Scholarly article on topic 'Experimental Performance of Heating System with Building-integrated PVT (BIPVT) Collector'

Experimental Performance of Heating System with Building-integrated PVT (BIPVT) Collector Academic research paper on "Earth and related environmental sciences"

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{"Water type PVT" / "building-integrated PVT" / "heating system" / "thermal efficiency" / "electrical efficiency" / "heating performance"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Jin-Hee Kim, Se-Hyeon Park, Jun-Gu Kang, Jun-Tae Kim

Abstract Photovoltaic-thermal (PVT) collector is a single device that combines a photovoltaic module and a solar thermal collector, producing thermal energy and generating electricity simultaneously. PVT collectors produce more energy per unit surface area than side-by-side PV modules and solar thermal collectors. They can be classified into air-type and water-type collectors according to the thermal medium used for collecting heat in the collector. The water-type PVT collector tends to have better thermal performance than the air-type PVT collector. The thermal energy from the water-type PVT collectors can be used in buildings for hot water and heating. It is important to understand the overall energy performance of building heating systems that work in conjunction with PVT collectors to demonstrate the potential of their building applications. In this paper, the energy performance of a building heating system combined with a water-type PVT collector integrated into the roof of an experimental unit is analyzed.

Academic research paper on topic "Experimental Performance of Heating System with Building-integrated PVT (BIPVT) Collector"

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Energy Procedia 48 (2014) 1374 - 1384

SHC 2013, International Conference on Solar Heating and Cooling for Buildings and Industry

September 23-25, 2013, Freiburg, Germany

Experimental performance of heating system with building-integrated PVT (BIPVT) collector

Jin-Hee Kima, Se-Hyeon Parkb, Jun-Gu Kangb, Jun-Tae Kimc*

aGreen Energy Technology Research Center, Kongju National University, South Korea bGuaduate School, Departments of Architectural Engineering, Kongju National University, South Korea cDepartments of Architectural Engineering, Kongju National University, South Korea

Abstract

Photovoltaic-thermal (PVT) collector is a single device that combines a photovoltaic module and a solar thermal collector, producing thermal energy and generating electricity simultaneously. PVT collectors produce more energy per unit surface area than side-by-side PV modules and solar thermal collectors. They can be classified into air-type and water-type collectors according to the thermal medium used for collecting heat in the collector. The water-type PVT collector tends to have better thermal performance than the air-type PVT collector. The thermal energy from the water-type PVT collectors can be used in buildings for hot water and heating. It is important to understand the overall energy performance of building heating systems that work in conjunction with PVT collectors to demonstrate the potential of their building applications. In this paper, the energy performance of a building heating system combined with a water-type PVT collector integrated into the roof of an experimental unit is analyzed.

© 2014TheAuthors. Published by ElsevierLtd.

Selectionandpeerreview bythe scientificconferencecommitteeofSHC 2013under responsibilityofPSEAG

Keywords : Water type PVT, building-integrated PVT, heating system, thermal efficiency, electrical efficiency, heating performance

1. Introduction

The high temperatures of PV (photovoltaic) modules reduce the efficiency of the PV system to which they belong. In particular, building-integrated photovoltaic (BIPV) systems appear to be more vulnerable to the PV

* Corresponding author. Tel.: +82-41-521-9333; fax: 82-41-521-8653. E-mail address: jtkim@kongju.ac.kr

1876-6102 © 2014 The Authors. Published by Elsevier Ltd.

Selection and peer review by the scientific conference committee of SHC 2013 under responsibility of PSE AG doi:10.1016/j.egypro.2014.02.155

module temperature, as they are attached to building surfaces. The photovoltaic/thermal (PVT) concept offers an opportunity to increase the overall efficiency of the solar device by utilizing heat exhausted from the PV module of the BIPV system. It is well known that PVT systems enhance PV efficiency by PV cooling, which can be achieved by circulating a colder fluid, water or air, at the underside of the PV module. A PVT collector can be classified into the air type or water type according to thermal medium used. The water-type PVT collector has better thermal performance than the air-type PVT collector. The thermal energy from water-type PVT collector can be used in buildings for domestic hot water and space heating.

