A NEW MEASUREMENT AND EVALUATION METHOD FOR DSC OF PCM SAMPLES

Transcription

1 A NEW MEASUREMENT AND EVALUATION METHOD FOR DSC OF PCM SAMPLES H Mehling, E Günther, S Hiebler, Bavarian Center for Applied Energy Research (ZAE Bayern), Walther-Meißner-Str. 6, D Garching, Germany. mehling@muc.zae-bayern.de L F Cabeza and C Castellón Centre GREA Innovació Concurrent Edifici CREA, Universitat de Lleida, Pere de Cabrera s/n, Lleida (Spain) Phone: , Fax: ABSTRACT The determination of the enthalpy of PCM as a function of temperature with sufficient accuracy in enthalpy and temperature is crucial in PCM technology. A very common method used for calorimetric measurements on PCM is the heat-flux DSC (hf-dsc) with constant heating and cooling rates. When using a hf-dsc, the heat flux sensor has to be calibrated and the measurement procedure depends on the calibration mode, which is heat capacity or enthalpy calibration. Each of the two calibration modes has advantages but also disadvantages with respect to PCM as sample. We therefore developed a new measurement procedure that is optimized with respect to PCM. The new method and its advantages compared to measurement procedures are described in this paper. We have applied the new procedure to a commercial PCM, paraffin, RT27, using two different hf-dsc instruments. The results are presented and compared to data from measurements with procedures. 1. BACKGROUND Introduction The determination of the enthalpy of PCM as a function of temperature with sufficient accuracy in enthalpy and temperature is crucial in PCM technology [Castellón et al, 2008]. A very common method used for calorimetric measurements on PCM is the heat-flux DSC (hf- DSC) in the dynamic mode. Usually, heating and cooling segments at constant rates ( dynamic method ) are performed in a measurement. A typical temperature program and corresponding signal shows figure 1. The peaks indicate strong thermal effects of the PCM sample at the corresponding temperatures. In a hf-dsc, the heat flux in or out of the sample is calculated from the measurement of the temperature difference at a heat conducting thermal resistance. For calibration, materials with a known thermal effect are used. This can be a known heat capacity or the enthalpy of a phase change. The measurement procedure depends on the calibration mode for the heat flux. Two common calibration modes are heat capacity (also called heat flow rate) calibration and enthalpy (also called heat) calibration. In this paper we describe these methods as they are used today, as well as their advantages and disadvantages with respect to PCM as sample. Then we present a new method that was developed by us. It is optimized with respect to PCM, taking into account the special properties of PCM and the need to determine the enthalpy of PCM as a function of temperature with sufficient accuracy.

2 Figure 1. Typical signal and temperature evolution during a dynamic hf-dsc measurement with constant heating and cooling rate. The dynamic and isothermal segments are marked by vertical lines. The new method is based on enthalpy calibration, but takes into account also sensible heat. A comparison of experimental results obtained by the common and our new method is presented. Common methods for the determination of thermal effects with a hf-dsc General principle of a hf-dsc A heat flux DSC determines the amount of heat absorbed or released by a sample upon a change of the sample temperature [Speyer 1994, Wunderlich 2001]. For this, the temperature development of the sample crucible in a furnace is compared to the temperature of an empty reference crucible in a symmetric position, as shown in figure 2. Figure 2: Sketch of the furnace of a hf-dsc. The sample and the reference are in symmetric positions in the furnace. The temperatures are measured below the bottom of the crucibles (arrows) and the detected signal U corresponds to their difference. In a measurement segment with constant heating or cooling, thermal effects of the sample lead to a deviation in the sample temperature from the reference temperature. This temperature difference T is detected ( signal ) and related to the properties of the sample by T = R C s C ) β (1) ( r Here, β [K/min] is the heating rate, R the thermal resistance [K/W] (equal to the sensitivity) and C the heat capacity [J/K] of the sample (s) and reference (r). A derivation of this basic formula is given and discussed in [Hemminger et al 1989]. From this temperature difference,

