Thermal Energy Storage Systems For Solar Cookers Engineering Essay

Thermal Energy Storage Systems For Solar Cookers Engineering Essay

The dismaying addition of the green house gas emanations and increase in fuel monetary value emphasizes the demand for more effectual and efficient alternate beginning of energy. Direct solar radiation is considered to be one of the most prospective beginnings of energy. Among the different energy terminal uses, energy for cookery is one of the basic and dominant terminal uses in developing states. Hence, there is a critical demand for the development of alternate and low-cost manner of cookery. Thermal energy storage is indispensable whenever there is a mismatch between the supply and ingestion of energy. Latent heat storage in a stage alteration stuff is really attractive because of its high storage denseness with little temperature swing. The pick of PCM, PCM encapsulation and HTF plays an of import function in add-on to heat transportation mechanism in the PCM. In this paper a PCM storage unit for a solar cooker was designed to hive away solar energy during sunshine hours and to be used during off sunshine hours. A fresh construct of PCM-based storage is presented.

2. Introduction

Due to flip rocketing fuel monetary values and of all time increasing demand of fossil fuels we are driven to utilize assorted beginnings of renewable energy. Among assorted energy demands, cooking constitutes a major portion. Household energy usage in developing states totaled 1090 Mtoe in 2004, about 10 % of universe primary energy demand [ 1 ] . This besides constitutes old and inefficient techniques, which result to increase in air pollution and harmful diseases to the individual cooking. Hence, there is a critical demand for the development of alternate and low-cost manner of cooking for usage in developing states. A fresh thermic 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. Surveies conducted to compare latent heat and reasonable heat storages have shown that a important decrease in storage volume can be achieved utilizing PCM compared to sensible heat storage. In a latent heat storage system, the reasonable constituent of the heat storage is kept low. This enables the system to be operated at low temperature ensuing in high efficiency of the solar energy aggregation system in renewable energy application. Latent heat storage media ( PCMs ) can hive away big measure of heat in a smaller weight and volume of stuff in comparing with reasonable heat storage media [ 2 ] . The pick of stage alteration stuff and heat transportation fluid plays a critical function in the efficiency of the thermic energy storage. The disadvantage of their heat money changer development is increasing the cost and complexness of thermic energy storage devices. In order to work out these jobs, both material probe and heat money changer development has been performed.

2.1 Literature reappraisal

The solar cookers are chiefly classified into indirect and direct solar cookers, this states the place of the cooker, with mention to the place it is placed. Indirect solar cooker is said to be convenient as the cookery unit is placed inside the house. The of import factor to be considered is the efficiency of cooking with thermic energy storage. An experiment on a cylindrical latent heat storage unit for the cookery pot of a solar cooker was designed and fabricated to hive away solar energy during sunshine hours. The stored energy was used to cook rice during the eventide. From the experimental consequences, one can reason that the storage of solar energy does non impact the public presentation of the solar cooker for midday cookery [ 2 ] . The solar aggregator plays an of import function in tackling the thermic energy from the Sun. Among the assorted aggregators design spherical or dish shaped solar aggregator has been proved efficient than level home base solar aggregators evacuated solar aggregators [ 3 ] .

2.1.1 Phase alteration stuff

The stage alteration stuff can be classified into organic and inorganic. They posses their ain thermophysical, physical and chemical belongingss. Organic stuffs has an advantage of no corrosiveness, low or none undercooling, Chemical and thermic stableness whereas inorganic has greater stage alteration heat content. The other of import factor to be considered is the latent heat energy of the stage alteration stuff [ 4 ] . The cooker public presentation is evaluated in footings of bear downing and dispatching times of the PCMs under different conditions. The public presentation of cooker was found to depend strongly on the solar strength, mass of the cookery medium, and the thermophysical belongingss of the PCM [ 5 ] . The thermophysical belongingss are runing temperature, latent heat of merger, specific heat, denseness of the stage alteration stuff in solid and liquid province. The PCM alterations from solid to liquid province upon warming and releases heat upon chilling down i.e altering from liquid province to solid province. Therefore this heats the heat transportation fluid and helps in cooking during off sunshine hours. Among the organic PCMs investigated D-mannitol proved to be the best suited for domestic cookery due to its optimal thaw point which is ideal for cooking. The other grounds for taking D-Mannitol as PCM are its less toxicity, low corrosiveness, stable and good latent heat of merger [ Table 1 ] .

