**2.1.2 Design of a new installation to implement the T-history method**

When analyzing errors with T-history, the most important factor is the precision in the temperature measurement. Thermal sensors used in previous implementations have been thermocouples, while Pt-100 was chosen for this new installation due to the higher precision: ±0.05ºC with a 4 threads assembly. However, Pt-100 has a longer response time, but will not affect the results provided that the response time is the same for all temperature measurements. This objective is achieved by using Pt-100 of the same manufacture set, and characteristics will be identical. Enthalpy is expressed in a mass unit basis; therefore the precision in mass measurements is as important as the precision in temperature measurements. A 0.1 mg precision scale is used to measure the mass of samples. The sample containers have been designed so that the method standards are fulfilled (Bi<0.1). Churchill-Chu (Marin & Monne, 1998) natural convection correlations for cylinders were used to calculate the suitable radius/length rate of the tubes. The chosen material was glass, since it allows the observation of the phase change process. Cylinders of 13 cm in length and 1 cm in diameter were used. A data logger was used with a RS-232 connection with 22 bits and 6 ½ resolution. A thermostatic bath (0.1 K precision) was used to fix the initial temperature of the samples. A calculation software, especially developed in Labview, was used to obtain the h-T curves. The new T-history implementation based its improvements on:


Examples of T-history analysis applied to two typical PCM (organic and inorganic) are shown in Figure 3. Typical phenomena as hysteresis or sub-cooling can also be observed.

The objective of analyzing organic and inorganic substances is to confirm the expected differences in behaviour: the inorganic PCM presents the sub-cooling phenomenon that occurs during cooling, presenting more hysteresis and quite higher stored energy density when compared to organic PCM.

The procedure used was: mass measurements of the samples and sample containers using a precision scale, then the Pt-100 were introduced into the samples (one into the PCM and one into the water), and the tubes were inserted into the thermostatic bath at the desired initial temperature. The initial temperature depends on the PCM to be tested as well as if it is for a heating or a cooling test. For a heating test, the initial temperature must be lower than the phase change temperature. For a cooling test, it must be higher. Once the temperature inside

PCM-Air Heat Exchangers: Slab Geometry 433

The same procedure to select the appropriate method to obtain the enthalpy vs. temperature curves was followed to find the method for thermal conductivity measurement in liquid and in solid phases. The most commonly used method is the hot wire method (Watanabe, 2002), nevertheless the temperature of the sample is measured with low accuracy and there is also the difficulty in measuring solid samples. A stationary parallel plate method (Mills et al., 2006) solves the problem of accuracy in temperature measurements, but in the liquid phase, convective movements affect the results. The Laser Flash is the only method that allows measuring the thermal diffusivity and sample temperature with accuracy, both in liquid and solid phases. It is based on a laser pulse that comes into contact with one surface of the sample and the temperature evolution on the opposite surface is measured by an infrared detector; therefore, the thickness of the sample must be perfectly determined. A mathematical evaluation of the temperature evolution allows the determination of the

may be obtained:

2 1/2

*p*

1.38 L t

where *L* is the sample thickness and *t1/2* is the time elapsed until half the temperature

Although we have focused on enthalpy and thermal conductivity, there are other important properties and issues to consider such as: encapsulation compatibility (plastic-paraffin; salt hydrated-metal), toxicity, flammability, corrosion, thermal cycling, rheology, density, and

The specific study system corresponds to a PCM-air heat exchanger acting as a TES unit. The unit is basically composed of PCM plates, the casing, and a fan that blows the air that circulates inside the equipment between the plates (see Figure 5). Although the set up could be arranged horizontally to reduce pressure drop and electrical consumption, the vertical distribution was a requirement because of the very first application (for temperature maintenance in telecom shelters, it should be a stand-alone system, hooked outside the

An important aspect in the design of PCM-air heat exchangers is the selection of an appropriate geometry of the PCM macroencapsulation. It is necessary to consider what will be the requirements that the storage system must satisfy and that will depend on the application. The heat transfer rate (absorbed or released), and the operation time, are two of the factors that generally will be considered. At least there are three typical options to select the shape of the macroencapsulation: plates, cylinders, and spheres. Here, plate shape is

façade, with the ability to plug in with a conventional chiller).

