| Three main parameters of solar direct radiant floor heating system, which are solar fraction, the average temperature of the supply and return water, and the match between the heat storage tank and collector area, are chosen within a recommended range in the current project design. It is easy to cause an uneconomical system or a not saving system. Therefore, these three parameters are optimized and researched in the paper.First, the solar fraction is optimized by the use of economic analysis. Mathematic and economic models related to the solar fraction are established in the first. Then, taken the typical villa and typical residential as the objects of study, and taken the typical cities as the examples, the solar fraction of solar direct radiant floor heating system is researched. The result shows that the optimal solar fraction is existed in Taiyuan, Lhasa, Beijing, Shenyang, etc, and it is the same for the villa and residential,for example, the solar fraction are 33.31%,33.85% for the typical villa and typical residential in Taiyuan, and it is also found that the economic effect is not significant when the system is applied in Harbin, Urumqi, Wuhan, Shanghai and other areas. And the impact on the optimal results caused by the price of solar collector, the life of solar collector, the prices of conventional energy, and the rate of government subsidy is analyzed, in which government susidys have the most evident influence on the results, for example, the optimal solar fraction can be increased by 4.7%-67.2%.Secondly, the average temperature of the supply and return water is optimized with the method of numerical simulation and experimental research. In the simplified, a completed mathematical is established. Then thelowest standard of thermal environment is put forward in this paper, and the optimal verage temperature of the supply and return water is simulated by Fluent in Taiyuan, Lhasa, Beijing, and Shenyang, for example, when the value of the tube spacing is 200mm, the optimal average temperature of the system is 25℃in Taiyuan. In the same time, the average temperature is also researched when the indoor heating design temperature are 18℃and 20℃for these area. An experimental platform is established in order to verify the validity of numerical simulation, experimental tests were carried out, and the laboratory is taken as an object to be simulated used the same mathematical models. The result shows that the simulation error is in the allowable range. Therefore the mathematical models and numerical simulation are correct and effective.Finally, the match between the heat storage tank and collector area is optimized in this paper. Dynamic mathematical model of the heat storage tank is established combined with the optimization results of the first two parameters. The hourly water temperature is calculated by the computer program developed by VBA language, the average efficiency of collectors, the total number of hours when the minimum supply water temperature can not be guaranteed, the tank temperature changed hourly, and the startup time of the tank are obtained under different specific value(it is the match too) during heating period. Considered the above various factors and the cost of the tank, the result shows that the optimal specific value of the system for the typical residential is 100:1 in Taiyuan, and the appropriate range is from 75:1 to 125:1.In summary, not only the exact methods for determining the three main parameters is offered in this paper, but also some valuable conclusions are obtained in this paper. The optimization and researches not only make the system save more costs and more energy, but also settles a foundation for the promotion and popularization of solar direct radiantfloor heating in China.
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