Thermophysical Properties

Melting point ( °C )

Heat of merger ( kJ/kg )

Density ( kg/l ) ( at 20 °C )

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 use of PCM and heat transportation fluid depends on developing agencies of container. The PCM encapsulation with different geometries of capsules has its ain advantages and disadvantages. The PCM container should run into the demands of strength, corrosion opposition, thermic stableness and act as barrier to protect the PCM from interaction with the environment. The PCM containment should besides supply structural stableness and easy managing [ 7 ] .

2.1.3 Thermal energy storage system

The thermic energy storage system houses the PCM encapsulations and the topographic point where the heat transportation takes topographic point.

The system should be compact, big 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 big figure of charging/discharging rhythms without loss in public presentation and storage capacity ;

The drawbacks of Telluriums are the corrosiveness and escape of the HTF and the commixture of PCM to the HTF. The other disadvantage of TES is big size and big temperature swing during the add-on and extraction of energy [ 6 ] .

2.1.4 Probe Of Spherical Encapsulation

The spherical capsules are chosen in the system studied because they give the best public presentation, and their installing into assorted armored combat vehicles is rather easy because of their geometry. The energy flux exchanged is relative to the difference of temperature between the fluid and the inside of the spherical capsule. The heat flux depends on the province of the PCM stuff i.e. if the PCM is wholly liquid, wholly solid or if the two stages are present. Melting of PCM begins on the outer surface of the spherical ball and afterwards it grows concentrically to the Centre. The continuance of dismissed energy decreased with addition in flow rate. A thermic energy storage system using stage alteration stuff for rapid heat discharge was studied numerically and by experimentation. In the numerical surveies, the PCM was encapsulated in four different capsules ( sphere, cylinder, home base and tubing ) for look intoing the effects of geometrical conFig.urations. The effects of the capsule diameter and shell thickness and the null fraction on the public presentation of the heat storage system were besides investigated. The experiment was conducted by utilizing a commercial home base heat money changer as the heat storage armored combat vehicle. It was found that the spherical capsule showed the best heat release public presentation among the four types of investigated capsules, whereas the cannular capsule with low null fraction was non ideal for rapid heat release of the thermic energy stored in the PCM. The heat release public presentation decreased in the order of domain, cylinder, home base and tubing [ 7 ] .

2.1.4.1 Traveling boundary jobs

The analysis of heat transportation jobs in runing and hardening procedures, called traveling boundary jobs in scientific literature, is particularly complicated due to the fact that the solid- liquid boundary moves depending on the velocity at which the latent heat is absorbed or lost at the boundary, so that the place of the boundary is non known a priori and signifiers portion of the solution [ 6 ] .

2.1.5 Heat Transfer Fluid

Heat Transfer Fluid is a non-corrosive, non-fouling, paraffinic heat transportation fluid that is formulated to supply fast and efficient heat transportation with temperatures up to 315C. Heat Transfer Oil ‘s operability temperature is from 20A°F ( -7A°C ) to a maximal movie temperature of 600A°F ( 315A°C ) Heat Transfer Oil is blended from the finest choice high viscousness index badly solvent refined, badly hydro treated 100 % pure paraffin base oils available.

The most normally proposed replacements for H2O are crude oil based oils and liquefied salts. The heat capacities are 25-40 % of that of H2O on a weight footing. However, these replacements have lower vapor force per unit area than H2O and are capable of operating at high temperatures transcending 300A°C. However, it can be limited due to stableness and safety grounds and high cost. In add-on, it is extremely caustic, and there is a trouble in incorporating it at high temperatures [ 8 ]

A pump circulates the HTF through the insulated pipes to the PCM storage, and so sum he cooking unit. This is a closed cringle rhythm. During sunshine hours, HTF transfers its heat to the PCM and stored in the signifier of latent heat, through the spherical encapsulated heat money changer. This stored heat is utilized to cook the nutrient in the eventide clip or when sun strength is non sufficient to cook the nutrient. Noon cooking did non impact the cookery in the eventide and flushing cooking utilizing PCM storage was found quicker than midday cooking [ 8 ] . Out of assorted commercial HTF investigated Silicone oil ( a?’50A°C to +250A°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

Property

Operating Range ( °C )

-25 to 290

45 to 315

-10 to 325

-10 to 316

-10 to 315

-18 to 288

-50 to 250

Autoignition Temperature ( °C )

343

404

407

349

328

379

460

Initial Boiling Point ( °C )

301

378

382

301

300

357

250

Flash Point ( °C )

177

146

171

193

190

229

315

Therminol – Solutia Inc.

