development of a solar thermal cooking system

ISBN 0 7334 0392 1

DEVELOPMENT OF A SOLAR THERMAL COOKING SYSTEM

G.L. Morrison, J. Di and D.R. Mills1

Report No 1993/FMT/1

ABSTRACT The development of a hot plate cooking system powered by solar thermal energy is described. The system uses a concentrating evacuated tubular collector to supply thermal energy to a high temperature store so that heat can be supplied for cooking at any time of day. The system incorporates a passive downward energy transfer system between the solar collector and the store. The cooking system can be installed indoors and the collectors mounted on any convenient roof, without the need for pumps or thermosyphon loops to transport energy from the solar collector to the store.

1 School of Physics, University of Sydney

Development of a Solar Thermal Cooking System 1

INTRODUCTION Solar cooking systems have been developed in many diverse forms ranging from hot box ovens to focussing dish cookers. Hot box oven systems have been widely accepted in India and other countries where there is a shortage of energy and pressure on bio-mass resources [1-5]. The success of the solar hot box oven is demonstrated by the sale of more than 200,000 systems in India [5], under a government sponsored scheme. Solar cookers in which the cooking pot is placed at the focus of a concentrating mirror [6] have not been widely adopted due to the need to continually adjust the orientation of the concentrator. Hot box ovens and concentrating solar cookers are cheap and effective, however they are limited to cooking during clear sky periods and require the cook to work outdoors. In this project a solar cooking system with similar characteristics to an electric hot plate cooking surface has been developed in order to provide a renewable energy cooking system that could be acceptable in western households. The primary requirement for adoption of energy conservation devices in energy rich societies is that the consumer is offered a better way of achieving some objective, or an alternative way of working that does not require significant changes of existing practice. For a solar cooking system to be adopted in a western kitchen the following objectives need to be satisfied.

• cooking can be carried out at any time of day • the cooking method must be similar to conventional cooking systems • no change in the appearance of the cooking appliance • a reduction in the use of conventional energy

To satisfy these objectives a solar stove with an energy storage system and conventional energy back-up has been developed. To match existing appliance quality the solar cooking surfaces have been integrated into a conventional electric cooking appliance so that the solar product has the appearance and function of a conventional cooking system. The cooker was purposefully made as similar as possible to an electric hot plate so that the technology would not be rejected due to non-standard appearance or the need to change cooking methods. This has been achieved and a solar cooking system with energy storage has been developed with appearance and power transfer rates similar to a conventional electric hot plate cooking top. To achieve high reliability, the solar collector and energy transport system have been designed to operate without moving components. To remove the need for a pump between the solar collector and the energy store a passive downward heat transport system was developed.


SYSTEM DESCRIPTION The primary elements of the solar stove are - high temperature solar collector consisting of a non-tracking concentrator with

evacuated tubular absorber - passive downward heat transport system to transfer energy from the solar collector to

the energy store - thermal energy storage unit with conventional energy backup. - a thermal energy transfer system between the store and the cooking surface. - conventional hot plate surface modified to work with thermal energy input To minimise operation and maintenance costs a totally passive system was developed. The only moving part or control device in the system (other than the standard regulating knob on the stove) is a solenoid valve to control the daily cycle of the downward heat transport system, the system components are shown in Fig 1. SYSTEM OPERATION The solar collector heats a small header tank containing water until it is hotter than the energy store. Steam is then transferred down (or along) a small diameter tube to the energy store where it condenses in a pressurised water storage medium. The energy storage unit consists of a high temperature pressurised water vessel mounted separately from the stove. Working fluid is returned to the solar collector header tank at the end of the day by a reverse pressure difference developed when the solar collector header tank temperature drops below the storage tank temperature. Heat is transferred from the store to the cooking surface by a heat pipe system controlled by the standard rotary control on the front of the stove. A steam heated cooking surface was developed in the form of a drop-in replacement for a conventional electric hot plate so that solar heated cooking surfaces could be easily integrated with existing bench top cooking appliances. SOLAR COLLECTOR An evacuated tubular solar collector was selected to supply the high temperatures required for cooking. An asymmetric concentrator (fig 2) was used to increase the solar input into each tube and to match the seasonal output of the collector with the seasonal load pattern. The concentrator shape [7] can be tailored to match the solar collector output to any required seasonal load pattern. The collectors used in this project were developed for a domestic application where the load peaked significantly in winter. The collector array incorporates a small header tank that is heated by natural circulation of liquid or liquid/steam between the


