First of all, I would like to especially thank my supervisor, Professor Seok-Hun Yoon, who has taught, helped, encouraged and supported me over the past two years. I would also like to thank several professors who taught me at KMU, including Professor You Teak Kim, Nam Chung Do, Jae Sung Choi, Cheol Oh. I would like to thank Professor Yun Chul Jung in the Department of Navigation and my Rector, Professor Dang Van Uy, my Dean, Professor Nguyen Dai An and my senior, Dr.
Finally, I would like to express my deep gratitude to my parents, my sisters, my brothers and all my friends in Vietnam for their love and patience. In this study, a closed-loop oscillating solar collector was constructed to experimentally investigate the effect of working fluid filling ratio, cooling water flow rate, and air gap thickness between absorber plate and glass cover on top heat loss and performance of the collector. To absorb and transport thermal heat energy from the heating to the cooling section, closed loop oscillating heat pipes were used in combination with an absorber plate/copper plate with a black chrome coating.
Top heat loss of the collector was determined based on temperatures of absorber plate, glass cover and ambient air measured and recorded by MV2000-Yokogawa recorder via K-type thermocouples. Top heat loss of the collector was determined at the air gap thicknesses of 5mm, 15mm, 25mm and 35mm. The results show that flow rates of cooling water of 0.15l/min and 0.30l/min give the collector better performance than that of 0.45l/min.
Keywords: air gap thickness, closed loop oscillating heat pipes (CLOHP), highest loss coefficient, highest heat loss, filling ratio (FR), cooling water flow rate (CWFR).
Introduction
Jompakdee [3] studied the effect of working fluids (R123, Ethanol), inner diameter and number of turns of closed-end oscillating heat pipes on the heat flux. Sehgal [5] researched the optimization of heat loss in normal and inverted flat plate collector configurations. Heat loss was minimized by optimizing the separation of the absorber plate to glass cover, providing the low emission surface on the collector back and spacing an additional reflector supported on glass wool insulation behind the back.
The result showed that the effective thermal conductivity of this kind of heat pipe was 1000-2000 times greater than the conventional thermal conductivity of copper. As is the case here, the effect of air gap thickness, working fluid FR, and cooling water velocity on top heat loss and performance of a CLOHP flat plate solar collector has not been experimentally investigated. Heat loss in flat solar collectors (33-50%) occurs due to convective loss (22-30%), radiation loss (5-7%) from the absorber surface to the cover and radiation loss.
To improve collector efficiency, convective and radiative heat loss must be minimized, especially heat loss through the lid/top loss. In this research, a CLOHP solar collector was built to study the effect of air gap thickness, and to investigate the effect of FR of working fluid, flow rate of cooling water on the highest heat loss and performance of the collector in more detail.
Theory analysis
Convective heat transfer coefficient
- Heat transfer coefficient from absorber plate to cover ( )
- Heat transfer coefficient from cover to ambient air ( )
The convective heat transfer coefficient between the absorber plate and the cover, , inclined at an angle to the horizontal can be calculated as [7]. The Nusselt number, , for the air between the absorber plate and the glass cover is calculated as the expression below [7]. 1 The exponent "+" in formula (2) means that only the positive value of the term in square brackets should be considered, and zero should be used for the negative value, and , the inclination angle ranges between 00-750.
The Nusselt number, , in formula (4) is the maximum value of three quantities separated by a comma, and is the ratio of the inclined length of the collector plate to the gap between the cover and the absorber plate.
Radiative heat transfer coefficient
- From plate to cover ( )
- From cover to ambient ( )
Top loss coefficient
In addition, OHP was used to transport thermal heat energy from the heating to the cooling section. Therefore, the top heat loss also depends on the thermal heat transport capacity of the heat pipes. In other words, the heat loss at the top can be affected by the working fluid filling ratio and perhaps by the cooling water flow rate.
Collector design and experimental set-up
Collector design
Experimental set-up
Before loading working fluid into the heat pipes, air was extracted with vacuum pump to pressure of 10-3KPa. During experiments, pressure inside the heat pipes was also recorded by recorder via pressure transmitter. To simulate the Solar, twelve 300W halogen lamps were used and attached to a flat plate parallel to the collector.
