Труды Всемирного конгресса Международного общества солнечной энергии

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

782

Fig. 2: Schematic sketch of the energy system before

installation of the solar heating system.

Both the technical unit and the heat storage unit are built

into 60 x 60 cm units by METRO THERM A/S. Due to this

prefabrication, the installation of the system is easy and the

risk of installation mistakes is reduced. The design of the

units provides good operation conditions for the

condensing natural gas boiler and for the solar collectors

and high energy savings for the system.

Figure 3 shows a schematical sketch of the two units as

well as the solar collector loop and space heating system.

Solar Store Unit

Technical Unit

Fig. 3: Schematic sketch of the solar/natural gas heating

system.

Figure 4 shows the house after installation of the solar

heating system.

Fig. 4: Demonstration house with collectors on the roof and

the solar tank and technical unit in the basement.

3. ENERGY MEASUREMENTS

3.1 Monitoring Systems

The space heating demand, the hot water consumption, the

heat loss of the circulation pipe, the natural gas

consumption and the electricity consumption of the natural

gas boiler and the heating system were measured in periods

before and after installation of the solar heating system. Further,

the heat transferred from the natural gas boiler to the hot water

tank was measured before installation of the solar heating

system. Furthermore, the heat produced by the solar collectors,

the heat produced by the boiler as well as the electricity

consumption of the solar heating system were measured in the

period after installation of the solar heating system.

Based on the measurements it is possible to estimate the

energy savings of the solar heating system.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

783

3.2 Measured Energy Quantities

The measurements started in August 2004 and will be

continued until December 2007. Not all the energy

quantities mentioned in section 3.1 were measured during

the whole measuring period. A constant energy quantity of

10.67 kWh per m

natural gas is assumed.

The total heat demand of the house, that is: Space heating

demand + domestic hot water consumption + heat loss from

circulation pipe, and the natural gas consumption month by

month appear from figure 5. It is clear that both the heat

demand and the natural gas consumption is much lower after

the installation of the solar heating system than before. The

reasons for the reduced natural gas consumption are the

decreased heat demand due to warm weather in the autumn

of 2006 and in the start of 2007 and the new energy system

with solar collectors and an improved natural gas boiler.

Further, it is noticed that the difference between the natural

gas consumption and the total heat demand in the winter

months is lower after installation of the solar heating system

than before. In the spring, summer and autumn the natural

gas consumption is lower than the total heating demand after

installation of the solar heating system. Before installation of

the solar heating system the natural gas consumption is for

all months higher than the total heat demand.

Fig. 5: Total heat demand and natural gas consumption for

the house during the measuring period.

Figure 6 shows the measured utilization of the natural gas

as a function of the total heat demand month by month. The

utilization of the natural gas is defined as: (space heating

demand + hot water consumption + heat loss from

circulation pipe)/natural gas consumption. The utilization

of natural gas is strongly increased by the solar heating

system, especially for low heat demands. Further, the

utilization of natural gas after installation of the solar

heating system is much higher in the sunny spring than in

the less sunny autumn.

Fig. 6: Utilization of the natural gas as a function of the

total heat demand.

Based on the total monthly measured heat demand and the

curves of figure 6 it is possible to estimate the natural gas

consumption for the house for all months without and with

the solar heating system, if only the heat demands are

measured. Consequently, the energy savings for the house

achieved by the solar heating system can be determined for

all months, where the total heat demand is measured.

However, so far the utilization curve for the solar heating

system is not covering July, August and September due to a

lack of measurements in these months. Therefore energy

savings for the summer months are not estimated with a

good accuracy.

The yearly natural gas savings for the solar heating system

is for the period May’05-April’06 estimated to 4100 kWh.

This corresponds to a reduction of the natural gas

consumption of 14.4%. Additional electricity savings of 60

kWh are estimated. The total estimated yearly energy

savings are 4200 kWh, corresponding to about 620 kWh

per m² collector. For other periods with durations of one

year the energy savings will be different. For increasing

heat demand the energy savings will increase.

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

784

The measurements will be continued. Therefore the

accuracy of the estimates of the energy savings will be

improved, and the variations of energy savings from one

year to another caused by weather variations and variations

of user habits can be determined.

4. CONCLUSION

Measurements of the heat demand and energy consumption

in a demonstration house before and after installation of a

new developed solar heating/natural gas heating system

based on a condensing natural gas boiler and 6.75 m² solar

collectors have been carried out.

