- Email: [email protected]
UTCs): Energy, exergy, and electrical-to-thermal rational approaches
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 110 (2014) 636–647 www.elsevier.com/locate/solener
Experimental study of performance of Photovoltaic–Thermal Unglazed Transpired Solar Collectors (PV/UTCs): Energy, exergy, and electrical-to-thermal rational approaches M. Gholampour, M. Ameri ⇑, M. Sheykh Samani Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Energy and Environmental Engineering Research Center, Shahid Bahonar University of Kerman, Kerman, Iran Received 6 June 2014; accepted 5 September 2014
Communicated by: Associate Editor Brian Norton
Abstract Photovoltaic panel associated with Unglazed Transpired Collector (PV/UTC) can convert solar energy into thermal and electrical energy. In the present paper, a UTC capable of combing with PV panels is designed, constructed and tested at Shahid Bahonar University in Kerman, Iran. The performance of the PV/UTC and UTC systems are evaluated based on the simple first law, first law defined as a function of electrical-to-thermal ratio number, and the second law efficiencies. The obtained results showed that mounting PV panel on the UTC can result in photovoltaic cooling, depending on the mass flow rate value of the air passed through the transpired plate. A critical radiation level based on the useful exergy gain is also presented and it is drawn that the greater number of PV panels would cause a decrease in critical radiation level. Also, the results show that the electrical-to-thermal rational and exergetic analyses are very important to design PV/UTC systems. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: PV/UTC; Electrical-to-thermal ratio number; First law efficiency; Second law efficiency
1. Introduction Solving the energy problem is going to take a lot of social action combined with government support. Rising energy costs is lastly beginning to force global leaders to study alternatives and provide the funding to create changes. Concerns like global warming are becoming a major reality and beginning worldwide concerns about pollution and consumption. Currently, solar air systems such as Unglazed Transpired Collectors (UTCs) increasingly ⇑ Corresponding author at: Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran. Tel.: +98 3412111763; fax: +98 3412120964. E-mail address: [email protected] (M. Ameri).
http://dx.doi.org/10.1016/j.solener.2014.09.011 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
aim at reducing a large part of heat demand in a building with the roof and the facade serving as air collectors. Generally, in UTCs, the air is preheated by passing through a perforated and blackened absorber plate due to negative pressure produced by the fans. The wind heat loss is decreased by the suction occurring at the transpired plate. This reduction could make a significant contribution towards no need for a cover, and in turn, results to cost reduction of the collector. In recent years, several studies about UTC systems have been carried out by many researchers. Kutscher et al. (1993) did theoretically some research on heat loss mechanisms in flat UTCs and found that the heat loss occurring due to natural convection and wind can be ignored. Using
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
637
Nomenclature A cp E_ el;net E_ fan E_ L E_ pv E_ solar E_ useful hair,in hair,out Ir IR Ifan Ipv m_ r Tair,in
plate area (m2) specific heat capacity of air (J kg1 K1) net electrical power gained by system (W) fan power (W) energy loss of the process (W) PV power (W) solar radiant energy (W) useful energy gain (W) enthalpy of the inlet air (J kg1) enthalpy of the outlet air (J kg1) incident solar radiation (W m2) irreversibility and exergy loss of the process (W) fan current (A) photovoltaic panel current (A) mass flow rate (kg s1) electrical-to-thermal ratio number inlet air temperature (K)
development a simple computer model, they were able to predict the UTC efficiency. Results of a number of installed and monitored UTCs were presented by Hollick (1994). Experimentally, he examined the effect of suction velocity on the UTC efficiency. Kutscher (1994) used experiments to develop correlations for heat exchanger effectiveness and pressure drop for air pass through the transpired plate. The author tested flat UTCs with low porosity and triangle hole layout. Summers (1995) conducted an analytical model for TRNSYS software which was able to calculate energy saving and life cycle solar saving according to the P1, P2 method. The author assumed the wind and natural convection heat losses are negligible. Using a commercial computational fluid dynamic code, Gunnewiek et al. (1996) numerically examined the flow distribution in large area UTCs. They presented plots which exhibit the nonuniformities of suction velocity occurring in large-area collectors. With the aim of decreasing the computational cost of UTC simulation, Dymond and Kutscher (1997) developed a 2D model based on the pipe network analysis to examine the flow and temperature distribution. On the basis of experimental measurements, Van Decker et al. (2001) presented a predictive model to estimate heat exchanger effectiveness in three regions including the front of the plate, the hole, and the back of the plate with square layout. Based on the numerical and experimental results, Gawlik and Kutscher (2002) presented a correlation for wind heat loss from corrugated plate with suction. This formula has been presented for both separate and attached flow regimes. Gawlik et al. (2005) made an attempt to experimentally and numerically study the effect of thermal conductivity of the transpired plate. They stated that it has a small effect on the thermal performance of UTCs. Leon and Kumar (2007) developed a mathematical model and
Tair,out outlet air temperature (K) Tamb ambient temperature (K) Tsun sun temperature (K) Vair outlet air velocity (m s1) Vfan fan voltage (V) Vpv photovoltaic panel voltage (V) X_ air;in exergy of inlet air (W) X_ air;out exergy of outlet air (W) X_ el;net net electrical exergy gain (W) X_ solar solar radiant exergy (W) X_ useful useful exergy gain (W) X_ useful;ac actual useful exergy gain (W) Greek symbols g first law efficiency gII second law efficiency
carried out parametric studies based on the first law efficiency by varying parameters such as solar radiation and suction velocity. Greig et al. (2012) conducted experiments on flow distribution in a channel with corrugated surface and showed that there are strong turbulence regions even at the low Reynolds number. With the purpose of optimizing the thermal performance of the UTCs, Badache et al. (2013) planned an experiment method and found that the mass flow rate and absorber coating are the main effective parameters. Gholampour and Ameri (2014a,b) developed computer model was validated against experimental data and studied UTC on the basis of energetic and exergetic approaches. They found that if the hole pitch was lower than 16 mm and the hole diameter was higher than 1 mm, fan power can be neglected at high radiation levels. Also, they showed how the exergy analysis is very important in designing the UTCs. Systems recognized as photovoltaic/thermal (PV/T) can efficiently, economically, and simultaneously convert solar energy into electricity and useful thermal energy and help to solve energy concern and global warming. Many researchers (Brinkworth et al., 2000; Zondag et al., 2003; Tonui and Tripanagnostopoulos, 2007; Chow et al., 2009; Shahsavar and Ameri, 2010; Shahsavar et al., 2011; Gholampour and Ameri, 2014a,b) have investigated the energy and exergy performances of PV/T air systems. Solar cells can be combined with UTC and act as a PV/T air system (PV/UTC system). In this system, UTC by capturing the transmitted energy from the photovoltaic panel and removing waste heat from the PV panel can improve the PV panel performance. In recent years, few researchers experimentally studied the performance of the PV/UTC systems based on the energetic approach. Hollick (1998) experimentally studied a PV/UTC system with corrugated
638
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
plate with two mass flow rates higher than 0.01 kg m2 s1 and found that the PV/UTC system has higher energy efficiency than either the PV panel or UTC on their own. He also showed that the PV panel has lower temperature in PV/UTC system, compared with PV panel alone. Naveed et al. (2006) carried out an experimental and economical study on PV/UTC with 0.011 kg m2 s1. They stated that combining PV panel with UTC can reduce PV temperature and simple payback period in the range of 3–9 °C and 8 years, respectively. Athienitis et al. (2011) designed and developed a prototype PV/UTC, in which 70% of UTC area was covered by PV panel. The authors showed that the thermal efficiency of UTC is higher than the combined thermal plus electrical efficiency, but the equivalent thermal efficiency of PV/UTC is 7–17% higher. All of the aforementioned studies were performed on the basis of energy analysis. Both PV/UTC and UTC systems consume electrical energy to draw the air through the transpired plate. Since thermal and electrical energies do not have the same worth, the importance of exergy analysis becomes more pronounced. In this paper, the performance of the systems are investigated based on the simple first law, equivalent thermal efficiency as a function of electrical-to-thermal ratio number, and the second law efficiencies. PV/UTC and UTC systems are designed, manufactured, and tested in a geographic location of Kerman, Iran. Based on the experimental results, UTC and PV/UTC systems are compared without neglecting fan power. A critical radiation level is also defined based on the actual exergy gain and all systems are compared in terms of this approach. 2. Description of the experimental set-up and measurement
Fig. 1. (a) Schematic view of the experimental set-up and (b) photograph of the fabricated PV/UTC at Shahid Bahonar University of Kerman.
