Energy Conversion and Management

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1 Energy Conversion and Management 50 (2009) Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: PCM air heat exchangers for free-cooling applications in buildings: Experimental results of two real-scale prototypes Ana Lazaro *, Pablo Dolado, Jose M. Marín, Belen Zalba Aragón Institute for Engineering Research (I3A), Thermal Engineering and Energy Systems Group, Torres Quevedo Building, C/María de Luna 3, Zaragoza, Spain article info abstract Article history: Received 24 September 2008 Accepted 20 November 2008 Available online 6 January 2009 Keywords: Thermal energy storage PCM Free-cooling Heat transfer Experimental Prototype Latent heat storage using phase change materials (PCM) can be used for free-cooling. In this application low air temperature is used to solidify the PCM during the night and then during the next day, the inside air of a building can be cooled down by exchanging heat with PCM. Short times for charging and discharging the PCM are required. PCM have in general low thermal conductivity, therefore the heat exchanger design is very important to fulfil free-cooling requirements. This paper presents an experimental setup for testing PCM air real-scale heat exchangers and the results for two real-scale prototypes. Results show that a heat exchanger using a PCM with lower thermal conductivity and lower total stored energy, but adequately designed, has higher cooling power and can be applied for freecooling. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Free Cooling using thermal energy storage (TES) system with PCM (phase change material) is especially interesting in climates with high daily temperature variations. Due to the fact that even in these climates, the outside temperature only reaches low levels for a few hours, heat exchange between air and PCM has to be efficient enough to solidify all the PCM during these hours. Different geometries for PCM to air heat exchange have been proposed. Arkar [1] studied spheres filled with a paraffin wax with a phase change around 20 C. Turnpenny [2] tested cylindrical geometry of heat pipes to achieve free-cooling. Results showed that with this geometry a 15 K difference of temperature between air and PCM was needed to have melting times shorter than 10 h. Zukowski [3] tested plastic bags containing PCM with 10 K difference of temperature between air and PCM to charge the system in 3 h. Other authors proposed different solutions to enhance heat transfer. Stritih [4] used metallic fins in a rectangular wall to enhance heat transfer between PCM and water. Graphite and other heat conducting materials have been studied to enhance heat conduction inside the PCM [5,6]. At the University of Zaragoza, Zalba [7] studied rigid encapsulation of PCM in slabs. Results showed that PCM thickness is crucial for controlling of heat transfer power. Based on these outcomes, * Corresponding author. Tel.: ; fax: address: ana.lazaro@unizar.es (A. Lazaro). two different real-scale prototypes of air to PCM heat exchangers have been designed and tested. This work presents the results and some conclusions about free-cooling design with PCM systems. 2. Experimental setup for testing prototypes An experimental setup was designed to study different air to PCM heat exchangers. A closed air loop setup is used to simulate indoor conditions. Setup design was based on ANSI/ASHRAE STAN- DARD method of testing active latent-heat storage devices based on thermal performance [8]. This setup, shown in Fig. 1, is constituted of: Inlet air conditioner allowing the simulation of different operating modes (5 kw air chiller and 4.4 kw electrical resistance) Air flow measurement Difference between inlet and outlet air temperatures measurement (thermopile) Inlet and outlet air temperature and humidity measurement PCM and air channels temperature measurement (over 20 thermocouples) Data logger and data screening Air ducts and gates PID controller The energy balance of air between the inlet and the outlet of the prototype is used for cooling evaluation (Eq. (1)). As the main parameters are the air flow and the air temperature difference /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.enconman

