Temperature regulation of concentrating photovoltaic window using argon gas and polymer dispersed liquid crystal films

Highlights

• Optical and thermal performance were analysed at laboratory environment for LCPV SEH system.

• Output of the system is demonstrated using different strategies, Argon filled and PDLC on the top of the module.

• Operating temperature was reduced by 10 °C and 4 °C for both states respectively.

• Increment in power was 37 mW and 47 mW respectively.

Abstract

Low concentrating photovoltaic (LCPV) system has been studied extensively, which showed excellent potential for the building integration application. However, such a system suffers from higher operating temperatures due to the concentrated light exposed into the solar cell. In this work, two different methods have been used to regulate the operating temperature of the solar cell without the interference of any other external mechanism. Two concepts were used to study the operating temperature of the solar cells are: i) use of Argon gas within the concentrator element, ii) incorporation of polymer-dispersed liquid crystal films (PDLC) on top of the module. In both cases, the power was improved by 37 mW–47 mW when temperature was reduced by 10 °C and 4 °C for the Argon gas-filled module and PDLC integrated module, respectively. In addition, the temperature effect of the PDLC integrated module showed a unique nature of reduction of the short circuit current due to the orientation of the liquid crystal particle, which increased at a higher temperature. The current study, therefore, shows the greater potential of improving the operating efficiency and reduction of solar cell temperature, without the need for additional pumping power such as needed for photovoltaic thermal application.

Introduction

In recent times, the low concentrating photovoltaic (LCPV) by using optical concentrators such as lenses mirror to focus incident radiation on the smaller area is an appealing application in photovoltaic industry for low cost and higher yield solar power generation. LCPVs trim down the cost of the PV system by reducing the cell area and replacing expensive semiconductor material by inexpensive semiconducting material [[1], [2], [3], [4]]. LCPV does not require expensive tracking system, supply more power for a limited area which reduces the necessity of large areas fragile silicon PV, and simpler system for building integration [[5], [6], [7]]. In general concentration ratio of LCPV for building integration remain <10 [8].

Non-imaging compound parabolic concentrator (CPC) is one of the most studied LCPV system, suitable for building integration [9], due to its higher acceptance angle [10,11]. CPC includes reflective or refractive mirror [6] or dielectric [12] type; symmetric [13] or asymmetric [9]; 2-dimesional [14] or 3-dimensional [15] type optics.

Most studied CPC geometry for building integration is 2D symmetric and asymmetric type which accept all the incident rays within the acceptance angle however it rejects all other incident radiation which is outside the acceptance angle [16,17]. The lumped electrical model was designed with 2D asymmetric CPC (trough) to measure their IV curves in an indoor laboratory. This model was integrated successfully to examine the coupled optical-thermal-electrical performance of CPC [14]. In northern latitude location, dielectric asymmetric compounds parabolic type showed un-stability at higher acceptance angle. Further work was needed to investigate the effect of change in the solar spectrum over time by considering the refractive index of the dielectric material and dispersive absorption coefficient followed by low-cost manufacturing process [9]. Limited concentration obtained from 2d geometry can be increased by using 3d CPC. However, 3d geometry offers circular exit and entry aperture which produce more losses and circular shapes PV cells are not popular. Thus, to enable the use of 3d CPC for rectangular or square shape crystalline silicon PV cells, crossed compound parabolic concentrator (CCPC) was developed which is 3d in nature and intersection of two 2d CPC crates this shape. A 3D crossed compound concentrator based on dielectric was used to analyse energy transformation and energy output of a low concentrated PV system [12]. Another 3D CCPC based PV module was fabricated to determine optical and electrical performance for building façade integration [11]. To improve further the concentration ratio, there are however other LCPV optic designs which require further investigation such as the Square-Elliptical-Hyperbolae (SEH) optic by Sellami et al. [18]. which is shown in Fig. 1. This SEH has elliptic entry to the square exit. This system had a concentration ratio of 6x, the optical efficiency of 55% and an acceptance angle of 50°, however, the concentration of incident light was non-uniform at the solar cell. SEH based spaced type LCPV BIPV window can be the next generation system which will have triple advantages into the building application: i) maintaining daylighting within the building envelope, ii) reduce thermal load of the building and iii) generate electricity within the building envelope. Thus 2D, 3D CPC are not compact, and they need to truncate the upper reflector section to maximize the efficiency of the BIPV. This new design is compact enough and feasible for BIPV application and can be accurately integrated into transparent façade, windows or roofs.

An important parameter that can negatively affect the performance of the PV cell is the temperature sensitivity of the solar cell. PV cells experience high thermal energy due to the absorption of incident solar radiation that is not converted into electricity. Several researchers have investigated the comparison between theoretical temperature and different parameters that come from solar cell output [19,20]. Bett et al. calculated theoretical values’ illustrating that temperature coefficient is dependent on the dominant recombination process. Another group from NREL measured experimentally the metastable changes due to light exposure results as the temperature dependence of the fill factor of CIGS that modified thin-film cells [21]. It is well studied the efficiency of the solar cell, overall power output and open-circuit voltage decreases due to the increase in solar cell temperature [22]. Variation in external quantum efficiency (EQE) also seen due to temperature variation in different regions of spectra. The influence of temperature of PV cell on different mechanisms fill factor (FF), open-circuit voltage (V_oc), short circuit current (J_sc) are well known. The fill factor temperature sensitivity can be possibly due to technical issue such as contact resistance [23]. Electrical performance of a typical PV module is 18–24% of the incident solar radiation, dependent on climate condition and type of solar cell used in the module. The rest of the incident radiation is converted into heat that can be primarily cause of increment of temperature and reduce the efficiency of the PV module [24].

Enhanced temperatures of crystalline silicon based solar cell under solar radiation is an important issue [25,26]. This becomes worse when the light is concentrated; specifically for the silicon solar cell and light concentrated on the solar cell surface, a significant rise in temperature is evident [27]. Thermal regulation of crystalline PV cells is thus essential for concentrating system. Thermal regulation using active air and water cooling and passive air cooling is possible and has been demonstrated as an effective and economical way. The proper length of the fins to pint out the harassment of the power of the PV system was inquired during the passive cooling by phase change material [28]. However, water- and air-cooling techniques need the additional cost of pump or fan to maintain the output of the system. In the case of water, evaporation is another obvious drawback to giving lower efficiency [29]. The incorporation of phase change materials into PV systems with concentrator photovoltaics can enhance the performance of the system in warmer weather of UAE [30]. Phase change materials absorb thermal energy as latent heat and can be used to maintain the temperature of the PV system at suitable phase transition temperature [31,32].

Previously, phase change material (PCM) type temperature regulation has been demonstrated by Sharma et al. [33] for low concentrator system. BIPV façade systems for temperature regulation with the induction of complicated airflow and convective heat transfer through natural way has been reported [34].

Simple method without compromising electrical efficiency of PV panels is natural cooling using free convection to remove heat from the back of the PV modules. A heat sink for CPV using normal grade aluminium on the back of the PV module to remove heat by thermal conduction has been examined. With a concentration of 500 suns, this system was capable to reduce cell temperature by up to 10 °C [35]. For passive cooling of PV cells heat pipes were used; 38 °C temperature was reduced [36]. Hydronic cooling technique was induced with the PV system to minimize the operating temperature; consequently, 23 °C reduction was evident with an increase of cell efficiency of 3.26% [37].

Spaced type concentrating BIPV window systems are mostly double glazed which further needs modification to enable it for heating load dominated location. Replacing the air between two glass panes with inert gas [[38], [39], [40]] or vacuum [[41], [42], [43]] can offer higher thermal insulation. Spaced type concentrating BIPV windows also offer static transparency which can be modulated by using smart switchable materials. Current switchable materials are electrically, thermally or optically activated, where electrically activated types such as electrochromic (EC), polymer dispersed liquid crystal (PDLC), and suspended particle device (SPD) respectively, are favourable due to their user control behaviour [44]. PDLC type smart materials become transparent in the presence of AC power supply and translucent while no power is applied [[45], [46], [47]]. The translucent state provides a lower transmission but provides full privacy from viewing which is suitable for glare control. PDLC glazing as a suitable candidate for building façade application has been the subject of many researchers [48]. PDLC glazing performance for overcast cloudy day was also examined and was acceptable to control glare [48]. However, no such work reported for the use of PDLC for CPV applications. The reliability and endurance of concentrated BIPV technology with PDLC requires more research and use of alternative materials for novel applications.

The integration of argon gas as insulating material within the panes of double-glazed window is another significant process to enhance the thermal insulating performance. Double glazed windows with argon gas filling approach has lion’s share in window technology and can represent an effective energy saving solution. U-value assessment of commercially available argon double-glazed window was examined numerically, experimentally, and theoretically. The experimental results were quite ample from environmental chamber tests showing good accordance with theoretical datasheet [49]. Another work was reported to present comparison of thermal performance of coated double-glazing and non-coated double-glazing with same argon gas filling ratio [50]. The U-value of non-coated double-glazing reduced about 3.7% and 13% for coated double-glazing. Moreover, the relative energy with low emissivity coating of 0.13 was saved about 51% making this system suitable to meet requirement of building energy efficiency.

Although, this approach has downside of drastically reduction in amount of solar radiation to pass through the glass due to insulating material. However, this condition has advantageous and disadvantageous behaviour at different latitudes such as it gives favourable output at medium latitude such as central Europe and unfavourable output at higher latitude areas e.g. Scandinavian countries.

In this work, an LCPV window prototype containing the SEH CPV module was modified using Argon gas within the double-glazed window cavity to improve insulation characteristics and PDLC film covers to control the daylighting and operating temperature. This developed system has been used for testing of PV module performance for different parameters such as efficiency, FF, power output and operating temperature of the system. Presence of argon gas and polymer dispersed liquid crystal (PDLC) film’s thermal behaviour of PV cells in a concentrating BIPV window has been investigated under indoor condition.