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Thermal Energy Storage Systems For Solar Cookers Engineering Essay

The alarming increase of the green house gas emissions and increase in fuel price emphasizes the need for more effective and efficient alternative source of energy. Direct solar radiation is considered to be one of the most prospective sources of energy. Among the different energy end uses, energy for cooking is one of the basic and dominant end uses in developing countries. Hence, there is a critical need for the development of alternative and affordable mode of cooking. Thermal energy storage is essential whenever there is a mismatch between the supply and consumption of energy. Latent heat storage in a phase change material is very attractive because of its high storage density with small temperature swing. The choice of PCM, PCM encapsulation and HTF plays an important role in addition to heat transfer mechanism in the PCM. In this paper a PCM storage unit for a solar cooker was designed to store solar energy during sunshine hours and to be used during off sunshine hours. A novel concept of PCM-based storage is presented.

2. INTRODUCTION

Due to sky rocketing fuel prices and ever increasing demand of fossil fuels we are driven to use various sources of renewable energy. Among various energy demands, cooking constitutes a major part. Household energy use in developing countries totaled 1090 Mtoe in 2004, almost 10% of world primary energy demand[1]. This also constitutes old and inefficient techniques, which result to increase in air pollution and harmful diseases to the person cooking. Hence, there is a critical need for the development of alternative and affordable mode of cooking for use in developing countries. A novel thermal energy storage system has been proposed. Of the two types of heat storage i.e Sensible heat energy storage and Latent heat energy storage, the latter is preferred. Studies conducted to compare latent heat and sensible heat storages have shown that a significant reduction in storage volume can be achieved using PCM compared to sensible heat storage. In a latent heat storage system, the sensible component of the heat storage is kept low. This enables the system to be operated at low temperature resulting in high efficiency of the solar energy collection system in renewable energy application. Latent heat storage media (PCMs) can store large quantity of heat in a smaller weight and volume of material in comparison with sensible heat storage media[2]. The choice of phase change material and heat transfer fluid plays a vital role in the efficiency of the thermal energy storage. The disadvantage of their heat exchanger development is increasing the cost and complexity of thermal energy storage devices. In order to solve these problems, both material investigation and heat exchanger development has been performed.

2.1 Literature review

The solar cookers are mainly classified into indirect and direct solar cookers, this states the position of the cooker, with reference to the position it is placed. Indirect solar cooker is said to be convenient as the cooking unit is placed inside the house. The important factor to be considered is the efficiency of cooking with thermal energy storage. An experiment on a cylindrical latent heat storage unit for the cooking pot of a solar cooker was designed and fabricated to store solar energy during sunshine hours. The stored energy was used to cook rice during the evening. From the experimental results, one can conclude that the storage of solar energy does not affect the performance of the solar cooker for noon cooking[2]. The solar collector plays an important role in harnessing the thermal energy from the sun. Among the various collectors design spherical or dish shaped solar collector has been proved efficient than flat plate solar collectors evacuated solar collectors[3].

2.1.1 Phase change material

The phase change material can be classified into organic and inorganic. They posses their own thermophysical, physical and chemical properties. Organic materials has an advantage of no corrosiveness, low or none undercooling, Chemical and thermal stability whereas inorganic has greater phase change enthalpy. The other important factor to be considered is the latent heat energy of the phase change material[4]. The cooker performance is evaluated in terms of charging and discharging times of the PCMs under different conditions. The performance of cooker was found to depend strongly on the solar intensity, mass of the cooking medium, and the thermophysical properties of the PCM[5]. The thermophysical properties are melting temperature, latent heat of fusion, specific heat, density of the phase change material in solid and liquid state. The PCM changes from solid to liquid state upon heating and releases heat upon cooling down i.e changing from liquid state to solid state. Thus this heats the heat transfer fluid and helps in cooking during off sunshine hours. Among the organic PCMs investigated D-mannitol proved to be the best suitable for domestic cooking due to its optimum melting point which is ideal for cooking. The other reasons for choosing D-Mannitol as PCM are its less toxicity, low corrosiveness, stable and good latent heat of fusion [Table 1].

