CHAPTER 3 EXPERIMENTAL INVESTIGATION

Transcription

1 58 CHAPTER 3 EXPERIMENTAL INVESTIGATION In the present work, a solar system integrated with a cooking unit through a thermal storage tank is developed, to test the performance of the system components during the charging and discharging processes. The major components involved in the present setup are, a solar parabolic trough collector, a thermal energy storage system, a cooking unit, and a double pipe heat exchanger. The proper selection of the heat transfer fluid circulated through these system components, and the identification of a suitable PCM to store the latent heat in the storage system are essential to maximize the efficiency of the system. The selection of the HTF, PCM and various system components, and the construction of the experimental setup, and the methodology adopted in the conduct of the experiments are presented in this chapter in detail. 3.1 SELECTION OF THE HEAT TRANSFER FLUID AND PHASE CHANGE MATERIAL The details about the heat transfer fluid (HTF) and phase change material (PCM) identified in the present work are presented in this section Heat Transfer Fluid In the present work, Therminol 55 oil is selected as the HTF and the sensible heat storage material, considering the properties of the fluid, like the maximum withstand temperature, coefficient of thermal expansion,

2 59 viscosity, thermal capacity, boiling point, flash point and fire point. The selected HTF is a synthetic oil intended for use in the liquid phase, for indirect process heating in various industries. The thermal stability of a HTF is one of the most important considerations in the selection of a fluid for operation under specific heat transfer conditions. The reason for using Therminol 55 is that, when compared to mineral oils it exhibits thermal stability that is markedly superior, resulting in a favorable cost / performance ratio. Therminol 55 offers outstanding high temperature performance up to 315 o C and delivers efficient, dependable, and uniform process heat over long periods of time. The selected HTF appears as a clear yellow liquid, and is compatible with all types of mineral oils; also, it can be added / mixed in all proportions with all other branded mineral oils. The properties of Therminol 55 are given in Table 3.1(Reference 211). Table 3.1 Properties of Therminol 55 a Composition Property Max. bulk temperature Kinematic 40 C 25 C Thermal 27 C Flash point (ASTM D-92) Fire point (ASTM D-92) Auto ignition temperature (ASTM D-2155) Boiling 1013 mbar Coefficient of thermal expansion Value Mixture of synthetic hydrocarbons 315 C 36.8 mm 2 /s (cst) 875 kg/m W/mK 193 C 238 C 366 C 351 C / C a Supplier: Solutia Chemicals India Pvt. Ltd., Mumbai, India

3 Phase Change Material After making a detailed search of all the commercially available phase change materials, sugar alcohols were identified as suitable material for use as PCMs on the basis of their melting temperature and latent heat of fusion. Moreover, sugar alcohols are extremely safe to be used as a sweetening agent. Since, it has already been noted that sugar alcohols have good thermal reliability, other parameters that are essential for effective heat storage need to be considered. Two such parameters are the melting temperature and the latent heat of fusion. Figure 3.1 shows a scatter diagram depicting the melting temperatures and heat of fusion of various sugar alcohols. Figure 3.1 Scatter diagram showing melting points and heat of fusion for various sugar alcohols From Figure 3.1, it is seen that the sugar alcohols have high latent heat storage capacity and melting temperature, and therefore, they are promising PCMs for practical latent heat thermal energy storage applications performed at o C. It can be clearly noted from the figure that,

