Experimental study of heat storage in a PCM incorporated into a residentiel premises walls

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Experimental study of heat storage in a PCM incorporated into a residentiel premises walls

Article · March 2015

DOI: 10.1109/IRSEC.2014.7059904

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Experimental study of heat storage in a PCM incorporated into a residentiel premises walls

Yassine BOUZLOU¹, Amina MOURID¹, Mustapha EL ALAMI¹, Mostafa NAJAM¹ and Mustapha FARAJI¹

1Thermal Group, LPMMAT Laboratory, Physics Department, Faculty of Sciences Aïn Chock Hassan II – University of Casablanca

Casablanca, Morocco

Bouzlouyassine@gmail.com , m.elalami@etude.univcasa.ma

Abstract— to improve building energy efficiency, Phase change materials (PCM) can be applied in building envelope to conserve heat energy. The cubicle with PCM panels can automatically absorb indoor redundant heat, which can greatly reduce the load of Heating, Ventilation and Air-Conditioning (HVAC) systems and save electric energy.

In experiments, a cell is constructed in the faculty of science Aïn Chock, at the Hassan II University of Casablanca (cavity with 3x3x3 m³). The internal side walls of the cell are covered by PCM panels. Thermocouples are erected above adequately to measure different temperatures used in the quantification of the thermal flux stored in the PCM or passing outwards.

Determination of these fluxes is based on known correlations of the convective heat transfer coefficient. This latter which constitutes the main objective of our works, will be defined experimentally in future works.

Keywords: Phase change material (PCM) wallboards; Latent heat; Energy efficiency; Heat storage; Buildings;

I. INTRODUCTION

Currently, the modern lifestyle is causing a huge problem of energy consumption. The climatic conditions generate a situation of discomfort, which requires the use of more and more increasing air conditioning systems. This causes worldwide scientific and technical mobilization. The research guides its efforts on deepening basic themes for the development of new techniques and technologies to conserve energy consumed by air conditioners [1].

Consequently, there is a need for heat storage so that the excess heat produced during supply periods can be stored for use during the demand periods. To this end, one of the most important techniques for thermal energy storage is the latent PCM energy storage; appear to be promising given their great heat storage capacity and their low heat losses during the storage period [2]. The PCMs are increasingly used for insulation and heat storage in buildings. It has been a central topic in research for the last 20 years. Because of the high thermal mass of PCM walls, they are capable of minimizing the effect of large fluctuations in ambient temperature on the inside temperature of a building [3]. They can therefore be very effective in shifting the heating and cooling load to off peak electricity periods. Cabeza [4] conducted an experimental set-

up to test phase change materials and to evaluate the energy storage in the walls by encapsulating PCMs and the comparison with conventional concrete without PCMs for the locality of Puigverd of Lleida (Spain) in real conditions. The results of this study show the energy storage in the walls by encapsulating PCMs and the comparison with conventional concrete without phase change materials leading to an improved thermal inertia as well as lower inner temperatures.

These results demonstrate a real opportunity in energy savings for buildings. The thermal inertia, seen in all the experiments, shows that all the PCM included in the cell walls freezes and melts in every cycle. The results also showed that night cooling is important to achieve this full cycle every day.

The aim of this paper is to evaluate the heat stored in the phase change material (PCM) by calculating the convective heat flux along the inner surface of the cell, and conductive thermal flux through the PCM layer, and then we calculate the convective heat transfer coefficients for external building surfaces (hc,,ext). These results are a clearly synonymous of energy efficiency in buildings, by contributing to the reduction of the energy consumption associated to active systems and or using more efficiently the available energy sources.

II. EXPERIMENTAL SETUP

A. Description of the cavity with PCM

The test cell is typical housing rooms built to scale 1 and exposed to weather conditions of Casablanca (in-situ) (Fig.1).

The phase change material used is encapsulated, in panels, by using thin Aluminum coverage. The PCM is a polymer blend of paraffin wax and provides the functionality panel with a melting temperature of about 22°C (thermal comfort temperature).