Xceltherm – Radco Industries

Merck Millipore

( The values are based on the specification from the several commercial HTF web sites )

Table 2. HTF Investigation [ 12-18 ]

2.1.6 Dispatching

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

2.1.6.1 Changeless flow rate dispatching

In changeless flow rate discharging, the flow rate is kept at a changeless degree for the continuance of the discharging period. Thermal energy is extracted from the TES system at a changeless flow rate regardless of the fact that the energy extracted from the storage becomes less as discharging continues, the flow rate is non varied to provide to its bead. Higher flow rates are likely to increase the rate of heat extraction. Dispatching with this method is instead uncontrolled since no effort is made to set the flow rate to bring forth a needed burden power.

2.1.6.2 Controlled burden power dispatching

In controlled power discharging, the flow rate is varied to keep a peculiar burden power. This method is much similar to the operation of any normal electrical hot home base where the cooking power can be varied by seting a boss.

Dispatching simulations were carried out utilizing two different methods. The first method was used to dispatch the TES system at a changeless flow rate. The consequences show a high rate of energy extraction every bit good as a rapid autumn in the temperature of the nutrient utilizing the extracted energy. Such a rapid autumn in temperature is unwanted for the cookery procedure. The 2nd method was used to dispatch the TES system at a variable flow rate to keep a controlled discharging burden power. The consequences indicate a lower rate of energy extraction every bit good as a H2O and oil temperature that is maintained at an about changeless degree when the peak temperature is approached. The 2nd method of dispatching is more good to the cookery procedure. Dispatching exergy efficiencies were surprisingly higher than dispatching energy efficiencies, demoing that the initial exergy stored was utilized more efficaciously than the initial energy stored. These anticipations are to be farther tested on an experimental platform. It is besides suggest the experimental testing of different designs of the heat use devices to heighten the heat extraction rate from the TES system [ 9 ] .

2. Methodology

The thermic energy storage system has been designed on the scenario of day-to-day cooking demand of a underdeveloped state, in this instance India has been taken for survey and the thermic energy storage system is to designed harmonizing to the day-to-day cookery demands of a household.

2.1. System design & A ; Design Calculations

The sum of fuel ( LPG ) needed to cook for four individuals is identified [ 10 ] and based on the energy demand PCM required is calculated. Using the calorific value of LPG the energy required to cook for a household of four is determined. Assuming the heat losingss from the storage armored combat vehicle and shrieking system as 20 % the entire solar energy required is calculated. Therefore utilizing the heat collected in the oil, alteration in temperature, specific heat of silicone oil the mass of oil inside the storage armored combat vehicle in calculated. Similarly mass of PCM is calculated. The volume of PCM by mass of PCM over denseness of PCM and likewise the entire volume of the storage armored combat vehicle is calculated. The figure of encapsulation is assumed to be 30 so that it PCM can be spread over every bit. The system consists of a a solar dish aggregator connected to the PCM storage armored combat vehicle and to the cookery unit. A oil reservoir is used to hive away the heat transportation fluid and a gate valve is used to modulate the flow of HTF. ( Fig.1 ) The heat money changer ( PCM storage armored combat vehicle ) is made up of chromium steel steel 304. It has a diameter of about 405mm and a tallness of 455mm. It consists of three separation home bases. The first home base from the underside is measured for 40mm and the other two home bases are kept at a distance of 110mm. The spherical encapsulations are besides made up of chromium steel steel 304 holding a diameter of 100mm and a wall thickness of 1mm. Merely 75 % of each spherical encapsulations are filled with stage alteration stuff ( D-Mannitol ) , due to positive volumetric enlargement on thaw of PCM.

During the off sun radiance hours, the heat energy stored in the PCM is discharged and is conducted to HTF. During this procedure as the heat is being liberated by the PCM, a stage alteration takes topographic point 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 occasion the the heat nowadays in the HTF is utilised for cooking intent. The flow of HTF to the solar receiving system is diverted straight to the heat money changer. This is done with the aid of gate valves. This is done to cut down the heat loss in the system. Insulation plays a major function in thermic energy storage. Ceramic wool is used as an insularity stuff. It is selected because it is economical and suits the best. The PCM container has been farther insulated with the aid of aluminum sheet. Thus it reduces natural convection.

Fig. 1. An spending of the complete Solar cookery device with PCM storage.

