WATER AND PCM-WATER STORES FOR DOMESTIC HOT WATER AND SPACE HEATING APPLICATIONS

Transcription

1 WATER AND PCM-WATER STORES FOR DOMESTIC HOT WATER AND SPACE HEATING APPLICATIONS Cristian Solé, Marc Medrano, Martí Comellas, Miquel Nogués, Luisa F. Cabeza Centre GREA Innovació Concurrent Edifici CREA, Universitat de Lleida, Pere de Cabrera s/n, Lleida (Spain) Phone: , Fax: ABSTRACT The University of Lleida built an experimental installation to test the behaviour of a phase change material (PCM) inside a real solar system to provide the domestic hot water (DHW) demand. A specific and constant demand was daily drawn-off from the store to discharge the system. Some advantages were experimentally observed, such as the capability of the PCM to reheat the amount of cold water surrounding the PCM after a partial unload. The temperature of the water surrounding the PCM kept constant or decreased more slowly than the water with no interaction with the PCM. New experiments were performed with a DHW demand widely used by Trnsys users to test the behaviour of the PCM in real conditions. The demand used was from Jordan and Vagen, where the flow rate and the time of occurrence of the demand are selected by statistical means. 1. INTRODUCTION It is well-known that the world energy demand increases every year and the developed countries are the major consumers. Fossil fuels are the primary energy source for most countries, and their variable price highly affects the economy of the different countries. Thus, it is necessary to find new energy sources. Also the energy management takes an important role in this situation. Moreover, it seems clear that one of the ways to decrease CO 2 emissions and to reach the values agreed in the Kyoto protocol is to promote more and more the use of renewable energies and to decrease the energy demand. In this complicated situation, storage appears as a powerful option and solution to help to solve the energy problem in the world. The use of renewable energies together with storage systems can help to decrease the energy demand of the modern societies. On the other hand, efficient and compact storage energy systems seem to be one of the key parameters to promote the use of renewable sources. Due to the mismatch between energy generation and demand, the storage set-up is essential. Phase Change Materials (PCM) seem to be one of the most promising techniques that might lead to this high energy storage performance. A PCM is a material which stores or supplies heat at its melting/solidification temperature using its high thermal energy storage density per unit volume as a consequence of its latent heat, which is higher than the sensible heat. It is possible to use the latent heat of solid-gas, solid-liquid and liquid-gas transformation, however, only the solid-liquid transformation is used due to its lower volume variation.

2 In simulation of solar domestic hot water systems, the demand profile of Jordan and Vagen is widely used by Trnsys users. In this study this profile have been applied in a domestic hot water experimental system to test the behaviour of a phase change material (PCM) in the top part of the storage tank and to check if important diferences can be observed with respect to another profile used in previous experiments. 2. EXPERIMENTAL SET-UP To test the use of PCM in a real system, an experimental solar stand was constructed at the University of Lleida with such a goal. Figure 1 and Figure 2 show pictures of the installation. The stand had four thermal solar collectors, two hot water tanks of 146 L and an electrical heater outside the tanks, which allowed electrical heating with a known power when needed. The two water tanks, from the Spanish manufacturer Lapesa, were identical, but one had been modified to insert the PCM modules. The right tank was equipped with thermocouples as indicated in Figure 2 to measure the temperature in the water at different levels. This allowed to check whether the water in the tank is still stratified. The thermocouples were fixed to the vertical pipe that extends over most of the tank interior and acts as cold water inlet (Mehling et al. (2003)). Figure 1. Solar thermal collectors

3 Figure 2. Hot water tanks from Lapesa and PCM modules Four PCM aluminium modules were located at the upper part of the tank as shown in Figure 2 resulting in a total amount of 5.55 kg. The pilot plant can work continuously with the solar system, knowing that the primary pump operates when the water temperature of the tank is lower than the temperature from the collectors. On the other hand, the system can work isolated from the solar loop with the electrical heater, allowing the repeatability of the experiments in the same conditions. PCM USED The temperature of the water to be stored as domestic hot water is about 60 ºC, therefore the melting temperature of the PCM should be around 60 ºC. In the market different PCM with this melting temperature can be found (Zalba et al. (2003)) and finally a composite of 90% sodium acetate trihydrate and 10% graphite was chosen for the experiments presented here. High heat transfer rate in the PCM is crucial for the final performance of the system and graphite resulted to be the best way to enhance the heat transfer in this application. It was also chosen by its suitable thermal data, and its low price. The data of the PCM graphite compound was given by the manufacturers with density of kg/l, a melting point of 58 ºC, a heat capacity of 2.5 kj/kg K, an enthalpy of kj/kg, and a thermal conductivity of 2 5W/m K. The melting point and the enthalpy were tested in our laboratory with a Mettler Toledo DSC 822e. 3. EXPERIMENTS PERFORMED Different demands were applied to the system in order to simulate an ordinary household performance. The system supplied hot water at 45 ºC when it was required. On the other hand and to be able to reproduce the experiments, the system was charged electrically during the

