study of medium temperature solar thermal applications

International Journal of Applied Research and Studies (iJARS)

ISSN: 2278-9480 Volume 2, Issue 5 (May - 2013)

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*Authors Copy; Restricted to Personal Use Only any manipulation will be against copy Right Policy @ iJARS

Manuscript Id: iJARS/478 1

Review Paper

Study of Medium Temperature Solar Thermal Applications

Authors:

1Parimal S. Bhambare*, 2Dr. G. V. Parishwad

Address For correspondence:

1Mechanical Engineering Department, MIT Academy of Engineering, Alandi(D), Pune

2Mechanical Engineering Department, College of Engineering, Pune

Abstract— Solar energy is widely used for a variety of

process heat and electricity generation applications. It is

essential to apply solar energy for a wide variety of

applications and provide energy solutions by modifying the

energy proportion, improving energy stability, increasing energy sustainability, conversion reduction and hence enhance

the system efficiency. In the work presented here, a brief

study of a few medium temperature solar thermal applications

up to 2400C pertaining to domestic and industrial applications

has been considered. Typical applications in the range

included here are water heating, air drying and dehydration,

refrigeration and air conditioning, steam generation system

and solar cookers.

A brief description about the solar thermal technology utilised,

fundamentals and applications in industry has been presented

here.

Keywords — Medium temperature, concentrator, collector,

process heating.

I. INTRODUCTION

Solar thermal energy is used as process heat for different

domestic and industrial applications [1,2] in medium and

medium to high temperature ranges. These applications

includes: hot water supply, desalination, sterilization,

pasteurization, drying, space heating and cooling,

refrigeration, distillation, washing and cleaning and

polymerization. All these applications lies in temperature range between 60 to 2800C [3]. Solar thermal collectors are

used for harnessing this solar energy. These collectors are

special type of heat exchangers, which absorb the solar

radiations, and convert it to heat which is further transferred to

the fluid flowing through the collector. These are of two types:

concentrating or sun tracking (Single and two axis) and non-

concentrating or stationery type (Refer Table 1). A non-

concentrating collector has the same area for intercepting and

for absorbing solar radiation, whereas a sun-tracking concentrating solar collector usually has concave reflecting

surfaces to intercept and focus the sun’s beam radiation to a

smaller receiving area, thereby increasing the radiation flux. A

detailed review of these collectors is presented by Soterius

Kaliogirou, 2004 [4]. Non-concentrating or stationery

collectors are suitable for low (Flat Plate, FPC and Advanced

Flat Plate Collector, AFP) to medium (Evacuated tube, ETC

and Compound Parabolic, CPC) temperature applications

while concentrating type are suitable for medium (Parabolic

trough (PTC), Fresnel, Scheffler and Cylindrical trough) to

high temperature (Paraboloid and Heliostat) applications as

they produce higher temperature [4, 5]. This paper presents a comprehensive review of the current

status of utilization of solar energy in industrial and domestic

applications. TABLE I Type of solar collectors [3]

Motion Collector Type Absorber

Type

Concentration

Ratio

Indicative

Temperature

Range

Stationary Flat Plate Collectors (FPC) Flat 1 30-80

Evacuated Tube Collector (ETC) Flat 1 50-200

Compound parabolic collector (CPC) Tubular 1-5 60-240

Single-axis

tracking

Linear Fresnel reflector (LFR) Tubular 10-40 60-250

Parabolic trough collector (PTC) Tubular 15-45 60-300

Cylindrical trough collector (CTC) Tubular 10-50 60-300

Two-axes

tracking

Parabolic dish reflector (PDR) Tubular 100-1000 100-500

Heliostat field collector (HFC) Tubular 100-1500 150-2000

Note: Concentration ratio is defined as the aperture area divided by the receiver/absorber

area of the collector

[email protected] *Corresponding Author Email-Id

mailto:[email protected]



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Fig. 1 shows the optimum collector area for different type of

solar collectors with demand temperature ranges.

