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Before the wide spread use of PCM products can be initiated there are a number of technical issues that need to be better

understood and resolved. One of them is the long-term performance of PCM products and the other is encapsulation and

containment of PCM for use in building applications. This paper will focus on the fist of these two issues.

The concept of using PCM materials for the application for the thermal energy storage in houses is not new (Kedl and

Stovall, 1989). The pioneering effort to apply PCM materials in residential housing was done by Telkesin 1947 (Telkes,

1947).Presently, a revival of this approach is observed (Schmidt), (Szymocha, 2007). The application of PCM based

heat storage systems not only resolves the problem of high density heat storage between energy supply (day) and demand

(night) but also may improve the performance and reliability of the heating systems by providing a more constanttemperature profile for heat storage. In the near future, PCM materials will play an important role as a heat buffer in

many applications that will make energy utilization much more practical and efficient. In the next decade, significantly

increased research activity in this area is expected.

PCM MATERIALS

When a PCM changes state from solid to liquid, or vice versa, it absorbs a large quantity of thermal energy while its

temperature remains relatively constant. These latent heat storage systems are preferred due to the large energy storage

density and practically isothermal nature of the storage process. Table 1 provides heat storage capacity potential

comparison between some common heat storage materials. For example, the PCM in form of the calcium chloride

hexahydrate (CCH) can store 9 (based on mass) or 14 (based on volume) times more heat than water when applied in

temperature range of 25 - 30C.

Table 1 Comparison of Heat Storage Capacity (in temperature range from 25 to 30C)

Material Heat storage capacity

by weight kJ/kg by volume kJ/dm3

Concrete 4.4 8.8

Water 21 21

Wax (n-Octadecane, C18H38) 212 175

Hydrated salt, CaCL26H2O 196 294

As a result the application of PCM materials can significantly reduce the overall volume or size of the heat storage

system. The significant volume reduction makes it possible to implement PCM materials into building elements such as

walls, floors or ceilings. There are many PCM materials available, with a phase change temperatures covering a widerange of applications. Some of them can operate at temperatures as high as 600C.

Hydrated salts, because of their low costs are very attractive for use as thermal storage in residential buildings.

However, some of them have a tendency for super-cooling, segregation and long-term performance deterioration, and

this might limit their long term practical application. When applying PCM materials in a building one of the most

important aspects is the long term stability and performance of the PCM over thousands of cycles. The long-term

stability of the organic materials has been confirmed. However, for inorganic materials (such as salt hydrates) there is

the possibility for some aging of PCM materials which will have some impact on the long term heat storage capacity.

Having the ability to test and confirm the long-term stability of potential PCM products is very important. It is predicted

that the implementation of PCM heat storage systems in residential, commercial, industrial and agriculture sectors will

be an evolving and growing trend in the continuing effort to develop high performance energy efficient buildings and

structures.

METHODS FOR MEASURING OF HEAT STORAGE CAPACITY

To determine the specific heat andlatent heat of materialsa number of methods such as differential thermal analysis

(DTA) or differential scanning calorimetry (DSC) are commonly used (Li, J, et al, 1985). These methods are very well

developed and accurate but they use only small micro-samples of the test material. When evaluating the performance of

these materials for commercial scale use a test method that can accommodate a larger sample may provide additional

information on the long-term stability of the product. It is believed that a very small sample, taken out of the large

testing specimen and out of testing system, might not reflect properly the bulk materialcharacteristics. During long-term

performance testing, when undergoing huge number for freezing/melting cycles, large PCM specimens often are subject


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to settling or stratification and are typically not perfectly homogeneous. For large size samples, testing using a non-

invasive method for heat of fusion and specific heat determination is necessary.

An interesting, recently developed simple method, known as T-history method, for the specific heat and thermal

conductivity determination was proposed by Zhang (Zhang et al., 1998). In this method, temperature-time relationship

curves of tested PCM sample are used to determine their thermo-physical properties. To enhance the measurement

accuracy, the method has been further modified (Zhang, Jiang, 1999), (Hong et al., 2004), (Marin et al., 2003, 2005).

However, this method being very simple, is relatively time consuming and troublesome, when applied for long-term

cycling evaluation. A specific problem is identification of the boundaries of the phase change period (the beginning andthe end), which is critical for accurate heat of fusion value determination.

A new method, called a double cell, was developed by the authors with designation for long-term PCM materials

testing. The double cell method is calorimetric type of method that allows measurement and comparison of the heat

storage capacity over numerous cycles. The testing strategy with cycling apparatus relies on alternative symmetric

heating and cooling under controlled conditions of a relatively large PCM sample (1-2 kg) that is located inside a well

insulated enclosure. While not as accurate a method for the determination of the heat storage capacity of small

specimens it is believed to provide good information for researchers and developers of commercialor larger scale

products who want to see the overall heat storage performance of bulk samples over a large number of cycles.

