Experimental study of carbon nano tube/oil nanofluid in dish concentrator using a cylindrical cavity receiver: Outdoor tests

Highlights

• A wounded cylindrical cavity receiver was experimentally considered.

• MWCNT/oil nanofluid and pure oil were tested as the working fluid.

• The average thermal efficiency was determined as 63.9% for the nanofluid.

• The average thermal efficiency was determined as 56.44% for the pure thermal oil.

• It was observed, thermal performance improved equal to 13.12% by application nanofluid.

Abstract

The application of nanofluids is accounted as an effective way for improving the thermal performance of solar systems. In the current study, MWCNT/thermal oil nanofluid was experimentally tested as a solar heat transfer fluid. A dish collector, using a wounded cylindrical cavity receiver, was considered as the solar system. The main aim of this work was the experimental investigation of the cylindrical cavity’s performance, using MWCNT/thermal oil nanofluid. Two experimental models were suggested for the prediction of the cavity’s thermal efficiency, by considering MWCNT/thermal oil nanofluid and pure thermal oil in the steady-state period. It was indicated that the heat loss coefficient of the cylindrical cavity receiver decreased by the application of nanofluid. The results revealed the average thermal efficiencies of 63.9% and 56.44% for MWCNT/thermal oil nanofluid and pure thermal oil, respectively. The thermal efficiency was changed form 60.04% to 64.76%, by using the nanofluid, in the steady-state period. Also, it was observed that the thermal performance of the cavity receiver was improved by 13.12%, by using MWCNT/thermal oil nanofluid. Based on the obtained results, the application of MWCNT/oil nanofluid is recommended in the solar dish concentrators with cavity receiver.

Introduction

Nowadays, the negative effects of fossil fuel consumption has become as one of the global challenges. One effective solution for reducing the environmental problems is the application of renewable energy technologies for producing heat or power. The concentrating solar technology is one of the promising techniques, which could be used in the thermal power plants or industrial thermal systems [1], [2], [3]. The parabolic dish concentrator is accounted as an effective solar technology for converting solar heat flux into the thermal energy. In the dish collector, different kinds of absorbers could be utilized [4], [5]. Cavity receivers, due to their particular configuration, have the highest thermal performance among other kinds of receivers [6].

Some researchers have studied the cavity receivers based on theoretical and experimental studies. Fang et al. [7] numerically studied the thermal performance of a cavity receiver. They concluded that the highest heat loss accrued when the wind blew in the parallel direction. Loni et al. [8], [9] theoretically studied rectangular and cylindrical cavity receivers. They used thermal oil as the solar heat transfer fluid, and presented the optimum structural parameters and operational conditions for obtaining the highest thermal performance. Le Roux et al. [10], within a numerical study, considered a dish collector with a cavity receiver; air was applied as the solar heat transfer fluid. They concluded that the thermal efficiency had an inverse relation with the diameter of the cavity inner tube. Also, Loni et al. [11], [12] considered the first and second laws of thermodynamics on the cavity receiver. The influence of different structural dimensions of the solar receiver was evaluated in their research. Ma [13] conducted an experimental study for the investigation of the cavity thermal heat losses. Uzair et al. [14] numerically evaluated the thermal heat losses from a cavity receiver. The results revealed that the orientation of the dish-receiver influenced the thermal performance. Reddy [15] theoretically evaluated the free convection heat losses from a cavity receiver. Their results predicted a Nusselt number for the investigated cavity receiver. Prakash et al. [16] conducted a research on a cylindrical cavity receiver, and assessed the impacts of the inlet temperature, wind speed, and the cavity inclination angle. Loni et al. [17] predicted the thermal performance of a parabolic dish collector with cavity receiver. They applied the ANN method for the prediction of the thermal performance of the cavity receiver. Wu et al. [18] experimentally investigated a cylindrical cavity receiver under the variation of cavity inclination, aperture ratio, and heat flux. The results indicated that the investigated parameters had a significant effect on the convection heat losses. In another work, Wu et al. [19] experimentally studied the mixed convection heat losses for a cylindrical cavity receiver.

