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Cool storage has been considering an efficient and cost-effective means to enhance the behavior of a refrigerator. However, its drawback in temperature control of the refrigerator compartment has not received enough attention. The controllable two-phase loop thermosyphon has the prospect of solving this problem. In this study, a novel cool-storage refrigerator integrated with a controllable two-phase loop thermosyphon is built to investigate its precise temperature control capacity. The phase change material is optimized by the orthogonal experiment method. The controllable two-phase loop thermosyphon presents the optimal cooling performance for the fresh food compartment when the filling ratio is 27.0%. As the average temperature of the fresh food compartment increases, the one on–off cycle of the controllable two-phase loop thermosyphon maintains at 32.0 min, and the operation ratio varies from 86.3% to 13.6%. The temperature control accuracy can be improved from 2.1 °C to 0.6 °C and the overall time of one on-off cycle decreases from 49.8 min to 22.4 min. The operation ratio gradually increases from 7.2% to 38.6% as the ambient temperatures increase. The results demonstrate that the controllable two-phase loop thermosyphon is a feasible and effective way to improve the temperature control performance of the cool-storage refrigerator.

The solar cells commonly adopted in the photovoltaic/thermal (PV/T) have negative temperature coefficients, leading to a significant decrement in electrical efficiency as cell’s temperature exceeds 80 °C, and thus the PV/T systems are mainly used at low-temperature applications. A new type of InGaN/GaN multiple quantum wells (InGaN/GaN MQWs) cells have attracted increasing interest in past few years. The cells have positive or near-zero temperature coefficients and are a promising option in medium-high temperature applications. It is the first time that InGaN/GaN MQWs cells are proposed in a PV/T system, and evacuated flat plate PV/T (EFP-PV/T) collectors are employed. A mathematical model is established to investigate the performance of EFP-PV/T collectors using two kinds of InGaN/GaN MQWs cells at high temperatures. Performance comparison with PV/T systems using conventional solar cells is also conducted. As the operating temperature increases to 150 °C, the efficiency of traditional cell-based PV/T collector decreases to 5.21%, while the efficiency of Type-1 InGaN/GaN MQWs has a minor drop from 4.34% to 4.16% and it increases to 2.07% for Type-2 InGaN/GaN MQWs. The characteristics of large absorption coefficient, radiation-hard, and superior thermal stability, and positive or near-zero temperature coefficients make InGaN/GaN MQWs cells suitable for use in high-temperature PV/T systems.

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.

The direct steam generation (DSG) solar power system using two stage accumulators and cascade steam-organic Rankine cycle (RC-ORC) has remarkably enlarged storage capacity. It can facilitate stable power generation and address the challenges of conventional DSG systems. Regenerator is generally an issue worthy of discussion in organic Rankine cycle (ORC) systems. However, its influence on the newly proposed DSG system has not been investigated yet and is expected to be appreciable. Introducing a regenerator affects not only the ORC efficiency, RC-ORC efficiency, heat exchanger area, but also heat storage capacity, discharge duration, discharge efficiency, aperture area of collectors and the net profit ( Δ P ). Detailed performance comparison between the DSG systems without/with regenerator is carried out in this paper. The results indicate that at a given power output, aperture area is reduced by the regenerator especially for MM, R365mfc and pentane due to the increment in ORC, RC-ORC and discharge efficiencies, as well as the decrement in heat input. Discharge duration is shortened by 0.01–1.78 h depending on ORC fluids. R365mfc exhibits the maximum Δ P (4.19 ∼ 6.48 million USD), followed by MM and pentane. On the contrary, Δ P is negative for benzene (−5.61 ∼ -4.31 million USD).

