INTRODUCTION
Solar water heaters, many of the type now widely used in the daily life of solar water heaters, according to the different collector plate can be divided into two-and vacuum-tube-type [1]. One all-glass vacuum tube solar water heaters, occupied most of the domestic market.
However, solar water heaters trend of development, flat-panel solar water heaters with development potential, Song Aiguo [2,3], and so that theoretically flat solar collectors on the average efficiency of vacuum tube solar collectors - Higher. Despite the current flat-panel solar water heaters and vacuum tube solar water heaters, compared to an average higher coefficient of heat loss, the high temperature of the low efficiency, but the cellular and heat pipes used in flat-panel technology combined with solar collector made of Cellular heat pipe plate solar water heaters can reduce the average coefficient of heat loss and increase the average high temperature of the efficiency.
Flat cellular heat pipe solar water heaters through the transparent cover and honeycomb sandwich under way to reduce heat loss on the use of heat pipe thermal diode characteristics of the evening heat loss significantly reduced. We developed a cellular heat pipe plate solar water heaters, and with the purchase of the all-glass vacuum tube solar water heaters under the same conditions were compared studied. This article will introduce the experimental study compared the results.
An experimental device and method
This study developed by cellular hot-plate collector of the cross section diagram as shown in Figure 1.
Flat cellular heat pipe solar collectors from insulation materials and components, Jin Zhukuang box, transparent glass plate, honeycomb sandwich, with fins of the heat pipe. Its features in a transparent cover Below is a cellular, heat pipes and fins of heat-placed cellular Below, effective lighting area of 1.4 m 2. Its working principle: when the sun radiation through the cellular structure with a transparent cover, its heat absorbing surface was coated with the coating fin with the absorption of heat, heat pipe reached operating temperature, through an internal working fluid (water) phase Heat transfer process will be changed to the accumulator.
All glass vacuum tube solar water heaters, lighting effective area of 1.7 m 2. Two adjacent collector place, with the inclination for the installation of 45 °.
In order to facilitate comparison cellular heat pipe solar water heaters and all-glass vacuum tube solar water heaters thermal properties, the two major experiments on the average efficiency and heat loss factor for testing. Testing instruments, precision instrument with the GB requirements. All test data from Hewlett-Packard 3497 A collection and data acquisition system connected to the computer, the computer every 3 min automatic data collection time.
Experiment, solar water heaters thermal properties of the main reference national testing standards GB/T12915-91 [4] (domestic solar water heaters thermal performance test method) and GB4271-84 [5] (flat-type solar collector thermal performance test methods ), Of which thermal performance indicators including instantaneous efficiency, the average efficiency of heat loss coefficient.
Experiments using the following formula to calculate the average efficiency:
- Said that on the day can be a useful absorbed by the day divided by the sum of the cumulative sun exposure, that is, the average efficiency.
Two water heaters, the average efficiency as shown in table 1. Can be seen from Table 1: cellular heat pipe plate solar water heaters, low-temperature (accumulator in the water temperature of less than 65 ℃), the average efficiency of all-glass vacuum tube solar water heaters on average higher efficiency; High-temperature (accumulator in the water temperature is higher than 65 ℃) cellular heat pipe plate solar water heaters on the average efficiency of all-glass vacuum tube solar water heaters on the average to low efficiency. This is because the relatively high temperature, cellular heat pipe plate solar water heaters heat loss than the vacuum tube solar water heaters, it's efficiency on average than in the high temperature of the vacuum tube-low. However, the temperature is less than 65 ℃ in the circumstances, the cellular heat pipe plate solar water heaters on average have higher efficiency. As a result of the cellular technology, effectively curbed a natural convection heat loss and heat panels on the outside of the loss of infrared radiation, heat pipe cellular plate solar water heaters on average to increase efficiency.
The average coefficient of heat loss is the description of the water heater another important indicator of performance, it is that in the absence of solar radiation under the conditions of a certain period of time, units of time, unit area lighting, solar water heaters, water temperature and the temperature of the temperature difference between the units Average heat loss. The average coefficient of heat loss-definition:
Where: U L --- average coefficient of heat loss, W / (m 2 ℃);
t m --- Average water temperature, ℃;
M --- solar water heaters, water quality, kg;
C p --- water heat, J / (kg ℃);
t 1 --- initial average water temperature, ℃;
t 4 --- the end of the average water temperature, ℃;
A --- solar water heaters, lighting area, m 2;
t a --- average temperature, ℃;
Δτ --- time interval, s;
。 Subscript represent different moments of data measurements.
The data processing and future generations-in (2), a cellular heat pipe-plate collector and all-glass vacuum tube collector of the coefficient of heat loss as shown in table 2.
From Table 2 can be seen in the cellular heat pipe-plate solar water heaters to heat loss coefficient of less than all-glass vacuum tube solar water heaters heat loss factor. This is because the heat pipe with a one-way heat transfer characteristics, played a role in thermal diodes, the heat loss decreased significantly.
Figure 2 shows for the summer under ideal weather conditions at a temperature of the water tanks in the curve. From the map, we can directly see that at night all-glass vacuum tube solar water heater temperature drop than cellular heat pipe solar water heaters large. In the evening, cellular heat pipe plate solar water heaters accumulator in the hot water heat through the outward transfer, and because of the vacuum tube, water, but also through the vacuum tube outward lost some heat. This is further evidence of the heat pipe technology applications in solar water heaters, water heaters can significantly reduce the heat loss.
3 Conclusion
(1) cellular heat pipe plate solar water heaters, the most notable feature is also the application of cellular and heat pipe technology. It was found that: when the temperature is less than 65 ℃ for its efficiency on average 48.3 percent, than the all-glass vacuum tube solar water heaters (42.2%) higher. Although more than 65 ℃, the average efficiency of its all-glass vacuum tube solar collectors should be slightly lower, but the latter easily scaling the wall, falling into the average efficiency will reduce. The cellular heat pipe plate solar water heaters through the heat pipe, the heat transfer of latent heat and difficult to scale, the average efficiency of its relatively stable.
。 (2) experiment in the cellular heat pipe plate solar water heater, the average coefficient of 1.16 heat loss than the all-glass vacuum tube solar water heaters, the average coefficient of 2.24 to heat loss much smaller.
。 (3) Flat solar water heaters use of cellular and heat pipe technology, its thermal properties have been improved, has a practical and promotional value.
。 Thanks: The experiment used in all-glass vacuum tube solar water heaters by large groups of Zhejiang, Zhejiang United States and the United States and the solar water heater industry Limited, in which their support and help to express my sincere thanks.