4 61 D-Mannitol has a slightly lower value of the latent heat of fusion than Erythritol; however, it has a higher melting point than Erythritol. D-Mannitol that has a phase change temperature of 166 to 169 o C is chosen for the present storage application, as it is available easily, and also the heat requirement at o C is very useful to carry out cooking, like frying of food stuffs at higher temperatures. D-Mannitol is a natural polyol (sugar alcohol) having a low molecular weight of (Reference 212). Sugar alcohols are carbohydrates and part of their chemical structure resembles that of sugar, and part of it resembles alcohol. The axisymmetric chemical structure of D-Mannitol is shown in Figure 3.2. Table 3.2 summarizes the thermophysical properties of D-Mannitol. It is most commonly used in the pharmaceutical and food industries, due to its characteristic sweet taste and comparatively low energy content. Consequently it makes a suitable substitute for sugar in the food industry and provides sweetness to drugs. The potential use of D-Mannitol as a phase change material is supported by its thermal properties. Its ability to retain heat effectively in its liquid state affirms its capability for use as a PCM. Furthermore, D-Mannitol s relatively high density of 1.49 g/cm 3 allows a large quantity of D-Mannitol to be packed into a small volume. This, coupled with its high specific heat of fusion allows for large amounts of energy to be absorbed for storage. As regards the cost, D-Mannitol is quite cheap, selling at 1000 Indian Rupees (US $ 25) per kg (Reference 213). The feasibility study of D-Mannitol as a PCM candidate from three thermo-physical property view points, namely, enthalpy of fusion, melting temperature and decomposition temperature, was carried out by Kumaresan et al (2011) and, is presented in the following section.

5 62 Figure 3.2 Axisymmetric chemical structure of D-Mannitol Table 3.2 Thermo-physical properties of D-Mannitol a Property Melting Temperature Heat of Fusion Value C kj/kg Density 1490 kg/m 3 Decomposition Temperature 300 C Chemical structure C 6 H 8 (OH) 6 a Supplier : Sisco Research Laboratories Pvt. Ltd., Mumbai, India DSC and TG/DTG/DTA analysis of D-Mannitol D-Mannitol Extrapure AR [CAS No ] having a labelled purity of 99% mass fraction, was purchased from Sisco Research Laboratories Pvt. Ltd. Mumbai, India, and used without further purification. The D-Mannitol was handled in a dry N 2 atmosphere, to prevent contamination by moisture. All the experiments were carried out at the Sophisticated Analytical Instrumentation Facility, Indian Institute of Technology Madras, Chennai, India.

6 63 The melting temperature and enthalpy of fusion were determined by the DSC. The Netzsch DSC 204 manufactured by Netzsch, Germany was used for the analysis. A quantity of mg of D-Mannitol was tested in an open aluminium crucible. The temperature range employed during the experiment was from C to C. The heating rate was kept constant at 10 o C/min. The experiment was repeated for different heat input rates. The thermal stability decomposition temperature and percentage of mass change of D-Mannitol were studied using the TG-DTA. The instrument used was the Netzsch STA 409 C/CD manufactured by Netzsch, Germany mg of D-Mannitol was tested in an alumina crucible in an N 2 atmosphere. The temperature range used during the experiment was from C to C. The heating rate was maintained at 0.5, 1.0, 2.0 and 10 o C/min for the DSC and 10 o C/min for the TG/DTG/DTA studies. The DSC experiment was repeated five times for each heating rate, and the data obtained were averaged. Figure 3.3(a-d) shows the results obtained from the DSC analysis at various heating rates. It is seen from Figure 3.3a that a sharply endothermic peak, corresponding to the melting process has an onset point of C. The curve then peaks at a temperature of C. The area under the peak denotes the enthalpy of fusion of D-Mannitol. The value obtained for the enthalpy of fusion is J/g. The change in the melting temperature and enthalpy of fusion were observed for different low heating rates of 0.5, 1.0 and 2.0 C /min. The marginal change in the melting point temperature and enthalpy of fusion in the DSC curves were observed in Figures 3.3(b-d) due to the heating rate variation.

