CHAPTER 2 LITERATURE SURVEY

Transcription

1 20 CHAPTER 2 LITERATURE SURVEY Various methods of the passive cooling of buildings were discussed in the previous chapter. The present research focuses on the free cooling of buildings integrated with the PCM-based latent heat storage system. Hence, a detailed literature survey has been made on the various studies carried out by researchers on free cooling. Extensive numerical modeling and the CFD analysis along with experimental investigations were also carried out in the present research. The literature survey, therefore, includes the various numerical studies available in the literature along with various Phase change materials available for free cooling. At the end of the literature survey, various heat transfer and other problems encountered in the design of a suitable phase change storage system are also discussed. 2.1 SURVEY ON FREE COOLING METHODS From the literature, it is clear that the first experiment on free cooling/ventilation cooling was reported by Turnpenny et al (2000 part A). In this work, the coldness of the night air is stored in the PCM and discharged during the day time. Heat pipes are embedded in the PCM to enhance the heat transfer between the air and the PCM as shown in Figure.2.1. A theoretical modeling of the proposed system is also done in this work. The heat transfer rate was approximately 40 W over a melting period of 19 hours for a temperature difference between the air and the PCM of 5º C. An improvement in the design of the same system was reported by

2 21 Turnpenny et al (2000 part B). A ceiling fan model with 3 blades with a sweep diameter of 1200 mm and air movement of 3 m 3 /s is used. At night the cool outside air is drawn in and passed over the heat pipe using the ceiling fan blowing downwards. The warm air is let out through the exit vent. During the day time the vents are closed and the ceiling fan blows air downward to cool the room. The heat transfer rate was measured and found to be 200 W, which was sufficient to take care of the summer load. Figure 2.1 System proposed by Turpenny et al The next comprehensive work on free cooling was done by Yanbing et al (2003). In this work at night, the outdoor cool air is blown through the phase change material package bed system to charge the coldness of air to the PCM as shown in Figure.2 2. During the daytime, heat is transferred to the LHTES system, and the coldness stored by the PCM at night is discharged to the room. The air flow rate was controlled to meet the different cooling load demands during the daytime. The room air temperature is reduced in the night ventilation system because of free cooling.

3 22 Figure 2.2 System proposed by Yanbing et al The first feasibility study of a free cooling system was done by Zalba et al (2002 and 2004). In this work, an experimental installation of a flat plate PCM encapsulate as shown in Figure 2.3 was used, for PCMs with a melting temperature around 20-25º C. The major advantages of a flat plate encapsulate are (i) the melting and freezing process of a PCM on a plate surface is symmetric, (ii) the heat transfer in the PCM can be controlled with the selected thickness of the encapsulation and (iii) a high area-to-volume ratio of storage is obtained. The system parameters analyzed are the energyto-volume ratios during charging and discharging. The temperatures of the air during melting (discharging) of the PCM studied were 28 and 30º C. The temperatures of the air selected during freezing (charging) of the PCM were 16 and 18º C. In order to achieve the maximum heat transfer, the air flow rate was selected to get a Biot number close to one.

4 23 Figure 2.3 Heat Exchanger set up of Zalba et al The solidification process was faster (i) when the thickness of the encapsulates was lower, (ii) the temperature difference between the air and the melting temperature of the PCM was higher and,(iii) the air flow rate was higher. The solidification time was lesser and melting time was more because of the free convection in the liquid phase. Marin et al (2005) made some improvements in the heat exchanger made by Belen Zalba by including graphite compounded material with paraffin films for heat transfer enhancement in the PCM. Due to the graphite addition, thermal conductivity is increased in the PCM without much reduction in the energy storage. Other advantages of adding graphite are the decrease in the sub cooling of salt hydrates and the decrease of volume change in the paraffin. These plates contain alternately the PCM and the composite with the PCM embedded in a graphite matrix as shown in Figure 2.4. It was found that there was a great reduction in the time, about one half in the case of heat exchanger using the graphite matrix as compared with those using the PCM only. But the reduction of the energy stored was

