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Heat collecting elements (HCEs) are the core components in the parabolic trough collector (PTC) system because photothermal conversion of the whole system occurs in the HCEs. However, considerable heat loss from the HCEs at high operating temperature exerts seriously negative impact on the photothermal conversion efficiency of the PTC system and subsequent application systems. To effectively reduce the heat loss and thus enhance the overall performance of the PTC system, in our previous work, we proposed three kinds of novel HCEs by partially depositing different IR-reflector coatings on the inner and outer surfaces of the glass envelope. The infrared (IR)-reflector of actual transparent conductive oxide (TCO) film, IR-reflector with a fixed cutoff wavelength of 2.5 μm, and the IR-reflector with optimal cutoff wavelength showed extremely effective roles in the reduction of heat loss in HCEs. In this paper, the comprehensive energy and exergy performances of these three novel HCEs in a real 72 m small-scale PTC system are further investigated by the mathematical models established. Additionally, the comparisons among overall performances of the proposed HCEs under different direct solar irradiances are also carried out. The results show that the simulated data yields good consistence with the experimental results, and that all three of the novel HCEs achieve superior overall performance compared with the conventional HCEs. The PTC system installing the novel HCEs with the IR-reflector coating which possesses the optimal cutoff wavelength has the best energetic and exergetic efficiencies, which are significantly improved by 25.2% and 28.1% compared with the conventional HCEs at the solar irradiance of 800 W/m2 and inlet temperature of 580 °C. Moreover, the proposed novel HCEs have a much superior performance at lower solar irradiance. The performance-enhanced PTC system will play a significantly positive role in the performance improvement of the heating and cooling of buildings in the future. [ABSTRACT FROM AUTHOR]

• A novel tower receiver with an air curtain was proposed and designed. • A numerical model based on the finite volume method was built and validated. • Optimum operating strategy and configurations of air nozzles were investigated. • Total heat loss is effectively reduced by 9.60 %. • Thermal efficiency is significantly enhanced by 0.49 %. External-type solar tower receiver, mounted on the top of a tower with an elevation of dozens or even hundreds of meters, is faced with unavoidably massive convective heat loss due to high operating temperature and high-speed wind at high altitude. To effectively reduce the convective heat loss, a novel solar tower receiver with an air curtain is proposed and designed. In this study, the comprehensive thermal performance of plant-scale tower receivers is numerically studied based on the finite volume method. The impacts of different configurations of air nozzles on the thermal performance of the solar tower receiver are investigated. Furthermore, techno-economic analyses on the Solar Two power plant are also carried out. The results show that, in scenario of incoming wind speed of 15 m/s, the air curtain exerts the best roles at the air-jet angle of 45°, a distance of 1.50 m from the receiver surface, and air-jet velocity of 16 m/s in an operating mode of switching off the air nozzles at the windward and leeward sides. Compared with the original receiver, the heat loss of the proposed receiver is effectively reduced by 9.60 %, and the efficiency and electricity power production are enhanced by approximately 0.49 %. [ABSTRACT FROM AUTHOR]

An evacuated flat-plate photovoltaic/thermal (E-PV/T) collector was proposed. The inner space of the E-PV/T collector is vacuumed to suppress non-radiative heat losses, thus increasing thermal efficiency of the collector. Therefore, the E-PV/T collector has the potential to simultaneously deliver electricity and heat at high temperatures. A mathematic model was developed to evaluate the performance of the E-PV/T collector. The effect of some key parameters (e.g., initial water temperature in the water tank, vacuum degree, long-wave panel emissivity, and temperature coefficient of solar cells) on the performance of the E-PV/T system was investigated and the results were compared with a normal flat-plate PV/T (N-PV/T) system. Results suggest that the vacuum helps to enhance the total efficiency by nearly 10% points in high-temperature conditions (>80 °C). The vacuum degree of the upper space exerts a greater effect on system efficiencies compared to that of the lower space. Lower long-wave panel emissivity and greater temperature coefficient of the solar cell promote the performance of the collector. By lowering the long-wave panel emissivity from 0.95 to 0.05, the total efficiency soars from 26.82% to 61.20%. This study may help to guide parametric optimization and operation strategy of flat-plate PV/T collectors for high-temperature applications. • An evacuated flat-plate photovoltaic/thermal (E-PV/T) collector was proposed. • The non-radiative heat loss is deeply suppressed owing to the vacuum. • The total efficiency is increased by nearly 10% points (>80 °C). • The thermal efficiency at zero-reduced temperature is 45.7% for the E-PV/T system. • Long-wave emissivity of the PV/T panel profoundly affects the thermal efficiency. [ABSTRACT FROM AUTHOR]