  2

of the sample (equation 2) and by measuring the heat capacity *cp* with a

, (2)

*c* , (3)

**2.2 Thermal conductivity** 

thermal diffusivity

increment is achieved, and *ρ* is the density.

**2.3 Other properties to consider** 

volumetric expansion.

**2.4 Geometry** 

DSC, also the thermal conductivity

Fig. 3. T-history results for an organic PCM (left) and for an inorganic one (right).

the PCM and water is fixed, the tubes are inserted into the air enclosure and the measurement starts. The enthalpy was calculated as shown previously in equations 1.

Detailed information on the raw data and calculations can be found in Lazaro, 2009. An example of the outputs window (Labview application) of an arbitrary T-history test is shown in Figure 4.

Fig. 4. Calculation outputs of a typical T-history test: measured temperatures (up), PCM enthalpy (down-left) and PCM specific heat (down-right).

#### **2.2 Thermal conductivity**

432 Heat Exchangers – Basics Design Applications

Fig. 3. T-history results for an organic PCM (left) and for an inorganic one (right).

shown in Figure 4.

the PCM and water is fixed, the tubes are inserted into the air enclosure and the measurement starts. The enthalpy was calculated as shown previously in equations 1.

Detailed information on the raw data and calculations can be found in Lazaro, 2009. An example of the outputs window (Labview application) of an arbitrary T-history test is

Fig. 4. Calculation outputs of a typical T-history test: measured temperatures (up), PCM

enthalpy (down-left) and PCM specific heat (down-right).

The same procedure to select the appropriate method to obtain the enthalpy vs. temperature curves was followed to find the method for thermal conductivity measurement in liquid and in solid phases. The most commonly used method is the hot wire method (Watanabe, 2002), nevertheless the temperature of the sample is measured with low accuracy and there is also the difficulty in measuring solid samples. A stationary parallel plate method (Mills et al., 2006) solves the problem of accuracy in temperature measurements, but in the liquid phase, convective movements affect the results. The Laser Flash is the only method that allows measuring the thermal diffusivity and sample temperature with accuracy, both in liquid and solid phases. It is based on a laser pulse that comes into contact with one surface of the sample and the temperature evolution on the opposite surface is measured by an infrared detector; therefore, the thickness of the sample must be perfectly determined. A mathematical evaluation of the temperature evolution allows the determination of the thermal diffusivity of the sample (equation 2) and by measuring the heat capacity *cp* with a DSC, also the thermal conductivity may be obtained:

$$\alpha = \frac{1.38}{\pi^2} \frac{\text{L}^2}{\text{t}\_{1/2}},\tag{2}$$

$$
\mathcal{A} = \alpha \rho c\_p,\tag{3}
$$

where *L* is the sample thickness and *t1/2* is the time elapsed until half the temperature increment is achieved, and *ρ* is the density.

#### **2.3 Other properties to consider**

Although we have focused on enthalpy and thermal conductivity, there are other important properties and issues to consider such as: encapsulation compatibility (plastic-paraffin; salt hydrated-metal), toxicity, flammability, corrosion, thermal cycling, rheology, density, and volumetric expansion.

#### **2.4 Geometry**

The specific study system corresponds to a PCM-air heat exchanger acting as a TES unit. The unit is basically composed of PCM plates, the casing, and a fan that blows the air that circulates inside the equipment between the plates (see Figure 5). Although the set up could be arranged horizontally to reduce pressure drop and electrical consumption, the vertical distribution was a requirement because of the very first application (for temperature maintenance in telecom shelters, it should be a stand-alone system, hooked outside the façade, with the ability to plug in with a conventional chiller).