2.2. Solidworks Modeling

The thermic energy storage system has been modeled utilizing the package named Solidworks harmonizing to the findings in the computation, the storage armored combat vehicle is divided into beds, which holds in topographic point the spherical capsules which shops the stage alteration stuff. The spherical capsules are sealed with a simple prison guard on top of them. The bottom portion is for good sealed and has an mercantile establishment pipe for the motion of the heat transportation fluid to the cookery unit and the upper portion of the heat money changer has been kept air tight with aid of bolts and has an recess pipe for the heat transportation fluid to flux. The theoretical account of the heat money changer and spherical encapsulation are Fig.. 2 and Fig.. 3 severally. Fig. 2 explains the assorted characteristics of the heat money changer from the HTF recess to the HTF mercantile establishment. Fig. 3 is the certain spherical encapsulation which houses the PCM.

Fig. 2 Thermal energy storage system

Fig. 3 Sealed PCM encapsulation

2.3. CFD Modeling

The CFD mold is done based on the design. That is the sum of energy to be stored to the system and the recess and mercantile establishment conditions of the thermal fluid. With advanced geometry and engagement tools in a powerful, flexible, tightly-integrated, and easy-to-use interface, GAMBIT ( pre processing package ) can dramatically cut down pre- processing times for engaging applications. The theoretical account imported from SOLIDWORKS is used for engaging. Using a practical geometry sheathing and advanced clean-up tools, imported geometries are rapidly converted into suited flow spheres.

2.4. CFD Analysis

The conjugate heat transportation analysis is done as per the boundary conditions. The boundary conditions are defined In the analysis based on the on the job conditions, belongingss of heat transportation fluid and the belongingss of PCM.

From the CFD analysis we can acquire an thought of the distribution of heat energy around the thermic energy storage system, in order to obtain the public presentation features of thermic energy storage system, the geometric parametric quantities of heat money changer is calculated. The information ‘s are obtained with regard to the recess fluid temperature and mass flow rate. Heat loss computation and energy stored in PCM should be calculated.

3 Consequences and Discussions

3.1 CFD Modeling

GAMBIT is a state-of-the-art pre-processor for technology analysis. With advanced geometry and engagement tools in a powerful, flexible, tightly-integrated, and easy-to-use interface, GAMBIT can dramatically cut down pre- processing times for engaging applications. The theoretical account imported from SOLIDWORKS is used for engaging. Using a practical geometry sheathing and advanced clean-up tools, imported geometries are rapidly converted into suited flow spheres. A comprehensive set of highly-automated and size map driven engagement tools ensures that the best mesh can be generated.

Sphere

Nodes

Elementss

Spherical encapsulation

21553

64537

Default sphere

41160

164588

Outer wall

18495

56661

Centrifuge home base

22277

89736

All spheres

103485

375522

Table 3 Mesh study

Fig. 4 Meshed theoretical account

3.1.1 Specifying the purgative of the theoretical account

In CFX, geometry can be imported from SOLIDWORKS utilizing native format, and the mesh of control volumes is generated automatically. This synergistic procedure is the 2nd pre-processing phase and is used to make input required by the Solver. The mesh files are loaded into the natural philosophies pre- processor, CFX-Pre. The physical theoretical accounts that are to be included in the simulation are selected.

Material belongingss and boundary conditions

Inlet fluid temperature

180 °C

Initial temperature

45 °C

Mass flow rate

5kg/s

Fluid viscousness

0.8 cP

Thermal conduction therminol-55

0.1199 W/mK

Thermal conduction D-mannitol

0.279 W/mK

Table 4 Material belongingss and boundary conditions of Tellurium

The conjugate heat transportation analysis is done as per the boundary conditions. The boundary conditions are defined In the analysis based on the on the job conditions, belongingss of heat transportation fluid and the belongingss of PCM.

Solving the CFD job

The partial differential equations are integrated over all the control volumes in the part of involvement. This is tantamount to using a basic preservation jurisprudence ( for illustration, for mass or impulse ) to each control volume.

These built-in equations are converted to a system of algebraic equations by bring forthing a set of estimates for the footings in the built-in equations.

The algebraic equations are solved iteratively. An iterative attack is required because of the non-linear nature of the equations, and as the solution approaches the exact solution, it is said to meet. For each loop an mistake, or residuary, is reported as a step of the overall preservation of the flow belongingss.

