EVALUATING THE EFFECT OFWEATHER CONDITIONS ON THE SOLAR FRACTION OF SOLAR ASSISTED HEATING SYSTEM ĐÁNH GIÁ ẢNH HƯỞNG của điều KIỆN THỜI TIẾT đến hệ số NĂNG LƯỢNG mặt TRỜI của hệ THỐNG NHIỆT NĂNG LƯỢNG mặt TRỜI

Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV

EVALUATING THE EFFECT OF WEATHER CONDITIONS ON THE
SOLAR FRACTION OF SOLAR ASSISTED HEATING SYSTEM
ĐÁNH GIÁ ẢNH HƯỞNG CỦA ĐIỀU KIỆN THỜI TIẾT ĐẾN HỆ SỐ NĂNG
LƯỢNG MẶT TRỜI CỦA HỆ THỐNG NHIỆT NĂNG LƯỢNG MẶT TRỜI
Le Minh Nhut
Ho Chi Minh City University of Technology and Education, Vietnam


ABSTRACT
The solar fraction of a solar assisted heating system with 26 m2 evacuated tube
collectors in Jeju Island, South Korea is presented in this paper. The set values of constant
water mass flow rate in the collector loop and heating panel loop are 6l/min and 16l/min,
respectively. The experiments were carried out on three different days such as fair day,
intermittent cloud sky day and overcast sky day to evaluate the effect of weather conditions
on the solar fraction, as well as the contribution of total useful heat gain of solar collectors for
domestic hot water production and space heating. The experimental results shown that the
solar fractions of fair day, intermittent cloud sky day and overcast sky day are 43.2%, 17.3%
and 0%, respectively.
Keywords: thermal performance, evacuated tube collectors, flow, solar fraction,
weather conditions.

TÓM TẮT
Bài báo trình bày hệ số năng lượng mặt trời của hệ thống nhiệt năng lượng mặt trời có
diện tích bộ thu ống chân không là 26m2 đặt tại đảo Jeju của Hàn Quốc. Lưu lượng nước
trong vòng lặp của bộ thu năng lượng mặt trời và sưởi ấm là không đổi và có giá trị cài đặt lần
lượt là 6l/min và 16l/min. Thí nghiệm được thực hiện trong ba ngày khác nhau gồm ngày
quang mây, ngày có mây và ngày mây mù hoàn toàn nhằm đánh giá sự ảnh hưởng của thời
tiết đến hệ số năng lượng mặt trời và sự đóng góp của năng lượng mặt trời cho sưởi ấm và gia
nhiệt nước nóng. Kết quả thí nghiệm cho thấy hệ số năng lượng mặt trời đạt được trong ba

ngày thí nghiệm lần lượt là 43.2%, 17.3% và 0%.
Từ khóa: hiệu suất nhiệt, bộ thu ống chân không, lưu lượng, hệ số năng lượng mặt trời,
điều kiện thời tiết.

1. INTRODUCTION
The demand for energy is increasing across the globe, resulting in the depletion of fossil
fuel resources, the increase of CO 2 , SO x , and NO x emissions to the atmosphere, and an
increase in energy expenditures for countries importing fossil fuel. For these reasons, many
governments have decided to strengthen their national efforts to increase the utilization of
renewable energy sources. Especially, research on solar energy has concentrated on solar
thermal systems for space heating, cooling, and water heating; most of the attention has been
focused on solar assisted heating systems, which have been well developed in many countries
for many years [1-2]. The effects of various parameters such as solar collector area, initial
water temperature, and volume of storage tank on the thermal performance of solar assisted
heating system were analyzed by Nhut and Park [3]. Jordan [4] presented the influence of the
domestic hot water load profiles on the fractional energy saving of a solar combisystem. The
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Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV
analysis results show that, the highest values of the electric energy demand of pumps for
draw-offs are in the early afternoon while the differences of the electric energy demand of
pumps for morning and noon profiles differ by about 1%-point. Groenhout et al [5] developed
a novel design for a solar domestic hot water heating system. The system performance was
evaluated according to the heat loss characteristics of the flat plate collectors. These authors
concluded that the overall heat transfer coefficients of an advanced solar collector are 30-70%
lower than from conventional flat plate designs. The operation of vacuum solar collectors
connected to warm water storage tank was also investigated by Goergiev [6]. The outlet
temperature and inlet temperature of solar collectors as well as the water temperature in the
storage tank were compared and evaluated based on the calculated and experimental data.