TESTING SYSTEM DESCRIPTION

In the double cell method the heat storage capacity is determined by measurement of the amount of the thermal energyrequired to melt the PCM sample under well specified conditions. The diagram of the testing apparatus is shown in

Figure 1.

Figure 1. Thermal cycling apparatus for the long-term PCM testing top view.

The PCM material, with a total weight of 2,000 g, is split in two equal size portions, enclosed in two plastic pouches and

put into well thermally insulated box in a vertical position. The pouches are separated by a thin metal membrane and are

sandwiched between two heating/cooling copper plates. The symmetrically heated or cooled pouches are equipped with a

Buffer Air

Temperature

Outlet

Inlet

PCM

Sample B

PCM

Sample A

Cooling

system

Heating

system with

controller

Data

logger

Thermal

insulation

envelopes

Temperature

sensors


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set of temperature sensors located on both sides of the pouches. The pouches are subjected to alternative heating

(electrical heating) and cooling (liquid cooling) cycles. During heating, the temperature controller automatically adjusts

the temperature of the heating plates to the preset temperature (e.g. 60C for the heating cycle). The heating cycle lasts

as long as required to reach the predetermined temperature in the internal space between PCM samples. Usually this

temperature is set about 5C above the phase change temperature of the material being tested. It is assumed that once all

temperature sensors on sample exceed the heating set point temperature complete PCM melting is achieved. After the

melting phase is complete the electrical heating system is shut-down and the cooling system is activated. The cooling

liquid, with a temperature of about 8C, is circulated through tubing welded to the copper plates. The cooling cycle is

continued until the temperature of the internal space between PCM samples decreases to approximately 6C below the

level of the phase change temperature and solid state of PCM sample is achieved. Once the low temperature limit is

achieved the cooling cycle is considered to be completed, cooling water is shut-of and the electrical heating system is

activated. Both, low and high temperatures can be set by the controller independently. Only during the heating period is

the energy consumption measured and recorded for the purpose of calculating the PCM heat storage capacity.

Measurements of the electrical energy consumed for heating is simple and accurate. The electrical energy measurements,

corrected for sensible heat of PCM sample, are used for the latent heat determination. The latent heat measurements are

performed every one hundred cycles and are averaged over 10 cycles to improve accuracy.

Before testing, the apparatus requires calibration to determine the system constant. The heat storage capacity and

transient heat loss of the well insulated test apparatus is empirically captured in the system constant. The determination

of the system constant is done with a specimen with known and stable heat storage capacity. The system constant does

not very much between cycles or samples and can be accounted for in all heat storage calculations. Before testing,

during sample preparation phase, for the each PCM specimen a small sub-sample of PCM was withdrawn and the heat of

fusion was measured independently by DSC method. A value for the latent heat from the DSC method was used as a

reference initial point.

The testing system is fully automated and does not require withdrawing any PCM sub-samples for analysis. As a result,

it is very convenient for long-term PCM cycling tests. Large PCM sample size gives the ability to investigate the

temperature gradient impact, freezing and cooling kinetics, observe super-cooling phenomenon and settling or

stratification effects. The system, when verified with paraffin, which is known for the long-term stability, proved very

good measurements reproducibility. During the initial stages of testing there was a problem with occasional pouch

leakages. To prevent such leakages, double-wall pouches were implemented and, practically, the leakages were

eliminated.

DISCUSSION AND RESULTS ANALYSIS ON A LONG-TERM PERFORMANCE

An example of the energy consumption curves form the system when testing a calcium chloride hexa-hydrate (CCH)

based PCM is shown in Figure 2. The amount of electrical energy required for performing one cycle of PCM melting

(after corrections for sensible heat and system constant) was used for latent heat determination and used as a heat storage

criterion. The computer controlled measurements of the consumed electrical energy (in Joules) was taken every second.

This one second information was totaled every three minutes and logged to accurately measure the energy consumed

over a 3 minute time interval.

The thermocouples located on both PCM samples made it possible to record timerelated temperature profiles. A multi-

channel data acquisition system collected and stored temperature readings every minute and the data was used for

determining when the PCM was fully melted or frozen. The temperature information from the various thermocouples

also allowed profiling the temperature changes throughout the sample by plotting temperature readings versus time. A

map to the thermocouple placement is given in Figure 3. All of the thermocouples used for determining the temperature

profile of the PCM were located on the inner surface of both specimens away from the copper heating/cooling plates.

