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International Journal of Applied Research and Studies (iJARS)

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

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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 ofprocess heat and electricity generation applications. It isessential to apply solar energy for a wide variety of

applications and provide energy solutions by modifying theenergy proportion, improving energy stability, increasingenergy sustainability, conversion reduction and hence enhance

the system efficiency. In the work presented here, a briefstudy of a few medium temperature solar thermal applicationsup 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. INTRODUCTIONSolar thermal energy is used as process heat for different

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

medium to high temperature ranges. These applicationsincludes: hot water supply, desalination, sterilization,pasteurization, drying, space heating and cooling,

refrigeration, distillation, washing and cleaning andpolymerization. All these applications lies in temperaturerange between 60 to 280

0C [3]. Solar thermal collectors are

used for harnessing this solar energy. These collectors arespecial type of heat exchangers, which absorb the solar

radiations, and convert it to heat which is further transferred tothe 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 andfor absorbing solar radiation, whereas a sun-trackingconcentrating solar collector usually has concave reflectingsurfaces to intercept and focus the suns beam radiation to asmaller receiving area, thereby increasing the radiation flux. A

detailed review of these collectors is presented by SoteriusKaliogirou, 2004 [4]. Non-concentrating or stationerycollectors are suitable for low (Flat Plate, FPC and AdvancedFlat Plate Collector, AFP) to medium (Evacuated tube, ETC

and Compound Parabolic, CPC) temperature applications

while concentrating type are suitable for medium (Parabolictrough (PTC), Fresnel, Scheffler and Cylindrical trough) to

high temperature (Paraboloid and Heliostat) applications asthey produce higher temperature [4, 5].

This paper presents a comprehensive review of the currentstatus of utilization of solar energy in industrial and domesticapplications.

TABLE IType of solar collectors [3]

Motion Collector Type Absorber

Type

Concentration

Ratio

Indicative

Temperature

Range

Stationary Flat Plate Collectors (FPC) Flat 1 30-80Evacuated Tube Collector (ETC) Flat 1 50-200

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

Single-axistracking

Linear Fresnel reflector (LFR) Tubular 10-40 60-250Parabolic 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/absorberarea of the collector

[email protected]*Corresponding Author Email-Id
mailto:[email protected]:[email protected]:[email protected]

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

solar collectors with demand temperature ranges.

Fig. 1Optimum 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 transferfluid (HTF) utilizing solar irradiation, which is further to theapplication as process heat. HTF forms the working fluid forthe system. On the contrary an indirect system has two

working fluids used in the system. As shown in Fig.1, atypical indirect heating system consists of mainly five majorcomponents namely, solar collector, HTF storage tank, boiler,pump for circulating the HTF and a heat engine to convertheat to mechanical energy [4, 6]. The efficiency of a solar

thermal conversion system is about 70% when compared to asolar electrical direct conversion system which has an

efficiency of 17% [7].

Fig. 2Schematicof 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 heatengine will be replaced by the respective application system.

III.INDUSTRIAL ENERGY SYSTEMAn Industrial system composed of four major components

namely: power supply, production plant, energy recovery andcooling systems [6, 8]. Fig. 3 shows the block diagram of the

industrial energy system. Power supply provides energy to thesystem 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 thermalenergy can be utilized directly as a source of energy, partly or

completely, for running a process in the system.

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

IV.SOLAR THERMAL APPLICATIONSSolar 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 anoverview 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 temperaturesthermal energy up to 100

0C, 27% up to 200

0C and the

remaining applications need high temperature in steel, glass

and ceramic industry [6]. Table III shows few of potentialindustrial processes and the required temperatures for theiroperations.

Industrial energy analysis shows that solar thermal energyhas enormous applications in low (i.e. 202000C), mediumand medium-high (i.e. 802400C) temperature levels [3].

Almost all industrial processes require heat in some parts oftheir processes. Most common applications for solar thermalenergy used in industry are the solar water heaters, solar

dryers, space heating and cooling systems and waterdesalination.

