| The dish Stirling thermal power system is characterized by taking solar energy with a dish-condenser as thermal resource to maintain Stirling engines. The system has the advantages of high efficient, non-pollution, flexible structure and so on, which is suited to provide powerful electricity for space station and remote areas, such as islet, border post and so on. However, solar energy is an intermittent energy. The output power of the system decrease rapidly, or even shutdown directly after direct radiation is interrupted (when clouds appear, the sun goes down, or orbiting the earth shadow).To solve this problem, molten salt solar storage energy devices on space power station has been built by NASA and other organizations. Compared with space station, dark clouds appear randomly and last uncertainly on the ground. For civilian use and large area promotion, high-power and low cost are also wanted. So, for the Dish Stirling thermal power system that service on the ground, a new molten salt thermal storage device is needed. At present, studies at home and abroad are still blank.Therefore, based on the single and multiple cylinder displacer piston dish Stirling thermal power system, target for civil use, put particular emphasis on cost and high power, the molten salt heat collection and storage device of dish Stirling thermal power system (work on the ground) was designed. Combine with the advantages of sodium heat pipe, a heat pipe collection and storage device of dish power system was improved. Besides, thermal packaging process was posted to decrease the requirements of high temperature mechanics performance of thermal storage cavity. That will prolong service life and ensure the system run safely. Compared with the thermal storage device that designed by NASA, the material cost of which lowed35.6%.In order to analyze the performance of the designed thermal storage device and optimize structure, based on the Fluent CFD platform, the influence of receiver cavity structures (aperture shield type, wind skirt, open cavity), parameters (height, diameter, aperture size) and assemble angles on temperature field of the cavity and the air around it, radiation field and air nature convection field was simulated. The relationships between above-mentioned factors, radiation heat flow density and temperature on the cavity inside surface were analyzed. The results showed that the height and diameter of the heat receiver cavity have obviously influence on radiation heat flow density, temperature and its mean square error. A correlation of the optimal heights and diameters was proposed. The radiation fields and temperature fields of the three kinds of structure heat receiver cavities are similar. Complicated nature convection appeared around the thermal storage cavity. An air band with high velocity of1.5m/s is formed near the outside surface of thermal storage cavity. The aperture shield cavity is the best structure. The conical reflector with30°half cone angle is suggested. The average radiation heat flow density and temperature on the cavity inside surface decreased exponentially with the increase of aperture shield diameter. The heat loss is the lowest when the angle of the heat receiver cavity between-π/4and π/4. The gradient of pressure is evident in the cavity, but pressure is homogeneous around it.Under the forced convection of environmental wind, the influence of wind speed, the angle of the receiver cavity on the radiation field, temperature field, pressure field and forced convection felid were discussed. Inside heat receiver cavity, the pressure is homogeneous. What’s more, the whole cavity was filled with convection zone. On the outside of the cavity, positive pressure on the windward side and negative pressure on the leeward side is shown. Swirl flow formed on the leeward side and the feature of nature convection disappeared. The influence law of wind speed and the angle of heat receiver cavity on the temperature of cavity inside surface are announced. For a certain wind speed, the temperature on the inside surface of the heat receiver cavity increases with the increase of angle θ∈[-π/2,π/4]. The heat loss is the lowest when θ is π/4. At last, the heat loss calculation rules of the thermal storage device were analyzed.Thermal storage material is the core of the molten salt phase change thermal storage device. The choice criterions of molten salt thermal storage material which is used on dish Stirling thermal power system were are summarized. Hundreds of molten salt (melting point between100℃～1000℃) were sorted out. Based on the standards of dish Stirling thermal power system, NaCl, NaF-60%KF and NaF-22CaF2-13MgF2were chosen as thermal storage material. The device, which is used for measuring molting volume change rate and conductivity, has the disadvantages of complicated structure, operation trivial and so on. So, the self-heating probe instrument and method, the equipment of measuring inorganic heat conduction coefficient were invented. The measuring results are consistent with reported or simulated values.The value of molten salt thermal storage medium will be realized only when it is contained in the self-designed thermal storage cavity and united with dish Stirling thermal power system. Therefore, combining the characteristic of on land and civil use,’the scheme of thermal collector and storage with two grade energy’ was put forward to avoid’waiting too long’ or ’temperature too high’ of convectional scheme. The energy balance relationships of thermal collector, storage and consumption were analyzed. A program of’The designing of multi-cylinder double-acting double crank dish Stirling engine molten salt phase change thermal storage system’ was written in language C on Vc++2012platform. The program is a collection of the Stirling engine structure design, operating characteristic analysis, thermal storage choose, the design of thermal collector and storage, the analysis of heat loss and weight, cost budgeting and so on. A system of40kW and20kW dish Stirling thermal power system with molten salt thermal storage device was designed with the program. Critical parapets of Stirling engine were agreed with reported values. The relationships among crank angle, volume, pressure and P-V were shown. The algorithm employed has a good convergence. The program is helpful to reduce the system design work strength and improve the design efficiency.The most crucial problem of the thermal storage device is the corrosion of molten salt on the cavity material, In order to analyze the corrosion behaves of alloy in molten NaCl thermal storage medium, one kind of typical alloy from Fe-based, Cr-based, Ni-based, Co-based respectively and Fe-24.4wt.%Al alloy were chosen as thermal storage container material. The corrosion behaviors of them in850℃molten NaCl at short time (15h) and long time (192h) were studied. The results showed that the corrosion characteristic of the selected alloy follows linear rule. Mass loss rate of Cr-based is the largest and Fe-24.4wt.%Al is the smallest on unit surface at unit time. After corroded15h, corrosion feature of Fe-based and Fe-24.4wt.%Al samples are not obvious. Intergranular corrosion feature on Cr-based sample is evident. α phase and y phase are well-distributed. Corrosion feature is not obvious for Ni-based sample. But the content of Cr decreased severely near sample surface within15μm. The content change of Fe and Ni is not obvious. A corrosion layer about15μm formed on Co-based sample. After corroded192h, no-uniformity corrosion happened on Fe-based sample and uniformity corrosion happened on the other kinds of samples. The content of Cr decreased near corrosion layer. Intergranular corrosion happened within the whole Co-based sample. Al2O3is formed on the surface of Fe-24.4wt.%Al sample, and there is no iron oxide on sample surface. A new phase formed near sample surface on the base of Fe-24.4wt.%Al sample for the deletion of Al. Cr escapes from the corrosion system as gas CrCl3, which is the primary cause of mass loss. Consequently, Cr should be avoid adding in thermal storage cavity material of NaCl.The study lays theoretical foundation and provides technical support for industria-lization of dish Stirling thermal power system with molten salt thermal storage device. |
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