Thermophysical Properties

Melting point(ºC)

Heat of fusion (kJ/kg)

Density (kg/l) (at 20 ºC)

Cost (US$/kg)

Material

Erythritol

120

339.8

1.45

5.0

D-mannitol

166-168

316.4

1.52

6.7-7.5

Galactitol

188-189

351.0

1.47

4.75-5.0

Xylitol

93-94.5

263.3

1.52

6.7-8.3

D-sorbitol

96.7-97.7

185.0

1.50

1.1

Table 1. PCM Investigation[11]

2.1.2 Encapsulation of PCM

Successful utilization of PCM and heat transfer fluid depends on developing means of container. The PCM encapsulation with different geometries of capsules has its own advantages and disadvantages. The PCM container should meet the requirements of strength, corrosion resistance, thermal stability and act as barrier to protect the PCM from interaction with the environment. The PCM containment should also provide structural stability and easy handling[7].

2.1.3 Thermal energy storage system

The thermal energy storage system houses the PCM encapsulations and the place where the heat transfer takes place.

The system should be compact, large storage capacity per unit mass and volume.

Ability to charge and discharge with largest heat input/output rates but without less temperature gradients which helps to undergo large number of charging/discharging cycles without loss in performance and storage capacity;

The drawbacks of TES are the corrosiveness and leakage of the HTF and the mixing of PCM to the HTF. The other disadvantage of TES is large size and large temperature swing during the addition and extraction of energy[6].

2.1.4 Investigation Of Spherical Encapsulation

The spherical capsules are chosen in the system studied because they give the best performance, and their installation into various tanks is quite easy because of their geometry. The energy flux exchanged is proportional to the difference of temperature between the fluid and the interior of the spherical capsule. The heat flux depends on the state of the PCM material i.e. if the PCM is entirely liquid, entirely solid or if the two phases are present. Melting of PCM begins on the outer surface of the spherical ball and afterwards it grows concentrically to the centre. The duration of discharged energy decreased with increase in flow rate. A thermal energy storage system employing phase change material for rapid heat discharge was studied numerically and experimentally. In the numerical studies, the PCM was encapsulated in four different capsules (sphere, cylinder, plate and tube) for investigating the effects of geometrical conFig.urations. The effects of the capsule diameter and shell thickness and the void fraction on the performance of the heat storage system were also investigated. The experiment was conducted by using a commercial plate heat exchanger as the heat storage tank. It was found that the spherical capsule showed the best heat release performance among the four types of investigated capsules, whereas the tubular capsule with low void fraction was not ideal for rapid heat release of the thermal energy stored in the PCM. The heat release performance decreased in the order of sphere, cylinder, plate and tube[7].

2.1.4.1 Moving boundary problems

The analysis of heat transfer problems in melting and solidification processes, called moving boundary problems in scientific literature, is especially complicated due to the fact that the solid– liquid boundary moves depending on the speed at which the latent heat is absorbed or lost at the boundary, so that the position of the boundary is not known a priori and forms part of the solution[6].

2.1.5 Heat Transfer Fluid

Heat Transfer Fluid is a non-corrosive, non-fouling, paraffinic heat transfer fluid that is formulated to provide fast and efficient heat transfer with temperatures up to 315C. Heat Transfer Oil’s operability temperature is from 20°F (-7°C) to a maximum film temperature of 600°F (315°C) Heat Transfer Oil is blended from the finest select high viscosity index severely solvent refined, severely hydro treated 100% pure paraffin base oils available.

The most commonly proposed substitutes for water are petroleum based oils and molten salts. The heat capacities are 25-40% of that of water on a weight basis. However, these substitutes have lower vapour pressure than water and are capable of operating at high temperatures exceeding 300°C. However, it can be limited due to stability and safety reasons and high cost. In addition, it is highly corrosive, and there is a difficulty in containing it at high temperatures [8]

A pump circulates the HTF through the insulated pipes to the PCM storage, and then tot he cooking unit. This is a closed loop cycle. During sunshine hours, HTF transfers its heat to the PCM and stored in the form of latent heat, through the spherical encapsulated heat exchanger. This stored heat is utilized to cook the food in the evening time or when sun intensity is not sufficient to cook the food. Noon cooking did not affect the cooking in the evening and evening cooking using PCM storage was found faster than noon cooking[8]. Out of various commercial HTF investigated Silicone oil (−50°C to +250°C) suits the best for cooking [Table 2].