5 24 between 12% and 20%, based on the storage volume occupied by the graphite. Figure 2.4 PCM-Graphite arrangement of Marin et al Nagano et al (2003) studied the potential for a manganese nitrate hexa - hydrate mixture, an inorganic PCM as a candidate for cooling to store the cold suitable for the free cooling temperature range. The thermal response, mass required, toxicity and corrosion properties of this material are studied in detail. It was found that the thermal properties of manganese nitrate hexa - hydrate offered a high potential as the PCM for TES in cooling systems. Almost all the chlorides are effective in modulating the melting point of manganese nitrate hexa -hydrate. MnCl 2.4H 2 O is used as an effective additive for the modulation of the melting point, reduction of super cooling and heat of fusion. Takeda et al (2004) developed a ventilation system utilizing thermal energy storage using phase change material granules. In this work an experimental ventilation system (shown in Figure. 2.5) that ensures a direct heat exchange between the ventilation air and the granules containing the phase change material (PCM) was fabricated and tested. The temperature of

6 25 outlet air is measured when the inlet air temperature was varied periodically to simulate changes in the outdoor ambient air temperature. The results showed that the outlet air temperature was stabilized and remained within the phase change temperature range. Packed PCM granules are kept in a rectangular parallel- piped duct made of 100 mm thick thermal insulation boards as shown in Figure.2.5. The duct is installed vertically and the PCM granules are packed in the center of the duct. Granules made by RUBITHERM GmbH were used as the PCM granules in the experiment. The granules have a particle diameter of 1 3 mm and consist of 65% ceramic materials and 35% paraffin hydrocarbon by weight. The packed bed has a high capacity to stabilize the diurnal fluctuations of the outdoor air temperature. A ventilation system for reducing the ventilation load was examined through computer simulation for eight representative cities of Japan. This revealed how different climatic temperature conditions would affect the required heat storage capacity. Figure 2.5 Conceptual system proposed by Takeda et al

7 26 Nagano et al (2004) embedded the PCM directly on the floor boards in the form of granules, several millimeters in diameter. This PCM packed bed is permeable to air, and so it is suitable for use in floor supply air conditioning systems. During the night, the circulation of cool air through the under floor space allows cool energy to be charged to the concrete slab, floor board and the PCM packed bed. During the day time cool energy can be used to remove the cooling load in the room. This method shown in Figure 2.6 is superior compared to a sensible storage system, because the building s thermal mass storage capacity is limited. Figure 2.6 Conceptual system proposed by Nagano et al Arkar and Medved (2005) studied the influence of the thermal property data of the phase change material on the result of a numerical model developed for a packed bed storage system used for free cooling. A packed bed numerical model was modified to take into account the non-uniformity of the PCM s porosity and the fluid s velocity, which is due to the small tube-to-

8 27 sphere diameter ratio. Based on the parametric analysis, a free cooling system was suggested by the same authors, (Arkar et al 2007) which comprises of a single cylindrical LHTES containing an optimized diameter of spheres with an encapsulated PCM, with a small pressure drop. In this study as shown in Figure 2.7, the LHTES is filled with spheres encapsulated with the PCM. The storage aspect ratio, L/D, is 1.5 with a small pressure drop, and thus, a low electrical power of the fans. Two LHTESs were used in this system, one operating with ambient air and the other with recirculated air. Thus, the overall cooling efficiency of free cooling, using the latent heat storage integrated system of a low energy building is increased. Figure 2.7 Daytime and night time free cooling operation mode as proposed by Arkar and Medved Medved and Arkar (2007) studied the free-cooling potential for different climatic locations in Europe. The size of the LHTES was optimized on the basis of the calculated cooling degree-hours. Six representative cities were selected in Europe, that cover a wide range of different climatic conditions. Numerical investigations of the free-cooling potential were made for a time period of 3 summer months, and the optimal mass was finalized for

9 28 the studied system. For a comparison of the free-cooling efficiency, the cooling degree-hours (CDH) was determined for the same time period. The LHTES optimization was made for selected parameters, such as the PCM s phase change temperature range, the PCM s melting temperature, and the ratio of the PCM s mass to the air volume flow rate. Based on the outcome of the experiments of Zalba et al, two different real-scale prototypes of air-to-pcm heat exchangers were designed and tested by Lazaro et al (2009) following the ANSI/ASHRAE standard (Method of testing the active latent-heat storage devices based on thermal performance). In this method, in order to obtain accuracy in the measurement of air flow and the temperature difference between the inlet and the outlet, precision thermopiles were used in the measurement of the inlet and outlet temperatures. (a) (b) Figure 2.8 Encapsulates used by Lazaro et al (a) Pouches (b) Flat panels Two prototypes used in their work were tested for the heat transfer between the air and the PCM. The prototype 1 shown in Figure 2.8 (a) uses