Based on the measurements the energy savings by the solar

heating system were estimated to 4200 kWh per year,

corresponding to 620 kWh/m² collector per year. The

energy savings will vary from year to year. In years with a

high heat demand the energy savings will be high. In years

with a low heat demand the energy savings will be lower

5. REFERENCES

(1) T. Larsson, “Enkätundersökning om energibesparing

och drift med solfångare”, Intern rapport 00:00, Örebro

University, 2000.

(2) A. Thür, L.J. Shah & S. Furbo, “Energy savings for

solar heating systems”, Solar Energy Vol. 80, Issue 11,

pp. 1463-1474, 2006.

(3) S. Furbo, L.J. Shah, C. Holm Christiansen & K. Vinkler

Frederiksen, “Kedeleffektiviteter for oliefyr og natur-

gaskedler i enfamiliehuse”, report R-072. Department

of Civil Engineering, Technical University of Denmark,

2004.

(4) A. Thür, “Compact Solar Combisystem. High

Efficiency by Minimizing Temperatures”, report R-160,

Department of Civil Engineering, Technical University

of Denmark, 2007.

EXPERIMENTAL STUDIES OF SOLAR HEAT PIPE USED TO OPERATE ABSORPTION

CHILLER IN CONDITIONS OF VIETNAM

LE CHI HIEP

Ho Chi Minh City University of Technology

268 Ly Thuong Kiet st., Dist.10, Ho Chi Minh City, Vietnam

lechihiep@gmail.com

HOANG AN QUOC

Ho Chi Minh City University of Technical Education

01 Vo Van Ngan st., Thu Duc Dist, Ho Chi Minh City, Vietnam

hanquoc@hcmute.edu.vn

HOANG DUONG HUNG

Danang University of Technology

54 Nguyen Luong Bang St, Lien Chieu Dist, Danang City, Vietnam

hdhung@ud.edu.vn

ABSTRACT

Several models of solar heat pipe have been fabricated and

tested. The experiments show that the flat plate model

could be used to operate absorption chiller in the climate of

southern part of Vietnam. Two main advantages of the

selected solar heat pipe are low cost and easy fabrication at

local conditions. It is expected that the selected solar heat

pipe could attract attention of the community to develop the

application of solar energy in Vietnam.

Based on the current demand, the paper presents the

experimental studies of the first generation of low cost

solar heat pipe. The paper also discusses the ability of

application of solar air conditioning in Vietnam and

recommends the suitable diagram mixing solar energy with

other heat source to operate stably the system.

INTRODUCTION

Currently, in Vietnam, solar energy is exploited mainly to

supply hot water, besides it is also used to supply people in

remote areas with electricity in small scale. But, because of

hot climate, solar energy is now also evaluated as one of

possible and available energy resources that can be used to

satisfy the air conditioning demand to fulfil the energy

conservation policy of the country.

One of the appropriate systems that can produce the needed

cooling capacity for air conditioning purpose is absorption

chiller which has been manufactured largely in

commercialization scale. Thus, the rising problem now is

the solar device that can be used to supply absorption

chiller with hot water at high enough temperature but at

low enough cost, and its fabrication must not need high

technology and high investment. Obviously, it is not easy to

fulfil at the same time these contradictions.

In order to find out the most appropriate option in

conditions of Vietnam, the experiments have been done on

two models: (i) tubular solar heat pipe and (ii) flat plate

solar heat pipe. Because of difficulties in keeping the

vacuum, tubular solar heat pipe is divided in two options:

evacuated and non-evacuated, whereas flat plate solar heat

pipe is non-evacuated one.

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

786

DESCRIPTION OF EXPERIMENTAL MODELS

2.1

Tubular Solar Heat Pipe

The experimental model consists of 6 heat pipes, each is

put in a 100mm diameter and 2000mm length glass tube.

The working fluid inside the heat pipe is pure water. The

drawing and the construction of the tested tubular solar heat

pipe are shown in Figures 1 and 2, respectively.

2000

?00

Rubber

Selective Absorber

Evaporator

Condenser

Evacuated Tube

Glass Tube

Fig. 1

Fig. 2

The arrangement of measuring devices and the total tested

system are presented in Figures 3, 4 and 5.

TC3

TC4

TC5

Fig. 3

TC1

TC2

TC3

TC4 TC5

Circulating pump

Hot Water

Storagered Tank

Solar collector

Data acquisition

Fig. 4

Fig. 5

2.2 Flat Plate Solar Heat Pipe

The flat plate solar collector fabricated at 1050mm width

and 1650mm length consists of 11 heat pipes. The diameter

and the length of evaporator section (including adiabatic

section) are 13mm and 1500mm, the diameter and the

length of condenser section are 25mm and 100mm,

respectively. The tested flat plate solar heat pipe are shown

in Figues 6 and 7.