A direct numerical simulation has been performed to study instantaneous behavior in lead-bismuth eutectic flows past a vertical, backward-facing step. A turbulent forced convection case and two cases of mixed convection, the first buoyancy-aided flow at a Richardson number Ri of 0.1 and the second buoyancy-opposed flow at $$Ri=0.02$$ , are simulated and discussed. The Reynolds number based on the bulk velocity and step height is 4805. A uniform heat flux is imposed on the expansion wall behind the step. In the forced convection case, the numerical results reveal two characteristic unsteady flow phenomena. The first is vortex-shedding motion along the separating shear layer, while the second is wall-normal flapping of the shear layer. These unsteady motions have significant influences on the thermal field. The vortex-shedding motion induces some streak-like low-temperature structures on the heated wall, while the flapping motion induces oscillation of the maximum temperature on the wall. In the mixed convection cases, buoyancy alters the flow field substantially. The two unsteady flow phenomena noted above constitute motions inherent to backward-facing step flow. Buoyancy plays a material role in vortex development, affecting vortex ranges and time-scales. While the vortex shedding frequency is insensitive to buoyancy, the frequency of the flapping motion increases with the buoyancy. These results contribute to an improved understanding of separating and reattaching flows, especially in association with buoyancy and temperature fluctuations. The data serve to aid future development and validation of improved heat-flux modeling of low-Prandtl-number fluids.

Due to the structure of photovoltaic/thermal (PV/T) collectors, non-uniform distribution of solar irradiance and PV cell temperature is inevitable. In traditional models, uniform or 1D distribution of irradiance and temperature were adopted widely to analyze the output performance, which cannot obtain an accurate description on the dynamic behavior of the PV/T system. Thus, a novel 2D irradiance-temperature coupling model for the performance characterization of flat-plate PV/T system was proposed in this study. Results suggest that the non-uniform distribution of irradiance exerted a dramatic effect on photovoltaic efficiency but a modest influence on photothermic efficiency. As the ratio of frame shadow on PV cells changed from 0% to 80%, the photovoltaic efficiency decreased by 42.26% and the photothermic efficiency increased by 9.81%. By contrast, the flow rate had a stronger effect on the photothermic efficiency than the photovoltaic efficiency. As the flow rate increased from 0.005 kg/s to 0.03 kg/s (velocity changing from 0.0112 m/s to 0.0674 m/s in the copper tube), the photovoltaic efficiency increased from 9.46% to 9.54% and the photothermic efficiency improved from 32.62% to 35.83%. Moreover, a parallel electrical circuit layout has a greater total output with a value of 2594.99 MJ/m2.

The solar concentrator will cause the non-uniform illumination distribution, which will eventually have a profound impact on photovoltaic (PV) cells. For a crystalline silicon photovoltaic cell, the electrical efficiency with solar concentrator is usually lower than that with uniform illumination distribution. In fact, there is a coupling relationship between the solar concentrating illumination distribution and the PV cell. In this paper, a crystalline silicon photovoltaic cell under non-uniform illumination distribution is indicated, whose electrical efficiency is not negatively affected by the non-uniform illumination distribution. In order to verify that the concentrating PV has a high electrical efficiency, a comparison between the PV under non-uniform and uniform illumination distributions is presented. A series of performance indicators such as the surface voltage distribution, the current density distribution, and the PV conversion efficiency between them are compared. The results show that with the same total illumination intensity, the electrical output of this concentrating PV module with the non-uniform illumination distribution is not reduced, but is 3.4 mW higher than that of the PV cell under the uniform illumination profile, which will provide reference for the design of highly efficient solar concentrating PV.