7 64 Figure 3.3(a d) DSC curves of D-Mannitol The TG-DTG curve is shown in Figure 3.4, and it is seen that the mass loss of the sample is completed in one stage. D-Mannitol remains stable below C. The result showed that D-Mannitol can store thermal energy without any mass loss when C is the maximum operating temperature. Beyond this temperature, the D-Mannitol begins to lose mass. The rate of mass loss reaches a maximum at about 365 C, and almost all its mass is lost at about 397 C. Figure 3.4 TG-DTG curve of D-Mannitol

8 65 The simultaneously recorded TG and DTA curve is shown in Figure 3.5. On the TG curve a single weight loss step is marked at C. The DTA curve in Figure 3.6 shows the three endothermic and two exothermic heat effects during decomposition in an inert atmosphere (nitrogen). The first endothermic effect of the main degradation of the sample was observed, as expected, at C, followed by an exothermic effect at C. Consequently the peak, which corresponds to a temperature of C and C, indicates the melting temperature and highest available temperature of D-Mannitol, in the given experimental conditions. This is seen to be consistent with the interpretation of the TG-DTG curve done earlier. The subsequent exothermic peak at C, and explicit endothermic peaks at C and C observed in the DTA curve may correspond to the loss of the different volatile products created during the thermal degradation of D-Mannitol. Figure 3.5 TG-DTA curve of D-Mannitol

9 66 Figure 3.6 TG-DTA curve of D-Mannitol with peak points indicated The above analysis proves that the high melting temperature, decomposition temperature and latent heat of fusion of D-Mannitol, make it an excellent phase change material for solar cooking applications. 3.2 SELECTION AND DESCRIPTION OF SYSTEM COMPONENTS The preliminary design for the evaluation of the cooking energy requirement, and the assumptions made in the selection of the PTC based on the energy requirement, are given in detail in Appendix 3. The selection of the system components is made based on the energy requirement for cooking for a residential house Parabolic Trough Collector The size of the solar parabolic trough collector (PTC), selected based on the energy requirement for the present cooking system requirement is given in Appendix 3. A photographic view of the PTC used in the experimental investigation is shown in Figure 3.7a. The key components of a

10 67 PTC include the collector structure, the receiver element, the heat collecting element also called the absorber tube at the focal axis of the parabola, and the drive system (tracking unit), which delivers thermal energy to its point of use. The PTC has a reflecting surface, which consists of six parabolic mirrors (1.25 m 2 each), with an aperture area of 7.5 m 2 which concentrates the incoming solar radiation to the absorber tube. The curve of a parabola is such that the light travelling parallel to the axis of a parabolic mirror will be reflected to a single focal point from any place along the curve. The absorber tube assembly consists of an absorber tube which is coated with heat resistant black paint, and is surrounded by a borosilicate glass cover envelope, with an annular gap of 1.5 cm. The receiver element or mirrored surface extends linearly into the trough shape running the length of the trough, that focuses the sun light along the length of the absorber tube through which the HTF is pumped. The HTF captures the solar energy in the form of heat that can be used for cooking application. The black paint on the pipe surface increases the absorptivity of the incident solar irradiance and reduces the reflectivity. The structural skeleton of the solar parabolic trough collector is the concentrator structure. The concentrator structure: Supports the mirrors and absorber tube, maintaining them in an optical alignment Withstands external forces, such as the wind Allows the collector to rotate, so that the mirrors and the absorber tube can track the sun. The PTC rotates around the horizontal N-S axis, and a single axis tracking is adopted in the E-W direction to track the sun to obtain the maximum energy incidence. The tracking unit consists of a timer control, AC motor (960 rpm, 1.5 kw) and an 125:1 reduction gear box. The timer control

11 68 shown in Figure 3.7b, having an on/off switch at a calculated rate specified with a fixed interval, helps to ensure that the collector remains pointed towards the sun, and moves at a speed of rotation of 0.25 /min. The specifications of the PTC system are detailed in Table.3.3. Figure 3.7 Photographic view of (a) the PTC (b) the timer control Table 3.3 Specification of the PTC Collector aperture area ( A a ) 7.5 m 2 Collector length ( L ) Aperture width ( W a ) 3.00 m 2.5 m Absorber tube diameter (d) 0.06m Focal distance ( f ) Concentration ratio (CR) 12 Tracking Mechanism Collector Material Absorber tube material Support structure material m Mechanical (Semi Automatic) 6 mm thick Saint Gobain glass Stainless Steel coated with heat resistant black paint Mild steel

