounts of PCM significantly increase the storage capacity of the top layer and improve performance of the storage for special load profiles. 273

2 The new concept presented here is therefore to place a PCM-module into the top of a hot water storage tank with stratification as shown in fig. 1. The advantages of the stratification still remain in this new system, but the addition of a PCM-module gives higher storage density in the top layer. Fig. 1. Domestic Hot Water (DHW) store with PCM module (left) and numerical model with a simulation result showing compensation of heat loss in the water in the top parts of the tank (right). If vertical heat transfer is small, as is usually the case in storage with stratification, this has two effects. First heat loss in the top layer can be compensated to have hot water for longer times without supply (fig. 1) and second, the PCM module can reheat part of the water automatically after a complete and fast unloading of the tank. The advantages of such a heat storage system become obvious in an example. A cylindrical PCM-module is inserted into the top of a cylindrical, hot water heat store. The storage design shall be: The PCM-module has half the diameter of the store and a quarter of its height. Then, 1/16 (about 6%) of the total volume of the heat store is PCM. In the top layer (defined by the vertical extent of the PCM module), one quarter of the volume is PCM and three quarters are water. The PCM shall be NaOAc 3H 2 O, which has a phase change enthalpy of H = 330 kj/l, a heat capacity of about 4.0 kj/l C and a melting point of 58 C. User profile: The maximum temperature in the heat store shall be 65 C, The minimum temperature for use shall be 50 C. The case studied now is cooling down of the storage by heat loss to the ambient without heat supplied. The heat store is assumed to be initially at 65 C. While cooling down by heat loss to the ambient (fig. 1 right), at 58 C water temperature in the top layer the PCM-module starts to give off the latent heat. If vertical heat transfer is small, then the heat transfer is mainly from the PCM-module to the water in the top layer. We then get for the cases with and without PCM: Without the PCM-module: the heat stored in the top layer which can be lost to the ambient before the water gets too cold to be used can be calculated from the temperature drop (15 C) and the heat capacity of water (4.1kJ/L C). It is VTL 62kJ/L (VTL = volume top layer). With the PCM-module: the heat stored in the top layer is VTL 98kJ/L in the PCM and VTL 41kJ/L in the water (less than before due to the volume of the PCM). The energy stored in the top layer with the PCM-module is therefore more than twice as large as without PCM! Therefore, if there is no heat supply to the store for a long time, the water in the top layer with 3/16 (19%) of the stores total volume can be held warm by the PCM-module for twice as long as without the PCM-module. This can be a significant advantage under certain load profiles. The idea was first developed and studied at the ZAE Bayern, Germany [1,2], where a tank was constructed and its performance was tested with the addition of a PCM module on top of it. This work 274

3 included experimental results and numerical simulations of the system using an explicit finitedifference method (fig. 1) and proved the feasibility of the concept. It was also shown that heat transfer in the PCM modules is crucial. Therefore, a PCM graphite composite with high thermal conductivity was used. However, the material available at that time had two restrictions. Only less suitable PCMs than NaOAc 3H 2 O could be used and the composite came in plate shape which makes it difficult to fill into modules of some other geometry. These problems have now been solved through the development of a new composite that is produced in a different way. To test the performance under real operating conditions, a complete solar heating system has been constructed at the University of Lleida, Spain. The solar system constructed consists of two identical circuits with solar collector and storage to be able to test storage in water and in water with PCM under identical conditions. The solar system and the first experimental results performed in it are presented here. Experiments shown here were designed to test the material in modules and to find a suitable geometry and number of modules. Experiments using the solar collectors as heat source and under realistic demand cycles will follow soon. 2. EXPERIMENTAL 2.1. Description of the solar system For the first experiments shown here, the right tank was equipped with thermocouples as indicated in fig 2 to measure the temperature in the water at different levels. This allows comparison of measured and simulated data (fig. 1). The thermocouples were fixed to the vertical pipe that extends over most of the tank interior and is used to supply cold water when hot water is taken out at the top of the tank. To allow controlled heating an electrical heater was used instead of the solar collectors. Figure 2. Installed DHW tanks and PCM-modules (left) and geometric dimensions (cm) including position of thermocouples (center and right) The PCM and the PCM module First tests at the ZAE Bayern have shown that the heat transfer in the PCM is crucial for the performance of the new concept. Therefore a PCM graphite composite was selected as storage material. Until now PCM graphite composites were based on a solid graphite matrix which comes in plates and cannot be filled easily into complicated shapes. A new production process developed at SGL TECHNOLOGIES GmbH to form a PCM-graphite compound has now overcome this disadvantage. The result is a granular PCM-graphite compound. The production by compounding is a 275

