CHAPTER - 4. Improvement in Energy Efficiency of a Solar Photovoltaic Panel by Thermal Energy Recovery

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1 CHAPTER - 4 Improvement in Energy Efficiency of a Solar Photovoltaic Panel by Thermal Energy Recovery Summary: As explained in chapter 2, the electrical efficiency of solar photovoltaic (PV) panel decreases with increase in its temperature because of its negative temperature coefficient. The conventional solar PV panel has the conversion efficiency of only 5-17%; this means, about 83-95% of incident energy is wasted and the proposition of recovering energy from solar PV panel can tap more thermal energy than electrical energy generated by PV panel itself. The heat was transferred by direct contact heat exchange with flowing water from top of the panel and bottom of the panel. Direct contact heat exchange from top surface was found more efficient in recovering energy as well improving the performance of PV panel. The refraction of light as it passes through the water layer straightens the incident radiation. The straightened radiation along with lower temperature of PV panel synergistically increases photovoltaic conversion efficiency. The computational fluid dynamics simulation of PV panel temperature closely resembled the experimental data. There is a potential to recover energy at larger scale for large scale solar PV installations. Thus, the present work proposes the win-win scenario of improved panel performance by controlling its temperature and recovery of thermal energy for alternate applications. Hiren D. Raval 52 PhD Thesis

2 Published peer-reviewed International Journal paper: Hiren D. Raval*, Subarna Maiti*, Ashish Mittal (2014) Computational fluid dynamics analysis and experimental validation of improvement in overall energy efficiency of a solar photovoltaic panel by thermal energy recovery, Journal of renewable and sustainable energy 6, pp , ISSN Research gap identification As discussed in literature review (chapter-2), many researchers have attempted photovoltaic panel cooling. Despite the extensive research on heat transfer from solar PV panel, modelling and experimental validation of solar panel heat transfer with water cooling from top surface with overall energy perspective remains the research gap. This chapter addresses the heat transfer aspects from photovoltaic panel cooling to increase the panel efficiency and develop understanding on energy recovery aspects to address the following research questions: 1. Can the temperature of solar photovoltaic panel be validated with theoretical temperature based on the computational fluid dynamics simulation with and without cooling of the photovoltaic panel? 2. Can the overall energy efficiency of converting solar radiation to electricity and captured thermal radiations be calculated with cooling and the same can be compared without cooling? 3. Is there any other phenomenon apart from cooling that may lead to increase in energy efficiency? 4.2 Experimental Heat transfer from solar photovoltaic panels poses the challenge that the panel efficiency should improve, understandably there should not be any obstruction in incident solar radiation over the panel. Hiren D. Raval 53 PhD Thesis

3 The direct contact heat exchanger system was designed with the coolant being water since radiations are incident from top; the heat exchange from was planned to control the temperature of PV panel. All the sides and back surface of the panel were properly insulated with calcium silicate insulation. Rationale was to utilize the maximum thermal energy and minimize the losses of thermal energy, at the same time achieve higher photovoltaic conversion efficiency Materials Solar PV panel -70 Wp, frame structure, Rheostat, water tank, Thermocouples, pyranometer (Kipp & Zonen CM4 pyranometer) Method The PV panel was kept at 20 o inclination in southward direction to get the optimal access to solar radiation with reference to the location Bhavnagar, India, Co-ordinates: o N and o E as shown in figure 1. One PV panel was provided cooling from top, whereas the other panel was kept without cooling. The variable resistance system (Rheostat) was used to measure the V-I (Voltage- Current) performance of PV panel. As shown in figure 4.1, the system where, cooling was provided from top comprised of the perforated pipe over its length at top, perforations being 2 mm in diameter. The flow rate of cooling water was varied from 1 liter per minute to 2 liter per minute and the V-I performance of the PV panel was evaluated. The water at outlet was drained out in a tank open to atmosphere and was then recirculated using a DC (direct current) Kemflo make pump. The nominal flow rate of the pump was 1 Lmin -1. Two pumps are operated for getting flow of 2 Lmin -1. FIGURE 4.1: Water flowing from top of the solar photovoltaic panel Hiren D. Raval 54 PhD Thesis

