Numerical And Experimental Study On Novel Parabolic Trough Solar Receivers Optimized Based On Negative Energy Flow Region

Parabolic trough collector(PTC)is the most commercialized and technically mature way in the high-temperature solar thermal utilization,which is popularly used in the industrial utilizaiton.Parabolic trough heat-collection elements(HCEs)are the key components of the PTC used for the absorption and conversion of the solar energy.Usually,the HCE's operating temperature can reach 400?550?;moreover,the higher running temperature of HCE would be obtained under the current trend of higher concentration ratio in CSP.However,HCEs are faced with a technical problem to be solved in the case of high operating temperatures,that is a large amount of radiation heat loss emitted from the HCEs,which has an exponentially biquadratic increase with its operating temperature.This phenomenon significantly decreases the system efficiency and economy.For above problem,a novel heat transfer mathematical model was established in this paper.'Negative Energy flow Region'(NER)in the HCE was found for the first time,and novel structural and coating optimization strategies were proposed for effectively reduce the heat loss of the HCEs.The novel strategies have a great breakthrough in the design of circumferential symmetry structure in HCE,and open up a new idea and method for the research of reducing heat loss.Furthermore,three kinds of the novel HCEs(NHCEs)were also designed and investigated.The main achievements of this thesis are outlined as follows:1.In the case of high temperature of HCEs,the traditional heat transfer analysis method based on average parameters cannot accurately simulate HCEs' heat transfer characteristics and capture the overlapping phenomenon of spectral bands,besides,the traditional heat transfer method does not consider the uneven solar irradiance distribution around HCEs.In view of the above shortcomings of traditional heat transfer methods,a novel heat transfer analysis method based on spectrum-space coupling was established by combining spectral parameters and spatial solar irradiance characteristics,which can accurately analyze the heat transfer characteristics in HCEs and realize the heat transfer analysis in spectral level.Based on the novel heat transfer method,we proposed the concept of NER in the HCE,which reveals the undiscovered hidden heat loss phenomenon.Based on the chracteristics of NER,we put forwarded the novel structural and coating optimization strategies for reducing the heat loss of the HCE.2.A structural optimization method of introducing a radiation shield(RS)into the vacuum annular in NER of the HCE was proposed.The spectral coating and structural parameters were designed and optimized.We designed and trail-manufactured small and standard novel HCEs with a RS(NHCE-RS).The experimental results showed that an effective reduction in heat loss and enhancement in efficiency of the NCHE-RS were achieved compared with CHCE.In the case of absorber temperature of 550?,the heat loss of the NHCE-RS was relatively reduced by 28.0%compared with CHCE.In the case of the inlet temperature of 350 ? and DNI of 600 W/m~2,the thermal efficiency of the NHCE-RS was relative enhanced by 4.9%.3.The spectral thermal radiation model and the heat collection in PTC model were built,and the simulation date was verified with the experimental results.The comparisons showed that the root mean square deviations(RMSDs)between the theoretical values and experimental values are 5.6%and 0.5%;which illustrated that the established models can accurately predict the thermal performance of the HCEs.Based on the verified models,we investigated the influences of the different parameters and climate conditions on the NCHE-RS.The simulation results showed that a superior thermal performance of the NHCE-RS was achieved in high running temperature and areas with low DNI,the largest comprehensively daily efficiencies of NHCE-RS were possessed in Hefei.In the case of the inlet temperature of 580 °C and DNI of 800 W/m~2,the thermal and exergetic efficiencies of the NHCE-RS can be improved by 14.9%and 14.5%.4.A structural optimization method of introducing a transparent selectively radiation shield(TRS)into the vacuum annular in NER of the HCE was proposed.We designed and optimized the novel HCEs with the TRS(NHCE-TRS).The results showed that the thermal and exergetic efficiencies of the NHCE-TRS can be improved by 9.2%and 10.8%in the case of the inlet temperature of 580 ? and DNI of 800 W/m~2.5.A coating optimization method of covering IR-Reflector(IRR)on the inner and outer surfaces of glass envelope in NER of the HCE was proposed.The optimal cutoff wavelength of the ideal IRR was redesigned and obtained,and novel HCEs with different IRRs(NHCE-IRR)were designed.The simulation results showed that the optimal cutoff wavelength of IRR mostly depends on the DNI and absorber temperature.NHCE-IRR with an IRR which has the optimal cutoff wavelength possessed the best thermal performance;the thermal and exergetic efficiencies of the NHCE-TRS can be improved by 25.2%and 18.1%compared with CHCE in the case of the inlet temperature of 580 ? and DNI of 800 W/m~2.