Fig. 1 Optimum collector area for different collectors and demand

temperatures [3]

II. SOLAR THERMAL CONVERSION SYSTEM

A Solar thermal conversion system can be of direct or

indirect type. Direct heating system heats up the heat transfer

fluid (HTF) utilizing solar irradiation, which is further to the

application as process heat. HTF forms the working fluid for

the system. On the contrary an indirect system has two

working fluids used in the system. As shown in Fig.1, a

typical indirect heating system consists of mainly five major components namely, solar collector, HTF storage tank, boiler,

pump for circulating the HTF and a heat engine to convert

heat to mechanical energy [4, 6]. The efficiency of a solar

thermal conversion system is about 70% when compared to a

solar electrical direct conversion system which has an

efficiency of 17% [7].

Fig. 2 Schematic of Indirect Solar Thermal Conversion System [4]

Thus solar thermal conversion system plays a very

important role in domestic as well as industrial sector [7].

System shown in Fig. 2 is used for producing power from

solar energy. For process heat applications boiler and the heat engine will be replaced by the respective application system.

III. INDUSTRIAL ENERGY SYSTEM

An Industrial system composed of four major components

namely: power supply, production plant, energy recovery and

cooling systems [6, 8]. Fig. 3 shows the block diagram of the

industrial energy system. Power supply provides energy to the

system with use of either electrical, gas, coal or gas. This

energy is utilized to run different subsystems, controller units,

switches, etc. in the system for its operation. Solar thermal

energy can be utilized directly as a source of energy, partly or

completely, for running a process in the system.

Fig. 3 Block diagram of typical industrial energy system [6, 8]

IV. SOLAR THERMAL APPLICATIONS

Solar thermal systems not only harness solar irradiations

but also store and provide, heat to HTF (usually air or water)

used in domestic and industrial applications. Table II gives an

overview of solar energy applications, system technologies

and type of systems commonly used in industry.

Industry utilizes fossil fuels for satisfying their thermal

energy requirements partially or completely. About 13% of

thermal industrial applications require low temperatures

thermal energy up to 1000C, 27% up to 2000C and the

remaining applications need high temperature in steel, glass

and ceramic industry [6]. Table III shows few of potential

industrial processes and the required temperatures for their operations.

Industrial energy analysis shows that solar thermal energy

has enormous applications in low (i.e. 20–2000C), medium

and medium-high (i.e. 80–2400C) temperature levels [3].

Almost all industrial processes require heat in some parts of

their processes. Most common applications for solar thermal

energy used in industry are the solar water heaters, solar

dryers, space heating and cooling systems and water

desalination.

With solar thermal energy replacing the fossil fuels for

industrial processes not only reduces dependency on conventional fuels but also minimizes greenhouse emissions

such as CO2, SO2, NOx [8]. Nevertheless, there are some

challenges for integration of solar heat into a wide variety of

industrial processes due to the periodic, dilute and variable

nature of solar irradiation [9].



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TABLE II Solar energy applications, system technologies and type of

systems commonly used in industry [3]

Solar Energy

applications

Solar system technology Type of system

SWH Thermo syphon systems Passive

Integrated collector storage Passive

Direct circulation Active

Indirect water heating systems Active

Air systems Active

Space heating and

cooling

Space heating and service hot water Active

Air systems Active

Water systems Active

Heat pump systems Active

Absorption systems Active

Adsorption systems Active

Mechanical systems Active

Solar refrigeration Adsorption units Active

Absorption units Active

Industrial heat

demand process

Industrial air and water systems Active

Steam generation Active

Solar desalination Solar stills Passive

Multi stage flash (MSF) Active

Multi effect boiling (MEF) Active

Vapor compression Active

Solar thermal power

systems

Parabolic trough collector systems Active

Parabolic tower systems Active

Parabolic dish systems Active

Solar furnaces Active

Solar chemistry systems Active

All solar thermal applications in industry can be classified

in following manner [6], 1. Hot water or steam demand process

2. Drying and dehydration process

3. Preheating

4. Concentration

5. Pasturization and sterilization

6. Washing and cleaning

7. Chemical reactions

8. Industrial space heating

9. Textile

10. Food

11. Building 12. Plastic

13. Chemistry

14. Business establishment

A. Solar Water Heating (SWH) System

SWH system provides an effective technology for

converting solar energy into thermal energy.

