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Experimental study on a direct water heating PV-T technology
Solar Energy 176 (2018) 604–614
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Experimental study on a direct water heating PV-T technology a,b,⁎,1
Jiajun Cen , Roan du Feu William Janssena a b c
a,b,1
a,c
a
T a
, Matus E. Diveky , Catriona McGill , Oliver Andraos ,
Desolenator B.V., Koningsplein 11, 6224EB Maastricht, the Netherlands The Grantham Institute & Department of Chemical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ London, UK Laboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, CH-8093 Zurich, Switzerland
A R T I C LE I N FO
A B S T R A C T
Keywords: PV-T system design Water heating
This paper details a field study and a theoretical model of a PhotoVoltaic-Thermal (PV-T) system consisting of a solar PV panel with a thermally insulated water reservoir underneath. Unlike conventional PV-T systems, water is in direct contact with the glass solar PV panel. Thus, metallic tubular heat exchangers are omitted in this design. During operation, the PV-T panel is tilted, and cold water is pumped into the reservoir from the side closest to the ground. This achieves an active cooling of the PV panel maintaining an optimal electrical efficiency. Generated electricity is used to operate pumps and run the control system, while excess electricity is stored in a battery to be utilised as desired. The system and the inlet water absorb solar thermal energy and as a result they increase in temperature. In our field study, we explore the viability of this system as a self-powered, off-grid, solar collector and find that it can provide enough hot water of approximately 80 °C for a household of four in areas where average daily solar irradiance is >4.5 kWh/m2. We varied (1) the exposure angle, (2) PV panel type and (3) reservoir depth and found that in the limited ranges covered by our experiments the optimised configuration is with (1) an exposure angle of 14.7°, (2) a bifacial mono-crystalline solar PV panel and (3) a reservoir depth of 12 mm (given a fixed inlet water flow rate). The theoretical model of the device that is built, tracks energy losses with time and outputs the average reservoir temperature at each five-minute time-step. We validated this model with the obtained data during the field study. Then, this model is used to perform a sensitivity analysis on the parameters in testing and beyond (such as primarily insulation types and thicknesses), to provide a direction for further development and improvement.
1. Introduction Recently, the emphasis of research on harvesting solar energy has been on developing new types of photovoltaic cells with higher electrical efficiencies or in the design of novel solar concentrators. The focus of such research is either on harvesting high quality solar electric energy or the lower quality solar thermal energy. Less research has been devoted to improving PV-T technologies, which harvest both solar electric and thermal energy, and their designs. With the rapid drop in cost price of existing ‘off-the-shelf’ PV panels in the last decade, the opportunity has arisen to reconfigure these PV panels into PV-T panels. This opens the way for applications which require both electrical and thermal energy such as (1) heating/cooling of buildings, (2) heating of water for household applications and (3) desalination. The final of these topics will be covered specifically in a forthcoming paper. The justification for PV-T technology is sound: the electrical
conversion efficiency of a PV panel reduces with an increase of temperature (Skoplaki and Palyvos, 2009). Therefore, combining a thermal collector with a PV panel should have the dual effect of collecting thermal energy to produce hot water while simultaneously cooling the panel, and therefore increasing its electrical efficiency. The market for this combined technology is still small, but already the variety of possible applications is becoming clear for both large- and small-scale systems. For example, small-scale systems have already been integrated into some homes in the UK and France where the electricity output is used to power the home and the collected hot water for central heating or domestic hot water production (Good et al., 2015). In Sweden, the heat from large-scale PV-T systems has been used to improve the performance of undersized boreholes (Good et al., 2015). More recently, and of particular interest here, hot water production from such PV-T systems has been proposed as the first stage in a desalination process (Janssen, 2015).
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Corresponding author at: Desolenator B.V., Koningsplein 11, 6224EB Maastricht, the Netherlands. E-mail address: [email protected] (J. Cen). 1 Both authors contributed equally. https://doi.org/10.1016/j.solener.2018.10.062 Received 21 July 2018; Received in revised form 19 October 2018; Accepted 22 October 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
I L Lref Ltran Lconf
Lconr Lrad Tres Tf
Tr Tamb Tsky σ QreMD QreSR ∊p Rf hf hr si ki
solar irradiance (W/m2) total energy loss (J) energy loss to reflection off the front face of the device (J) energy loss to transmittance through the front face of the device (J) convective heat loss through the front face of the device (J) convective heat loss through the rear of the device (J) radiative losses through the front face of the device (J) reservoir temperature (°C) front face temperature (°C)
rear face temperature (°C) ambient temperature (°C) sky temperature (°C) Stefan-Boltzmann Constant (W/m2 K4) acrylic reflection coefficient at midday acrylic reflection coefficient at sunrise acrylic emissivity front face thermal resistance (W/m·K) front face heat transfer coefficient (W/m2 K) rear face heat transfer coefficient (W/m2·K) insulation thickness (m) insulation thermal conductivity (W/m·K)
efficiencies of 64% (Dubey and Tiwari, 2008), 50–63% (Saitoh et al., 2003), and 50% (He et al., 2006). Recently, other PV-T system designs have been considered, modelled, and built, although the literature on them is far less extensive. One such example is the PV-T concentrator (Tyagi et al., 2012), an experimental design which produced high thermal and electrical efficiencies of 58% and 11% , respectively (Coventry, 2005). The experimental research presented here tests the feasibility of a new design of PV-T solar collector that can be readily built by attaching a reservoir under an off-the-shelf solar panel. It is entirely self-powered, can be used off-grid, can provide hot water for a family of four for purposes such as showering and cleaning, and in which any excess electricity generated can by stored in a battery. The PV-T converter proposed differs to the traditional PV-T system in that it functions by flowing water across the back of the entire PV panel, through a reservoir that lies between the panel and a metal plate, as opposed to through a tubular heat exchanger affixed to the back of the PV panel. This method has the advantage that the water absorbs solar energy directly as well as absorbing the thermal energy which is transferred from the PV panel. This is a design that has not yet been tested in the literature. A similar design has been modelled computationally (Prakash, 1994), reporting efficiencies of 9.2% (electrical) and 64% (thermal), but this is the first time such a design has been built and trialled experimentally. The efficiencies of the system in harvesting solar energy and converting it into both electrical and thermal energy are calculated. The performance of a PV panel with a large temperature gradient across its
To date, the vast majority of proposed liquid PV-T systems are of a very similar design. An insulated casing is fitted on the back of a solar panel, and a metal sheet is attached to the back of that panel to absorb and transfer solar thermal energy. A tubular heat exchanger is run back and forth across the metal sheet, heating the water as it passes under the PV panel. Many theoretical and numerical models have been made of such systems. One such model found that running a heat exchanger along 1 m of a PV panel with a flow rate of 0.3 ml/s could raise water temperature from 27 °C to 57 °C (Bergene and Løvvik, 1995), which would result in approximately 1 L of ~60 °C water being produced per hour. This system had a thermal efficiency of 58% and an electrical efficiency of 11.1%. The same study found that increasing flow rate further did not result in any significant gain in the electrical efficiency of the PV panel. A second model of a ∼1 m2 collector in Zondag et al. (2002), with a higher flow rate of 16 ml/s showed a lower rise in outflow temperature, from an ambient 20 °C to a peak of 32.5 °C. This system had lower thermal and electrical efficiencies of 33% and 6.7% , respectively. A more detailed study looked at installing a specific PV-T design on rooftops Kalogirou (2001). This model involved 5 m2 of PV panel with water flowing through a heat exchanger at 7 ml/s, and showed a daily production of 59 L of 50 °C water, with a 7.7% electrical efficiency, giving an overall efficiency of the system of 32%. Experimental studies were also carried out using this traditional PVT system, with one particular study finding that very high efficiencies, of ∼12.5% (electrical) and ∼68% (thermal), were achievable, giving an overall efficiency of approximately 80% (Tripanagnostopoulos et al., 2002). Other experimental studies have shown lower overall
Fig. 1. Sketch of the PV-T system configured with an exposure angle of θ = 17° and dimensions in millimetres. There are four temperature sensors installed (marked with black triangles). From left to right (lowest to highest) they are T1, T2, T3 and T4. 605
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surface is so far undocumented. This is the first report in a study that will investigate the further application of this system in the promising technology of water desalination through distillation, in which producing water of as high a temperature as possible is paramount. Therefore, the highest attainable temperatures within the reservoir are measured, along with the volume of hot water (>80 °C) that can be produced. The device is tested in the field in Cyprus and the parameters of exposure angle, reservoir depth are varied with the aim of maximising hot water production and thermal efficiency. Different PV panel types are also tested with the aim of maximising electrical efficiency. Finally, a theoretical model of the system is presented, and validated with the experimental data. The model calculates energy losses of different forms throughout the day to produce a temperature profile of the reservoir with time. This can be used to investigate further improvements to the presented device by performing sensitivity analysis on parameters that were not covered by the experimental testing. The result is an efficient, self-powering, novel PV-T system design capable of providing hot water to a family that can be built by attaching a reservoir to an off-the-shelf PV panel, along with a model of the system that can be used to tune a set of parameters to the users needs and to the average ambient conditions in their location.
control system, while any excess energy is stored in batteries. Bifacial PV panels are worth considering as they will capture sunlight reflected off the metallic back surface of the reservoir, as well as due to the internal reflection gains associated with them. In these solar panels the photovoltaic wafers are sandwiched between low-iron tempered glass with ARC (EN 12150) with a thickness of about 5.3 mm. The length and width of the PV panel used is approximately 1675 mm by 1001 mm, and their operational temperature range is −40 °C to 85 °C. To build the described PV-T system, the anodized aluminium frame around the PV panel glass is removed. This is placed into a stainless steel 316L reservoir with an EPDM rubber seal running around the reservoir edge, between the panel and the reservoir back. To insert temperature sensors and feed water into the cavity between the glass PV panel and the metal back of the reservoir, stainless steel 316L tubes are pinched through the EPDM rubber. Finally, a specially designed metallic frame of 1694 mm × 1048 mm is placed over the reservoir edges and fixed using nuts and bolts. Equal pressure needs to be applied around the entire EPDM seal to prevent water leakage. Special care needs to be taken where the seal is pierced by a stainless steel 316L tube. The solar panel and two AGM 12V-60Ah batteries are connected and controlled by a Victron energy BlueSolar charge controller (MPPT 75 115) (Victron Energy Blue Power, 2018). With the Victron controller, the amount of electricity generated by the panel can be registered. A Hukseflux SR05-DA2 (Hukseflux Thermal Sensors, 2018) pyranometer is installed to log the total solar irradiance (W/m2). Furthermore, four temperature sensors are installed to measure the temperature of water inside the reservoir, the so-called reservoir temperature (as illustrated in Fig. 1), and two thermocouples are installed to measure temperature of the inlet water tank and of the outlet water tank. To pump water in and out of the reservoir a peristaltic pump is placed close to the inlet location. The underside of the reservoir is insulated using a 1.5 cm styrofoam sheet, and the sides of the reservoir are insulated with a 2 mm Armaflex insulation layer. A sheet of acrylic glass is placed over the top of the reservoir to create an insulating layer. This acrylic has a low thermal conductivity of 0.2 W/(mK), a 92% transmission of visible light (3 mm thickness) and real refractive index of 1.4905 (Coventry, 2005) at 589 nm.