The elements of the experimental set-up and a photograph of the constructed PV/UTC with the overall dimensions of 102 100 6 cm that will provide preheated air and electricity are shown in Fig. 1a and b, respectively. The UTC is made using a 0.5 mm thickness steel sheet painted with a matt black dye. Using three layers of glass wool, silicon-rubber, and wood, the back and sides of the air channel casing are well insulated thermally to make the energy loss as small as possible. In order to reduce unwanted heat loss and airflow the ends and the sides of the UTC are welded and sealed with silicon-rubber. The air is drawn through the plate by two 6.6-W DC fans, whose rated voltage and current are 12 V and 0.55 A respectively. The correct and accurate measurements are essential to analyze obtained experimental results. In order to measure the incident solar radiation intensity, a CMP 6 Kipp and Zonen pyranometer (±4 W m2 accuracy) is mounted on the surface parallel to the UTC surface in such a manner that it does not cast a shadow onto the transpired plate. As can be seen in Fig. 1a, a nozzle is mounted at the end of air channel to assess higher accuracy of the air velocity measurement. The air velocity is measured at the end of
nozzle by a multifunction measuring instrument Testo 435 (Testo, Germany) equipped with a vane probe (2.36 in. diameter, 0.25–20 m s1, ±0.1 m s1 accuracy). The vane probe is located at the outlet according to the manufacturer’s recommendation. T-type thermocouples calibrated using constant temperature bath are used to measure all temperatures. As shown in Fig. 1a, the outlet air temperature is measured using a thermocouple located after the fan in the air channel. A thermocouple fixed in the shade underneath the UTC measured the ambient temperature. All temperatures are recorded using an 8-channel Thermometer/Datalogger (TES PROVA-800, China) with ±0.5 °C accuracy. Wind speed is measured by a NRG-40 anemometer (5–25 m s1, ±0.14 m s1 accuracy) which is mounted on the surface parallel to the collector surface in such a manner as to not cast a shadow onto the collector plate. The transpired plate can be modeled as a heat exchanger (Kutscher (1994)). Van Decker et al. (2001) found that the transpired plate with triangular hole pattern provides better performance from the heat exchanger effectiveness standpoint. An attempt is made to utilize transpired plate
M. Gholampour et al. / Solar Energy 110 (2014) 636–647 Table 1 Detailed specifications of the UTC. Parameter
Value
Transpired plate material Plate thickness Dimension of the transpired plate Transpired plate coating Back insulation Side insulation Sealant Air channel casing material UTC tilt Absorptivity of the transpired plate Reflectivity of the transpired plate Transpired plate surface area Hole pitch Hole diameter Mass flow rate Plenum depth
Steel 0.5 mm 102 100 cm Matt black dye Glass wool, Silicon-rubber, Wood Glass wool, Silicon-rubber, Wood Silicon-rubber Steel 5° 0.9 0.1 1.02 m2 13 mm 1 mm 0.009 (kg s1 m2) 6 cm
with triangular hole layout in this study. Gholampour and Ameri (2014a,b) stated that fan power can be neglected at high radiation levels if the hole pitch is lower than 16 mm and the hole diameter is higher than 1 mm. Hole pitch 13 mm and hole diameter 1 mm are selected for the present study. The detailed specifications of the UTC are listed in Table 1. The plenum depth shown in Table 1 represents the normal distance between the transpired plate and back wall of the UTC. To produce both useful heat and electricity, the UTC can be combined with photovoltaic panel. Compared with flat plate collectors, UTCs consume considerable electrical power to draw air through the transpired plate. In PV/ UTCs, photovoltaic panels can make up the electrical power consumed by fan and produce electrical energy by sacrificing some thermal energy. The following configurations of systems, schematized in Fig. 2, are investigated in this study.
639
Configuration I: UTC. Configuration II: UTC + one PV panel on it (1PV/ UTC). Configuration III: UTC + two PV panel next it (2PV/UTC). The experimental set-up of PV/UTC (Config. II) is schematized in Fig. 3. As can be observed in Fig. 3, DC fans are fed by a DC source. In order to achieve the maximum PV power at high radiation levels (at midday), a 5.5 ohm linear resistor is connected to the PV panel as resistive load in PV/UTC systems. The voltage and current of PV module, DC fans, and resistive load are measured by several DT-9205 type multimeter (±1.0% accuracy). To measure the PV panel temperature, a T-type thermocouple is fixed behind the PV panel. The detailed electrical specifications of the DC fan, PV panel, and resistor are tabulated in Table 2. Finally, the system is mounted at a tilt angle of 5° on a single mobile track and tested outdoor to obtain experimental data. The experiments have been performed during June 2013 to August 2013 at Shahid Bahonar University campus under the meteorological conditions in Kerman, Iran (30°N latitude; 57°E longitude). The measured data was recorded at time intervals of 20 min. 3. Analysis In this section, an attempt is made to evaluate the energy and exergy analyses of the studied systems and uncertainty of the main parameters. To better comprehend both energy and exergy analyses, the working principle is explained first. Ambient air drawn by fans is heated in contact with solar energy-absorbing transpired plate. In PV/UTC systems, part of incident solar radiation is absorbed by PV
Configuration I: UTC
Configuration II: 1PV/UTC
Configuration III: 2PV/UTC Fig. 2. Tested configurations.
640
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
law of thermodynamics in quasi steady state results the energy balance of system in the following relationship: _ air;in þ E_ solar ¼ mh _ air;out þ E_ el;net þ E_ L mh
ð1Þ
where m_ (kg s1) and h (j kg1) are the mass flow rate and enthalpy of the air, respectively. E_ solar (W) is the solar radiant power falling on the system, and E_ L (W) is the power loss of the process. E_ el;net (W) denotes the net electrical power gained by the system and can be negative, positive, or zero. The net electrical power gain can be written as: E_ el;net ¼ E_ pv E_ fan
Fig. 3. Schematic view of the experimental set-up of the PV/UTC (Config. II-a).
Table 2 Detailed electrical specifications of the DC fan, PV panel, and resistor. PV panel specifications PV panel type Maximum power (nominal) Short-circuit current Open-circuit voltage Cell number Temperature coefficient of maximum power Temperature coefficient of short-circuit current Temperature coefficient of open-circuit voltage Weight Height Width Thickness
Polycrystal-Si 45 W 2.98 A 20.5 V 36 cells in series 0.00046 W K1 0.0007 A K1 0.038 V K1 7.7 kg 97.7 cm 46.2 cm 1.1 cm
DC fan specification Fan type Rated current Rated voltage Rated power
Axial DC fan 0.55 A 12 V 6.6 W
Resistor specifications Resistor type Resistor number Rated power Rated resistance
Ceramic cement resistor 4 resistors in parallel 40 W 5.5 X
panel and converted into electricity and heat and the other part, transmitted through the PV panel, is absorbed by UTC. In PV/UTC systems the air passing behind the PV panel can cause reduction of the operating temperature of the PV panel, hence PV panel efficiency and electrical power output increase. 3.1. Energy analysis Energy analysis is one of the approaches to evaluating the performance of the system (first law of thermodynamic). The energy balance equation should be written to do energy analysis. The studied systems can be treated as a control volume with inlet and outlet streams. The first
ð2Þ
In which E_ pv (W) and E_ fan (W) are the PV power and fan power, respectively. For UTCs, the PV power is zero (hence, the net electrical power gain is negative). It should be noted that the PV power is the power consumed just by resistive load in. Taking into account that air acts as an ideal gas and by substituting Eqs. (2) into (1), the energy balance equation can be rewritten as: E_ solar ½m_ cp ðT air;out T air;in Þ þ E_ pv E_ fan ¼ E_ L
ð3Þ
The bracket term in Eq. (3) denotes the useful energy gained by the system. The useful energy gain E_ useful (W) can be obtained by using the energy balance equation and considering the pressure of the fan power: E_ useful ¼ m_ cp ðT air;out T air;in Þ þ E_ pv E_ fan
ð4Þ
The first law efficiency is given the ratio of the useful energy to the received energy. Based on the energy balance equation, the solar energy can be taken into account as the received energy. According to these expressions, the first law efficiency can be given as: useful energy g¼ received energy _ p ðT air;out T air;in Þ þ E_ pv E_ fan mc ð5Þ ¼ IrA In which Ir (W m2) is the incident solar radiation on the system. A (m2) is the whole area of the system on which the solar radiant energy falls. The defined first law efficiency gives rise to a fundamental problem; fan power, PV power, thermal energy ðm_ cp ðT air;out T air;in ÞÞ, and solar radiation energy do not have the same worth and quality. Considering the same quality for the thermal energy and solar radiation energy and defining the equivalent thermal energy for the fan and PV power is the conventional way to solve this problem. The equivalent thermal energy that can be employed to determine the electrical-to-thermal ratio. In this paper, this ratio is defined as follows: r¼
E_ el _Ethermal
ð6Þ
To make an improvement in design of systems like PV/UTC systems, this ratio number acts an important role. To determine the value of this ratio, there are different
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
approaches such as coal power station, exergy, open market, renewable energy market, and GHG emissions. Coventry and Lovegrove (2003) found that this ratio changes between 1 based on the simple first law efficiency to 17 based on the simple exergy analysis. In this study, in addition to exergy analysis, the effect of this ratio has been studied in the range of 1–9. Note that the first law efficiency defined based on the ratio number is called equivalent thermal efficiency in this paper. 3.2. Exergy analysis Exergy analysis gives a better view of the system performance, especially when both thermal and electrical energies are apparent in the energy balance equation. The exergy balance equation for the system as a control volume can be formulated as: X_ air;in þ X_ solar ¼ X_ air;out þ X_ el;net þ IR
ð7Þ
In which X_ air;in and X_ air;out are the exergy of inlet and outlet air, respectively. X_ solar is the exergy of solar radiation falling on the system. X_ el;net is the net electrical exergy gain (equal to the net electrical power gain), and IR is the irreversibility and exergy loss of the process. Note that the air channel casing is assumed to be adequately insulated. The exergy balance equation can be rewritten as: ð8Þ X_ solar X_ air;out X_ air;in X_ el;net ¼ IR The bracket term in Eq. (7) presents the useful exergy gained by the system. Considering air as an ideal gas and by neglecting air pressure drop or fan power, the useful exergy gain provided by the system is given by: T air;out _ p T air;out T air;in T amb ln X_ useful ¼ mc þ E_ pv T air;in ð9Þ Note that the electrical exergy is equal to the electrical energy. The actual useful exergy gain considering air pressure drop is: X_ useful;ac ¼ X_ useful E_ fan
ð10Þ
Estimating the solar radiant exergy is a crucial issue investigated by many researchers. Among these, Petela (1964) presented the following correlation for the solar radiant exergy: " 4 # 4T 1 T amb amb X_ solar ¼ IrA 1 þ ð11Þ 3 T sun 3T sun in which, Tsun is the sun temperature equal to 6000 K. The second law efficiency can be written as: gII ¼
X_ useful;ac X_ solar
ð12Þ
where X_ useful;ac and X_ solar can be calculated by using Eqs. (9) and (10), respectively.