3 the heat flux, the specific heat capacity c p (T) and the enthalpy h(t) are calculated. The raw signal of a hf-dsc is the temperature difference, given in K, or a thermocouple voltage, given in µv. The instrument has to be calibrated to assure a correct determination of the temperature and the heat flux. Temperature calibration The value that is used for the temperature axis of the acquired data is measured by the sensor below the reference crucible. This sensor is a thermocouple that is highly reproducible; calibration is only necessary from time to time. The calibration is done by comparing measured phase change temperatures of materials to literature values [Gmelin et al 2000]. The temperature calibration is usually directly applied to the measurement data and already included in the signal output by the instrument. Determination of c p using the heat capacity calibration A c p determination based on a calibration with a heat capacity comprises three measurements, here called empty line, line, and sample line. To take into account possible asymmetries, first a measurement where both the sample crucible (sc) and the reference crucible (rc) are empty is performed; the resulting data are called empty line. T = R C C ) β (2) empty ( sc rc The reference crucible is empty for all following measurements, too. In a second measurement, the material is put into the sample crucible and the measurement is repeated, giving the line. T = R C + C C ) β (3) ( sc rc This is effectively the caloric calibration of the system. Finally, a third measurement is done, now with the sample in the sample crucible; the resulting data are called sample line. T = R C + C C ) ß sample ( sc sample rc (4) In the evaluation, the effect of the crucibles is eliminated by taking the differences between / sample lines to the empty line: T sample/, only = T = R ( C sample/ sc = R ß C + C T sample/ sample/ empty C rc ) ß R ( C sc C rc ) ß (5) The calibration is taken into account by taking the sample relative to the lines. Then:

4 T T sample, only, only C = C sample c = c p,sample p, m m sample (6) The temperature differences T can be obtained from the voltage signals. This conversion T(U) cancels out in eq. (5), and the voltage signals U can be used directly. Finally, the specific heat of the sample is found by rearranging eq. (5) and becomes c p,sample = c p, m m sample U U sample U U empty empty (7) From the known masses of sample and reference, and the specific heat capacity of the, the specific heat capacity of the sample is calculated at any point of time. The conversion between time and temperature is finally done via the recorded temperature ramp. Determination of h using the enthalpy calibration Another approach to calibration is using materials with a well known melting enthalpy. The procedure starts with a set of calibration measurements on these materials (figure 3a). An empty line measurement is not done. Instead, a baseline is constructed from the beginning to the end of the peak. From eq. (1) follows that, for a material in the sample crucible and an empty reference crucible, T = R C β (8) Figure 3. Schematic procedure for enthalpy calibration. At a fixed heating rate, the melting enthalpies of several materials are measured (a). By comparison with literature values, the sensitivity at the melting temperatures is determined and the sensitivity function is found by interpolation (b). The integral over the peak area in the measured signal, indicated by the shaded areas in figure 3, is proportional to the melting enthalpy: T dt = R C β dt = R C dt / dt dt = R H (9)