Flat plate collectors are the central component of any solar

water heating system. The efficiency of the system depends on

the performance of the flat plate collector.

TABLE III Heat demand in industries with temperature ranges [6]

Industry

Process

Temperature (oC)

Dairy Pressurization 60-80

Sterilization 100-120

Drying 120-180

Concentrates 60-80

Boiler feed water 60-90

Tinned food Sterilization 110-120

Pasteurization 60-80

Cooking 60-90

Bleaching 60-90

Textile Bleaching, dyeing 60-90

Drying, degreasing 100-130

Dyeing 70-90

Fixing 160-180

Pressing 80-100

Paper Cooking, drying 60-80

Boiler feed water 60-90

Bleaching 130-150

Chemical Soaps 200-250

Synthetic rubber 150-200

Processing heat 120-180

Pre-heating water 60-90

Meat Washing, sterilization 60-90

Cooking 90-100

Beverages Washing, sterilization 60-80

Pasteurization 60-70

Flours and by-products Sterilization 60-80

Timber by-products Thermo diffusion beams 80-100

Drying 60-100

Pre-heating water 60-90

Preparation pulp 120-170

Bricks and blocks Curing 60-140

Plastics Preparation 120-140

Distillation 140-150

Separation 200-220

Extension 140-160

Drying 180-200

Blending 120-140

Hence all the research in SWH is focussed on performance

improvement of flat plate collectors [7]. The flat plate

collector absorbs solar radiations and converts it into heat

energy. This heat is then absorbed by HTF flowing through

the tubes of the collector. This heat can be then stored or used

directly.

In solar water heating systems, potable water can either be

heated directly in the collector (direct systems) or indirectly

by a heat transfer fluid that is heated in the collector, passes through a heat exchanger to transfer its heat to the domestic or

service water (indirect systems). Fig. 5 and Fig. 6 show both



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the systems [3]. The heat transfer fluid is transported either

naturally (passive systems) or by forced circulation (active

systems). Natural circulation occurs by natural convection

(thermosyphoning), whereas for the forced circulation systems pumps or fans are used. Except for thermosyphon and

integrated collector storage (ICS) systems, which need no

control, solar domestic and service hot water systems are

controlled using differential thermostats. Fig. 4 shows a

typical SWH system [3].

Five types of solar energy systems can be used to heat

domestic and service hot water: thermosyphon, ICS, direct

circulation, indirect, and air. The first two are called passive

systems as no pump is employed, whereas the others are

called active systems because a pump or fan is employed in

order to circulate the fluid [4].

Most of the industries use low pressure hot water for different applications below 1000C depending on their heat

requirements. When temperatures above 1000C is required

pressurized system is required which makes system cost to

increase. For medium temperature applications (above 1000C)

mineral oils are used. However, higher cost, tendency of

cracking and oxidation are few issues associated with such

systems [9].

Fig. 4 Block diagram of SWH system [5]

SWH systems are used in textile industries to supply hot

water up to 800C for dyeing, bleaching and washing purposes

[6]. Built in storage type solar water heaters are introduced in

Pakistan textile industries’ saving about 17.13 MJ of fossil

fuel energy and subsequently improving the performance [10].

Balaji Foods and Feeds Industry from India installed a

1MW SWH system with thermal energy storage system for getting about 11000 litre/day of hot water for an egg powder

making plant.

The process consists of washing, pasteurizing, fermenting

and maintaining a room at 550C. The temperature requirement

of hot water varies between 40 to 800C at different stages of

process.

Fig. 5 Direct circulation SWH system, DT: Differential Thermometer [4]

Fig. 6 Indirect circulation SWH system, DT: Differential Thermometer [4]

The system saved about 261 kL of furnace oil per year. The

system saved environment from emissions gasses viz., 9.45

tons of SO2, 675 tons of CO2, and 562.5 tons of CO produced

from burning of furnace oil annually [12]. The system is

shown in Fig. 9.

Fig. 7 Solar-oil integrated heating plant, S: storage tank, C: solar collector

bank [11]

SWH systems supply hot water for washing and cleaning of bottles in bottle washing plant. Fig. 8 shows a process layout

of the plant with temperature ranges [8].