641
3.3. Uncertainty analysis Measurement instruments, experimental conditions and environment, instrument calibration, and recording data are some factors causing errors and uncertainties in the experiments. To measure the air velocity, solar radiation, temperatures, voltage, and current, appropriate measurement instruments are used in this study. The uncertainty of the main parameters occurring during measurements, is dealt with by the help of the method proposed by Kline and McClintock (1953): " 2 #1=2 n X @R wR ¼ wx ð13Þ @xi i i¼1 where wR and wxi are the uncertainties associated with the parameter R and independent variables (xi), respectively. In addition, n is the number of the independent variables. From the measured data, mass flow rate, fan power, PV power, thermal energy, first law efficiency, and second law efficiency are calculated. The mean values for all the days together for DT, Tamb, Tout, Vair, Ir, Vpv, Vfan, Ipv, and Ifan are found to be 15.24 °C, 27.3 °C, 42.39 °C, 2.32 m s1, 650 W m2, 8.37 V, 8.37 V, 1.74 A, and 0.33 A, respectively. The uncertainty of the main calculated parameters are listed in Table 3. 4. Critical radiation level One limitation of solar heaters is that they cannot supply the sufficient thermal energy to heat the operating fluid at every radiation level and overcome heat losses. The radiation level that makes the thermal useful energy gain zero is known as critical radiation level (Duffie and Beckman, 1991). This definition is based on the thermal useful energy gain for solar heaters. In this paper, the critical radiation level based on the actual exergy gain and equivalent thermal useful energy gain is redefined.
Table 3 The uncertainty of main parameters. Parameter
Unit
Comment
Uncertainty in the temperature measurement Outlet air temperature Ambient air temperature PV panel temperature
°C °C °C
±0.5 ±0.5 ±0.5
in the air velocity measurement in the measurement of solar energy
m s1 W m2
±0.1° ±4
in in in in in in in in
% % kg s1 W W W % %
0.5 0.8 0.0003 5.8 0.138 0.025 0.9 1.2
Uncertainty Uncertainty radiation Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty
the the the the the the the the
measurement of voltage measurement of current mass flow rate thermal energy PV Power fan power first law efficiency second law efficiency
642
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
5. Results and discussion In this section, an effort is made to investigate the behavior of PV/UTC and UTC systems. Neglecting the effects of wind velocity and ambient temperature on the systems’ response facilitates comparison among systems. Experiments was performed at successive days to satisfy this assumption. Thus, in this paper, all plots are pictured as a function of solar radiation and appropriately interpolated. A confident conclusion can be drawn by using this strategy. It should be noted that the results obtained from the systems were conducted on hours with clear sky condition. 5.1. Air temperature rise From the air temperature rise standpoint, a comparison among all tested systems is made and pictured in Fig. 4. As expected, installing PV panel on UTC reduces the air
temperature rise at every radiation level (Fig. 4a). This can be ascribed to the fact that part of incident solar energy absorbed by PV panels in PV/UTC systems. Therefore, the air is heated less in PV/UTC systems. Also, it can be concluded from this figure that the air temperature rise of PV/UTC systems varies non-linearly against solar radiation, but varies linearly for UTC systems similar to the experimental results obtained by Hollick (1994). Fig. 4b and c exhibits the effect of mass flow rate on the air temperature rise of UTC and PV/UTC, respectively. These figures illustrate that the air temperature rise decreases as the mass flow rate increases. This behavior becomes more pronounce when there is no PV panel on the UTC. There is also another point that should be mentioned: low dispersion of the data points, observed in all plots, can be attributed to errors occurred in the experiments and the neglecting effects of the ambient temperature and wind velocity on the systems’ response.
Fig. 4. Air temperature rise as a function of solar radiation (a) all tested configurations with 0.009 kg s1 mass flow rate, (b) all tested configurations with 0.0215 kg s1 mass flow rate, and (c) 1PV/UTC at different mass flow rates.
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
5.2. Photovoltaic panel temperature PV panel temperature strongly depends on wind velocity, ambient temperature, and solar radiation. Therefore, the PV panel temperature is hourly studied as the term of PV panel and ambient temperature difference. Fig. 5a and b shows the effect of mass flow rate on the capability of cooling PV panel. As can be seen in these figures, it can be derived that compared with PV alone, the PV/UTC system with 0.009 kg s1 mass flow rate is not efficient from the cooling capability standpoint. This result is very interesting when it can be seen that compared the PV alone, the PV/UTC with 0.0215 kg s1 mass flow rate is able to cool PV panel more. To put it simply, the value of mass flow rate specifies whether or not the air flow sucked through UTC is capable of cooling the PV panel located on it. In the case of Config. III (2PV/UTC), it can be observed in Fig. 5c that the PV panel in 1PV/UTC system has higher temperature than both PV panels in 2PV/UTC system. This is can be attributed to
643
the fact that in 2PV/UTC system, the air passing between the PV panels and UTC has lower temperature than that in 1PV/UTC system. 5.3. Photovoltaic power The two parameters determining the current–voltage (I–V) characterization curve of a PV panel are PV panel temperature and solar radiation. As mentioned before, the PV temperature depends on the solar radiation, ambient temperature, and wind velocity. In Photovoltaic–thermal (PV/T) systems, as well as wind velocity, all heat transfer convection mechanisms influence the PV panel temperature. Also, the current–voltage (I–V) characterization curves of the DC fans and resistor are powerfully determinant. Therefore, due to existence of large number of effective factors, the study of photovoltaic power is so difficult. But based on these interpretations, under similar experiment conditions, the PV panel temperature can be used as an appropriate indicator to investigate the hourly
Fig. 5. Hourly variation of PV panel and ambient temperature difference (a) PV panel alone and 1PV/UTC with 0.009 kg s1 mass flow rate, (b) PV panel alone and 1PV/UTC with 0.0215 kg s1 mass flow rate, and (c) 1PV/UTC and 2PV/UTC with 0.0215 kg s1 mass flow rate.
644
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
variation of PV power. Also, it should be noted that the PV power decreases as the PV panel temperature increases. Using PV temperature as an indicator (Fig. 5c) can explain how PV panel located down the UTC has higher PV power in 2PV/UTC system (Fig. 6).
to thermal energy gain. The fact is that the thermal exergy is lower than electrical power. Negative values for the second law efficiency at low solar radiation (see Fig. 8a) exhibit that these systems consume more electrical exergy for fans than the exergies recovered.
5.4. First law efficiency
5.6. Electrical-to-thermal ratio number
For all tested configurations, the simple first law efficiency (r = 1) as a function of solar radiation is shown in Fig. 7. The thick lines, seen in this figure, are related to the systems 0.0215 kg s1 mass flow rate. All efficiency lines are interpolated from experimental data point. To keep the figure legible, the experimental data points are not displayed. It can be inferred from Fig. 7 that UTC is more efficient than PV/UTC systems from the simple first law efficiency standpoint. Also, a linear reduction pattern for UTC system can be observed whereas the first law efficiency of PV/UTC follows a parabolic pattern when the first law efficiency is plotted against solar radiation (Fig. 7). One needs to note that in UTC case, with the increase of solar radiation, the first law efficiency decreases in spite of the fact that the air temperature rise (i.e. useful thermal energy gain) increases (see Fig. 4a). This can be imputed to the fact that with the increase of solar radiation, received energy (i.e. solar radiant energy) grows more than useful energy. As depicted in Fig. 7, PV/UTC systems with fewer PV panel have better performance.