• Spectral thermal radiation model based on the spectral parameters was built and validated. • Negative energy-flow region (NER) partially around the solar receiver was discovered. • Quantitative analyses on NER by the proposed negative energy-flow index were investigated. • A novel optimization strategy by introducing a radiation shield into the receiver was proposed. • The heat loss of the novel receiver was effectively reduced by 35.9%. Parabolic trough solar receivers are the key heat-collecting elements (HCEs) in parabolic trough collectors (PTCs). However, the HCEs emit considerable heat loss at high operating temperature, which exerts significantly negative impact on the thermal performance of the HCE and PTC. For accurately calculating the thermal radiation in HCEs, a novel spectral thermal radiation model based on the spectral parameters was established and validated. Comparison between the simulation results and experimental data demonstrated the model yielded satisfactory consistency. Integrating the achieved spectral thermal radiation with circumferentially maldistributed solar irradiance, a rarely mentioned phenomenon of negative net heat gain and negative energy-flow region (NER) partially around the HCE were discovered. NER exercises adverse influences on the thermal performance of the HCE. In this study, quantitative analyses on the NER by the proposed negative energy-flow index (NEI) were investigated. The effects of parameters, namely, absorber temperature, solar irradiance, on the NER and HCE were studied as well. Besides, a novel optimization strategy by introducing a radiation shield for effectively reducing the heat loss was proposed. The performance of the proposed novel HCE with an inner radiation shield (NHCE-RS) was also investigated. Results showed NEI can scientifically analyze the negative net heat gain and thermal performance in NER, and NHCE-RS has superior thermal performance at higher absorber temperature and lower solar irradiance. The heat loss of the NHCE-RS was effectively reduced by 35.9% at the absorber temperature of 600 °C and solar irradiance of 500 W/m2. [ABSTRACT FROM AUTHOR]

Abstract Evacuated collector tubes (ECTs), the key components of the parabolic trough collector, suffer considerable heat loss at high operating temperature. Based on the characteristic of uneven distribution of solar irradiance around the absorber tube, novel ECTs covered with infrared (IR)-reflectors on the partial area of glass envelope were proposed to effectively reduce the heat loss and enhance the performance. Two kinds of ideal IR-reflectors, namely, the first one with a fixed cutoff wavelength, the second one with a variable and optimal cutoff wavelength were proposed. A real material of transparent conductive oxide (TCO) film was also used as an IR-reflector in novel ECTs for the analysis. The mathematical models relying on spectral parameter calculation method were established for comprehensively investigating the overall performance of the novel ECTs. Moreover, the influences of TCO film reflectivity and emissivity on the ECTs were investigated. Results demonstrated that the novel ECTs with TCO films, the first ideal films, and the second ideal films achieved obvious superior thermal performance compared with the conventional ECTs. Their relative heat loss reductions at an absorber temperature of 600 °C reached 18.7, 25.3, and 43.8%, the heat-collecting efficiencies were relatively enhanced by 7.2, 10.8, and 16.7%, respectively. Highlights • Novel collector tubes with spectrum-selective glass envelope are proposed. • Mathematical models with spectral radiant distribution are built and validated. • Optimal cutoff of the ideal IR-reflector is accurately figured out. • The heat loss of the novel collector tube is reduced by around 43.8%. • Heat-collecting efficiency of the novel collector tube is raised by 16.7%. [ABSTRACT FROM AUTHOR]

Prospects for parabolic trough collector are growing as the market increasingly values concentrated solar-thermal utilization. Parabolic trough solar receivers, the key components of parabolic trough collector system, seriously suffer degradation of photothermal conversion performance at high operating temperature due to considerable emissive heat loss, which exerts significantly negative influence on the overall performance and development of parabolic trough collector and subsequent thermal utilization systems. This study examines the spectral emissive heat loss and circumferential heat transfer characteristics around the parabolic trough solar receiver. In this framework, a new concept is proposed, i.e. the negative thermal-flux region in which negative net heat gain occurs, accordingly enlightening and giving birth to new optimization strategies for reducing emissive heat loss of the parabolic trough receiver. A novel parabolic trough receiver with an inner radiation shield in the negative thermal-flux region is designed, manufactured and comprehensively tested. The results show a validity of the existence of the negative thermal-flux region and great potential of new optimization methods to achieve breakthrough enhancement of heat-collecting performance in parabolic trough collector system. Compared with the prototype solar receiver, the heat loss of proposed solar receiver is effectively reduced by 28.1% at absorber temperature of 600 °C, the heat-collecting and exergetic efficiencies are significantly enhanced by 12.9 and 17.6% at the solar irradiance of 600 W/m2 and inlet temperature of 550 °C. • Negative thermal-flux region occurring in solar receiver is discovered. • New optimization strategies different from the conventional ones are concluded. • Comprehensive experiments about the proposed solar receivers are carried out. • Heat loss, thermal and exergetic efficiency models with high precision are established. • The proposed optimization method has great potential for the system performance enhancement. [ABSTRACT FROM AUTHOR]