1020

7 0

1 650

7 0

2 5

9 1

1 500

100

Fig. 6

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

787

Fig. 7

In all experiments, temperatures are measured by using

thermoelectric couples with 5mm length sensor. The

recorded data are sent immediately to Multi Data

Acquisiton System (20 inputs) which is connected directly

to a computer.

DISCUSSION

Experiments have been done on the two models. The mean

ambient temperature is around 31

C - 33

C and the wind

speed is 1m/s - 3m/s. In order to evaluate the efficiency of

each option, Tables 1 and 2 show the values which are

considered as the mean of recorded data, Figures 8 and 9

present the variation of water temperature on time.

TABLE 1: TUBULAR SOLAR HEAT PIPE

Water temperature at the

middle of the tank (

C )

Time

Non-evacuated Evacuated

Total solar

radiation

(W/m

)

8:00 30 30 370

8:30 50 55 500

9:00 67 72 650

9:30 73 78 720

10:00 79 82 750

10:30 83 88 800

11:00 85 90 850

11:30 89 92 850

12:00 90 93 890

12:30 90 94 890

13:00 89 95 850

13:30 89 95 850

100

Time

Temperature (C

)

Non Evacuated

Evacuated

Fig. 8: The variation of water temperature on time / tubular

solar heat pipe.

TABLE 2: FLAT PLATE SOLAR HEAT PIPE

Time

Water temperature at

the middle of the tank (

C )

Total solar

radiation

(W/m

)

8:00 30 350

8:30 49 450

9:00 67 490

9:30 76 655

10:00 79 780

10:30 85 790

11:00 88 810

11:30 90 850

12:00 92 890

12:30 93 870

13:00 93 820

13:30 94 810

Tables 1, 2 and Figures 8 and 9 lead us to the primary

remark that the efficiency of evacuated tubular solar heat

pipe is higher than that of non-evacuated tubular solar heat

pipe. But, in comparison with flat plate solar heat pipe, the

efficiency of evacuated tubular solar heat pipe is only a

little higher than that of flat plate solar heat pipe. The

experiments show that, in general, evacuated tubular solar

heat pipe can supply hot water at temperature higher than

C from around 9:40AM, whereas non-evacuated tubular

solar heat pipe can supply hot water at high enough

temperature only after 10:30AM. Table 2 also shows that

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

788

flat plate solar heat pipe can supply hot water at 79

C at

around 10:00AM. To make everything clearer, the

efficiency comparison of tested options is presented in

Figure 10.

100

:30

10:00

:30

11:3

:30

13:3

Time

Temperature (C

)

Fig. 9: The variation of water temperature on time/flat plate

solar heat pipe.

0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,1

(T-Ta)/I

Collector efficency (%

Heat pipe Evacuated tubular

collector

Heat pipe non-evacuated

tubular collector

Flate Heat

e collector

Fig. 10: The efficiency comparison.

Based on the primary tested data, the experiments on flat

plate solar heat pipe have been done. In this case, the

collector was connected with a 40litre tank, the results

show that - from 9AM to 2PM - the mean increase of water

temperature is around 30

C - 85

C/unit, the mean collector

efficiency is around 42.5%.

CONCLUSIONS

Due to difficulties in keeping the vacuum, the flat plate

solar heat pipe should be selected to supply absorption

chiller with hot water. Main advantages of flat plate solar

heat pipe are low cost and easy fabrication, that means flat

plate solar heat pipe can be fabricated at any in-country

small workshop. This option suits the current situation in

Vietnam where there has not been yet any industry

referring to solar collector.

The experiments show that it is possible to supply small

absorption chiller with hot water produced by flat plate

solar heat pipe. In order to operate stably and to reduce the

solar collector surface, solar absorption chiller system

should include additional heat source.

It is determined that solar energy itself can operate the

system from 10AM to 2PM, before 10AM and after 2PM the

system can be operated by mixing solar energy and

additional heat source. Besides cooling, hot water supply for

household purposes is also considered as another function of

the system as well as of the selected solar heat pipe.

The total cost of the tested unit (W h L = 1050mm h

1650mm) is around 230USD. It is expected that the total

cost of the flat plate solar heat pipe could be reduced

significantly whenever solar collector is manufactured

largely in commercialization scale.