As mentioned before, the electrical-to-thermal ratio number acts an important role in design of systems like PV/UTC systems. A selection of obtained results related to this ratio is presented in Fig. 9. The first interesting result that can be derived is that with the increase of ratio number, for the UTC system, the reduction trend for r = 1 (i.e. simple first law efficiency) is changed to a growing trend for r = 9 (Fig. 9a). For UTC system, the growing trend for r = 9 is similar to the trend for this system for second law efficiency (Fig. 9a). This can be attributed to the worth inequality between the thermal and electrical energy. The same as exergy concept, the ratio number equal to 9 shows that the electrical energy worth is more significant than thermal energy worth. Generally, with the increase of ratio number, the systems’ response approaches to similar responses obtained based on the second law efficiency. The obtained results for 1PV/UTC and 2PV/UTC (Fig. 9b and c) also confirm this fact.
5.5. Second law efficiency
As discussed before, the determination of the critical radiation level based on the actual exergy gain is important. It can be deduced form Fig. 8 that the critical radiation level depends on the number of PV panels. The greater
Fig. 8 exhibits the second law efficiency as a function of solar radiation for all tested configurations. As can be observed, all PV/UT systems follow a parabolic trend while UTC follows a growing linear trend. It can also be concluded that PV/UTC systems are preferable from the second law efficiency standpoint. This can be imputed to the fact that the PV/UTC produces electrical power in addition
Fig. 6. Hourly variation of PV power for 2PV/UTC with 0.0215 kg s1 mass flow rate.
5.7. Critical radiation level
Fig. 7. First law efficiency vs. solar radiation for all tested configurations.
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
645
Fig. 8. Second law efficiency vs. solar radiation (a) all tested configurations with 0.009 kg s1 mass flow rate and (b) all tested configurations with 0.0215 kg s1 mass flow rate.
Fig. 9. Equivalent thermal efficiency vs. solar radiation at different ratio numbers (a) UTC with 0.009 kg s1 mass flow rate, (b) 1PV/UTC with 0.0215 kg s1 mass flow rate, and (c) 2PV/UTC with 0.0215 kg s1 mass flow rate.
646
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
Fig. 10. Mean efficiency for UTC, 1PV/UTC, and 2PV/UTC systems at different mass flow rates (a) r = 1, (b) r = 4, (c) r = 9, (d) second law.
number of PV panels yields a decrease in critical radiation level. Generally, 2PV/UTC system, due to having PV panel and producing more electrical power, has the lowest critical radiation level. Also, it can be derived that since the mass flow rate increasing would lead to an increase in the fan power, it yields an increase in critical radiation level (see Fig. 8). 5.8. Comparison of systems For better comparison of the systems from the first and second law efficiency standpoints, the mean efficiencies (at the interval of 300–900 W m2) are shown in Fig. 10 at different mass flow rate for various electrical-to-thermal ratio numbers and second law efficiency. As can be concluded from this figure, the UTC with higher mass flow rate is the most efficient system when the comparison criteria is the equivalent thermal efficiency with ratio numbers 1 and 4. It should be noted that Coventry and Lovegrove (2003) suggested that the most suitable approach for a PV/thermal system should use 4.2 as a ratio based on the
renewable energy market. In the case of ratio number 4, the value of mass flow rate specifies whether or not the greater number of PV panels yields to better performance. From the equivalent thermal efficiency with ratio number 9 and second law efficiency standpoints, the greater number of PV panels would lead to an improvement in the performance. All systems with higher mass flow rate have better performance from the equivalent thermal efficiencies viewpoints while the second law efficiency of all systems decreases as the mass flow rate increases. 6. Conclusions A detailed experimental study was carried out to evaluate the energetic and exergetic efficiencies of three types of UTC and PV/UTC systems .The uncertainty of main parameters occurring during the measurements was also conducted. Based on the results obtained from the experiments, the following conclusions have been drawn: (1) for all PV/UTC systems, the air temperature rise follows a parabolic pattern while it follows a reduction linear pattern for
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
UTC system. (2) The value of mass flow rate specifies whether or not the air flow sucked through UTC is capable of cooling the PV panel located on it. (3) PV/UTC systems are preferable from the second law efficiency standpoint due to producing electrical power in addition to thermal energy gain. (4) Negative values of the second law efficiency exhibit that the system uses more electrical exergy for fans than exergiy that it recovers at low solar radiation. (5) With the increase of electrical-to-thermal ratio number, the systems’ response approaches to similar responses obtained based on the second law efficiency. (6) The greater number of PV panels would lead to a decrease in critical radiation level. (7) UTC with higher mass flow rate is the most efficient system when the comparison criteria is the first law efficiency for ratio number equal 1 and 4. (8) All systems with higher mass flow rate have better performance in the terms of the equivalent thermal efficiencies viewpoints while the second law efficiency of all systems decreases as the mass flow rate increases. References Athienitis, A.K., Bambara, J., O’Neill, B., Faille, J., 2011. A prototype photovoltaic/thermal system integrated with transpired collector. Sol. Energy 85, 139–153. Badache, M., Rousse, D.R., Halle´, S., Quesada, G., 2013. Experimental and numerical simulation of a two-dimensional unglazed transpired solar air collector. Sol. Energy 93, 209–219. Brinkworth, B.J., Marshall, R.H., Ibarahim, Z., 2000. A validated model of naturally ventilated PV cladding. Sol. Energy 69, 67–81. Chow, T.T., Pei, G., Fong, K.F., Lin, Z., Chan, A.L.S., Ji, J., 2009. Energy and exergy analysis of photovoltaic–thermal collector with and without glass cover. Appl. Energy 86, 310–316. Coventry, J.S., Lovegrove, K., 2003. Development of an approach to compare the ‘value’ of electrical and thermal output from a domestic PV/thermal system. Sol. Energy 75, 63–72. Duffie, J.A., Beckman, W.A., 1991. Solar Engineering of Thermal Processes. John Wiley & Sons, New York. Dymond, C., Kutscher, C., 1997. Development of a flow distribution and design model for transpired solar collectors. Sol. Energy 60, 291–300. Gawlik, K., Kutscher, C., 2002. Wind heat loss from corrugated, transpired solar collectors. J. Sol. Energy Eng. 124, 256–261. Gawlik, K., Christensen, C., Kutscher, C., 2005. A numerical and experimental investigation of low-conductivity unglazed, transpired solar air heaters. J. Sol. Energy Eng. 127, 153–155.
647
Gholampour, M., Ameri, M., 2014a. Design considerations of unglazed transpired collectors: energetic and exergetic studies. J. Sol. Energy Eng. 136, 031004. Gholampour, M., Ameri, M., 2014b. Energy and exergy study of effective parameters on performance of photovoltaic/thermal natural air collectors. J. Sol. Energy Eng. 136, 031001. Greig, D., Siddiqui, K., Karava, P., 2012. An experimental investigation of the flow structure over a corrugated waveform in a transpired air collector. Int. J. Heat Fluid Flow 38, 133–144. Gunnewiek, L.H., Brundrett, E., Hollands, K.G.T., 1996. Flow distribution in unglazed transpired plate solar air heaters of large area. Sol. Energy 58, 227–237. Hollick, J.C., 1994. Unglazed solar wall air heaters. Renewable Energy 5, 415–421. Hollick, J.C., 1998. Solar cogeneration panels. Renewable Energy 15, 195– 200. Kline, S.J., McClintock, F., 1953. Describing uncertainties in singlesample experiments. Mech. Eng. 75, 3–8. Kutscher, C.F., 1994. Heat exchange effectiveness and pressure drop for air flow through perforated plates with and without crosswind. J. Heat Trans. 116, 391–399. Kutscher, C.F., Christensen, C.B., Barker, G.M., 1993. Unglazed transpired solar collectors: heat loss theory. J. Sol. Energy Eng. 115, 182– 188. Leon, M.A., Kumar, S., 2007. Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors. Sol. Energy 81, 62–75. Naveed, A.T., Kang, E.C., Lee, E.J., 2006. Effect of unglazed transpired collector on the performance of a polycrystalline silicon photovoltaic module. J. Sol. Energy Eng. 128, 349–353. Petela, R., 1964. Exergy of heat radiation. J. Heat Trans. 86, 187–192. Shahsavar, A., Ameri, M., 2010. Experimental investigation and modeling of a direct-coupled PV/T air collector. Sol. Energy 84, 1938–1958. Shahsavar, A., Ameri, M., Gholampour, M., 2011. Energy and exergy analysis of a photovoltaic–thermal collector with natural air flow. J. Sol. Energy Eng. 134, 011014. Summers, D.N., 1995. Thermal simulation and economic assessment of unglazed transpired collector systems. In: Wisconsin Energy Bureau, University of Wisconsin USA. Tonui, J.K., Tripanagnostopoulos, Y., 2007. Air-cooled PV/T solar collectors with low cost performance improvements. Sol. Energy 81, 498–511. Van Decker, G.W.E., Hollands, K.G.T., Brunger, A.P., 2001. Heatexchange relations for unglazed transpired solar collectors with circular holes on a square or triangular pitch. Sol. Energy 71, 33–45. Zondag, H.A., de Vries, D.W., van Helden, W.G.J., van Zolingen, R.J.C., van Steenhoven, A.A., 2003. The yield of different combined PV– thermal collector designs. Sol. Energy 74, 253–269.
