Труды Всемирного конгресса Международного общества солнечной энергии

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EXPERIMENTAL STUDY OF ELECTRICITY GENERATION BY PHOTOVOLTAIC

THERMAL COLLECTORS: COMPARISON WITH CONVENTIOAL

PHOTOVOLTAIC PANELS

Jiachen Su, Shujuan Wang¢, Changhe Chen

Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education,

Department of Thermal Engineering, Tsinghua University

Beijing 100084, P. R. China

wangshuj@tsinghua.edu.cn

ABSTRACT

A hybrid photovoltaic/thermal collector (PV/T) is designed

to improve electrical efficiency and supply thermal energy

and electrical energy simultaneously. In this work, A PV/T

system was built, which includes PV/T collector, water tank,

pump, measurement apparatus, and data collecting system.

A conventional PV panel is installed for comparison. The

study focuses on the electrical, thermal efficiency of the

system, and the comparison of different components

temperature. The results show that electrical efficiency of

PV/T is remarkably improved by the cooling comparing

with that of PV; meanwhile the thermal energy produced is

also considerable. The highest water tank temperature for

21 days indicates there is an upper limit of water

temperature which should be considered for practical use of

hot water. Some suggestions for the optimization of the

design of PV/T system are also given.

1. INTRODUCTION

The electrical efficiency of solar cells is generally

temperature dependent and it is reduced when the

temperature increases. The following equation shows how

the efficiency decreases as the temperature increases

[1]:

(- )nn TTμ=+ (1)

Hybrid photovoltaic-thermal (PV/T) system is designed to

maintain relative low temperature to improve electrical

efficiency by flowing either water or air below photovoltaic

cell, as well as take advantage of the thermal energy, thus

convert solar energy into both electrical and thermal energy,

increase the overall energy conversion efficiency.

Different kinds of hybrid PV/T systems have been

proposed in the past. Kern and Russell [2] gave the concept

of PV/T collector using water or air as the cooling fluid.

The performance of a hybrid PV/T is evaluated in the

research of Huang et al. [3], and compared to a solar

collector. Raghuraman [4] and Yasushi Morita [5] et al.

studied the numerical method predicting the performance

of liquid and air PV/T flat plate collectors. Sound papers on

PV/T collectors have been written by H. A. Zondag et al.

[6], where nine different kinds of design of PV/T collectors

are studied and compared.

In this work, a PV/T system is constructed and its

performance has been investigated under current Beijing

environment, aiming to provide both electricity and hot

water. The analysis is carried out for electrical and thermal

efficiency of PV/T; comparison is made with a photovoltaic

cell (PV) on the photovoltaic conversion characteristic and

PV temperature.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

873

2. EXPERIMENTAL PV/T SYSTEM

The PV/T system is shown schematically in Fig. 1 and the

cross-sectional view of the components of the system is

presented in Fig. 2. The PV/T system consists of a PV cell

(126 W), and an insulated cylindrical water storage tank of

capacity 150L. The PV cell is attached by a flat solar heat

collector (1.5 m

), where water circulates through pipes to

extract heat generated by PV cell. The system uses thermal

insulation material to avoid thermal losses form the non-PV

cell system surfaces. The PV/T is connected to the water

tank through insulated pipes. A water pump of 25 W is used

to circulate the water.

Fig. 1: Schematic diagram of hybrid PV/T system.

Fig. 2: Cross-sectional view of PV/T.

The experiment was carried out outdoors in Beijing, China

for determination of the performance of PV/T system. The

thermal efficiency (

η ) of the system is a function of the

solar radiation (

G ), the input water temperature ( T ) and

the ambient temperature (

T ). The electrical efficiency

η of the system depends mainly on the incoming solar

radiation and the PV cell temperature (

T ) [7].

The electrical efficiency is calculated by:

VGAη = (2)

Where I and V are the current and the voltage of PV cell,

and A the PV/T area.

The thermal efficiency is calculated by:

()/mC T T GAη =− (3)

Where

m is the water mass flow rate, C the water

specific heat,

T and T are the output and input water

temperature, respectively.

3. RESULTS AND DISCUSSIONS

3.1 The Comparison of PV/T and PV Cell Temperature

The main purpose of PV/T design is to decrease the PV cell

temperature thus enhance the electricity conversion

efficiency. For comparison, a traditional PV cell is

constructed to demonstrate the improvement of temperature.