Deng et al [7] proposed a solar assisted heating system with a CO 2 heat pump, in which the
CO 2 heat pump is used as an auxiliary heater. The study was conducted on the representative
range of outside temperature from − 5 oC to 5 oC. The domestic hot water demand referred to
occur just at three time points: 7:00 am, 12.00 am, and 8:00 pm while the space heating
demand is a lumped parameter that depends on the heat losses of the building, occupants’
behavior, and so on. Authors concluded that, for an application with floor heating, the
COP heating of the CO 2 heat pump increased from 2.17 to 2.49 when the outside temperature
varied from − 5 oC to 5 oC. The study results of the optimized system also indicated that the
average heating COP for the entire heating season is 2.38 and solar fraction is 69.0%. A
model to determine the performance of a combined solar thermal heat pump hot water system
was also developed by Panars [8]. The experimental results shown that the amount of
auxiliary energy saving on annual basis can reach to 70% for the climate data of Athens.
In this study, a solar assisted heating system for residential house is developed to
evaluate the effect of under real weather conditions at Jeju Island, South Korea on the solar
fraction, as well as the contribution of total useful heat gain of solar collectors for domestic
hot water production and space heating.
2. SYSTEM DESCRIPTION AND EXPERIMENTAL SETUP
A schematic diagram of the solar assisted heating system for residential house is shown
in Fig.1. The system is designed for installation on the roof of a residential building at Jeju
Island in South Korea. It consists of solar collectors, a water storage tank, a boiler, panels for
heating, and a personal computer for data acquisition.
The operation of the system can be described as follows. For the collector loop, when
the difference between the outlet temperature of the collector and the bottom water
temperature of the storage tank is higher than the set value of ΔT on , the collector pump is
switched on, and will be switched off if this value is lower than the value of ΔT off . For
domestic hot water, if the outlet temperature does not reach the required temperature (which is
additionally heated by the boiler and supplied to the user), the city water was pre-heated at the
heat exchanger inside the storage tank. For space heating, the hot water in the storage tank at
temperature T s is supplied to spaces through the panels buried in the floor of each room. If the
temperature T s is sufficiently high, the energy is taken from the storage tank; however, if the

temperature T s is lower than the required temperature, the boiler is switched on and hot water
is supplied directly to the panels. In this system, the domestic hot water mass flow rate for a
single family house with four residences was measured by a magneto-hydrodynamic flow
meter (uncertainty is ±0.5%). The global solar irradiance and ambient temperature are
measured by a pyranometer (uncertainty is ±1%) and a thermocouple located behind the solar
collectors (type K, uncertainty is ±0.5%), respectively (Fig.2). The eight K-type
thermocouples are used to measure the inlet temperatures and outlet temperatures of solar
collectors, panels for heating, domestic hot water at the storage tank, and the water
temperature of the storage tank and the ambient temperature.

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Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV
Solar collector

Pyranometer

Solar
collector

Domestic
hot water
Heating

Boiler

Storage
tank

Figure 2: Pyranometer instrument

was installed on the collector
surface of the solar assisted heating
system for residential house

Figure 1: Schematic diagram
of the solar assisted heating
system for residential house

3. EXPERIMENTAL RESULTS AND DISCUSSION
In these experiments, the useful heat gain Q u of the solar collectors is calculated using
the following equation:
Qu = mc p (Tco − Tci )