ScienceDirect Solar Energy 110 (2014) 636–647 www.elsevier.com/locate/solener
Experimental study of performance of Photovoltaic–Thermal Unglazed Transpired Solar Collectors (PV/UTCs): Energy, exergy, and electrical-to-thermal rational approaches M. Gholampour, M. Ameri ⇑, M. Sheykh Samani Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Energy and Environmental Engineering Research Center, Shahid Bahonar University of Kerman, Kerman, Iran Received 6 June 2014; accepted 5 September 2014
Communicated by: Associate Editor Brian Norton
Abstract Photovoltaic panel associated with Unglazed Transpired Collector (PV/UTC) can convert solar energy into thermal and electrical energy. In the present paper, a UTC capable of combing with PV panels is designed, constructed and tested at Shahid Bahonar University in Kerman, Iran. The performance of the PV/UTC and UTC systems are evaluated based on the simple first law, first law defined as a function of electrical-to-thermal ratio number, and the second law efficiencies. The obtained results showed that mounting PV panel on the UTC can result in photovoltaic cooling, depending on the mass flow rate value of the air passed through the transpired plate. A critical radiation level based on the useful exergy gain is also presented and it is drawn that the greater number of PV panels would cause a decrease in critical radiation level. Also, the results show that the electrical-to-thermal rational and exergetic analyses are very important to design PV/UTC systems. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: PV/UTC; Electrical-to-thermal ratio number; First law efficiency; Second law efficiency
1. Introduction Solving the energy problem is going to take a lot of social action combined with government support. Rising energy costs is lastly beginning to force global leaders to study alternatives and provide the funding to create changes. Concerns like global warming are becoming a major reality and beginning worldwide concerns about pollution and consumption. Currently, solar air systems such as Unglazed Transpired Collectors (UTCs) increasingly ⇑ Corresponding author at: Department of Mechanical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran. Tel.: +98 3412111763; fax: +98 3412120964. E-mail address: [email protected] (M. Ameri).
http://dx.doi.org/10.1016/j.solener.2014.09.011 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
aim at reducing a large part of heat demand in a building with the roof and the facade serving as air collectors. Generally, in UTCs, the air is preheated by passing through a perforated and blackened absorber plate due to negative pressure produced by the fans. The wind heat loss is decreased by the suction occurring at the transpired plate. This reduction could make a significant contribution towards no need for a cover, and in turn, results to cost reduction of the collector. In recent years, several studies about UTC systems have been carried out by many researchers. Kutscher et al. (1993) did theoretically some research on heat loss mechanisms in flat UTCs and found that the heat loss occurring due to natural convection and wind can be ignored. Using
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
637
Nomenclature A cp E_ el;net E_ fan E_ L E_ pv E_ solar E_ useful hair,in hair,out Ir IR Ifan Ipv m_ r Tair,in
plate area (m2) specific heat capacity of air (J kg1 K1) net electrical power gained by system (W) fan power (W) energy loss of the process (W) PV power (W) solar radiant energy (W) useful energy gain (W) enthalpy of the inlet air (J kg1) enthalpy of the outlet air (J kg1) incident solar radiation (W m2) irreversibility and exergy loss of the process (W) fan current (A) photovoltaic panel current (A) mass flow rate (kg s1) electrical-to-thermal ratio number inlet air temperature (K)
development a simple computer model, they were able to predict the UTC efficiency. Results of a number of installed and monitored UTCs were presented by Hollick (1994). Experimentally, he examined the effect of suction velocity on the UTC efficiency. Kutscher (1994) used experiments to develop correlations for heat exchanger effectiveness and pressure drop for air pass through the transpired plate. The author tested flat UTCs with low porosity and triangle hole layout. Summers (1995) conducted an analytical model for TRNSYS software which was able to calculate energy saving and life cycle solar saving according to the P1, P2 method. The author assumed the wind and natural convection heat losses are negligible. Using a commercial computational fluid dynamic code, Gunnewiek et al. (1996) numerically examined the flow distribution in large area UTCs. They presented plots which exhibit the nonuniformities of suction velocity occurring in large-area collectors. With the aim of decreasing the computational cost of UTC simulation, Dymond and Kutscher (1997) developed a 2D model based on the pipe network analysis to examine the flow and temperature distribution. On the basis of experimental measurements, Van Decker et al. (2001) presented a predictive model to estimate heat exchanger effectiveness in three regions including the front of the plate, the hole, and the back of the plate with square layout. Based on the numerical and experimental results, Gawlik and Kutscher (2002) presented a correlation for wind heat loss from corrugated plate with suction. This formula has been presented for both separate and attached flow regimes. Gawlik et al. (2005) made an attempt to experimentally and numerically study the effect of thermal conductivity of the transpired plate. They stated that it has a small effect on the thermal performance of UTCs. Leon and Kumar (2007) developed a mathematical model and
Tair,out outlet air temperature (K) Tamb ambient temperature (K) Tsun sun temperature (K) Vair outlet air velocity (m s1) Vfan fan voltage (V) Vpv photovoltaic panel voltage (V) X_ air;in exergy of inlet air (W) X_ air;out exergy of outlet air (W) X_ el;net net electrical exergy gain (W) X_ solar solar radiant exergy (W) X_ useful useful exergy gain (W) X_ useful;ac actual useful exergy gain (W) Greek symbols g first law efficiency gII second law efficiency
carried out parametric studies based on the first law efficiency by varying parameters such as solar radiation and suction velocity. Greig et al. (2012) conducted experiments on flow distribution in a channel with corrugated surface and showed that there are strong turbulence regions even at the low Reynolds number. With the purpose of optimizing the thermal performance of the UTCs, Badache et al. (2013) planned an experiment method and found that the mass flow rate and absorber coating are the main effective parameters. Gholampour and Ameri (2014a,b) developed computer model was validated against experimental data and studied UTC on the basis of energetic and exergetic approaches. They found that if the hole pitch was lower than 16 mm and the hole diameter was higher than 1 mm, fan power can be neglected at high radiation levels. Also, they showed how the exergy analysis is very important in designing the UTCs. Systems recognized as photovoltaic/thermal (PV/T) can efficiently, economically, and simultaneously convert solar energy into electricity and useful thermal energy and help to solve energy concern and global warming. Many researchers (Brinkworth et al., 2000; Zondag et al., 2003; Tonui and Tripanagnostopoulos, 2007; Chow et al., 2009; Shahsavar and Ameri, 2010; Shahsavar et al., 2011; Gholampour and Ameri, 2014a,b) have investigated the energy and exergy performances of PV/T air systems. Solar cells can be combined with UTC and act as a PV/T air system (PV/UTC system). In this system, UTC by capturing the transmitted energy from the photovoltaic panel and removing waste heat from the PV panel can improve the PV panel performance. In recent years, few researchers experimentally studied the performance of the PV/UTC systems based on the energetic approach. Hollick (1998) experimentally studied a PV/UTC system with corrugated
638
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
plate with two mass flow rates higher than 0.01 kg m2 s1 and found that the PV/UTC system has higher energy efficiency than either the PV panel or UTC on their own. He also showed that the PV panel has lower temperature in PV/UTC system, compared with PV panel alone. Naveed et al. (2006) carried out an experimental and economical study on PV/UTC with 0.011 kg m2 s1. They stated that combining PV panel with UTC can reduce PV temperature and simple payback period in the range of 3–9 °C and 8 years, respectively. Athienitis et al. (2011) designed and developed a prototype PV/UTC, in which 70% of UTC area was covered by PV panel. The authors showed that the thermal efficiency of UTC is higher than the combined thermal plus electrical efficiency, but the equivalent thermal efficiency of PV/UTC is 7–17% higher. All of the aforementioned studies were performed on the basis of energy analysis. Both PV/UTC and UTC systems consume electrical energy to draw the air through the transpired plate. Since thermal and electrical energies do not have the same worth, the importance of exergy analysis becomes more pronounced. In this paper, the performance of the systems are investigated based on the simple first law, equivalent thermal efficiency as a function of electrical-to-thermal ratio number, and the second law efficiencies. PV/UTC and UTC systems are designed, manufactured, and tested in a geographic location of Kerman, Iran. Based on the experimental results, UTC and PV/UTC systems are compared without neglecting fan power. A critical radiation level is also defined based on the actual exergy gain and all systems are compared in terms of this approach. 2. Description of the experimental set-up and measurement
Fig. 1. (a) Schematic view of the experimental set-up and (b) photograph of the fabricated PV/UTC at Shahid Bahonar University of Kerman.
