EXPERIENCES WITH LOWEX PCM CHILLED CEILINGS IN DEMONSTRATION BUILDINGS

Transcription

1 EXPERIENCES WITH LOWEX PCM CHILLED CEILINGS IN DEMONSTRATION BUILDINGS T. HAUSSMANN, P. SCHOSSIG, L. GROSSMANN Fraunhofer ISE, Heidenhofstraße 2, Freiburg, 79111, Germany +49 (0)761/ , ABSTRACT In several demonstration and commercial projects new chilled ceilings with integrated PCM as dispersed latent heat storage were installed for high efficient low exergy cooling in buildings. Different combinations of ceilings and cold sources were realized to show the main advantages of using PCMs for each combination. The aim of the research project was to develop new technologies to reduce the primary energy needs for cooling in buildings and to create the possibility to cover this energy demand with environmental heat or cold sources. The best approach for these targets is adding thermal mass to a construction material which allows decoupling the cooling demand from the cold production. This allows to shift the cold production from day to night, where most cold sources work more efficient due to lower night air temperatures. LowEx cooling concepts especially with low power or expensive cold sources, for example borehole heat exchangers, often need storage capacity to be able to cover the cold demand. 1. INTRODUCTION The aim of the project called PCM-aktiv, funded by the german federal ministry of economics and technology, was to develop a low exergy (LowEx) cooling system with integrated phase change materials (PCM). LowEx cooling concepts work with small temperature differences between cooling device and room temperature. Therefore these cooling devices need high heat storage capacities or a high power output of the cold source. Integrating a PCM in the cooling device is an easy way to increase the heat storage capacity with little additional mass and volume. Additionally a PCM-storage could be used as a distributed storage, without space needed for a central heat storage device. The distributed heat storage should be integrated into the building as near to the user as possible. In first order, the inner surfaces of a building should be used to install the PCM. Here the energy fluxes which have to be stored and released are small enough to handle with a PCM-storage and the cooling surface gets activated passive. In this case no active components or energy is needed to store or release the heat. The PCM begins melting when the room temperatures reaches the melting range and within the melting process a huge amount of thermal energy gets stored in the PCM isothermal. When the temperature drops below the melting range the PCM crystallizes and releases the stored heat. If the cooling demand in a building exceeds the passive cooling capacity of the PCM, overheating occurs. In this case an active cooling device could be used to cover the excess cooling demand. Combining PCM with conventional cooling surfaces allows both, reducing the energy demand for cooling in buildings and covering the remaining demand with environmental cold sources. The first system we wanted to develop in cooperation with our partners was a chilled ceiling. In a previous project construction materials for passive cooling, especially different plaster types, have been developed. Combining these plasters with capillary tubes allows setting up chilled ceilings active cooled with water. At the end of the project we realized chilled PCM ceilings in several buildings to show the benefit achieved by using PCM and to demonstrate the new cooling technology.

2 2. CONCEPT OF PCM IN CHILLED CEILINGS In our project we installed two chilled ceilings in different buildings to demonstrate the technology. A third ceiling was installed in another commercial retrofit of a building. If a chilled PCM ceiling should be used in a building the control is very important. Depending on the cold source used for regeneration, the PCM has to fulfill different functions to maximize energy savings which have to be considered when setting up the control: 1. Passive cooling and peak shifting Adding PCM to a chilled ceiling increases the thermal storage capacity in a small temperature range. During the day the room heats up due to internal and external loads. When the room temperature reaches the melting range of the PCM it begins to melt and stores a huge amount of energy isothermal and the room heats up much slower. This effect can be used to shift the period of time of active cooling from day to night, or in best case active cooling can be avoided completely. Especially in buildings with an occupation only during the day (e.g. 08:00am 18:00pm), overheating after this period of time is acceptable. During the night the stored heat has to be released. Due to the melting range of the PCM the air temperature in the room has to drop below 19 C and sufficient ventilation is needed to regenerate the PCM passive (night air ventilation). In most cases passive cooling is not possible and the PCM has to be regenerated with a cooling device. Most cold sources work more efficient during the night due to the lower air temperatures. Peak shifting is important for every kind of cold source to reduce the time of operation of the chilled ceiling but especially compression chiller benefit most from cooling during night. For example producing the cold by a compression chiller at 10K lower ambient temperatures during night helps to reduce the chiller energy consumption between 10 to 30%. To demonstrate this combination a chilled PCM ceiling served by a compression chiller was installed in a lab at our project partner DAW. compression chiller Figure 1. Combination of a chilled PCM ceiling with compression chiller. The main benefits achieved by using PCM in the ceiling are reducing the time of operation and shifting the cold production from day to night with higher efficiency due to lower temperatures at night. 2. Accumulating cooling power Conventional cooling concepts produce cold on demand and must be able to cover the peak demand during the day. Especially during night a long unused period of time is available. To use this period of time a cold storage, for example a PCM, is necessary. The storage allows accumulating the produced cold over time and covers during the day the basic cooling demand in the room. Only the excess demand during the day has to be covered directly by the chiller. This allows using chiller with lower cooling power output. Also a conventional water storage tank can be used but a dispersed PCM-storage in the chilled ceilings has several advantages: - No control regime or active components are needed to activate the basic cooling process.

