DETERMINATION OF THE HEAT STORAGE CAPACITY OF PCM AND PCM-OBJECTS AS A FUNCTION OF TEMPERATURE. E. Günther, S. Hiebler, H. Mehling

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1 DETERMINATION OF THE HEAT STORAGE CAPACITY OF PCM AND PCM-OBJECTS AS A FUNCTION OF TEMPERATURE E. Günther, S. Hiebler, H. Mehling Bavarian Center for Applied Energy Research (ZAE Bayern) Walther-Meißner-Str. 6, Garching, Germany Tel: guenther@muc.zae-bayern.de 1. BACKGROUND Phase change materials (PCM) are thermal storage materials with a high storage density for small temperature range applications. One such application is the storage of natural cold of the night to use during daytime, called free cooling. Very high energy efficiency can be reached as the cold has not to be generated. Those systems are especially interesting for lightweight buildings, which heat up fast in hot days due to the small thermal mass of the building structure. This is considered a very promising application for PCM, as the market for cooling of buildings is expanding rapidly and free cooling systems can be retrofitted in existing buildings [Mehling 2002, Zalba 2004]. The temperature differences available for loading and unloading the storage are typically less than 5 C. The design of a thermal storage system within this narrow temperature range has to be founded on reliable and high resolution material data. The accurate determination of the PCM's heat storage capability as a function of temperature is crucial. Another very important factor in the design of free cooling applications is the heat transfer between storage material and ambient air. Heat transfer can be enhanced by adding fins inside or outside the encapsulation and increasing mass flow of the heat transfer medium. Direct measurements on entire PCM objects are needed to confirm calculations [Arkar 2005]. In this paper we report on our work to improve useful measurement methods for the analysis of PCM for free cooling. 2. DIFFERENTIAL SCANNING CALORIMETRY (DSC) The standard measurement method for thermal analysis is differential scanning calorimetry (DSC). This measurement method is based on the detection of differences in the thermal responses that a reference and sample show when simultaneously subjected to a temperature program. Constant heating and cooling ramp (dynamic mode) The most common operating mode for DSC is a ramp with constant heating rate. Typical heating or cooling rates for the measurement of specific heat are 2 to 10 K/min. The signal is proportional to the temperature difference between sample and reference. A typical signal and temperature plot during a heating ramp is shown in figure 1. The specific heat of the sample as a function of temperature c p (T) is determined with the help of a standard material. From that data, the storage capacity can be derived by integration.

2 Figure 1: Typical heat flow and temperature evolution during a constant heating rate dynamic DSC measurement. The accuracy of DSC measurements of standard materials is discussed in detail by Richardson [Richardson 1997] and Rudtsch [Rudtsch 2002]. Concerning DSC measurements of PCM, the accuracy of the measurement is dominated by the heating rate and sample size. DSC with standard heating rates is not optimal for high resolution measurements on PCM. While changing phase during a measurement, the sample is far away from thermal equilibrium. In contrast to materials without a phase change or with high thermal conductivity, a significant temperature gradient is created inside the sample. This can lead to deviations of several K of the indicated heat storage capacity with respect to temperature. The deviation depends on sample size and heating rate, as well as on the sample s heat storage capacity and thermal conductivity. The thermal properties of the sample are usually not precisely known. Therefore, the deviation cannot be eliminated by mathematical means. Figure 2: Effects of sample mass and heating rate variation in dynamic DSC measurements on c p (T)-curves. Experimentally, the deviation can be reduced by using smaller heating and cooling rates or samples. In figure 2, results of measurements that were done on the same material but with different sample weights and heating rates are shown. The end of the phase change peak in the c p (T)-curve is shifted from 26 C (0.5 K/min, small mass) to about 30 C (2K/min. large mass). However, a reduction of the deviation can only be done at the expense of a weaker signal/noise ratio. The increasing noise can be clearly observed in our measurements. The best possible combination