An example of the profiles recorded by the temperature sensors are shown in Figure 4. Analysis of the graph shows that

the temperature of the heating plate remains fairly constant at around 60C during the heating cycle. The figure also

shows that there is a significant temperature gradient in vertical direction inside the samples and slight changes in a

cooling plates temperature (~10C 3C) during the freezing cycle. It is evident that melting begins at the top of the


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sample and propagates towards the bottom. The solidification progresses in the opposite direction (i.e. from the bottom

to the top) and at the end of the cycle a cavity inside of PCM is created. The heating period is significantly shorter (~60

min) then cooling period (~175 min) and the overall time for each cycle was approximately 4 hours. The cycle time of

the system could be improved somewhat by chilling the inlet water further or by increasing its flow rate. The large

temperature gradient in the sample does suggest some errors in calculating the latent heat storage capacity of the material

using a single or small number of cycles even when the specific heat of the sample is known and accounted for.

However, the pattern of the temperature records is very well repeated over 100s to 1000s of cycles. The purpose of the

test method is to determine the long term stability and performance of the material over a large number of cycles so

errors or uncertainty in a few cycles are averaged out over time. From the temperature profiles, it can be seen that testedCCH samples have a very limited tendency for super-cooling.

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600

Time, min

PowerW

Figure 2. Record of energy readings for energy consumption for 3 heating cycles for an internally formulated CCH

based PCM material at approximately 1500 cycles.


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Figure 3: Simple mapping of the thermocouple locations on the inside bag surface for each PCM sample usedin the double wall testing apparatus.

0

10

20

30

40

50

60

70

0 100 200 300 400 500

Time, min

Te

mperature,

Front plate

Back plate

T 1b

T 2b

T 3t

T 4t

T 5b

T 6b

T 7tT 8t

Water in

Water out

Figure 4: Example of the temperature profiles distributions during the heating and cooling cycles for an internally

formulated CCH based PCM material.

To determine the long-term stability of a PCM samples the heat storage capacity was evaluated every 100 cycles by

measuring the amount of energy used for the heating cycle (an average value for 10 cycles was used). Knowing the

established constant for the cycler, the heat storage capacity was calculated and recorded over numerous cycles. A

number of internally formulated calcium chloride hexahydrate based PCMs were evaluated (L-6 A, B, C) as well as

some commercially available formulations (TEAP-29 PCM). All formulations had a phase change temperature of

approximately 27C. Examples of the long-term stability testing results for the three samples are shown in Figure 5.

Heating plates

temperatures

Bottom set

of sensors

Top set

of sensors


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0

50

100

150

200

0 500 1000 1500 2000 2500

Cycle

Heatstoragecapacity,kJ/kg

TEAP

L-6C

L-6B

Figure 5. Long term heat storage capacity measurements for TEAP 29 and ARC CCH formulation L-6B and L-6C

PCM samples

The important part of the testing program was verification of the performance and the long-term stability of the

encapsulated PCM elements in conditions simulating the real application in terms of the sample size, orientation and the

temperature range. The significant size of the PCM samples created the possibility for simulation of real system

behavior with its negative effects like: temperature gradient development, super-cooling and settling or stratification.

Initial long-term tests results revealed some variation in the heat storage capacity. When the cycling curves were time

synchronized it was evident that changes are related to system disturbances. More in depth analysis found that unstableoperational conditions of the system (cooling water temperature and ambient air temperature) were main reasons for

some of the variation. After taking account of these factors the quality of the test results improved. Better control of the

ambient air and cooling water temperatures as well as improved insulation around the control specimen would improve

the overall accuracy and performance of the double wall test system.

From the performed tests it can be concluded that the performance of ARC internally formulated L-6 series CCH

samples is similar to the TEAP 29 sample which was used as reference sample. After 2,000 freeze-thaw cycles, TEAP

29 lost about 10% of original heat storage capacity. Two thousands cycles, assuming 100 cycles per year, represents

about 20 years of the system operation time. For sample L-6C, the heat storage capacity is more stable than for the

TEAP 29. After 1,600 cycles sample L-6C has showed very limited decrease (


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To improve the overall quality and stability of the readings from the system, improvements in the ambient air and water

cooling temperature control as well as improved insulation would be required.

Based on the data obtained the following is concluded:

The double cell system performs well and is a convenient apparatus for the long-term bulk PCM samplestesting;

The reference inorganic TEAP sample of PCM performs well showing good stability over 2000 cycle period; The ARCs manufactured PCM samples (Series L-6) shows good stability when compared to the reference

TEAP sample and with a slightly higher heat overall heat storage capacity value.

It has been concluded from the long-term test results that no changes in the heat storage capacity and in the melting

temperature for investigated calcium chloride hexahydrate PCM material was identified when exposing the sample for a

large number of thermal cycles (2000 cycles).

Acknowledgements

This work was performed by ARC, Edmonton and was financially supported by Alberta Innovation and Science and

Natural Resources Canada (NRCan).

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Telkes, M. Solar house heating a problem of heat storage, Heat. Vent., 46, 68, 1947

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