An important aspect in the design of PCM-air heat exchangers is the selection of an appropriate geometry of the PCM macroencapsulation. It is necessary to consider what will be the requirements that the storage system must satisfy and that will depend on the application. The heat transfer rate (absorbed or released), and the operation time, are two of the factors that generally will be considered. At least there are three typical options to select the shape of the macroencapsulation: plates, cylinders, and spheres. Here, plate shape is

PCM-Air Heat Exchangers: Slab Geometry 435

operating modes (5 kW air chiller and 4.4 kW electrical resistance); 2) Air flow measurements; 3) Difference between inlet and outlet air temperature measurements (thermopile); 4) Inlet and outlet air temperature and humidity measurements; 5) PCM and air channels temperature measurements (31 thermocouples); 6) Data logger and data

The energy balance of air between the prototype's inlet and outlet is utilized to evaluate the cooling (equation 4). As the main parameters are the air flow and the air temperature difference between the inlet and the outlet, the accuracy depends on the precision when

> air air through HX air air through HX p *Qm h m c T* ·

 Air temperature difference: thermopile. There were difficulties to overcome in this measurement: a long period of time with little temperature difference; the temperature distributions along the air ducts due of its dimensions; and accuracy, which is required since it is a main parameter of evaluation. A thermopile was chosen as it is

 Air humidity: 2 sensors were used to measure air humidity at the inlet and outlet. Latent energy variation was negligible in the air energy balance for cooling power

Two real-scale prototypes of PCM-air heat exchangers were constructed and incorporated into the experimental setup to characterize them. Initially tests were conducted with the equipment filled with bags of a hydrated salt PCM (prototype 1). Subsequently, the bags were replaced by plates of a paraffin based PCM, and the unit was tested filled with plates. These two geometries were arranged vertically and parallel to the airflow. The casing of the heat exchanger unit used in both cases was the same. PCM thickness was a critical parameter to obtain the required cooling rates (Dolado et al., 2007). Vertical position was a requirement; therefore a metallic grid was used to force PCM thickness below a maximum in vertical position. The experimental setup built to test this kind of heat exchangers is shown in Figure 6. Tests using a constant inlet air temperature setpoint were accomplished. Figure 7 (left) shows the cooling power evolution in prototype 1. Results showed that the cooling rates were very low and the total melting times were double the melting design time (2h). Different air flow rates were tested. As it can be seen in figure 8 (left), the air flow influence on melting times and cooling rates were negligible (in the figure *HH:mm* denotes time, hours:minutes). Cooling power does not increase by a rise of air flow rates. 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

recommended by the ANSI/ASHRAE standard to overcome those difficulties. Air flow: energy balance of electrical resistances. The air temperature changes during tests, therefore most of air flow measurement methods are not suitable for transitory measurements. Mass flows depend only on the fan velocity; therefore they are

measured by applying an energy balance to the electrical resistances.

The reader can find more information on the experimental setup in Lazaro, 2009.

· ·

(4)

screening; 7) Air ducts and gates; 8) PID controller.

measuring these parameters. The methods used are:

evaluation.

**3.2 Two prototypes** 

selected because it has been a deeply studied geometry since London & Seban, 1943. It involves: 1) Easy-to-control PCM thickness, which is a crucial design factor as it allows regulating elapsed times of the melting and solidification; 2) Uniformity of the PCM thickness and, therefore, of the phase change process; 3) Simplicity of the manufacturing process (both small scale and large scale) and versatility of handling (transportation, installation ...); 4) Commercial accessibility in a wide variety of plate-shaped encapsulations in different materials, both metallic and plastic.

Fig. 5. PCM panels and air flow system (left); PCM-air heat exchanger (right).

Finally, the rigid metallic plate encapsulation has been selected to avoid both compatibility issues (Lazaro et al., 2006) as well as leakage problems detected previously (Lazaro, 2009) when using pouches.

#### **2.5 Heat transfer mechanisms: basics**

The basics of the heat transfer in PCM are compiled by Zalba et al., 2003, and discussed in a very understanding way by Mehling & Cabeza, 2008. The authors describe the basics of the heat transfer by means of: 1) Analytical models; 2) Numerical models; 3) Modelling; 4) Comparison of models vs. experimental; 5) Methods to improve the heat transfer.
