The sun (∼6,000 K) and outer space (∼3 K) are two significant renewable thermodynamic resources for human beings on Earth. The solar thermal conversion by photothermal (PT) and harvesting the ...coldness of outer space by radiative cooling (RC) have already attracted tremendous interest. However, most of the PT and RC approaches are static and monofunctional, which can only provide heating or cooling respectively under sunlight or darkness. Herein, a spectrally self-adaptive absorber/emitter (SSA/E) with strong solar absorption and switchable emissivity within the atmospheric window (i.e., 8 to 13 μm) was developed for the dynamic combination of PT and RC, corresponding to continuously efficient energy harvesting from the sun and rejecting energy to the universe. The as-fabricated SSA/E not only can be heated to ∼170 °C above ambient temperature under sunshine but also be cooled to 20 °C below ambient temperature, and thermal modeling captures the high energy harvesting efficiency of the SSA/E, enabling new technological capabilities.
•Combination of cascade organic Rankine cycle and two-tank storage is novel.•Technical challenges associated with wet steam turbines are overcome.•Stable power output is guaranteed over a wide range ...of solar radiation.•A cycle efficiency of 27.4% is achievable at a hot tank temperature of 250 °C.•The system is potentially more cost-effective than a conventional DSG plant.
Direct steam generation (DSG) solar power systems have the potential to improve heat collection performance and reduce capital cost. One challenge of the DSG solar thermal power technology is the unsteadiness of steam generation and power conversion under fluctuating solar radiation. A novel concentrated solar power generation system is proposed. It has three features: two-phase water/steam as heat transfer fluid, two-tank water storage, and cascade organic Rankine cycle (CORC) with a mixing chamber as power block. Steam is produced in the solar field and condensed in a high temperature tank, while an organic fluid replaces water for power conversion. The system enables smooth cycle operation by resilient control strategy and can tackle the challenge associated with wet steam turbines. It can tolerate lower purity of water/steam that only serves as the heat transfer fluid, thereby reducing the technical requirement. Thermodynamic performance in the normal operation condition and heat discharge process are assessed. The influences of ORC working fluid and storage tank size are examined. Results indicate that the mixing chamber temperature plays a crucial role in thermal efficiencies of both charge and discharge processes, storage capacity, and overall performance. A CORC efficiency of about 27.4% is achievable. The equivalent heat-to-power efficiency ranges from 13.35% to 18.81%, depending on the ORC fluid and volume of the storage tank. The novel system has an efficiency comparable to a conventional DSG system while a lower technical requirement in heat collection and power generation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZRSKP
This paper proposes a comprehensive thermodynamic and economic model to predict and compare the performance of concentrated solar power plants with traditional and novel receivers with different ...configurations involving operating temperatures and locations. The simulation results reveal that power plants with novel receivers exhibit a superior thermodynamic and economic performance compared with traditional receivers. The annual electricity productions of power plants with novel receivers in Phoenix, Sevilla, and Tuotuohe are 8.5%, 10.5%, and 14.4% higher than those with traditional receivers at the outlet temperature of 550°C. The levelized cost of electricity of power plants with double-selective-coated receivers can be decreased by 6.9%, 8.5%, and 11.6%. In Phoenix, the optimal operating temperature of the power plants is improved from 500°C to 560°C by employing a novel receiver. Furthermore, the sensitivity analysis of the receiver heat loss, solar absorption, and freeze protection temperature is also conducted to analyze the general rule of influence of the receiver performance on power plants performance. Solar absorption has a positive contribution to annual electricity productions, whereas heat loss and freeze protection temperature have a negative effect on electricity outputs. The results indicate that the novel receiver coupled with low melting temperature molten salt is the best configuration for improving the overall performance of the power plants.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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.
Based on the simulated non-uniformity solar radiation flux distribution of the absorber by the Soltrace software using the Monte Carlo Ray-Trace Method, an innovative parabolic trough solar receiver ...that employs two solar selective coatings with different properties on the outer surface of the absorber is proposed. The concentration ratio and absorber temperature that influence optimal cut-off wavelengths of the solar selective coatings are quantitatively analyzed to optimize the property of the coating. The optimal cut-off wavelength increases with the concentration ratio, but drops with the increasing absorber temperature. The heat transfer process of receivers is numerically simulated to predict the thermal performance of evacuated receivers based on spectrum parameters heat transfer model. Heat loss simulation results show that: the double-selective-coated receiver can reduce heat loss and boost the collecting efficiency significantly compared with PTR70 receiver. When the temperature of absorber is 500 °C, the double-selective-coated receiver can reduce heat loss by 157.8 W/m and increase the collecting efficiency from 64.7% to 68.1%. The System Advisor Model annual simulation results indicate that double-selective-coated receivers can decrease the levelized cost of electricity of concentrating solar plants by 2.78%–7.34%, and increase electricity production by 2.94%–8.21% compared with traditional PTR70 receivers.