2. Description of the PV-T system The PV-T system proposed here consists of a PV panel with a thermally insulated water reservoir underneath as illustrated in Fig. 1. The water reservoir is filled and the panel is configured at an angle towards the sun. The reservoir can either be held full of stagnant water, or cold water can be pumped into it from the bottom and as it flows upward in direct contact with the PV panel it is heated by sunlight. This pumping can be continuous or it can be triggered when the top of the reservoir reaches a set temperature. This allows water within a desired temperature range to be collected. The water exits the system from the top of the reservoir and is stored in a water tank. Flow rate through the reservoir is on average 6 ml/s. In this experiment, three types of PV panel are used. They are monocrystalline (SW 270 mono) (Solar World, 2018a), poly-crystalline (SW 250 poly) (Solar World, 2018b) and bifacial mono-crystalline (SW 280 Duo) (Solar World, 2018c) panels from SolarWorld. The electricity generated by the PV panel is used to power the water pumps and the
Fig. 2. Images of the experimental setup in Limassol (Cyprus) in summer 2017. (a) A close image of the PV-T system. (b) Layout of the three PV-T systems with wooden boxes to keep inlet water around ambient temperature. 606
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3. Experimental setup
4.2. Solar exposure angle
Three equivalent PV-T systems were shipped to and assembled in Limassol, Cyprus, for experimental work from July to September 2017. The three systems, shown in Fig. 2, were aligned parallel to each other such that they all faced directly south with an initial exposure angle of 12.7° from horizontal. The goal of this field experiment was threefold; to maximise hot water (80 °C) production from the devices, to calculate and optimise achievable thermal and electrical efficiencies, and to demonstrate the feasibility of the devices as self-powering solar collectors. Prior to any experimental investigation, the PV-T devices were benchmarked to ensure the same electrical and thermal performance under the same conditions. This lasted for three days, during which time reservoirs were left stagnant and reservoir temperatures were recorded across the day in five minute intervals. This time step is used throughout all the experiments. After consistency in performance between the devices had been confirmed, the parameters shown in Table 1 were tested. Solar exposure angle was tested first. During this time one of the devices was suffering from leakage problems that were subsequently fixed, therefore only two devices could be used for testing. The two devices were set to 11.3° and 14.7° and left with stagnant water for three days, exposure angles were then switched and left for a further three days. PV panel type was tested second, during this test each device was fitted with a different PV panel as shown in Table 1. For this test all three devices were used, as the leakage problem was resolved. Devices were left stagnant and data was collected for five days. The third test was for depth of the water reservoir. Each device was fitted with a bifacial panel and set to an exposure angle of 11.3°, then the devices were each fitted with a water reservoir of a different depth as shown in Table 1. Reservoirs were initially left stagnant and data was collected for three days, then the reservoirs were run in continuous flow mode, where water was pumped through the system once the reservoir reached 80 °C, for a further three days. Finally the best performing reservoir, the 12 mm reservoir, was tested with the continuous flow mode for five hours, starting at (a) 9 am and (b) 11 am. The flow rate was consistent at 6 ml/s
Fig. 4 shows temperature data collected at different exposure angles of (a) at 14.7° and (b) at 11.3° from horizontal. Angles higher than 14.7° were not tested due to the pressures exerted by water inside the reservoir and the resulting risk of solar panel glass breakage. The two temperature profiles that are shown in Fig. 4 are from the same device with two different exposure angles. The profiles are largely similar, however, a larger temperature gradient is formed when the exposure angle is higher (Fig. 5). When the angle of the PV-T unit is set to 14.7° (Fig. 4a) the temperature difference within the reservoir reaches 19 °C (equivalent to a gradient of 14 °C/m) by 11:00 where it remains until 14:00. When the angle is set to 11.3 °C however, the temperature difference is lower at 14 °C (a gradient of 10 °C/m), and shorter-lived, lasting under 2 h. This is as expected, as increasing the angle of the PV-T unit inhibits water mixing and enhances temperaturelayer formation. 4.3. Comparison PV panels Three different PV panels from SolarWorld were tested on the devices. A mono-crystalline panel (SW 270 mono) (Solar World, 2018a), a poly-crystalline panel (SW 250 poly) (Solar World, 2018b) and a bifacial mono-crystalline panel (SW 280 Duo) (Solar World, 2018c); the panels had performance statistics as in Table 3. PV output data was collected across five days to assess the performance of these panels, see Fig. 6a for an example day. The bifacial panel consistently outperformed the other two panels, peaking around 180 W, while the mono- and the poly- peaked around 160 W. Similarly, looking at the electrical efficiencies of each panel (Fig. 6b), the efficiency of the bifacial panel was typically 1.5–2% higher than the other two. All panels reached peak efficiency around 11:00 then suffered from a drop in efficiency of ∼1.5% during the hottest part of the day. The average efficiency of the mono-crystalline panel was 12.4%, the poly-crystalline 12% and the bifacial panel 13.4%. The observed electrical efficiencies were, for the poly- and bifacial panels, exactly in line with expected efficiencies when considering both the parameters in Table 3 and the acrylic sheet that was placed over the PV panel (with a 92% light transmission rate). This was despite the fact that the panels were operating under a high temperature gradient, the effects of which were previously unknown (It should be noted that the surface temperature of the PV panels is lower than the ∼80 °C temperature of the water reservoir). The mono- panel however, consistently underperformed and exhibited very similar performance to the poly-crystalline panel.
4. Results 4.1. Ambient conditions The reservoirs were set to a stagnant mode and the thermocouples (Fig. 1) were used to measure reservoir temperatures under different weather conditions. Fig. 3 shows temperature profiles for the same PV-T system on two different days. The shape of the temperature profiles are typical, increasing from approximately 7:00 to a peak at 14:00 then dropping off. This demonstrates that the devices continue to gain energy from the sun for an hour after the peak in solar irradiance, which occurs around 13:00. After this the difference between the reservoir temperature and the ambient temperature becomes too high resulting in a significant heat flux in the outward direction which leads to thermal losses. On days with high ambient temperatures and high solar irradiance (as in Fig. 3a) the top of the reservoir can reach 90 °C for up to 3 h, and remain above 80 °C for over 5 h. On cloudy or colder days with low solar irradiance, as in Fig. 3b, temperatures in the reservoir may never reach 80 °C. The difference in maximum reservoir temperatures between the two days in Fig. 3 was ∼15 °C. Table 2 compares the effects of ambient temperature and solar irradiance on the reservoir temperatures. Days 1 and 2 (Table 2) have similar ambient temperatures but different solar irradiance, while days 2 and 3 display different solar irradiance but similar ambient temperatures. From these data it is clear that the difference of 5 °C in ambient temperature has comparable impact on reservoir temperatures to a 87 W/m2 difference in solar irradiance.
4.4. Reservoir depth and hot water production. The systems were set up with three different reservoir depths of d = 12 mm, 15 mm and 20 mm. The exposure angle was set to 11.3° and bifacial panels were used. Each reservoir thickness was tested in the stagnant mode for three days, and then in the conditional flow mode for another three days, i.e. water above 80 °C was collected from the top of the reservoir and replaced with ambient water from the bottom, the results of which are shown in Table 4. Fig. 7 shows typical temperature profiles within the three reservoirs of varying thickness. Thicker reservoirs with higher volumes of water were seen to heat up slower, and reach lower maximum temperatures (Table 4). We can see that with the thinnest reservoir water Table 1 Parameters varied for experimental testing. PV type Reservoir depth Exposure angle
607
mono12 mm 11.3°
poly15 mm 12.7°
bifacial 20 mm 14.7°
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Fig. 3. Temperature profiles of the same device on 2 different dates: (a) with maximum ambient temperature of 36 °C and maximum irradiance of 918 W/m2; (b) with maximum ambient temperature of 31 °C and maximum irradiance of 850 W/m2.
1 1 hour period and again peaked at 7.1 L per half hour. 2 Fig. 9 shows temperature profiles within the best performing (12 mm) reservoir, during (a) collection of water above 80 °C, (b) continuous pumping of water for five hours from 11:00, and (c) continuous pumping of water for five hours from 9:30.
Table 2 Solar irradiance, ambient temperature (at 2 pm), and maximum water temperature (∼2 pm) across three different days.