(1)

where m, c p are the mass flow rate and specific heat coefficient of the water in the
collector loop, respectively. While T co , T ci are the outlet temperature and inlet temperature of
the collectors, respectively.
The solar fraction is often used in order to evaluate the thermal performance of the solar
assisted heating system for residential house, which is the amount of energy provided by the
solar energy divided by the total energy is supplied by the solar system and the auxiliary
boiler. The experimental results of this research are shown as follows.
Figure 3 shows the operation of the solar collectors in case of a fair day. The collector
pump continuously operated from 9:15 a.m until about 16:00 p.m, thereby, the collection solar
energy also began at that time. When the collector pump operated, the outlet temperature of
the solar collectors gradually increased in the morning and then gradually decreased in the
afternoon while the inlet temperature of the solar collectors was continually increased and just
decreased when the collector pump turned off. The outlet temperature of the collector reached

the highest value was 80.3oC at the time of 13:45 p.m. The temperature difference between
the inlet and outlet of collector was approximately 2oC to 15oC. The water temperature in the
thermal storage tank was gradually increased ranges from 9:15 a.m until about 16:00 p.m. The
increase of the water temperature in the thermal storage tank depends on many factors, such
as useful heat gain of solar collector transferred to the thermal storage tank, heat loss of the
thermal storage tank, the heat demand of domestic hot water and space heating.
Figure 4 shows the characteristic on energy saving and consumption in case of a fair
day. The detail was given as in Table 1. The difference between the total heating supply
(consist of useful heat gain of solar collectors and auxiliary heat of boiler) and the total heat
demand (consist of domestic hot water and space heating demand) is 5.87(kWh). This is due
to the heat loss of the pipe and surround environment, the heat to change the internal energy
of the material during the heat transfer process and the rest part is stored in the thermal
storage tank. The solar fraction in case of a fair day is 43.2%.
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50000

800

Outdoor temp
Collector inlet temp
Storage tank temp
Collector outlet temp
Flow rate
Solar radiation

45000

700

40000

600
500
400
300
200

35000
30000

Q, kcal

120
110
100
90
80
70
60
50
40
30
20
10
0

Solar radiation [W/m2]

Temperature, Flow rate [oC, l/min]

Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV

25000
20000
15000
10000

100

5000
0

0
0

4

8

12
16
Time [hour]

20

Heating

24

Boiler

Solar energy Hot water

Figure 4: Characteristic on
energy saving and consumption in
case of a fair day

Figure 3: Operation of solar
collectors in case of a fair

Table 1: Energy saving and consumption with solar energy in case of a fair day
Energy (kWh) Energy (kcal)
Solar fraction (%)
Solar energy

31.35

26,873

Boiler

41.18

35,337

Domestic hot water supply

11.9

10,232

Heat supply for space heating

54.76

47,096

43.2

Flow rate
Collector outlet temp
Storage tank temp
Collector inlet temp
Outdoor temp
Solar radiation

800

90000

700

80000
70000

600
500
400

300
200
100

60000

0
0

4

8

16
12
Time[hour]

20

24

Q, kcal

120
110
100
90
80
70
60

50
40
30
20
10
0
-10

Solar radiation [W/m2]

Temperature, Flow rate [oC, l/min]

100000

50000
40000
30000
20000
10000
0

Heating

Boiler Solar energy Hot water

Figure 6: Characteristic on
energy saving and consumption
in case of an intermittent cloud
sky day

Figure 5: Operation of solar
collectors in case of an intermittent
cloud sky day

Table 2: Energy saving and consumption with solar energy in case of an intermittent
cloud sky day
Energ
Energy
Solar fraction
y(kWh)
(kcal)
(%)
Solar energy

22.06

18,978

Boiler

105.7

90,917

Domestic hot water supply

8.62

7,409

107.34

92,315

Heat supply for space heating

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Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV
Figure 5 shows the operation of the solar collectors in case of an intermittent cloud sky
day. The collector pump operated from 11:43 a.m until about 15:47 p.m but was not
continuously, thereby, the collection solar energy also began at that time. In the period from
12:38 a.m to 13:02 p.m, the collector pump was switched off. This is because the sudden
decreasing of the global solar irradiance on the collector surface led to the outlet temperature
of the collector was reduced less than the water temperature of the thermal storage tank. The
temperature difference between the inlet and outlet of collector was approximately 2 oC to
14.1 oC. The water temperature in the thermal storage tank strongly oscillated. This is due to
the demand of the domestic hot water supply for users and the hot water supply for space
heating is not continuous. Figure 6 shows the characteristic on energy saving and
consumption in case of a fair day. The detail was given as in Table 2. The difference between
the total heating supply (consist of useful heat gain of solar collector and auxiliary heat of
boiler) and the total heat demand (consist of domestic hot water and space heating demand) is
11.8(kWh). This is due to the heat loss of the pipe and surround environment, the heat to
change the internal energy of the material during the heat transfer process and the rest part
was stored in the thermal storage tank. The solar fraction in case of a fair day is 17.3%.
Figure 7 shows the operation of the solar collectors in case of an overcast sky day. The
collector pump was not operated. This is due to the global solar irradiance came to the

