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 công nghệ toàn quốc 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 nhutlm@hcmute.edu.vn 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 lượng mặt trời hệ thống nhiệt lượng mặt trời có diện tích thu ống chân không 26m2 đặt đảo Jeju Hàn Quốc Lưu lượng nước vòng lặp thu lượng mặt trời sưởi ấm không đổi có giá trị cài đặt 6l/min 16l/min Thí nghiệm thực ba ngày khác gồm ngày quang mây, ngày có mây ngày mây mù hoàn toàn nhằm đánh giá ảnh hưởng thời tiết đến hệ số lượng mặt trời đóng góp lượng mặt trời cho sưởi ấm gia nhiệt nước nóng Kết thí nghiệm cho thấy hệ số lượng mặt trời đạt ba ngày thí nghiệm 43.2%, 17.3% 0% Từ khóa: hiệu suất nhiệt, thu ống chân không, lưu lượng, hệ số lượng mặt trời, điều kiện thời tiết INTRODUCTION The demand for energy is increasing across the globe, resulting in the depletion of fossil fuel resources, the increase of CO , 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 643 Kỷ yếu hội nghị khoa học công nghệ toàn quốc 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 heat pump, in which the CO heat pump is used as an auxiliary heater The study was conducted on the representative range of outside temperature from − oC to 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 heat pump increased from 2.17 to 2.49 when the outside temperature varied from − oC to 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 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 644 Kỷ yếu hội nghị khoa học công nghệ toàn quốc 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 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 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 shows the characteristic on energy saving and consumption in case of a fair day The detail was given as in Table 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% 645 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 Solar radiation [W/m2] Temperature, Flow rate [oC, l/min] Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV 25000 20000 15000 10000 100 5000 0 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 16 12 Time[hour] 20 24 Q, kcal 120 110 100 90 80 70 60 50 40 30 20 10 -10 Solar radiation [W/m2] Temperature, Flow rate [oC, l/min] 100000 50000 40000 30000 20000 10000 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 646 17.3 Kỷ yếu hội nghị khoa học công nghệ toàn quốc khí - Lần thứ IV Figure 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 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 shows the characteristic on energy saving and consumption in case of a fair day The detail was given as in Table 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 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 shows the characteristic on energy saving and consumption in case of an overcast sky day The detail was given as in Table 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 -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 16 12 Time[hour] 20 80000 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 647 Kỷ yếu hội nghị khoa học công nghệ toàn quốc 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 Boiler 188.23 161,840 Domestic hot water supply 10.01 8,612 Heat supply for space heating 163.5 140,542 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: nhutlm@hcmute.edu.vn; nhutlm@jejunu.ac.kr 648 ... 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... 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. .. 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