2
Solar irradiance (W/m ) Ambient temperature Maximum reservoir temperature
Day 1
Day 2
Day 3
830 36 °C 80 °C
917 35 °C 88 °C
912 30 °C 80 °C
5. Theoretical analysis of design A theoretical model is developed to inform further improvements to the device, and is validated with the gathered testing data. The model takes the energy landing on the device in the form of solar irradiance, I (W/m2), as its starting point and calculates energy losses across each five minute time-step to find the total energy that reaches the water in the reservoir. The temperature of the water reservoir is then calculated at each time-step assuming all remaining energy after the calculated losses is converted to heat. The model is validated using solar irradiance, ambient temperature, and reservoir temperature data gathered over a six day period during the experimental testing. The validated model can be used to better understand the energy losses of the device, and to perform sensitivity analyses on a set of design parameters, some
temperatures reached 80 °C at ∼12:00 allowing the hot water to be collected and replaced with cold while the system is still heating. This is up to two hours earlier than with the 15 mm thick rubber reservoir, while the 20 mm thick reservoir sometimes did not reach 80 °C at all. The rate of water production (litres per half hour collection period) can be seen in Fig. 8. Production can be seen to peak between 13.00 and 13.30 for all three systems. For the 12 mm reservoir production spanned a three hour period and peaked at 10.7 L per half hour, for the 15 mm reservoir production spanned a 2 1 hour period and peaked at 2 7.1 L per half hour, while for the 20 mm reservoir production spanned a
Fig. 4. Temperature profiles of the same device under exposure angles of (a) 14.7° and (b) 11.3° to the horizontal ground. Each exposure angle was tested for three days. 608
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Fig. 5. Temperature difference throughout the day between the top and bottom of the water reservoir for two different exposure angles: 14.7° (blue) and 11.3° (orange). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 3 SolarWorld PV panel operational efficiencies and temperatures. All panels are rated between −40 and 85 °C
Table 4 Reservoir depths and corresponding volumes, temperatures attained, and hot water produced.
mono-
poly-
bifacial
Reservoir depth
12 mm
15 mm
20 mm
Efficiency at Toperational Toperational
15.2%
13.9%
16.7%
46 °C
46 °C
48 °C
Efficiency drop beyond Toperational
0.45%
0.41%
0.43%
Reservoir volume (L) Average maximum (stagnant) water temperature Average 80 °C water produced (L/day)
18.2 93 °C 18.7
22.7 87 °C 12.3
30.3 83 °C 5.5
Fig. 6. (a) PV output (W) and (b) electrical efficiency on 7th September 2017 from solar PV panels using 3 different types of photovoltaic wafers: mono-crystalline (SolarWorld SW 270 mono), poly-crystalline (SolarWorld SW 250 poly), and bifacial (SolarWorld 280 Duo). 609
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Fig. 7. Temperature profiles across the same day for each of the three different reservoir thicknesses of (a) 12 mm, (b) 15 mm, and (c) 20 mm.
Tresn = Tresn − 1 +
Δt·(A·In − Ln) , cp·ρ·V
Ln = Lrefn − Ltrann − Lconfn − Lradn − Lconrn , 2
(1)
3
where A = 1.6 (m ) and V(m ) are the front surface area and the volume of the reservoir, ρ = 995.7 kg/m2 s the density of water at 30 °C, cp = 4179 J/(kg·K) is the specific heat of water and the time-step, Δt , is 200 s. The losses are calculated with he following equations:
Lrefn = QreSR −
QreSR·In ·(Irrn·A) Imax ·(1 + QreMD )
(2)
Ltrann = In·A·(1 − T)
(3)
Lconfn = h fn ·A·(Tfn − Tan )
(4)
Lconrn = hrn ·A·(Trn − Tan )
(5)
(
)4 − (Tsky
Lradn = ∊p ·σ·( Tfn + 273.3
n
)
+ 273.3 4 )·A
(6)
where QreSR = 0.02 and QreMD = 0.25 are the reflection coefficients of the acrylic face at midday and sunrise respectively, Imax (Wm−2) is the peak solar irradiance across the day, T = 0.88 is the front-face transmittance, σ = 0.000000056703 W/(m2 K4) is the Stefan-Boltzmann Constant and ∊p = 0.9 is the acrylic emissivity. The sky temperature, Tskyn (°C), is calculated using Swinbank’s Formula, modified to include deviation correction as in Alados-Arboledas et al. (1988). h fn , the front face heat transfer coefficient is calculated as described by Duffie and Beckman (1991) (including Rayleigh number calculation as described in Chandrasekhar (1981), Lin et al. (1989)), and rear face heat transfer coefficient, hrn , is calculated as described by Churchill and Chu (1975), Maddox and Issam (1989) for laminar flow against a plane. Trn and Tfn are the rear and front face surface temperatures. The rear face temperature is calculated classically by:
Fig. 8. Hot water (>80 °C) production rate per half hour collection period, for each reservoir depth, on 24 August 2017.
of which are difficult or costly to test. This is of upmost importance for the future development and improvement of this device. The total energy loss at time-step n, Ln , is made up of losses to reflection off the front face of the device, Lrefn , losses to transmittance through the front face, Ltrann , losses to heat convection through the front, Lconfn and rear, Lconrn of the device, and radiative losses through the front of the device, Lradn , any additional losses are assumed to be negligible. The remaining energy is entirely converted into heat within the water reservoir, giving a reservoir temperature at time-step n, Tresn , of:
Fig. 9. Temperature profiles when pumping water through the 12 mm reservoir, (a) only pumping to collect 80 °C water, (b) continuous pumping 11:00–16:00, (c) continuous pumping 9:30–14:30. 610
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Trn =
(ki·Tresn) + (si·hrn ·Tan) ki + (si·hrn )
where Rfront = 0.042 is the front-face thermal resistance. The above process is repeated for 20 iterations at each time-step.
(7)
where the insulation thickness si = 0.025 m, and the insulation’s Thermal Conductivity, ki = 0.022 W/(m·K). The front face temperature is calculated using the following iterative process:
Tfn (0) =
5.1. Model validation Model feed data was collected on six consecutive days in the testing period, and the model was run for each of these days. The model output is validated with actual reservoir temperatures witnessed on the six test days, as shown in Fig. 10. The model can be seen to agree well with the experimental data across all six test days. The model is particularly good at predicting the peak temperature attained in the day, while the warm up and cool down periods are
Tresn + Tan 2 1
Tfn (i + 1) = Tfn (i) + 4 ·(Rfront ·A·∊ρ ·σ·Ta4n ·Rfront ·A·h fn ·Tan + Tresn
(8)
− Rfront ·A·∊ρ ·σ·Tfn (i) 4 − Tfn (i)·(Rfront ·A·h fn + 1))
Fig. 10. Ambient temperatures and solar irradiance for six consecutive days in September, with model predictions of reservoir temperature and observed reservoir temperature for those days. 611
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bifacial mono-crystalline panel outperforms both the mono-crystalline and poly-crystalline solar panels, which both performed similarly. Moreover, the bifacial and the poly- panels performed in line with their specifications despite temperature differences as high as 19 °C (Section 4.2) across the panel. Drops in efficiency were dependent on the maximum temperature of the PV panel and not on any gradient across it. The primary reason for the increased efficiency of the bifacial panel is that it can collect solar energy hitting it from either side. The devices used have a metallic back sheet, so solar rays that penetrate the PV panel can be reflected back off this sheet back onto a solar cell. Purely from the standpoint of power production, the bifacial panel is the sensible choice. 6.1.3. Reservoir depth There is a strong correlation between the depth of the reservoir and both temperatures within it and the volume of hot water it can produce. Shallower reservoirs get hotter faster, have a higher temperature gradient, and produce more 80 °C water (Table 4 & Fig. 7). With the 12 mm reservoir, 80 °C water can be collected and replaced with cold from around 12:00. At this time the thermal flux of the system is still high enough to heat up the new cold water to 80 °C. For thinner reservoirs the production therefore lasts for longer, and beyond this the peak production rate is higher (Fig. 8). With the larger reservoir depths, thermal energy gathered during the day is wasted. This can be seen by looking at the refreshment rate of a reservoir across a day of hot water production. The 12 mm reservoir has a volume of 18.2 L and produces as average of 18.7 L, giving a refreshment rate of 1.03 reservoirs per day. Therefore all the thermal energy already captured in the water when production began (at 12:00) was collected. The 20 mm reservoir however has a volume of 30.3 L and produces only 5.5 L, giving a refreshment rate of 0.18 reservoirs per day, and so the thermal energy stored in the rest of the water goes to waste. It could be concluded, therefore, that the thinner the water reservoir the greater water production will be. For example, it was calculated that under the conditions seen here a temperature of 90 °C could be reached by 11:00 if a reservoir depth of 9 mm was used. This conclusion is true to a point, however, practical limitations exist on the depth of the reservoir which at times have to hold heat exchangers of a certain thickness, for example.
Fig. 11. Difference between model and actual data across the six test days used to feed the model.
predicted with slightly less accuracy. This is highlighted in Fig. 11 where the temperature difference between the model and the experimental data is shown. It can be seen that the model has a tendency to under-predict temperatures in the morning by up to 5.5 ° C, and over predict in the afternoon by up to 4 °C. 6. Discussion 6.1. Parameters 6.1.1. Exposure angle It is desirable to have a steep temperature gradient within the reservoir so that the water at the top can be allowed to reach as high a temperature as possible for the same total thermal energy. As seen in Section 4.2 (Fig. 5), by setting the angle of the PV-T unit to 14.7° a temperature difference of 19 °C was achieved within the reservoir, equivalent to a temperature gradient of 14 °C/m. In comparison, with an angle of 11.3°, the temperature difference reached only 14 °C, giving a temperature gradient of 10 °C/m. Optimal performance of solar panels is achieved when the panel is tilted such that sunlight falls perpendicularly onto its surface. This angle changes throughout the year with the changing position of the sun. Field measurements were taken in Cyprus during July and August in 2017, so the optimal exposure angles for PV power production were 19° and 27° respectively [20]. However, for our tests we limited the exposure angle to a maximum of 14.7° to avoid leakage as a result of pressure build-up in the lower section of the reservoir. The optimal angle for water production is not necessarily the same as for electricity production, but our results show that steeper angles correlate to higher temperature gradients within the reservoir.