10 29 aluminum pouches filled with an inorganic PCM and air is passed over it. The air was made to flow parallel to the pouches from the bottom to the top. When tested with a constant inlet temperature, the results showed that the cooling rates were low and the melting time is double the melting design time. The second prototype was designed using aluminum panels filled with an organic PCM as shown in Figure 2.8 (b). The set up was tested with different air flow rates in prototype 2 and it was observed that it has influence on the melting time and cooling power. This indicates that the conduction inside the PCM will control the heat transfer compared to heat convection to air. An empirical model for a real-scale prototype of a PCM-air heat exchanger is discussed by Lazaro et al (2009). From the experimental results, an empirical model for simulating the thermal behavior in the tested heat exchanger in different cases was prepared for evaluating the technical viability of its application. Since the thermal properties of the PCM vary with temperature, a PCM-heat exchanger design must be based on the transient analysis. This work shows that the PCM selection criteria must include the power demand. 2.2 MODELING OF THE PHASE CHANGE STORAGE SYSTEM The use of phase change materials for the storage of energy has received considerable attention in recent years. The numerical approach of solving phase change problems is categorized as Temperature based models or variable domain models and Enthalpy based models or fixed domain models. In the temperature based models, also called as variable domain models the volume of each region changes with respect to time and the temperature is the sole dependent variable. The energy conservation equations are written separately for each region and the solutions of these equations are coupled through the energy balance at the interface. The major disadvantage in the temperature based model is the continuous tracking of the interface by solving simultaneously all the three energy equations.

11 30 The enthalpy method which was introduced in the 1940s is widely employed in modeling phase change problems. The advantage is that it can accommodate materials that change their phase over a temperature range. In addition, the phase change problem can be reduced to a single equation in terms of enthalpy. There is no boundary condition to be satisfied at the interface and the total domain volume does not change with respect to time. Meyer (1973) showed that the enthalpy model can also be used for solving the problems where the material phase change occurs with a negligible temperature range ( o C). Voller and Cross (1981) showed that the accuracy of the scheme is dependent on the choice of the temperature range considered for phase change, elemental control volume and time step. Shamsunder and Sparrow (1975) have developed an integral relation for the enthalpy model without assigning any phase change temperature range for the analysis of multidimensional conduction phase change problems where the phase change occurs at a fixed temperature. Here, the value of T is recovered from the H-T relationship while tracking the node is required to identify the two phase and single phase nodes. Date (1991) has generalized the HT relationship in such a way that no tracking is required, and he has solved the finite difference equation by the Tri Diagonal Matrix Algorithm (TDMA). Velraj et al (1997) modified this relationship to accommodate the materials having a range of phase change temperatures. Later, Date (1991) devised a procedure to estimate the exact location of the interface within the phase change node and the appropriate nodal temperature at the phase change, based on the value of solid fraction calculated for the node. Voller (1985) has proposed a new formulation for the enthalpy method based on separating the enthalpy into latent and sensible heat components. The numerical scheme incorporated suitable source terms in the governing equations. A nodal latent value is assigned to each cell according to its temperature. When the phase change takes place, the latent heat absorption, or release, is reflected as a source or sink term in the energy equation. The zero-velocity condition,

12 31 which is required as a liquid region transforms to solid, is effected by adding a source term in the momentum equation. This model is identified as the enthalpy-porosity model. Voller (1990) developed a new implicit enthalpy solution scheme that requires no under relaxation or over relaxation depending on the problem. Swaminathan and Voller (1990) have proposed a generalized enthalpy method that incorporates both the apparent heat capacity and source based methods in it. Zivkovic and Fujii (2001) simulated the transient behavior of a phase change material for both cylindrical and rectangular geometries, and their rectangular geometry showed a good agreement with the experiment. Rady and Mohanty (1996) had applied an enthalpy-porosity fixed grid method to the melting and solidification of pure metals in a rectangular cavity. Lacroix (1993) developed a theoretical model to predict the transient behavior of a shell-and-tube storage unit with the PCM on the shell side and the heat transfer fluid (HTF) circulating inside the tube. Results showed that the shell radius, the mass flow rate, and the inlet temperature of the HTF are important parameters. Patrick and Lacroix (1998) numerically estimated the thermal behavior of multi-layer heat storage unit. The model is based on the conservation equation of energy for the PCM and the fluid heat transfer. They concluded that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, mass of the PCM, and fluid mass flow rate and temperature. Kurklu et al (1996) developed a numerical model for the prediction of the thermal performance of a PCM, polypropylene tube, utilizing air as the heat transfer fluid. Velraj et al (1999) carried out an experimental analysis and numerical modeling of inward solidification in a finned vertical tube for a latent heat storage unit. The influence of various parameters on the performance of the system has been studied. Elgafy et al (2004) developed a computational model to investigate and predict the thermal performance of high melting