A considerable amount of research has been conducted regarding the water-PVT collector, which is similar in terms of its manufacturing to a conventional solar thermal collector. Most studies of performance estimations examined various absorber plate types, such as the sheet-and-tube, fully-wetted and box channel types. Various types of water PVT collectors have been suggested, such as a channel type PVT collector [1], a PVT collector with polymer absorbers [2], thermosyphon PVT collectors [3,4,5] and a PVT collector with a sheet-and-tube absorber[6]. Glazed and unglazed PVT collectors were compared by Tripanagnostopoulos et al. [7] and Chow et al. [8,9]. Bergene and Lovvik [10] thoroughly analyzed the electrical and thermal efficiencies of a water-type PVT system and the energy conversion between different factors. In another study [11,12], experimental and theoretical performances were examined with respect to a water-type flat plate PVT collector. Fujisawa and Tani [13] evaluated the effective energy of a PVT collector depending on the presence of a glass cover.

Various designs of water-type PVT systems have since been proposed, and the theoretical and experimental performances of these PVT systems have been evaluated. In addition, research has been actively carried out on PVT systems linked to conventional heating and cooling facilities. Moreover, economic feasibility studies have been presented, including a calculation of the payback period and the effectiveness of different PVT systems.

Some papers focusing on a building energy system with PVT collectors dealt with the heating performance of the system as well as the performance of the PVT collector when it was linked to the heating system. A study of PV/T systems with TRNSYS was published by Kalogirou [14, 15], dealing with the modeling and simulation of a hybrid PV/T collector, linked to a thermal storage tank. They analyzed the solar fraction during the heating and cooling seasons. In addition, they studied simulation models of a PVT collector according to the operation mode, thermosyphon and pump in circulation. One study [16] analyzed the heating energy performance of a PVT collector with a corrugated polycarbonate and rectangular channel absorber. Fraisse et al. [17] pointed out that the low operating-temperature-requirement (35°C) of the Direct Solar Floor (DSF) system is highly suitable for the application of a PVT water system. Using the TRNSYS simulation program, they studied such an application for a glazed collector system. Another study [18] described a space and tap-water-heating system with a roof-sized PVT array combined with a ground-coupled heat pump. The system performance, when applied to a one-family Dutch dwelling, was evaluated through the TRNSYS simulation program.

More recently, an extensive analysis of a PVT heat pump system running at variable pump speeds was performed in China. An experimental investigation of an unglazed PVT evaporator system prototype with R-22 as the refrigerant was done. The winter-day test showed a peak COP at 10.4 and an average value of 5.4 at a 20°C condenser water supply temperature [19, 20]. Also, mathematical models based on a distributed parameter approach were developed [21, 22]. The simulation results show that at a fixed compressor speed and refrigerant flow, and with a condenser water supply of 30°C, the PVT evaporator arrays showed an overall efficiency level in the range of 0.64 ~ 0.87, thermal efficiency of 0.53 ~ 0.64, and a PV efficiency range of 0.124 ~ 0.135. Pei et al. [23] conducted a comparative study of the merits of a glass cover. The result of their analysis indicated that a single-glass cover is able to raise the photothermic exergy efficiency, but on the other hand has an adverse effect on the photovoltaic exergy efficiency. Chow et al. [4] studied a BiPVT/w system applicable to a multistory apartment building for water pre-heating purpose. Later on, they [24] installed modular box-structure PVT/w collectors onto the SW-facing façade of an experimental chamber. The experimental results demonstrated that the space cooling load is reduced by 50% during a peak summer condition. Erdil et al. [25] carried out experimental measurements of an open-loop PVT/w domestic water pre-heating system. Another study [26] analyzed the performance of PVT collectors in domestic heating and cooling systems. The thermal efficiency of their PVT collector system was found to be approximately 9% lower than a conventional solar thermal collector.

In this paper, the experimental performance of a heating system combined with a building-integrated PVT (BIPVT) collector was analyzed. For this paper, a water-type unglazed BIPVT collector of 1.5kWp PV was installed onto the experimental unit and was incorporated with a heating system.

2. Experiment

2.1. Unglazed BIPVT collector

A water-type unglazed BIPVT collector was developed for this study. The BIPVT collector consists of PV modules in combination with heat extraction units made of aluminum, with no additional glass cover. The collector was thermally protected with 50mm of glass-wool insulation behind the thermal collecting plate. For the BIPVT collector, the aluminum sheet-and-tube absorber was attached onto the back side of the PV module using a thermal conduction adhesive (Fig. 1). The PV modules used for the collectors were 240Wp mono-Si PV modules which show an electrical efficiency rating of 16.7% under standard test conditions (STC). Previous research [27] found that the electrical efficiency of a BIPVT collector in an outdoor experimental condition was 16.4%, its thermal efficiency coefficient at zero reduced temperature was 0.73, and its heat loss coefficient is 15W/m2 K. The performance and details pertaining to BIPVT collector are shown in Table 1.