6.2. Potential as a PV-T system The electrical efficiency of the device reaches 13.4% when fitted with the bifacial panels. The proportion of the remaining incident solar radiation that is converted into thermal energy depends on the state in which the device is functioning. When the devices are filled with stagnant water, solar energy is converted into thermal energy and the water is heated until ∼14:00 (Fig. 3a). Taking 31st August 2017 as an example, at 14:00 the average temperature within the reservoir was 89 °C, the maximum temperature was 96 °C, and the water contained 12.5% of the total energy to land on the solar panel that day (22% of the energy that had hit it so far). After this point, losses from the reservoir became greater than the energy absorbed, and the temperature dropped. In order to capture more thermal energy in this environment, further insulation would be needed. Heat loss back through the PV panel itself will always result in temperatures dropping after a certain point. This is dependent on the thermal conductivity of the insulation material and the temperature difference between the reservoir and the outside. This, of course, is not the state in which the device would usually be functioning, but it is a good demonstration of the amount of energy that it can hold at any given moment. When water is flowing through the device the results are significantly different. For two days water was pumped through pre-filled devices at a rate of 6 ml/s (Fig. 9(b) & (c)). Thermal efficiencies were then calculated using the equation:
6.1.2. Solar panel for PV-T systems When choosing a PV panel for a PV-T system described here, it is desirable to have a panel that is as thin as possible to allow maximum penetration of solar radiation into the reservoir. This has to be coupled with the fact that the panel must be strong enough to withstand the pressure from the water reservoir beneath it, tempered glass is, therefore, a must. Comparing the three PV panels used in this study, it is clear that the 612
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̇ p (To − Ti )/(Apv G ), ηth = mC
pump and a more complex control system). Therefore, as well as providing enough hot water for a family, and powering itself, the proposed system would provide all additional power generated for household needs. The total material cost of this system for our experiment is about 1000EUR excluding VAT. This has potential to become economically feasible for a single family. By comparison, the traditional solar collectors when it first was introduced in Cyprus and Greece cost around 950USD (Michaelides et al., 1999), which is equivalent to 1150 EUR today. With economies of scale (e.g., purchase parts for >500 units) and the choice of cheaper parts (e.g., use a thermosetting polymer to make the reservoir instead of Stainless Steel 316L), we estimate that the total material cost can be cut down to around 600EUR. This price could be brought down by a further ∼ 50 EUR by using the cheaper poly-crystalline PV panel, which produces enough power to run the system, however this would come at the cost of less electricity generation which can be used for applications.
(9)
where ṁ is the mass flow rate through the device (kg/s), Cp is the specific heat of water (J/kg K), To and Ti are the outlet and inlet temperatures of the water respectively (K), Ap v is the upward facing surface area of the PV panel (m2), and G is the solar irradiance (W/m2). When run for five hours during the middle of the day the thermal efficiencies of the devices averaged 53.4%. This, combined with the electrical efficiency of 13.4%, gives an overall PV-T efficiency of 66.8%. These efficiencies are not greatly affected by ambient conditions and remain largely the same each day. Comparing these efficiencies to the literature discussed in Section 1 we see that the device is already performing competitively, despite being a prototype where further gains can still be made. The calculated thermal efficiency will likely also be dependent on both the flow rate and hours of operation, which are recommended as the subject of further testing. The maximum achievable thermal efficiency however will be determined by the set of energy losses included in the theoretical model, some of which have the potential to be reduced. Additional gains in efficiency could be made, for example, by increasing positive heat flux in the morning through reducing the thickness of the glass PV panel, or reducing negative heat flux in the afternoon through increasing the thickness of the insulating air gap between the glass panel and the acrylic sheet. The acrylic glass itself transmits infrared wavelengths of less than 2.5 μ m but absorbs and hence blocks higher wavelengths up to about 25 μ m, while heat irradiated by the water reservoir at 90 °C peaks at approximately 8 μ m (Planck’s law), therefore this material is well suited for this purpose. However, over the course of the testing period this sheet was seen to degrade, warp, and yellow in colour, potentially reducing the transmittancy of solar irradiation as well as compromising the integrity of the insulating layer of air. Therefore tests should be done on alternative materials that exhibit similar conductive and optical properties, to find an ideal insulation layer that is more durable after repeated sun exposure. One such option would be a poly-carbonate, which has a similar thermal conductivity ∼0.19 W/(mK).
6.4. Theoretical model Various aspects of the device are untested and can still be improved. Beyond the two testing parameters of reservoir depth and exposure angle the model can be used to perform sensitivity analysis on the following parameters: PV panel dimensions (and so, with reservoir thickness, also reservoir volume), front-face insulation thickness, frontface insulation material, air-gap between PV panel and front-face insulation, rear-face insulation thickness and rear face insulation material. Although the device as proposed here can readily be recreated and furher tested by the reader, it is recommended that insulation and material types are investigated using the theoretical model before doing so. For example, Fig. 12 below shows the losses at each time-step as predicted by the model (transmittance losses through the front face are constant at 12%). Combined losses amount to over 100% at around 2 pm when the device starts to cool down, and Fig. 12 shows that this is largely down to the front-face convective losses. Thorough sensitivity analyses of various material types and thicknesses on this loss, and indeed on all the losses, could potentially bring energy loss down and increase the performance of further devices without the need for additional test periods. If a user was planning on building such a device in a known
6.3. Feasibility as an off-grid, self-powering, solar collector. The temperature of hot water required for domestic consumption is much lower than the 80 °C water collected through most of these experiments, and usually lies around 40–45 °C. Consumption per person per day varies greatly and is dependent upon season and latitude. In the U.K., for example, average daily consumption is considered to be between 30 and 50 L of 45 °C water (CERNUNNOS, 2018) (120–200 L for a family of four), while in warmer climates this will be significantly less. The PV-T system proposed here, when pumping continuously for five hours, from 11am until 4 pm, produced 108 L of water at an average of 59 °C (Fig. 9(b)). This is energy equivalent to 185 L of 45 °C hot water at the same ambient temperature. If 45 °C water was to be collected directly, collection would begin between 9:00 and 9:30, and could be expected to continue until around 16:30–17:00. It must be taken into account that most hot water will be required in the morning and evenings, while such a solar collector can only collect during the day, and so an extremely well-insulated hot water tank may be required. Once this has been considered, it is clear that such a system is capable of providing hot water for a family of four. Such a system requires little power to run, and any excess electricity could be either stored in a battery or directed straight into a house, for example. In these experiments two standard Victron batteries were connected in series for this purpose. A control system would be required to start and stop water pumping at given temperatures, and to control and direct the PV output, while the pump itself consumes less than 5 W. Such a system could be turned on and off automatically at sunrise and sunset, and through the day could safely be assumed to run on 25 W (the power consumption witnesses here when running only the inlet
Fig. 12. Energy losses at each time step. Transmittance through the front face is left off as it is a constant at 12%. 613
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References
location, this model could be fed with average temperature and solar irradiance data from that location and then used to assess the ideal device parameters before the device was built, and indeed to assess whether the device is worth building in that location. Additionally, if a user had different needs to those explored here, for example a greater volume of water required but at a lower temperature, then the model could be used to choose tune the device to those needs prior to building the device.
Alados-Arboledas, L., Vida, J., Jiménez, J., 1988. Effects of solar radiation on the performance of pyrgeometers with silicon domes. Atmos. Ocean. Technol. 5, 666–670. Bergene, T., Løvvik, O.M., 1995. Model calculations on a flat-plate solar heat collector with integrated solar cells. Sol. Energy 55, 453–462. CERNUNNOS. Sizing a hot water tank for solar boilers. Available at < http://cernunnoshomes.co.uk/technology/boilers-explained/sizing-a-hot-water-tank/ > (last retrieved on 18-02-2018). Chandrasekhar, S., 1981. Hydrodynamic and Hydromagnetic Stability. Dover Books on Physics Series, Dover Publications. Churchill, S.W., Chu, H.H., 1975. Correlating equations for laminar and turbulent free convection from a vertical plate. Int. J. Heat Mass Transfer 18 (11), 1323–1329. Coventry, J.S., 2005. Performance of a concentrating photovoltaic/thermal solar collector. Sol. Energy 78, 211–222. Dubey, S., Tiwari, G., 2008. Thermal modeling of a combined system of photovoltaic thermal (PV/T) solar water heater. Sol. Energy 82, 602–612. Duffie, J., Beckman, W., 1991. Solar Engineering of Thermal Processes. John Wiley and Sons, New York. Good, C., Chen, J., Dai, Y., Hestnes, A.G., 2015. Hybrid photovoltaic-thermal systems in buildings a review. Energy Procedia 70, 683–690. He, W., Chow, T.-T., Ji, J., Lu, J., Pei, G., Chan, L.-s., 2006. Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water. Appl. Energy 83, 199–210. Hukseflux Thermal Sensors. Hukseflux SR05-DA2. Available at < https://www. hukseflux.com/sites/default/files/product_manual/SR05_manual_v1610.pdf > (last retrieved on 18-02-2018). Janssen, W.F.J., 2015. Method and device for treating a fluid. Available at < http:// www.freepatentsonline.com/y2015/0251923.html > (last retrieved on 18-02-2018). Kalogirou, S.A., 2001. Use of TRNSYS for modelling and simulation of a hybrid pvthermal solar system for Cyprus. Renew. Energy 23, 247–260. Lin, H.T., Yu, W.S., Yang, S.L., 1989. Free convection on an arbitrarily inclined plate with uniform surface heat flux. Wärme - und Stoffübertragung 24, 183–190. Maddox, D., Issam, 1989. Single- and two-phase convective heat transfer from smooth and enhanced microelectronic heat sources in a rectangular channel. J. Heat Transfer 111, 1045–1052. Michaelides, I., Kalogirou, S., Chrysis, I., Roditis, G., Hadjiyianni, A., Kambezidis, H., Petrakis, M., Lykoudis, S., Adamopoulos, A., 1999. Comparison of performance and cost effectiveness of solar water heaters at different collector tracking modes in Cyprus and Greece. Energy Convers. Manage. 40, 1287–1303. Prakash, J., 1994. Transient analysis of a photovoltaic-thermal solar collector for cogeneration of electricity and hot air/water. Energy Convers. Manage. 35, 967–972. Saitoh, H., Hamada, Y., Kubota, H., Nakamura, M., Ochifuji, K., Yokoyama, S., Nagano, K., 2003. Field experiments and analyses on a hybrid solar collector. Appl. Therm. Eng. 23, 2089–2105. Skoplaki, E., Palyvos, J., 2009. On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations. Sol. Energy 83, 614–624. Solar World. Sunmodule Plus SW 270 Mono. Available at < http://www.solarworld.de/ fileadmin/downloads_new/produkt/sunmodule/datenblaetter/es/mono/mono_270_ es.pdf > (last retrieved on 18-02-2018). Solar World. Sunmodule SW 250 Poly. Available at < http://www.solarworld.de/ fileadmin/downloads_new/produkt/sunmodule/datenblaetter/nl/poly/poly_240250_nl.pdf > (last retrieved on 18-02-2018). Solar World. Sunmodule Bisun SW 280 DUO. Available at < https://www.solarworldusa.com//media/www/files/datasheets/archive/sunmodule-bisun-duo-solar-paneldatasheet.pdf > (last retrieved on 18-02-2018). Tripanagnostopoulos, Y., Nousia, T., Souliotis, M., Yianoulis, P., 2002. Hybrid photovoltaic/thermal solar systems. Sol. Energy 72, 217–234. Tyagi, V., Kaushik, S., Tyagi, S., 2012. Advancement in solar photovoltaic/thermal (PV/ T) hybrid collector technology. Renew. Sustain. Energy Rev. 16, 1383–1398. Victron Energy Blue Power. BlueSolar MPPT 75 115. Available at < https://www. victronenergy.nl/solar-charge-controllers/mppt7510 > (last retrieved on 18-022018). Zondag, H., de Vries, D., van Helden, W., van Zolingen, R., van Steenhoven, A., 2002. The thermal and electrical yield of a PV-thermal collector. Sol. Energy 72, 113–128.