3 of sample mass and heating rate is dependent on the individual instrument. Typical values for commercial DSC are heating rates of K/min and samples of 10-25mg. Isothermal steps mode Another possibility to obtain data of the heat storage capability is running the DSC in isothermal steps mode. In this case, the furnace is heated stepwise in given temperature intervals. The sample is following the steps with some time delay, and a signal is detected. When the signal goes back to zero, the sample is in thermal equilibrium with the furnace. This is when the next step can be heated. The area below a peak is proportional to the heat absorbed by the sample in the relevant step. An example for an isothermal steps ramp and signal is given in figure 3. Figure 3: Typical heat flow and temperature evolution during an isothermal steps measurement. The absorbed heat is determined stepwise and attributed to the temperature interval of the respective step of the ramp. The heat storage capacity or enthalpy curve h(t) is then obtained by adding the values of the steps. The temperature resolution of the acquired data is equal to the step size. An isothermal steps measurement with the DSC requires more programming and more time for the measurement itself. The evaluation is more complex than that of a dynamic c p -measurement as presented above. The big advantage compared to the dynamic mode is that the uncertainty in temperature is precisely known, as it is confined to the step size. Reducing the step size leads to better temperature resolution. 3. T-HISTORY METHOD The T-history method was proposed by Zhang [Zhang 1999] as a cheap and easy way to evaluate candidate PCM regarding their phase change enthalpy. The method was improved by various authors [Marín et al. 2003], and is now suitable for the determination of temperature dependent c p. The setup of our T-history installation and a typical measurement are shown in figure 4.

4 Figure 4: Schematic drawing of the T-History instrument as set up (left), and a typical output of a T-history measurement (right). Cooling and heating of sample, reference and ambient are shown. Two identical crucibles containing the sample and a reference are placed in an insulated box. The temperatures of the surrounding, the reference and the sample are recorded. Then, the surrounding temperature is changed instantaneously. The response of the sample and the reference to this change is dependent on their thermo-physical properties. With the help of the reference material, the thermal resistance between crucible and surroundings is calibrated. The specific heat and enthalpy of the sample material as functions of temperature can then be determined. Contrary to DSC, in a T-history measurement the signal is not acquired from the temperature difference between sample and reference, but from the respective temperature differences to the surrounding. So the signal/noise ratio can be kept high even for very slow measurements. This is why this method is suitable for the acquisition of nearequilibrium data also of large samples. The measurement of large samples is important for two reasons. First, many PCM are compound material and only large samples can assure that the measurement is done on the correct composition. Second, many PCM show subcooling that can be heavily dependent on the sample size. In a typical application, PCM is used in packs of at least 100 ml. The sample size in the measurement should be similar, so that results concerning subcooling are relevant for the application case. Our installation is designed for heating and cooling experiments in the range of -5 C to 80 C. In the current setup, the sample volume is 20 ml, but also larger samples of about 100 ml have been used. 4. ANALYSIS OF PCM-OBJECTS When dealing with encapsulated PCM as used in heat storage for free cooling, heat transfer inside the PCM, as well as through the encapsulation and to the air is of great interest. Reliable data solely based on mathematical calculations are hard to obtain, so another experimental installation has been designed and set up. Figure 5 shows a sketch of the setup. Figure 5: Schematic drawing of the air flow chamber and measurement instruments.

5 The central component of the installation is a test chamber, through which air of defined temperature and velocity is blown. With the help of this installation, free cooling components can be investigated concerning heat conduction and power performance; effects of subcooling on power output can be observed directly. Variations of heat conductive elements and encapsulations can be tested under conditions as relevant for free cooling applications. A typical data recording of a cooling run is shown in figure 6. Figure 6: Typical results of an air flow chamber cooling experiment. Temperatures of chamber inlet, chamber outlet and ambient as well as air flow are recorded (right). 5. RESULTS AND DISCUSSION The results of the calorimetric measurements are presented in enthalpy-temperature diagrams. This representation allows the user to read easily the storage capacity in the temperature interval as needed for his particular installation. For the discussion of the presented measurement methods, we did experiments on two different materials. PCM A is an organic material, while PCM B is an inorganic material with a tendency to subcool. Results PCM A Measurements on PCM A with the DSC dynamic and steps method and the T-history-method are shown in figure 7. Figure 7: Results of the enthalpy determination of bulk PCM with three different methods (left). Both heating and cooling curves are presented for each method. The sample masses, m, and the maximal distance between heating and cooling curves, T, are given for each method (right).