12 Thermal Energy Storage System In the present experimental study, a separate storage tank filled with HTF and PCM encapsulated internally finned spherical balls are used to store the heat. The reason for the selection of such a storage tank filled with PCM balls and HTF is that, the poor thermal conductivity of the PCM, varies the resistance to heat transfer during charging and discharging. This effect is most significant during the discharging process. During its solidification on the convective heat transfer surface, the solidified layer of the PCM acts as an insulator, and further, as the thickness of the solidified layer increases with respect to time, the resistance to the heat transfer between the HTF and the liquid PCM in the storage tank increases. This, in turn, decreases the heat transfer rate appreciably and causes a non-uniform rate of discharging characteristics in the storage tank, which may restrict its usage for any application. This type of combined sensible and latent heat storage system increases the heat storage capacity, compared to the sensible heat storage system, and eliminates the problems that are usually encountered in a separate LHS unit. A cylindrical thermal energy storage (TES) tank shown in Figure 3.8 having a diameter of 550 mm (d o ) and 1100 mm (h) length made of 5 mm thick MS plate is kept vertically, with two plenum chambers on the top and bottom of the tank, and a perforated distributor plate is provided on the bottom of the tank to achieve a uniform flow of the HTF. The tank is provided with an upper opening for loading and unloading the PCM balls, and provision is also made to drain the oil at the bottom side. The overfill vent is provided at the tank top cover, which allows the HTF vapor to escape during the charging / discharging process. The storage tank is provided with high temperature withstanding oil sight glass, having two valves, one at the top and the other at the bottom. This facilitates to know the HTF level inside the

13 70 storage tank. An immersed type electric heater, having a capacity of 6 kw is provided at the bottom of the tank for an auxiliary purpose. A thermostat control unit is kept in the storage tank to maintain the temperature of the HTF near the outlet of the storage tank at a constant level. Figure 3.8 TES tank The storage tank is well insulated with glass wool of 0.15 m thickness and the pipe lines are also insulated with the same material, 0.10 m thick, and covered with an aluminium sheet cladding. The storage tank in the present analysis is located between the energy collecting unit (solar collector) and the application unit (cooking units).

14 Spherical ball fabrication and encapsulation of PCM The poor thermal conductivity of the PCM necessitates its encapsulation in a small size, with a large surface area for heat exchange. It is observed from the various analyses carried out by the researchers, that the spherical configuration has higher heat storage capacity than any other configuration, and an internally finned container is quite advantageous in terms of enhancing the heat transfer. In the present investigation, an internally finned spherical container is considered, and this section is used to explain the fabrication procedure adopted for the finned spherical ball. Figure 3.9 shows the various steps involved in making the PCM container. A stainless steel sheet metal of thickness 1 mm is formed into two hemispherical shells as shown in Figure. 3.9a. Inside the hemispherical shells two semi-circular fins are welded (Figure.3.9b) and these two shells are placed orthogonally (Figure.3.9c) and finally welded to form a spherical ball (Figure.3.9d). A circular opening of an inner diameter of 9 mm along with a nipple having a height of 7 mm is provided at the top for PCM filling. The total number of balls required to store the evaluated heat in the storage system is estimated as 126, based on the evaluation of the PCM requirement, shown in Appendix 4. These stainless steel balls are produced following the above procedure.

15 72 (a) Hemispherical Shell and Plate (fin) (b) Hemispherical balls welded with fin (c) Orthogonal fin arrangement (d) Welded PCM ball Figure 3.9(a d) Fabrication of spherical ball

16 73 The PCM D-Mannitol shown in Figure 3.10a is heated to its melting point ( oC), and poured into the ball in liquid form as shown in Figure 3.10b and c respectively. The PCM is filled 85 to 90% of its volume in order to accommodate the volumetric expansion during phase change, and each ball has 0.41 kg of the PCM. The PCM filled in a ball is allowed to solidify (Figure 3.10d) and the nipple is inserted in the circular hole and welded perfectly to prevent leakage. Finally, the total numbers of 126 PCM filled balls are kept inside the storage tank as shown in Figure a c Figure 3.10(a d) Method of PCM encapsulation b d

