Effect of time interval of water-supply on heating performance of solar low temperature floor heating system

In rural area of China, due to low income, it has been popularly used for winter-heating with straws and coal. The heating method is so backward that it is promising for heating with solar energy. Since the heat gain from sun is limited to specific area, it is essential to improve the efficiency of solar energy utilization. Some researchers found that on-off modes of circulating pump took obvious effects on heating performance of central heating. In this paper, three different kinds of on-off modes were applied to the solar floor heating system in order to discover the influence of on-off modes. The experiment was conducted during the winter in a single-storey building in Zhangma Village, Minqing county, Gansu province, China. The total area of the building is 117 m2, the residential area of about 103 m2, whose actual heating areas is 64 m2. The heating system consists of 6 sets of vacuum tube solar collectors. Each collector consists of 40 vacuum tubes in Φ58 mm × L 1 800 mm with a 400 L water tank. The heating period was 17:00-23:30 every day in Dec 2015. The system had the same water-flowing duration of 8 min and three different intermittent durations, 5 min intermittent durations on Dec 3th, 11 min on Dec 25th, and 6 min on Dec 30th. Various parameters, such as the solar radiation intensity, the supply and return temperature of the system, the tank temperature, the ambient temperature, the flow rate and the ambient wind speed, were all measured by various sensors in the experiment. All measured parameters were acquisited and recorded automatically by the Agilent 34970A every 10 s. The variation law of difference of supply and return temperature during the heating period was analyzed, the heat supply, heat loss rates, energy consumptions, the solar fractions and average heating efficiency of the system all were calculated, the correlations between various factors with the heat supply were analyzed. The experimental results reveal the following: With the same water-flowing durations, the longer the discontinuous time, the greater the difference of supply and return temperature. Compared with the time interval of 5 min, the solar fraction of the time interval of 6 min was increased by 5.27%, heat loss rate of the system decreased by 2.79%, and the operating energy consumption decreased by 6.67%. The time interval of 6 min had the highest average heating efficiency of the system in 3th to 29th in Dec 2015. Various factors discussed were significantly correlated with the heat supply. The supply temperature and the ambient temperature had greater impact on the heat supply, but the flow rate and the ambient wind speed had less impact on it. According to the operation modes of the solar floor heating system, the water-flowing time was longer than the displacing time of water stored in the pipes under the floor. Not only the water supply displaced the hot water in the pipes completely every time, but also there was some high temperature water flowing through the return pipe. In the further operation, the water-flowing time should be reduced, so that the water-flowing time could be equal to the displacing time of water in the pipes, and the high temperature water was prevented from losing excessive heat through the return pipe. ©2018, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.