Design and Research of Solar Medium Temperature Heat Pipe Receiver

In recent years, in order to reduce the cost of parabolic trough solar power station, researchers have proposed to replace the double loop system of traditional trough solar power station with direct steam generation (DSG) system (including two loops of heat transfer oil loop and water circulation). After eliminating the heat transfer oil loop, the system efficiency is significantly improved.

But it also brings a series of new problems: the circumferential temperature difference of receiver heat absorption pipe in DSG system is large, and the impact of steam water mixture on the pipeline leads to poor receiver reliability, bending, vibration and even damage to the glass sleeve. The heat pipe has excellent isotherm and the separation of evaporation section and condensation section, which can solve the problem of receiver in DSG system and improve the reliability of receiver. The author uses the medium temperature heat pipe to replace the ordinary heat sink, independently develops the solar medium temperature heat pipe receiver for DSG system, and studies the performance of the medium temperature heat pipe and the medium temperature heat pipe receiver through simulation tests.

1 Structure of solar medium temperature heat pipe receiver

The medium temperature heat pipe receiver is composed of a medium temperature heat pipe and a glass sleeve. The evaporation section of the heat pipe is covered with a single-layer glass sleeve. One end of the evaporation section is connected with the glass sleeve through a glass metal seal, and the other end is supported by a support to form the heat absorption section of the receiver; The condensation section of the heat pipe extends into the jacket to form the heat release section of the receiver. The outer surface of the heat pipe evaporation section is coated with a high-temperature selective absorption coating as a heat absorption layer. The heat pipe evaporation section is separated from the condensation section, and the heat absorption section and heat release section of the receiver are also separated accordingly, as shown in Figure 1.

2 Simulation Test Research

During the operation of parabolic trough solar collector, the ratio of heat flux received by the side facing the concentrator to the side facing the concentrator is 62:1, which is also the main reason for the excessive circumferential temperature difference of the receiver. In the test, the electric furnace is used to simulate the heating conditions of the medium temperature heat pipe receiver. Two layers of glass fiber strips with a thickness of 4[nln are added on the upper surface of the heat pipe evaporation section (i.e. the part where the heat pipe is located in the furnace) to block the radiation heat transfer of the electric furnace to the heat pipe, so as to realize the simulation of the actual working conditions of the medium temperature heat pipe receiver. In the test of the medium temperature heat pipe receiver, the working inclination angle of the heat pipe is 4 ℃.

2.1 simulation test system

The test system includes metering pump, pulse damper, electric furnace, cooler, back pressure valve and data acquisition system, as shown in Figure 2. The pulse damper is used to balance the flow fluctuation generated by metering pump and ensure the stability of water pressure and flow in the pipeline. The back pressure valve plays the role of back pressure and regulates the pipeline pressure in front of the valve. The measurement system consists of Agilent data acquisition instrument, thermocouple, computer and pressure gauge The flowmeter is composed of heat pipe temperature, inlet and outlet water temperature, system pressure and flow. The arrangement of thermocouples is shown in Figure 3. K-type thermocouples are arranged along the axial direction and the circumferential direction of the three cross sections of the heat pipe wall to measure the axial temperature and circumferential temperature distribution of the heat pipe wall. E-type thermocouples are arranged at the inlet and outlet of the jacket. The thermocouples are directly welded on the heat pipe wall, and the temperature measuring points are covered with a thickness of 2-3 mm To avoid the influence of furnace radiation on the accuracy of thermocouple temperature measurement.

Fig. 2 Schematic diagram of test system

Fig. 3 thermocouple arrangement

Heat pipe transmission power Q:

Where: C is the specific heat capacity, J / (kg ·℃); RH is the mass flow, kg / S;

Heat transfer coefficient of heat pipe evaporation section:

Where: Pi is the average temperature of the heat pipe evaporation section, which is obtained by measuring the average temperature of each point directly above the pipe wall of the heat pipe evaporation section and each point on three cross sections; tile is the temperature of the heat pipe insulation section; ah is the surface area of the heat pipe evaporation section.

Heat transfer coefficient of condensation section of heat pipe:

Where: TC is the average temperature of the condensation section of the heat pipe, which is obtained by measuring the pipe wall temperature immediately below the condensation section of the heat pipe insulation section; AhpC is the surface area of the condensation section.

The temperature distribution trends of the three sections at different positions of the evaporation section are different. Therefore, the average value of the maximum circumferential temperature difference of the three sections is selected as the performance evaluation parameter of the heat pipe.