Flow structure in a water-in-glass evacuated tube solar water heater

An Experimental Study on Evacuated Tube Solar Collector for Heating of Air in India

Effect of the collector tilt angle on thermal efficiency and stratification of passive water in glass evacuated tube solar water heater

SYNOPSIS A proprietary heat-pipe Evacuated Tube Solar Water Heater (ETSWH) originally designed for collection of solar energy using forced fluid circulation was operated using thermosyphon fluid circulation. The thermal performance of this solar water heating system was monitored from October 2006 to June 2007. The ETSWH array was inclined with the expected collector outlet at a higher datum level than the expected collector inlet to provide a hydrostatic pressure differential across the manifold. During the monitoring period it was observed that thermosyphon flow was not always in the expected direction, with flow reversal occurring when the collector inlet temperature was greater than that of the outlet. When the evacuated tube solar water heater manifold was inclined at 1° to the horizontal, reverse fluid flow was observed to occur for 69% of the monitored diurnal periods. Diurnal reverse circulation lowers system efficiency by reducing thermal stratification in the hot water storage tank via convective entrainment and mixing. The thermal performance of the ETSWH was monitored continuously from January 2007 to June 2007 with the manifold inclined at 5° to the horizontal. Over this time period it was found that fluid flow reversal arose for 22% of the diurnal periods considered, resulting in a 47% improvement in diurnal storage efficiency compared to when the system had its manifold inclined at 1°. The long term diurnal storage efficiency of the optimised system inclined at 5” was measured as 66%.

Comparative experimental investigation of two evacuated tube solar water heaters of different configurations for domestic application of Baghdad- Iraq

Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles

Performance study of fog desalination system coupled with evacuated tube solar collector

Performance improvement of water in glass evacuated tube solar water heaters has received attention from many researchers yet the effect of geometric parameters on the performance of the system has not been fully explored. In this work, the effect of collector tube length, diameter and tilt angle on the temperature and velocity distribution was studied using computational fluid dynamics (CFD). The results of the CFD model were validated against experimental data. The collector tube length was found to have a significant influence on both the temperature in the storage tank and the velocity distribution. The longest collector tube (2000 mm) achieved the highest temperature compared to collector lengths of 1800 mm and 1600 mm. A larger collector tube diameter of 52 mm enhanced the average temperature in the storage tank compared to collector tube diameters of 42 mm and 47 mm. The effect of tilt angle on velocity distribution was analysed using tilt angles of 10, 23, 30, and 45 degrees and the results showed that the average flow velocity for lower tilt angles was high, however, the maximum flow velocity which occurs at regions close to the boundary between the collector tube and the storage tank was higher for high tilt angles.

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

Effect of the transient energy input on thermodynamic performance of passive water-in-glass evacuated tube solar water heaters

The climate change because of global warming and worldwide energy scarcity are prompting almost all the countries of the world to look for alternate energy sources like nuclear and renewable. Because of infrastructure and other reasons, developed countries can tap into nuclear energy but developing country like Bangladesh is not fortunate enough to have that option. Consequently, the only option that is left open to developing countries is renewable energy. Among renewable energy solar, wind, hydro, geo-thermal and wave energies are most reliable, which do not cause carbon emission. Solar energy can be used in many purposes like water heating, air heating etc. Among the various types of solar collector, evacuated tube solar collector is one of the most effective water heating device. An evacuated tube solar water heater was constructed from locally available materials and investigated in the department of Mechanical Engineering of KUET. In the first attempt, the highest. storage tank temperature obtained was only 47°C. In the present attempt, cylindrical parabolic reflectors were placed under each evacuated tube. Consequently, the maximum water temperature obtained at outlet to collector was 8I°C and 76°C in the storage tank in the month of April-May.

Analyzing an evacuated tube solar water heating system using twin-nano/paraffin as phase change material

Measurement and simulation of flow rate in a water-in-glass evacuated tube solar water heater

This paper evaluates the characteristics of water-in-glass evacuated tube solar water heaters including assessment of the circulation rate through single ended tubes. A numerical model of the fluid flow inside a single ended evacuated tube has been developed assuming no interaction between adjacent tubes in the collector array. The numerical model is firstly validated by means of literature data. Then, the performance in terms of circulation mass flow rate in different heat flux condition and at different inclination angle of the collector are evaluated. Moreover, a sensitivity analysis on the effect of the assumptions of different reference parameters and of the choice of the values of the fluid thermophysical properties is carried out and a quantitative analysis of the expected uncertainty is presented. Finally, an extension of previous literature correlations in terms of non-dimensional Reynolds and Rayleigh numbers is proposed. Regarding heating boundary conditions, the circumferential heat distribution was found to be an important parameter influencing the flow structure and circulation rate through the tube.

Under transient climatic conditions, solar water heaters using heat pipes are more effective at capturingincident solar radiation than other equivalent sized solar water heaters. The cost must be reduced toimprove uptake of such systems. To investigate two methods were considered by this study: thermosyphonfluid flow and reflective concentrators. A physical reconfigurable laboratory model of the manifold andassociated condensers of a heat-pipe-evacuated tube system were fabricated; fluid circulation was viathermosyphonic action, particle imaging velocimetry derived velocity maps and the use of concentratorswas simulated. When condenser spacing was doubled, the Nusselt number increased by 43%, the velocityby 55% but the heat transfer efficiency of the model manifold decreased by 9%. Potential annual energysavings of 10207 GWh could be realized if such systems could be successfully fabricated. Keywords: thermosyphon; fluid flow visualization; solarenergy; particle imaging velocimetry;compound parabolicconcentrators; evacuated tube solar water heaters

The water-in-glass evacuated collectors are made up of parallel circular tubes. They are installed with some inclination angle to the horizontal. The thermal performance of water-in-glass evacuated tube solar water heater heavily depends on weather conditions. The analysis of the sensitivity of the model parameter and weather conditions on heat transfer process is extremely important to install a solar water heater system in order to achieve its maximum efficiency. The evaluation of the sensitivity of the system parameters is done by considering one parameter after another while keeping the remaining fixed. Further to the analysis of the heat transfer process, the average heat transfer coefficient and the average natural circulation flow rate are calculated.
 The fluid flow is assumed to be unsteady, two-dimensional, laminar and incompressible. The heat and fluid flow are analyzed using the Navier-Stokes equations and temperature equation for an incompressible fluid, subject to density variation with temperature. The discretization of the governing equations is done by Finite Volume Method (FVM). The Open FOAM computational fluid dynamic software with PISO-SIMPLE algorithm is used for the simulation.
 The results show that the heat transfer process is improved when there is a moderate level inclination angle. Further, it is found that when the ratio of tube length to diameter is high, the heat transfer process is improved. The solar radiation input highly affects the performance of a solar water heater. The cold-water inlet temperature does not directly affect the buoyancy induced flow, but it influences the temperature gain.
 The angle of the solar rays vary within the daytime, however it does not affect the performance of the solar water heater since an evacuated-tube has a circular absorbing surface, it passively tracks the sun throughout the day. 
 These results recommend using moderate level tube inclination angle and high ratio to improve the performance of a solar water heater.