7. Conclusion This field study explored the viability of a recently developed PV-T system as a self-powered, off-grid, solar collector. This constitutes the first time that such a device has been tested experimentally, and it has demonstrated the capability to provide hot water of approximately 80 °C for a family of four, as well as providing excess electricity towards household applications, assuming an average daily solar irradiance of >4.5 kWh/m2. Electrical and thermal efficiencies of 13.4% and 53.4% are reported, which compete well with the traditional PV-T system. Calculation of the electrical efficiency is straightforward. It is the power produced by the PV panel normalised by the solar irradiance multiplied by the exposure surface area. The reported thermal efficiency however is strongly dependent on the flow rate and the average temperature difference of the water one aims to achieve. The maximum achievable thermal efficiency could only be estimated by considering energy losses. A theoretical model of the device was built, which tracks energy losses with time and outputs the average reservoir temperature at each five-minute time-step. This model was validated with the obtained data during the field study. This model performs well and has the functionality to inform these future improvements through a sensitivity analysis of parameters and their effect on different types of energy loss. This would bypass the need for lengthy and expensive additional testing. An immediate potential improvement includes varying insulation material and thickness, for example, changing the acrylic sheet over the panel in our experiment for a more suitable material. The experimental device has demonstrated the capacity to produce over 20 L of 80 °C + water in a day, and under good conditions, the reservoir held water of over 95 °C. There is still room for improvement here. The rapid drop in cost price of existing ‘off-the-shelf’ PV panels in the last decade has provided the opportunity to reconfigure these PV panels into PV-T panels, thus opening the way for applications which require both electrical and thermal energy such as (1) heating/cooling of buildings, (2) heating of water for household applications and (3) desalination. We will cover the latter topic in a forthcoming paper. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.solener.2018.10.062.
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Experimental study on a direct water heating PV-T technology a,b,⁎,1
Jiajun Cen , Roan du Feu William Janssena a b c
a,b,1
a,c
a
T a
, Matus E. Diveky , Catriona McGill , Oliver Andraos ,
Desolenator B.V., Koningsplein 11, 6224EB Maastricht, the Netherlands The Grantham Institute & Department of Chemical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ London, UK Laboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, CH-8093 Zurich, Switzerland
A R T I C LE I N FO
A B S T R A C T
Keywords: PV-T system design Water heating
This paper details a field study and a theoretical model of a PhotoVoltaic-Thermal (PV-T) system consisting of a solar PV panel with a thermally insulated water reservoir underneath. Unlike conventional PV-T systems, water is in direct contact with the glass solar PV panel. Thus, metallic tubular heat exchangers are omitted in this design. During operation, the PV-T panel is tilted, and cold water is pumped into the reservoir from the side closest to the ground. This achieves an active cooling of the PV panel maintaining an optimal electrical efficiency. Generated electricity is used to operate pumps and run the control system, while excess electricity is stored in a battery to be utilised as desired. The system and the inlet water absorb solar thermal energy and as a result they increase in temperature. In our field study, we explore the viability of this system as a self-powered, off-grid, solar collector and find that it can provide enough hot water of approximately 80 °C for a household of four in areas where average daily solar irradiance is >4.5 kWh/m2. We varied (1) the exposure angle, (2) PV panel type and (3) reservoir depth and found that in the limited ranges covered by our experiments the optimised configuration is with (1) an exposure angle of 14.7°, (2) a bifacial mono-crystalline solar PV panel and (3) a reservoir depth of 12 mm (given a fixed inlet water flow rate). The theoretical model of the device that is built, tracks energy losses with time and outputs the average reservoir temperature at each five-minute time-step. We validated this model with the obtained data during the field study. Then, this model is used to perform a sensitivity analysis on the parameters in testing and beyond (such as primarily insulation types and thicknesses), to provide a direction for further development and improvement.
1. Introduction Recently, the emphasis of research on harvesting solar energy has been on developing new types of photovoltaic cells with higher electrical efficiencies or in the design of novel solar concentrators. The focus of such research is either on harvesting high quality solar electric energy or the lower quality solar thermal energy. Less research has been devoted to improving PV-T technologies, which harvest both solar electric and thermal energy, and their designs. With the rapid drop in cost price of existing ‘off-the-shelf’ PV panels in the last decade, the opportunity has arisen to reconfigure these PV panels into PV-T panels. This opens the way for applications which require both electrical and thermal energy such as (1) heating/cooling of buildings, (2) heating of water for household applications and (3) desalination. The final of these topics will be covered specifically in a forthcoming paper. The justification for PV-T technology is sound: the electrical
conversion efficiency of a PV panel reduces with an increase of temperature (Skoplaki and Palyvos, 2009). Therefore, combining a thermal collector with a PV panel should have the dual effect of collecting thermal energy to produce hot water while simultaneously cooling the panel, and therefore increasing its electrical efficiency. The market for this combined technology is still small, but already the variety of possible applications is becoming clear for both large- and small-scale systems. For example, small-scale systems have already been integrated into some homes in the UK and France where the electricity output is used to power the home and the collected hot water for central heating or domestic hot water production (Good et al., 2015). In Sweden, the heat from large-scale PV-T systems has been used to improve the performance of undersized boreholes (Good et al., 2015). More recently, and of particular interest here, hot water production from such PV-T systems has been proposed as the first stage in a desalination process (Janssen, 2015).
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Corresponding author at: Desolenator B.V., Koningsplein 11, 6224EB Maastricht, the Netherlands. E-mail address: [email protected] (J. Cen). 1 Both authors contributed equally. https://doi.org/10.1016/j.solener.2018.10.062 Received 21 July 2018; Received in revised form 19 October 2018; Accepted 22 October 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
I L Lref Ltran Lconf
Lconr Lrad Tres Tf
Tr Tamb Tsky σ QreMD QreSR ∊p Rf hf hr si ki
solar irradiance (W/m2) total energy loss (J) energy loss to reflection off the front face of the device (J) energy loss to transmittance through the front face of the device (J) convective heat loss through the front face of the device (J) convective heat loss through the rear of the device (J) radiative losses through the front face of the device (J) reservoir temperature (°C) front face temperature (°C)
rear face temperature (°C) ambient temperature (°C) sky temperature (°C) Stefan-Boltzmann Constant (W/m2 K4) acrylic reflection coefficient at midday acrylic reflection coefficient at sunrise acrylic emissivity front face thermal resistance (W/m·K) front face heat transfer coefficient (W/m2 K) rear face heat transfer coefficient (W/m2·K) insulation thickness (m) insulation thermal conductivity (W/m·K)
efficiencies of 64% (Dubey and Tiwari, 2008), 50–63% (Saitoh et al., 2003), and 50% (He et al., 2006). Recently, other PV-T system designs have been considered, modelled, and built, although the literature on them is far less extensive. One such example is the PV-T concentrator (Tyagi et al., 2012), an experimental design which produced high thermal and electrical efficiencies of 58% and 11% , respectively (Coventry, 2005). The experimental research presented here tests the feasibility of a new design of PV-T solar collector that can be readily built by attaching a reservoir under an off-the-shelf solar panel. It is entirely self-powered, can be used off-grid, can provide hot water for a family of four for purposes such as showering and cleaning, and in which any excess electricity generated can by stored in a battery. The PV-T converter proposed differs to the traditional PV-T system in that it functions by flowing water across the back of the entire PV panel, through a reservoir that lies between the panel and a metal plate, as opposed to through a tubular heat exchanger affixed to the back of the PV panel. This method has the advantage that the water absorbs solar energy directly as well as absorbing the thermal energy which is transferred from the PV panel. This is a design that has not yet been tested in the literature. A similar design has been modelled computationally (Prakash, 1994), reporting efficiencies of 9.2% (electrical) and 64% (thermal), but this is the first time such a design has been built and trialled experimentally. The efficiencies of the system in harvesting solar energy and converting it into both electrical and thermal energy are calculated. The performance of a PV panel with a large temperature gradient across its
To date, the vast majority of proposed liquid PV-T systems are of a very similar design. An insulated casing is fitted on the back of a solar panel, and a metal sheet is attached to the back of that panel to absorb and transfer solar thermal energy. A tubular heat exchanger is run back and forth across the metal sheet, heating the water as it passes under the PV panel. Many theoretical and numerical models have been made of such systems. One such model found that running a heat exchanger along 1 m of a PV panel with a flow rate of 0.3 ml/s could raise water temperature from 27 °C to 57 °C (Bergene and Løvvik, 1995), which would result in approximately 1 L of ~60 °C water being produced per hour. This system had a thermal efficiency of 58% and an electrical efficiency of 11.1%. The same study found that increasing flow rate further did not result in any significant gain in the electrical efficiency of the PV panel. A second model of a ∼1 m2 collector in Zondag et al. (2002), with a higher flow rate of 16 ml/s showed a lower rise in outflow temperature, from an ambient 20 °C to a peak of 32.5 °C. This system had lower thermal and electrical efficiencies of 33% and 6.7% , respectively. A more detailed study looked at installing a specific PV-T design on rooftops Kalogirou (2001). This model involved 5 m2 of PV panel with water flowing through a heat exchanger at 7 ml/s, and showed a daily production of 59 L of 50 °C water, with a 7.7% electrical efficiency, giving an overall efficiency of the system of 32%. Experimental studies were also carried out using this traditional PVT system, with one particular study finding that very high efficiencies, of ∼12.5% (electrical) and ∼68% (thermal), were achievable, giving an overall efficiency of approximately 80% (Tripanagnostopoulos et al., 2002). Other experimental studies have shown lower overall
Fig. 1. Sketch of the PV-T system configured with an exposure angle of θ = 17° and dimensions in millimetres. There are four temperature sensors installed (marked with black triangles). From left to right (lowest to highest) they are T1, T2, T3 and T4. 605
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surface is so far undocumented. This is the first report in a study that will investigate the further application of this system in the promising technology of water desalination through distillation, in which producing water of as high a temperature as possible is paramount. Therefore, the highest attainable temperatures within the reservoir are measured, along with the volume of hot water (>80 °C) that can be produced. The device is tested in the field in Cyprus and the parameters of exposure angle, reservoir depth are varied with the aim of maximising hot water production and thermal efficiency. Different PV panel types are also tested with the aim of maximising electrical efficiency. Finally, a theoretical model of the system is presented, and validated with the experimental data. The model calculates energy losses of different forms throughout the day to produce a temperature profile of the reservoir with time. This can be used to investigate further improvements to the presented device by performing sensitivity analysis on parameters that were not covered by the experimental testing. The result is an efficient, self-powering, novel PV-T system design capable of providing hot water to a family that can be built by attaching a reservoir to an off-the-shelf PV panel, along with a model of the system that can be used to tune a set of parameters to the users needs and to the average ambient conditions in their location.
