| The global energy and ecological crisis give threat to the survival and development of human beings,development and utilization of renewable energy have become the research hot spot,and solar energy has received more attention.Generally,there are two ways that converts solar power to electric power: solar photovoltaic and solar thermal power generation.Solar photovoltaic power generation using a small part of the visible radiation which the sun sends out that make semiconductor materials produce photovoltaic effect,converting solar power to electric power.One of solar thermal power generation collects solar energy using solar concentrated thermal system to heat the working medium and driving the thermal power generator to generate electricity,it belongs to the high temperature of solar energy utilization;the other is solar thermoelectric generation,using the sun radiant energy as heat source of semiconductor thermoelectric module,and the Seebeck effect of thermoelectric material will change the sun’s thermal directly to electricity,it belongs to the medium temperature of solar energy utilization.As the demands for solar medium temperature utilization more and more and the development of thermoelectric materials performance,and the solar thermoelectric generation is a green and environmental protection way to generate electricity and becomes a research hotspot in recent years,this way can use full spectrum of sun compared with solar photovoltaic generation,and also it is a solid-state device,which increases its reliability and reduces maintenance、does not produce any waste.This paper combines solar concentrated thermal technology with semiconductor thermoelectric generation technology to design a concentration solar thermoelectric generation device,the key technologies involved in the device and the performance of thermoelectric coupling are researched.(1)The sun’s energy density is low,solar concentrated technology can enhance the level of solar thermal conversion and realize the solar thermal utilization effectively.Combining solar concentrated thermal technology with semiconductor thermoelectric generation technology to design a concentration solar thermoelectric generation device,the device is mainly include solar concentrator 、 heat collection plate which coated with solar selective coatings 、 TEM(Thermoelectric Module)、 cooling system 、 the sun tracking control system 、 MPPT(Maximum Power Point Tracking)control et.al,converting solar power to electric power.(2)Using the parabolic concentrator which is line focus,the incident solar rays reflected and then gathered into the focal line,forms a focal plane on the heat collection plate which area is lesser,making the hot side of TEG has high temperature thermal.Simulation the device model in optics software of Trace Pro,the results show that the designed device can concentrated solar better,after heat collection plate absorbed,it can be used as the heat source of TEG;when the incidence ray is vertical,the ray absorption rate of receiver reaches 98.20%.At zero deflection angle,under the incident irradiation of 748 W,receiver absorption irradiation of 588.6W,optical efficiency of system is 78.58%.The reflection loss will increase as the deflection angle increasing,and the optical efficiency will drop.(3)When the device applies in actual,the heat collection plate will shelter from parabolic concentrator,some of the light can not arrive heat collection plate due to the fact that light travels in straight lines and some is blocked,so the bottom of heat collection plate produces shadow area,resulting in the decreased overall output power.The shadow area mathematical model is established,and simulation the change rule of shadow area in different deviation angle in Trace Pro.Based on the construction model,parameters of shadow area were acquired,thus optimal compensation size and setting angle of planar reflector were obtained,after that the compensation design of planar reflector were made and verify its correctness.(4)The sun tracking system can adjust the position of concentrator according to the change of sun,it ensure that the open face of condenser and incident sunlight is always perpendicular to each other,improving the utilization rate of solar energy greatly.This paper design verticalhorizontal double rotor active tracking based on PLC,there are four illuminance sensors symmetrical located in the up and down,left and right sides of condenser for acquisition of solar illuminance real-time,and sent the collection data into the PLC storage and comparison,finally output comparison results to control the step motors by PLC.Stepper motors control the transmission device and the transmission device driven the condenser tracking sun automaticly.The hardware and software debugging results show that the sun tracking control system can realize effective tracking of sun,meet the needs of this device for solar power.(5)If want to keep the device is in a state of maximum power output always,it must be MPPT control for TEG.Through comparative analysis,it selects the conductance incremental method as MPPT control algorithm,established MPPT control submodel of conductance incremental method under the environment of Matlab/Simulink,simulation waveform shows that the conductance incremental method can tracking the maximum power point of system quickly and effectively.The research adopts centralized-distributed-hybrid MPPT control scheme,each TEM has an independent distributed unit,and after TEMs in series or in parallel,they will connect with a centralized unit,finally,the output terminal of each distributed unit in parallel with the output terminal of each centralized unit.Centralized unit and distributed unit work together torealize the maximization output power of TEMs.The results of test show that the output power of TEG with MPPT is achieved fast,accurately and steadily,and in thirty minutes working time the average power adds 3.2W.(6)The thermal performances of concentration solar thermoelectic generation are analyzed based on energy balance and three dimensional temperature field model.The mathematical models of solar-thermal energy conversion,thermal-electric energy conversion,thermal dissipation process are set up from the angle of energy balance,doing simulation and numerical computation for this energy equation under MATLAB condition,and analysisthe affect ofconcentrated ratio and solar irradiance for the performance of system,the numerical simulation results can optimize the design of device.ANSYS finite element three-dimensional temperature field model is set up,analyzed thermoelectric coupling performance of a single PN junction,gets the temperature field and electric potential fielddistribution trend,energy distribution of heat collection board surface and the temperature field distribution of heat sink.Compared the output power under different temperature of simulatong with device test powers,The results show that theoretical values a bit higher than experimental values,it proves the validity of test results.(7)Test analysis were carried out on the device,the instantaneous test for water cooling device analysis the cooling water flow rate,load change,and the MPPT control which effect the performance of the device,the results show that when flow quantity is less than 8L/min,temperature of heat pipe condensation and TEG cold side are drop rapid,but when the flow quantity is greater than 8L/min,the temperature change is flat out.Output power is rising along with the increase of temperature difference,the maximum power is 30.2W.The optimal match load is near resistance when temperature difference is low,the optimal load increases gradually along with the increase of temperature difference,the optimal load resistance is 40Ω with temperature difference is 150 K.The device reach maximum output power 30.2W when operate12 min with MPPT,and 29.1W when operate 18 min without MPPT,the output power of TEG with MPPT is achieved fast,accurately and steadily.Whole day tests for the device have done during the 08:00~16:00 on June 16,2015,running 8 hours the electric quantity is 222.4W·h.The thermoelectric conversion efficiency is up to 5.4% and the maximum efficiency of the device is4.1%.The test for wind cooling device analysis the wind speed which is main factor that influences the output performance of the device and verify whether the device can provide power supply for a small grow seedlings greenhouse in facility horticulture in Northeast Agricultural University.the results show that increasing wind speed made the temperature of heat-collecting plate,the hot side temperature of TEG all drop and total power is 340.2W·h in 6 hours.The device generates electricity 52.01 hours and the electric quantity is 2.74 k W·h in ten days,it can satisfy the needs of power supply for a small grow seedlings greenhouse.(8)Using solar energy engineering economy,and gets the annual earnings expression of solar installation from the perspective of present value.The results show that the rise ofconventional energy sources is a very good positive factor for the development and utilization of solar energy.Power conversion efficiency of solar photovoltaic generation and thermoelectric generation are analyzed.The conversion efficiency of solar cell can not be improved in a short time,The low conversion efficiency of thermoelectric generation mainly limited by the properties of thermoelectric material,optimal value of thermoelectric materials are not more than one,when it reaches two,compared with the photovoltaic conversion efficiency has a certain competitive advantage.The worth encouraging is that starting from the basic law of thermodynamics for all theoretical research,it has no limit for thermoelectric coefficient optimal value so far.In the long term,the solar thermoelectric power generation device which make up of the condenser and high value coefficient will has economically competitive for many applications. |
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