2 440 A. Lazaro et al. / Energy Conversion and Management 50 (2009) Nomenclature Q _m air HX Dh air c Pair DT T surface cooling power of PCM heat exchanger (kw) air flow (kg/s) h air inlet hair outlet (kj/kg) specific heat of air T air inlet Tair outlet (K) kj kgk average of surface temperature of PCM ( C) DT thermoplie kg da temperature difference of air between inlet and outlet measured using a thermopile (K) mass of dry air (kg) between the inlet and the outlet, accuracy depends on the precision in measuring these parameters. The methods used are _Q ¼ _m airhx Dh air _m airhx c Pair DT ð1þ Air temperature difference: Thermopile. There are three difficulties to solve in this measurement: a long period of time with small temperature difference, temperature distributions in air ducts because of its dimensions and accuracy is needed because it is a main parameter of evaluation. ANSI/ASHRAE standard recommends a thermopile to solve this problem. It was chosen because it solves these difficulties. Thermocouples are designed to measure direct temperature difference between two temperature junctions. Since a thermopile is constructed by using several junctions in series of calibrated thermocouple wire, the output signal is amplified by the number of junctions, so sensibility is increased. In this case, a six junction thermopile is used. In order to have representative measurements of air temperature difference, each junction is located at the centre of equal cross section areas. Therefore, the temperature difference is measured involving the complete sectional area. Precision is then 0.51 C, better than when using two Pt100 (0.65 C) and also sensibility and temperature distribution evaluation are improved. Air flow: energy balance of electrical resistances. Air temperature changes during a test, therefore most of air flow measurements methods are not suitable for transitory measurements. Mass flow depends only on fan velocity, therefore it is measured by applying an energy balance on electrical resistance. The maximum power is set to electrical resistances. The energy consumption is measured with a 1% uncertainty and air temperature difference caused in the air by passing through the electrical resistances is measured by a thermopile with an accuracy of 0.51 C. Air humidity: two sensors are used to measured air humidity at the inlet and the outlet. The maximum air humidity variation during a test was kg/kg da. It is lower than the 0.2% of total stored energy, therefore, it was confirmed that latent energy variation is negligible in energy balance of air for cooling power evaluation. Table 1 resumes this evaluation. 3. Prototype 1 Prototype 1 was designed using aluminium pouches filled with an inorganic PCM. PCM thickness was a critical parameter to obtain the required cooling rates [9]. Vertical position was a requirement, therefore a metallic grid was used to force PCM thickness below a maximum in vertical position. Fig. 2 shows this configuration. Air flows parallel to the pouches from bottom to top. Tests using a constant inlet air temperature were accomplished. Fig. 3 shows the cooling power evolution in prototype 1. Results Fig. 1. Experimental setup for air PCM heat exchangers at University of Zaragoza. Table 1 Uncertainty of measurements. Uncertainty of measurements DT thermopile ( C) 0.51 Air flow (kg/s) 0.02 Fig. 2. Prototype 1 configuration.

3 A. Lazaro et al. / Energy Conversion and Management 50 (2009) Fig. 6. Torn aluminium pouches and PCM leakages in the surfaces of pouches. Fig. 3. Cooling rate evolution in prototype 1 during tests with different inlet air temperatures. Fig. 7. Bent metallic grid. Indicating that, contrary to what was at first designed, heat transfer by conduction inside the PCM resistance is dominant. The prototype was opened to confirm the diagnosis, and PCM leakages were found out. Fig. 5 shows the prototype walls soaked with PCM. Some pouches were torn (Fig. 6) and the metallic grid Fig. 4. Cooling rate evolution with constant inlet air temperature in prototype 1 during tests with different air flow rates. showed that the cooling rates were very low and the total melting times were double the melting design time. Different air flow rates were tested. As it can be seen in Fig. 4, the air flow influence on melting times and cooling rates were negligible. Cooling power does not increase by a rise of air flow rates. Fig. 8. Aluminium panel containing PCM. Fig. 5. PCM leakages inside the prototype 1. Fig. 9. Cooling rate evolution in prototype 2 during tests with different constant inlet air temperatures.

4 442 A. Lazaro et al. / Energy Conversion and Management 50 (2009) was deformed by the pushing force of the solidification process of the PCM inside pouches (Fig. 7). PCM thickness was twice higher than the designed, causing a higher and dominant heat transfer resistance by conduction inside the PCM. Therefore melting times were higher and flow rate had almost no influence. This prototype does not fulfil requirements and was discarded. 4. Prototype 2 The second prototype was designed using aluminium panels filled with organic PCM. Fig. 8 shows the panels that were used in prototype 2. Configuration was also vertical. Air flows were parallel to the panels from top to bottom. Due to the fact that the PCM in prototype was organic, it presents lower thermal conductivity that PCM in prototype 1. Furthermore, the total stored energy in prototype 2 is also lower than in prototype 1. The first tests were accomplished using constant inlet air temperature. As it can be seen in Fig. 9, cooling rates were higher than those obtained with prototype 1 and the melting times were half the melting times with prototype 1. Different air flow rates in prototype 2 were tested and it was observed that it has influence on the melting time and cooling power. For the lowest air flow rate, it can be seen that the power evolution Fig. 12. Cooling rate evolution in prototype 2 during tests with constant heating power. Heating power is expressed as a percentage of the total heating capacity of resistances (4.4 kw). Table 2 Main characteristics of prototypes. Prototype 1 Prototype 2 PCM Commercial inorganic Commercial organic Stored energy (kj) 31,584 24,395 Thermal conductivity of solid PCM [W/ (m K)] curve changes its shape and is more similar to prototype 1. This indicates that heat transfer by conduction inside the PCM starts to be relevant when compared to heat convection to air like in prototype 1. Fig. 10 shows this result. The first results of prototype 2 were satisfactory, so more test were planned to evaluate its behaviour under real conditions. Two types of experiments were accomplished: constant rise of inlet air temperature and constant heating power. Fig. 10. Cooling rate evolution with constant inlet air temperature in prototype 2 during tests with different air flow rates. Fig. 11. Cooling rate evolution in prototype 2 during tests with constant rise of inlet air temperature. Fig. 13. Example of comparison between the melting times for both prototypes during test with the same fixed inlet air temperature.