13 32 point phase change material during its melting and solidification processes within a cylindrical enclosure. In this model the phases are assumed to be homogeneous, and a source term, S, arising from the melting or solidification process is considered as a function of the latent heat of fusion and the liquid phase fraction. Trp et al (2005) carried out a theoretical and experimental heat transfer analysis of the shell and tube heat exchanger with the cold fluid inside and solidification outside. The numerical results were used as a guideline for the design of a latent thermal energy storage system. An alternative formulation called the apparent heat capacity method is employed to solve the melting and solidification problem by including the effect of the PCM storage through apparent heat capacity model in the energy equation. Various shapes of the apparent heat capacity like rectangular and triangular profiles were studied by Beasley et al (1989) and Lamberg et al (2004). Arkar and Medved (2005) simulated the heat transfer in the cylindrical packed bed by using the apparent heat capacity method utilizing the DSC results as a model of a the heat capacity value. Hed and Bellander (2006) developed a mathematical model for flat plat PCM air heat exchanger for free cooling considering the shape of the c p (T) curve. 2.3 PHASE CHANGE MATERIALS USED IN FREE COOLING For efficient free cooling, it is necessary to select a PCM that is suitable for the climatic conditions, and to determine the optimal mass of the PCM for the selected geometry and performance parameters of the LHTES system. The desirable properties of the PCMs are the high latent heat of fusion, high thermal conductivity, small volume change during phase change, and least sub-cooling while freezing, should possess chemical stability, be non-toxic and cheap. The PCM used for free cooling should have melting temperatures ranging from 15º C to 30º C. Detailed discussions about various types of the PCM and their properties are given in various reviews (Zalba

14 ), Sharma (2009), Sharma (2005), Tyagi (2007) and Kenisarin (2007). The PCMs are classified as organic, inorganic and eutectic materials. The inorganic PCMs (Tables 2.1 and 2.2) have a higher thermal conductivity and energy storage density. Also, they are noninflammable and cheap. But because of their corrosiveness, super cooling and phase segregation during the phase change, these materials are not usually used for free cooling. To overcome these problems, normally nucleating and thickening agents are to be added with the inorganic PCMs. Due to the absence of these problems organic PCMs (Table 2.3) become attractive. However, flammability, volume change and lower heat conductivity are concerns in recent studies. Organic PCMs are classified as paraffin and non paraffin. Euctectic or Non euctectic mixtures of organic and inorganic PCMs could be used to get the desired melting point. Most of the experiments conducted so far in free cooling use commercial grades of PCMs (Table 2.4) available from the manufacturers for which the properties are available on the websites. Table 2.1 Inorganic PCMs for free cooling Compound Melting point ( o C) Heat of fusion (kj/kg) References KF. 4H2O Naumann (1989), Abhat (1983) Mn(No 3 ) 2. 6H 2 O Nagano (2004) CaCl 2. 6H 2 O Dincer (2002), Lane (1980), Tyagi (2008), Kimura (1984) Na 2 SO 4. 10H 2 O Naumann (1989), Abhat (1983)

15 34 Table 2.2 Inorganic Eutectics for free cooling Compound 48% CaCl % NaCl+0.4% KCl+47.3% H 2 O 47% Ca(NO 3 ) 2. 4H 2 O+53% Mg(NO 3 ) 2. 6H 2 O Melting Point point ( o C) Heat of fusion (kj/kg) References Abhat (1983) Abhat (1983) 60% Na(CH 3 COO) Li (1991) 3H 2 O+40% CO(NH 2 ) 2 Table 2.3 Organic PCMs for free cooling Compound Melting Temperature Point ( o C) Heat of fusion (kj/kg) References Capric acid Dincer (2002), Lane (1980) Capric Lauric acid Capric acid - Myristic acid Capric acid - Palmitic acid Capric acid - Stearic acid Karunen (1991), Dimaano (2002), Hawes (1993) Karunen (1991), Sari (2005) Karunen (1991), Pieppo (1991) Karunen (1991), Sari (2005) Paraf n C 16 C Zalba (2002), Marin (2003) Paraf n C 13 C Zalba (2002), Marin (2003) Dimethyl sabacate Feldman (1986) Polyglycol E Dincer (2002), Lane (1980) 1-Dodecanol Hawes (1993) Vinyl stearate Feldman (1986) Hexadecane Lazaro (2006)