5 By integrating the peaks of the measurements, we get R at the melting temperatures of the materials. The peaks of the materials are very narrow, and the sensitivity is taken as constant within the peak width. By interpolation, the sensitivity over a larger temperature range is determined (figure 3b). To evaluate a sample measurement, its peak area is again defined using a baseline constructed from the beginning to the end of the peak, the same way as in the enthalpy calibration. Then, the value of the peak area is determined as in eq. (9), but now taking into account the variation of the interpolated sensitivity over the peak width, resulting in H = T / R( T ) dt (10) This is a procedure used for the analysis of many thermal effects. The result is however not satisfying for PCM in two respects: first, the result is an integral value of the melting enthalpy and not the enthalpy as a function of the temperature; second, by integrating the peak without measuring an empty line the sensible heat is not taken into account. We will explain why this is a serious problem in the next section. 2. DEVELOPMENT OF A NEW MEASUREMENT AND CALIBRATION PROCEDURE FOR HF-DSC OPTIMIZED FOR PCM The problem For heat storage applications, it is necessary to determine the sum of both latent and sensible heat as a function of temperature. With the current way of using the enthalpy calibration as described above, only the peak integral can be determined. That means that the sensible heat is not taken into account, and that there is no temperature resolution within the peak [Richardson 1997]. Using the heat capacity calibration, it is possible to determine the sum of latent and sensible heat as a function of temperature. However, this approach has also some disadvantages. First, three measurements are necessary, resulting in much experimental effort. Second, the sensitivity and therefore the accuracy are not optimized for PCM. The sensitivity determined using heat capacity calibration is valid only for small signals. As figure 1 shows, the signal within the melting peak can easily be 10-times higher than outside of the peak. This is due to the high thermal storage capacity of the PCM which is not typical for other materials. In the temperature range of the peak, where the majority of the stored energy is concentrated and that is most important for the use of PCM, the sensitivity determined using heat capacity calibration must be extremely extrapolated beyond the signal range of the measured calibration points. The difference between both sensitivities, from heat capacity calibration (for small signals) and from enthalpy calibration (for large signals), according to our experience is about 5% - 10%; the typical range according to DIN is 2-10%. The reason why the sensitivity data differ is that some effects are not linear, so linear extrapolation leads to errors [Hiebler 2007]. New method The correct empty line is very important for the accuracy of the heat capacity calibration. In this calibration, the empty line is determined by a measurement and subtracted from the signal of sample and in eq. (5). This procedure assumes that the only difference between the three measurement runs is the crucible content. This assumption implicitly requires the signal of the empty crucible to be constant over a longer period of time. In reality, this

6 stability is not perfect. Typically, there is a certain drift of the empty line that can be experimentally observed by repeated measurements with an empty crucible. If the and sample signals are weak, as it is the case for small heating rates, the drift of the empty line can be in the order of magnitude of the signals and cause large errors. Contrary to an error in the sensitivity, the error in empty line does not scale with the signal, and is therefore worse for small signals. If the measured empty line is so sensible to errors, and therefore of limited usefulness, it is probably not worse to construct the empty line instead. This is the basis for the new measurement and evaluation procedure. For the construction, the empty line is determined according to ASTM E 1269, using the signals at isothermal state and connecting them as shown in figure 4. Figure 4. Construction of the empty line according to ASTM. The signal values of the isothermal segments are connected by a straight line. The only assumption used in the construction is the linearity of the empty line during the dynamic segment. A big advantage of the proposed method is the reduced experimental effort, as only the sample measurement has to be done, and the empty line is constructed. 3. EXPERIMENTAL VERIFICATION OF THE NEW PROCEDURE To test the evaluation procedure we used a commercial PCM called RT27 from Rubitherm GmbH. The material data of this paraffin are a melting temperature of about 28 C and a phase change enthalpy of 179 kj/kg in the range from 19 C to 34 C. The experimental verification of the method, developed at the ZAE Bayern, was carried out at the ZAE Bayern with a Netzsch DSC 204 Phönix and at GREA (University of Lleida) with a Mettler Toledo DSC 822e, using a similar setup but different models of DSC instruments. 4. RESULTS AND DISCUSSION The RT27 measurements are evaluated and the total enthalpy change in the temperature interval C is compared with regard to a agreement between heating and cooling measurement agreement between different heating / cooling rates agreement with reference data (steps method, manufacturer data)