evacuated tubular solar absorbers and the header tank. When the temperature of the water in the header tank is greater than the energy store the water in the header tank boils and energy is transferred from the collectors to the energy store by steam transfer. Steam separation in the header tank is achieved by appropriate design of the header tank connections. PASSIVE HEAT TRANSPORT The heat transport system consists of a simple small bore tube joining the top of the header tank in the collector array with the bottom of the energy storage tank, both tanks are pressurised. The system operates as a cyclic passive downward heat transport system. Liquid return to the top tank is achieved without the need for a pump, by using the pressure difference that develops when the collector cools down at night. The heat transport system has the following stages of operation over a daily cycle (Fig 3). i) The system starts with liquid in the solar collector header tank. ii) When the liquid in the header tank is hotter than the energy store, it boils and vapour passes down the heat transfer pipe and condenses in the larger mass of water in the energy storage tank. iii) When all the fluid has passed to the lower tank a reversal of temperature conditions must be introduced to produce a vapour pressure difference sufficient to return the condensate to the upper tank. The thermodynamic cycle of the heat transport system is shown in Fig 4. Step 1-2 boiling process in the solar collector header tank Step 2-3 vapour transport between the header tank and the energy store via the connecting pipe Step 3-4 condensation and liquid accumulation in the energy storage tank. Step 4-5 liquid return from the energy store to the header tank. To achieve conditions for liquid return to the solar collector, the header tank temperature must be reduced below the energy storage tank temperature. This will occur at night or during cloudy periods, when the collector cools down faster than the energy storage tank. The heat transport system is a heat pipe with liquid reservoirs at either end, liquid return to the collectors is achieved by a simple plumbing arrangement and vapour pressure, rather than a standard heat pipe wick. Due to the large vapour pressure that is available, a very high lift of the returning liquid can be achieved. If this system is used as a downward heat transport system the vertical recycle height that can be supported is a function of the working fluid and


the temperature difference that can be developed between the two reservoirs. For a water based system and energy store temperatures above 100°C, recycle lifts significantly greater than 10m can be achieved. ENERGY STORE The thermal energy storage system is a pressurised water vessel sized to hold 10MJ thermal energy, for a temperature swing of 30K. A 70L pressurised tank, holding 50L of water in the morning and 60L at the end of the day was selected. The mass in the energy store increases during the day due to steam transfer form the solar collector to the energy store. The energy transfer from the store to the cooking surface is achieved by steam passing from the top of the store and condensing in the space beneath the cooler cooking plate. The energy store is located below the hot plate so that the condensate can drain back from the cooking plate to the energy store Fig 5. The energy storage tank must have a clearance space above its water content to allow rapid generation of steam for the transfer to the cooking plate. To ensure the cooking system is continuously available the energy storage tank has an auxiliary heater operating on off-peak electric supply. In addition to facilitating the use of renewable energy for cooking, the energy store of this system also limits the peak electrical demand when the system is operated by the electric booster. HOT PLATE The design of the hot plate and the heat transport system between the store and the hot plate is critical to the success of the system, since the temperature difference between the energy store and the cooking surface must be minimised. A gravity return heat pipe system is used to transfer heat from the energy store to the cooking surface. The cooking surface is constructed from a copper plate in good thermal with a steam pipe or steam heated cavity. When a pot is placed on the cooking surface and the valve in the transfer line is opened, steam passes from the top of the energy store through the transfer pipe and condenses in the cavity beneath the cooking surface. The condensate is returned to the storage tank by gravity provided the cooking surface is at least 100 mm above the water level in the energy store. Considerable development was required to design a steam condensation space under the cooking surface that did not accumulate condensate during cooking and hence block the steam supply. A range of designs with counter flow steam and condensate in one pipe connected between the energy store and the cavity hot plate system were built and tested (Fig 6). All single pipe configurations were found to suffer from condensate accumulation in the cooking plate cavity. The most effective design was found to be a two pipe configuration with a coiled tube steam condenser soldered under the cooking surface Fig 7. The steam flow between the energy store and the hot plate is arranged with a steam input pipe from the top of the energy store to the centre of the spiral tube beneath the hot plate and a condensate return pipe from the outlet of the spiral tube to the bottom of the energy store.