Simulation The solar intensity was recorded with the MV 2000 Yokogawa recorder via LP-PYRA-50-Pyranometer and adjusted by adjusting the transformer. First, the effect of the thickness of the air gap between the absorber plate and the glass cover on the top heat loss was investigated. The temperatures of the absorber plate, glass cover and ambient air were recorded and then determined by averaging.
The top loss coefficient and the top heat loss rate were calculated from the recorded temperatures as shown in Chapter 2. Second, the effect of the working fluid charge ratio and the cooling water flow rate on the top heat loss and collector performance was investigated. The peak loss rate was determined to evaluate the effect of working fluid charge ratio and cooling water flow rate on collector performance.
Temperatures of all components and pressures inside the heat pipes were recorded and determined during a steady state period.
Results of experiment and discussion
Effect of the air gap thickness on thermal top heat loss
This shows that the temperature of the absorber plate strongly depends on the thickness of the collector air gap. The effect of air gap thickness on the convection heat transfer coefficient from the absorber plate to the glass cover is shown in Fig. The convective heat transfer coefficient reaches a maximum at an air gap thickness of 5 mm and a minimum at 15 mm in most ranges of solar radiation intensity.
At this air gap thickness, the convective heat transfer coefficient is equal to the conduction heat transfer coefficient, because the Nusselt number in this case is equal to one. Therefore, the convective heat transfer coefficient is very high at 5 mm of air gap thickness. At an air gap thickness of 15 mm the Nusselt number is slightly greater than unity, but at this thickness it is sufficient to achieve a high thermal conduction resistance that gives a low heat loss coefficient.
Therefore, the total top loss coefficient is minimized at an air gap thickness of 15 mm as shown in figure. The change in the top heat loss rate is similar to the change in the total top loss coefficient and is shown in Figure. The heat loss at the top is minimal with an air gap thickness of 15 mm in almost all solar radiation areas.
Because the convective heat transfer coefficient and the total top loss coefficient are minimum at 15 mm of air gap thickness. The effect of air gap thickness on collector performance can be judged by the ratio of peak loss to incident solar energy. The change of this ratio versus the thickness of the air gap in wide regions of solar radiation intensity is shown in Fig.
In all ranges of radiation intensity, this ratio tends to decrease, increase and then decrease in later case of the air gap thickness. Due to the change in rate of top loss at different air gap thicknesses is shown in Fig. The higher the solar radiation intensity, the smaller the ratio of peak loss to incident solar radiation.
Effect of FR on top loss and thermal performance of the collector
Since the length of the heating section of the heating pipes in this study is longer than in some previous studies. This is also the reason why it makes the temperature of the absorber plate different at different filling conditions as shown in fig. Total collector peak loss coefficient versus working fluid fill ratio is shown in Fig.
It shows that the peak loss coefficient of the solar collector depends intensively on the filling ratio of the working fluid. The effect of filling ratio on solar collector performance can be estimated by the ratio of the rate of peak loss to incident solar energy. The thermal performance of the collector depends not only on the filling ratio, but also on the solar irradiation intensity.
The thermal efficiency of the collector versus the filling ratio at different solar intensities is shown in the figure. As analyzed above, the level of top loss has a dominant effect on collector performance. This is because the peak collector loss rate reaches a minimum at FR 60% and 70% in these solar intensity ranges.
The effect of the cooling water flow rate on the thermal efficiency of the collector is shown in figures. This is because at higher flow rates the bubbles within the cooling section of the heat pipes can collapse so quickly that it causes an imbalance between vapor bubbles and liquid slugs. Steam bubbles have the function of making pressure wave to transport liquid slugs from the heating to the cooling part of the heat pipes.
Therefore, it gives low thermal efficiency of the solar collector at high flow rate of cooling water. In this research, experiments were conducted on a closed-loop oscillating heat pipe flat plate solar collector to investigate the effect of air gap thickness between absorber plate and glass cover, filling ratio of working fluid and flow rate of cooling water on peak heat loss and performance of the collector. Thermal peak loss coefficient and peak loss of the solar collector depend on both the thickness of the air gap and the solar radiation intensity.
The optimal air gap thickness of the collector is 15 mm for most solar intensity ranges. At this air gap thickness, the collector can deliver the highest performance due to the smallest top loss.
