Experiment on performance of all-glass vacuum tube solar array heating system

Abstract: As a kind of conversion device of solar energy, the solar collector is the most important part of the solar heating system. Among various solar collectors, all-glass vacuum tube solar collector is regarded as more favourable than other collectors in both technical and economic perspectives, so domestic and foreign experts have studied several aspects of it. But the current studies usually focus on the heating performance of the solar system during the whole heating season and the influence factors of the collecting efficiency of the all-glass vacuum tube solar collector, and there is barely research on the hourly and dynamic heating performance of solar heating system in a monomer building under different operating ways. With the purpose of studying the above problems, an all-glass tube solar heating system is fabricated on a monomer building, combined with a low-temperature floor radiation heating. The system is composed of 6 groups of standpipe all-glass vacuum tube solar collectors which have uniform structure parameters, a low-temperature floor radiation heating device, a circulating pump, a valve, a conductor and other accessories. Every group of solar collector comprises 40 all-glass vacuum tubes with the external diameter of 58 mm and the length of 1 800 mm, and a storage tank with the volume of 400 L, which is installed on a rack with an angle of 45° facing south. The contour aperture area of solar collector is about 3.85 m2, so the total contour aperture area of the array is about 23.1 m2. The monomer building locates in Minqin County, Gansu Province, China. Its building area is 117 m2 and actual heating area is 87 m2. The operation mode of system is as follows: Daily 17:30-23:00 is set to be heating time; during this period, the controller controls the water pump to circulate hot water at a constant flow rate, stop for 5 min every operating for 8 min. In the experiment, the values of various parameters, such as the solar irradiance, the inlet and outlet temperatures of collector array, the tank water temperature, the ambient temperature, the circulating water flow rate and the wind speed, are measured by different sensors. All measured variables are collected and recorded automatically by Agilent 34970A data acquisition instrument every 10 s. The testing period was from November 24th to December 5th, 2015. Then, many important parameters such as the total heat loss coefficient of storage tank, the collection efficiency of solar collector array, the solar energy utilization and the solar heating fraction of the system, and hourly variation of building heat load are theoretically and experimentally investigated. Furthermore, in the actual operation state, the heating effect of the solar heating system is analyzed, and the improvement proposals of operating strategy are provided. The results show that the total heat loss coefficient of storage tank in this system is 25.82-31.53 W/℃, the collection efficiency of solar collector array is 38%-72%, and the solar energy utilization and the solar heating fraction of the system are 37.1% and 48.3%, respectively; only 54.6% of heat collected by the solar collector is used, the remaining heat is emitted to the environment, and thus the heat loss of system accounts for a large proportion of the total collected heat; under the actual operating state, the heat supply is much more than the heat consumption of building, and especially in the initial period of heating, heat supply reaches more than 10 times that consumed by building, and heat supply and heating time are excessively concentrated. As a consequence, improvement proposals of operating strategy are provided for the solar heating system of the monomer building to reduce the water flux of heating system and advance the heating time.