6 For each method, results from heating and cooling experiments are given. The distance between heating and cooling curve is a measure of the uncertainty that is associated with the measurement method. When heating, the surface of the sample is the point with the highest temperature; when cooling, it is the point with the coolest temperature. The temperature sensors of DSC and T-history measure this surface temperature, attributing the heat flux to the extreme temperatures in the heating and the cooling cases. So the enthalpy curve of the sample in thermodynamic equilibrium is inside the area confined by the heating and cooling curves. For the isothermal steps method, the uncertainty in temperature is fixed by the step size, in the graph indicated by horizontal error bars. The mass of the sample is given for each method. The upper limit of the phase change temperature range of the investigated PCM is found to be about 28 C, but no sharp bend is found at the lower part of the enthalpy curve. This is typical for non-pure substances, as are most PCM. The curves of dynamic DSC were determined with a heating/cooling rate of 0.5 K/min. The temperature deviation is confined by the heating and cooling curves to 2 K. The heating and cooling curves for the isothermal steps method are very similar, and the uncertainty in T is equal to the step size, here 1 K. This is a little less than the deviation of T-history, which is 1.2 K. However, the sample mass of the T-history measurement is about 1000 times the mass of the DSC measurements. Results PCM B Measurements on bulk PCM B were done with the T-history method, and encapsulated samples of PCM B were investigated using the air flow chamber. DSC measurements on PCM B were not done because of the subcooling of the sample. The results are presented in figure 8. Figure 8: Stored energy as a function of time as determined with the air flow chamber (left). Encapsulated PCM B was used as storage material in this measurement. The bulk material was investigated with the T-history method, and the results of the two methods are compared (right). The air flow experiments were done using a storage of four PCM packs, with a mass of about 3 kg each pack. At an air flux of 33 m³/h, power output during the phase change was about 50 W. The total energy stored is attributed to the temperature interval between the beginning and end of the measurement (heating: 17 C 35 C, cooling 16 C - 34 C). Looking at the T-history data, a subcooling of about 5 K was detected at the bulk material. The phase change enthalpy is distributed over a wider temperature interval compared to PCM A. The distance between heating and cooling curves is 3.5 K. The total phase change enthalpy as predicted by T-history is in very good agreement with the data of the air flow chamber.

7 6. CONCLUSION In this paper, three methods for the determination of enthalpy data of phase change materials (PCM) are discussed concerning their precision and suitability for a range of materials. Dynamic differential scanning calorimetry (DSC) has to be used with care for the determination of storage capacity of PCM because of this method s susceptibility to larger temperature errors when sample size or heating / cooling rates are too large. DSC measurements using the isothermal steps method are an improvement, but they can only be used for homogeneous samples without subcooling. The T-history method extends the range of accessible materials to inhomogeneous PCM, also with subcooling. Results of T-history measurements are superior to dynamic DSC and can compete with isothermal steps mode DSC. In addition, a method for the experimental characterization of encapsulated PCM-objects designed for the use in air flow systems is presented. Results are compared to T-history predictions and are found to agree very well. ACKNOWLEDGMENTS We wish to express our deep appreciation to Rubitherm GmbH and SGL CARBON GmbH for the provision of samples for this investigation. REFERENCES Arkar, C. & Medved, S. (2005) Influence of accuracy of thermal property data of a phase change material on the result of a numerical model of a packed bed latent heat storage with spheres. Thermochimica Acta 438, Marín, J.M., Zalba, B., Cabeza, L.F. and Mehling, H. (2003) Determination of enthalpy-temperature curves of Phase Change Materials with the T-history method Improvement to temperature dependent properties. Measurement Science & Technology 14, Mehling, H., Manara, J. & Körner, W. (2002). Potential to improve the thermal comfort in buildings using latent heat storage materials under climatic conditions of Germany. in: A. Sayigh (Ed.), World Renewable Energy Congress VII, Cologne, Germany, Richardson, M.J. (1997) Quantitative aspects of differential scanning calorimetry. Thermochimica Acta 300, Rudtsch, S. (2002) Uncertainty of heat capacity measurements with differential scanning calorimeters. Thermochimica Acta 382, Zalba, B., Marín, J.M., Cabeza, L. & Mehling, H. (2004). Free-cooling of buildings with phase change materials. International Journal of Refrigeration 27, Zhang, Y., Jiang, Y. & Jiang, Y. (1999). A simple method, the T-history method, of determining the heat of fusion, specific heat and thermal conductivity of phase-change materials. Measurement Science and Technology 10,