3.2 CFD analysis

In order to obtain the public presentation features of thermic energy storage system, the geometric parametric quantities of heat money changer is calculated. The information ‘s are obtained with regard to the recess fluid temperature and mass flow rate. All the public presentation parametric quantities are carried out for the no swirl status. It was made a specific geometry for the heat money changer in dimensions of 440mm diameter and 400 millimeter tallness 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 bear downing and dispatching are as shown in Fig. 6. Contours of temperature can explicate the public presentation of thermic energy storage system clearly. Certainly temperature ‘s profiles are derived flow ‘s profiles. Using contours of temperature that is exhibited diagrammatically. The Fig. 6 illustrates temperature profiles of the stage alteration stuff throughout the heat money changer. The temperature at layer-1 from the top is 428K to 448K. The temperature at bed -2 is relatively lesser than layer-1, but there is non much fluctuation in the overall mean temperature, but at layer-3 the temperature distribution is mean. But the overall temperature distribution throughout the heat money changer is about equal to the input temperature.

Fig. 7 illustrates the temperature distribution in the heat transportation fluid. The temperature of the heat transportation fluid is bit by bit reduced from the top due to the good heat transportation that occurs in between the heat transportation fluid and stage alteration stuff. It ensures that the spherical length encapsulation is effectual.

The graph shows the bear downing profile of the unstable recess and mercantile establishment temperature. The temperature of the heat transportation fluid is bit by bit increased with regard to the clip. It explains the thermic energy in the heat transportation fluid is observed by the stage alteration stuff severally.

Fig. 6 Temperature distribution of the stage alteration stuff ( bear downing rhythm )

Fig. 7 Temperature distributions throughout the HTF

4 Decision

An thorough literature study has been done on the assorted types of cooking units and the working rule of solar cookers is analysed. Storage of thermic energy utilizing Phase Change Materials is studied. The PCM armored combat vehicle is so designed as per the energy demand, that is, to cook for a household of four. Having completed the design and analysis of the bear downing features of D-Mannitol in the designed fluidized bed heat money changer it is possible to utilize it for cooking during off sunshine hours. This theoretical account ca be fabricated and existent clip informations analysis of bear downing and discharging of the energy could be found.

5 Recognition

Apart from the attempts of myself, the success of any undertaking depends mostly on the encouragement and guidelines of many others. I take this chance to show my gratitude to the people who have been instrumental in the successful completion of this undertaking. I would wish to demo my greatest grasp to Prof. Philip C Eames. I ca n’t state thank you plenty for his enormous support and aid. I feel motivated and bucked up every clip I attend his meeting. Without his encouragement and counsel this undertaking would non hold materialized. The counsel and support received from all the members who contributed and who are lending to this undertaking, was critical for the success of the undertaking. I am thankful for their changeless support and aid.

Appendix -1

DESIGN CALCULATION

LPG required to cook nutrient per individual per twenty-four hours = 0.078 kilogram

Therefore for four individuals = 0.312 kilogram

Calorific Value of LPG = 51912.56 kJ/kg

Cooking efficiency utilizing LPG as fuel = 53.6 %

Energy required to cook nutrient utilizing LPG

for four individuals per twenty-four hours = 0.312*51912.56*0.536

= 8681.44 kJ

Assuming 20 % heat loss in the storage armored combat vehicle & A ; pipe Line,

Entire 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 armored combat vehicle = Heat collected in oil / CPa?†T

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

Mass of oil in storage armored combat vehicle = 33.86 kilogram

D- Osmitrol is used as the PCM to hive away energy with latent heat of merger 316.4 kJ/kg.

Heat collected in PCM = 0.6*10417.72

= 6250.63 kJ

Mass of PCM in storage armored combat vehicle = 6250.63 / 316.4

= 19.75 kilograms

Entire volume of PCM = ( mPCM/I?PCM )

= 19.75/1520

= 0.0129 M3

Entire Volume of Storage system = ( moil/I?oil ) + ( mPCM/I?PCM )

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

= 0.0530 M3

Assume that figure of encapsulation is 30

Volume of each PCM ball = 0.0145/30

= 0.00048 M3

Radius of PCM ball = 0.048 m

= 48 millimeter

Wall thickness of the PCM ball = 4 millimeter

Volume of each PCM

encapsulation = 0.00058 M3

Entire volume of PCM

encapsulation = 0.01766 M3

Volume of PCM storage armored combat vehicle = 0.01766 + 0.0478

= 0.0625 M3

Assume tallness of PCM armored combat vehicle = 0.4 m

Radius of PCM armored combat vehicle = 0.22 m