With solar thermal energy replacing the fossil fuels for

industrial processes not only reduces dependency on

conventional fuels but also minimizes greenhouse emissionssuch as CO2, SO2, NOx [8]. Nevertheless, there are some

challenges for integration of solar heat into a wide variety ofindustrial processes due to the periodic, dilute and variablenature of solar irradiation [9].

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TABLE IISolar 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 classifiedin following manner [6],1. Hot water or steam demand process2. Drying and dehydration process3. Preheating4. Concentration5. Pasturization and sterilization6. Washing and cleaning7. Chemical reactions8. Industrial space heating9. Textile10. Food11. Building12.

Plastic13. Chemistry

14. Business establishmentA. 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)

airy ressurization 60-80

terilization 100-120

rying 120-180

oncentrates 60-80

oiler 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 performanceimprovement of flat plate collectors [7]. The flat platecollector absorbs solar radiations and converts it into heat

energy. This heat is then absorbed by HTF flowing throughthe tubes of the collector. This heat can be then stored or useddirectly.

In solar water heating systems, potable water can either be

heated directly in the collector (direct systems) or indirectlyby a heat transfer fluid that is heated in the collector, passesthrough 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 (activesystems). Natural circulation occurs by natural convection

(thermosyphoning), whereas for the forced circulation systemspumps or fans are used. Except for thermosyphon andintegrated collector storage (ICS) systems, which need nocontrol, solar domestic and service hot water systems are

controlled using differential thermostats. Fig. 4 shows atypical SWH system [3].

Five types of solar energy systems can be used to heat

domestic and service hot water: thermosyphon, ICS, directcirculation, indirect, and air. The first two are called passivesystems as no pump is employed, whereas the others arecalled 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 heatrequirements. When temperatures above 100

0C 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 ofcracking and oxidation are few issues associated with suchsystems [9].

Fig. 4Block diagram of SWH system [5]

SWH systems are used in textile industries to supply hotwater up to 800C for dyeing, bleaching and washing purposes

[6]. Built in storage type solar water heaters are introduced inPakistan 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 forgetting 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. 5Direct circulation SWH system, DT: Differential Thermometer [4]

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

The system saved about 261 kL of furnace oil per year. Thesystem saved environment from emissions gasses viz., 9.45

tons of SO2, 675 tons of CO2, and 562.5 tons of CO producedfrom burning of furnace oil annually [12]. The system isshown in Fig. 9.

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

bank [11]

SWH systems supply hot water for washing and cleaning ofbottles in bottle washing plant. Fig. 8 shows a process layoutof the plant with temperature ranges [8].

Active SWH systems has been used in dairy industries forwashing and cleaning, pasteurization, boiler feed water (60

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falling rate drying where mass flow of moisture from interior

is decreased continuously. C is the critical point where surfaceis not any more saturated and the falling rate period starts. In

point E there is still moisture inside the product, moisturecontent movement takes place slowly by diffusion and dryingcan stop e.g. at point D when the final moisture content isreached [17].

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

Direct or open air solar drying technique is used formillennia by mankind for preserving food and agricultural

products. This is a simple technique with few majordisadvantages such as uncontrolled and slow rate of operation,environmental and weather condition dependency,

contamination, dusting, fermentation, attacks by birds andinsects and other unfavourable conditions. On the other handindirect air heating has only disadvantage as higher initialcost. It involves some thermal energy collecting devices anddryers of special techniques. Higher drying rate, controlleddrying, 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 around600C but it may reach to about 800C for a few. Table IVshows drying data before and after solar drying for few

agricultural and food products with drying air temperature[18].

Table IVDrying 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 showsnumerous types of solar dryers have been designed and

implemented for drying of agricultural and food dryingapplications. A brief review of them has been presented byArun Mujumdar [18] and A.A. El-Sebaii et.al [19].

Fig. 12Classification of solar dryer [17]

Solar energy for wastewater sludge drying is another area

of application for solar drying. Both direct and indirectmethods of solar drying are used for the process. Fig. 13shows the schematic of solar assisted wastewater sludge dryer

[20].

