Experimental Study On Heat Transfer With Molten Salt In Trough Solar Collector System

The main types of the Solar thermal power generation are as following: solarpower tower, parabolic trough, solar power dish and linear fresnel. Among these types,parabolic trough has proven to be the ripest in technique and most widely used one. Atpresent according to statistics, The capacity of the operational Solar Thermal PowerStations is more than2100MW, and the proportion of the parabolic trough is about92%. Among these Solar Power systems, thermal oil is widely used as heat transferfluid, which has many disadvantages, such as high costs, high pressure and low upperlimit working temperature (400℃)。The application of molten salt heat transfer fluid (HTF) in a trough solar thermalpower system has several obvious advantages. For instance, for a trough plant withsalt, it may be possible to raise the solar field output temperature to450-500℃,thereby increasing the Rankine cycle efficiency of the power block steam turbine toover40%, compared to393℃with the current high-temperature oil and a cycleefficiency of37.6%. Moreover, molten salt is cheaper and more environmentallybenign than thermal oil. However, the disadvantage of molten salt as heat transferfluids is their relatively high melting point, typically140-240℃. The operational riskof a freeze-up if the process temperature drops unexpectedly adds cost to systems thatuse salt as heat transfer fluids.In this paper, a kind of new molten salt (melting point84.5℃, upper limitworking temperature550℃) was developed by our team and was used as heat transferfluids in a trough collector system developed by our team. The system have realizedsuccessful operation over2000hrs. The measurement of the molten salt flow wasrealized using the ultrasonic flow meter.Experiments were made to test the heat loss of collector tube with molten salts.The heat loss data at the different inlet temperature between180℃to500℃wereobtained in the night which had no wind. Experimental results show that with theincrease of the operating temperature of the molten salt, the heat loss of the tubegradually. The heat loss of the tested tube is more larger than the tube of the PTR70which is manufactured by the Schott companyThe forced-convection heat transfer experimental between the water and salt hasbeen did in the double-pipe heat exchanger. Overall heat transfer coefficients and average forced convective heat transfer coefficients were obtained by Wilson methodand Heat transfer correlations were also obtained. Comparing it with the well-knownColburn equation, Dittus-Boelter equation, the experimental results have a goodagreement with the classical heat transfer equations.In this paper, the heat loss of the collector tube has been measured using a newelectric heating method which is developed by us. The heat loss data at the differentinner temperature were obtained and comparison with foreign collector tubes heat lossdata by national renewable laboratory in USA were also made. Experimental resultsshow that the measuring equipment which has a simple structure and easy to operatecan have a good evaluation in measuring the heat loss of the collector tube.