HTF

Therminol - 55

Therminol - 59

Therminol - 62

Xceltherm 600

Xceltherm CA

Xceltherm 445 FP

107742 Silicone oil

PROPERTIES

Operating Range(ºC)

-25 to 290

45 to 315

-10 to 325

-10 to 316

-10 to 315

-18 to 288

-50 to 250

Autoignition Temperature(ºC)

343

404

407

349

328

379

460

Initial Boiling Point (ºC)

301

378

382

301

300

357

250

Flash Point (ºC)

177

146

171

193

190

229

315

Therminol - Solutia Inc.

Xceltherm - Radco Industries

Merck Millipore

(The values are based on the specification from the respective commercial HTF websites)

Table 2. HTF Investigation [12-18]

2.1.6 Discharging

The discharging of the heat energy from the PCM to the HTF during the off sunshine hours can be done in two ways, constant flow rate discharging and controlled load power discharging.

2.1.6.1 Constant flow rate discharging

In constant flow rate discharging, the flow rate is kept at a constant level for the duration of the discharging period. Thermal energy is extracted from the TES system at a constant flow rate regardless of the fact that the energy extracted from the storage becomes less as discharging continues, the flow rate is not varied to cater to its drop. Higher flow rates are likely to increase the rate of heat extraction. Discharging with this method is rather uncontrolled since no attempt is made to adjust the flow rate to produce a required load power.

2.1.6.2 Controlled load power discharging

In controlled power discharging, the flow rate is varied to maintain a particular load power. This method is much similar to the operation of any normal electrical hot plate where the cooking power can be varied by adjusting a knob.

Discharging simulations were carried out using two different methods. The first method was used to discharge the TES system at a constant flow rate. The results show a high rate of energy extraction as well as a rapid fall in the temperature of the food using the extracted energy. Such a rapid fall in temperature is undesirable for the cooking process. The second method was used to discharge the TES system at a variable flow rate to maintain a controlled discharging load power. The results indicate a lower rate of energy extraction as well as a water and oil temperature that is maintained at an almost constant level when the peak temperature is approached. The second method of discharging is more beneficial to the cooking process. Discharging exergy efficiencies were surprisingly higher than discharging energy efficiencies, showing that the initial exergy stored was utilized more effectively than the initial energy stored. These predictions are to be further tested on an experimental platform. It is also suggest the experimental testing of different designs of the heat utilization devices to enhance the heat extraction rate from the TES system[9].

2. METHODOLOGY

The thermal energy storage system has been designed on the scenario of daily cooking need of a developing country, in this case India has been taken for study and the thermal energy storage system is to designed according to the daily cooking needs of a family.

2.1. System design & Design Calculations

The amount of fuel (LPG) needed to cook for four persons is identified [10] and based on the energy requirement PCM required is calculated. Using the calorific value of LPG the energy required to cook for a family of four is determined. Assuming the heat losses from the storage tank and piping system as 20% the total solar energy required is calculated. Thus using the heat collected in the oil, change in temperature, specific heat of silicone oil the mass of oil inside the storage tank in calculated. Similarly mass of PCM is calculated. The volume of PCM by mass of PCM over density of PCM and similarly the total volume of the storage tank is calculated. The number of encapsulation is assumed to be 30 so that it PCM can be spread over equally. The system consists of a a solar dish collector connected to the PCM storage tank and to the cooking unit. A oil reservoir is used to store the heat transfer fluid and a gate valve is used to regulate the flow of HTF.(Fig.1) The heat exchanger (PCM storage tank) is made up of stainless steel 304. It has a diameter of about 405mm and a height of 455mm. It consists of three separation plates. The first plate from the bottom is measured for 40mm and the other two plates are kept at a distance of 110mm. The spherical encapsulations are also made up of stainless steel 304 having a diameter of 100mm and a wall thickness of 1mm. Only 75% of each spherical encapsulations are filled with phase change material (D-Mannitol), due to positive volumetric expansion on melting of PCM.

During the off sun shine hours, the heat energy stored in the PCM is discharged and is conducted to HTF. During this process as the heat is being liberated by the PCM, a phase change takes place inside the spherical encapsulation (i.e.) the melted PCM (D-mannitol) starts to solidify. As this continues, the HTF flows through the cooking panel and at this juncture the the heat present in the HTF is utilised for cooking purpose. The flow of HTF to the solar receiver is diverted directly to the heat exchanger . This is done with the help of gate valves. This is done to reduce the heat loss in the system. Insulation plays a major role in thermal energy storage. Ceramic wool is used as an insulation material. It is selected because it is economical and suits the best. The PCM container has been further insulated with the help of aluminum sheet. Thus it reduces natural convection.