This article introduces the results of a thermodynamic-economic-environmental analysis of conventional and integrated solar gas turbine trigeneration power plants based on parabolic trough collectors. The trigeneration plants are required to produce electricity with 90 MWe (from steam-turbines), 2500 kg/s of chilled water at 7 °C and 10 bars, and 34.8 kg/s of industrial process steam at 500 °C and 27.6 bars. The hourly and yearly performance of the considered plants with different gas turbine and solar field sizes have been examined and presented. In addition, a conceptual procedure to identify the optimal solar integration configuration has been developed and presented. Furthermore, the off-design behavior and regional potential of the optimally solar integration configuration have been assessed. The study reveals that the optimal configuration is the integration of 126 ha of parabolic trough collector’s solar field (46.2 ha of the total active aperture area) with the trigeneration plant of 130 MWe gas turbine size, which gives a levelled electricity cost of 5.75 USȻ/kWh with 114 k-tonne reduction of the annual CO2 emissions. Moreover, the study shows that the most proper location to utilize the solar hybrid power plants is in locations with high levels of solar irradiance and low ambient temperature.

Compared with conventional silicon solar cell, concentrating photovoltaic cell has a higher incident illumination intensity and non-uniform illumination intensity distribution. There is a close relationship between the illumination intensity and the resistance and shading losses of front metal fingers. In this paper, according to the non-uniformity of the illumination distribution formed by the concentrator, the effects of different finger numbers and spacing on the performance of low-concentrating photovoltaic cells were studied. First, low-concentrating photovoltaic cell modules with different finger numbers and spacing were established, and efficiency of these modules was compared to determine the optimal finger numbers and spacing of the front metal fingers. Then, effects of the shading losses, finger resistance and lateral spreading resistance on the performance of the low-concentrating photovoltaic cell were analyzed. Results show that for the specific illumination intensity distribution of the low-concentrating photovoltaic cell when the numbers and spacing of the front metal fingers are optimized, efficiency of the low-concentrating photovoltaic cell can be increased from 13.805% to 13.837%. Additionally, it can be concluded that the optimization of the number and spacing of the front metal fingers in the crystalline silicon solar cell is an efficient way to improve the electrical performance of the concentrating photovoltaic cell.

This article presents the results of a thermodynamic-economic-environmental analysis of integrating linear Fresnel reflector (LFR) with a tri-generation system. The optimal integrated solar field size has been identified and the pertinent reduction in CO2 emissions due to solar integration is estimated. For the considered tri-generation plant (that is required to produce 110 MWe of electricity from steam turbines, 45461 m3/day of freshwater and 2300 kg/s of chilled water), the study revealed that the optimal configuration is the integration of 83.6 hectares of LFR solar field with the tri-generation plant of 130 MWe, which gives a levelized electricity cost of 6.37 USȻ/kWh with 96.40 k-tonne reduction of the annual CO2 emission. The study also revealed that the integration of LFR technology with a conventional tri-generation system (TGS) in high insolation regions has more economic feasibility compared to equivalent TGS integrated with CO2 capturing technology while achieving the same emissions reduction result.

As an entirely passive cooling strategy, radiative cooling (RC) is an appealing approach to draw heat from the terrestrial surface to outer space. RC can be easily and effectively realized during the nighttime with the absence of sunlight, which coincidently mismatches the operation time of solar collectors. Therefore, it is of possibility and significance to unite the RC device and solar collector to extend the working period and function. In the present work, a novel combined photo-thermal and radiative cooling (PT-RC) module was proposed, experimentally and numerically investigated. By applying separated heating and cooling components, the PT-RC module demonstrated a daytime stagnation temperature of 159.8 °C and a nighttime one of 0.6 °C through a consecutive 24 h experimental duration. By contrast, a conventional photo-thermal (PT) module as a reference showed the highest panel temperature of 153.5 °C during the daytime and the lowest panel temperature of 7.9 °C in the nighttime. Further modeling was conducted to evaluate the thermal performance of the proposed PT-RC module compared with stand-alone PT module and spectrally coupled PT-RC module under different working conditions. This dual-function collector may contribute a potential solution for sustainable energy technology in Zero Energy Building applications.