4 well-known technology and the production of the compound can therefore be done with standard equipment. In contrast to the infiltration into a pre-pressed matrix, this process allows also the use of PCMs that are usually repelled by a graphite surface and therefore cannot be infiltrated into a prepressed matrix. Here we used a compound of about 90Vol% NaOAc H 2 O and 10% graphite. The PCM was choosen because of its suitable thermal data and also because of its low price. The data of the PCM graphite compound were determined by measurement and are: Density: 1,35-1,4 kg/l Melting point 58ºC Heat capacity: ca. 2,5 kj/kgk; Enthalpy: kj/kg Thermal conductivity 2-5 W/m K The PCM modules used were two commercial aluminum bottles (fig 2 left). They were filled with almost identical amounts (1,40 and 1,62 kg) of the new PCM-graphite composite material. The bottles were not filled completely, so that they would float automatically in the top layer of the tank. This results in a smaller effect than possible because not all of the volume of the module is used for PCM. However, this way experiments are much easier to perform. The data of the PCM modules are Diameter: 8,8 cm, height: 31,5 cm. Total PCM amount: 3,0 kg, which results in a total latent heat of approximately 600kJ Expected effect on the performance of the heat storage The expected effect can be calculated according to the discussion in the introduction and the data from fig. 2 right. We assume again, as a simplification of the problem, that heat transfer in the vertical direction is small due to thermal stratification. The heat released by the PCM-modules is then predominantly heating a layer of water of the same height as the PCM modules as shown in fig. 1 (right). The height of the PCM module is approximately 30cm (fig. 2 right). Using the data for the tank we calculate that: The PCM modules extend through ¼ of the height of the tank (exactly 31,5cm out of 125cm). The PCM modules have 2 1/20 of the cross sectional area of the tank (8,8 2 /39 2 1/20 each), that is a total of 1/10. The PCM modules fill 1/4 1/10=1/40 (only 2,5%) of the total tank volume. The PCM modules fill 1/10 of the 30cm high upper (top) layer that will be heated by them. The remaining volume of this layer is water with a total amount of 32 liters. The heat capacity of the water in the upper layer is then 32 4,1kJ/ºC=132kJ/ºC. Therefore the 600 kj stored as latent heat in the modules can heat the water around them by 600kJ:132kJ/ºC=4,5ºC. The expected effect on the performance of the storage tank are then: The heat stored in the PCM modules can potentially compensate heat loss and delay the cool down of the water layer around them by a time where the remaining layers have cooled down by 4,5ºC. If the tank is unloaded in a short time, the heat released by the PCM modules can reheat the water around them by 4,5ºC. These effects were then be experimentally tested. 2. MEASUREMENTS 2.2. Compensation of heat loss on stand still The purpose of this experiment is to test for how long the period with hot water is extended using the PCM modules if there is no heat supplied to the storage, that is on stand still of the heat supply. This for example could be a few days without sun in the case of solar collectors. Therefore, the tank was initially heated to 70ºC and the heater then shut off. During the cool down period, the temperature in the water was measured at different levels as indicated in fig. 2. In addition, two thermocoulpes (PCM 1 and PCM2) were fixed on the outside of the PCM modules. This allowed a control of the PCM temperature, however it has to be pointed out that it is a temperature slightly lower than the one within the PCM. 276

