Performance experiments for flat plate solar water heater based on micro heat pipe array

Abstract

A novel flat plate solar water heater based on the micro heat pipe array (MHPA) was invented, and the experiments were carried out on the MHPA heat collector and water heater to test their thermal performance. Owing to its distinctive structure, the MHPA has good heat transfer ability, high reliability, high compressive strength and low cost. The structure and character of the novel collector with MHPA as the high-efficiency heat transfer element are as follows. First, the contact surface between the absorber and the MHPA is so large that the heat resistance is smaller than the traditional one, thus its thermal transfer capability is greatly improved. Second, the condenser section of the MHPA is connected closely to the heat exchanger with dry type. The heat pipes do not contact with water, which precludes scaling and leakage in the collector. Third, the MHPA uses low freezing points of the refrigerants, which makes the collector more suitable for extremely cold areas. Fourth, the MHPA is made of aluminum instead of copper, which could reduce the fabrication cost significantly. Fifth, the unique heat exchanger could exclude water easily, which could prevent the frozen cracking of the collector in winter. Consequently, compared with other conventional evacuated glass tube solar water heaters, the novel flat plate solar water heater offers the additional benefits of high compressive strength, light weight, high reliability and durability, easy integration into buildings, and resistance to freezing during the winter months. Compared with traditional sheet-and-tube structure flat plate solar water heaters, this novel solar water heater also demonstrates many advantages: lower cost, absence of welding, resistance to freezing. The experiments focused on the novel collector's instantaneous efficiency. The test results for the collector's instantaneous efficiency show that the slope and intercept of the instantaneous efficiency curve are 4.7 and 0.80 respectively, which are 11.0% and 22.3% superior to the technical data of the Chinese national standard. The high maximum instantaneous efficiency can be attributed to high transmittance of the glass cover and good absorption of the absorber, as well as great performance of micro heat pipe array. Due to the excellent isothermal characteristic of the MHPA, the absorber have uniform temperature field so that the heat loss by natural convection in the collector is reduced, thus the novel flat plate solar collector have relatively low heat loss factor. The low emissivity of absorber film also reduces the radioactive heat transfer between the absorber and glass cover. Finally, a detailed experimental study focusing on the thermal performance for the novel flat plate solar water heater based on the MHPA is conducted during daytime over several days. The test results show that the effective heat gain per contour aperture area for the novel flat plate solar water heater are higher than that of the Chinese national standard, and all of the average daily thermal efficiency are higher than 60%. Therefore, the flat plate solar water heater based on the MHPA has the broad application prospect because of its excellent performance.