All-glass Evacuated Tube Solar Collector Research Articles

Performance investigation of an all glass evacuated tube solar collector with an innovative heat exchanger design using TiO2-H2O nanofluids- an experimental study

ABSTRACT The objective of the current study is to experimentally analyze the thermal performance of an all-glass evacuated tube solar collector (ETSC) with an innovative heat exchanger design by employing TiO2-H2O nanofluid as a heat transmission fluid. Different higher volume concentration (v/c) of TiO2 nanoparticles, i.e. 0.83%, 0.9%, and 0.94% are examined to explore the impact of the nanofluids on the ETSC performance in comparison with only water based ETSC. An outdoor experimental investigation has been carried out using nanofluid at 0.6, 0.7 and 0.8 l/min volumetric rates of flow and its impact using serpentine copper tube coil having uniform pitch distance of 14.3 cm with aluminum fins attached on both side of coil having diameter 12 mm and thickness 1 mm, respectively. The study indicates that thermal performance of ETSC increases with the rise in volumetric flow rate with higher v/c of nanoparticles. For this condition, the temperature rise of water using nanofluid is higher than simple water-based ETSC system, recording an increase in outlet temperature by 4.21%. The maximum thermal efficiency obtained is 52.33% at 0.9% v/c and 0.8 l/min. A comparative assessment between the obtained results and some of the published results reveals that by using serpentine copper coil attached with aluminum fins, with higher concentrations of TiO2 nanoparticles and maintaining higher volumetric flow rate, thermal efficiency of ETSC can be much improved.

Straight-through all-glass evacuated tube solar collector for low and medium temperature applications

A straight-through all-glass evacuated tube collector (ETC) made of high-quality borosilicate glass was developed for large-scale low and medium temperature solar hot water systems. It consists of an inner and outer tube without a free end and was shown to be mechanically stable with a thermal expansion coefficient of (3.3 ± 0.1) × 10−6 K−1. Due to turbulent conditions in the straight-through all-glass tube, a higher heat transfer rate could be achieved than in a Dewar-tube ETC, leading to increased heat extraction and decreased heat losses. The photo-thermal efficiency was improved, indicating that a straight-through tube could be more cost-effective. At the same time, active flow in the tube can result in a better water quality as the accumulation time in the tube was short.

Experiment and simulation study on convective heat transfer of all-glass evacuated tube solar collector

Solar collector with horizontal double-row all-glass evacuated tubes has been extensively implemented in the solar water heating system engineering. The temperature distribution and stratification of horizontal double-row all-glass evacuated tube collector with 24 evacuated tubes have been studied. Validation of instantaneous efficiency under different declination angle θm were studied by means of experiments. Moreover, numerical simulations are carried out for four different declination angle θm (0°/2°/4°/6°). The results showed that the declination angle θm has significant effect on energy conversion efficiency, flow patterns and stratification inside evacuated tubes. When θm > 0, with the declination angle increased, the instantaneous efficiency also increased, nevertheless, the temperature stratification weakened and the heat loss coefficient demonstrated no significant change. Declination angle 6° allowed to achieve significant higher temperatures, nevertheless, along with inactive area at the bottom of evacuated tubes appears. If declination angle θm < 0, inversion phenomenon appears. With the increase of |θm|, more heat is trapped in the sealed end of the evacuated tube, which is not conducive to the flow heat transfer in evacuated tube solar collector.

Heat transfer and thermal efficiency of a lab-fabricated ferrofluid-based single-ended tube solar collector under the effect of magnetic field: An experimental study

Heat transfer and thermal efficiency of a single-ended all-glass evacuated tube solar collector (SEETSC), as a special type of heat exchanger in the solar thermal systems, can be improved using ferrofluids, as working fluid, under the effect of the magnetic field. The present experimental study investigates the natural convection, and thermal efficiency of a lab-fabricated ferrofluid (Mn–Zn Fe2O4/water) based single-ended tube solar collector, as a simplified model of the SEETSCs, in the presence of non-uniform magnetic field. Effect of the nanoparticles volume fraction (0.0–0.5%), input heat flux (qin′′=50–350 W/m2) and the magnetic field generated by several identical permanent magnets (remanent magnetization (Br) = 0.02–1.0 T) on the heat transfer and thermal efficiency of the collector is investigated. The results show that applying a magnetic field causes not only compensation of the heat transfer reduction due to the high viscosity values but also improvement of it. The maximum enhancements in the obtained average Nusselt number and thermal efficiency are respectively about 73.0% and 47.0% for the 0.5 vol% ferrofluid at qin′′ = 350 W/m2 and Br = 1.0 T. According to this study, it can be feasible to enhance the thermal efficiency of the SEETSCs-based solar thermal systems using the ferrofluid under the effect of magnetic field.