•A novel parabolic trough receiver with double selective coating was proposed.•A heat transfer model was established and verified by NREL experimental data.•The cut-off wavelength and heat flux distribution of receivers were studied.•The heat loss and collecting efficiency of receivers were analyzed and compared.•The annual simulation was conducted to evaluate solar power plant performance.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Conventional parabolic trough solar receivers are widely used to harvest heat energy at temperatures ranging from 400 °C to 550 °C. However, high temperatures cause excessive heat loss in solar ...receivers. Two types of novel solar receivers with an inner metal radiation shield (RS), one with solar selective absorbing coating on the outer surface and one without, were proposed and constructed to improve the thermal performance of solar receivers. Experiments were conducted in an enthalpy difference lab, and mathematical models with spectral radiant distributions were established to predict the thermal performance of the solar receivers. A comparison between the simulated and experimental results showed satisfactory consistencies. Predictions were obtained using the models with the root mean square deviation of less than 6%. The novel solar receiver without solar selective absorbing coating on the outer surface of the RS showed superior performance at absorber temperatures exceeding 550 °C. At the absorber temperature of 600 °C, the percentage of heat loss reduction of the receiver with solar selective absorbing coating and of that without reached 23.4% and 24.2%, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•A spectral self-regulating solar receiver with Cr2O3/VO2/SiO2 coating is proposed.•Comprehensive heat transfer mathematical models are established and validated.•Heat loss is effectively reduced by ...18.2% at the absorber temperature of 600 °C.•Thermal efficiency is enhanced by 4.8% at the inlet temperature of 580 °C.•Great potential for the application in the solar-thermal utilization is harvested.
A significant decrease in the solar-thermal conversion efficiency of parabolic trough collectors occurs at high operating temperatures, mainly due to the massive radiant heat loss from the parabolic trough solar receiver incurred in this case. In this paper, a novel parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating is proposed to effectively reduce the radiant heat loss and improve the overall performance of solar receivers. The thermochromic layer, deposited on the glass envelope’s inner surface in the negative thermal-flux region, has a reversible transition from a monoclinic (M) phase to a rutile (R) phase at the critical temperature of 68 °C. The occurrence of the phase transition in the thermochromic coating induces beneficial self-regulation of the spectrum-selectivity characteristics of the glass envelope, i.e., transmittance, reflectance, and absorptance (emittance) of glass envelope to solar and infrared radiation, for enhancing the thermal performance of solar receivers. Comprehensive heat transfer models based on the finite volume and spectral radiant heat transfer methods are established in this study. It is validated the models can predict the thermal performance of the solar receivers with high accuracy. In this framework, the total heat loss, thermal efficiency, and exergy efficiency of parabolic trough collector systems integrated with the proposed novel solar receivers are investigated and analyzed. Besides, the effects of transmittance of thermochromic coating (M phase) and emittance of thermochromic coating (R phase), two key parameters, on the overall performance of proposed solar receivers are also studied. The results show that no matter under the M phase or R phase, the thermochromic coating can play unique advantages to improve the thermal performance of the proposed solar receivers. The heat loss and thermal efficiency of the proposed solar receivers are effectively reduced and enhanced by 18.2% and 4.8% at the absorber temperature of 600 °C and inlet fluid temperature of 580 °C, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•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%.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The heat pipe evacuated tube solar collectors, which is non-concentration, no-tracking and working at medium temperatures, suffer from low efficiency and thus rare application. The heat loss exerts ...serious negative influence on its performance at high operation temperatures. Thus, heat shields were introduced between the absorber plate and the glass tube to decrease the heat loss. Experimental and theoretical work were conducted to evaluate the thermal performance of the novel and traditional heat pipe evacuated tube solar collectors. According to the experimental results, the heat shields improved thermal efficiency at inlet water temperature of approximately 20 °C–150 °C, and the solar collector performed better at higher inlet water temperatures. The thermal efficiency of the novel one was enhanced by 11.8% at the inlet temperature and solar radiation of approximately 150 °C and 820 W/m2, respectively. Moreover, the novel one’s efficiency coefficients in the instantaneous efficiency curves, which can reflect the decrease rate of instantaneous thermal efficiency, resulted in decrease of 28.4% and 29.9%, respectively. Meanwhile, simulated results showed that, the thermal efficiency increments of novel collector over traditional one increased at weaker solar radiation and lower ambient temperature at each inlet water temperature.
•Novel collector tubes with heat shields introduced have been proposed.•Experimental and theoretical work were conducted of novel and traditional collectors.•The thermal efficiency was enhanced by up to 11.8% in experiment.•More improvement at lower values of solar radiation and water flow rate.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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.
•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%.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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