3 - A conventional cold water storage must be sized relative big due to the low usable temperature difference, or the cold has to be stored at lower temperatures with higher losses and worse production efficiency. - Often environmental cold sources are not able to generate cold at such low temperatures. A chilled PCM ceiling makes the usage of environmental cold sources with low power, high installation costs and low costs of operation (e.g. borehole heat exchangers) possible. A chilled PCM ceiling in combination with directly attached borehole heat exchangers was realized in a commercial retrofit project of the Engelhard & Bauer printing plant in Karlsruhe. Figure 2. Combination of chilled PCM ceiling with environmental cold sources. Accumulating the produced cold in PCM over a long period of time during the night allows using cold sources with rel. high supply temperatures or low power output. 3. Combination of temporally alternating consumers Shifting the cold production from day to night and cooling a room with a dispersed PCM storage allows sharing the cold source with other consumers during the day. Especially cold sources with low costs of operation or the use of waste heat or cold can benefit from temporally alternating consumers. Also the area chilled by an existing plant can be enhanced by adding a PCM chilled ceiling. To show the advantages of this combination a chilled PCM ceiling cooling 5 offices during night and convectors cooling an open-plan office during day served by a combined cold, heat and power generator was installed in 2008 at Fraunhofer ISE. adsorption chiller-unit CHP Combined Heat and Power unit Figure 3. Combination of temporally alternating cold consumers with a combined cold, heat and power generator. Better capacity utilization and enhancing chilled area with the same cold source are the main benefits. 3. CHILLED CEILINGS WITHIN THE PROJECT PCM AKTIV In the previous project PCM-Passiv construction materials with integrated PCM have been developed for passive cooling concepts. The aim of the project PCM-Aktiv was to combine these construction materials with capillary tubes to generate an active cooling device. In the first approach we decided to develop chilled ceilings based on gypsum plaster and chilled

4 ceilings based on smoothing cement with implemented PCM. Microencapsulated paraffin with a melting range from C was used as PCM. Two chilled ceilings were realized within the project PCM-Aktiv. In general both ceilings were constructed in the same way. The main differences were: different cold sources, PCM construction materials and layer thickness. For monitoring the ceilings under real usage we installed several temperature sensors in up to 3 different depths of the PCM-layer. Additional we measure the room air temperature in several positions, the ambient temperature and characteristic water temperatures. Figure 4. Cross-section of chilled ceilings with PCM construction materials (plaster or smoothing cement) and capillary tubes for active cooling m² Chilled ceilings in 5 offices at Fraunhofer ISE At Fraunhofer ISE a chilled ceiling with integrated PCM was installed in five offices in the year 2008 with an overall surface area of 100m². In this case the PCM layer consists of a PCM-gypsum plaster produced by our project partner Maxit. The layer thickness of the plaster is about 3cm with a density of 950kg/m³. The latent heat storage capacity is about 16J/g and the overall storage capacity in a 6K temperature range is nearly 162Wh/m². For comparison, without PCM the same ceiling will just be able to store 62Wh/m² in the plaster layer. Figure 5. Impressions of the implementation of the chilled PCM ceilings: left: assembled capillary tubes, middle: covering with PCM plaster, right: finished ceiling The cold source (CCHP) was planned and installed in another EU-funded project called Polysmart (POLYgeneration with advanced Small and Medium scale thermally driven Airconditioning and Refrigeration Technology). A scheme of the combined cold, heat and power generator (CCHP) is given in figure 6. In first order the CHP unit generates electricity. In this process generated waste heat is used to heat the offices during winter directly and in summer the heat is used to serve two adsorption chillers which convert heat to cold at 14 C. Two