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

A silk cocoon solar assisted drying has been presented byPanna Lal Singh [21]. The optimum temperature for theprocess is about 60-800C. The tenacity of the silk threadobtained for solar dried cocoon and electrical dried cocoon

were about 0.77 N and 0.75 N respectively. The NPV (netpresent value) of solar dryer is found to be more stable asagainst the escalation rate in electricity as compared to the

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

Industries which involve drying process usually use hot air

or gas with a temperature range between 1400C and 220

0C.

Solar thermal systems can be integrated with conventionalenergy supplies in an appropriate way to meet the system

requirements. Heat storage seems to be necessary whensystem is required to work in the periods of day when there isno irradiation [3].

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

C. Solar Refrigeration and Air ConditioningWith solar thermal energy absorption, adsorption, solid and

liquid desiccant and solar-electrical technologies are used forsolar refrigeration and air conditioning system. The mainadvantages of solar cooling systems concern the reduction ofpeak loads for electricity utilities, the use of zero ozone

depletion impact refrigerants, the decreased primary energyconsumption and decreased global warming impact [22]though reduction of green house gases up to 50% [23].

Absorption refrigeration systems are adopted mostfrequently for solar cooling over other systems. It requiresvery low or no electrical input and for the same coolingcapacity, the physical dimensions of an absorption

refrigeration system are usually smaller than that of anadsorption refrigeration system due to the high heat and masstransfer coefficient of the absorbent. In addition, the fluidity ofthe absorbent gives greater flexibility in realizing a morecompact and/or efficient system [24]. It was counted that

about 59% of the solar cooling systems in Europe were solarabsorption cooling systems. In China, almost all the large-scale solar cooling demonstration projects during the lasttwenty years were based upon absorption systems [22].

The most usual combinations of fluids include lithiumbromide-water (LiBrH2O) where water vapour is the

refrigerant and ammoniawater (NH3H2O) systems whereammonia is the refrigerant. Fig. 15 shows the basic principleof operation for absorption refrigeration system.

The NH3H2O system is more complicated than the LiBrH2O system. The NH3H2O system requires generatortemperatures in the range of 1251700C with air-cooled

absorber and condenser and 951200C when water-cooling is

used. The coefficient of performance (COP), which is definedas the ratio of the cooling effect to the heat input, is between0.6 and 0.7 [4]. The LiBrH2O system operates at a generator

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

Temperature in the range of 70950C with water used as a

coolant in the absorber and condenser and has COP higher

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than the NH3H2O systems. The COP of this system is

between 0.6 and 0.8. A disadvantage of the LiBrH2O systemsis that their evaporator cannot operate at temperatures much

below 50C since the refrigerant is water vapour. Commerciallyavailable chillers for air conditioning systems, use LiBr-H2Oabsorption 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 pressurizedhot water temperature ranging from 80 to 150

0C. The COP of

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

double effect chillers operate with higher temperature of heatsource which ranges from 155-205

0C. COP of double effect

chillers is higher and it is about 0.9-1.2 [4]. Fig. 16 shows asingle-effect absorption cooling system

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

Storing cool energy during sunshine hours in a cool thermalenergy storage tank, either in a sensible heat form or in a

latent heat using Cool Thermal Energy Storage (CTES) isused in industries for process cooling, food preservation andbuilding 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, lowerequipment cost and more effective when lower grade energysuch as solar energy is used. Zhang et al. [26] presented a

simulation study of silica gel-water solar adsorptionrefrigeration system using MATLAB Simulink as tool. Fig. 19

Fig. 17Solar 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 40-85

0C but below 100

0C to prevent degradation of silica gel.

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

Fig. 19Solar assisted air conditioning system [27]

Solar assisted air conditioning systems generally based onsolar absorption refrigeration. Sabina et al. [27] from their

performance evaluation study shown that integrating chilledwater 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 CO 2

throughout the summer period. Schematic of the system isshown in Fig. 19.

A variety of solar collectors are used in the solar

refrigeration system. Flat plate collectors are sufficient toachieve temperatures below 100

0C. But for temperatures

above 1000C evacuated tube collectors, compound parabolic

collectors or concentrating collectors are used. Table V showsthe details of the collectors, storage method and applications

for the solar refrigeration systems.