16 35 Table 2.4 Commercial PCMs for free cooling Name Melting Heat of fusion temperature (kj/kg) Density Manufacturing Company s Name RT20 Paraffin (0.75) Rubitherm RT26 Paraffin (0.76) RT27 Paraffin (0.75) ClimSel C 23 Salt hydrate Climator AB ClimSel C 24 Salt hydrate Climsel C 32 Salt hydrate STL 27 Salt hydrate S27 Salt hydrate E17 Salt hydrate E19 Salt hydrate E21 Salt hydrate E30 Salt hydrate Mitsubishi chemical Cristopia Environmental Process Limited SELECTION OF THE PCM For efficient free cooling it is necessary to select a PCM that is suitable for the climatic conditions assumed, and also consider the various parameters like the melting point, heat storage capacity and density. Diurnal temperature variation of the selected location is very important while selecting the phase change material. Hence the monthly average maximum and minimum temperatures for the selected city (Bangalore), are obtained from the statistical data provided by ISHRAE, India (shown in APPENDIX 7). Towards the selection of PCM, various organic, inorganic and eutectic phase change materials discussed in the above section has been considered as the candidate for the present application. Inorganic PCMs were not

17 36 considered because of their corrosiveness, super cooling and phase segregation during the phase change. These problems were not experienced by the researchers with organic PCMs. Hence it was decided to select RT 27, which is a commercial grade paraffin available with RUBITHERM, Germany. The properties of RT 27 provided by the manufacturer are given in Table.2.5. The latent heat value of PCM alone is given from DSC analysis which is different from manufacturer s data (180 kj/kg). In order to confirm the important properties of the PCM such as the phase change temperature and latent heat, RT 27 was tested using a differential scanning calorimeter. Figures 2.11.a and 2.11.b show the characteristic curves obtained during the solidification and melting of the PCM. It is seen from the figure that the selected PCM has a solidification range of 25-27º C and a melting range of 26-29º C, which is suitable for the selected site. (a) (b) Figure 2.9 Results of the DSC analysis for RT27 a) Solidification b) Melting

18 37 Table 2.5 Thermo physical properties of paraffin Sl No Property Unit Value 1 Density (kg/m 3 ) 870(S) - 750(l) 2 Specific heat J/kg K 1800(S) (l) 3 Melting point º C 28º C 4 Freezing point º C 26º C 5 Solidification range º C 25º C - 27º C 6 Melting range º C 27º C - 29º C 7 Latent heat of fusion J/kg Thermal conductivity W/m K 0.20(S) (l) 2.5 HEAT TRANSFER PROBLEMS AND DESIGN CONSIDERATIONS IN FREE COOLING Free cooling works well in places where the atmospheric diurnal temperature range is more than 15º C. This temperature range is achievable in desert and interior regions. For places where the diurnal temperature range is less than 15º C, adopting the free cooling concept requires careful design consideration. Hence, the selection of the PCM and achieving the required heat transfer for free cooling requires careful consideration Thermal Resistance of air and the PCM In all the free cooling applications air is used as the heat transfer fluid and the PCM is used as the storage material. The surface heat transfer coefficient with the air as the working medium is normally of a low value. Usually, fins are provided on the air side to increase the surface area of the heat transfer. During the initial part of the solidification and melting, the conduction resistance offered by the PCM will be very low, and hence, high heat transfer can be achieved by having a higher surface heat transfer coefficient (i.e.) by circulating the air at higher velocity. But at later stages the