Some researchers experimentally examined the thermal performance of dish concentrators with cavity receiver. Rafeeu and Kadir [20] experimentally evaluated a dish concentrator for different dimensions, using water. Their results revealed that the higher thermal efficiency was achieved by using higher concentrator depth and more efficient reflective materials. Skouri et al. [21] experimentally investigated a parabolic dish concentrator, as a case study. Reddy et al. [22] experimentally studied the thermal performance of a dish concentrator, using water. Mawire and Taole [23] experimentally considered the energy and exergy performance of a special design of dish collector, using a cylindrical cavity receiver. Their results reported the heat loss factor and the optical efficiency for the considered setup. Xiao et al. [24] experimentally evaluated a two-step dish collector, using a cavity receiver, for rotating a micro gas turbine. Madadi et al. [25] evaluated the energy and exergy efficiency of a dish collector, by using cylindrical and conical cavity receivers. They concluded that the cylindrical cavity receiver had higher energy and exergy performance, compared to the conical cavity receiver. Azzouzi et al. [26] experimentally and numerically evaluated a tubular cavity receiver. Pavlovic et al. [5] numerically and experimentally investigated a dish collector, equipped with a spiral receiver. They numerically investigated different solar heat transfer fluids such as water, Therminol VP-1, and air. Their results showed that water was the most appropriate for the low-temperature applications; while the Therminol VP-1 was suitable at the higher temperature applications.

There are some experimental and numerical research works about the testing of nanofluids as the solar heat transfer fluid. Edalatpour and Solano [27] numerically investigated a flat plate collector, using Al₂O₃/water nanofluid. The results indicated that the outlet temperature decreased by increasing the Reynolds number at a fixed nanoparticle volume fraction. Khullar et al. [28] numerically investigated a nanofluid-based concentrating parabolic solar collector (NCPSC). Also, Al₂O₃/Therminol VP-1 was used as the solar heat transfer fluid. The results showed a performance enhancement of 5–10% for the investigated NCPSC, compared to a conventional parabolic solar collector. Loni et al. [29] thermodynamically modelled a cavity receiver, by using different types of nanofluids. They concluded that Cu/thermal oil nanofluid had the best thermodynamic results on the investigated solar system. Pavlovic et al. [30] numerically studied a dish collector with a spiral receiver, and examined the effects of different nanofluids. He et al. [31] experimentally evaluated the thermal performance of a vacuum tube solar collector. This research team used two kinds of nanofluids, including TiO₂/water and carbon nanotube/water, under the sunny and cloudy weather conditions. They concluded that the carbon nanotube/water nanofluid showed a higher performance, compared to the TiO₂/water. Meibodi et al. [32] experimentally investigated the thermal performance of a flat plate collector, using SiO₂/ethylene glycol (EG)–water nanofluid. The effect of the nanofluid mass flow rate on the collector’s thermal performance was evaluated. Li et al. [33] evaluated the thermal performance of a tubular solar collector using three different types of nanofluids (Al₂O₃/water, ZnO/water, and MgO/water). They reported that the collector’s thermal performance had the highest amount, when using ZnO-H₂O nanofluid with 0.2% volume fraction. Madadi et al. [34] conducted energy and exergy analyses for a parabolic dish collector with cavity receiver and Al₂O₃/water nanofluid. The results indicated that the thermal and exergy efficiencies of the investigated collector were enhanced by increasing the nanofluid’s concentration.

It could be extracted from the literature review that there no experimental work has been reported about the application of oil-based nanofluid in the cavity receivers. Therefore, the novelty of the current study is the experimental investigation of a cylindrical cavity receiver, using MWCNT/thermal oil nanofluid. The main objective of the current study is to determine the thermal performance of the cavity receiver, by using MWCNT/thermal oil nanofluid. Also, two models are suggested for the prediction of the cavity’s thermal efficiency versus $\frac{T_{\mathit{in}}-T_{\mathit{amb}}}{I_{\mathit{beam}}}$, by using MWCNT/thermal oil nanofluid and pure thermal oil. The results of this work elucidate the positive influence of the application of MWCNT/thermal oil nanofluid on the dish concentrator with cavity receiver.