Fig. 3 shows the temperature variation of PV/T and

traditional PV cell. Ambient temperature and water tank

temperature are also given for the reference. It can been

seen that the temperature of PV/T is always higher than that

of PV cell and the maximum difference 7.6 ć was

achieved by the cooling effect during the day time.

0 100 200 300 400

Time, minute (From 9:41AM to 17:00 PM)

Temperature,

Temperature of the bottom of PV/T panel

Temperature of the top of PV/T panel

Ambient temperature

Water tank temperature

PV cell temperature

Fig. 3: Comparison of several typical temperatures.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

874

100

200

300

400

500

600

700

0 100 200 300 400 500

Time, minute (From 9:41 am to 17:00 PM)

Irradiation,W/m2

Fig. 4: Irradiation during the day time for the reference,

which is typical in the summer of Beijing.

3.2 Efficiency Analysis

The PV/T system and PV cell was tested for 21 days in the

natural environment in May and June, Beijing. Comparing

PV/T and PV cell, as expected, the increase in the electrical

efficiency of PV/T is remarkable, from 0.36% to 2.6%. The

reason is certainly due to the structural difference; and thus

it has become clear that the inlet temperature and heat

transfer resistance introduced in the cell-heat collector

interface are the key design points in the study of the

optimum structure of PV/T system.

Fig. 5: Electrical efficiency of PV/T and PV cell for 21

days in May and June, 2007.

Fig. 6 indicates that the thermal efficiency of PV/T system

varies from 36% to 61%, due to different weather

conditions, especially solar irradiance. It is also need to be

noted that the PV/T system has a lower thermal efficiency

than the conventional thermal collector. This is because the

conversion of solar energy to electricity and increased heat

loss on the surface with the glass cover missing compared

Thermal efficiency

Fig. 6: Thermal efficiency of PV/T for 21 days in May and

June, 2007.

with the thermal collector. However, the PV/T system can

produce a remark able amount of thermal energy and also

electricity using equal surface area, especially when the

thermal energy and electricity are both needed and

installation room is limited.

3.3 Water Tank Temperature Analysis

It is clear from Fig. 7 that there is an upper limit on the

water tank temperature, basically no more than 45 ć

which should be considered when it comes to the

application of hot water. Whether the PV/T design is

successful or not mainly depends on the cooling effect that

can be achieved for the solar cells. So low water

temperature desired by cooling and the thermal energy

demand (comparatively high temperature) are a paradox to

some extent. Anyway this problem could be solved when

the PV/T is integrated into a system whose thermal energy

demands of temperature is sufficiently low to insure the

desired cooling effect, like floor heating systems and water

supply of swimming pool, etc..

Day

Water tank temperature

Fig. 7: The highest water tank temperature that can be

attained in a day.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

875

4. CONCLUSIONS

A hybrid PV/T system was successfully constructed by

pasting crystalline silicon solar cells onto a flat-plate heat

collector and conventional PV cell was also installed for

comparison. Experiment was carried out outdoors in May

and June in Beijing and 21 days’ data was collected for

analysis.

A comparison of the PV/T system to a conventional PV cell

showed increased electrical efficiency; the main purpose of

PV/T design and the need of structure optimization made

the inlet temperature and heat transport between PV and

collector become key design points in the future.

The thermal efficiency of PV/T is lower than conventional

heat collector, but this can be compensated by the

advantage of PV/T system that can provide multi-energy

forms and architecture uniform.

The improvement of electrical efficiency is due to the

cooling of the PV cells by low temperature water

circulation. The designed final water temperature should

therefore be seriously considered in the application of hot

water.

5. NOMENCLATURE

n the maximum power point efficiency of the

photovoltaic collector at reference conditions

μ the photovoltaic efficiency temperature coefficient

T the photovoltaic cell temperature (ć)

T the reference temperature (ć)

η thermal efficiency

η electrical efficiency

G total global solar radiation (W/m

)

T inlet water temperature to collector (ć)

T outlet water temperature to collector (ć)

T ambient temperature (ć)

T PV module temperature (ć)

charge current of PV cell to storage battery (A)

V charge voltage of PV cell to storage battery (V)

the PV/T area (m

)

m water mass flow rate (m

/h)

C the water specific heat ( kJ/kg K⋅ )

6. REFERENCES

(1) Duffie, J. A. Beckman, W. A., 1991. Solar Engineering

of Thermal Processes, second ed. John Willey and Sons

Inc., New York, pp. 777, 250-326

(2) E. C. Kern Jr, and M. C. Russell, combined

Photovoltaic & thermal hybrid collector system, Proc.