control system, while any excess energy is stored in batteries. Bifacial PV panels are worth considering as they will capture sunlight reflected off the metallic back surface of the reservoir, as well as due to the internal reflection gains associated with them. In these solar panels the photovoltaic wafers are sandwiched between low-iron tempered glass with ARC (EN 12150) with a thickness of about 5.3 mm. The length and width of the PV panel used is approximately 1675 mm by 1001 mm, and their operational temperature range is −40 °C to 85 °C. To build the described PV-T system, the anodized aluminium frame around the PV panel glass is removed. This is placed into a stainless steel 316L reservoir with an EPDM rubber seal running around the reservoir edge, between the panel and the reservoir back. To insert temperature sensors and feed water into the cavity between the glass PV panel and the metal back of the reservoir, stainless steel 316L tubes are pinched through the EPDM rubber. Finally, a specially designed metallic frame of 1694 mm × 1048 mm is placed over the reservoir edges and fixed using nuts and bolts. Equal pressure needs to be applied around the entire EPDM seal to prevent water leakage. Special care needs to be taken where the seal is pierced by a stainless steel 316L tube. The solar panel and two AGM 12V-60Ah batteries are connected and controlled by a Victron energy BlueSolar charge controller (MPPT 75 115) (Victron Energy Blue Power, 2018). With the Victron controller, the amount of electricity generated by the panel can be registered. A Hukseflux SR05-DA2 (Hukseflux Thermal Sensors, 2018) pyranometer is installed to log the total solar irradiance (W/m2). Furthermore, four temperature sensors are installed to measure the temperature of water inside the reservoir, the so-called reservoir temperature (as illustrated in Fig. 1), and two thermocouples are installed to measure temperature of the inlet water tank and of the outlet water tank. To pump water in and out of the reservoir a peristaltic pump is placed close to the inlet location. The underside of the reservoir is insulated using a 1.5 cm styrofoam sheet, and the sides of the reservoir are insulated with a 2 mm Armaflex insulation layer. A sheet of acrylic glass is placed over the top of the reservoir to create an insulating layer. This acrylic has a low thermal conductivity of 0.2 W/(mK), a 92% transmission of visible light (3 mm thickness) and real refractive index of 1.4905 (Coventry, 2005) at 589 nm.
2. Description of the PV-T system The PV-T system proposed here consists of a PV panel with a thermally insulated water reservoir underneath as illustrated in Fig. 1. The water reservoir is filled and the panel is configured at an angle towards the sun. The reservoir can either be held full of stagnant water, or cold water can be pumped into it from the bottom and as it flows upward in direct contact with the PV panel it is heated by sunlight. This pumping can be continuous or it can be triggered when the top of the reservoir reaches a set temperature. This allows water within a desired temperature range to be collected. The water exits the system from the top of the reservoir and is stored in a water tank. Flow rate through the reservoir is on average 6 ml/s. In this experiment, three types of PV panel are used. They are monocrystalline (SW 270 mono) (Solar World, 2018a), poly-crystalline (SW 250 poly) (Solar World, 2018b) and bifacial mono-crystalline (SW 280 Duo) (Solar World, 2018c) panels from SolarWorld. The electricity generated by the PV panel is used to power the water pumps and the
Fig. 2. Images of the experimental setup in Limassol (Cyprus) in summer 2017. (a) A close image of the PV-T system. (b) Layout of the three PV-T systems with wooden boxes to keep inlet water around ambient temperature. 606
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3. Experimental setup
4.2. Solar exposure angle
Three equivalent PV-T systems were shipped to and assembled in Limassol, Cyprus, for experimental work from July to September 2017. The three systems, shown in Fig. 2, were aligned parallel to each other such that they all faced directly south with an initial exposure angle of 12.7° from horizontal. The goal of this field experiment was threefold; to maximise hot water (80 °C) production from the devices, to calculate and optimise achievable thermal and electrical efficiencies, and to demonstrate the feasibility of the devices as self-powering solar collectors. Prior to any experimental investigation, the PV-T devices were benchmarked to ensure the same electrical and thermal performance under the same conditions. This lasted for three days, during which time reservoirs were left stagnant and reservoir temperatures were recorded across the day in five minute intervals. This time step is used throughout all the experiments. After consistency in performance between the devices had been confirmed, the parameters shown in Table 1 were tested. Solar exposure angle was tested first. During this time one of the devices was suffering from leakage problems that were subsequently fixed, therefore only two devices could be used for testing. The two devices were set to 11.3° and 14.7° and left with stagnant water for three days, exposure angles were then switched and left for a further three days. PV panel type was tested second, during this test each device was fitted with a different PV panel as shown in Table 1. For this test all three devices were used, as the leakage problem was resolved. Devices were left stagnant and data was collected for five days. The third test was for depth of the water reservoir. Each device was fitted with a bifacial panel and set to an exposure angle of 11.3°, then the devices were each fitted with a water reservoir of a different depth as shown in Table 1. Reservoirs were initially left stagnant and data was collected for three days, then the reservoirs were run in continuous flow mode, where water was pumped through the system once the reservoir reached 80 °C, for a further three days. Finally the best performing reservoir, the 12 mm reservoir, was tested with the continuous flow mode for five hours, starting at (a) 9 am and (b) 11 am. The flow rate was consistent at 6 ml/s
Fig. 4 shows temperature data collected at different exposure angles of (a) at 14.7° and (b) at 11.3° from horizontal. Angles higher than 14.7° were not tested due to the pressures exerted by water inside the reservoir and the resulting risk of solar panel glass breakage. The two temperature profiles that are shown in Fig. 4 are from the same device with two different exposure angles. The profiles are largely similar, however, a larger temperature gradient is formed when the exposure angle is higher (Fig. 5). When the angle of the PV-T unit is set to 14.7° (Fig. 4a) the temperature difference within the reservoir reaches 19 °C (equivalent to a gradient of 14 °C/m) by 11:00 where it remains until 14:00. When the angle is set to 11.3 °C however, the temperature difference is lower at 14 °C (a gradient of 10 °C/m), and shorter-lived, lasting under 2 h. This is as expected, as increasing the angle of the PV-T unit inhibits water mixing and enhances temperaturelayer formation. 4.3. Comparison PV panels Three different PV panels from SolarWorld were tested on the devices. A mono-crystalline panel (SW 270 mono) (Solar World, 2018a), a poly-crystalline panel (SW 250 poly) (Solar World, 2018b) and a bifacial mono-crystalline panel (SW 280 Duo) (Solar World, 2018c); the panels had performance statistics as in Table 3. PV output data was collected across five days to assess the performance of these panels, see Fig. 6a for an example day. The bifacial panel consistently outperformed the other two panels, peaking around 180 W, while the mono- and the poly- peaked around 160 W. Similarly, looking at the electrical efficiencies of each panel (Fig. 6b), the efficiency of the bifacial panel was typically 1.5–2% higher than the other two. All panels reached peak efficiency around 11:00 then suffered from a drop in efficiency of ∼1.5% during the hottest part of the day. The average efficiency of the mono-crystalline panel was 12.4%, the poly-crystalline 12% and the bifacial panel 13.4%. The observed electrical efficiencies were, for the poly- and bifacial panels, exactly in line with expected efficiencies when considering both the parameters in Table 3 and the acrylic sheet that was placed over the PV panel (with a 92% light transmission rate). This was despite the fact that the panels were operating under a high temperature gradient, the effects of which were previously unknown (It should be noted that the surface temperature of the PV panels is lower than the ∼80 °C temperature of the water reservoir). The mono- panel however, consistently underperformed and exhibited very similar performance to the poly-crystalline panel.
4. Results 4.1. Ambient conditions The reservoirs were set to a stagnant mode and the thermocouples (Fig. 1) were used to measure reservoir temperatures under different weather conditions. Fig. 3 shows temperature profiles for the same PV-T system on two different days. The shape of the temperature profiles are typical, increasing from approximately 7:00 to a peak at 14:00 then dropping off. This demonstrates that the devices continue to gain energy from the sun for an hour after the peak in solar irradiance, which occurs around 13:00. After this the difference between the reservoir temperature and the ambient temperature becomes too high resulting in a significant heat flux in the outward direction which leads to thermal losses. On days with high ambient temperatures and high solar irradiance (as in Fig. 3a) the top of the reservoir can reach 90 °C for up to 3 h, and remain above 80 °C for over 5 h. On cloudy or colder days with low solar irradiance, as in Fig. 3b, temperatures in the reservoir may never reach 80 °C. The difference in maximum reservoir temperatures between the two days in Fig. 3 was ∼15 °C. Table 2 compares the effects of ambient temperature and solar irradiance on the reservoir temperatures. Days 1 and 2 (Table 2) have similar ambient temperatures but different solar irradiance, while days 2 and 3 display different solar irradiance but similar ambient temperatures. From these data it is clear that the difference of 5 °C in ambient temperature has comparable impact on reservoir temperatures to a 87 W/m2 difference in solar irradiance.