The elements of the experimental set-up and a photograph of the constructed PV/UTC with the overall dimensions of 102 100 6 cm that will provide preheated air and electricity are shown in Fig. 1a and b, respectively. The UTC is made using a 0.5 mm thickness steel sheet painted with a matt black dye. Using three layers of glass wool, silicon-rubber, and wood, the back and sides of the air channel casing are well insulated thermally to make the energy loss as small as possible. In order to reduce unwanted heat loss and airflow the ends and the sides of the UTC are welded and sealed with silicon-rubber. The air is drawn through the plate by two 6.6-W DC fans, whose rated voltage and current are 12 V and 0.55 A respectively. The correct and accurate measurements are essential to analyze obtained experimental results. In order to measure the incident solar radiation intensity, a CMP 6 Kipp and Zonen pyranometer (±4 W m2 accuracy) is mounted on the surface parallel to the UTC surface in such a manner that it does not cast a shadow onto the transpired plate. As can be seen in Fig. 1a, a nozzle is mounted at the end of air channel to assess higher accuracy of the air velocity measurement. The air velocity is measured at the end of
nozzle by a multifunction measuring instrument Testo 435 (Testo, Germany) equipped with a vane probe (2.36 in. diameter, 0.25–20 m s1, ±0.1 m s1 accuracy). The vane probe is located at the outlet according to the manufacturer’s recommendation. T-type thermocouples calibrated using constant temperature bath are used to measure all temperatures. As shown in Fig. 1a, the outlet air temperature is measured using a thermocouple located after the fan in the air channel. A thermocouple fixed in the shade underneath the UTC measured the ambient temperature. All temperatures are recorded using an 8-channel Thermometer/Datalogger (TES PROVA-800, China) with ±0.5 °C accuracy. Wind speed is measured by a NRG-40 anemometer (5–25 m s1, ±0.14 m s1 accuracy) which is mounted on the surface parallel to the collector surface in such a manner as to not cast a shadow onto the collector plate. The transpired plate can be modeled as a heat exchanger (Kutscher (1994)). Van Decker et al. (2001) found that the transpired plate with triangular hole pattern provides better performance from the heat exchanger effectiveness standpoint. An attempt is made to utilize transpired plate
M. Gholampour et al. / Solar Energy 110 (2014) 636–647 Table 1 Detailed specifications of the UTC. Parameter
Value
Transpired plate material Plate thickness Dimension of the transpired plate Transpired plate coating Back insulation Side insulation Sealant Air channel casing material UTC tilt Absorptivity of the transpired plate Reflectivity of the transpired plate Transpired plate surface area Hole pitch Hole diameter Mass flow rate Plenum depth
Steel 0.5 mm 102 100 cm Matt black dye Glass wool, Silicon-rubber, Wood Glass wool, Silicon-rubber, Wood Silicon-rubber Steel 5° 0.9 0.1 1.02 m2 13 mm 1 mm 0.009 (kg s1 m2) 6 cm
with triangular hole layout in this study. Gholampour and Ameri (2014a,b) stated that fan power can be neglected at high radiation levels if the hole pitch is lower than 16 mm and the hole diameter is higher than 1 mm. Hole pitch 13 mm and hole diameter 1 mm are selected for the present study. The detailed specifications of the UTC are listed in Table 1. The plenum depth shown in Table 1 represents the normal distance between the transpired plate and back wall of the UTC. To produce both useful heat and electricity, the UTC can be combined with photovoltaic panel. Compared with flat plate collectors, UTCs consume considerable electrical power to draw air through the transpired plate. In PV/ UTCs, photovoltaic panels can make up the electrical power consumed by fan and produce electrical energy by sacrificing some thermal energy. The following configurations of systems, schematized in Fig. 2, are investigated in this study.
639
Configuration I: UTC. Configuration II: UTC + one PV panel on it (1PV/ UTC). Configuration III: UTC + two PV panel next it (2PV/UTC). The experimental set-up of PV/UTC (Config. II) is schematized in Fig. 3. As can be observed in Fig. 3, DC fans are fed by a DC source. In order to achieve the maximum PV power at high radiation levels (at midday), a 5.5 ohm linear resistor is connected to the PV panel as resistive load in PV/UTC systems. The voltage and current of PV module, DC fans, and resistive load are measured by several DT-9205 type multimeter (±1.0% accuracy). To measure the PV panel temperature, a T-type thermocouple is fixed behind the PV panel. The detailed electrical specifications of the DC fan, PV panel, and resistor are tabulated in Table 2. Finally, the system is mounted at a tilt angle of 5° on a single mobile track and tested outdoor to obtain experimental data. The experiments have been performed during June 2013 to August 2013 at Shahid Bahonar University campus under the meteorological conditions in Kerman, Iran (30°N latitude; 57°E longitude). The measured data was recorded at time intervals of 20 min. 3. Analysis In this section, an attempt is made to evaluate the energy and exergy analyses of the studied systems and uncertainty of the main parameters. To better comprehend both energy and exergy analyses, the working principle is explained first. Ambient air drawn by fans is heated in contact with solar energy-absorbing transpired plate. In PV/UTC systems, part of incident solar radiation is absorbed by PV
Configuration I: UTC
Configuration II: 1PV/UTC
Configuration III: 2PV/UTC Fig. 2. Tested configurations.
640
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
law of thermodynamics in quasi steady state results the energy balance of system in the following relationship: _ air;in þ E_ solar ¼ mh _ air;out þ E_ el;net þ E_ L mh
ð1Þ
where m_ (kg s1) and h (j kg1) are the mass flow rate and enthalpy of the air, respectively. E_ solar (W) is the solar radiant power falling on the system, and E_ L (W) is the power loss of the process. E_ el;net (W) denotes the net electrical power gained by the system and can be negative, positive, or zero. The net electrical power gain can be written as: E_ el;net ¼ E_ pv E_ fan
Fig. 3. Schematic view of the experimental set-up of the PV/UTC (Config. II-a).
Table 2 Detailed electrical specifications of the DC fan, PV panel, and resistor. PV panel specifications PV panel type Maximum power (nominal) Short-circuit current Open-circuit voltage Cell number Temperature coefficient of maximum power Temperature coefficient of short-circuit current Temperature coefficient of open-circuit voltage Weight Height Width Thickness
Polycrystal-Si 45 W 2.98 A 20.5 V 36 cells in series 0.00046 W K1 0.0007 A K1 0.038 V K1 7.7 kg 97.7 cm 46.2 cm 1.1 cm
DC fan specification Fan type Rated current Rated voltage Rated power
Axial DC fan 0.55 A 12 V 6.6 W
Resistor specifications Resistor type Resistor number Rated power Rated resistance
Ceramic cement resistor 4 resistors in parallel 40 W 5.5 X
panel and converted into electricity and heat and the other part, transmitted through the PV panel, is absorbed by UTC. In PV/UTC systems the air passing behind the PV panel can cause reduction of the operating temperature of the PV panel, hence PV panel efficiency and electrical power output increase. 3.1. Energy analysis Energy analysis is one of the approaches to evaluating the performance of the system (first law of thermodynamic). The energy balance equation should be written to do energy analysis. The studied systems can be treated as a control volume with inlet and outlet streams. The first
ð2Þ
In which E_ pv (W) and E_ fan (W) are the PV power and fan power, respectively. For UTCs, the PV power is zero (hence, the net electrical power gain is negative). It should be noted that the PV power is the power consumed just by resistive load in. Taking into account that air acts as an ideal gas and by substituting Eqs. (2) into (1), the energy balance equation can be rewritten as: E_ solar ½m_ cp ðT air;out T air;in Þ þ E_ pv E_ fan ¼ E_ L
ð3Þ
The bracket term in Eq. (3) denotes the useful energy gained by the system. The useful energy gain E_ useful (W) can be obtained by using the energy balance equation and considering the pressure of the fan power: E_ useful ¼ m_ cp ðT air;out T air;in Þ þ E_ pv E_ fan
ð4Þ
The first law efficiency is given the ratio of the useful energy to the received energy. Based on the energy balance equation, the solar energy can be taken into account as the received energy. According to these expressions, the first law efficiency can be given as: useful energy g¼ received energy _ p ðT air;out T air;in Þ þ E_ pv E_ fan mc ð5Þ ¼ IrA In which Ir (W m2) is the incident solar radiation on the system. A (m2) is the whole area of the system on which the solar radiant energy falls. The defined first law efficiency gives rise to a fundamental problem; fan power, PV power, thermal energy ðm_ cp ðT air;out T air;in ÞÞ, and solar radiation energy do not have the same worth and quality. Considering the same quality for the thermal energy and solar radiation energy and defining the equivalent thermal energy for the fan and PV power is the conventional way to solve this problem. The equivalent thermal energy that can be employed to determine the electrical-to-thermal ratio. In this paper, this ratio is defined as follows: r¼
E_ el _Ethermal
ð6Þ
To make an improvement in design of systems like PV/UTC systems, this ratio number acts an important role. To determine the value of this ratio, there are different
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
approaches such as coal power station, exergy, open market, renewable energy market, and GHG emissions. Coventry and Lovegrove (2003) found that this ratio changes between 1 based on the simple first law efficiency to 17 based on the simple exergy analysis. In this study, in addition to exergy analysis, the effect of this ratio has been studied in the range of 1–9. Note that the first law efficiency defined based on the ratio number is called equivalent thermal efficiency in this paper. 3.2. Exergy analysis Exergy analysis gives a better view of the system performance, especially when both thermal and electrical energies are apparent in the energy balance equation. The exergy balance equation for the system as a control volume can be formulated as: X_ air;in þ X_ solar ¼ X_ air;out þ X_ el;net þ IR
ð7Þ
In which X_ air;in and X_ air;out are the exergy of inlet and outlet air, respectively. X_ solar is the exergy of solar radiation falling on the system. X_ el;net is the net electrical exergy gain (equal to the net electrical power gain), and IR is the irreversibility and exergy loss of the process. Note that the air channel casing is assumed to be adequately insulated. The exergy balance equation can be rewritten as: ð8Þ X_ solar X_ air;out X_ air;in X_ el;net ¼ IR The bracket term in Eq. (7) presents the useful exergy gained by the system. Considering air as an ideal gas and by neglecting air pressure drop or fan power, the useful exergy gain provided by the system is given by: T air;out _ p T air;out T air;in T amb ln X_ useful ¼ mc þ E_ pv T air;in ð9Þ Note that the electrical exergy is equal to the electrical energy. The actual useful exergy gain considering air pressure drop is: X_ useful;ac ¼ X_ useful E_ fan
ð10Þ
Estimating the solar radiant exergy is a crucial issue investigated by many researchers. Among these, Petela (1964) presented the following correlation for the solar radiant exergy: " 4 # 4T 1 T amb amb X_ solar ¼ IrA 1 þ ð11Þ 3 T sun 3T sun in which, Tsun is the sun temperature equal to 6000 K. The second law efficiency can be written as: gII ¼
X_ useful;ac X_ solar
ð12Þ
where X_ useful;ac and X_ solar can be calculated by using Eqs. (9) and (10), respectively.
