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 fract
Trang 1EVALUATING 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
nhutlm@hcmute.edu.vn
ABSTRACT
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 T ẮT
Bài báo trình bày hệ số năng lượng mặt trời của hệ thống nhiệt năng lượng mặt trời có
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
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 CO2, SOx, and NOx 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
Trang 2analysis 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 CO2 heat pump, in which the
CO2 heat pump is used as an auxiliary heater The study was conducted on the representative range of outside temperature from − 5 oC to 5oC 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
behavior, and so on Authors concluded that, for an application with floor heating, the
COPheating of the CO2 heat pump increased from 2.17 to 2.49 when the outside temperature varied from − 5 oC to 5oC 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 ΔTon, the collector pump is switched on, and will be switched off if this value is lower than the value of ΔToff 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 Ts is supplied to spaces through the panels buried in the floor of each room If the temperature Ts is sufficiently high, the energy is taken from the storage tank; however, if the temperature Ts 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
Trang 3
3 EXPERIMENTAL RESULTS AND DISCUSSION
In these experiments, the useful heat gain Qu of the solar collectors is calculated using the following equation:
p
where m, cp are the mass flow rate and specific heat coefficient of the water in the collector loop, respectively While Tco, Tci 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%
Figure 2: Pyranometer instrument was installed on the collector surface of the solar assisted heating system for residential house
Pyranometer Solar collector
Figure 1: Schematic diagram
of the solar assisted heating system for residential house
Storage tank
Heating
Boiler
Domestic
hot water
Solar
collector
Trang 4Table 1: Energy saving and consumption with solar energy in case of a fair day
Energy (kWh) Energy (kcal) Solar fraction (%)
43.2
Table 2: Energy saving and consumption with solar energy in case of an intermittent
cloud sky day
Energ y(kWh)
Energy (kcal)
Solar fraction (%)
17.3
Figure 5: Operation of solar
collectors in case of an intermittent
cloud sky day
-10
0
10
20
30
40
50
60
70
80
90
100
110
120
24 20 16 12 8 4 0
Flow rate Collector outlet temp Storage tank temp Collector inlet temp Outdoor temp
Time[hour]
0 100 200 300 400 500 600 700 800
Solar radiation
Figure 6: Characteristic on energy saving and consumption
in case of an intermittent cloud sky day
0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
Hot water Solar energy Boiler
Heating
0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000
Hot water Solar energy Boiler
Heating
Figure 4: Characteristic on energy saving and consumption in
case of a fair day
0
10
20
30
40
50
60
70
80
90
100
110
120 Outdoor temp
Collector inlet temp
Storage tank temp
Collector outlet temp
Flow rate
24 20 16 12 8 4
0
2 ]
o C
Time [hour]
0 100 200 300 400 500 600 700 800
Solar radiation
Figure 3: Operation of solar
collectors in case of a fair
Trang 5Figure 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
Figure 8: Characteristic on energy saving and consumption in case of an overcast
sky day
0 20000 40000 60000 80000 100000 120000 140000 160000 180000
Hot water Boiler
Heating
Figure 7: Operation of solar
collectors in case of an overcast sky
day
-10
0
10
20
30
40
50
60
70
80
90
100
24 16
12 8 4
2 ]
Outdoor temp
Collector inlet temp
Storage tank temp
Collector outlet temp
Flow rate
o C
Time[hour]
0 100 200 300 400 500 600 700 800
Solar radiation
Trang 6Table 3: Energy saving and consumption with solar energy in case
of an overcast sky day
Energy (kWh) Energy (kcal) Solar fraction (%)
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
230-233
[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: nhutlm@hcmute.edu.vn; nhutlm@jejunu.ac.kr