Fig. 1. An outlay of the complete Solar cooking device with PCM storage.

2.2. Solidworks Modeling

The thermal energy storage system has been modeled using the software named Solidworks according to the findings in the calculation, the storage tank is divided into layers, which holds in place the spherical capsules which stores the phase change material. The spherical capsules are sealed with a simple screw on top of them. The bottom part is permanently sealed and has an outlet pipe for the movement of the heat transfer fluid to the cooking unit and the upper part of the heat exchanger has been kept air tight with help of bolts and has an inlet pipe for the heat transfer fluid to flow. The model of the heat exchanger and spherical encapsulation are Fig.. 2 and Fig.. 3 respectively. Fig. 2 explains the various features of the heat exchanger from the HTF inlet to the HTF outlet. Fig. 3 is the sealed spherical encapsulation which houses the PCM.

Fig. 2 Thermal energy storage system

Fig. 3 Sealed PCM encapsulation

2.3. CFD Modeling

The CFD modeling is done based on the design. That is the amount of energy to be stored to the system and the inlet and outlet conditions of the thermic fluid. With advanced geometry and meshing tools in a powerful, flexible, tightly-integrated, and easy-to-use interface, GAMBIT (pre processing software) can dramatically reduce pre- processing times for meshing applications. The model imported from SOLIDWORKS is used for meshing. Using a virtual geometry overlay and advanced clean-up tools, imported geometries are quickly converted into suitable flow domains.

2.4. CFD Analysis

The conjugate heat transfer analysis is done as per the boundary conditions. The boundary conditions are defined In the analysis based on the working conditions, properties of heat transfer fluid and the properties of PCM.

From the CFD analysis we can get an idea of the distribution of heat energy around the thermal energy storage system, in order to obtain the performance characteristics of thermal energy storage system, the geometric parameters of heat exchanger is calculated. The data’s are obtained with respect to the inlet fluid temperature and mass flow rate. Heat loss calculation and energy stored in PCM should be calculated.

3 Results and Discussions

3.1 CFD Modeling

GAMBIT is a state-of-the-art pre-processor for engineering analysis. With advanced geometry and meshing tools in a powerful, flexible, tightly-integrated, and easy-to-use interface, GAMBIT can dramatically reduce pre- processing times for meshing applications. The model imported from SOLIDWORKS is used for meshing. Using a virtual geometry overlay and advanced clean-up tools, imported geometries are quickly converted into suitable flow domains. A comprehensive set of highly-automated and size function driven meshing tools ensures that the best mesh can be generated.

Domain

Nodes

Elements

Spherical encapsulation

21553

64537

Default domain

41160

164588

Outer wall

18495

56661

Separator plate

22277

89736

All domains

103485

375522

Table 3 Mesh report

Fig. 4 Meshed model

3.1.1 Defining the physic of the model

In CFX, geometry can be imported from SOLIDWORKS using native format, and the mesh of control volumes is generated automatically. This interactive process is the second pre-processing stage and is used to create input required by the Solver. The mesh files are loaded into the physics pre- processor, CFX-Pre. The physical models that are to be included in the simulation are selected.

Material properties and boundary conditions

Inlet fluid temperature

180 ºC

Initial temperature

45 ºC

Mass flow rate

5kg/s

Fluid viscosity

0.8 cP

Thermal conductivity therminol-55

0.1199 W/mK

Thermal conductivity D-mannitol

0.279 W/mK

Table 4 Material properties and boundary conditions of TES

The conjugate heat transfer analysis is done as per the boundary conditions. The boundary conditions are defined In the analysis based on the working conditions, properties of heat transfer fluid and the properties of PCM.

Solving the CFD problem

The partial differential equations are integrated over all the control volumes in the region of interest. This is equivalent to applying a basic conservation law (for example, for mass or momentum) to each control volume.

These integral equations are converted to a system of algebraic equations by generating a set of approximations for the terms in the integral equations.

The algebraic equations are solved iteratively. An iterative approach is required because of the non-linear nature of the equations, and as the solution approaches the exact solution, it is said to converge. For each iteration an error, or residual, is reported as a measure of the overall conservation of the flow properties.