641
3.3. Uncertainty analysis Measurement instruments, experimental conditions and environment, instrument calibration, and recording data are some factors causing errors and uncertainties in the experiments. To measure the air velocity, solar radiation, temperatures, voltage, and current, appropriate measurement instruments are used in this study. The uncertainty of the main parameters occurring during measurements, is dealt with by the help of the method proposed by Kline and McClintock (1953): " 2 #1=2 n X @R wR ¼ wx ð13Þ @xi i i¼1 where wR and wxi are the uncertainties associated with the parameter R and independent variables (xi), respectively. In addition, n is the number of the independent variables. From the measured data, mass flow rate, fan power, PV power, thermal energy, first law efficiency, and second law efficiency are calculated. The mean values for all the days together for DT, Tamb, Tout, Vair, Ir, Vpv, Vfan, Ipv, and Ifan are found to be 15.24 °C, 27.3 °C, 42.39 °C, 2.32 m s1, 650 W m2, 8.37 V, 8.37 V, 1.74 A, and 0.33 A, respectively. The uncertainty of the main calculated parameters are listed in Table 3. 4. Critical radiation level One limitation of solar heaters is that they cannot supply the sufficient thermal energy to heat the operating fluid at every radiation level and overcome heat losses. The radiation level that makes the thermal useful energy gain zero is known as critical radiation level (Duffie and Beckman, 1991). This definition is based on the thermal useful energy gain for solar heaters. In this paper, the critical radiation level based on the actual exergy gain and equivalent thermal useful energy gain is redefined.
Table 3 The uncertainty of main parameters. Parameter
Unit
Comment
Uncertainty in the temperature measurement Outlet air temperature Ambient air temperature PV panel temperature
°C °C °C
±0.5 ±0.5 ±0.5
in the air velocity measurement in the measurement of solar energy
m s1 W m2
±0.1° ±4
in in in in in in in in
% % kg s1 W W W % %
0.5 0.8 0.0003 5.8 0.138 0.025 0.9 1.2
Uncertainty Uncertainty radiation Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty
the the the the the the the the
measurement of voltage measurement of current mass flow rate thermal energy PV Power fan power first law efficiency second law efficiency
642
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
5. Results and discussion In this section, an effort is made to investigate the behavior of PV/UTC and UTC systems. Neglecting the effects of wind velocity and ambient temperature on the systems’ response facilitates comparison among systems. Experiments was performed at successive days to satisfy this assumption. Thus, in this paper, all plots are pictured as a function of solar radiation and appropriately interpolated. A confident conclusion can be drawn by using this strategy. It should be noted that the results obtained from the systems were conducted on hours with clear sky condition. 5.1. Air temperature rise From the air temperature rise standpoint, a comparison among all tested systems is made and pictured in Fig. 4. As expected, installing PV panel on UTC reduces the air
temperature rise at every radiation level (Fig. 4a). This can be ascribed to the fact that part of incident solar energy absorbed by PV panels in PV/UTC systems. Therefore, the air is heated less in PV/UTC systems. Also, it can be concluded from this figure that the air temperature rise of PV/UTC systems varies non-linearly against solar radiation, but varies linearly for UTC systems similar to the experimental results obtained by Hollick (1994). Fig. 4b and c exhibits the effect of mass flow rate on the air temperature rise of UTC and PV/UTC, respectively. These figures illustrate that the air temperature rise decreases as the mass flow rate increases. This behavior becomes more pronounce when there is no PV panel on the UTC. There is also another point that should be mentioned: low dispersion of the data points, observed in all plots, can be attributed to errors occurred in the experiments and the neglecting effects of the ambient temperature and wind velocity on the systems’ response.
Fig. 4. Air temperature rise as a function of solar radiation (a) all tested configurations with 0.009 kg s1 mass flow rate, (b) all tested configurations with 0.0215 kg s1 mass flow rate, and (c) 1PV/UTC at different mass flow rates.
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
5.2. Photovoltaic panel temperature PV panel temperature strongly depends on wind velocity, ambient temperature, and solar radiation. Therefore, the PV panel temperature is hourly studied as the term of PV panel and ambient temperature difference. Fig. 5a and b shows the effect of mass flow rate on the capability of cooling PV panel. As can be seen in these figures, it can be derived that compared with PV alone, the PV/UTC system with 0.009 kg s1 mass flow rate is not efficient from the cooling capability standpoint. This result is very interesting when it can be seen that compared the PV alone, the PV/UTC with 0.0215 kg s1 mass flow rate is able to cool PV panel more. To put it simply, the value of mass flow rate specifies whether or not the air flow sucked through UTC is capable of cooling the PV panel located on it. In the case of Config. III (2PV/UTC), it can be observed in Fig. 5c that the PV panel in 1PV/UTC system has higher temperature than both PV panels in 2PV/UTC system. This is can be attributed to
643
the fact that in 2PV/UTC system, the air passing between the PV panels and UTC has lower temperature than that in 1PV/UTC system. 5.3. Photovoltaic power The two parameters determining the current–voltage (I–V) characterization curve of a PV panel are PV panel temperature and solar radiation. As mentioned before, the PV temperature depends on the solar radiation, ambient temperature, and wind velocity. In Photovoltaic–thermal (PV/T) systems, as well as wind velocity, all heat transfer convection mechanisms influence the PV panel temperature. Also, the current–voltage (I–V) characterization curves of the DC fans and resistor are powerfully determinant. Therefore, due to existence of large number of effective factors, the study of photovoltaic power is so difficult. But based on these interpretations, under similar experiment conditions, the PV panel temperature can be used as an appropriate indicator to investigate the hourly
Fig. 5. Hourly variation of PV panel and ambient temperature difference (a) PV panel alone and 1PV/UTC with 0.009 kg s1 mass flow rate, (b) PV panel alone and 1PV/UTC with 0.0215 kg s1 mass flow rate, and (c) 1PV/UTC and 2PV/UTC with 0.0215 kg s1 mass flow rate.
644
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
variation of PV power. Also, it should be noted that the PV power decreases as the PV panel temperature increases. Using PV temperature as an indicator (Fig. 5c) can explain how PV panel located down the UTC has higher PV power in 2PV/UTC system (Fig. 6).
to thermal energy gain. The fact is that the thermal exergy is lower than electrical power. Negative values for the second law efficiency at low solar radiation (see Fig. 8a) exhibit that these systems consume more electrical exergy for fans than the exergies recovered.
5.4. First law efficiency
5.6. Electrical-to-thermal ratio number
For all tested configurations, the simple first law efficiency (r = 1) as a function of solar radiation is shown in Fig. 7. The thick lines, seen in this figure, are related to the systems 0.0215 kg s1 mass flow rate. All efficiency lines are interpolated from experimental data point. To keep the figure legible, the experimental data points are not displayed. It can be inferred from Fig. 7 that UTC is more efficient than PV/UTC systems from the simple first law efficiency standpoint. Also, a linear reduction pattern for UTC system can be observed whereas the first law efficiency of PV/UTC follows a parabolic pattern when the first law efficiency is plotted against solar radiation (Fig. 7). One needs to note that in UTC case, with the increase of solar radiation, the first law efficiency decreases in spite of the fact that the air temperature rise (i.e. useful thermal energy gain) increases (see Fig. 4a). This can be imputed to the fact that with the increase of solar radiation, received energy (i.e. solar radiant energy) grows more than useful energy. As depicted in Fig. 7, PV/UTC systems with fewer PV panel have better performance.