17 74 Figure 3.11 PCM filled spherical balls in storage tank Cooking Units In order to use the solar energy for cooking applications, it is essential to design and develop a suitable cooking unit that allows the HTF to circulate from the collector / storage tank through the cooking unit. In the present investigation, two types of such vessels, namely, the bowl type (tava) and the flat type cooking units, are specially devised for cooking applications, as shown in Figure The tava type unit consists of a double walled annular portion through which the HTF is circulated. It has an inner bowl of dimensions 290 mm diameter and 105 mm depth, and an outer bowl of 350 mm diameter and 135 mm depth, with an annular gap of 30 mm between the vessels. This unit has an axially placed inlet at the bottom, and the outlet is located horizontally at a vertical depth 60 mm from the top. The flat plate cooking unit is of a hollow box configuration, made of 5 mm thick mild steel plates with 45 mm spacing between the top and bottom plates. The cooking is carried out at the top plate having the dimension of 400 x 400 mm, and it is exposed to the ambient. The entry and exit of the hollow portion are connected with a variable area duct on both sides, to establish the connection with the piping circuit. Finally, these

18 75 cooking units are fixed with a metal stand, that provides the height required for convenient cooking. Figure 3.12 Cooking units Positive Displacement Pump The HTF is circulated in an experimental system with the help of a positive displacement pump. An external gear pump is selected, based on the smaller size to minimize the heat loss to the ambient during the operation, higher withstanding temperature, required head and capacity, and noiseless operation. A photographic view of the selected gear pump is shown in Figure The specifications are as follows: Speed : 1440 rpm Capacity : 50 m 3 /h Head Power : 30 m : 1.5 hp

19 76 Figure 3.13 External gear pump Double Pipe Heat Exchanger In the present experimental study, a double pipe heat exchanger as shown in Figure 3.14a is used to measure the mass flow rate of the HTF, circulated in a circuit. The HTF is circulated through the inner pipe of 32 mm diameter with 3 mm thickness. The cold water stored in a separate water tank with a sight glass tube provision for the measurement of the water level, shown in Figure 3.14b, is circulated through the annulus of the pipe. The outer pipe is of 90 mm diameter with 5 mm thickness. The water storage tank is connected with a double pipe heat exchanger through a hose. The hot and cold fluids enter at opposite ends (counter flow arrangement). The unknown HTF mass flow rate is calculated from the energy balance made for the hot and cold fluids circulated in the heat exchanger, by using the measured mass flow rate of the cold water from the storage tank. Figure 3.14 Photographic view of the (a) double pipe heat exchanger (b) cold water storage tank

20 EXPERIMENTAL SETUP The experimental setup consists of a PTC, a PCM based storage tank, cooking units, double pipe heat exchanger and a circulating pump. The layout of the experimental setup is shown in Figure This experiment was conducted at Anna University, Chennai (longitude 80.21, latitude 13.01), a city located in the southern part of India. The absorber tube transfers the solar radiation received from the collector to the HTF, which circulates through the tube. The glass cover envelope reduces the thermal radiation and convection heat loss to the free air, which moves around the absorber tube. The rubber corks are incorporated at the ends to achieve an air tight enclosure. A flexible corrugated stainless steel hose is used to connect the absorber tube and the metal pipe, through which the HTF is circulated to the storage tank. The HTF (Therminol 55) is charged in the piping circuit, and it was ensured that there is no leak in the oil flow path. The HTF from the PTC enters the storage tank at the top and leaves from the bottom. A positive displacement pump maintains the circulation of the HTF in a closed circuit, through connecting pipes between the absorber tube in the collector and the storage tank, during the charging process. A counter flow double pipe heat exchanger is provided between the PTC and TES tank, to measure the mass flow rate of the HTF. The two cooking units selected for the present work are located between the storage tank and the positive displacement pump. The parabolic trough collector is bypassed during the discharging process, and the pump maintains the circulation of the HTF in a closed circuit, through connecting pipes between the storage tank and the cooking units. Each cooking unit is separately provided with an inlet and outlet valve. This provision allows the by-passing of any one of the cooking units during the experimentation, by circulating the HTF through the other cooking unit. The HTF then leaves through the outlet of the cooking units and is re-circulated through the storage tank during the discharging process. The tava type cooking unit is suitable for both boiling and frying applications. The flat type