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Table VStages 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

collector2. Evacuatedtube collector3. Concentrated

collector

1. Sensible

2. Latent

3. Thermo-Chemical

1. Absorption

2. Adsorption

3. Desiccant4. Ejector

1. Sensible

2. Latent

3. Thermo-Chemical

1. Air conditioning

( i) office

(ii) Hotel(iii) Building(iv) Laboratory2. Food

preservation(i) Vegetables

(ii) Fruits(iii) Meat and Fish

3. Processindustries

(i) Dairy(ii) Pharmaceutical

(iii) Chemical

Solar PV

(electrical)

1. Vapor

Compression2. Thermo-

electric

D. Solar Steam Generation SystemsLow temperature is used in industrial applications,

sterilization, and for powering desalination evaporations.Parabolic trough collectors (PTC) are mainly employed forsolar steam generation. Three concepts are used to produce

solar steam namely, the steam flash, In-situ or direct andunfired boiler. In steam flash method, pressurized hot waterfrom collector is flashed in separate vessel to produce steam.In direct or in situ method two phase flow is passed in thecollector to produce steam. Unfired boiler system uses heat

transfer fluid which passes through the collector, is transferredto an unfired boiler where steam is generated by heatexchange to water [4]. Figs. 20, 21 and 22 shows theschematic of above systems.

Fig. 20Schematic of steam flash system [4]

E. Solar CookersSolar 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 anenclosed area. This absorbed heat is used for cooking orbaking various kinds of food. In solar cookers internal boxtemperatures can be achieved up to 3000C. Solar cookers

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

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

come in many shapes and sizes, etc., but all cookers trap heatin some form of insulated compartment [28, 29]. In most of

these designs the sun actually strikes the food for cooking.

Fig. 23Classification 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 cookerswithout storage are further classified into direct and indirect

solar cookers depending upon heat transfer mechanism to thecooking pot. Direct type make use of solar energy directly incooking process while indirect type uses heat transfer fluid totransfer heat from collector to cooking pot [29].

Direct type cookers are broadly classified into box type andconcentrating type cookers. Fig. 24 summarises different typesof box type cookers while Fig. 25 summarises different types

of concentrating type solar cookers.

Fig. 24Box 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. 25Concentrating 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 beingused to collect heat and transfer it to the cooking pot. Solarcookers with flat plate collector, evacuated tube collector and

concentrating type collector are commercially availablecookers under this category. The various types of indirect typesolar cookers are shown in Fig. 25.

Solar cookers with thermal storage use thermal energy storagematerial to store thermal energy. This stored heat can be usedto 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 aresome of the common thermal energy storage material used for

sensible heat storage. While acetamide, stearic acid,acetanilide, coconut oil, polyethylene, salt hydrate, etc. are theexamples of few latent heat storage material or phase changematerials (PCM) used in solar cookers for thermal energy

storage [28, 29].

Fig. 26Indirect 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. CONCLUSIONA brief overview of different solar thermal application inmedium temperature applications has been presented here to

elaborate the extent of the applicability of solar thermalenergy to industrial applications. Solar heat for industrial

processes has a great potential to curb the demand forconventional energies which reduce our dependence onimported fuels and to reduce CO2 emissions. However, theoverall efficiency depends on the proper integration of the

different systems and appropriate design of the solarconcentrators/collectors.Efforts in the direction of improvement in the efficiencies ofthe solar collecting systems such as reducing the top losscoefficient by introducing aerofoil design shape for glass

covers for SWH systems, system design with minimumnumber of components, utilization of less energy intensivematerials for manufacturing of the system, etc. should be

employed to make the system more cost effective (reducingpay-back period) and environmental friendly in terms ofreduction in terms of CO2emissions in order to penetrate inthe industries.

System design engineers, manufacturers, solution providers,service engineers and material providers should consider solarinstallations as a sustainable energy development. Besides,

government should encourage utilisation of solar thermal

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