The sinter cooling flue gas expelled from the end of an annular cooler was taken as the heat source of an organic Rankine cycle (ORC) system, and R123, R245fa, R600, R601 and R601a were selected as the working fluids of the ORC system. The effects of evaporation temperature and superheat degree of working fluid, as well as the pinch point temperature difference in the evaporator on the system thermal performance, were analyzed in detail. The results show that the system net output power and exergy efficiency for different working fluids first increase and then decrease with an increase in the evaporation temperature and decrease with an increase in the superheat degree and pinch point temperature difference. The change in pinch point temperature difference has no effect on the system thermal efficiency. For a given operational condition, the system thermal efficiency and exergy efficiency of R123 are the maximum, while the system total irreversible loss of R245fa is the maximum. When the evaporation temperature is greater than 110 °C, the system net output power of R600 is the maximum. The ORC system could obtain the maximum net output power and exergy efficiency through the adjustment of evaporation temperature of working fluid.

The compound parabolic concentrator photovoltaic (CPC-PV) cell module is composed of two symmetrical mirrors and a crystalline silicon solar cell. In this study, two new CPC-PV cells based on known CPC-PV cell are established by changing the location and number of bus-bar in crystalline silicon solar cell. The effect of different location and number of bus-bar on the electrical performance of CPC-PV cells was investigated. The numerical simulation of three CPC-PV cells is performed using a two-dimensional finite element model. The electrical performance of three CPC-PV cells are compared, and surface voltage distribution, current density and internal current flow of the crystalline silicon solar cell for each CPC-PV cell are analyzed. The results show that compared with the known CPC-PV cell with bus-bar placed on the right side of the crystalline silicon solar cell, the electrical efficiency of the CPC-PV cell when the bus-bar is placed in middle region of the crystalline silicon solar cell is relatively improved by 14.76%. Finally, it is concluded that the bus-bar optimization based on the non-uniform illumination intensity distribution formed by the concentrator will help to improve the electrical performance of the CPC-PV cell.

The parameter study of sinter waste heat recovery in vertical tank was conducted numerically by using energy and exergy analysis, and the experimental data obtained from a homemade experimental apparatus was applied to verify the reliability of numerical model. Based on the first and second laws of thermodynamics, the effects of flow rate of cooling air (FRCA) and inlet temperature of cooling air (ITCA), as well as the inner diameter of cooling section (IDCS) and height of cooling section (HCS), on the sinter cooling process were analyzed in detail. The results show that the average deviation between the experimental data and calculation values is 4.93%, and the model reliability is verified. The enthalpy exergy of outlet air tends to increase first and then decrease with increasing the FRCA and ITCA, while increasing the IDCS only leads to the increase in enthalpy exergy of outlet air. For a given operational condition, the enthalpy exergy of outlet air can reach a maximum value with increasing the HCS. The vertical tank could obtain the maximum enthalpy exergy of outlet air through the adjustments of FRCA and ITCA, as well as the HCS.

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.

The demands of cooling, heating and electricity in residential buildings are varied with seasons. This article presented a seasonal solar combined cooling heating and power (CCHP) system based on evacuated flat-plate collectors and organic Rankine cycle. The heat collected by evacuated flat-plate collectors is used to drive the organic Rankine cycle unit in spring, autumn and winter, and drive the double-effect lithium bromide absorption chiller in summer. The organic Rankine cycle condensation heat is used to yield hot water in spring and autumn, whereas supply heating in winter. The system thermodynamic performance was analyzed. The results show that the system thermal efficiency in spring, autumn and winter, ?sys,I, increases as organic Rankine cycle evaporation temperature, T6, and evacuated flat-plate collectors outlet temperature, T2, decrease. The maximum ?sys,I of 67.0% is achieved when T6 = 80?C and T2 =100?C. In summer, the system thermal efficiency, ?sys,II, increases first and then decreases with the increment of T2. The maximum ?sys,II of 69.9% is obtained at T2 =136?C. The system output performance in Beijing and Lanzhou is better than that in Hefei. The average output power, heating capacity, hot water and cooling capacity are 50-72 kWh per day, 989-1514 kWh per day, 49-57 ton per day and 1812-2311 kWh per day, respectively. The system exergy efficiency increases from 17.8-40.8% after integrating the organic Rankine cycle unit.