5 consumers are attached to the cold source: during the day (07:00am 07:00pm) convectors cool an open-plan office (120m²) on demand and during night (07:00pm 07:00am) the produced cold is used to cool down the chilled PCM ceilings and the offices to be prepared for the next day. Figure 6. Scheme of the used combined cold, heat and power generator to serve two temporally alternating consumers. Cooling the offices passive during the day is the most important target to increase energy efficiency by realizing the principle of temporally alternating cooling. With the same cold source the cooled area can be enhanced and the capacity utilization of the CHP increases. The power output of the CCHP is not big enough to serve both consumers at a time. If the cooling demand exceeds the storage capacity of the chilled PCM ceilings overheating can occur volume flow ambient temperature 3 2,5 temperature [ C] air temperatures temperature plaster (surface) temperature plaster (back side) 2 1,5 1 volume flow [m³/h] :00 supply temperature 02:00 04:00 06:00 08:00 10:00 ambient temperature temperature plaster (back side) air temperature (height 0.2m) volume flow 12:00 time 14:00 16:00 18:00 20:00 temperature plaster (surface area) air temperature (height 1.95m) supply temperature 22:00 0,5 0 17/07/08 00:00 Figure 7. Measurements at Fraunhofer ISE show peak shifting into the night is possible and overheating can be avoided.

6 Figure 7 shows measured temperatures in one office during a representative day. At 07:00am the pump was turned off and during the whole day no active cooling is needed until the scheduled regeneration starts at 07:00pm. The control was programmed to start cooling during the day if the room air temperature reaches 26 C and the regeneration during night should run until the surface temperature of the ceiling drops below 19 C. Both switching temperatures were not reached. The room temperature rises up to 25 C which is within the comfort range, but at the upper end. A problem is the surface temperature of the ceiling which doesn t drop below 19 C during night. That implies that the PCM storage could probably not be fully discharged. The cooling power of the ceiling is not big enough to dissipate the stored heat and additionally the internal loads (PCs, Phones ) in the offices during night with a supply temperature of 16 C. Here lower supply temperature may be needed. This is mainly caused by the building type with good insulation, heavy outside walls, small windows and no ventilation. Without a cooling device the room temperature varies only in a range of 2K within 24 hours, measured after installing the ceilings but without water circulation. Looking at the ceiling temperatures the influence of the PCM can be seen. During the melting process the rising of the temperatures slows down till all PCM is melted. After this the temperature rises as fast as before. Around 22 C a significant change in the temperature gradient can be seen caused by the PCM. How a reference ceiling without PCM would behave is unknown at the moment, but extrapolating the temperature gradient indicates that the period of passive cooling can be extended up to two or three hours m² Chilled ceiling in a lab at DAW Within the project PCM-Aktiv a second chilled ceiling in combination with a compression chiller was installed in a lab at DAW. Figure 8 shows some pictures of the installation. Here a smoothing cement with approximately 40mass% PCM is used with a layer thickness of only 1cm. The heat storage capacity is around 155Wh/m² within a 6K temperature range. For comparison, without PCM the same ceiling is able to store around 40 Wh/m². Figure 8. Installation of a chilled ceiling at DAW. Left: mounting of capillary tubes, right: finished ceiling This ceiling was realized to show the possibility to enhance the energy efficiency of conventional cooling technologies by using PCM construction materials. The main benefits are again shifting cold production from day to night and reducing the need for an active cooling by storing excess heat in the melting process of the PCM. The orientation of the lab is north/east but due to higher internal loads and many windows it tends to overheat quickly. Occupied is the lab from 07:00am 17:00pm. Overheating after this period of time is acceptable.

7 Figure 9. Measurement of a chilled ceiling at DAW. In principle the possibility to shift the cooling demand can be shown, but the control strategy is not optimal. A time dependence control is to implement. Figure 9 shows measured temperatures in the lab. A simple control which only reacts on the measured air temperature in the room is used to control active cooling. If the room air temperature reaches 23 C cooling will start and gets deactivated if the room air temperature drops below 21 C. Again it is possible to cool the room over a longer period of time passive what indicates that in principle shifting the cooling period into the night is possible. A problem is that the passive cooling period doesn t correspond to the occupation time in the lab. In the worst case ( ) the PCM is nearly fully melted in the morning, and the ceiling is forced to start active cooling earlier. It is necessary to implement time dependence to the control to make sure that the PCM starts fully crystallized in the morning. Also different measurements have shown that the ceilings react very sensitive on little changes in the control values. Lowering the values to 19/21 C leads to a continuously activity of the ceiling, because the power output is not big enough. On the other hand raising the set point to 22/24 C leads to more uncomfortable temperatures in the room Chilled ceiling in a retrofit of a printing plant The third ceiling was realized in a commercial retrofit project of an office building. Besides expanding the office area (new floor on top of the old building), better building envelope, and increasing the employees comfort in these offices by for example better daylight usage and better air quality, the building should be cooled and heated as much as possible with night air and waste heat recovery. Because of structurally reasons the retrofit has to be done as light weight construction. To realize a full passive cooling concept more cold storage capacity is needed. An open chilled PCM ceiling was installed in combination with directly attached borehole heat exchangers which enhances the heat storage capacity to be able to cover the excess heat demand with environmental cold. To enhance the ability of passive cooling the melting range of the PCM used is between 23 C and 25 C. These higher temperatures allow more often regenerating the PCM with forced ventilation during the night. On the other hand higher melting temperatures lead to a lower power output while passive cooling during the day. The concrete ceiling in the basement was also equipped with capillary tubes but the