Fig. 1. Conceptual drawing of a BIPVT collector

Table 1. BIPVT collector performance and details

Subject

Performance and detail

Maximum power(Pmax) 240W

Electrical efficiency 16.4%

Thermal efficiency coefficient [FR(xa)] 0.73

Heat loss coefficient [FRUL] 15W/mJ K

Size 1656*997*50 mm

2.2. Heating system with BIPVT collectors

For the heating system combined with the BIPVT collector, an experimental unit was built, including a test room and a service room (Fig. 2). The slope of the roof is 30°, and it was orientated toward the south. The BIPVT collectors were placed on the roof. The power generation capacity was 1.5kWp, and the collecting area was 8.64 m2. The system consisted of one array composed of six serially connected modules with a maximum current of 8.15A and a maximum voltage of 179.58 V.

In order to use the thermal energy from the BIPVT collectors for heating in the unit, the roof-integrated BIPVT system was installed onto an experimental unit and was incorporated with a heating system. The heating system for the unit with the BIPVT collector was configured with a thermal storage tank of 500 liters, an auxiliary boiler, an

inverter and a fan-coil unit (FCU) to release the heat. RTD thermocouples were used to measure their temperatures, and two flow meters were installed for measuring of the collector supply and heating supply. A data acquisition instrument was also connected to record all of the data related to the thermal and electrical performance of the BIPVT collector and the outdoor conditions. A schematic diagram of the heating system with the BIPVT collector is shown in Fig. 3. In order to evaluate the heating performance of the heating system with the BIPVT collector, the experiment was performed under the conditions described below during October of 2012.

The pump, controlled by a differential temperature controller (DTC), circulates the heat-transfer fluid from the BIPVT collector through the heat exchanger in the thermal storage tank and back to the BIPVT collectors. When the temperature of the BIPVT collectors exceeds that of the tank bottom by 4°C, the differential temperature controller switches the circulating pump on. When the temperature of the BIPVT collectors drops to 2°C above the thermal storage tank temperature, the differential temperature controller stops the pump. The water flow rate of the BIPVT collector was maintained at 10LPM (liter/min). For the heating performance experiment, the heating load of a 100 m2 building was assumed to be 17 kW. Then, the assumed value of the heating load was consumed by the FCU from 1:00 p.m. to 7:00 a.m. The supply flow rate of the FCU was 5 LPM.

BIPVT collectors

Fig. 2. View of the experimental unit

BIPVT Modules

(1,656L X 997W X 30H 12EA)

Glycol Tank

Fig. 3. Schematic diagram of the heating system with BIPVT collectors

3. Results and discussion

3.1. Thermal and electrical performance of the BIPVT collector

With the experimental results of the heating system with the BIPVT collectors, the thermal and electrical performances of the BIPVT collector were analyzed.

The thermal efficiency is determined as a function of the solar radiation (G), the mean fluid temperature (Tm) and the ambient temperature (Ta). The thermal efficiency is calculated as the ratio of the incoming solar radiation on the BIPVT collector (Qj) and the heat gain energy from the BIPVT collector (Q2) by the following equation:

nth thermal efficiency [-]

Apvt collector area [m2]

To collector outlet water temperature [ C]

Tt collector inlet water temperature [ C]

m mass flow rate [kg/s]

Cp specific heat [J/kg K] G irradiance on the collector surface [W/m2]

The thermal efficiency of the PVT collectors was conventionally calculated as a function of the ratio AT/G, where AT = Tm - Ta. Here, Tm and Ta are the PVT collector's mean fluid temperature and the ambient temperature, respectively, and G is the solar radiation on the collector surface. Hence, AT denotes the measurement of the temperature difference between the collector and its surroundings relative to the solar radiation. The thermal efficiency of the BIPVT collector, nh, is expressed as

Vth =Vo ~a1(~G) (2)

where no is the thermal efficiency at zero reduced temperature, and a! is the heat loss coefficient.

To illustrate the measurement results of the BIPVT collector, the thermal performance is expressed by Fig. 4. The thermal efficiencies of the BIPVT collector can be expressed with the relational expression n th =

0.52-7.99 (AT/G).

Thus, the collector's thermal efficiency (no) at zero reduced temperature is 52%, which indicates relatively high performance; however, the heat loss coefficient (a}), which can have the effect of reducing the thermal efficiency, is 7.99 W/m2 K. The average thermal efficiency is about 30%.

The electrical efficiency depends mainly on the incoming solar radiation and the PV module temperature. It is calculated with the following equation:

lei = -T-G (3)

The electrical efficiency of the BIPVT collector with heating system under the outdoor condition is shown in Fig. 5. The performance of BIPVT collector can be expressed with the following relational expressions: nel = 0.169-0.002 (AT/G). The highest electrical efficiency of the BIPVT collector is 16.9% under outdoor test conditions and with the given X axis coefficients (AT/G).