HEAT TRANSPORT SYSTEM SIZING Solar Collector Liquid Inventory (Energy transfer per cycle) The latent heat of vaporisation of water is temperature dependent (hfg = 2258 kJ/kg at 100°C and 1947 kJ/kg at 200°C), hence the liquid inventory that must be in the upper header tank in the morning is a function of the operating temperature and the collector area. For typical Sydney irradiation conditions a useful energy output in the range 5 to 8 MJ/m2 day could be expected. Hence a liquid inventory of 4L/m2 is required. The net energy exchange per diurnal cycle depends on the heat loss from the upper tank during the night and cloudy phases of the cycle. The worst case condition is when the hot liquid is returned to the upper header tank at the end of the heat transfer cycle and cools to ambient temperature before the start of the next heating cycle. The energy returned to the upper tank when liquid return occurs varies from 15% at 100°C to 40% at 200°C (Fig 8). This heat loss can be minimised by delaying the return of liquid to the solar collector header tank until just prior to the start of the next heating cycle. Although this is easily achieved by using a valve in the transfer line, it increases the complexity of the system as a control device is required to operate the valve at the required time. The net energy transfer per kg of mass transfer is shown in Fig 9 as a function of system operating temperature. The lower limit assumes that all the energy in the hot liquid returned during the re-cycle operation, is lost before the next cycle begins. The upper limit assumes no loss in the re-cycle operation. Downward Heat Transport Tube Size The primary criteria for transfer pipe sizing is to minimise the temperature difference between the evaporator and the condenser during the heating stage of the cycle. The pipe diameter required is influenced by the design temperature, since the density of steam and hence the transfer velocity is a strong function of temperature. If the transfer pipe is well insulated there will be single phase flow in the pipe, and the exit conditions could be slightly superheated (point 3 in Fig 4). If the pipe is not well insulated, or if it is operating at a low power transfer level, there may be condensation in the pipe. The pressure and temperature drop along the transfer pipe can be evaluated by analysing heat loss and compressible factors governing flow in the pipe. If the wetness state of steam in the pipe is greater than 95% then the pressure drop along the pipe can be analysed on the assumption that saturation conditions apply at all points. The steam conditions along the pipe during the energy transfer operation can be analysed by dividing the pipe into a series of segments and computing the pressure drop due to friction in each segment. The density and temperature variations being determined on the basis of saturation conditions at each point in the pipe. Typical temperature drops across the transfer pipe are shown in Figs 10-13 for power transfers of 1 to 100kW and pipe lengths of 5 to 10m. The transfer pipe diameter required for a maximum temperature drop of 5K, is shown in Table 1 for a range of power transfers, pipe lengths and operating temperatures. A small diameter transfer tube is required to ensure that the liquid can be returned to the header tank as a solid plug, rather than as a bubbly mixture of steam and liquid.