4 4.3 CFD Simulation PV panel was exposed to solar radiation out of which a fragment is getting converted into electricity. The energy balance across the solar PV panel is given by, Rate of accumulation of heat = Rate of heat input Rate of heat output + Rate of heat genera ρ av Cp av dt/dt = q sw - q lw - q conv Pout...(4.1) ρ av = Average Density of PV panel and glass Cp av = Average Specific heat of PV panel and glass T = Temperature of PV panel (It is assumed that temperature of PV panel and glass above it are same) q lw = long wave radiation q sw = short wave radiation q conv = Heat loss by convection P out = Power output of PV panel -q lw = A σ [(1+CosƟ)/2*Є sky T sky 4 + (1-CosƟ/2)* Є ground T ground 4 ЄpanelT panel 4 ]...(4.2) σ = Stephen Boltzman Constant Є = Emissivity Ɵ = Angle between panel and ground q sw = αφa...(4.3) q conv = A (hc natural +hc forced )(T panel T env )...(4.4) Substituting (4.2), (4.3) and (4.4) in (4.1) ρ av Cpav dt/dt = A σ [(1+CosƟ)/2*Є sky T sky 4 + (1-CosƟ/2)* Є ground T ground 4 Є panel T panel 4 ] + αφa - A (hc natural +hc forced )(T panel T env ) P out Є sky = T Є ground = Emissivity of concrete =0.94 Є panel = 0.85 Ɵ = 20 Hiren D. Raval 55 PhD Thesis

5 A = Area of panel =0.628 sq. m. σ = 5.67 X 10-8 W/m 2 K 4 ρ av = 3015 kg/m3 Cp av = J/g o C hc natural = 1.31 (T glass - T air ) 1/3 hcforced = v The ANSYS Computational fluid dynamics software was used to simulate this model. Assumptions made in simulating the model using ANSYS CFD tool are depicted below. A constant water thickness of 2 mm above the panel is considered. The flow is steady. The water temperature varies with time (Water absorbs the solar radiation). Infrared: 50%, Visible radiation: 40% and ultraviolet: 10% of the total incident radiation. Absorptivity of opaque material is 80%. Glass transmissivity: 80%, Absorptivity: 20% and no reflectivity. Pipe material does not absorb radiation. Simulations were carried out applying boundary conditions with ANSYS CFD software until the solution converges.the photovoltaic panel comprised of the following different layers, physical properties of each layer is given in table 4.1[1, 2]. TABLE 4.1: Physical properties of the constituents of PV panel Layer Thickness t (m) Thermal Conductivity K Density (kg/m 3 ) Specific heat Cp (J/kg K) (w/m K) Tedlar Rear contact 10X EVA 500X PV Cell 225X ARC 100X Glass Hiren D. Raval 56 PhD Thesis

6 As there are perforations in the feed water pipe, the geometry was created by considering 8 inlets and 4 outlets of water from the panel. Air domain has been considered surrounding the physical geometry. Thereafter, geometry was also created by considering 1 slit type inlet and 1 outlet to match the experimental condition. Figure 4.2 shows the meshed domain of 8 inlets, 4 outlets system and figure 4.3 shows the sliced domain of this system made using ANSYS CFD software. Skewness of mesh is 0 and orthogonal quality is 1. FIGURE 4.2: Meshed domain of 8 inlets 4 outlets system FIGURE 4.3: Sliced domain of 8 inlets 4 outlet system Figure 4.4A shows the schematic of model and 4.4B shows the schematic of model mesh. Meshing becomes denser near boundaries as the software will solve equations at the crossing points. The denser meshing near boundaries will improve the quality of solution. Hiren D. Raval 57 PhD Thesis