As shown in Fig. 5, the electrical efficiency is very sensitive to the X axis coefficients especially compared to the thermal efficiency. These facts indicate that in spite of same condition of X axis coefficients, the BIPVT collector have the non-uniform electrical performance by influence of the PV temperature on the BIPVT collector besides that of the ambient temperature, mean fluid temperature and solar radiation. The experimental results show that the electrical efficiency of the BIPVT collector decreased according to the increase of the BIPVT back temperature, from 25 °C to 45 °C, under the test conditions (Fig. 6). From these experimental results, it was found that the BIPVT collectors with the heating system better performed with the electrical efficiency of maximum 18.9%.

Here, Im and Vm are the current and the voltage of the PV module operating under a maximum power

Fig. 4. Thermal efficiency of the BIPVT collector with the heating system.

Fig. 5. Electrical efficiency of the BIPVT collector with the heating system.

Fig. 6. Electrical efficiency according to the BIPVT backside temperature

3.2. Daily thermal characteristic of the heating system

The thermal performance of the system during a clear day is shown in Fig. 7. It indicates that the temperature of the thermal storage tank rose to 40°C from 18°C through the heat gain from the BIPVT collectors. The temperature of the thermal storage tank and the BIPVT collector continuously rose from 9 a.m. to 3 p.m. in spite of reduction of the solar radiation starting at 12 p.m. Moreover, the temperature of the FCU inlet as the heating supply rose by more than 40°C from 2 p.m. to 3 p.m. From 4 p.m. to 7 a.m. on the next day, the water temperature of the FCU inlet and thermal storage tank decreased slowly as it was used for the heating load. These results indicate that when the hot water temperature for the heating supply is about 40°C, from 2 p.m. to 3 p.m., the heating energy can be solely supplied by the heat gain from the BIPVT collector system. From the experimental results, it was also found that the average temperature difference between the BIPVT collector inlet and outlet is 3.3 °C, with maximum of 4.4 °C.

The heat gain from the BIPVT collectors and its thermal efficiency are presented in Fig. 8. In this figure, the thermal efficiency and heat gain of the BIPVT collector increase according to the increase in the solar radiation from 8 a.m. However, from 11 a.m., the thermal efficiency continuously decreases in spite of the increase in the solar radiation. This occurs due to the increase in the inlet temperature of the BIPVT collectors, as caused by the temperature increase of the thermal storage tank. It was found that the total heat gain from the collector is 9.7 kWh while the average thermal efficiency of the system is 30%.

Fig. 7. Thermal performance of the heating system with the BIPVT collector

10.000

.2 6.000

Solar radiation Thermal heat gain -Thermal efficiency

8 9 10 11 12 13 14 15

Time (hr)

Fig. 8. Heat gain of the heating system with BIPVT collector

4. Conclusion

In this paper, the performance of a heating system combined with water-type PVT collector integrated into the roof of an experimental unit was analyzed.

According to the experimental results, for the heating system with the BIPVT collector, it was confirmed that the respective thermal and electrical efficiency of the BIPVT collector are 30% and 17%, on average. In particular, regarding the electrical efficiency, it showed a high performance level of more than 16% while the heating system

with the BIPVT collector was running. This occurred because the relatively low temperature fluid circulates into the BIPVT collector from the thermal storage tank while the BIPVT collectors generate hot water. Therefore, a heating system with a BPVT collector is very favorable for increasing the energy performance of buildings. It was also confirmed that the water temperature of the thermal storage tank rose by 40°C and that this could be utilized as a heat source for heating; this can reduce the required energy in a building with such a BIPVT collector. In conclusion, a heating system with a BIPVT collector is very useful in buildings, and its effectiveness was confirmed in this study.

Acknowledgements

This work was supported by a grant from the Human Resources Development Project of the Korea Institute of Energy Technology Evaluation and Planning (No.20114010 203040) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A3015046)

References

[1] Zondag AH, Vries DW, Helden WGJ. Zolingen RJC. The yield of yifferent combined PV-thermal collector designs. Solar Energy 2003; 74(3): 253-269.

[2] Sandnes B, Rekstad J. A Photovoltaic/Thermal collector with a polymer absorber plate. Solar Energy 2002; 72(1): 63-73.

[3] Garg HP, Agarwal RK. Experimental study on a hybrid photovoltaic thermal solar water heater and its performance predictions. Energy Conversion and Management 1994; 35(7): 621-633.

