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Theoretical background and literature review
In this chapter, the shortage in performance of PV modules, due to high operating temperature is explained, and the necessity of developing PVT collectors is pointed out. The concept of PVT collectors and their types are mentioned in details. A short literature review about the design and energetic balance of PVT collectors is presented.
Limitation of PV systems
Each type of solar cells in the PV systems has a certain cut-off absorption wavelength corresponding to the cells band gap energy. Any incident photon with a wavelength longer than the cut-off wavelength will not be absorbed by the solar cells. That means photons of longer wavelength (their energy is below the band gap energy) do not generate electron-hole pairs to produce electricity, but only dissipate their energy as heat in the cells. Most of PV cells convert 8 to 20 % of the incident solar radiations into electricity depending on the type of cells and the operating conditions, and more than 50 % of the incident solar energy is transferred to heat energy. As a result, the PV cells temperature may increase to more than 30 ᵒC above the ambient temperature depending on the later and the wind speed. Increasing the cells temperature leads to reducing VOC voltage and thus reducing the cells electrical efficiency. Figure 2.1 shows an example of an I-V curve of a PV module presenting the effect of cell temperature on the electrical performance of the module (arrangement of cells). For example, by increasing the silicon (mono-crystalline) based cell temperature from the ambient value of 25 ᵒC to 85 ᵒC, the VOC decreases by 18 % which negatively affects the electrical performance of solar system, Häberlin, Heinrich,Book p136, (2012).
Fig.2.1 I-V curve of a PV module with different cell/modules temperatures and 1000 W/m2 insolation.
Isc increases slightly, while VOC decreases considerably with increasing the temperature. Red circles indicate
Cooling the PV modules with a fluid stream (coolant) is a good strategy to improve the modules performance. Conceptually the better design is to re-use the heat energy extracted by the fluid. Then the total energy yield per unit area of the modules can be considerably improved. These are the incentives leading to the evolvement of PVT hybrid solar technology.
Basic concept of PVT collectors
PVT hybrid collectors can supply electrical and thermal energy simultaneously, and has merits of higher energy gain and effective utilization of installation area compared to only PV panels or solar thermal collectors with the same capacity. A PVT collector mainly consists of two layers: A glazed or unglazed PV layer at the top and a thermal absorber layer at the bottom which absorbs heat from the PV layer. The absorber layer has an inlet and outlet for fluid circulation. The fluid, which may be liquid (such as water or water-glycol mixture) or air, is circulated through the absorber material to extract heat from it. Fig.2.2 shows a simple structure of flat plate PVT collector.
Extracted heat from the PV modules serves as a second benefit along with the increased performance of the PV modules. For low temperature applications such as domestic hot water heating, pool heating, and air heating in ventilation systems, PVT collectors are attractive in terms of efficiency and performance. Furthermore PVT collectors provide architectural uniformity at the building facade and are aesthetically better than the two separated arrays of PV and solar thermal collectors being placed side by side to get the same output of one PVT collector.
There are several types of PVT collectors and they are classified with respect to the fluid used, shape of the collector and whether it’s integrated into a building or not. Moreover, having glazing on the collector is another factor that distinguishes a PVT collector’s characteristics. The main two types of PVT collectors, according to the shape, are flat plate PVT collectors, concentrating PVT collectors, and they can use either water (PVT water collectors) or air (PVT air collectors) as fluid. Flat plate PVT collectors are very similar to the well-known flat plate thermal collectors, but they have a PV layer above the thermal absorber as shown in Fig.2.2. In concentrating PVT collectors, concentrators are used to increase the irradiance level on PV modules which leads to use less PV panel area and hence reducing the total cost of the collector. A low concentrating PVT water collector of building integrated type of 50 m2 glazed area (mono-crystalline Si solar cells) was investigated by Brogren et al., (2002), where low cost aluminum foil re flectors in parabolic shape were used with a concentration ratio of 4.3. The annual electricity production was considerably higher by about two times, and the thermal energy was higher by 3 to 4 times than the electrical energy produced by conventional PV systems, with no concentration. The collector was installed in Stockholm, Sweden and was able to produce 800 kWh of heat and 200 kWh of electricity per m2 solar cell area. The hot water had a temperature of about 50 ᵒC.