Working fluids can play a critical role in the working of an organic Rankine cycle system. A direct vapor generation solar organic Rankine cycle embedded with phase change material storage is analyzed in this study. The system comprised of an array of evacuated flat plate collectors, phase change material based thermal storage, expander, condenser, and organic working fluid pump. The storage tank model is modeled using a finite difference method in MATLAB programming environment while the 1D model of ORC system is used to evaluate the system performance. After a careful screen, 12 dry and isentropic working fluids were selected and their impact on the performance of the heat storage tank and the overall system is evaluated. The results show that the system efficiencies increase and decrease with the increment and decrement in the critical temperature of the working fluid. Moreover, the rise and fall of working fluid temperature, phase change material temperature, and the quantity of energy stored and released generally increase with an increase in the critical temperature of the working fluid. At the evaporation temperature of 10 °C higher and lower than the melting point temperature of the phase change material, Benzene has achieved the highest system efficiencies of 10.7% & 10.4% during charging and discharging mode, respectively. However, the maximum the rise and fall of working fluid temperature, phase change material temperature, and the quantity of energy stored and released during charging and discharging mode is attained by Heptane which is found to be 5.35 °C & 7.34 °C, 0.48 °C & 0.44 °C and 13.81 MJ & 23.04 MJ, respectively. Heptane has shown overall best performance among the selected working fluids and found to be feasible for phase change material storage based direct vapor generation solar ORC system.

Loop thermosyphon (LT) is a two-phase closed device that provides an alternative solution to overcome the freezing problem associated with the traditional photovoltaic/thermal (PV/T) system. The overall performance of a LT-PV/T system is significantly affected by volume-filling ratio. To reveal this effect, a prototype of the LT-PV/T system was designed and constructed in this paper; and long-term outdoor tests under 24%, 32%, 40% and 48% volume-filling ratios were carried out. The results show that the daily average photothermal performance of the LT-PV/T system presented a trend of increased first and then decreased with the volume-filling ratio increasing on the selected sample days. The highest performance was generated under the 32% filling condition. The solar evaporator had an even temperature distribution when the volume-filling ratio was no less than 32%; the temperature difference of the solar evaporator decreased with the volume-filling ratio increased, the values were 1.5 °C, 1.2 °C and 0.6 °C, respectively. Performances of the collector and system were linear fitted based on the outdoor tests, the fitting results showed that both of them shared the same variation trend, the optimum volume-filling ratio lied in between 32% and 40% of the whole system volume, but nearer to 32%.

For rooftop building-integrated photovoltaic (BIPV) technology, photovoltaic (PV) modules are typically mounted on the sunny side of a rooftop to receive a high amount of solar irradiance, whereas the opposite side of the rooftop will have free space. This study proposed a novel strategy for building-integrated PV and radiative cooling (RC) system, namely BIPV−RC system, by covering the sunny side of a rooftop with PV modules and its free side with all-day RC modules to integrate solar energy collection and RC utilization into a single building unit. A mathematical model was developed for the proposed BIPV−RC system and a case study was conducted in two tropical cities (Karachi, Pakistan and Riyadh, Saudi Arabia). Results show the total electricity and cooling output of the BIPV−RC system in Riyadh is 462.1 kWh·m −2 and 1315.3 MJ·m −2 , respectively, approximately 20.7% and 94.0% higher than those in Karachi. Moreover, a comparative study between the BIPV−RC system and the common BIPV and building-integrated RC (BIRC) system was performed and results indicate that the annual total energy output of the BIPV−RC system is nearly 79.1% and 16.8% higher than that of BIPV and BIRC system, respectively.