IEEE Photovoltaic Specialist, Washington DC,

USA, (1978) 1153-1157

(3) Huang, B. J., Lin, T. H., Hung, W. C., Sun, F. S., 2001.

Performance evaluation of solar photovoltaic/thermal

systems. Solar Energy 70, 443-448

(4) P. Raghuraman, Analytical predictions of liquid and air

photovoltaic/thermal flat plate collector performance, J.

Sol. Energy Eng., 103 (1981) 291-298

(5) Yasushi Morita, Toru Fujisawa, and Tatsuo Tani,

Moment Performance of Photovoltaic/Thermal Hybrid

Panel (Numerical Analysis and Exergetic Evaluation).

Electrical Engineering in Japan, Vol. 133, No. 2, 2000

(6) H. A. Zondag, D. W. de Vries, W. G. J. van Helden, et al.

The yield of different combined PV-thermal collector

designs. Solar Energy, 2003 (74): 253-269

(7) S. A. Kalogirou, Y. Tripanagnostopoulos. Hybrid PV/T

solar systems for domestic hot water and electricity

production. Energy Conversion and Management 47

(2006) 3368-3382

POTENTIAL APPLICATION OF COMBISYSTEM FOR AN AUSTRALIAN

CLIMATIC REGION

E. Halawa

F. Bruno

W. Saman

Sustainable Energy Centre - University of South Australia

Mawson Lakes SA 5095 - Mawson Lakes – Adelaide, Australia

edward.halawa@unisa.edu.au,

frank.bruno@unisa.edu.au,

wasim.saman@unisa.edu.au

ABSTRACT

Australian climate is characterised by its more sunny days

with mild winter condition compared to Europe. Solar

water heating systems have been in the market for many

years to provide domestic hot water to main cities in

Australia. The market for these systems have been

constantly growing and relevant Australian Standards have

been developed. This trend is expected to remain in the

near future. The paper reports on the results of a recent

numerical study on the potential application of a solar

combisystem to provide both space heating and domestic

hot water (DHW) for a two storey house in Adelaide,

Australia. The house space heating requirement was

estimated using AccuRate, a building energy rating

software developed in Australia, whilst the domestic hot

water requirement is based on the Australian Standard AS

4234. The system modelling was carried out using the

TRNSYS simulation package. In the study, the thermal

performance of flat plate and evacuated type collectors are

compared. A number of factors such as effects of tank size,

collector slopes and system configuration are investigated.

1. INTRODUCTION

In Australia, space heating represents about 38% and

domestic water heating about 27 % of the energy demand

of residential buildings. [AGO, 1999].

Solar water heating is a well-established technology. In

1999/2000 the number of solar hot water heaters installed

in domestic sector in Australia reached 36000 units,

consisting of 32000 electric boosted systems and 6000 gas

boosted systems (Ellis & Associates, 2001). In Australia

more market penetration of this technology is expected due

to improved public environmental awareness and

government’s financial inducements (Lee et al., 2003).

Solar space heating systems, on the other hand, have not

gained market acceptance due mainly to high initial cost. In

addition, only a small amount of total annual heat collected

by the system is eventually utilised due to the short period

of the heating season throughout the year.

Combisystems - which provide both space heating and

domestic hot water - have been introduced in homes in a

number of European countries. About one quarter of solar

collectors installed in central Europe in 2001 are used for

combisystems (Frei, 2003). The total area of collectors

installed in 2001 in Sweden was larger for combysystems

than for DHW (Weiss, 2003).

The combisystem’s appeal in Europe is largely due to the

fact that residential space heating requirements are much

higher (up to 3 – 5 times) than for DHW, especially in

middle and northern Europe (Gajvert & Fiedler, 2003).

Australian conditions are different. As mentioned earlier,

Australian heating energy requirements are only slightly

above DHW requirements. Also, compared to Europe, the

majority of Australian regions enjoys relatively large

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

877

number of sunny days throughout the year even in regions /

cities where heating energy requirements are dominant such

as Adelaide (South Australia) and Melbourne (Victoria).

This paper presents the results of a study on the potential

application of a solar combisystem to provide both space

heating and DHW for an environmentally friendly two

storey house in Adelaide, Australia.

2. SYSTEM DESCRIPTION

Fig. 1 shows the schematic of the system modelled. The

system consists of a collector array, a hot water storage

tank and gas post-boost system in the pipe outlets, piping

for hot water and cold water from the mains and the two

loads.