TABLE 1

Transfer Pipe Diameter for 5K Temperature Drop

Pipe Diameter (nominal OD mm) Power Transfer Temperature 150°C Temperature 100°C

kW Length 5 m Length 10 m Length 5 m Length 10 m

1 4.8 6.4 7.9 7.9 10 9.5 12.5 15.9 19 100 19 25.4 31.7 38

PERFORMANCE TESTS OF DOWNWARD HEAT TRANSPORT SYSTEM Heat Pipe Prototype A downward heat transport system was constructed between two 20 L reservoirs separated vertically by 7m. The transfer pipe was 9m long and 4.4mm internal diameter. Heat was introduced into the upper reservoir by an electric heater to simulate a solar collector (fig 14). The system was instrumented with thermocouples in the upper and lower reservoirs, and the lower reservoir was mounted on scales so that the mass transfer could be monitored. The system was tested for a power transfer of 1.9kW and evaporator temperatures of 130°C to 150°C. Comparisons of measured and predicted temperature drop along the transfer pipe are shown in Fig 15, good agreement was obtained between the measurements and the steam transfer model based on the assumption of saturated steam conditions along the pipe. The heat pipe performance from a cold start to steady state operation is shown in Fig 16. Although the mass transfer rate does not reach a steady state value until approximately 60 minutes after start-up, there was noticeable mass transfer within the first 20 minutes. Mass transfer on start-up is small due to the low density of steam for the cold start conditions. For the transfer pipe diameter used in this prototype (4.4mm), the transfer pipe model based on saturation conditions indicates that an evaporator temperature of at least 90°C is required for 1kW power transfer. Hence the power transfer in the prototype pipe was limited by compressibility conditions until the evaporator temperature was greater than 90°C. Evaporator Configuration During start-up and for low collector temperature operation the vapour velocity in the transfer pipe may be very high and care must be taken to ensure that liquid is not entrained and carried directly to the bottom reservoir. A range of steam separator configurations were tested in the prototype system (Fig 17). Configuration (a) relied on gravity to separate the vapour/liquid produced by boiling action in the collector. Testing of this configuration showed that liquid was not being separated by the simple T junction plumbing system. A carefully designed steam separator was also tested, configuration (b), and found to avoid direct


transfer of liquid for all operating conditions. Although configuration (b) satisfied the operating requirements it is a non-standard fitting and hence expensive to construct and install. The third configuration (c) that was investigated used only standard plumbing fittings and relied on the separation of liquid from the vapour stream by gravity as it passed over the surface of the liquid reservoir. This configuration was found to be as effective as the specially constructed steam separator.

Re-cycle Operation The re-cycle period of the prototype system is shown at the end of the test shown in Fig 16. When the power into the upper reservoir was turned off the temperature dropped, and liquid started to re-cycle (7m rise) as soon as the upper reservoir temperature was below the lower reservoir temperature. The re-cycle operation was a gradual process and took typically 15 minutes to return 10L of liquid through the 4.45mm pipe (return velocity < 1m/s). The return time in the test rig was limited by the low thermal mass in the bottom reservoir. If the bottom reservoir was part of a larger energy store the re-cycle process would occur faster. The energy lost as a result of re-cycling hot liquid to the collector at the end of the day can be eliminated by blocking the return operation until just prior to the start of the next heating cycle. Such a control was implemented in the prototype system by installing a steam solenoid valve in the bottom of the transfer line, and delaying the liquid return for 6 hours, as shown in fig 18. During the test shown in fig 18 a heating cycle was completed at approximately 1 hour and the solenoid was then closed. The temperature of the upper reservoir was then cooled to 15°C over the next hour. The system was then allowed to cool down until the lower reservoir was 110°C. For these temperature differences the pressure differential across the pipe was approximately 100kPa compared to 70 kPa required to balance the 7m vertical lift. When the solenoid valve was opened the liquid returned to the upper tank at a rate of 10L per 20 minutes which is only slightly slower than the return rate observed in the high temperature test shown in Figs 16. Transient Operation The analysis and tests outlined above are for operation at high power transfer (maximum temperature drop typically 5K). During application in a solar heating system the heat pipe will be operating over a wide range of conditions and for low power transfer there may be significant condensation in the heat pipe. If the condensation results in a steam wetness quality below 0.95 the presence of a water film on the walls of the tube will increase the friction and temperature drop along the tube. To assess the operation of the heat pipe for a wide range of operating conditions a transient model was developed to compute steam conditions in the pipe in terms of heat pipe wall insulation, inlet temperature and power transfer. The pipe model can be coupled to a transient solar collector model to determine the long term performance of the solar collector downward heat transport system and energy store, including transient operation due to unsteady solar conditions.