7 4.4 Results and Discussions: Initial part of this section discusses the results with 8 inlets, 4 outlet system; the later part simulates the system with single inlet and single outlet. FIGURE 4.4A: Schematic of the model FIGURE 4.4B: Schematic of the model mesh Figure 4.5 shows the simulation results of temperature on the back side of the panel as well as on the top of the panel. As there is insulation on the back side, the heat cannot escape from the back. However, the temperature is within control with the average temperature near 36 o C at the back side except the extreme bottom corner point where the temperature reached upto 60 o C. The temperature is particularly higher at the bottom corners of the panel where water cannot flow and remain stationary whereas; the temperature is well under control about 27 o C on the top side as visible from the image. The purpose of cooling from top side is to cool the array surface and insulation at the back side prevents heat loss. The results show that the cooling is effective as the average temperature does not reach very high on the back surface. Hiren D. Raval 58 PhD Thesis

8 Direct solar irradiance: 800 w/m2 Inlet cooling water temperature: 30 o C FIGURE 4.5: Temperature of photovoltaic panel back surface and top surface Total Inlet flow: 1 Liter/minute FIGURE 4.6: Velocity of water over photovoltaic panel Figure 4.6 shows the velocity of water over photovoltaic panel. The velocity is low (less than 0.05m/s) over the panel, however it is higher at entry and exit points. The central part is photovoltaic panel over which the velocity profile is shown. The temperature of photovoltaic panel as shown in Figure 4.5 shows that this velocity was good enough to control the temperature. Hiren D. Raval 59 PhD Thesis

9 Direct solar irradiance: 800 w/m2 Inlet cooling water temperature: 30 o C FIGURE 4.7: Temperature of cross section of photovoltaic panel Figure 4.7 shows the temperature of photovoltaic panel across the cross-section. It is explicit that the temperature increases at the bottom because of insulation. When the thermal energy is transferred from top and it is not allowed to escape from bottom, it is understandable that the temperature of the back side will increase. The insulation is efficient to prevent heat loss and heat is transferred from top. Direct solar irradiance: 800 w/m2 Inlet cooling water temperature: 30 o C FIGURE 4.8: Temperature of cross section in z axis of photovoltaic panel Figure 4.8 shows the temperature of cross section in z axis of photovoltaic panel. The temperature varies because of natural convection of air above the panel. Hiren D. Raval 60 PhD Thesis

10 Total Inlet flow: 1 Liter/minute FIGURE 4.9: Velocity vector above the PV panel surface Figure 4.9 shows the velocity vectors above photovoltaic panel surface. Velocity is higher at inlet and outlet. At inlet, it is higher because the flow is coming from the small opening and at the outlet, the flow converges to a small exit. There were number of perforations over the length of pipe. Thus, to, simulate the actual experimental condition with more precision, the meshed domain with single inlet and single outlet is considered as shown in figure 4.10 with the velocity remaining nearly same as the actual experimental velocity. FIGURE 4.10: Meshed domain with single inlet/single outlet The meshing characteristics: Skewness: 0, Orthogonal quality: 1 The figure 4.11 demonstrates the velocity vectors along the streamline of water. The velocity of water is low i.e. approximately m/s for both the cases 1 LPM and 2 Hiren D. Raval 61 PhD Thesis

11 LPM. However, the increase in velocity is quick for 2 LPM flow when the flow converges to the exit point. Inlet flow: 1 Liter/minute Inlet flow: 2 Liter/minute FIGURE 4.11: Velocity vectors for the flow 1 LPM and 2 LPM at 13:00 Hrs The snapshot in Figure 4.12A and 4.12B demonstrates the comparison of PV panel temperature with 1 LPM and 2 LPM flow respectively at 10:00 Hrs and 13:00 Hrs. It can be seen from the simulation results that larger area of PV panel remains at lower temperature with increased flow rate, and higher flow rate is particularly required to maintain the temperature when solar radiation intensity is higher at 13:00 Hrs. When comparing the panel temperature without cooling as shown in Figure 4.13, it becomes clear that the panel temperatures are well within control with cooling. The panel temperature without cooling reaches upto 76 o C and the changes at different locations in the panel as a result of natural convection are indicated in figure The panel temperature is controlled within 40 o C by cooling even with lower flow rate of 1 LPM. Hiren D. Raval 62 PhD Thesis