A novel hybrid parabolic trough power system that integrates direct steam generation and molten salt systems is proposed in this study to stably and efficiently operate solar power plants. The feedwater from the power block is preheated and evaporated in a low temperature direct steam generation solar field. A high-temperature solar field using molten salt as heat transfer fluid is employed to superheat and reheat the steam to 540 °C. The heat transfer model based on energy conversion and balance is applied to numerically investigate the overall performance of various power plants. This paper presents a comparative study of the effects of different locations, steam turbines, and system configurations on the outputs of the power plants. The hybrid systems exhibit favorable system stability and reliability and an excellent overall performance. The molten salt loop percentages of hybrid systems with and without reheat section are only 45.1% and 37.4%, which lead to low energy loss in the operation period and low molten salt freezing protection energy. The annual thermal energy productions of the hybrid systems with and without the reheat section in Tonopah are 685.1 and 691.7 GWh, and the percentages of energy required for freezing protection of those systems are 3.2% and 2.7%, respectively. The hybrid systems with reheat section exhibit optimal electricity production and economic performances compared with those of other configurations. The electricity production increments of novel systems in Tonopah and Lhasa are 14.0% and 14.8%, respectively, compared with those of molten salt systems.

Passive sub-ambient daytime radiative cooling has been demonstrated in previous studies by rigorous spectrum-tailored coolers with high solar reflection and strong thermal emission. In this work, a general strategy to change the relative position between the radiative coolers and the sun was proposed to decrease solar radiation reaching the coolers. In this way, passive sub-ambient daytime radiative cooling was well demonstrated on the basis of two commonly available coolers, coolers A and B. Cooler A was fabricated by attaching a 20 μm-thick polydimethylsiloxane film on a reflective metal substrate, and cooler B was a commercial reflective mirror. A field test was conducted for coolers A and B toward north sky in Hefei (117°E, 32°N), China. Results show that the average temperature drops below ambient temperatures of coolers A and B are 3.2 °C and 7.4 °C, respectively, which indicate that the proposed strategy can achieve passive sub-ambient daytime radiative cooling. Furthermore, the feasibility of the proposed strategy for daytime radiative cooling was also proved using a universal thermal model. This work provides a new insight into passive sub-ambient daytime radiative cooling and opens up a new avenue for common materials to potentially achieve sub-ambient daytime radiative cooling.

The loop thermosyphon with active heat transfer control capability is a promising solution for the temperature management of the fresh food compartment of cool-storage refrigerators or other precise temperature control devices. However, the frequently start-stop operating mode of the loop thermosyphon has not received enough attention, its active temperature management behaviours still requires further investigation. In this study, a controllable loop thermosyphon used in the temperature control of a fresh food compartment is developed and its start-stop is controlled by the temperature controller with the assistance of two magnetic valves. The performances of the controllable loop thermosyphon in long-term and start-stop operating modes are tested. The controllable loop thermosyphon with 35% filling ratio present highest heat transfer rate, reliable heat transfer capability, and good stability for the varying temperatures in the fresh food compartment. Based on this filling ratio, the heat transfer rate remains higher than 40 W during the long-term operating test. When the controllable loop thermosyphon operates in the start-stop mode, the start-stop cycle decreases from 39.4 min to 27.2 min and the operating ratio increases from 12.6% to 53.7% as the temperature of the fresh food compartment rises. Particularly, when the start-stop cycle reduces from 42.6 min to 18.1 min, the temperature control accuracy is improved from 2.0 °C to 0.4 °C. The results demonstrate that the controllable loop thermosyphon is promising as a novel precise temperature management device and is qualified for the use in cool-storage refrigerators.