POWER TRANSFER BETWEEN STORE AND HOT PLATE The operation of the energy transfer system between the store and cooking surface was determined by evaluating the time to boil 2L of water as a function of energy store temperature. Boiling tests were used to quantify the power transfer by measuring the rate of change of temperature of a pot containing 2L of water, as a function of energy store temperature. Typical boiling performance is shown in Fig 19 for a store temperature of 140°C. The variation of initial power transfer with energy store temperature is shown in Fig 20. The energy transfer rates for the steam transport system are comparable to conventional electric and gas cooking systems. The time to boil 2L of water is shown in Fig 21 as a function of store temperature, the boiling cycle times are also comparable to conventional cooking systems. COOKING TESTS A series of boiling and frying operations were also evaluated and typical cooking times are shown in Table 2. These tests show that for effective frying the energy store temperature must be higher than 160°C. Boiling or steaming cooking operations can be carried out in times similar to a conventional electric hot plate for store temperatures down to 130°C.

TABLE 2

Cooking Times for Steam Heated Cook Top

Cooking operation Energy store Temperature

Time required Minutes

Cook 0.5kg rice in 0.75kg water

150°C 17

Fry 3 eggs and 0.25kg onions

150°C 8

Steam 1 kg rice in 1.3kg water

160°C 17

Fry 0.5kg rice, 0.25kg green beans and 0.25kg shrimps

175°C 8

Fry 1kg sausages

175°C 10

Fry 4 eggs and 0.25kg onions

175°C 9

Fry 0.8kg beef

180°C 9

Fry omelette, 3 eggs, 0.25kg onions

180°C 8


PROTOTYPE SOLAR STOVE A solar powered hot plate cooking top was constructed as a direct replacement element for an electric hot plate and fitted in a standard electric stove as shown below. The standard electric stove only required minor modifications to convert it to solar operation. The energy store incorporated an off-peak electric back-up to ensure sufficient energy is always available in the store at the start of each day.

Solar energy store and electric stove with front hot plates converted for solar input

Solar collector and converted stove


CONCLUSIONS A solar powered hot plate cooking system incorporating energy storage has been developed and demonstrated to have similar cooking characteristics as a conventional electric hot plate cooking top. The solar hot plate has been designed as a direct replacement item for an electric element and a standard electric stove was converted to solar operation with only minor modification. The solar stove includes an energy store so cooking times are not restricted. The steam transfer cooking system has been shown to have cooking times similar to an electric hot plate for most cooking operations. The stove can also be operated on off-peak electricity, thus reducing electricity grid peak loading for locations where peak loads are due to domestic demand. REFERENCES 1. Kuhnke, K. "Solar Cookers for Developing Countries, A Worldwide Study", ISES Conference, Advances in Solar Energy Technology, Hamburg, 2676-2682, 1987. 2. Magney, G.K. "Providing Solar Cookers for Afghan Refugees", ISES Conference, Advances in Solar Energy Technology, Hamburg, 2688-2692, 1987. 3. Grupp, M. "Solar Cooking - Lessons from the Past, Hopes for the Future", World Renewable Energy Congress, Reading, 1325-1328, 1990. 4. Magney, G.K. "Keys to Successful Solar Cooking", ISES World Congress, Colorado, 3707-3712, 1991, 5. Department of Non-Conventional Energy Sources, Ministry of Energy, India, Annual Report, 1990-91. 6. Bernard, R. "Safe Solar Cooking", World Renewable Energy Congress, Reading, 854 - 857, 1990. 7. Mills, D.R. & Giutranich, J.E. "New Ideal Concentrators for Distant Radiation Sources", Solar Energy, V23, 85-89, (1979) 8. Di, J.F "Solar Stove With Downward Heat Transport System" M.Eng.Sci Thesis, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney Australia, 1992.