12 Direct solar irradiance: w/m 2 Inlet cooling water temperature: 30 o C Direct solar Irradiance: W/m 2 Inlet cooling water temperature: 35 o C FIGURE 4.12A (Top) and 4.12B (Bottom): PV panel temperature with cooling at 10:00 Hrs 13:00 Hrs respectively Direct solar irradiance at 10:00 Hrs: W/m 2 at 13:00 Hrs: W/m 2 FIGURE 4.13: Panel temperature without cooling 10:00 Hrs and 13:00 Hrs Hiren D. Raval 63 PhD Thesis

13 Direct solar Irradiance: W/m 2 Inlet cooling water temperature: 30 o C Direct solar Irradiance: W/m 2 Inlet cooling water temperature: 33.5 o C FIGURE 4.14: Temperature across the cross section of PV panel at 10:00 Hr and 15:00 Hrs The figure 4.14 shows the temperature across the cross section of PV panel at 10:00 Hrs and 15:00 Hrs. It can be seen that the temperature of the bottom layer near insulation increases despite the panel top surface is maintained at close to 35 o C. This is because the heat is not getting wasted from the bottom on account of insulating layer at bottom. This also ensures the heat transfer direction from bottom up across the layers of PV panel as the day progresses. Hiren D. Raval 64 PhD Thesis

14 Direct solar irradiance: W/m 2 FIGURE 4.15: Temperature of panel cross section without cooling at 15:00 Hrs The figure 4.15 indicates the effect of natural convection. The air adjacent to PV panel gets heated as the temperature of PV panel increases and the density of air decreases at higher temperature. The light air climbs up the panel and increases the temperature at the top part of panel as compared to the bottom as visible in figure Thus, natural convection alters the PV panel temperature at different places. The ambient conditions on the days of experiment are as shown in table 4.2. TABLE 4.2: The environmental conditions on the days of experiment Condition 14 May 1 June Insolation (7 Hrs to 1730) W/m W/m 2 Average wind speed 1.4 m/s 1.4 m/s Average insolation (24 hrs) W/m W/m 2 Average ambient temperature o C o C The performance of PV panel is assessed by V-I Performance. In figure 4.16, the performance of PV panel has been demonstrated, where the peak power produced by PV panel with and without cooling are compared. It is clear that the peak power produced by PV panel improves with cooling where about 10% improvement in power output is observed at 13:00 hrs. The pump consumes 5 W power. Therefore, the net power produced Hiren D. Raval 65 PhD Thesis

15 with cooling has to be reduced by 5 W in each case. However, the gravity operated systems can also be designed where; net power will be the same as power produced. Peak Power produced by Solar PV panel (Watt) Time (Hours) Peak power Peak power without cooling With Cooling FIGURE 4.16: Performance comparison of PV panel with and without cooling Cooling water flow:1 LPM Total energy generated over the day with cooling was 333 watt-hour, whereas the total energy generated without cooling was 303 watt-hour. Temperature of PV panel with and without cooling -Cooling water flow: 1 LPM Temperature 40 of PV panel ( o C) Time of the day 1600 Avg.Temp. without cooling (Experimental) Average temperature of panel without cooling (by CFD simulation) Average temperature of panel with cooling (Experimental) Average temperature of panel with cooling (by CFD simulation) FIGURE 4.17: Temperature of PV panel with and without cooling It is explicit from figure 4.17 that there is a significant decline in PV panel temperature as a result of cooling from top surface at 1 LPM flow. The experimental results are compared Hiren D. Raval 66 PhD Thesis