19 38 variable conduction resistance offered by the PCM is very high, and hence the velocity of air circulation can be reduced to match the PCM resistance with air resistance. Most of the researchers have used paraffin (organic PCM) as the phase change material which has a very low thermal conductivity. Though it has no segregation problem in repeated cycling normally encountered in inorganic salt hydrates, the thermal conductivity is less. In order to compensate the low thermal conductivity heat transfer enhancement techniques like the introduction of fins (Agyenim et al 2009, 2010), inserting the metal matrix in the PCM (Hafner et al 1999), packing the PCM with lessing rings (Velraj et al 1997, 1999, Seeniraj et al 2008), introduction of graphite in the PCM base material in fibrous (Fukai et al 2000, 2002, 2003) and shape stabilized form, and the use of heat pipes (Turnpenny et al 2000 a, b) as shown in Figure.2.9, are employed Effect of Geometry on the Encapsulation Container In a free cooling operation, the charging time available in the accelerated mode will be very less (3-4 hours when the ambient temperature is low, usually in the early morning hours), and hence, there is a restriction in the solidification time, solidification thickness and heat transfer surface. Several studies have been made in various configurations like the flat plate, cylindrical, shell and tube, and spherical encapsulation as shown in Figure With flat plates it is possible to achieve more surface area per unit volume of storage material, with low PCM thickness for reducing the solidification time. According to Zalba et al (2004) the flat plate configuration with the channel width of 15 mm resulted in a charging time of 4 hours and a discharging time of 6 hours, which is reasonably acceptable for free cooling. Also they have less weight and volume. PCM Cylindrical pipes have lesser

20 39 fabrication difficulty, comparable heat transfer characteristics and lower heat loss rate. In a shell and tube heat exchanger the transfer of heat takes place in the axial and radial directions, with an increased area of convective heat transfer. PCM balls have larger surface area per unit volume compared to the cylindrical geometry of the length equal to the diameter. Heat transfer and pressure drop can be controlled by selecting the size of the balls. Arkar et al. (2005) have used 25 mm balls and Takeda et al (2004) used granulated (micro encapsulated) PCM of 3mm size. In the later case, the charging time is 1.5 hours, which is well suited for the free cooling application. However, this increases the pressure drop, and hence the pumping power of the fan used. Optimizing the size of the encapsulates/granules is essential for improving the energy efficiency. PCM pouches and panels were tested by Lazaro et al. (2009) and the panel was found to be superior to the pouch Energy Efficient Charging and Discharging In free cooling applications the lowest temperature during the day is available for 3 to 4 hours in the early morning. This period is utilized to charge the cool energy in the PCM. The initial part of the charging can be accelerated with a higher surface heat transfer coefficient. However, during the later part of the charging, a higher heat transfer coefficient may not be useful, as the conductive resistance of the PCM becomes dominant. Hence, the fan speed can be reduced during the later part of the charging process, so that the reduced heat transfer coefficient is in conjunction with the inside variable conductive resistance of the PCM. Hence, while designing a free cooling system, an energy efficient multiple speed fan should be used to achieve energy efficient charging and discharging.

21 40 Figure 2.10 Heat transfer enhancement techniques in PCM Figure 2.11 (Continued)

22 41 Figure 2.11 Various Types of PCM Encapsulations Effect of Phase Change Temperature When the selected PCM has a distinct Phase change temperature, the PCM cannot accommodate the swing in the ambient air temperature. Hence the same PCM cannot be used in all the seasons. According to Medved and Arkar (2005), a PCM with a range of phase change temperatures is suitable for free cooling as it absorbs the heat in varying inlet temperatures and accommodates the swing in the ambient air temperature on a large number of days. Thus, it is construed that the selection of the PCM for year round thermal management is difficult, since there is a large swing in the atmospheric temperature in all the seasons. Pasupathy et al. (2008) suggested two layers of the PCM with different melting temperatures for year round thermal management in the roof top of the building. A similar concept of using multiple PCMs is also used for free cooling for year round thermal management.