21 78

22 79 configuration is selected to prepare some of the Indian dishes like dosa, chappathi, etc. The K (chromel-alumel) type thermocouples are placed at the provisions made at the inlet and outlet of the heat exchanger to measure the temperature of the HTF and the cold medium. Also, the thermocouples are placed in the TES tank, the inlet and outlet of the PTC to measure the temperature of the HTF, which is used to evaluate the performance of the system. A hollow aluminium rod with a small groove along the length is placed vertically in the storage tank, and five holes are drilled at various heights through which the thermocouples are inserted into the aluminium bar as shown in Figure 3.16, so as to avoid the contact of the sensors with the PCM balls. Thermocouples are also kept inside the PCM balls, at various depths in the storage tank. All the thermocouples were connected to the data acquisition system (Agilent make, Model No: 34970A) for continuous monitoring. Figure 3.16 Thermocouple in aluminium bar The direct beam component of solar irradiance during the experiment was measured by using the pyrheliometer (Hukseflux make, Model No: DR01).

23 EXPERIMENTAL METHODOLOGY Charging Process Initially a stagnation test is performed in the PTC, to evaluate the maximum temperature achievable by the HTF in the absorber tube, when it is in a stagnant condition. Experiments are carried out during the month of April in Chennai (longitude 80.21º, latitude 13.01º), India, to determine the performance of the collector-storage system during the charging process. During the experiment, the intensity of the solar radiation is measured using a pyrheliometer, at time intervals of 15 minutes. The charging experiments are conducted by circulating the HTF through the PTC and the storage unit. The temperature of the HTF at the inlet and outlet of the PTC, and the temperature variation of the HTF at five different heights in the storage tank, are measured continuously using a data acquisition system. The experiments are conducted until hrs Discharging Process In order to conduct the discharge experiment, the HTF heated by PTC up to 116 o C in the storage tank is further heated by an electric heater up to a temperature of 175 o C. After the HTF reaches 175 o C, the temperature of the HTF near the outlet of the storage tank is maintained at the same level using a thermostat control unit. During the discharging process, the HTF from the TES tank is circulated through the cooking units to study the performance and discharge characteristics of the storage system. The performance of the two cooking units is investigated separately. During the experiment with the tava type cooking unit, a known quantity of olive oil is taken in the cooking unit, and the HTF is allowed to circulate through the annular portion of the cooking unit. The temperature of the HTF entering and leaving the cooking unit and

24 81 the temperature of the olive oil are measured by using thermocouples, and the readings are recorded using the data acquisition system. An experiment is also conducted with an LPG stove in a conventional cooking vessel for the same temperature raise of the olive oil, to compare the performance. During the experiment with the flat plate cooking unit, one of the major South Indian food items of a dosa was prepared on the flat plate by using the dosa batter. The dosa batter was prepared by using the mixture of soaked rice, urad dal (black gram) and water. The mixture was spread over the center portion of flat plate by using the ladle in the form circle. A little amount of edible oil was trickled around the edges of dosa. The hot HTF was circulated through the flat plate cooking unit that supplied the heat to the dosa from the bottom side. Before the edges of dosa start browning due to this heat transfer, it is flipped to heat the other side of dosa also. The number of dosa prepared in a given interval of time and the energy utilized in preparing a dosa using the loss of water during the preparation, were noted to estimate the performance of the cooking unit. Separate experiment was conducted to determine the heat loss coefficient (U L ) of the insulated storage tank, to evaluate the heat retention time of the heat storage tank. Several experiments were conducted to check the repeatability of the readings. The results along with the evaluated parameters are analyzed and discussed in Chapter 5.