Generally, air or water is used as a fluid to extract the heat from the PVT collector. Water and air are preferred coolants rather than refrigerants since the overall cost of the entire system increases due to the capital cost and maintenance cost for the refrigerant loop. Water is the most effective fluid to collect the heat from the PV panel and absorber due to its high heat capacity and thermal conductivity.
Parameters affecting PVT performance
In PVT collectors, the electrical and the thermal efficiencies increase to certain extend as the mass flow rate of the fluid increases. However, increasing the mass flow rate results also in decreasing the outlet temperature of the fluid and increasing the required electrical energy for the pump or the fan that moves the fluid through the PVT system. Therefore, a good designing of a PVT collector requires choosing the right value of the fluid mass flow. There are additional fundamental parameter to be considered in the PVT collector design, such as collector area, the length to width ratio of the collector and the depth of cooling duct, which affect significantly the energetic performance of the collector (Brinkworth, 2006, Farshchimonfared et al., 2015).
PVT cost compared to PV systems
The costs of PVT and PV systems depend on the developed technology and varying over time, but mostly, PVT collectors are more expensive compared to PV and thermal collectors. For example, Tripanagnostopoulos et al., (2002) conducted outdoors tests on both PVT air and water collectors of different design configurations for horizontal-mounted applications. They found that PVT air collectors are around 5 % higher in production costs than the PV modules. This would be around 8 % for PVT water collectors with pc-Si cells, and around 10 % when the entire system costs were considered. The outcome of their experimental tests gave a range of thermal efficiency from 38 % to 75 % for PVT air collectors and 55 % to 80 % for PVT water collectors, based on the steady state noon-hour measurements in the University of Patra (at 38.2 ᵒ N) in Greece. During the experiment, the collectors were arranged in parallel rows and keeping a distance between rows to avoid shading, and then low-cost booster diffuse Aluminum reflectors (concentration ration = 1.5) were placed between the adjacent rows to increase the received radiation at collector surfaces.
PVT water collectors
In this type of PVT collectors, water or a mixture of water (i.e. water-ethylene-glycol) is circulated through the collector to extract heat from it. Using a PVT water collector is advantageous if there is a demand for hot water. Flat structure of the collector allows simple design and convenient integration on a building, usually rooftops; therefore PVT flat water collectors are more popular compared to concentrating PVT water collectors. Furthermore, in PVT water collectors, hot water may be stored in an external tank for a period of time which imposes drawback of leakage and freezing that may occur in extreme conditions, Arif Hasan et al., (2010). However, these can be prevented by robust construction and by using water-ethylene-glycol mixture. PVT water collectors may either be glazed or unglazed. An extra layer of glass on top of the PV cells results in less heat losses and greater thermal performance, whereby reducing electrical performance by some extent due to the reason that PV cells perform worse in higher temperatures. Unglazed PVT collectors have higher electrical performance and lower thermal performance compared to glazed ones because of greater heat losses resulting in lower operating temperatures, Ramos, (2010).
PVT water collectors have been presented in the literature with different design depending on the cooling method. Fig.2.3 shows the cross sectional views of common PVT water collector designs. The sheet-and-tube design (Fig.2.3.a) provides the simplest way to construct a PVT collector by integrating a standard PV panel to a thermal collector without any modification. Even, the annual ef ficiency of this design is about 2 % worse compared to other designs; it can be a good choice due to its simplicity. The cooling channel below transparent PV design (Fig.2.3.c) gives the best efficiency, Zondag, (2003). The configuration of a channel PVT collector with the channel on top of the PV (Fig.2.3.d) imposes constraints on the choice of the collector fluid; so that the absorption spectrum of the fluid should be sufficiently different from the PV absorption spectrum in order to allow the PV to receive the incoming radiation. The effect of spraying water upon the PV cells is investigated in terms of cells temperature variation and reflection losses showing an increase in cells efficiency on average of 3.26 %, (Amery and Abdolizadeh, 2009).