23 42 While freezing most of the PCMs experience sub-cooling. Hence, the available temperature potential between the air and the PCM will become much less, which reduces the heat transfer rate. Hence the ratio of sensible heat to latent heat stored in the PCM material, which is normally defined as the Stefan number, should be as low as possible Effect of Insulation Insulation plays an important role in retaining the stored heat in the system, especially when operating with a low diurnal temperature difference. If the day time ambient temperature is high, and the system and ducts are not insulated, the heat loss to the surroundings will be more through the walls and through the ducts used for conveying the cold air to the room. In tropical climates, rather than keeping the heat exchanger outside it is better to keep the PCM inside the building, using PCM panels and pouches for reducing the heat losses to the ambient air Fan Energy Consumption The energy required to drive the fan should be as low as possible with the maximum heat transfer. For achieving this objective, the volume flow rate, pressure drop, and charging time should be reduced in the designed system. The volume flow rate of air is controlled by controlling the velocity of the flow. The velocity of the air flow in the heat exchanger considered in the designed system should be less, for energy efficient operation. As the solidification progresses, increasing the velocity of the air does not play any role in reducing the solidification time. Also at higher velocities more energy is wasted as turbulent production and dissipation, without increasing the heat transfer rate.

24 Effect of Geographical Location and Seasons on Free Cooling Potential The ambient temperature of a place is dependent on the seasonal climate and its geographical location. The comfort temperature of the room ranges from 21º C to 25º C. The free cooling concept is site specific and climate dependent. Free cooling is suitable for the interior and desert regions. The benefit is less in the coastal area, because temperature moderation is done by the sea and land breeze, and in places where the diurnal temperature variation is less. The effectiveness of the free cooling does not depend on the average temperature of a place, but it is a strong function of the amplitude of the ambient temperature swing. The melting temperature of the PCM should be higher in a warmer climate. The optimum PCM phase change temperature as given by Arkar et al (2006) is T p = T a + 2 K. The variation of the ambient temperature of the air should be on both sides of the melting temperature of the PCM, so that the atmosphere can act as the heat sink to reject the heat during the night time. If during summer the ambient temperature of a place does not fall below the melting temperature of the PCM, it will not be solidified, and will not be ready for the next day s operation. To estimate the efficiency of free cooling, a concept called as cooling degree hours was introduced by Arkar et al (2006). CDH N I 1 (T a T o ) (2.1) where n - number of days for which free cooling experiment is conducted T a - ambient temperature.

25 44 T o - Outlet temperature of the free cooling system - 1 hour If the outlet temperature of the LHTS system is reduced, the cooling potential by free cooling is increased, because of the increase in the cooling degree hours. 2.6 PROFILE OF THE PRESENT WORK The present research is subdivided into 3 major sub groups and they are presented in 3 chapters. The results of each work are presented in the respective chapters. In the first part, a numerical study and experiment on a single tube surrounded by the PCM shell is carried out. A transient numerical model is developed to investigate and predict the performance of a paraffin phase change material in the annular portion of the cylindrical container during its solidification and melting processes. The Enthalpy method of modeling is adopted, and the discretised non-dimensional form of governing equations and boundary conditions are solved by the implicit finite difference method by using MATLAB software. The temperature variation of the PCM along the two axes of the polar co-ordinates (r, z) and the time required for solidification are analyzed and presented. In the second part, the results of the numerical work obtained are used in the development of a novel modular heat exchanger that encapsulates the PCM in the shell portion of the module and passage for the flow of air through the tubes. These modules are stacked one over the other with air spacers between each module. This modular heat exchanger arrangement is suitable for free cooling applications where the diurnal temperature variation is low. The CFD modeling and analysis is carried out for a complex

26 45 configuration involving the flow and heat transfer studies for the fluid flowing through the tube in the heat exchanger module and the air spacer kept at the top and bottom of the module, in conjunction with the transient heat transfer analysis for the PCM encapsulated in the shell portion of the module, using the apparent heat capacity method using FLUENT software. The CFD results are validated with the experimental results. The CFD results presented in the work are very useful in understanding the flow and heat transfer phenomena in a modular heat exchanger selected for the free cooling application. The steady state analysis is useful to determine the pressure drop across the module and the spacers, and to know the flow and temperature variation of the heat transfer fluid in the module, so as to select the geometrical and flow parameters for a given surface temperature and inlet condition. The transient analysis results are useful to determine the PCM solidification characteristics and to verify the suitability of the selected geometrical dimensions. In the third part, an experimental investigation is made to study the performance of the modular heat exchanger selected for free cooling. Free cooling experiments were carried out in the range of m/s frontal velocity and tested for their performance in regions with low diurnal temperature variation. The PCM is selected based on the climatic condition of the region selected for the study. The performance of the building is tested for various heating load conditions of 1, 2 and 3 kws, and the effectiveness of the free cooling is evaluated. A CFD analysis has also been conducted for the charging condition carried out in the experiment and the results are presented.