4 night. Water circulated through an electrical heater and it stopped when the temperature at 90 cm (Figure 2) was at 65 ºC, assuring the melting of the PCM. DOMESTIC HOT WATER (DHW) DEMAND PROFILE The solar installation was designed in such a way that a computer program allowed control of the system to simulate an ordinary household performance. In previous experiments, a morning shower and an evening hot bath were simulated drawing-off the store during some minutes (Cabeza et al. (2006)). In these new set of experiments, the DHW demand applied was the widespread DHW demand used in simulations and in the TRNSYS environment and created by Jordan and Vajen (2001). In simulations performed previously, a demand of 200 L/day at 45 ºC was set in 6 minutes time-scale. These 200 L/day demand at 45 ºC is the total amount used by the user, which is the hot water extracted from the storage tank and the cold water from the tap to cool down the water at 45 ºC. The selected demand was obtained from a full month simulation. Table 1 shows the statistical demand of each day of one month. The 17 th day was chosen because the demand of hot water at 45 ºC of this day was lower ( L) than the volume of the tanks of the solar pilot plant (146 L). Thus, the water demand of this day at 45 ºC was assured. Table 1. DHW demand at 45ºC for each day of one month Day of the month Demand of the day (L) Day of the month Demand of the day (L) Day of the month Demand of the day (L) Day of the month Demand of the day (L) Day of the month Demand of the day (L) There were four draw-offs every day as showed in Figure 3. Every discharge has a specific amount of water used (hot water from the tank plus cold water from the net): 50 L for the first discharge, and 35 L, 20 L, and 30 L for the next three, which results in a total demand of 135 L in one day.

5 8 7 6 mass flow (L/min) time (h) Figure 3. DHW profile set in the experimental set-up 4. RESULTS Figure 4 shows the comparison of the accumulated energy stored in the experimental tanks. Results for the water tank (without PCM) are presented in solid line and results for the PCMwater tank are shown in dashed line. The green line is the value of the accumulated energy transferred to the tank through the internal coil heat exchanger of the tank, and it is seen clearly in the figure that the tank is charged during the night in 4 hours. Then, no more energy is transferred to the tank and this value keeps constant. The black line is the accumulated energy of the tank. For the water tank, it has the same value than the accumulated energy of the water (blue line) but for the PCM-water tank, the difference is due to the energy stored in the PCM (red line). The value of the accumulated energy in the tank, water and PCM changes throughout the experiment due to the DHW energy demand. Comparing the experiments performed without PCM and with PCM in the top part of the storage tank, more energy is transferred to the storage tank for the latter, as shows Figure 4, and as a consequence, more energy is stored in the PCM-water store due to the PCM. Figure 4 shows clearly that the energy stored in the water tank is nearly the same amount stored by the water in the PCM-water store. The difference between both storages is due to the PCM. The storage tank with PCM receives more energy than the water tank during the charging process, which is until the water temperature at 90 cm is at 65 ºC. This increase in the energy supplied to the store means that the energy stored in the PCM-water tank is 8.1% larger than in the water tank. About the total energy stored in the PCM-water tank, approximately 92.4% is due to the water and 7.6% is due to the PCM (Figure 4).