Active SWH systems has been used in dairy industries for

washing and cleaning, pasteurization, boiler feed water (60–



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850C), sterilization (130–1500C) and even for drying milk to

produce powder. In the production, milk and whey are spray-

dried in huge towers with air, which is heated from 1200C to

1800C [3, 13 &14]. Fig. 10 shows the layout out of SWH system for dairy application.

Domestic SWH systems are used for supplying hot water

for washing clothes, dishwashing, bathing and other cleaning

processes with temperature up to 650C.

Fig. 8 System layout of bottle washing plant [8]

Fig. 8 System layout of bottle washing plant [8]

Fig. 9 Control system layout, Balaji Foods and Feeds Industry, India [12]

India holds about 3.53 million square metres of SWH

systems are installed till June 2010 as per MNRE statistics [15]. Compared to world total SWH installations in 2005 was

about 2.1% of world installations and India has way to go

ahead in this area [16].

Fig. 10 Dairy plant with SWH [13]

B. Solar Air Drying and Dehydration

Drying (or dewatering) is a simple process of excess water

(moisture) removal from a natural or industrial product in order to reach the standard specification moisture content. It is

an energy intensive operation. Moisture content of foodstuff is

around 25–80%, but generally for agricultural products it is

around 70% [17, V Belesolis]. Moisture content of the food

stuff is reduced to increase its longer shelf life. Another case

of drying (or dewatering) is the total removal of moisture until

food has no moisture at all. Dehydrated food, when ready to

use, is re-watered and almost regains its initial conditions.

Convective drying, i.e. drying by flowing heated air

circulating either over the upper side, bottom side or both, or

across its mass is the widest among drying methods used

worldwide. Hot air heats up the product and conveys released moisture to atmosphere.

Two basic moisture transfer mechanisms are involved in

drying [17]:

1. Migration of moisture from the mass inside to the

surface.

2. Transfer of the moisture from the surface to the

surrounding air, in the form of water vapour.

Agricultural products drying using solar energy is the oldest

method used by mankind for preserving them. Generally these

methods can be classified into two categories:

(a) Direct, or open-air sun drying, the direct exposure to the sun.

(b) Indirect solar drying or convective solar drying.

Temperature is one of the major factor that affect taste,

colour, flavour, texture or nutritional values of the product.

Few products require pre-treatment before solar drying to

keep their flavour and texture.

Drying rate is an important factor for agricultural and other

food products drying. Fig. 11 shows the drying rate curve for

agricultural products. This shows three phases of drying: AB

is the time spent to heat up the material until drying

temperature is attained, BC is the constant-rate drying, CE the



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falling rate drying where mass flow of moisture from interior

is decreased continuously. C is the critical point where surface

is not any more saturated and the falling rate period starts. In

point E there is still moisture inside the product, moisture content movement takes place slowly by diffusion and drying

can stop e.g. at point D when the final moisture content is

reached [17].

Fig. 11 Drying rate curve for phase I, II and III [17]

Direct or open air solar drying technique is used for

millennia by mankind for preserving food and agricultural

products. This is a simple technique with few major

disadvantages such as uncontrolled and slow rate of operation,

environmental and weather condition dependency,

contamination, dusting, fermentation, attacks by birds and

insects and other unfavourable conditions. On the other hand

indirect air heating has only disadvantage as higher initial cost. It involves some thermal energy collecting devices and

dryers of special techniques. Higher drying rate, controlled

drying, increased productivity; no losses at all in terms of

quality are the few advantages of the technique to mention

[17].

Temperature plays important role in solar drying processes.

Average temperature of agricultural product drying is around

600C but it may reach to about 800C for a few. Table IV

shows drying data before and after solar drying for few

agricultural and food products with drying air temperature

[18].