Fig. 1: Schematic of the Proposed Solar CombiSystem.

Energy is delivered to the house through a water-to-air load

heat exchanger, namely a fan coil unit.

Based on the local solar hot water manufacturer’s advice

and on recent technology developments in Europe (Weiss,

2003), it was decided to serve both loads using a single

stratified storage tank. The water heated by the solar

collector is stored in the tank and is delivered as hot water

at the temperature of 50ºC. If solar energy heats the upper

portion of the tank above 50ºC then a bypass system takes

mains water and mixes it with the hot water to deliver it at

50ºC.

The gas post boost system maintains the temperature of

water entering the load heat exchanger at 60ºC. The boost

system is located in the pipe outlets and is of sufficient

capacity that it can supply all of the domestic hot water and

space heating energy requirements.

Water at 60ºC or higher is removed from the top of the tank,

circulated through a water-to-air load heat exchanger (LHX)

and returned to the tank. The flow rate of the water is

controlled so that it exactly meets the house heating load.

Since the heating load profile was provided (see Section 4),

no building modelling was incorporated. However, the set

room point temperature is assumed 21ºC and LHX heats

the air to 35ºC. This data is used to estimate the water flow

rate to heat the space.

The system simulation was carried out using the

TRNSYS 16 simulation package (TRNSYS 16, 2004).

3. HOUSE DESCRIPTION

The house has two storeys with a total floor area of 213 m

(first floor: 140 m

and second floor: 73 m

). It has reverse

brick veneer external walls insulated with R2.5 rockwool

batts with reflective foil The roof/ceiling of the house is

insulated with R4 rockwool batts.

The windows of the house are double glazed 4 mm clear

with 12 mm argon filled and low E film. It has solid

construction external doors.

All these and other detailed house specifications were input

to the AccuRate software to estimate the hourly energy

requirements of the house for a one year period. Adelaide

weather data was used in the simulation.

4. HEATING LOAD PROFILES

The monthly figures for DHW loads were derived from

Australian Standard 4234-1999 which provides seasonal

(monthly) load profile, hourly load profile and monthly

cold water temperatures.

Monthly space heating (SH) loads were estimated using an

Australian building energy rating software, AccuRate

(Hearne website, 2006). This software was used to model

the house because it has a built-in library which contains

local manufacturers’ data for various building materials

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

878

(walls, windows, etc) relevant to Australian conditions.

Fig. 2 shows monthly profiles of DHW and SH loads.

500

1000

1500

2000

2500

3000

3500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Load, MJ

DHW SH TOTAL

Fig. 2: Monthly DHW and SH load profiles.

Annual figures for each load type are: 12,546 MJ (DHW)

and 6,010 MJ (SH). Although annual SH load is less than

annual DHW load, on the monthly basis the SH load

represents the larger portion of peak winter loads of June –

August. The heating load for the months from April to

September represents 68.2% of the annual heating load

compared to 31.8% for summer months.

These two factors significantly affect the thermal

performance of the proposed combisystem.

5. COLLECTORS AND TANK SPECIFICATIONS

In the study, two types of collectors were studied and their

performances were compared. The first collector is of flat

plate (FP) type having the following instantaneous

efficiency curve coefficients: a

= 0.74, a

= 14.23

kJ/hgm

K, and a

= 0.056016 kJ/hgm

The second collector is of an evacuated tube (ET) type with

the following instantaneous efficiency curve coefficients: a

= 0.717, a

= 5.472 kJ/hgm

K, and a

= 0.0306 kJ/hgm

The system has the following values of incident angle

modifier (IAM):

ȱ 0 10 20 30 40 50 60 70 80 90

K 1.0 1.0 0.99 0.98 0.96 0.93 0.87 0.74 0.38 0

K 1.0 1.02 1.08 1.18 1.37 1.4 1.34 1.24 0.95 0

6. RESULTS AND DISCUSSIONS

Figures 3-5 show the simulation results for both systems

with different collector areas and slopes attached with a 315

L, 450 L or 630 L tank. The graphical presentation of the

results is such that one can easily locate a point in each

curve of each figure which gives a certain value of the

annual solar fraction.

Fig. 3: Annual Solar Fraction vs. Collector Area – 315 L

storage tank.

Fig. 4: Annual Solar Fraction vs. Collector Area – 450 L

storage tank.

Fig. 5: Annual Solar Fraction vs. Collector Area – 630 L

storage tank.