6 25000 Energy (kj) E_DPtot (w/opcm) E_tanctot (w/opcm) E_DPtot (wpcm) E_tanctot (w/pcm) E_watertot (wpcm) E_PCMtot (wpcm) Time (min) Figure 4. Comparison of the accumulated energy stored by the water and PCM-water tanks. Solid lines for the water experiment and dashed lines for the PCM-water experiment. Green line for the energy transferred through the internal coil heat exchanger; blank line for the accumulated energy in the tank, blue for the water and red for the PCM Temperature (ºC) T0 T30 T90 T110 T120 T0 T30 T90 T110 T120 TPCM Time (min) Figure 5. Comparison of the temperature profiles. Solid lines for the water experiment and dashed lines for the PCM-water experiment. T0 is the bottom water temperature, and T120 is the top water temperature. It is difficult to observe the reheating and the slower decreasing of the water temperature in contact with the PCM in Figure 5 but it is clearly shown in Figure 6. It is seen that the temperature at 90 cm from the base of the store keeps nearly constant at 56 ºC during approximately 150 minutes due to the phase change of the PCM. The big decrease in the temperature at 650 min approximately coincides with a draw-off of the tank, also observed in Figure 5 in the layer at 30 cm from the base of the storage tank. Figure 6 also shows the slower decrease in the temperature in the layers at 110 cm and 120 cm for the PCM-water storage tank. The maximum temperature difference between the layer at 110 cm in the water tank and PCM-water tank is approximately 1.3 ºC (the water tank layer is hotter) and it is produced after 800 minutes of the experiments. For the layer at 120 cm, the maximum difference is 1.5 ºC, respectively. The temperature decreases in the stores but it

7 does it more slowly in the PCM-water store. Approximately, 450 minutes later (that is over 1250 minutes), the temperature difference for the 110 cm layer at 1260 minutes approximately is just 0.6 ºC meanwhile it is 0.7 ºC in the 120 cm layer. The layer at 110 cm from the base of the tank cools down 11.1% slower for the PCM-water tank than in the water tank, and the layer at 120 cm does it 12.7% slower. Thus, it is clear than the PCM-water store cools down more slowly than the water store. 62 Temperature (ºC) cm 1.3 ºC 120 cm 1.5 ºC T90 T110 T120 T90 T110 T120 TPCM 110 cm 0.6 ºC 120 cm 0.7 ºC Time (min) Figure 6. Bigger view of the omparison of the temperature profiles at the phase change zone. Solid lines for the water experiment and dashed lines for the PCM-water experiment. T120 is the top water temperature 5. CONCLUSIONS The use of PCM in an experimental solar stand with the ordinary household performance was tested. Four PCM aluminium modules resulting in a total amount of 5.55 kg of PCM were located at the upper part of the tank. The PCM tested was composite of 90% sodium acetate trihydrate and 10% graphite with a melting point of 58 ºC and an enthalpy of kj/kg (data from the manufacturers). A demand of 135 L of hot water at 45 ºC was drawn-off from the 146 L storage tank divided in 50 L, 35 L, 20 L, and 30 L as shown in Figure 3. As expected, the PCM-water tank stores up to 8.1% more energy than the water tank. This increase in the energy stored is mainly due to the PCM. In the PCM-water tank, approximately 92.4% of the energy corresponds to the water and 7.6% to the PCM. Also a slower decreasing of the temperature is observed in the PCM-water tank. A maximum difference of 1.5 ºC and 1.7 ºC is observed in the 110 and 120 cm layer respectively between the water and PCM-water store when the phase change starts. After 450 minutes approximately, the difference between the water and PCM-water tank for the 110 and 120 cm layer is just 0.4 ºC and 0.6 ºC, respectively. The 110 cm layer cools down 11.1% slower in the

8 PCM-water tank than in the water tank meanwhile the 120 cm does it at a 12.7% smaller cooling rate. Thus, it is clear than the PCM-water store cools down slower than the water store. Comparing the DHW demand profile used here from Jordan and Vagen (with a selected day) with another different profile, no differences are observed in the results. ACKNOWLEDGEMENTS The work was partially funded by the Spanish government (project ENE C02-01/CON). Dr. Marc Medrano would like to thank the Spanish Ministry of Education and Science for his Ramon y Cajal research appointment. REFERENCES Cabeza L.F., Ibáñez M., Solé C., Roca J., Nogués M. (2006). Experimentation with a water tank including a PCM module, Solar Energy Materials and Solar Cells, Jordan U., Vajen K. (2001). Realistic domestic hot-water profiles in different time scales, IEA SHC. Task 26: Solar combisystems. Mehling H., Cabeza L.F., Hippeli S., Hiebler S. (2003). PCM-module to improve hot water heat stores with stratification. Renewable Energy 28 (2003) Nogués M., Cabeza L.F., Roca J., Illa J., Zalba B., Marín J.M., Hiebler S., Mehling H. (2002). Efecto de la Inserción de un Módulo de PCM en un Depósito de ACS. Anales de la Ingenieria Mecánica, vol Zalba B., Marín J.M., Cabeza L.F., Mehling H. (2003). Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering