Effect of a hybrid jet impingement/micro-channel cooling device on the performance of densely packed PV cells under high concentration

Abstract

This paper studies in deep the performance of a new hybrid jet impingement/micro-channel cooling scheme for densely packed PV cells under high concentration. The device combines a slot jet impingement with a non-uniform distribution of micro-channels. The Net PV output of the concentrator system, defined as the PV output less the pumping power, and its temperature uniformity are analyzed. The hybrid cooling scheme offers a minimum thermal resistance coefficient of 2.18 × 10⁻⁵ K m²/W with a pressure drop lower than in micro-channel devices. This characteristic involves that the Net PV Output of the PV receiver is higher when cooled by the hybrid design than when cooled by the micro-channel one. The chance to modify, at the design stage, the internal geometry of the hybrid cooling scheme allows improving the temperature uniformity of the PV receiver through the adequate distribution of the local heat removal capacity.

Introduction

As a consequence of the growing efficiency of high concentration PV cells, this technology is seen as one of the ways to reduce the cost of solar electricity. Nowadays, there are modules available in the market with punctual lenses that concentrate the sun on small PV cells. The distance between cells is very large and passive cooling techniques can keep the cells in an optimum operating range of temperature. In the case of devices that concentrate the sun, trough mirrors, on densely packed PV cells, it becomes necessary to apply an active cooling on the receiver.

On the one hand, for solar concentration ratios above 150 suns, Royne et al. (2005) exposed that the cooling device must have a thermal resistance coefficient lower than 10⁻⁴ K m²/W. On the other hand, the cooling device must maintain relatively good temperature uniformity. This characteristic affects the global performance of the PV receiver in two ways: (1) Chenlo and Cid, 1986, Royne and Dey, 2007 showed that the temperature non-uniformity reduced the whole efficiency of the PV receiver, although at a lower rate than the non-uniformity of illumination; (2) The lack of uniformity in temperature of the PV cells implies mechanical stress, due to the fact that the thermal expansion depends on the local temperature of the receiver. In addition to the large number of thermal cycles that take place in a high concentration PV device, this causes a thermal fatigue of the receiver and affects the reliability of the whole system.

For solar concentration ratios between 160.8 and 202.9, Zhu et al. (2010) showed that a liquid immersion cooling method offers good solutions to the issues presented above. Nevertheless, at higher concentration ratios, it is necessary to use the cooling schemes that are generally applied in electronic devices: jet impingement and micro-channels (Royne et al., 2005). These kinds of heat sinks reach the thermal resistance coefficient requested but do not offer adequate temperature uniformity on the receiver. Furthermore, the micro-channel cooling schemes cause high pressure drop. This entails a high requirement of pumping power that reduces the whole electricity production of the system. Royne and Dey (2007) explored the viability of arrays of impinging jets as a cooling device for densely packed PV cells. The system included a complex return architecture that improves the performance of the cooling device. The study concluded that high heat transfer coefficients were reached but that it was difficult to reduce the temperature nonuniformity inherent to jet impingement distributions.

Some studies tried to overcome the major disadvantages of both micro-channels (Steinke and Kandlikar, 2006) and jet impingement (Aldabbagh and Sezai, 2004). An interesting way of investigation is the development of hybrid jet impingement/micro-channel cooling schemes. Sung and Mudawar, 2006, Sung and Mudawar, 2008 concluded that the hybrid modules maintain a higher degree of temperature uniformity than other cooling schemes.

Along this line, Barrau, 2008, Barrau et al., 2009 developed a new hybrid jet impingement/micro-channel cooling scheme (Fig. 1).

At an early stage, an experimental study (Fig. 2) demonstrated the capacity of the proposed heat sink to achieve temperature profiles of the target object that may decrease in the fluid flow direction of the cooling fluid, thereby maintaining the global high heat removal capability inherent to micro-channel and jet impingement technologies (Barrau et al., 2010a). Also a three dimensional numerical study of this cooling scheme was achieved (Barrau, 2008). The numerical model was experimentally validated and applied to carry out a parametric study of the device. The results showed that, through an optimization procedure at the design stage, the distribution of the local heat removal capacity can be adapted to the specific needs of the application. For example, it was shown that uniform temperature profiles can be reached for both uniform and non-uniform heat flux distributions.

In the present work, the impact of the use of the new hybrid cooling scheme proposed by Barrau et al. (Barrau et al., 2009, Barrau et al., 2010a; Barrau, 2008) on the performance of densely packed PV cells is investigated. The specific trends of the cooling scheme are first described, using the numerical model that was previously validated by comparison with the experimental measurements. The Net PV output of the PV receiver, which is the power delivered by the PV cells less the pumping power of the cooling scheme, is calculated using the results of the numerical simulations and compared with the one of a PV receiver cooled by micro-channels. Finally the temperature uniformity of the PV receiver cooled by micro-channels and by the hybrid design is analyzed.