collector surface was too low during the day. The inlet temperature and outlet temperature of
the collector closed with the ambient temperature during the day. The water temperature in
the thermal storage tank strongly oscillated. This is due to the demand of the domestic hot
water supply for users and the hot water supply for space heating is not continuous. Figure 8
shows the characteristic on energy saving and consumption in case of an overcast sky day.
The detail was given as in Table 3. The difference between the total heating supply (consist of
useful heat gain of solar collector and auxiliary heat of boiler) and the total heat demand
(consist of domestic hot water and space heating demand) is 14.72(kWh). This is due to the
heat loss of the pipe and surround environment, the heat to change the internal energy of the
material during the heat transfer process and the rest part is stored in the thermal storage tank.
The solar fraction in case of an overcast sky is 0%. The heat demands of domestic hot water
and space heating are covered by the auxiliary energy source.

Temperature, flow rate[oC, l/min]

90
80
70
60
50

800

180000

700

160000

600

140000

500

120000

400

40

300

30
20

200

10

100

0
-10

Q, kcal

Outdoor temp
Collector inlet temp
Storage tank temp

Collector outlet temp
Flow rate
Solar radiation

Solar radiation[W/m2]

100

100000

60000
40000
20000

0
0

4

8

16
12
Time[hour]

20

80000

0

24

Heating

Boiler

Hot water

Figure 8: Characteristic on energy saving
and consumption in case of an overcast
sky day

Figure 7: Operation of solar
collectors in case of an overcast sky
day

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Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV
Table 3: Energy saving and consumption with solar energy in case
of an overcast sky day
Energy (kWh)
Energy (kcal)
Solar fraction (%)
Solar energy

0

0

Boiler

188.23

161,840

Domestic hot water supply

10.01

8,612

Heat supply for space heating

163.5

140,542

0

3. CONCLUSION
The experiment results of this research shown that the values of the solar fraction for the
cases of a fair day, an intermittent cloud sky day and an overcast sky day are 43.2%, 17.26 %
and 0%, respectively. However, the solar fraction of the solar assisted heating system is not
fixed value because of it depends on many factors such as the load, the collection and storage
tank sizes, the operation, and the climate.
To increase the solar fraction, the heat demand for domestic hot water production and
space heating should reduce in the night and increase in the daytime. This is because the heat

demand for the night is covered by the auxiliary energy source (boiler).
REFERENCES
[1] Nhut, L.M., & Park, Y.C., A simulation model for predicting the performance of solar
domestic hot water system. Advanced Materials Research, 2012, Vols. 512-515, p. 230233.
[2] Novo, A.V., Bayon, J.R., & et al., Review of seasonal heat storage in large basins: water
tanks and Grave-water pits. Applied Energy, 2010, Vol. 87, p. 390-397.
[3] Nhut, L.M., & Park, Y.C., A study on automatic optimal operation of a pump for solar
domestic hot water system. Solar Energy, 2013, Vol. 98, p. 448-457.
[4] Park, Y.C., & Nhut, L.M., Performance prediction of a solar hot water system with
change of circulating pump efficiency in solar collectors. International Conference on
Renewable Energies and Power Quality (ICREQ’13), Bilbao(Spain), March 20-22, 2013.
[5] Groenhout, N.K., Behnia, M., & et al., Experimental measurement of heat loss in an
advanced solar collector. Experimental Thermal and Fluid Science, 2002, Vol. 26, p.
131-137.
AUTHOR’S INFORMATION
Le Minh Nhut, Ph.D.
Department of Heat and Refrigeration Technology
Faculty of Vehicle and Energy Engineering
Ho Chi Minh City University of Technology and Education
No.1-Vo Van Ngan St., Thu Duc Dist., Ho Chi Minh City, Vietnam
Mobile: (+84)-978 446 968
Email to: ;

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