5 A. Lazaro et al. / Energy Conversion and Management 50 (2009) Temperature rise ramps were then set into the resistances controller. Fig. 11 shows the results for a constant increment rate of inlet air temperature. As it can be seen, the faster the rise, the higher cooling power and the lower melting time. For constant power tests, different heating powers of electrical resistances were fixed. Results are shown in Fig. 12. Prototype 2 was able to maintain a cooling capacity over 3 kw for approximately 1 1 h or approximately 1 kw for more than 3 h. 2 This result is useful to design the optimal operation mode depending on the application. 5. Conclusions Table 2 shows the main characteristics of the tested prototypes. In Fig. 13 the differences in melting times for both prototypes in the tests performed with the same inlet air temperature can be seen. Prototype 2 has an improved design. Although in prototype 2 the stored energy and thermal conductivity of PCM are lower than in prototype 1, the PCM thickness is fixed and then, the dominant heat transfer resistance is not conduction inside PCM. Cooling power is higher and melting times are shorter. That fulfils requirements for free-cooling applications. From these results, was concluded that for free-cooling applications, an effort should be made to design heat exchangers instead of to enhance PCM thermal conductivity. Until now, thermal conductivity enhancement of PCM supposes an additional increase in the cost of the PCM. The economical viability of thermal energy systems using PCM is a critical parameter for its application. As it was proved in this work, heat exchanger design can enhance thermal response of a system more than using a PCM with higher thermal conductivity. For total energy storage strategy, the duration time of the cooling capacity of PCM heat exchanger depends on the cooling power demand. Fig. 12 shows different durations for different power demands using prototype 2. Acknowledgement Authors would like to thank the Spanish government for partially funding this work, within the framework of research project: ENE C References [1] Arkar C, Vidrih B, Medved S. Efficiency of free cooling using latent heat storage integrated into the ventilation system of a low energy building. Int J Refriger- Revue Int Du Froid 2007;30(1): [2] Turnpenny JR, Etheridge DW, Reay DA. Novel ventilation cooling system for reducing air conditioning in buildings. Part I: testing and theoretical modelling. Appl Thermal Eng 2000;20(11): [3] Zukowski M. Experimental study of short term thermal energy storage unit based on enclosed phase change material in polyethylene film bag. Energy Convers Manage 2007;48(1): [4] Stritih U. An experimental study of enhanced heat transfer in rectangular PCM thermal storage. Int J Heat Mass Transfer 2004;47(12 13): [5] Marin JM, Zalba B, Cabeza LF, Mehling H. Improvement of a thermal energy storage using plates with paraffin graphite composite. Int J Heat Mass Transfer 2005;48(12): [6] Py X, Olives R, Mauran S. Paraffin/porous-graphite-matrix composite as a high and constant power thermal storage material. Int J Heat Mass Transfer 2001;44: [7] Zalba B, Marin JM, Cabeza LF, et al. Free-cooling of buildings with phase change materials. Int J Refrig-Revue Int Du Froid 2004;27(8): [8] ANSI/ASHRAE STANDARD Method of testing active latent-heat storage devices based on thermal performance. [9] Dolado P, Lazaro A, Zalba B, Marín JM. Numerical simulation of heat transfer in phase change materials (PCM) for building applications. Heat transfer in components and systems for sustainable energy technologies. Chambery, France (April 2007).