The thermal efficiency of PVT water systems is generally in the range of 45 – 70 % for unglazed to glazed panel designs.
PVT air collectors
In PVT air collectors, air is circulated through the collector instead of water. This type of collectors is attractive for applications where heated air is required. PVT air collectors provides some advantages such as they are cheaper and less complex compared to PVT water collectors [Arcuri, (2014); Bambrook et al., (2012)], where a simple fan can be used instead of a water pump. Moreover, there is no risk of freezing, or boiling at extreme weather conditions. On the other hand, PVT air collectors suffer from some major disadvantages such as having lower thermal performance characteristics compared to water types due to lower heat capacity and thermal conductivity of air compared to water. Furthermore, low density of air causes the transfer volume to be significantly higher than that of PVT water types. Thus, pipes with higher volume and greater bulk are needed which is not suitable for applications with low available area and is not aesthetically likable. Despite these disadvantages, PVT air collectors are suitable choices for hot air applications thanks to their lower cost. Fig.2.4 shows a single pass flat plate PVT collector, where the air is circulated in a duct at the back of PV modules to extract the heat.
PVT air collectors can also be uncovered or glass covered, where a covered collector has an additional glass cover over the PV modules to reduce heat loss from the top of the collector, Tonui et al., (2007). Variety of PVT air collector designs including single pass or double-pass, wall-mounted or roof mounted have been reported in the literature.
A number of experimental and analytical studies have been reported on PVT air collectors [Tonui et al., (2007), Chen et al., (2010), Mei et al., (2009), Candanedo et al., (2010)]. Tonui et al. (2007) investigated low cost improvements for PVT air collectors to enhance the heat extraction from the PV modules. The design contains two modifications involving the interposing of a thin (flat) metallic sheet at the middle of the air duct or channel (TMS configuration in Fig.2.5) and attaching rectangular fins, oriented parallel to the flow direction, at the back wall of the channel (FIN configuration in Fig.2.5), to enhance heat transfer from the channel walls to airflow. The PVT collector consisted of two identical polycrystalline pc-Si PV panels, of length 1 m, aperture area 0.4 m2 and rated power of 46 Wp, as ‘absorber’ plates. The air channel had a depth of 15 cm attached at the rear surface of each module. The air duct casings were constructed from a thermal insulator board of thickness 5 cm on the backside and the edges with a thin aluminum sheet as inner lining. Small inlet and outlet vents of diameter 5 cm (to fit to the flexible tubes from the air pump) were provided at the top and bottom of the air channels, respectively. The fins height and spacing distance was each equal to 4 cm. Fig.2.5 displays the three PVT designs developed by Tonui et al. (2007).
Fig.2.5 Cross-sectional view of PVT air collector models. Flow direction is perpendicular to the page, Tonui et al. (2007). REF: presents the PVT air collector without modification, TMS: presents the modified collector with thin metalic layer at the middle of the duct, FIN: presents the modified collector with fins in the duct.
Their results showed that the thermal efficiency was 25, 28, and 30 % for REF (collector without modifications), TMS, and FIN configurations, respectively. They mentioned that the modifications increased the thermal efficiency of the PVT air collectors by 12 % and 20 % for TMS and FIN designs, respectively compared to the REF design for both natural and forced airflow. In forced flow, the air flow rate was 60 m3h-1 corresponding to air velocity of 0.25 m/s. While in natural flow, the flow rate was 12.5 m3h-1 corresponding to air velocity of 0.06 m/s. The module temperature measured without air circulation for all the PVT air configurations was about 55-75 ᵒC at ambient temperature of about 30 ᵒC and insolation level of 700-800 W/m 2. With the air circulation, the measured PV module temperatures range from 45 to 65 ᵒC depending on the flow rate, weather conditions and time of the day.