4.4. Reservoir depth and hot water production. The systems were set up with three different reservoir depths of d = 12 mm, 15 mm and 20 mm. The exposure angle was set to 11.3° and bifacial panels were used. Each reservoir thickness was tested in the stagnant mode for three days, and then in the conditional flow mode for another three days, i.e. water above 80 °C was collected from the top of the reservoir and replaced with ambient water from the bottom, the results of which are shown in Table 4. Fig. 7 shows typical temperature profiles within the three reservoirs of varying thickness. Thicker reservoirs with higher volumes of water were seen to heat up slower, and reach lower maximum temperatures (Table 4). We can see that with the thinnest reservoir water Table 1 Parameters varied for experimental testing. PV type Reservoir depth Exposure angle
607
mono12 mm 11.3°
poly15 mm 12.7°
bifacial 20 mm 14.7°
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Fig. 3. Temperature profiles of the same device on 2 different dates: (a) with maximum ambient temperature of 36 °C and maximum irradiance of 918 W/m2; (b) with maximum ambient temperature of 31 °C and maximum irradiance of 850 W/m2.
1 1 hour period and again peaked at 7.1 L per half hour. 2 Fig. 9 shows temperature profiles within the best performing (12 mm) reservoir, during (a) collection of water above 80 °C, (b) continuous pumping of water for five hours from 11:00, and (c) continuous pumping of water for five hours from 9:30.
Table 2 Solar irradiance, ambient temperature (at 2 pm), and maximum water temperature (∼2 pm) across three different days.
2
Solar irradiance (W/m ) Ambient temperature Maximum reservoir temperature
Day 1
Day 2
Day 3
830 36 °C 80 °C
917 35 °C 88 °C
912 30 °C 80 °C
5. Theoretical analysis of design A theoretical model is developed to inform further improvements to the device, and is validated with the gathered testing data. The model takes the energy landing on the device in the form of solar irradiance, I (W/m2), as its starting point and calculates energy losses across each five minute time-step to find the total energy that reaches the water in the reservoir. The temperature of the water reservoir is then calculated at each time-step assuming all remaining energy after the calculated losses is converted to heat. The model is validated using solar irradiance, ambient temperature, and reservoir temperature data gathered over a six day period during the experimental testing. The validated model can be used to better understand the energy losses of the device, and to perform sensitivity analyses on a set of design parameters, some
temperatures reached 80 °C at ∼12:00 allowing the hot water to be collected and replaced with cold while the system is still heating. This is up to two hours earlier than with the 15 mm thick rubber reservoir, while the 20 mm thick reservoir sometimes did not reach 80 °C at all. The rate of water production (litres per half hour collection period) can be seen in Fig. 8. Production can be seen to peak between 13.00 and 13.30 for all three systems. For the 12 mm reservoir production spanned a three hour period and peaked at 10.7 L per half hour, for the 15 mm reservoir production spanned a 2 1 hour period and peaked at 2 7.1 L per half hour, while for the 20 mm reservoir production spanned a
Fig. 4. Temperature profiles of the same device under exposure angles of (a) 14.7° and (b) 11.3° to the horizontal ground. Each exposure angle was tested for three days. 608
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Fig. 5. Temperature difference throughout the day between the top and bottom of the water reservoir for two different exposure angles: 14.7° (blue) and 11.3° (orange). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 3 SolarWorld PV panel operational efficiencies and temperatures. All panels are rated between −40 and 85 °C
Table 4 Reservoir depths and corresponding volumes, temperatures attained, and hot water produced.
mono-
poly-
bifacial
Reservoir depth
12 mm
15 mm
20 mm
Efficiency at Toperational Toperational
15.2%
13.9%
16.7%
46 °C
46 °C
48 °C
Efficiency drop beyond Toperational
0.45%
0.41%
0.43%
Reservoir volume (L) Average maximum (stagnant) water temperature Average 80 °C water produced (L/day)
18.2 93 °C 18.7
22.7 87 °C 12.3
30.3 83 °C 5.5
Fig. 6. (a) PV output (W) and (b) electrical efficiency on 7th September 2017 from solar PV panels using 3 different types of photovoltaic wafers: mono-crystalline (SolarWorld SW 270 mono), poly-crystalline (SolarWorld SW 250 poly), and bifacial (SolarWorld 280 Duo). 609
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Fig. 7. Temperature profiles across the same day for each of the three different reservoir thicknesses of (a) 12 mm, (b) 15 mm, and (c) 20 mm.
Tresn = Tresn − 1 +
Δt·(A·In − Ln) , cp·ρ·V
Ln = Lrefn − Ltrann − Lconfn − Lradn − Lconrn , 2
(1)
3
where A = 1.6 (m ) and V(m ) are the front surface area and the volume of the reservoir, ρ = 995.7 kg/m2 s the density of water at 30 °C, cp = 4179 J/(kg·K) is the specific heat of water and the time-step, Δt , is 200 s. The losses are calculated with he following equations:
Lrefn = QreSR −
QreSR·In ·(Irrn·A) Imax ·(1 + QreMD )
(2)
Ltrann = In·A·(1 − T)
(3)
Lconfn = h fn ·A·(Tfn − Tan )
(4)
Lconrn = hrn ·A·(Trn − Tan )
(5)
(
)4 − (Tsky
Lradn = ∊p ·σ·( Tfn + 273.3
n
)
+ 273.3 4 )·A
(6)
where QreSR = 0.02 and QreMD = 0.25 are the reflection coefficients of the acrylic face at midday and sunrise respectively, Imax (Wm−2) is the peak solar irradiance across the day, T = 0.88 is the front-face transmittance, σ = 0.000000056703 W/(m2 K4) is the Stefan-Boltzmann Constant and ∊p = 0.9 is the acrylic emissivity. The sky temperature, Tskyn (°C), is calculated using Swinbank’s Formula, modified to include deviation correction as in Alados-Arboledas et al. (1988). h fn , the front face heat transfer coefficient is calculated as described by Duffie and Beckman (1991) (including Rayleigh number calculation as described in Chandrasekhar (1981), Lin et al. (1989)), and rear face heat transfer coefficient, hrn , is calculated as described by Churchill and Chu (1975), Maddox and Issam (1989) for laminar flow against a plane. Trn and Tfn are the rear and front face surface temperatures. The rear face temperature is calculated classically by:
Fig. 8. Hot water (>80 °C) production rate per half hour collection period, for each reservoir depth, on 24 August 2017.
of which are difficult or costly to test. This is of upmost importance for the future development and improvement of this device. The total energy loss at time-step n, Ln , is made up of losses to reflection off the front face of the device, Lrefn , losses to transmittance through the front face, Ltrann , losses to heat convection through the front, Lconfn and rear, Lconrn of the device, and radiative losses through the front of the device, Lradn , any additional losses are assumed to be negligible. The remaining energy is entirely converted into heat within the water reservoir, giving a reservoir temperature at time-step n, Tresn , of:
Fig. 9. Temperature profiles when pumping water through the 12 mm reservoir, (a) only pumping to collect 80 °C water, (b) continuous pumping 11:00–16:00, (c) continuous pumping 9:30–14:30. 610
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Trn =
(ki·Tresn) + (si·hrn ·Tan) ki + (si·hrn )
where Rfront = 0.042 is the front-face thermal resistance. The above process is repeated for 20 iterations at each time-step.
(7)
where the insulation thickness si = 0.025 m, and the insulation’s Thermal Conductivity, ki = 0.022 W/(m·K). The front face temperature is calculated using the following iterative process:
Tfn (0) =
5.1. Model validation Model feed data was collected on six consecutive days in the testing period, and the model was run for each of these days. The model output is validated with actual reservoir temperatures witnessed on the six test days, as shown in Fig. 10. The model can be seen to agree well with the experimental data across all six test days. The model is particularly good at predicting the peak temperature attained in the day, while the warm up and cool down periods are
Tresn + Tan 2 1
Tfn (i + 1) = Tfn (i) + 4 ·(Rfront ·A·∊ρ ·σ·Ta4n ·Rfront ·A·h fn ·Tan + Tresn
(8)
− Rfront ·A·∊ρ ·σ·Tfn (i) 4 − Tfn (i)·(Rfront ·A·h fn + 1))
Fig. 10. Ambient temperatures and solar irradiance for six consecutive days in September, with model predictions of reservoir temperature and observed reservoir temperature for those days. 611
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bifacial mono-crystalline panel outperforms both the mono-crystalline and poly-crystalline solar panels, which both performed similarly. Moreover, the bifacial and the poly- panels performed in line with their specifications despite temperature differences as high as 19 °C (Section 4.2) across the panel. Drops in efficiency were dependent on the maximum temperature of the PV panel and not on any gradient across it. The primary reason for the increased efficiency of the bifacial panel is that it can collect solar energy hitting it from either side. The devices used have a metallic back sheet, so solar rays that penetrate the PV panel can be reflected back off this sheet back onto a solar cell. Purely from the standpoint of power production, the bifacial panel is the sensible choice. 6.1.3. Reservoir depth There is a strong correlation between the depth of the reservoir and both temperatures within it and the volume of hot water it can produce. Shallower reservoirs get hotter faster, have a higher temperature gradient, and produce more 80 °C water (Table 4 & Fig. 7). With the 12 mm reservoir, 80 °C water can be collected and replaced with cold from around 12:00. At this time the thermal flux of the system is still high enough to heat up the new cold water to 80 °C. For thinner reservoirs the production therefore lasts for longer, and beyond this the peak production rate is higher (Fig. 8). With the larger reservoir depths, thermal energy gathered during the day is wasted. This can be seen by looking at the refreshment rate of a reservoir across a day of hot water production. The 12 mm reservoir has a volume of 18.2 L and produces as average of 18.7 L, giving a refreshment rate of 1.03 reservoirs per day. Therefore all the thermal energy already captured in the water when production began (at 12:00) was collected. The 20 mm reservoir however has a volume of 30.3 L and produces only 5.5 L, giving a refreshment rate of 0.18 reservoirs per day, and so the thermal energy stored in the rest of the water goes to waste. It could be concluded, therefore, that the thinner the water reservoir the greater water production will be. For example, it was calculated that under the conditions seen here a temperature of 90 °C could be reached by 11:00 if a reservoir depth of 9 mm was used. This conclusion is true to a point, however, practical limitations exist on the depth of the reservoir which at times have to hold heat exchangers of a certain thickness, for example.