[4] Chow TT, Chan ALS, Fong KF, Lo WC, Song CL. Energy performance of a solar hybrid collector system in multi-story apartment building. Proceeding of IMtechE Part A: Journal of Power and energy 2005; 219(1): 1-11.

[5] Chow T T, He W, Ji J, Chan ALC. Performance evaluation of photovoltaic-thermosyphon system for subtropical climate application. Solar Energy 2007; 81(1): 123-130.

[6] Zondag HA, Vries DW, Helden WGJ, Zolingen RJC, Steenhoven AA. Thermal and electrical yield of a PV-thermal collector. Solar Energy 2002; 72(2): 113-128.

[7] Tripanagnostopoulos Y, Nousia TH. Souliotis M, Yianoulis P. Hybrid photovoltaic/thermal solar Systems. Solar Energy 2002; 72(3): 217234.

[8] Chow TT, Pei G, Chan LS, Fong KFA Comparative study of PV glazing performance in warm climate. Indoor Built Environment 2009; 18(1): 32-40.

[9] Chow TT, Pei G, Fong KF, Lin Z, Chan A, Ji J. Energy and exergy analysis of photovoltaic- thermal collector with an without glass cover. Applied Energy 2009; 86(3): 310-316.

[10] Bergene T, Lovvik OM. Model calcu-lations on a flat-plate solar geat collector with integrated solar cells. Solar Energy 1995; 55(6): 453462.

[11] Garg HP, Agarwal RK. Experimental study on a hybrid photovoltaic thermal solar water heater and its performance predictions. Energy Conversion and Management 1994; 35(7): 621-633.

[12] Garg HP, Agarwal RK. Some Aspects of a PV/T collector/forced circulation flat plate solar water heater with solar cells. Energy Conversion and Management 1995; 36(2): 87-99

[13] Fujisawa T, Tani T. Annual exergy evaluation on photovoltaic-thermal hybrid collector. Solar Energy Materials and Solar Cells 1997; 47(1-4): 135-148.

[14] Kalogirou SA. Use of TRNSYS for modelling and simulation of a hybrid pv thermal solar system for cyprus. Renewable Energy 2001; 23(2): 247-260.

[15] Kalogirou SA, Tripanagnostopoulos Y. Hybrid PV/T solar systems for domestic hot water an electricity production. Energy Conversation Management 2006; 47(18-19): 3368-3382.

[16] Huang JA, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal system. Solar Energy 2001;70(5): 443-448.

[17] Fraisse G, Menezo C, Johannes K. Energy performance of water hybrid PV/T collectors applied to combined systems of Direct Solar Floor Type. Solar Energy 2007; 81(11): 1426-1438.

[18] Bakker M, Zondag HA, Elswijk MJ, Strootman KJ, Jong MJM. Performance and costs of a roof-sized PV/thermal array combined with a ground coupled heat pump. Sol Energy 2005; 78: 331-339

[19] Ji J, Liu K, Chow TT, Pei G, He W, He H. Performance analysis of a photovoltaic heat pump. Applied Energy 2008; 85(8): 680-693.

[20] Ji J, Pei G, Chow TT, Liu K, He H, Lu JP, et al. Experimental study of photovoltaic solar assisted heat pump system. solar Energy 2008; 82(2): 43-52.

[21] Ji J, Liu K, Chow TT, Pei G, He H. Thermal analysis of PV/T evaporator of a solar assisted heat pump. Energy Research 2007; 31(5): 525545.

[22] Ji J, He H, Chow TT, Pei G, He W, Liu K. Distributed dynamic modeling and experimental study of PV evaporator in a PV/T solar assisted heat pump. Heat Mass Transfer 2009; 52(5-6): 1365-1373.

[23] Pei G, Ji J, Chow TT, Liu H, Yi H. Comparative analysis of winter performance of PV-SAHP system with and without glass cover. Proceeding of IMtechE Part A: Power and energy 2008; 222(2): 179-187.

[24] Chow TT, He W, Ji J. An experimental study of facade-integrated photovoltaic/water-heating system. Applied Thermal Engineering 2007;27(1):37-45.

[25] Erdil E, Ilkan M, Egelioglu F. An experimental study on energy generation with a Photovoltaic(PV)-solar thermal hybrid system. Energy 2008; 33(8): 1241-1245.

[26] Vokas G, Christandonis NS. Hybrid photovoltaic-thermal systems for domestic heating and cooling a theoretical approach. Solar Energy 2006;80:607-15.

[27] Zondag HA, Borg N Van der, Eisenmann W. D8-6: PVT Performance measurement guidelines, Petten: ECN and Emmerthal: ISFH, 2005.