Testing of an experimental building integrated PVT (BIPVT) air system is done by Bazilian et al. (2001), where an air gap between the PV module and the building is used for circulating air to cool PV modules and the pre- heated air can be used for building thermal needs. Chen et al. (2010) investigated another type of BIPVT system thermally coupled with a ventilated concrete slab in a low energy solar house located in the cold climate of Quebec, Canada. The used BIPVT system consisted of 21 amorphous silicon 136 Wp PV modules bonded directly to metal roofing which covers an area of (6.2 m width x 10.4 m length). A shallow air duct with a depth of 0.038 m runs underneath the metal roofing and the back of the channel is composed of plywood and insulation. The use of low air flow rates (0-0.0056 kg/s m2) enables large temperature increases between the inlet and outlet air of the order of 40 ᵒC for a sunny day in winter. The PV modules were shown to be operating at high temperatures around 50-60 ᵒC under these conditions. A typical thermal efficiency of 20 % for the BIPVT system was reported. More PVT air collector designs have been presented in the literature with some of the main features implemented being single- pass or double-pass air flow, and using the PVT concept as either a building wall facade or a roof mounted unit. Hegazy (2000) performed an extensive investigation of the thermal, electrical, hydraulic and overall performance of four types of flat-plate PVT air collectors including one cooling channel above PV as Mode I (Fig.2.6.a), one channel below PV as Mode II (Fig.2.6.b), PV between single-pass channels as Mode III (Fig.2.6.c), and finally the double-pass design as Mode IV (Fig.2.6.d).
The numerical analysis showed that while Mode (I) has the lowest performance, the other three have comparable energy yields. The thermal efficiency of the PVT collector increased with increasing the air mass flow regardless of the collector configuration. The increment in the thermal efficiency was high before the air mass flow reached 0.02 kg/sm2, while the increment became weak after the air mass flow over passed 0.02 kg/s m2 as shown in (Fig.2.7). The thermal efficiency was 50 % at air mass flow of 0.02 kg/sm2.
The temperature difference between the inlet and outlet air of the collector decreased with increasing the air mass flow as shown in Fig.2.8. The temperature difference was 25 ᵒC at air mass flow of 0.02 kg/sm2.
Fig.2.8 Variation of maximum air temperature rise with air mass rate for PVT air collector configurations (I–IV) analyzed by Hegazy, (2000).
Fig.2.9 Hourly values of solar insolation and ambient temperature used in the computations, Hegazy, (2000).
Performance comparisons had been performed employing the hourly data of solar insolation and ambient temperature shown in Fig.2.9. These climate data were recorded at Minia University, Egypt for the summer day 24 June 1998, during which the prevailing wind speeds varied between 0.5 and 1.5 m/s.
Brinkworth, (2006) studied the optimization of the geometry of naturally ventilated PV cooling ducts to minimize the loss of PV modules ef ficiency. The outcome of this study determined that the optimum ratio of the duct length (L) to its hydraulic diameter (DH) is around 20 (or DH/L=0.05), but this value could not be used for a PVT air collector as the thermal energy output and fan power have not been investigated in the study.
Recently, Farshchimonfared et al., (2015) studied PVT air collectors to optimize the cooling channel depth, the air mass flow rate per unit collector area and the air distribution duct diameter considering the whole system performance. The optimization process aimed to maximizing the thermal energy output per unit collector area delivered to the building at a fixed value of the temperature rise (Tout-Tin= 10 ᵒC), and for different values of PVT air collector area Ac (10, 15, 25, 30 m2) and L/W ratio (0.5, 1, 1.5, 2), where W is the air duct width in m. The collector was linked to an air distribution system of a typical residential building in a climate with a mild winter (e.g. Sydney). The optimum value of air mass flow was almost constant and approximately equal to 0.021 kg/s m2 (Fig.2.10). The optimum depth (Dopt) value varies between 0.09 and 0.026 m (Fig.2.11) and the optimum air distribution duct varies between 0.3 and 0.5 m (Fig.2.12). The optimum depth increases as the collector L/W ratio and the collector area increase.