Mathematical model development and optimal design of the horizontal all-glass evacuated tube solar collectors integrated with bottom mirror reflectors for solar energy harvesting

Abstract As one of the inexhaustible energy sources, solar energy as a means to provide space heating has been a public interest for decades. Many stand-alone solar thermal technologies have come into practice to replace the out-of-date systems. However, conventional solar thermal systems present two drawbacks: (1) unsteady solar sources can lead to insufficient heating in the winter, and (2) the solar collectors can become overheated in the summer. Therefore, this study proposes a conceptual design of an integrated solar harvesting unit that consists of the horizontal all-glass evacuated tube solar collectors and bottom mirror reflectors to overcome the above drawbacks to the largest extent possible. To accomplish this, a generic mathematical model of this design unit was developed, followed by the model validation process and optimal design analysis. For cities in the severe cold and cold climate zones of northern China, the bottom mirror reflectors can be regarded as solar energy collection boosters during the heating season, which can contribute solar energy ranging from 40% to 80% of the total collected solar energy depending on the inclined angles of the solar collectors and reflectors. In the summer, using such integrated unit with the solar collectors tilted at an obtuse angle, the absorbed solar radiation can be reduced by 20%, which is beneficial to overheating prevention.

An experimental investigation on a small-sized parabolic trough solar collector for water heating in cold areas

When used in cold environment, the conventional flat plate solar collectors and all-glass evacuated tube solar collectors will cause various problems, such as large heat loss, low efficiency, freeze and tube-burst, which severely limit their applications. In this paper, a special small-sized parabolic trough solar collector (PTC), which could overcome the shortcomings of conventional solar collectors, was proposed for water heating in cold areas. An experimental platform was developed, and extensive tests were conducted to evaluate the characteristics of the proposed PTC. It was found that the thermal efficiency of the proposed PTC reached about 67% even under the condition of solar radiation of less than 310W/m2, indicating that the PTC could collect solar radiation efficiently. The results also showed that when the fluid temperature was under 100°C, the thermal efficiency was improved with increased fluid temperature, due to the increase of Reynolds number caused by the great variation of thermophysical parameters of the heat transfer fluid (HTF), which resulted from the increasing fluid temperature. Both the frosting on the reflector and the ambient wind velocity weakened the thermal efficiency greatly. The highest thermal efficiencies were respectively about 26% and 56% under condition of frosting and wind velocity of 5.0m/s. The comparison test showed the coated double glazing receiver had a longer temperature response delay to the deflected sunlight than that of the uncoated one. The tested PTC system operated normally in low fluid temperature condition, showing great anti-freezing property in cold environment.

Self-Cleaning and Antireflective Films for All-Glass Evacuated Tube Solar Collectors

Abstract SiO 2 antireflective nanoporous films were prepared by sol-gel process, combined with dip coating technology. The highest values of solar transmittance (96% in the broader spectrum range of 250 nm-2500 nm) are obtained. The static contact angle values increase after the treatment with 100% ethylchlorosilane and hexamethyldisilazane solutions. The antireflective films has to display long term stability as the operation conditions in solar collectors include low temperatures, high temperatures, and humid environment, all of which can degrade its optical properties.

A Study on Thermal Performance of a Novel All-Glass Evacuated Tube Solar Collector Manifold Header with an Inserted Tube

A novel all-glass evacuated tube collector manifold header with an inserted tube is proposed in this paper which makes water in all-glass evacuated solar collector tube be forced circulated to improve the performance of solar collector. And a dynamic numerical model was presented for the novel all-glass evacuated tube collector manifold header water heater system. Also, a test rig was built for model validation and comparison with traditional all-glass evacuated tube collector. The experiment results show that the efficiency of solar water heater with a novel collector manifold header is higher than traditional all-glass evacuated tube collector by about 5% and the heat transfer model of water heater system is valid. Based on the model, the relationship between the average temperature of water tank and inserted tube diameter (water mass flow) has been studied. The results show that the optimized diameter of inserted tube is 32 mm for the inner glass with the diameter of 47 mm and the water flow mass should be less than 1.6 Kg/s.