7 In figure 5, the results of the different evaluations of the measurement at ZAE Bayern are shown. The uncertainty in temperature indicated for the steps measurement is given by the step size [Günther et al, 2006]. The uncertainty of the temperature calibration is the same for both kinds of measurement and is not indicated in this graph. For the new method, heating and cooling curves agree perfectly in the total enthalpy change with results from steps measurements, as well as with data from the manufacturer and from the FhG ISE [Mehling et al 2006]. Figure 5: Comparison of data obtained at ZAE Bayern with the new method and with the steps method, data of the manufacturer, and from FhG ISE. In figure 6, results of the measurements at GREA are presented. Prior measurements using the heat capacity calibration show some disagreement between heating and cooling curves (figure 6a). Measurements on a new sample of the same material, using the proposed new method, lead to a consistent result at much less experimental effort (figure 6b). Figure 6: Results of measurements at GREA. The same material was measured with the conventional method (a) and the new method (b). There is, however, a notable difference in the total enthalpy change compared to the data from measurements at ZAE Bayern as shown in figure 5. The total enthalpy change differs by about 45 J/g out of 210 J/g or 21 %. This difference will be the subject of future investigations.

8 5. SUMMARY AND CONCLUSION The determination of the enthalpy of PCM as a function of temperature with sufficient accuracy in enthalpy and temperature is crucial in PCM application. Commonly, calorimetric measurements on PCM are done by heat-flux DSC (hf-dsc) with constant heating and cooling rates. When using a hf-dsc, the heat flux sensor has to be calibrated and the measurement procedure depends on the calibration mode, which is heat capacity or enthalpy calibration. Each of the two calibration modes has advantages but also disadvantages with respect to PCM as sample. In this paper, we proposed a different procedure especially designed for PCM that reduces the error with regard to the stored heat as a function of temperature and minimizes measurement effort. The new procedure, developed at the ZAE Bayern, was applied to a commercial PCM, paraffin RT27, at the ZAE Bayern and at GREA (University of Lleida). The results were compared with regard to agreement of heating and cooling curves, of different heating and cooling rates, and reference data. The comparison shows that inconsistencies as previously experienced with the procedures have disappeared using the method proposed in this paper. This has been achieved with a reduced effort: only the sample measurement is necessary and the empty line is constructed. For measurement automation, this is a big advantage: if an automatic sample changer is used, it is practically impossible to do several measurements on one crucible with different contents, as necessary for the heat capacity calibration method. Our first results indicate that the new method is promising to acquire enthalpy data of PCM with high accuracy at low experimental effort. More experience with this new method should be acquired and readers of this paper are encouraged to apply it to their measurement data and report the results. REFERENCES Castellón C., Günther E., Mehling H., Hiebler S. and Cabeza L. F., (2008) Determination of the enthalpy of PCM as a function of temperature using a heat-flux DSC A study of different measurement procedures and their accuracy Int. J. Energy Res. 2008; 32: Gmelin E and Sarge S M (2000) Temperature, heat and heat flow rate calibration of differential scanning calorimeters. Thermochimica Acta Günther E, Mehling H and Hiebler S (2006) Determination of the heat storage capacity of PCM and PCMobjects as a function of temperature. Proc. Ecostock - The Tenth Int. Conf. on Thermal Energy Storage (Stockton College, NJ, USA). Hemminger W F and Cammenga H K (1989) Methoden der thermischen Analyse (Springer Verlag, Berlin, Gemany) Hiebler, S. (2007) Kalorimetrische Methoden zur Bestimmung der Enthalpie von Latentwärmespeichermaterialien während des Phasenübergangs; Mehling H, Ebert H P, Schossig P. (2006) Development of for materials testing and quality control of PCM 7th IIR Conf. on Phase Change Materials and Slurries for Refrigeration and Air Conditioning, (Dinan, France) Richardson M (1997) Quantitative aspects of differential scanning calorimetry. Thermochimica Acta Speyer R F (1994) Thermal Analysis Of Materials. Marcel Dekker Inc., New York, USA Wunderlich B (2001) Thermal Analysis Encyclopedia of Materials: Science and Technology ed Buschow K et al. Elsevier Science Ltd., Oxford, UK, pp