The cubicle dimensions are 3m×3m×3m. The panel dimensions are 1×1.2×0.0052m³. Walls are composed of six layers, from inside to outside: PCM (0.5cm), air layer (1.2cm), Alveolar Bricks (7cm), air layer (14cm), Alveolar Bricks (7cm), Mortar 1cm. The north wall is equipped with a solid wood door (1mx2.10m) and a single glass (5mm) window with aluminum frame (1mx1m). There is about 42m² inner surface of the cell covered by the MCP (about 200kg MCP).

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Figure 1: View of the cell

The cubicle is equipped with a heater floor, connected to a solar thermal collector. Pending its start, we use an oil radiator, to heat the cell. It provides a constant heating power when turned on, and maintain the room temperature constant (Fig. 2).

B. Measurement Devices

The cell was instrumented with 44 thermocouples distributed adequately on the walls and in the cavity. A meteorological station was installed nearby (Fig. 3). It measures inlet and outlet temperatures, wind speed, global solar radiation, humidity and wind direction. All the instrumentation is connected to a data logger connected to a computer (Fig.2).

Figure 2: Experimental setup

Figure 3: Meteorological station

C. Data exploitation

The measured temperatures are used to determine the conductive heat flux in the air layer (width=12mm) for each wall (west, east, south and north) (equation 1). We neglected the convective flows in this layer and radiation heat exchange between air and walls. This flux will be used to evaluate the convective heat transfer coefficient on the outside walls when the regime is permanent and internal temperature is above the melting point of the PCM (equation 2). Referring to the Figure 4, the conductive heat flux through the air layer is:

𝑄_{𝑐𝑜𝑛𝑑} =𝜆𝑎𝑖𝑟

𝑒𝑎𝑖𝑟

(𝑇₂− 𝑇₃) (𝑊/𝑚²) (1) Where𝑄_{𝑐𝑜𝑛𝑑} is the lost heat flux by conduction, 𝜆_𝑎𝑖𝑟 thermal conductivity of air, 𝑒_𝑎𝑖𝑟thickness of air layer, Ti is the surface temperature, Figure 4.

ℎ_{𝑐,𝑒𝑥𝑡} = 𝑄𝑐𝑜𝑛𝑑

(𝑇₄− 𝑇_𝑒) (𝑊/𝑚². 𝐾) (2) To evaluate the heat stored in the PCM, we use equation 3;

the flux stored, is the difference between the flux exchanged by convection between the wall and the inner air (warmer than the wall of the room), and that which passes outwardly by conduction (equation 3).

𝑄𝑠𝑡𝑜𝑟𝑒𝑑= 𝑄𝑐𝑜𝑛𝑣,𝑖𝑛− 𝑄𝑐𝑜𝑛𝑑 (𝑊/𝑚²) (3)

With

𝑄𝑐𝑜𝑛𝑣,𝑖𝑛= ℎ𝑖. (𝑇𝑎− 𝑇1) (𝑊/𝑚²) (4)

hi : is the convective heat transfer coefficient along the

internal faces of the building. It is a fundamental unknown, in this work. We found some previous works suggesting definitions of this coefficient. We selected those proposed by ARSHRAE [5] giving h_i =9.1W/m².K for configuration similar to ours. It will be a subject of our future experimental

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will be given.

Figure 4: Heat transfer inside the wall with PCM

III. RESULTS AND DISCUSSION

In this section, the results of the experiment are presented for March, 9^th, 2014 between 23h and 1h. This period is chosen because it has the most unfavorable conditions for heating (absence of external heating).

A. Convective heat transfer coefficients for external building The external heat transfer coefficients are calculated using the measured temperatures of both inner and outer faces of the cell walls (west, east, south and north). Their variations with time are shown in Fig.5. The different curves of the cavity walls, the oscillations which are present primarily due to fluctuations of the wind speed. We note that all the curves of hc,ext present a maximum around midnight (t=00h32mn). Note, also, that the convective coefficients for the west and north walls are greater than the east and south ones with a difference that can reach up to 6 W/m².K. This is, may be due to the wind direction for the experience period (the station indicated that the wind direction is NW: North West). The convective heat transfer coefficient is influenced by several factors, such as the geometry of the building, the position at the building envelope, the building surface roughness, wind speed, wind direction, air flow patterns and surface to air temperature differences [6]. In urban areas, local air flow patterns around a building strongly depend on the arrangement and geometry of neighboring buildings [7] which strongly influence (h c, ext).