REFERENCES

(1) Jorge Facao and Armando C. Oliveira, “Simulation of

The Thermal Behaviour of A Hybrid Heat Pipe Solar

Collector”, 1

International Conference on Sustainable

Energy Technologies, 12-14 June 2002, Porto, Portugal

(2) G.Oliveti, N.Arcuri, “Solar radiation utilisability

method in heat pipe panels”, Solar Energy 1996,

Vol.57, No.5, pp.345-360

(3) Hee-Youl Kwak, Chang-Yong Choi, “Long term

thermal performance of evacuated tubular solar

collector system for industrial process heat in Korea”,

Korea Institute of Energy research, Korea

(4) Li, Z.F. and K. Sumathy, “Experimental Studies on a

Solar Powered Air Conditioning System with

Partitioned Hot Water Storage Tank,” Solar Energy, Vol.

71, No. 5, 2001, 285-297

EVALUATION OF THE THERMAL PERFORMANCE OF A SOLAR WATER HEATING

THERMOSYPHON VERSUS A TWO-PHASE CLOSED THERMOSYPHON USING

DIFFERENT WORKING FLUIDS

A. Ordaz-Flores

Posgrado en Ingeniería (Energía),

Universidad Nacional Autónoma de México

Privada Xochicalco s/n

Temixco, Morelos, 62580, México

O. García-Valladares, V. H. Gómez

Centro de Investigación en Energía,

Universidad Nacional Autónoma de México

Privada Xochicalco s/n

Temixco, Morelos, 62580, México

ABSTRACT

A water heating closed two-phase thermosyphon solar

system was designed and built. The system consists of a

flat plate solar collector coupled to a thermotank by a

continuous copper tubing in which the working fluid

circulates. The working fluid evaporates in the collector

and condensates in the thermotank transferring its latent

heat to the water through a coil heat exchanger. The tested

fluids are acetone and R134a. The thermal performance of

the proposed systems is compared with a conventional solar

water thermosyphon under the same operating conditions.

Advantages of a two-phase system include the elimination

of freezing, fouling, scaling and corrosion. Geometry and

construction materials are the same except for the closed

circuit presented in the two-phase system. Data were

collected from temperature and pressure sensors throughout

the two systems. Early results suggest that R134a may

provide a better performance than acetone for this kind of

systems.

1. INTRODUCTION

The increasing interest of preserving the non renewable

resources has led to focus on sustainable growing, based

mainly on using renewable energy. Renewable energy

sources are the Sun, biomass, hydrogen, wind, etc. The Sun

leads to thermosolar and photovoltaic technologies, mainly.

The current work has special interest in the thermosolar

technology, particularly in the Domestic Water Heating

Systems (DWHS).

Solar domestic water heating systems permit to diminish

the consumption of liquid gas and electricity, helping to

reduce the quantity of pollutants expelled to the atmosphere.

In 2004, Kalogirou [1] studied the environmental impact of

energy utilization and the potential benefits to swap

conventional for solar assisted systems. He estimated that,

for the case of solar water heating, one of the two most

widely used renewable energy, the savings would reach up

to 80%. Hence, the importance of solar water heating.

The most common currently available solar equipments are

the thermosyphons in which the water is heated in a flat

plate solar collector and storaged in a thermotank.

Problems like freezing, corrosion, scaling and fouling

presented in this kind of systems are eliminated in

two-phase systems [2].

In 1979, Soin et al. [2] described an experimental set up to

evaluate the performance of a solar collector with a phase

change working fluid. They used acetone and petroleum

ether as working fluids, because of their high boiling and

condensation heat transfer coefficient. They demonstrated

that the collector efficiency increases linearly with liquid

level.

A refrigerant, trichlorofluoromethane, was used by

Schreyer in 1981 [3] to evaluate the energy recovery in a

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

790

solar collector coupled to a heat exchanger, and the latter to

a storage tank. The primary loop was passive and the

secondary needed a recirculation pump. His system

recovered up to 83% energy at low collector temperature

difference.

Evaluation of R134a, among others, as replacing working

fluids of ozone depletion promoting chlorofluorocarbons,

was made by Calm and Didion [4]. They concluded that

there is no perfect fluid to prevent every environmental

impact. R134a has a high latent heat of vaporization, does

not contributes to ozone depletion but, yet low, does have

impact on global warming.

Ong and Haider-E-Alahi [5] studied the performance of a

heat pipe filled up with R134a, and found that the heat flux

transferred increased with high refrigerant flow rates, high

fill ratios and greater temperature difference between bath

and condenser.

More recently, Hussein [6,7] studied a two-phase closed

thermosyphon with the heat exchanger (condenser) in the

solar collector, however, he did not mention the working

fluid used. He carried out both experimental and numerical

tests and set some dimensionless variables to determine

adequate storage dimensions for the tank to improve the

solar energy gain.