Table IV Drying data for few agricultural products before and after solar

drying [18]

Product Moisture Percent (wb) Drying Air

Temperature (oC) Initial Final

Bananas 80 15 70

Barley 18-20 11-13 40-82

Beets 75-85 10-14 -

Cardamom 80 10 45-50

Cassava 62 17 70

Chilies 90 20 35-40

Coffee seeds 65 11 45-50

Copra 75 5 35-40

Com 28-32 10-13 43-82

Cotton 25-35 5-7 --

French beans 70 5 75

Garlic 80 4 55

Grapes 74-78 18 50-60

Green forages 80-90 10-14 --

Hay 30-60 12-16 35-45

Longan 75 20 --

Medicinal plants 85 11 35-50

Oats 20-25 12-13 43-82

Onions 80-85 8 50

Peanuts 45-50 13 35

Pepper 80 10 55

Potato 75-85 10-14 70

Pyrethrum 70 10-13 --

Rice 25 12 43

Rye 16-20 11-13 --

Sorghum 30-35 10-13 43-82

Soybeans 20-25 11 61-67

Spinach leaves 80 10 --

Sweet potato 75 7 75

Tea 75 5 50

Virgin Tobacco 85 12 35-70

Wheat 18-20 11-14 43-82

Solar dryers can be classified into different categories. Fig.

12 shows the different types [19]. Literature reviewed shows

numerous types of solar dryers have been designed and

implemented for drying of agricultural and food drying

applications. A brief review of them has been presented by Arun Mujumdar [18] and A.A. El-Sebaii et.al [19].

Fig. 12 Classification of solar dryer [17]

Solar energy for wastewater sludge drying is another area

of application for solar drying. Both direct and indirect

methods of solar drying are used for the process. Fig. 13

shows the schematic of solar assisted wastewater sludge dryer

[20].



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Fig. 13 Schematic of covered solar assisted wastewater sludge dryer [20]

A silk cocoon solar assisted drying has been presented by

Panna Lal Singh [21]. The optimum temperature for the

process is about 60-800C. The tenacity of the silk thread

obtained for solar dried cocoon and electrical dried cocoon

were about 0.77 N and 0.75 N respectively. The NPV (net

present value) of solar dryer is found to be more stable as

against the escalation rate in electricity as compared to the

same for electrical dryer. Fig. 14 shows the schematic of the

system.

Industries which involve drying process usually use hot air

or gas with a temperature range between 1400C and 2200C. Solar thermal systems can be integrated with conventional

energy supplies in an appropriate way to meet the system

requirements. Heat storage seems to be necessary when

system is required to work in the periods of day when there is

no irradiation [3].

Fig. 14 Schematic of forced convection solar assisted silk cocoon dryer [20]

C. Solar Refrigeration and Air Conditioning

With solar thermal energy absorption, adsorption, solid and

liquid desiccant and solar-electrical technologies are used for solar refrigeration and air conditioning system. The main

advantages of solar cooling systems concern the reduction of

peak loads for electricity utilities, the use of zero ozone

depletion impact refrigerants, the decreased primary energy

consumption and decreased global warming impact [22]

though reduction of green house gases up to 50% [23].

Absorption refrigeration systems are adopted most

frequently for solar cooling over other systems. It requires

very low or no electrical input and for the same cooling

capacity, the physical dimensions of an absorption

refrigeration system are usually smaller than that of an

adsorption refrigeration system due to the high heat and mass transfer coefficient of the absorbent. In addition, the fluidity of

the absorbent gives greater flexibility in realizing a more

compact and/or efficient system [24]. It was counted that

about 59% of the solar cooling systems in Europe were solar

absorption cooling systems. In China, almost all the large-

scale solar cooling demonstration projects during the last

twenty years were based upon absorption systems [22].

The most usual combinations of fluids include lithium

bromide-water (LiBr–H2O) where water vapour is the

refrigerant and ammonia–water (NH3–H2O) systems where

ammonia is the refrigerant. Fig. 15 shows the basic principle of operation for absorption refrigeration system.

