A numerical investigation on optimization of PV/T systems with the field synergy theory

Highlights

• The special cooling channel based on field synergy theory was proposed.

• The optimal structural parameters of the special channel were determined.

• The effect of solar radiation and mass flowrate on PV/T system was investigated.

Abstract

High cell temperature of PV modules would reduce electrical efficiency and hinder the development of photovoltaic application. Many potential cooling channels have been proposed for PV modules cooling but perform inefficiently, due to the insufficient heat transfer between the cooling water and PV modules. In current research, a PV/T system with a special cooling channel based on field synergy theory, was designed to optimize the performance of PV/T systems. The performance of the new PV/T system was studied numerically, and mathematical models were validated with the experimental data. Results indicated that electrical efficiency of PV modules increased by 0.5% (from 11.4% to 11.9%) when the solar radiation decreased from 1200 to 300 W/m². The electrical efficiency increased 1.11% (from 10.8% to 11.9%) as the inlet mass flowrate of cooling water increased from 0.0018 to 0.018 kg/s at the inlet temperature of cooling water of 20 °C. Additionally, the special channel with shark dorsal fin type sawtooth or regular type sawtooth, had the optimal parameter values, with the channel depth of 0.05 m, the sawtooth peak of 0.035 m and the sawtooth width of 0.01 m. The average cell temperature with the optimized channel was 6.05 °C lower than the conventional smooth channel, and the new cooling channel performed better with a high solar radiation.

Introduction

The development and utilization of renewable and environmental-friendly energy, have been concerned in recent years due to the aggravation of fossil energy depletion and global warming diffusion [1]. Solar energy belongs to an excellent renewable energy with the advantages of pollution-free, inexhaustibility, and huge amount. In order to utilize solar energy, PV modules are commonly used as a kind of equipment to convert solar radiation into electrical energy. However, a challenge, the limited performance of solar cells caused by a high cell temperature, was raised in the application of PV modules. When the cell temperature increases by 1 °C, the electrical efficiency of PV modules decreases by 0.3–0.5% [2], [3]. Therefore, some feasible solutions have been proposed to regulate the heat accumulated on the surface of PV modules [4]. Existing cooling technologies, including air cooling [5], water cooling [6], cooling with PCM [7] and special structure designs [8], have been discussed to reduce cell temperature. In a review study [8], it was reported that the thermal efficiency of PV modules with water cooling could reach up to 50–70%, while that of air cooling was only 30–50%. Also, it was pointed out that in the field of PV/T cooling, it was necessary to study the heat transfer enhancement. In addition to air/water cooling, a structure optimization of the cooling channels on the PV modules, targeting a larger heat transfer area and a higher Reynolds number, is another effective way to enhance the cooling performance and improve the electrical efficiency [9].

Many researchers tried to optimize structural configurations of cooling channels [10], including adding fins with different shapes, multi-channel forms, channel shapes and arrangement, to enhance the overall efficiency of PV/T systems [11]. Some literatures with optimized structural configurations were summarized as following: (1) Adding fins with different shapes [12]. Hussain et al. [13] carried out an experimental study on a PV/T thermal cooling channel with hexagonal shape fins. They reported that the electrical efficiency increased by about 0.2% and the thermal efficiency was improved by 20–70%. Özakin [14] performed a comparative study on PV efficiency of three kinds of cooling channels including the empty, the frequent and the sparse fins status. Results indicated that the exergy efficiency with sparse fins was approximately increased by 70%, and increased by 30% with frequent fins, compared with the empty status. (2) Multichannel forms. Sopian et al. [15] studied comparatively steady-state models of single-channel and double-channel PV/T based on air cooling respectively, and discovered that the combined efficiency of double-channel type was 10–15% higher than that of the single-channel one. Ooshaksaraei et al. [16] found that the PV/T system with double-path parallel flow of the cooling water, had the highest total energy efficiency (51–67%), followed by double-path counter flow (47–62%), single-path returning flow (42–56%), and single-path (28–49%). (3) Channel arrangement. Fudholi et al. [17] investigated the performance of PV/T systems with three different arrangements of cooling water flow channels including web flow absorber, direct flow absorber and spiral flow absorber. They found that the spiral flow absorber exhibited the highest performance, with a total efficiency of 68.4%, an electrical efficiency of 13.8% and a thermal efficiency of 54.6% at a solar radiation of 800 W/m² and a cooling water flowrate of 0.041 kg/s. Nahar et al. [18] simulated the performance of PV modules with a cooling flow pancake-shaped channel. Results demonstrated that the cell temperature was cooled down to an average cell temperature of 42 °C, and the electrical efficiency increased by 2% with the inlet velocity of cooling water from 0.0009 to 0.05 m/s. (4) Channel with different shapes. Baloch et al. [19] investigated the performance of PV modules with a cooling technique, called a converging channel heat exchanger, and found that the cell temperature reduced significantly to 45.1 °C in June and to 36.4 °C in December by using the converging cooling technique. The maximum percentage in improvement of power output was 35.5%, whereas the maximum percentage of increase in the conversion efficiency was 36.1%, compared to the performance of an uncooled PV module. Fudholi et al. [20] performed an energy analysis on a PV/T system with ∇-groove shape and found that the average electrical efficiency was 12.66%.

As analyzed above, many potential channel structures and theoretical analysis [21] have been proposed to cool down PV modules, but there is still a room to improve [22]. As well known, the field synergy between velocity and temperature gradient has a great influence on the heat transfer performance [23], [24]. Therefore, a special design of cooling channel based on the field synergy theory was proposed, which had a smaller synergy angle between the velocity gradient of cooling water and the temperature gradient on the surface of PV modules, to promote the overall efficiency of the PV/T system. Meanwhile, the effects of solar radiation, inlet flow rate of cooling water and structural parameters of channel on the performance of PV/T systems were discussed. The result provides a view for the cooling technology of PV modules based on the field synergy theory, which is helpful to further boost electricity yield of PV modules.