In the last decades, the use of traditional energy sources has been replacedcontinuously by sustainable energy due to global warming, depletionof fossil fuels, and diversification of energy matrices. In this way, solar energy has been attracting a lot of attention and stands as a promising source, which can be and has been applied for several applications as: agriculture, heat production, seawater distillation, electricity generation, and many others [1].
Due to their economic and technical viability and high performance, thermal solar collectors are extensively used. One of the most common and promising thermal collectors is the evacuated tube solar collectors (ETC) [1, 2]. These collectors can be applied as sources for organic Rankine cycle (ORC), absorption systems, hydrogen production, freeze recovery, thermoelectric generation, and others. Yaïci et al. [3] conducted an investigation of a solar-driven ORC system that works with fluid mixtures and used micro-cogeneration. They chose an evacuated tube flat plate solar collector to feed the system, due to its efficiency and delivered temperature. Parabolic trough collectors (PTC) are also diversely used for ORC applications [4]. Meraj et al. [5] performed an analysis as a case study in New Delhi. The results showed that there was no need to design the thermal and photovoltaic system for high capacities of the absorption system, and that the concentration ratio plays the main hole.
Pandya et al. [6] tested several solar collectors to determine the optimum heat source for an absorption cooling system. The authors tested the flat-plate solar collector (FPC), ETC, PTC, and compound parabolic collector (CPC). Results showed that ETC demands the lowest area, and FPC the lowest cost. Jiang et al. [7] made a numerical and experimental investigation on thermal and optical aspects on triangular solar air collectors. They evaluated three models: using insulation material, transparent cover plate and double transparent cover plate. The one using transparent cover plate has the highest optical efficiency and the different models are adaptable for different weather conditions. Karabuga et al. [8] conducted a thermodynamic analysis using ETC to power an ORC system, which produces electrical energy and generates hydrogen. The energy and exergy efficiencies of the whole system were about 51% and 16%, respectively. Imponenti et al.[9] used parabolic trough collectors to supply and control a freeze recovery. The results showed PTC plants are an opportunity for capital and operational cost-savings in these systems. Al-Tahaineh and AlEssa [10] used the evacuated tube solar collector to heat a cold zone in a thermoelectric generator (TEG). The results indicated that TEG applications have a promising potential for solar energy. The ETC outlet hot water temperature is one of the most significant parameters in the TEGs performance.
The most diverse commercial thermal solar collector used is the evacuated tube (ETC) [11]. Two main techniques are used to enhance the efficiency of ETC: geometrical modifications and the use of nanofluids and PCMs. Henein and Abdel-Rehim [12] investigated the performance of a heat pipe ETC using MgO/MWCNT nanofluid. The hybrid use of 50% MgO and 50% MWCNT presents the best performance for all tested flow rates. Alrowaili et al. [13] conducted an investigation of a CuO-Cu/water nanofluid coupling ETC and an energy storage system. The ETC presented high efficiency using hybrid nanofluid 2.5g CuO + 1.5g Cu in different flow rates. Ismail et al. [14] made a numerical comparative study about the use of circular and rectangular absorbers for ETC using Al2O3/water as a nanofluid. The best ETC configuration is using a circular absorber and the maximum improvement in the ETC efficiency using nanofluid was about 9%.
Eltaweel et al. [15] investigated the use of MWCNT/water as a nanofluid for ETC and FPC using various flow rates. The maximum energy efficiency found for the ETC was 55% at 0.02kg/s and with 0.05 wt%. According to the results, the enhancement in efficiency can reduce the collector area. Tabarhoseini and Sheikholeslami [16] investigated the entropy generation inside an ETC using CuO/water nanofluid. They concluded that the entropy generation using just water is higher than using nanofluid, the heat transfer entropy generation was reduced by about 6% using 5% CuO. Yeh et al. [17] tested a double spiral coil heat exchanger with phase change material (PCM) in a parabolic trough collector. The results show that maintaining the temperature difference between the working fluid and PCM it is possible to increase the duration of available hot water for domestic applications. The optimal configuration was found for 2.6 times longer discharging process and outlet temperature of 55°C.
Proposing a geometrical modification, Teles et al. [18], modelled numerically an eccentric solar collector with a small concentration. A maximum of 73% efficiency was obtained using this model modification. Seddaoui et al. [19] combined the configuration of FPC and ETC to create a new design of vacuum flat plate collectors. This model was tested experimentally and simulated numerically. The results showed that for high absorber emissivity values the new model was more efficient than the ETC, although for low absorber emissivity the ETC was more efficient. Almitani et al. [20] tested several geometries of a twisted turbulator effect inside a parabolic solar collector. They also tested nanofluids using MWCNT and MgO nanoparticles in different concentration ratios. The results showed that the use of the turbulator improved the efficiency of the collector and increased the Reynolds number from 10,000 to 25,000. Roshith and Varghese [21] made a numerical investigation of different tube geometry for a glass ETC. A high collector length-to-diameter ratio, inclination angle, and circumferential heat input lead to a high natural circulation flow rate. The effects of vacuum and solar film in a low-concentration eccentric collector were investigated [22]. The eccentric collector with both reflective film and vacuum reached a maximum efficiency of 89% and a minimum of 42%. Ismail et al. [23] evaluated experimentally the performance of a low-concentration concentric solar collector that works with the direct flow. The highest thermal efficiency reported was 68% for the model with concentration and vacuum.
So far, the research was dedicated to improving the performance of solar collectors by using thermal (use of nanofluids and PCM) and geometrical innovations. The literature review shows a research gap related to analyzing and comparing several geometrical innovations in concentric and eccentric evacuated tube solar collectors using different parameters. Based on this premise, this paper aims to carry out comparative analyses of evacuated tube solar collectors with different geometric configurations.
This paper presents the simultaneous outdoor experimental tests on eight solar collector configurations; eccentric, concentric with and without reflective film, and with and without vacuum in the annular space to identify the collector configuration that produces the best thermal performance. Further numerical simulations were conducted on the best solar collector configuration to investigate the most adequate materials for both the cover tube and the collector absorber.
The contribution of this study includes individual outdoor testing of the eccentric collector, simultaneous outdoor testing of both concentric and eccentric solar collectors’ configurations and validation of the developed numerical code with experimental results.