Determination of the heat storage capacity of PCM and PCM objects as a function of temperature

Transcription

1 Determination of the heat storage capacity of PCM and PCM objects as a function of temperature E. Günther, S. Hiebler, H. Mehling ZAE Bayern, Walther-Meißner-Str. 6, Garching, Germany

2 Outline Introduction Determination of heat storage capacity of PCM Differential Scanning Calorimetry (DSC) T-History method Characterization of PCM objects Air flux chamber Results and Comparison Summary and Conclusion

3 Quality of a measurement Good or bad quality of a measurement? It depends on the intention! Have a look at the application Is the data acquired by the measurement good enough to predict the behavior of the application with the required precision? A good measurement is precise enough effort & cost effective

4 Storage capacity, phase change temperature temperature phase change temperature useable storage capacity stored thermal energy with phase change operational temperature range

5 Degree of subcooling temperature melting & phase change temperature stored thermal energy nucleation temperature useable storage capacity with phase change with subcooling operational temperature range

6 Degree of subcooling temperature melting & phase change temperature stored thermal energy nucleation temperature degree of subcooling with phase change operational temperature range

7 Applications requirements Building applications (e.g. free cooling ) typical temperatures 18 C 26 C PCM air small temperature gradients difficult heat transfer high temperature resolution of storage capacity needed! ~ 1K or less

8 Pure material real PCM pure material real PCM storage capacity solid phase change liquid storage capacity temperature temperature sharp peak T m blurred peak storage capacity (T)

9 DSC general setup of a heat flux DSC DSC differential scanning calorimetry furnace reference sample temperature sensors

10 DSC dynamic mode typical recording

11 DSC dynamic mode typical result storage capacity as a function of temperature the temperature resolution seems to be ok variation of measurement parameters?

12 DSC dynamic mode deviations 3K temperature deviation sample mass heating rate use DSC with great care!

13 Temperature deviation & uncertainty constant heating / cooling rate determination of Q between non-isothermal states sensor is on the surface of the sample (outside the crucible) signal is attributed to an extreme temperature exact form of gradient is unknown the truth is somewhere between heating and cooling case

14 Temperature deviation & uncertainty constant heating / cooling rate determination of Q between non-isothermal states sensor is on the surface of the sample (outside the crucible) signal is attributed to an extreme temperature exact form of gradient is unknown how to reduce the gradient? the truth is somewhere between heating and cooling case

15 Amount of temperature deviation constant heating / cooling rate the larger the sample, the larger the gradient the larger the heating / cooling rate, the larger the gradient the lower the thermal conductivity, the larger the gradient

16 Amount of temperature deviation constant heating / cooling rate PCM are often signal vanishes for inhomogeneous too small samples wrong composition the larger the sample, the larger the gradient if sample is too small the larger the heating / cooling rate, the larger the gradient the lower the thermal conductivity, the larger the gradient

17 Amount of temperature deviation constant heating / cooling rate signal vanishes for too small HR the larger the sample, the larger the gradient the larger the heating / cooling rate, the larger the gradient the lower the thermal conductivity, the larger the gradient

18 Amount of temperature deviation constant heating / cooling rate the larger the sample, high for standard the larger materials, the gradient temperature gradient is the larger the heating very low / for cooling most rate, PCM the larger increased the gradient by phase change! the lower the thermal conductivity, the larger the gradient

19 Reducing temperature deviation heating / cooling with a steps program determination of Q between isothermal states the maximum gradient (uncertainty in T) depends on the step size the max. gradient is not dependent on the sample s properties! maximum temperature deviation is equal to step size

20 DSC isothermal steps typical recording

21 DSC isothermal steps typical result storage capacity as a function of temperature temperature resolution = step size variation of steps program, sample mass?

22 DSC isothermal steps variation of parameters variation of sample mass heating / cooling max / min temperatures m = mg no change in result! uncertainty in temperature = step size

23 DSC dynamic & steps methods results T method DSC, dynamic DSC, steps T 2 K 1 K using optimized parameters dynamic: ± 0.5K/min steps: 1K step size

24 DSC unsolved problems main problem: small sample mass many PCM are inhomogeneous (e.g. contain nucleators, graphite, gelling material) crucible size correct composition of the sample? storage capacity phase change temperature subcooling

25 T-History Method calorimetry of large samples large sample mass correct composition, subcooling slow measurement small thermal gradient heating and cooling measurements quantify temperature deviation & subcooling

26 Results DSC & T-history T uncertainty ± 0.6K good enough for free cooling

27 PCM objects air flow chamber aim and setup check if heat storage capacity calculated from material data (calorimetry) is ok for PCM objects experimental verification of heat transfer from encapsulated PCM objects to air

28 Air flow chamber measurement

29 Air flow chamber power output

30 Air flow chamber chamber content

31 Air flow chamber results energy stored method T-history air flow chamber typ. mass 15 g 2 kg

32 Summary & Conclusion Aim of our investigations determination of heat storage capacity for PCM with high resolution in temperature experimental verification of predicted properties of PCM objects PCM DSC with optimized parameters T < 1K only for homogeneous PCM without subcooling T-history T < 1.5K, suitable for all PCM PCM objects air flow chamber verification of properties of real-size objects energy storage capacity agrees within ±10% with T-history