Ground type influences the mean wind speed and turbulence intensity profiles [8, 9] which also influence (hc, ext) [6]. Its experimental determination seems of great importance, which we try to explain in the Figure 5, when the majority of these parameters are present.

Figure 5: Time variation of the convective heat transfer coefficients

Table 1: Avargae convective heat transfert coifficients : COMPARISON WITH PREVIOUS RESULTS

Model Wall

Hagishima &

Tanimoto[11]

Ashrae task group [12]

Jayamaha

& Al [13]

Liu &

Harris [14]

Our Results

South 14,68 15,62 9,28 11,22 9,52

West 14,68 15,62 9,28 11,22 11,29

Small differences between the experimental average convective heat transfer coefficient (our results) and the models (Liu & Harris, Jayamaha & Al), as shown in Table 1. These deviations are, certainly, due to the due to the location of the cavity and the type of ground that surrounds this latter, which are different from the conditions suited to each model, plus most of the model is based only on the wind speed without taking into consideration different temperature gradients.

B. Heat flux stored in PCM

To quantify the flux of heat stored in the MCP, we performed a specific heat balance on each wall of the cavity.

Indeed, we evaluated the convective flux exchanged between the inside air and the wall (using the coefficient hi given by [5]), as well as the flux passing outwards (energy losses), based on the characteristics of the air layer (12mm) we fitted in each wall. The value of the stored heat flux (Qstored ) for all the surfaces may be expressed by equation 3.

The figure 6 shows the three curves for the total flux received by the south wall (by convection), flux stored in the MCP, and lost flux to the outside (by conduction). Note that the average heat fluxes for this wall are:

Qconv,s = Qconv,in  35W/m² Qstored,s  25 W/m² Qls,s = Qcond  10W/m²

With Qconv,s, Qstored,s and Qls,s are, respectively, inner convective, stored and lost heat fluxes of the south wall.

Authors acknowledge the financial support provided by IRESEN

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Figure 6: Time Variation of heat fluxes for the south wall

For the same period of measurements and in the case of the west wall, the different fluxes (convective, stored and lost outwardly) vary in the same manner, as in the case of the south wall, figure 7. The phase change material plays an important role in the energy storage.

Quantitatively, the outward flux lost (Qls,w) remains virtually unchanged:

Qls,w = Qcond  10W/m²

Against, the other two fluxes have much decreased in comparison with the case of the south wall:

Qconv,w = Qconv,in  28 W/m² Qstored,w  18 W/m²

Qconv,w, Qstored,w are, respectively the convective and the stored fluxes for the west wall.

IV. CONCLUSION

This work falls within the scope of energy efficiency in building. The heat storage by phase change materials is one of the most promising methods to keep the thermal comfort in the building with a significant reduction of energy losses to the outside and as a result a significant savings in heating energy.

This work is the first approach that allows us to find excellent results:

 The results showed a significant reduction of heat flux through the wall with PCM, due to absorption of heat flux in this latter. It can therefore be concluded that PCM is effective for storage of heating gains, and improvement of thermal comfort.

Figure 7: Time Variation of heat fluxes for the west wall

 The heat stored is different depending on the orientation of the walls, climatic conditions, internal gains, and the temperature range over which phase- change occurs, and the latent heat capacity per unit area of the wall.

 The heat stored by phase change material can reduce the heating time, and by the way, reduce the consumption of electrical energy.

 The convective coefficient determined experimentally for each wall, allows for the heat balance of the cavity which will help us a lot in the future works.

In the continuation of this work, we will dispose, wisely, the PCM on the roof, for thermal comfort in summer.

ACKNOWLEDGMENT

Authors acknowledge the financial support provided by IRESEN: The “Institut de Recherche en Energie Solaire et Energies Nouvelles” Morocco.

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