For example, an annual fraction of 65% can be produced by

the FP system attached to a 315 L tank and tilted at 25º

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

879

with a collector area about 15 m

(see the top black circle

on the vertical red line, Figure 3). The same solar fraction

can be produced if the system is tilted 45º or 60º with 11 m

collector area.

Using the ET system, the same solar fraction can be

attained at a much reduced solar collector area at different

collector slopes.

In Fig. 4, the system with a 450 L tank, a solar fraction of

70% can be attained by the FP system tilted at 25º with a

collector area of nearly 18 m

; or, if tilted at 45 or 60º, the

same fraction can be attained with 12 m

collector area.

Using the ET system, the same solar fraction can be

attained with a collector area of about 8 – 9 m

at collector

slopes of 25 – 60º.

In Fig. 5, the system with a 630 L tank, a solar fraction of

75% can be attained by the FP system tilted at 25º with a

collector area of about 21 m

. If tilted at 45 or 60º, the same

fraction can be attained with a collector area of about 15 m

Using the ET system, the same solar fraction can be

attained with a collector area of about 9 – 11 m

at collector

slopes of 25 – 60ºC.

Effect of tank volume

From Figures 3 - 5 we can also see the effect of tank

volume on the performance of a system with a given

specification. For example, the points marked with a cross

star represent the FP system with a collector area of 14 m

tilted at either 45 or 60º. As shown, an increase in tank

volume from 315 L to 630 L increases the solar fraction

from about 67.5% to about 74%. However at higher tank

volumes (630 – 945) this effect is marginal (the results for

tank volumes above 630 are not shown).

Effect of Collector Slope

Four collector slopes were simulated to see how it affects

the systems’ performance. The 25º slope represents the

conventional roof pitch in South Australia, the 45º and 60º

represent the favourable slope angle in terms of solar gain

during winter. The 90º slope (vertical position) makes it

possible to position the solar system in the north wall of the

building.

As shown, for the FP system, 45º and 60º slopes give the

best solar fractions for fixed solar collector areas and tank

volumes. For the ET system, the 45º slope gives the highest

solar fraction for a fixed collector area and tank volume.

For both systems, the vertical position (90º) should be

avoided as it results in poor solar fractions.

Monthly System Performance

Figure 6 shows the monthly solar fraction of a system with

12 m

collector area mounted with 45º slope facing north

with a 450 L tank. The figure represents the trend of the

system’s thermal performance on a monthly basis in

general for both FP and ET systems.

As shown, the systems are able to provide most of the heat

energy requirements during summer, spring and autumn

and about 50% (FP) and 60% (ET) of the heating energy

requirements during the peak winter months of June –

August. The latter is mainly for winter space heating load.

The figure also shows a slightly better performance of the

ET system during the winter months compared with the FP

system.

0.0

0.2

0.4

0.6

0.8

1.0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Annual Solar Fraction

Fig. 6: Monthly Solar Fraction for A

= 12 m

, tank = 450

L, Slope = 45º.

It is worth comparing the results from the present study

with that from Europe. According to Charron et al. (2007),

small systems in the Netherlands consists of 4 to 6 m

collector with 0.3 m

storage tank. In Switzerland, Austria

and Sweden larger systems (with a solar fraction of 20 to

60%) typically use 15 – 30 m

of collector area with 1 – 3

thermal storage tank. The more favorable climatic

conditions for Australia make it possible to have smaller

systems with similar outputs.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

880

7. CONCLUSIONS

Thermal performance analysis of solar combisystem with a

flat plate solar water heating system and the solar

evacuated tube collector system has been carried out.

Following is the summary of the findings:

 As expected, the thermal performance of the

evacuated tube system is significantly better than the

flat plate system mainly due to the way the collector

surface absorbs the solar radiation and reduces heat

losses due to vacuum tube covering the absorber. This

results in a reduced collector area required to attain a

certain solar fraction.

 Both systems seem to have optimum performance when

tilted at 45º at different collector areas and tank volume.

At this slope, a FP system with 450 L tank with

collector area of 12 m

give the solar fraction of 70%.

The ET system attains the same solar fraction at a

collector area of 8 m

The analysis reported here deals only with theoretical

performance of both systems using technical specifications

available from both systems. The analysis does not deal

with long term performance, technical reliability and the

cost of each of the systems. The continuation of this work

involves developing combined systems for the provision of

hot water, heating and cooling to improve the year round

utilization of the solar heat collected and thus improve the

economic feasibility of the system.