8 power output of the borehole heat exchangers is not big enough to serve both ceilings at a time. It was planned to cool the basement ceiling during day and during night the PCM ceilings in the first floor were regenerated. The Fraunhofer ISE is monitoring the building in a project (EnSan) funded by the German federal ministry of economics and technology The chilled ceiling modules base upon PU-sandwich elements. Room sided the surface is covered with a gypsum plasterboard (Smartboard) and between plasterboard and PU-foam capillary tubes are installed and directly connected to borehole heat exchangers. Figure 10. Installation of a chilled ceiling in a retrofit project. Borehole heat exchanger directly couplet to the ceilings were used as cold source. pre retrofit planned Measured 2008 kwh/(m²a) kwh/(m²a) kwh/(m²a) Heating Cooling Table 1. Energy consumption for heating and cooling Table 1 shows that the measured energy consumption is much higher than planned. Several reasons lead to this result: A higher pressure drop in the water cycles in combination with an oversized pump lead to an insufficient cold distribution and reduced cooling efficiency. Additionally the ground temperature is higher than expected which reduces the power output of the borehole heat exchangers. The cold distribution inside the buildings is now repaired, but the lower efficiency of the borehole heat exchangers still exists. The solution which should now be realized is the installation of a heat pump connected to the borehole heat exchangers to support building heating and by the way cooling down the ground during winter. This monitoring project shows, that LowEx concepts are very sensitive for installation or planning failures. A monitoring helps to detect and fix these failures. 8 radiant cooling ceiling roof roof radiant cooling ceiling height [m] floor : room air temperature [ C] Figure 11. thermography picture of the chilled ceiling an temperature distribution in the building measured in different heights

9 Figure 11 shows on the left a thermography picture the roof showing the different temperatures of the PCM ceiling modules and the high insulated roof of the building. The graph on the right shows measured room temperatures in different heights of the building in the 2 nd floor. The chilled ceiling is much cooler than the high insulated roof behind. On floor level the temperatures are within the comfort range of the users mainly due to the cold radiation of the ceilings. 4. Conclusion Within the project PCM-Aktiv we developed together with our project partners chilled ceilings with integrated microencapsulated PCMs. Depending on the available cold sources the PCM has to fulfill different functions which help to reduce the cooling energy demand in buildings and cover the resulting demand with environmental cold sources. To demonstrate these advantages three chilled ceilings with different could sources or energy concepts were installed in demonstration projects and first measurements were done in A detailed Monitoring will follow during the next years. First results show, that the energy demand for cooling could be reduced so far by optimizing the control strategies. Shifting the energy demand from day to night by adding thermal mass to the building is a good solution to enhance the efficiency of most cold sources. Increasing the chilled area and the utilization capacity of a cold source is another important advantage achievable by using PCM in a chilled ceiling as dispersed storage. We also could show that PCM increases the possibility to cover remaining cold demand with environmental cold sources. Using longer periods of time during night to accumulate produced cold allows using cold sources with lower power output or higher supply temperatures. Measurements with chilled PCM ceilings also show that power output and response time are not negatively affected by the PCM. Especially the increased thermal mass only has an effect on the temperature gradient within the melting range. Outside the melting range the ceiling reacts as fast as a conventional chilled ceiling would behave. All these techniques help reducing the growing energy demand for cooling in buildings. The influence of the PCM could be shown with the measurements but can not be quantified at the moment. To quantify the energy saved by using PCM a reference system is needed, wherefore detailed simulations are necessary. On the other hand our experiences show that the LowEx cooling concepts are very failure sensitive. A monitoring should be done after the installation to make sure the system works as planned. Little failures during planning and installation can lead to significant higher energy consumption of the building which happened for example during the retrofit of the E&B printing plant. REFERENCES BINE Informationsdienst, Latentwärmespeicher, ISSN Kalz D., Pfafferott J., Schossig P. and Herkel S., Thermoaktive Bauteilsysteme mit integrierten Phasenwechselmaterialine eine Simulationsstudie,. Bauphysik, 29(1): Schossig P., Henning H.-M., Gschwander S., Haussmann T., , Microencapsulated phase-change materials integrated into construction materials, Solar Energy Materials & Solar Cells, 89(2/3): Schossig P., Henning H.-M., Haussmann T. and Raicu R., 2003, Phase change materials in constructions, Proc. Phase Change Material Slurry Scientific Conference and Business Forum, Yverdon-les-bains Switzerland, p