Finally, a thermosyphon heat-pipe solar collector was

studied by Esen and Esen [8] in 2005 to evaluate its

thermal performance using three different working fluids,

R134a, R407C and R410A. They found that the latter

offered the highest solar energy collection.

In this work, R134a was chosen due to its availability, low

cost and small impact to environment. On the other hand,

acetone was chosen as another working fluid because it

avoids the high pressures reached with the former.

2. EXPERIMENT

A water heating two-phase closed thermosyphon and a

conventional natural thermosyphon are compared

simultaneously. The two-phase system consists of a flat

plate solar collector coupled to a thermotank by a copper

tubing circuit in which the working fluid circulates. A

scheme of the systems is shown in Figure 1.

Fig. 1: Scheme of the two-phase closed thermosyphon and

the conventional thermosyphon.

Focusing on the fluid refrigerant behaviour, the solar

collector is the evaporator of the system and the copper coil

immersed in the thermotank is the condenser. The incoming

solar radiation makes the temperature of the refrigerant in

the collector to grow higher to reach the saturation liquid

state. From this point, the working fluid starts to evaporate

to reach the saturated vapour state and even the superheated

vapour zone. As the refrigerant has a higher temperature

than the water, the former donates its phase change latent

heat to the latter and leaves the thermotank as slightly

subcooled liquid to come back to the solar collector to

repeat the cycle.

Both systems have the same geometry, except for the coil

presented in the two-phase system. Each collector has an

absorption area of 1.62 m

and the volume capacity of each

thermotank is 160 l.

Refrigerant R134a and acetone were used as working fluids

in the two-phase system. Refrigerant R134a is one of the

replacing working fluids of chlorofluorocarbons since it

does not contribute to ozone depletion. At atmospheric

pressure, R134a evaporates at −26.1°C [9] and acetone at

56.05°C [10]. At their respective boiling points, their

enthalpies of vaporization are 216.98 kJ/kg for R134a and

501.03 kJ/kg for acetone. The combination of boiling point

temperature (the lowest, the best) and heat of vaporization

(the highest, the best) will show which of the fluids is more

suitable for these operating conditions. In early tests, the

two-phase system was loaded up to 78% when operating

with R134a and up to 63% when operating with acetone.

To carry out the experimental characterisation, it was

supposed that the thermotank is filled up with water during

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

791

the morning, with neither load nor any human intervention

until the next day in the morning when the whole water is

removed from the thermotank and filled up back again for

the next day of test.

The stratifications profile in each thermotank is monitored

by three sensors placed at different altitudes; this

arrangement permits to determine both the final

temperature reached in the water and the efficiency. The

temperatures and pressures of the working fluid in the

circuit are important to establish the state of the fluid

throughout the equipment, as well as the maximum

pressures of operation in the days of tests. An ambient

temperature sensor was used to monitor this parameter,

which is particularly important at night to estimate the

thermal losses during the night period. Finally, a spectral

pyranometer was used to monitor the incoming solar

radiation on the collector plane. A schema of the set of

sensors for the two-phase system is shown in Figure 2.

Fig. 2: Scheme of the set of sensors for the two-phase

system.

The test was carried out in the Solar Platform of the Centro

de Investigación en Energía of the Universidad Nacional

Autónoma de México, located in Temixco, Morelos State,

México, at 18º50.36’ N latitude and at 99º14.07’ W

longitude, with an altitude of 1219 m osl. The yearly

average ambient temperature is 23.09 ºC with a yearly

average solar irradiance on the horizontal plane of 20.28

MJ/m

. The tests began from the late days of May on until

the late days of June of 2007.

A data acquisitor was used to monitor and register the data

from the set of sensors. The typical one-day-long tests

began at 9:00 h and ended at 8:30 h of the next morning.

After 9:00 h, there was no human intervention during the

performance of the equipment. Daily, at 18:00 h, the data

was collected to establish the energy obtained from the Sun

during the radiation period and calculate the efficiency,

useful heat and the increase of water temperature in the

thermotank.

3. RESULTS

Figure 3 shows the radiation and the stratification profile of

the water temperatures of the experimental systems for one

day of test. The morning and the early afternoon were

sunny until around 3:00 pm with an average solar radiation

on the collector plane of 546.44 W/m

. The cloudy period

seems to diminish the gain of heat in the conventional

system more than in the phase change system.

Fig. 3: Radiation and stratification profile for one day of

test.

The efficiency and useful heat of the system are calculated

from the equations:

/ 100qqη ()=×

(1)

where

qIA tΔ()( )( )= (2)

and

qm c TΔ()( )( )= (3)

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

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