3.2 CFD analysis

In order to obtain the performance characteristics of thermal energy storage system, the geometric parameters of heat exchanger is calculated. The data’s are obtained with respect to the inlet fluid temperature and mass flow rate. All the performance parameters are carried out for the no swirl condition. It was made a specific geometry for the heat exchanger in dimensions of 440mm diameter and 400 mm height that is based on the basic design. As shown in Fig. 5

Fig. 5 Model of Heat Exchanger

3.2.1 Temperature distribution

The contours of temperature for charging and discharging are as shown in Fig. 6. Contours of temperature can explain the performance of thermal energy storage system clearly. Certainly temperature’s profiles are derived flow’s profiles. Using contours of temperature that is exhibited graphically. The Fig. 6 illustrates temperature profiles of the phase change material throughout the heat exchanger. The temperature at layer-1 from the top is 428K to 448K. The temperature at layer -2 is comparatively lesser than layer-1, but there is not much variation in the overall average temperature, but at layer-3 the temperature distribution is average. But the overall temperature distribution throughout the heat exchanger is almost equal to the input temperature.

Fig. 7 illustrates the temperature distribution in the heat transfer fluid. The temperature of the heat transfer fluid is gradually reduced from the top due to the good heat transfer that occurs in between the heat transfer fluid and phase change material. It ensures that the spherical length encapsulation is effective.

The graph shows the charging profile of the fluid inlet and outlet temperature. The temperature of the heat transfer fluid is gradually increased with respect to the time. It explains the thermal energy in the heat transfer fluid is observed by the phase change material respectively.

Fig. 6 Temperature distribution of the phase change material (charging cycle)

Fig. 7 Temperature distributions throughout the HTF

4 Conclusion

An exhaustive literature survey has been done on the various types of cooking units and the working principle of solar cookers is analysed. Storage of thermal energy using Phase Change Materials is studied. The PCM tank is then designed as per the energy requirement, that is, to cook for a family of four. Having completed the design and analysis of the charging characteristics of D-Mannitol in the designed fluidized bed heat exchanger it is possible to use it for cooking during off sunshine hours. This model ca be fabricated and real time data analysis of charging and discharging of the energy could be found.

5 Acknowledgement

Apart from the efforts of myself, the success of any project depends largely on the encouragement and guidelines of many others. I take this opportunity to express my gratitude to the people who have been instrumental in the successful completion of this project. I would like to show my greatest appreciation to Prof. Philip C Eames. I can't say thank you enough for his tremendous support and help. I feel motivated and encouraged every time I attend his meeting. Without his encouragement and guidance this project would not have materialized. The guidance and support received from all the members who contributed and who are contributing to this project, was vital for the success of the project. I am grateful for their constant support and help.

Appendix -1

DESIGN CALCULATION

LPG required to cook food per person per day = 0.078 kg

Therefore for four persons = 0.312 kg

Calorific Value of LPG = 51912.56 kJ/kg

Cooking efficiency using LPG as fuel = 53.6%

Energy required to cook food using LPG

for four persons per day = 0.312*51912.56*0.536

= 8681.44 kJ

Assuming 20% heat loss in the storage tank & pipe Line,

Total solar energy required (Q) = 9718.03 + (9718.03 *0.20)

= 10417.72 kJ

Assuming 60% Solar energy collected is stored in the PCM and 40% in the oil

Mass of oil in storage tank = Heat collected in oil / CP∆T

= (0.4* 10417.72) / (2.05*(180-120))

Mass of oil in storage tank = 33.86 kg

D- mannitol is used as the PCM to store energy with latent heat of fusion 316.4 kJ/kg.

Heat collected in PCM = 0.6*10417.72

= 6250.63 kJ

Mass of PCM in storage tank = 6250.63 / 316.4

= 19.75 kg

Total volume of PCM = (mPCM/ρPCM)

= 19.75/1520

= 0.0129 m3

Total Volume of Storage system = (moil/ρoil) + (mPCM/ρPCM)

= (33.86/845) + (19.75/1520)

= 0.0530 m3

Assume that number of encapsulation is 30

Volume of each PCM ball = 0.0145/30

= 0.00048 m3

Radius of PCM ball = 0.048 m

= 48 mm

Wall thickness of the PCM ball = 4 mm

Volume of each PCM

encapsulation = 0.00058 m3

Total volume of PCM

encapsulation = 0.01766 m3

Volume of PCM storage tank = 0.01766 + 0.0478

= 0.0625 m3

Assume height of PCM tank = 0.4 m

Radius of PCM tank = 0.22 m


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