Fig.2.10 Optimum air mass flow rate per unit collector area as a function of the length to width ratio (L/W) for collectors of area 10, 15, 25 and 30 m2, Farshchimonfared et al., (2015).
Fig. 2.11 Optimum depth as a function of the length to with ratio (L/W) for collectors of area 10, 15, 25 and 30 m2, Farshchimonfared et al., (2015).
Fig.2.12 Variation of the air distribution duct diameter as a function of the length to width ratio (L/W) for collectors of area 10, 15, 25 and 30 m2, Farshchimonfared et al., (2015).
Fig.2.13 Variation of the optimum depth to length ratio of the PVT air collectors, Farshchimonfared et al., (2015).
Fig.2.13 illustrates the variation of the optimum depth to length (Dopt/L) of PVT air collectors. The value (Dopt/L) was not constant and varied between 3.4 x 10-3 to 4 x 10-3 depending on the value of L/W and the collector area.
Bambrook et al. (2012) demonstrated a thermal energy output of around 15 kWhth /day (2.41 kWhth/m2 day), and an electrical output of 3.28 kWhel/day (0.53 kWhel/m2 day), on a sunny winter’s day in Sydney. The electrical and thermal outputs were generated by using a PVT air collector with an area of (Ac =6.22 m2 ), and air mass flow of 0.048 kg/s m2. The PVT air collector consisted of six PV modules with total output power of 0.66 kWp. The non-ventilated PVT air collector had a midday electrical efficiency of 10.6 % in sunny winter weather conditions. At 12:00 pm, the air cooled collector produced an electrical efficiency of 12.2 %, representing a relative electrical efficiency increase of 15%. The PVT air system had a daily thermal energy yield of 22.7 kWhth/kWp pv, and a daily PV electrical energy yield of 4.97 kWhel /kWppv . The fan energy requirement in that case was 0.15 kWh/day, which is smaller than the additional energy generated by the PV modules (0.25 kWh) due to the ventilation and operation at a lower temperature. The reference electrical energy output used for that case is the electrical energy output of the non-ventilated PVT system which is 3.03 kWh and corresponds to a PV electrical energy yield of 4.59 kWhel/kWppv (These values of electrical energy, with ventilation and without ventilation, were adjusted for an average Sydney sunny day solar insolation of 5.75 kWh/ m2 day).
Chan et al. (2010) demonstrated a heat output of 9.7 kW, and an electrical output of 2.41kW, produced by 64.5 m2 residential BIPVT with air flow rate of 0.036 kg/s m2. The fan power was 0.4 kW. The unventilated case generated an electrical output of 2.31 kW and no thermal output. That means an increase of 100 W in electrical output and 9.7 kW heat output of the BIPVT air system came at a price of 400 W for the fan power.
Table of contents :
1. Introduction
1.1 Motivation
1.2 Aims
1.3 Method
2. Theoretical background and literature review
2.1 Limitation of PV systems
2.2 Basic concept of PVT collectors
2.3 Parameters affecting PVT performance
2.4 PVT cost compared to PV systems
2.5 PVT water collectors
2.6 PVT air collectors
3. Experimental setup and measurements
3.1 Description of the installed PVT air collector
3.2 Description of the monitoring system
3.2.1 Calibration of thermocouple sensors
3.2.2 Pyranometer, wind and air velocity sensors
3.2.3 Voltage dividers and shunt resistances
3.2.4 Low pass filters
3.2.5 Data acquisition
4. Data analysis and results
4.1 Data analysis
4.2 Results
4.2.1 Thermal performance
4.2.2 Electrical performance
5. Discussion and conclusions
5.1 Results analysis
5.2 Future work
References
Appendices
Appendix I PV Solibro modules data sheet
Appendix II Inverters datasheet
AppendixIII Data logger’s datasheet
Appendix IV Pyranometer’s datasheet
Appendix V Wind speed senosr’s datasheet
Appendix VI Air velocity senosr’s datasheet