Fig 1. Solar stove with downward heat transport system

Fig 2. Asymmetric concentrator and evacuated tubular absorber


Fig 3. Operation stages of downward heat transport system

Fig 4. Downward heat transport system thermodynamic cycle

0

5

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15

20

25

0 500 1000 1500 2000 2500 3000 3500

Enthalpy kJ/kg

Pres

sure

bar

1 2

34

5

Evaporator during energy

Steam transfer pipe

Evaporator during refilling

Liquid line saturation line

Low temperature

High temperature

Working fluid return

Vapour transfer

Heat source

Heat extraction

Downward heat transport


Fig 5 Energy store and cooking surface connection


Fig 6. Steam heated cavity hot plate

Fig 7. Steam heated coil hot plate


Fig 8. Energy loss due to condensate return to solar collector (worst case)

Fig 9. Effect of re-cycled liquid heat loss on net energy tran

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100 120 140 160 180 200

Re-cycle temperature °C

Ene

rgy

loss

%

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1.8

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2.4

100 120 140 160 180 200

Evaporator temperature °C

Ene

rgy

tran

sfer

MJ/

kg

No heat loss

Total heat loss


Fig 10. Temperature drop for a 5m long transfer pipe with power transfer of 1 kW.


0

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50 100 150 200

Evporator temperature °C

Tem

pera

ture

dro

p K

3/4", 15.8mm5/8", 12.7mm1/2", 10.3mm3/8", 7.15mm

Pipe length = 5mPipe ID

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50 70 90 110 130 150 170 190


Tem

pera

ture

dro

p K

3/8" ,7.15 mm

5/16" ,6.35 mm

1/4" ,4.45mm

3/16", 3.17mm

Pipe length = 5 mPipe ID



Fig 13. Temperature drop for a 10m long transfer pipe with power transfer of 100 kW

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Tem

pera

ture

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p K

3/4", 7.15mm

5/8", 6.35mm

1/2:, 4.45mm3/8", 3.17mm

Pipe length = 10 mPipe ID

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50 100 150 200


Tem

pera

ture

dro

p K

34.8 mm28.5 mm22.2 mm15.9 mm

Pipe length 10mPipe ID


Fig 14. Downward heat transport evaluation rig


Fig 15. Measured and predicted temperature drop for a 9m transfer pipe

with power transfer of 1.9 kW

Fig 16. Measured performance of downward heat transport system pipe length = 9m, pipe diameter = 4 mm

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Computed temperature dropMeasured temperature drop

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°C

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s tr

ansf

er k

g

Evaporator temperature

Condenser temperature

Mass transfer


Fig 17. Steam separator configurations for solar header tank

VapoVapour

Water Vapour

Vapour

Water

Vapour

Steam separator

Vapour

Water

Vapour

(a)

(b)

(c)


Fig 18. Measured performance of downward heat transport system with liquid return delay

Fig 19. Steam hot plate cooking test

Temperatures and power transfer during 2L boiling operation

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0 1 2 3 4 5 6 7

Time hr

Tem

pera

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Maa

a kg

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Lower tank mass

Solenoid closed

Solenoid opened

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er k

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temperature

Hot plate temperature


Fig 20. Initial steam hot plate power transfer when a 2L water vessel

is placed on the cooking surface

Fig 21. Time required to boil 2L of water on steam heated hot plate

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Store temperature °C

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Store temperature °C

Tim

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of w

ater

(min

)

development of a solar thermal cooking system

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