The NH3–H2O system is more complicated than the LiBr–

H2O system. The NH3–H2O system requires generator

temperatures in the range of 125–1700C with air-cooled

absorber and condenser and 95–1200C when water-cooling is

used. The coefficient of performance (COP), which is defined

as the ratio of the cooling effect to the heat input, is between

0.6 and 0.7 [4]. The LiBr–H2O system operates at a generator

Fig.15 Basic principle of absorption refrigeration system [4]

Temperature in the range of 70–950C with water used as a

coolant in the absorber and condenser and has COP higher



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than the NH3–H2O systems. The COP of this system is

between 0.6 and 0.8. A disadvantage of the LiBr–H2O systems

is that their evaporator cannot operate at temperatures much

below 50C since the refrigerant is water vapour. Commercially available chillers for air conditioning systems, use LiBr-H2O

absorption systems with hot water or steam as the heat source.

In market two types of chillers are available, the single and

double effect. Single effect chillers operate with pressurized

hot water temperature ranging from 80 to 1500C. The COP of

the system varies little with heat source. On the other hand

double effect chillers operate with higher temperature of heat

source which ranges from 155-2050C. COP of double effect

chillers is higher and it is about 0.9-1.2 [4]. Fig. 16 shows a

single-effect absorption cooling system

Fig. 16 Single-effect absorption cooling system[22]

Storing cool energy during sunshine hours in a cool thermal

energy storage tank, either in a sensible heat form or in a

latent heat using Cool Thermal Energy Storage (CTES) is

used in industries for process cooling, food preservation and

building air conditioning systems [25]. Fig. 18 shows the solar

absorption chiller system with storage tank.

Compared to absorption, adsorption refrigeration system

shows advantages like no distillation (NH3-H2O system),

corrosion or crystallization (Li-Br system) problem, lower equipment cost and more effective when lower grade energy

such as solar energy is used. Zhang et al. [26] presented a

simulation study of silica gel-water solar adsorption

refrigeration system using MATLAB Simulink as tool. Fig. 19

Fig. 17 Solar absorption chiller with storage tank [25]

shows the structure of silica gel-water adsorption chiller

system. Hot water temperature is in the range of about 40-

850C but below 1000C to prevent degradation of silica gel.

Fig. 18 Structure of silica gel-water adsorption chiller [26]

Fig. 19 Solar assisted air conditioning system [27]

Solar assisted air conditioning systems generally based on

solar absorption refrigeration. Sabina et al. [27] from their

performance evaluation study shown that integrating chilled

water storage tanks with the solar assisted air conditioning

system it is possible to save 30% of water consumption, 20%

of electrical consumption and about 1.7 tons of CO2

throughout the summer period. Schematic of the system is

shown in Fig. 19.

A variety of solar collectors are used in the solar

refrigeration system. Flat plate collectors are sufficient to

achieve temperatures below 1000C. But for temperatures above 1000C evacuated tube collectors, compound parabolic

collectors or concentrating collectors are used. Table V shows

the details of the collectors, storage method and applications

for the solar refrigeration systems.



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Table V Stages and options in solar cooling technologies [25] Source Conversion Thermal

storage

(hot

energy)

Production

of cool

energy

Thermal

storage

(cool

energy)

Applications

Sun Solar Thermal

1. Flat plate

collector

2. Evacuated

tube collector

3. Concentrated

collector

1. Sensible

2. Latent

3. Thermo-

Chemical

1. Absorption

2. Adsorption

3. Desiccant

4. Ejector

1. Sensible

2. Latent

3. Thermo-

Chemical

1. Air conditioning

( i) office

(ii) Hotel

(iii) Building

(iv) Laboratory

2. Food

preservation

(i) Vegetables

(ii) Fruits

(iii) Meat and Fish

3. Process

industries

(i) Dairy

(ii) Pharmaceutical

(iii) Chemical

Solar PV

(electrical)

1. Vapor

Compression

2. Thermo-

electric

D. Solar Steam Generation Systems

Low temperature is used in industrial applications,

sterilization, and for powering desalination evaporations.

Parabolic trough collectors (PTC) are mainly employed for

solar steam generation. Three concepts are used to produce

solar steam namely, the steam flash, In-situ or direct and

unfired boiler. In steam flash method, pressurized hot water from collector is flashed in separate vessel to produce steam.