5 Fig. 3 shows the recorded temperature history. Except for the measurements at 0cm and 30cm at the bottom of the tank all the temperatures decline very similar at the beginning. The solidification of the PCM starts after slight subcooling, at about 54,5ºC. Then the water in the upper layer at levels 110cm and 120cm (fig. 2) remains at constant temperature because heat loss there is is now compensated for by the heat released by the PCM modules. At 60cm the temperature declines as before and at 90cm, which is close to the lower edge of the PCM modules (fig.2 right), vertical heat transfer influences the temperature decline. After about 10 hours the latent heat is released and the temperature in the upper water layer declines at a similar rate as in the other parts of the storage. The maximum difference in temperature between upper layer and lower parts which cool down as without PCM module is about 3ºC. This is reasonably close to the value of 4,5ºC calculated before, as it is seen in fig. 3 that some heat is released later to the water and vertical heat transfer also takes place (as the 90cm level shows) temperature ºC may 28-may 29-may 30-may 31-may 01-jun 02-jun time temperature ºC PCM2 PCM1 90 cm 110 cm 120 cm cm cm 50 30/5/03 0:00 30/5/03 12:00 31/5/03 0:00 31/5/03 12:00 date time Figure 3. Compensation of heat losses on cool down: total data (left) and important part (right). The effect on the storage performance is, as indicated by the arrows with two heads, about 10 hours with a maximum compensation of 3ºC Reheating of the top layer after complete unloading The second effect to be tested was reheating of water.this simulates a demand profile, where the hot water is completely taken out of the storage tank and replaced by cold water. If the time used for this process is short enough, the PCM module will only loose little heat. Most of its heat is then given of to the cold water which in turn is thereby partly reheated. Depending on the temperature of the cold water and other design parameters, this might be sufficient to allow a further unloading of the storage without heat supply from the outside. As calculated before, a reheating by about 4,5ºC can be expected in the upper water layer. An exact calculation is hard to do because natural convection due to the higher and time dependent temperature gradients will now also become significant and cause mixing. The experiment was again performed after heating the whole storage to 70ºC. At the beginning, water of about 20ºC was supplied to the tank bottom while hot water was taken out at the top (fig. 4). When the cold water reached the 120cm thermocouple the process was stoped. As fig. 4 shows, the water at the 110cm layer which was at 28ºC at this time heated up to 33ºC in about 15 minutes. This again close to the expected effect for the temperature rise which was calculated to be 4,5ºC. The time needed to reheat the water is short enough to improve for example solar hot water systems in households. The reheating can for example supply water for an additional shower if the tank was just emptied by other people. 277

6 70 65 temperature ºC cm 30 cm 60 cm 90 cm 120 cm PCM 1 PCM cm :50 16:00 16:10 time hh:mm Figure 4. Reheating of the water around the PCM modules. 3. CONCLUSIONS The final goal of this work was to bring the idea to improve hot water heat stores with stratification by inserting PCM-modules at the top closer to application in the market. Therefore, in contrast to previous work, this work used a new PCM graphite compound with optimized thermal properties and acceptable price and the tests were performed in a real solar system. The main results of this work are: The effects of the PCM module as they were calculated were also observed in the experiments. Only 3 kg of the PCM compound are enough to compensate for a heat loss of 3-4ºC in 36 liters of water. This is equivalent to the cool down in the real storage over a period of about 10 hours. Reheating of cold water after a fast cool down (unloading) of the storage was in minutes, which is considered to be fast enough for most applications. Acknowledgements The authors would like to acknowledge the companies Lapesa S.A., Atesa S.A., and SGL Carbon. This work was partially funded by the Spanish project DPI C02-02 (Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica ). References 1. Mehling H., Cabeza L. F., Hippeli S., Hiebler S.: PCM-module to improve hot water heat stores with stratification, IEA ECES IA Annex 17 1st Workshop, Garching (Germany), Mehling H., Cabeza L. F., Hippeli S., Hiebler S.: PCM-module to improve hot water heat stores with stratification, Renewable Energy (2003), 28 (5), pp Mehling H., Hiebler S., Ziegler F.: Latent heat storage using a PCM-graphite composite material, Proceedings of TERRASTOCK 2000, Stuttgart, Germany (2000). 4. Zalba B., Marín J. M., Cabeza, L. F., Mehling, H.: Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering (2003), 23, pp