16 with CFD simulation results and found in close conformity. When temperature reaches close to 60 o C at 14:00 hrs without cooling the panel, it is controlled well below 40 o C with cooling. Peak Power Produced by PV Panel (Watt) Time (Hours) Peak Power - With cooling Peak power- Without cooling FIGURE 4.18: Performance comparison of PV panel with cooling water flow-2 LPM and without cooling It is evident from the figure 4.18 that the peak power produced by PV panel improves as in the case of 1 LPM flow, however the improvement is substantial. There is 20% improvement in peak power produced at 13:00 hrs. Temperature of PV panel with and without cooling -Cooling water flow: 2 LPM Avg.Temp. without cooling (Experimental) Temperature of PV panel ( o C) Time of the day 1600 Average temperature of panel without cooling (by CFD Simulation) Average temperature with cooling (Experimental) Average temperature of panel with cooling (BY CFD simulation) FIGURE 4.19: Temperature of PV panel with and without cooling- Cooling water flow 2 LPM Hiren D. Raval 67 PhD Thesis

17 Fig indicates that the PV panel temperature decreases from 58 o C to 37 o C as a result of cooling with 2 LPM flow at 14:00 Hrs. The experimental results are in close conformity with the simulation results. In this way, the experimental results are in close confirmation with simulation results and it becomes explicit that panel performance improves as a result of cooling from top as the panel temperature is controlled below 40 o C. It is also important to know V-I performance of photovoltaic panel with and without cooling. The results below indicate the V-I performance of photovoltaic panel with and without cooling. V-I Performance of Panel at 1200 Hrs Current (Amp.) Voltage (V) With Cooling Without Cooling FIGURE 4.20: V-I performance of photovoltaic panel at 12:00 hrs Hiren D. Raval 68 PhD Thesis

18 Current (Amp.) V-I Performance of Panel at 1215 Hrs With Cooling Without Cooling Voltage (V) FIGURE 4.21: V-I performance of photovoltaic panel at 12:15 hrs Current (Amp.) V-I Performance of Panel at 1245 Hrs With Cooling Without Cooling Voltage (V) FIGURE 4.22: V-I performance of photovoltaic panel at 12:45 hrs Hiren D. Raval 69 PhD Thesis

19 V-I Performance of Panel at 1315 Hrs Current (Amp.) Voltage (V) With Cooling Without Cooling FIGURE 4.23: V-I performance of photovoltaic panel at 13:15 hrs V-I Performance of Panel at 1445 Hrs Current Amp Voltage (V) With Cooling Without Cooling FIGURE 4.24: V-I performance of photovoltaic panel at 14:45 hrs Hiren D. Raval 70 PhD Thesis

20 Current (Amp.) V-I Performance of Panel at 1515 Hrs Voltage (V) With Cooling Without Cooling FIGURE 4.25: V-I performance of photovoltaic panel at 15:15 hrs V-I Performance of Panel at 1545 Hrs Current (Amp.) With Cooling Without Cooling Voltage (V) FIGURE 4.26: V-I performance of photovoltaic panel at 15:45 hrs Figure demonstrates that V-I performance of photovolataic panel improves at higher resistance. At low resistance, current is slightly lower with the panel cooling, the relative current performance of panel improves at higher resistance; moreover voltage is substantially more with cooling. However, the actual applications are generally with higher resistance and therefore the performance with cooling is better. These data were taken over several days and average of the data has been reported. Hiren D. Raval 71 PhD Thesis

21 Refraction effect and improvement of panel performance The influence of refractive index in changing direction of the solar radiation has not been studied earlier. The reason of improvement in energy efficiency can also be attributed to the refraction effect because of flow of water above the panel. The angle of incident radiation in Bhavnagar was obtained by SOLPOS calculator. The angle changes as a result of refraction by the water layer. The refractive index of air and water and the incident angle are known. The angle after refraction can be calculated using Snell s law n 1 sin 1 = n 2 sin 2. Water has the refractive index [3]. Air has refractive index 1.0. The table 4.3 indicates the effect of refraction over the solar PV panel. TABLE 4.3: The Angle change as an effect of refraction by solar PV panel Time Angle Ө 1 Angle Ө 2 09: : : : : : : : : From the table 4.3, it is clear that the incident angle varied from to from Morning 09:00 hrs to evening 17:00 hrs. The angle changes after refraction and thus, the angle 2 varied from to In this way, the span of angle reduced from to In other words, the incident radiations were straightened as a result of refraction as shown in the figure These straightened radiations helped to improve the performance of the panel as they strike to panel at relatively larger angle as compared to the one without water layer on top. Hiren D. Raval 72 PhD Thesis