In direct or in situ method two phase flow is passed in the

collector to produce steam. Unfired boiler system uses heat

transfer fluid which passes through the collector, is transferred

to an unfired boiler where steam is generated by heat

exchange to water [4]. Figs. 20, 21 and 22 shows the

schematic of above systems.

Fig. 20 Schematic of steam flash system [4]

E. Solar Cookers

Solar cooker is an age old technology used worldwide for

cooking food. Principally solar cookers and ovens absorb solar

energy and convert it to heat which is captured inside an

enclosed area. This absorbed heat is used for cooking or

baking various kinds of food. In solar cookers internal box

temperatures can be achieved up to 3000C. Solar cookers

Fig. 21 Schematic of Direct or in situ steam generation system [4]

Fig. 22 Schematic of unfired boiler steam generation system [4]

come in many shapes and sizes, etc., but all cookers trap heat

in some form of insulated compartment [28, 29]. In most of

these designs the sun actually strikes the food for cooking.

Fig. 23 Classification of solar cookers [29]



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As shown in Fig.23 solar cookers are broadly classified into

solar cookers with storage and without storage. Solar cookers

without storage are further classified into direct and indirect

solar cookers depending upon heat transfer mechanism to the cooking pot. Direct type make use of solar energy directly in

cooking process while indirect type uses heat transfer fluid to

transfer heat from collector to cooking pot [29].

Direct type cookers are broadly classified into box type and

concentrating type cookers. Fig. 24 summarises different types

of box type cookers while Fig. 25 summarises different types

of concentrating type solar cookers.

Fig. 24 Box type cooker: (a) without reflector, (b) with single reflector, (c)

with double reflector, (d) with three reflectors (e) with four reflectors, (f) with

eight reflector [29]

Fig. 25 Concentrating type cooker: (a) panel cooker, (b) funnel cooker, (c)

spherical reflector, (d) parabolic reflector, (e) Fresnel concentrator and (f)

cylindro-parabolic concentrator [29].

In indirect type solar cookers, heat transfer fluid is being

used to collect heat and transfer it to the cooking pot. Solar cookers with flat plate collector, evacuated tube collector and

concentrating type collector are commercially available

cookers under this category. The various types of indirect type

solar cookers are shown in Fig. 25.

Solar cookers with thermal storage use thermal energy storage

material to store thermal energy. This stored heat can be used

to cook the food in case of cloudy environment or cooking

indoors or cooking off sunshine hours. Both sensible and

latent heat storage materials are used for storing the thermal

energy. Engine oil, vegetable oil or sand, granular carbon are

some of the common thermal energy storage material used for

sensible heat storage. While acetamide, stearic acid, acetanilide, coconut oil, polyethylene, salt hydrate, etc. are the

examples of few latent heat storage material or phase change

materials (PCM) used in solar cookers for thermal energy

storage [28, 29].

Fig. 26 Indirect type solar cooker: (a) with flat plate collector, (b) with

evacuated tube collector, (c) parabolic concentrators at Tirumala Tirupathi

Devasthanam and (d) spherical reflectors at Auroville [29].

V. CONCLUSION

A brief overview of different solar thermal application in

medium temperature applications has been presented here to

elaborate the extent of the applicability of solar thermal

energy to industrial applications. Solar heat for industrial

processes has a great potential to curb the demand for

conventional energies which reduce our dependence on

imported fuels and to reduce CO2 emissions. However, the

overall efficiency depends on the proper integration of the

different systems and appropriate design of the solar

concentrators/collectors. Efforts in the direction of improvement in the efficiencies of

the solar collecting systems such as reducing the top loss

coefficient by introducing aerofoil design shape for glass

covers for SWH systems, system design with minimum

number of components, utilization of less energy intensive

materials for manufacturing of the system, etc. should be

employed to make the system more cost effective (reducing

pay-back period) and environmental friendly in terms of

reduction in terms of CO2 emissions in order to penetrate in

the industries.

System design engineers, manufacturers, solution providers,

service engineers and material providers should consider solar installations as a sustainable energy development. Besides,

government should encourage utilisation of solar thermal



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systems in industries through their policies commensurate

with its large potential.

REFERENCES

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*Authors Copy

study of medium temperature solar thermal applications

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