As mentioned before, the electrical-to-thermal ratio number acts an important role in design of systems like PV/UTC systems. A selection of obtained results related to this ratio is presented in Fig. 9. The first interesting result that can be derived is that with the increase of ratio number, for the UTC system, the reduction trend for r = 1 (i.e. simple first law efficiency) is changed to a growing trend for r = 9 (Fig. 9a). For UTC system, the growing trend for r = 9 is similar to the trend for this system for second law efficiency (Fig. 9a). This can be attributed to the worth inequality between the thermal and electrical energy. The same as exergy concept, the ratio number equal to 9 shows that the electrical energy worth is more significant than thermal energy worth. Generally, with the increase of ratio number, the systems’ response approaches to similar responses obtained based on the second law efficiency. The obtained results for 1PV/UTC and 2PV/UTC (Fig. 9b and c) also confirm this fact.
5.5. Second law efficiency
As discussed before, the determination of the critical radiation level based on the actual exergy gain is important. It can be deduced form Fig. 8 that the critical radiation level depends on the number of PV panels. The greater
Fig. 8 exhibits the second law efficiency as a function of solar radiation for all tested configurations. As can be observed, all PV/UT systems follow a parabolic trend while UTC follows a growing linear trend. It can also be concluded that PV/UTC systems are preferable from the second law efficiency standpoint. This can be imputed to the fact that the PV/UTC produces electrical power in addition
Fig. 6. Hourly variation of PV power for 2PV/UTC with 0.0215 kg s1 mass flow rate.
5.7. Critical radiation level
Fig. 7. First law efficiency vs. solar radiation for all tested configurations.
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
645
Fig. 8. Second law efficiency vs. solar radiation (a) all tested configurations with 0.009 kg s1 mass flow rate and (b) all tested configurations with 0.0215 kg s1 mass flow rate.
Fig. 9. Equivalent thermal efficiency vs. solar radiation at different ratio numbers (a) UTC with 0.009 kg s1 mass flow rate, (b) 1PV/UTC with 0.0215 kg s1 mass flow rate, and (c) 2PV/UTC with 0.0215 kg s1 mass flow rate.
646
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
Fig. 10. Mean efficiency for UTC, 1PV/UTC, and 2PV/UTC systems at different mass flow rates (a) r = 1, (b) r = 4, (c) r = 9, (d) second law.
number of PV panels yields a decrease in critical radiation level. Generally, 2PV/UTC system, due to having PV panel and producing more electrical power, has the lowest critical radiation level. Also, it can be derived that since the mass flow rate increasing would lead to an increase in the fan power, it yields an increase in critical radiation level (see Fig. 8). 5.8. Comparison of systems For better comparison of the systems from the first and second law efficiency standpoints, the mean efficiencies (at the interval of 300–900 W m2) are shown in Fig. 10 at different mass flow rate for various electrical-to-thermal ratio numbers and second law efficiency. As can be concluded from this figure, the UTC with higher mass flow rate is the most efficient system when the comparison criteria is the equivalent thermal efficiency with ratio numbers 1 and 4. It should be noted that Coventry and Lovegrove (2003) suggested that the most suitable approach for a PV/thermal system should use 4.2 as a ratio based on the
renewable energy market. In the case of ratio number 4, the value of mass flow rate specifies whether or not the greater number of PV panels yields to better performance. From the equivalent thermal efficiency with ratio number 9 and second law efficiency standpoints, the greater number of PV panels would lead to an improvement in the performance. All systems with higher mass flow rate have better performance from the equivalent thermal efficiencies viewpoints while the second law efficiency of all systems decreases as the mass flow rate increases. 6. Conclusions A detailed experimental study was carried out to evaluate the energetic and exergetic efficiencies of three types of UTC and PV/UTC systems .The uncertainty of main parameters occurring during the measurements was also conducted. Based on the results obtained from the experiments, the following conclusions have been drawn: (1) for all PV/UTC systems, the air temperature rise follows a parabolic pattern while it follows a reduction linear pattern for
M. Gholampour et al. / Solar Energy 110 (2014) 636–647
UTC system. (2) The value of mass flow rate specifies whether or not the air flow sucked through UTC is capable of cooling the PV panel located on it. (3) PV/UTC systems are preferable from the second law efficiency standpoint due to producing electrical power in addition to thermal energy gain. (4) Negative values of the second law efficiency exhibit that the system uses more electrical exergy for fans than exergiy that it recovers at low solar radiation. (5) With the increase of electrical-to-thermal ratio number, the systems’ response approaches to similar responses obtained based on the second law efficiency. (6) The greater number of PV panels would lead to a decrease in critical radiation level. (7) UTC with higher mass flow rate is the most efficient system when the comparison criteria is the first law efficiency for ratio number equal 1 and 4. (8) All systems with higher mass flow rate have better performance in the terms of the equivalent thermal efficiencies viewpoints while the second law efficiency of all systems decreases as the mass flow rate increases. References Athienitis, A.K., Bambara, J., O’Neill, B., Faille, J., 2011. A prototype photovoltaic/thermal system integrated with transpired collector. Sol. Energy 85, 139–153. Badache, M., Rousse, D.R., Halle´, S., Quesada, G., 2013. Experimental and numerical simulation of a two-dimensional unglazed transpired solar air collector. Sol. Energy 93, 209–219. Brinkworth, B.J., Marshall, R.H., Ibarahim, Z., 2000. A validated model of naturally ventilated PV cladding. Sol. Energy 69, 67–81. Chow, T.T., Pei, G., Fong, K.F., Lin, Z., Chan, A.L.S., Ji, J., 2009. Energy and exergy analysis of photovoltaic–thermal collector with and without glass cover. Appl. Energy 86, 310–316. Coventry, J.S., Lovegrove, K., 2003. Development of an approach to compare the ‘value’ of electrical and thermal output from a domestic PV/thermal system. Sol. Energy 75, 63–72. Duffie, J.A., Beckman, W.A., 1991. Solar Engineering of Thermal Processes. John Wiley & Sons, New York. Dymond, C., Kutscher, C., 1997. Development of a flow distribution and design model for transpired solar collectors. Sol. Energy 60, 291–300. Gawlik, K., Kutscher, C., 2002. Wind heat loss from corrugated, transpired solar collectors. J. Sol. Energy Eng. 124, 256–261. Gawlik, K., Christensen, C., Kutscher, C., 2005. A numerical and experimental investigation of low-conductivity unglazed, transpired solar air heaters. J. Sol. Energy Eng. 127, 153–155.
647
Gholampour, M., Ameri, M., 2014a. Design considerations of unglazed transpired collectors: energetic and exergetic studies. J. Sol. Energy Eng. 136, 031004. Gholampour, M., Ameri, M., 2014b. Energy and exergy study of effective parameters on performance of photovoltaic/thermal natural air collectors. J. Sol. Energy Eng. 136, 031001. Greig, D., Siddiqui, K., Karava, P., 2012. An experimental investigation of the flow structure over a corrugated waveform in a transpired air collector. Int. J. Heat Fluid Flow 38, 133–144. Gunnewiek, L.H., Brundrett, E., Hollands, K.G.T., 1996. Flow distribution in unglazed transpired plate solar air heaters of large area. Sol. Energy 58, 227–237. Hollick, J.C., 1994. Unglazed solar wall air heaters. Renewable Energy 5, 415–421. Hollick, J.C., 1998. Solar cogeneration panels. Renewable Energy 15, 195– 200. Kline, S.J., McClintock, F., 1953. Describing uncertainties in singlesample experiments. Mech. Eng. 75, 3–8. Kutscher, C.F., 1994. Heat exchange effectiveness and pressure drop for air flow through perforated plates with and without crosswind. J. Heat Trans. 116, 391–399. Kutscher, C.F., Christensen, C.B., Barker, G.M., 1993. Unglazed transpired solar collectors: heat loss theory. J. Sol. Energy Eng. 115, 182– 188. Leon, M.A., Kumar, S., 2007. Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors. Sol. Energy 81, 62–75. Naveed, A.T., Kang, E.C., Lee, E.J., 2006. Effect of unglazed transpired collector on the performance of a polycrystalline silicon photovoltaic module. J. Sol. Energy Eng. 128, 349–353. Petela, R., 1964. Exergy of heat radiation. J. Heat Trans. 86, 187–192. Shahsavar, A., Ameri, M., 2010. Experimental investigation and modeling of a direct-coupled PV/T air collector. Sol. Energy 84, 1938–1958. Shahsavar, A., Ameri, M., Gholampour, M., 2011. Energy and exergy analysis of a photovoltaic–thermal collector with natural air flow. J. Sol. Energy Eng. 134, 011014. Summers, D.N., 1995. Thermal simulation and economic assessment of unglazed transpired collector systems. In: Wisconsin Energy Bureau, University of Wisconsin USA. Tonui, J.K., Tripanagnostopoulos, Y., 2007. Air-cooled PV/T solar collectors with low cost performance improvements. Sol. Energy 81, 498–511. Van Decker, G.W.E., Hollands, K.G.T., Brunger, A.P., 2001. Heatexchange relations for unglazed transpired solar collectors with circular holes on a square or triangular pitch. Sol. Energy 71, 33–45. Zondag, H.A., de Vries, D.W., van Helden, W.G.J., van Zolingen, R.J.C., van Steenhoven, A.A., 2003. The yield of different combined PV– thermal collector designs. Sol. Energy 74, 253–269.