As a high solar concentration technology, the solar tower power (STP) system is an appealing approach to generate high-grade thermal energy and achieve high thermal-to-electric efficiency. In this study, the authors notice the solar flux distribution characteristic of the central receiver and combine the advantages of lower average operation temperature of the direct steam generation (DSG) loop and higher efficiency of the molten salt (MS) loop. A hybrid solar tower system that involves steam and MS as the heat transfer fluids is proposed for improving the thermal efficiency of STP systems. The receiver of the hybrid system is divided into two sections, which are respectively designed for the MS and DSG loop, namely MS-DSG system. By comparing the DSG-MS system to the traditional system, the DSG-MS system demonstrates significant heat loss reduction of 31.8 GWh in Lhasa and 34.5 GWh in Tonopah, and the corresponding electricity outputs are improved by 6.22% and 5.82% with a MS receiver panel number of 8. The steam outlet quality of the DSG loop is insensitive to the overall performance of the systems. It is indicated that the steam quality can be adjusted for ensuring two-phase heat transfer stability and safe operation of the receiver. Moreover, the hybrid system also gives a flexible adjustment of thermal energy storage capacity by optimizing receiver panel number for different heat transfer fluid loop. • The hybrid solar tower system using steam and MS as the HTF is proposed. • Convection and radiation heat losses reduction of 24.8% and 42.4% are reported. • Electricity output of the plant in Lhasa is improved by 6.22%. • The hybrid system gives a flexible adjustment of the thermal storage. [ABSTRACT FROM AUTHOR]

• A performance-enhanced parabolic trough solar receiver is manufactured. • Indoor heat loss experiment based on heat equilibrium method is conducted. • Outdoor thermal efficiency experiment is carried out in two-axis tracking platform. • The heat loss of new receiver is reduced by around 28.0%. • The thermal efficiency of new receiver is effectively raised by 1.6–4.9%. Parabolic trough collectors (PTCs) are the most mature way to harvest high-temperature heat source and widely applied in solar thermal utilizations. Parabolic trough solar receivers as the heat-collecting elements (HCEs) are the key parts of PTC, but face with a knotty problem that is exploding radiative heat loss under high operating temperature, which exerts a significantly negative role on the overall performance of the PTC system. For effectively reducing the heat loss and improving the thermal performance of solar receiver, a structurally optimized HCE with an inner radiation shield was proposed, designed, and manufactured. Furthermore, the indoor heat loss and outdoor thermal efficiency testing were carried out in the Institute of Electrical Engineering, Chinese Academy of Sciences (IEECAS) to validate comprehensive thermal performance of the proposed HCEs. The results show that the radiation shield plays an effective role in reducing the heat loss and improving the thermal efficiency. The heat loss of the proposed HCE is significantly reduced by 28.0% compared to the conventional HCE at the absorber temperature of 550 °C. And the proposed HCE possesses superior performance at high operating temperature and low solar irradiance. In the case of inlet temperature of 350 °C and solar irradiance of 600 W/m2, the thermal efficiencies of proposed HCE and conventional HCE are 49.4 and 51.8% respectively, and the thermal efficiency of the proposed HCE is effectively enhanced by 4.9%. [ABSTRACT FROM AUTHOR]

• A spectral–spatial coupling analysis of receiver is carried out. • Negative thermal-flux region and local optimal cutoff wavelength are found. • A novel receiver using two selective coating is proposed. • Heat loss reduction of 41.0% is reported for the optimized receiver. • Thermal efficiency enhancements of 10.2%–40.2% of the receiver are presented. The parabolic trough system is one of the main technological routes to achieve high-temperature solar thermal conversion. The parabolic trough system is a mature technology and can be easily coupled with distributed systems. However, the parabolic trough receiver, which is a key component of the parabolic trough system, suffer from enormous radiation heat loss at high temperature. An analytical model based on spectral-spatial coupling distributed parameters is developed. Analytical results reveal that >40% surface area of the absorber is negative thermal-flux region and exposes the widely long-term thermal performance weakness in circumferentially uniform receiver design. A local optimal cutoff wavelength is reported. Results show that the exists asymmetrical design of the receiver can reduce radiation heat loss by approximately 41.0% and improve photothermal efficiency by 10.2–42.0% as solar irradiation varies from 1000 W/m2 to 200 W/m2 at 600 °C. The asymmetric design may be a promising choice for optimization of the receiver due to the strong heterogeneity of the solar flux distribution at high temperature. The discovery of negative thermal-flux region and local optimal cutoff wavelength also leads to the optimization of other concentrated solar technologies for improving photothermal performance. [ABSTRACT FROM AUTHOR]