22 o o FIGURE 4.27 Radiation at a given point straightened as a result of refraction The experiment was performed to justify this finding. Ice was placed on the back side of the PV panel and water layer on the top of the other identical panel to maintain the temperature of PV panel at 35 o C in both cases. The peak output observed with ice on the back side was Watt, whereas the peak output with water layer on top was Watt at same time 13:00 Hrs (7.6% increase in power output). This made it clear that the refraction effect played its role to improve the panel performance. Typically, if the increase in power output is about 20%, 7.6% may be attributed to refraction effect and 12.4% may be attributed to cooling. Overall energy efficiency: The energy efficiency with and without cooling was worked out as a case to understand the effect of cooling on overall energy efficiency. It has been observed that the efficiency was raised from 6.68% to 40.42% with cooling as shown in table 4.4. Hiren D. Raval 73 PhD Thesis

23 TABLE 4.4: Energy efficiency calculations (For 1 Liter per minute flow) Time 12:00 Hrs 14:00 Hrs 16:00 Hrs Inlet water temperature(tw in ) Outlet water temperature(tw out ) Energy contained by water(watt) Peak power without cooling (watt) Peak power with cooling (watt) Difference (watt) Total power saving (watt) Watt/m Solar panel area (sq.m) Power incident on the panel (watt) Energy efficiency without cooling Energy efficiency with cooling (electrical + thermal) In this way, there is a significant improvement in overall energy (thermal + electrical) efficiency with the cooling from top. The heated water can be used for some application e.g. feed water to reverse osmosis. The higher temperature feed water improves permeability of membrane and more product water can be generated from given membrane area. 4.5 Conclusion The following conclusions can be derived from the experiments to control the photovoltaic panel temperature and theoretical study by computational fluid dynamics analysis. Photovoltaic panel demonstrates the poorer performance at higher temperature, thus cooling of photovoltaic panel is necessary to retain/enhance its efficiency. The panel temperature could be effectively controlled by transferring heat from top at low flow rate of 1 and 2 Liters per minute. Insulation at the back surface and sides ensured that heat is not lost and transfer of heat is bottom-up in the photovoltaic Hiren D. Raval 74 PhD Thesis

24 panel. CFD analysis is in close confirmation with experimental validation for the panel temperature. It is demonstrated that the refraction of incident solar radiation while striking to PV panel through the water film is beneficial. Refraction narrows the span of incident radiation over the panel and narrowing the span of angle variation is better from the point of view of panel performance. This is proven by increased power output in the panel with water flowing from top. The rise in water temperature reported indicates that there is a potential to tap the thermal energy. The higher temperature water can be used for desalination systems like membrane distillation, reverse osmosis etc. Reverse Osmosis water flux increases with increase in feed water temperature. It is prudent to utilize the feed water to Reverse Osmosis as cooling water and utilize the captured thermal energy as soon as possible. Interdisciplinary approach of tapping thermal energy from photovoltaic panel and thereby controlling its temperature along with utilizing the tapped thermal energy for useful application such as Reverse Osmosis can increase the overall energy efficiency of photovoltaic powered Reverse Osmosis. In this study, the overall energy efficiency has been increased from about 6% to 40% by direct cooling the array surface of PV panel from top at 1 liter per minute flow. References 1. S. Armstrong, W.G. Hurley (2010) A thermal model of photovoltaic panels under varying atmosphetic conditions, Applied thermal engineering, Volume 30, p. 1488, ISSN G. Notton, C. Cristofari, M. Mattei, P.Poggi (2005) Modeling of a double glass photovoltaic module using finite differences, Applied thermal engineering, Volume 25(17-18), p. 2584, ISSN X. Han, Y. Wang, L.Zhu (2011) Electrical and thermal performance of silicon solar cells immersed in dielectric liquids, Applied energy, Volume 88, p. 448, ISSN Hiren D. Raval 75 PhD Thesis