8. NOMENCLATURE

= collector intercept efficiency

= negative of first order efficieny coefficient, kJ/hgm

= negative of second order efficieny coefficient,

kJ/hgm

= collector area, m

= incident angle modifier, longitudinal direction

= incident angle modifier, transversal direction

9. REFERENCES

(1) Australian Greenhouse Office, Australian Residential

Building Sector – Greenhouse Gas Emissions 1990 –

2010 – Executive Summary Report, 1999.

(2) Australian Standard AS 4234-1994: Solar Water

Heaters – Domestic and Heat Pump – Calculations of

Energy Consumption.

(3) H. Gajbert & F. Fiedler, 2003: Solar combisystems – A

State of the Art Report – PhD Course on Solar Heating

– Dipl. Civil Engineering, Technical University of

Denmark (DTU), July 2003.

(4) Hearne Website - http://www.hearne.co.nz/ -

Residential Building Thermal Performance Assessment

(2006) – viewed 20 June 2007.

(5) M. Ellis. & Associates, Analysis of Potential for

Minimum Energy Performance Standards for

Miscellaneous Water Heaters – Prepared for the

Australian Greenhouse Office – Final Report.

(6) R. Charoon, A. Athienitis, M. Passini, Integrated

Design and Optimization for Zero Energy Solar Homes,

www.nrcan.gc.ca – viewed 4 July 2007.

(7) T. Lee., D. Ferrari, E. Donnelly,: Active Solar Space

Heating for a Cool Temperate Climate: A Case Study,

ANZES Solar Energy Society Conference, 2003,

Melbourne.

(8) TRNSYS 16 Reference Manual, 2004.

(9) U. Frei, Solar Thermal Collectors – State of the Art and

Further Development. ISES Solar World Congress

2003, Göteborg, Sweden.

(10) W. Weiss, Solar Heating Systems – Status and Recent

Develpments. ISES Solar World Congress 2003,

Göteborg, Sweden.

DESIGN OF A CONTROL SYSTEM USED TO SIMULATE THE DAILY VARIATION OF

SOLAR RADIATION USING ELECTRICAL POWER

Ashmore Mawire

Department of Physics and Electronics

North West University

Mafikeng Campus

Private Bag X2046

Mmabatho 2735

South Africa

ashmawire02@yahoo.co.uk

Michael McPherson

Department of Physics and Electronics

North West University

Mafikeng Campus

Private Bag X2046

Mmabatho 2735

South Africa

ABSTRACT

In order to simulate the daily variation of solar radiation, a

Visual Basic computer program has been designed to

control the power supplied to an electrical hot plate. The

program employed a combined feedforward and feedback

structure. The design can be used as a solar radiation

simulator for the design of solar concentrators and for

characterizing a TES system.

1. INTRODUCTION

The daily variation of solar radiation is of importance in

solar thermal applications which include solar water

heating and solar cooking. Domestic solar water heaters

require a knowledge of the amount of solar radiation for

their efficiency to improve [1-2]. Conventional solar

cookers without heat storage also need a measurement of

the solar energy to characterize and optimize them

accordingly [3-5]. The disadvantages of solar cookers

without thermal energy storage (TES) are that the cook ing

cannot be carried out when the sun is not available, that the

operator is dangerously directly exposed to the sun’s rays

and that the cooking capacity cannot be maximized.

Solar cookers using TES eliminate these drawbacks [6-7].

It is necessary to characterize the TES capability even

without a collector/ concentrator to capture the incident solar

energy. This can be done by using electrical power which

heats up the heat transfer fluid (HTF) in the TES. A

prototype TES and cooking system and its test facility has

been developed [8]. The solar concentrator is being

simulated by an electrical hot plate that is in thermal contact

with a hollow copper spiral coil through which a HTF flows.

Two different solar radiation models to simulate the daily

variation of solar energy have been reported [9-10]. It is

essential to use any one of the models to enable the electrical

power to follow the daily variation of solar radiation.

Simulating the solar radiation with electrical power is also

advantageous in harsh climatic conditions [11] where it is

hard to fully utilize the available solar radiation.

2. SYSTEM DESCRIPTION

2.1 Hardware Setup

The hardware setup for power control is illustrated by the

diagram of Fig. 1. A computer connected to an HP 34970A

data-logger [12] via an RS 232 interface is used to

implement the power control algorithm. Using the

Standards Commands for Programmable Instruments (SCPI)

instructions it is possible to control the functionalities of

the data-logger by utilizing the computer. An analogue

signal that is generated by the digital to analogue converter

Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2

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