Experimental Studies on the Thermal Performance of the Wall-Type All-Glass Evacuated Tube Solar Water Heater

Based on the test methods for solar water heating systems, the useful energy output, water temperature rise and thermal efficiency of the wall-type all-glass evacuated tube collectors fixed at a large tilt angle (≥60o) have been conducted by the experimental test in Kunming. The results show that the average thermal efficiency varies from 35% to 50%, the useful energy output decreases from 13.1MJ to 8.8MJ and the water temperature rise decreases from 31.3 °Cto 20.9°C when the tilt angle increases from 60o to 90o. These results can provide some useful experimental data support for the application of the all-glass evacuated tube solar collector integrated into the wall of the high-rise buildings.

Performance and building integration of all-ceramic solar collectors

In this paper, a type of all-ceramic solar collector from cheap materials is introduced. The all-ceramic solar collectors are made from ordinary ceramic and vanadium-titanium black ceramic. The ordinary ceramic raw materials mean mainly porcelain clay, quartz, feldspar, etc. The material of the solar absorber coating is vanadium-titanium black ceramic, which has a stable value of solar absorptance in the range of 0.93–0.97. Some characteristics and performance analysis of all-ceramic solar system are given. For the purpose of comparison, three solar systems consisting of all-glass evacuated tube solar collectors, metal-flat-plat solar collectors, and all-ceramic solar collectors were built. The all-ceramic solar system has the highest thermal efficiency. The heating-rate trends of three solar systems are different in the test period. The all-ceramic solar system can integrate well with building roof. The appropriate approach for the integration between the all-ceramic solar system and building roof is given. When using the integrated approach, the pitched building roof only need basic concrete structural layer, insulating layer, waterproof layer, and leveling course.

Optimal tilt-angles of all-glass evacuated tube solar collectors

In this paper, a detailed mathematical procedure is developed to estimate daily collectible radiation on single tube of all-glass evacuated tube solar collectors based on solar geometry, knowledge of two-dimensional radiation transfer. Results shows that the annual collectible radiation on a tube is affected by many factors such as collector type, central distance between tubes, size of solar tubes, tilt and azimuth angles, use of diffuse flat reflector (DFR, in short); For collectors with identical parameters, T-type collectors (collectors with solar tubes tilt-arranged) annually collect slightly more radiation than H-type collectors (those with solar tubes horizontally arranged) do. The use of DFR can significantly improve the energy collection of collectors. Unlike the flat-plate collectors, all-glass evacuated tube solar collectors should be generally mounted with a tilt-angle less than the site latitude in order to maximize the annual energy collection. For most areas with the site latitude larger than 30° in China, T-type collectors should be installed with a tilt-angle about 10° less than the site latitude, whereas for H-type collectors without DFR, the reasonable tilt-angle should be about 20° less than the site latitude. Effects of some parameters on the annual collectible radiation on the collectors are also presented.

Theoretical analysis of the thermal performance of all-glass evacuated tube solar collectors with absorber coating on the outside or inside of the inner tube

Theoretical efficiencies (η) and thermal behaviour of all-glass Evacuated Tube solar Collectors with an Internal Absorber Film (ETCIAF), i.e. the absorber film deposited in the inner surface of the inner tube, are compared and contrasted with the traditional design of all-glass Evacuated Tube solar Collectors with an External Absorber Film (ETCEAF), using the absorber film on the external surface of the inner tube. The values of η of the ETCIAF are unacceptably lower than that of ETCEAF for any particular value of the heat transfer coefficient ( h b) for the annular space, except in the case of a highly leaky ETCEAF, with h b > 2.6 W/m 2 K. However, it is shown that the use of a transparent conductive coating with moderately low emittance ∼0.1−0.25 on the outside of the absorber tube of ETCIAF can offer efficiences ∼0.75−0.63, respectively, for f = 0.1 °C m 2/K, competing well ( η = 0.76) with the ETCEAF design operating under best conditions ( α = 0.91, ϵ = 0.05, and h b = 0.026 W/m 2 K).