Fig. 11. Difference between model and actual data across the six test days used to feed the model.
predicted with slightly less accuracy. This is highlighted in Fig. 11 where the temperature difference between the model and the experimental data is shown. It can be seen that the model has a tendency to under-predict temperatures in the morning by up to 5.5 ° C, and over predict in the afternoon by up to 4 °C. 6. Discussion 6.1. Parameters 6.1.1. Exposure angle It is desirable to have a steep temperature gradient within the reservoir so that the water at the top can be allowed to reach as high a temperature as possible for the same total thermal energy. As seen in Section 4.2 (Fig. 5), by setting the angle of the PV-T unit to 14.7° a temperature difference of 19 °C was achieved within the reservoir, equivalent to a temperature gradient of 14 °C/m. In comparison, with an angle of 11.3°, the temperature difference reached only 14 °C, giving a temperature gradient of 10 °C/m. Optimal performance of solar panels is achieved when the panel is tilted such that sunlight falls perpendicularly onto its surface. This angle changes throughout the year with the changing position of the sun. Field measurements were taken in Cyprus during July and August in 2017, so the optimal exposure angles for PV power production were 19° and 27° respectively [20]. However, for our tests we limited the exposure angle to a maximum of 14.7° to avoid leakage as a result of pressure build-up in the lower section of the reservoir. The optimal angle for water production is not necessarily the same as for electricity production, but our results show that steeper angles correlate to higher temperature gradients within the reservoir.
6.2. Potential as a PV-T system The electrical efficiency of the device reaches 13.4% when fitted with the bifacial panels. The proportion of the remaining incident solar radiation that is converted into thermal energy depends on the state in which the device is functioning. When the devices are filled with stagnant water, solar energy is converted into thermal energy and the water is heated until ∼14:00 (Fig. 3a). Taking 31st August 2017 as an example, at 14:00 the average temperature within the reservoir was 89 °C, the maximum temperature was 96 °C, and the water contained 12.5% of the total energy to land on the solar panel that day (22% of the energy that had hit it so far). After this point, losses from the reservoir became greater than the energy absorbed, and the temperature dropped. In order to capture more thermal energy in this environment, further insulation would be needed. Heat loss back through the PV panel itself will always result in temperatures dropping after a certain point. This is dependent on the thermal conductivity of the insulation material and the temperature difference between the reservoir and the outside. This, of course, is not the state in which the device would usually be functioning, but it is a good demonstration of the amount of energy that it can hold at any given moment. When water is flowing through the device the results are significantly different. For two days water was pumped through pre-filled devices at a rate of 6 ml/s (Fig. 9(b) & (c)). Thermal efficiencies were then calculated using the equation:
6.1.2. Solar panel for PV-T systems When choosing a PV panel for a PV-T system described here, it is desirable to have a panel that is as thin as possible to allow maximum penetration of solar radiation into the reservoir. This has to be coupled with the fact that the panel must be strong enough to withstand the pressure from the water reservoir beneath it, tempered glass is, therefore, a must. Comparing the three PV panels used in this study, it is clear that the 612
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̇ p (To − Ti )/(Apv G ), ηth = mC
pump and a more complex control system). Therefore, as well as providing enough hot water for a family, and powering itself, the proposed system would provide all additional power generated for household needs. The total material cost of this system for our experiment is about 1000EUR excluding VAT. This has potential to become economically feasible for a single family. By comparison, the traditional solar collectors when it first was introduced in Cyprus and Greece cost around 950USD (Michaelides et al., 1999), which is equivalent to 1150 EUR today. With economies of scale (e.g., purchase parts for >500 units) and the choice of cheaper parts (e.g., use a thermosetting polymer to make the reservoir instead of Stainless Steel 316L), we estimate that the total material cost can be cut down to around 600EUR. This price could be brought down by a further ∼ 50 EUR by using the cheaper poly-crystalline PV panel, which produces enough power to run the system, however this would come at the cost of less electricity generation which can be used for applications.
(9)
where ṁ is the mass flow rate through the device (kg/s), Cp is the specific heat of water (J/kg K), To and Ti are the outlet and inlet temperatures of the water respectively (K), Ap v is the upward facing surface area of the PV panel (m2), and G is the solar irradiance (W/m2). When run for five hours during the middle of the day the thermal efficiencies of the devices averaged 53.4%. This, combined with the electrical efficiency of 13.4%, gives an overall PV-T efficiency of 66.8%. These efficiencies are not greatly affected by ambient conditions and remain largely the same each day. Comparing these efficiencies to the literature discussed in Section 1 we see that the device is already performing competitively, despite being a prototype where further gains can still be made. The calculated thermal efficiency will likely also be dependent on both the flow rate and hours of operation, which are recommended as the subject of further testing. The maximum achievable thermal efficiency however will be determined by the set of energy losses included in the theoretical model, some of which have the potential to be reduced. Additional gains in efficiency could be made, for example, by increasing positive heat flux in the morning through reducing the thickness of the glass PV panel, or reducing negative heat flux in the afternoon through increasing the thickness of the insulating air gap between the glass panel and the acrylic sheet. The acrylic glass itself transmits infrared wavelengths of less than 2.5 μ m but absorbs and hence blocks higher wavelengths up to about 25 μ m, while heat irradiated by the water reservoir at 90 °C peaks at approximately 8 μ m (Planck’s law), therefore this material is well suited for this purpose. However, over the course of the testing period this sheet was seen to degrade, warp, and yellow in colour, potentially reducing the transmittancy of solar irradiation as well as compromising the integrity of the insulating layer of air. Therefore tests should be done on alternative materials that exhibit similar conductive and optical properties, to find an ideal insulation layer that is more durable after repeated sun exposure. One such option would be a poly-carbonate, which has a similar thermal conductivity ∼0.19 W/(mK).
6.4. Theoretical model Various aspects of the device are untested and can still be improved. Beyond the two testing parameters of reservoir depth and exposure angle the model can be used to perform sensitivity analysis on the following parameters: PV panel dimensions (and so, with reservoir thickness, also reservoir volume), front-face insulation thickness, frontface insulation material, air-gap between PV panel and front-face insulation, rear-face insulation thickness and rear face insulation material. Although the device as proposed here can readily be recreated and furher tested by the reader, it is recommended that insulation and material types are investigated using the theoretical model before doing so. For example, Fig. 12 below shows the losses at each time-step as predicted by the model (transmittance losses through the front face are constant at 12%). Combined losses amount to over 100% at around 2 pm when the device starts to cool down, and Fig. 12 shows that this is largely down to the front-face convective losses. Thorough sensitivity analyses of various material types and thicknesses on this loss, and indeed on all the losses, could potentially bring energy loss down and increase the performance of further devices without the need for additional test periods. If a user was planning on building such a device in a known
6.3. Feasibility as an off-grid, self-powering, solar collector. The temperature of hot water required for domestic consumption is much lower than the 80 °C water collected through most of these experiments, and usually lies around 40–45 °C. Consumption per person per day varies greatly and is dependent upon season and latitude. In the U.K., for example, average daily consumption is considered to be between 30 and 50 L of 45 °C water (CERNUNNOS, 2018) (120–200 L for a family of four), while in warmer climates this will be significantly less. The PV-T system proposed here, when pumping continuously for five hours, from 11am until 4 pm, produced 108 L of water at an average of 59 °C (Fig. 9(b)). This is energy equivalent to 185 L of 45 °C hot water at the same ambient temperature. If 45 °C water was to be collected directly, collection would begin between 9:00 and 9:30, and could be expected to continue until around 16:30–17:00. It must be taken into account that most hot water will be required in the morning and evenings, while such a solar collector can only collect during the day, and so an extremely well-insulated hot water tank may be required. Once this has been considered, it is clear that such a system is capable of providing hot water for a family of four. Such a system requires little power to run, and any excess electricity could be either stored in a battery or directed straight into a house, for example. In these experiments two standard Victron batteries were connected in series for this purpose. A control system would be required to start and stop water pumping at given temperatures, and to control and direct the PV output, while the pump itself consumes less than 5 W. Such a system could be turned on and off automatically at sunrise and sunset, and through the day could safely be assumed to run on 25 W (the power consumption witnesses here when running only the inlet
Fig. 12. Energy losses at each time step. Transmittance through the front face is left off as it is a constant at 12%. 613
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References
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7. Conclusion This field study explored the viability of a recently developed PV-T system as a self-powered, off-grid, solar collector. This constitutes the first time that such a device has been tested experimentally, and it has demonstrated the capability to provide hot water of approximately 80 °C for a family of four, as well as providing excess electricity towards household applications, assuming an average daily solar irradiance of >4.5 kWh/m2. Electrical and thermal efficiencies of 13.4% and 53.4% are reported, which compete well with the traditional PV-T system. Calculation of the electrical efficiency is straightforward. It is the power produced by the PV panel normalised by the solar irradiance multiplied by the exposure surface area. The reported thermal efficiency however is strongly dependent on the flow rate and the average temperature difference of the water one aims to achieve. The maximum achievable thermal efficiency could only be estimated by considering energy losses. A theoretical model of the device was built, which tracks energy losses with time and outputs the average reservoir temperature at each five-minute time-step. This model was validated with the obtained data during the field study. This model performs well and has the functionality to inform these future improvements through a sensitivity analysis of parameters and their effect on different types of energy loss. This would bypass the need for lengthy and expensive additional testing. An immediate potential improvement includes varying insulation material and thickness, for example, changing the acrylic sheet over the panel in our experiment for a more suitable material. The experimental device has demonstrated the capacity to produce over 20 L of 80 °C + water in a day, and under good conditions, the reservoir held water of over 95 °C. There is still room for improvement here. The rapid drop in cost price of existing ‘off-the-shelf’ PV panels in the last decade has provided the opportunity to reconfigure these PV panels into PV-T panels, thus opening the way for applications which require both electrical and thermal energy such as (1) heating/cooling of buildings, (2) heating of water for household applications and (3) desalination. We will cover the latter topic in a forthcoming paper. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.solener.2018.10.062.
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