Solar selective absorbing coatings that absorb solar irradiation and emit infrared radiation have a significant impact on the thermal efficiency of the receivers. The spectrum parameters heat transfer model and the non-ideal coating curve model are established. The cutoff wavelength of coatings is comprehensively optimized, and the effect of the optical properties of the coating on overall performance is analyzed. Results show that the optimal cutoff wavelength rises with the solar irradiation flux but decreases with increased absorber temperature. Sensitivity analysis results of the coating optical properties indicates that the thermal efficiency significantly decreases with increasing slope width. The change range of the thermal efficiency at the temperature of 200 °C is ±0.1%, resulting from a ±1 μm variation in slope width, whereas the range at 600 °C is ±6.5%. Spectral absorptivity analysis shows that the coating absorptivity has nearly same positive influence on thermal efficiency under different temperatures and irradiation fluxes, whereas emissivity analysis reveals an evidently different negative effect on the receiver performance. Finally, the annual optimal cutoff wavelength decreases with increasing temperature but increases with solar irradiation. The optimal cutoff wavelength decreases from 2.23 μm at 200 °C to 0.78 μm at 600 °C in Phoenix.

This paper presents the results of a thermo-economic analysis of integrating linear Fresnel reflector (LFR) with cooling, heat and power tri-generation plan (LFR-GTPP). The annual performance of the LFR-GTPP with different sizes of gas turbine and solar collector’s area have been examined and presented. Thermoflex + PEACE software were used for the economic-thermodynamic-environmental assessment of different integration configurations. For the considered tri-generation plant (produce 35.6 kg/s of steam (120.55 MW), 2500 kg/s of chilled water (24440 tons), and 90 MWe of steam turbines electricity), the study revealed that LFR-GTPP with gas turbine sizes in the range 130–190 MWe have more economic feasibility for integrating LFR. Furthermore, the study revealed that the integration of LFR system with a conventional gas turbine tri-generation power plant (GTPP) in locations with high solar radiation has more economic feasibility (with 7.6% reduction in LEC) compared to equivalent GTPP integrated with CO2 capturing technology while achieving the same CO2 emissions reduction. Moreover, a conceptual procedure to determine the optimal configurations of the LFR-GTPP has been developed and presented in this article. The results indicate that the proper location to apply optimal integration configuration is in regions with high levels of solar radiation and low ambient temperature.

Radiative cooling is treated as a promising clean alternative for cooling energy harvesting by dissipating heat to the sky mainly at night in a passive manner. Yet the cooling flux of a stand-alone radiative cooling system is far below a conventional vapor compression refrigeration system, which hinders its wider application. If the nocturnal radiative cooling process acts as an auxiliary function of the solar collector, the aforementioned limitation of radiative cooling technique can be addressed. Therefore, a dual-functional collector which can offer heat in daytime and cooling energy in nighttime was proposed in this study. This hybrid photo-thermal and radiative cooling collector used the cheap and accessible black acrylic paint as the panel coating. A photo-thermal and radiative cooling prototype was manufactured and investigated experimentally in heating and cooling modes. Results showed that the thermal efficiency at zero-reduced temperature and cooling efficiency at zero-dimensionless temperature difference were 63.0% and 58.3%, respectively under clear sky condition. Besides, the collector achieved a net radiative cooling flux of 55.1 W/m2 in a clear night. Moreover, the hybrid system gathered a daily heat of 8.64 MJ during a consecutive 8 h, and a nightly cooling energy of 0.99 MJ in a consecutive 11.5 h.

Efficiency improvement of household refrigerators exerts extensive influences on the reduction in residential energy consumption and alleviation of environmental pollution. Controllable loop thermosyphons (CLTs) are low-cost, high-precision heat transfer control devices that can be potentially applied in refrigerators as crucial energy saving and temperature management components. A novel refrigerator with CLTs is proposed in this study. The evaporator in fresh food compartment is removed, the cold loss of the topside freezer is partly reused by the CLTs to cool the fresh food compartment in intermittent mode, and the overall cold loss of the refrigerator is efficiently reduced. The energy saving behaviour of the refrigerator is numerically studied based on tested temperature boundary conditions. The energy saving mechanism is comparatively analyzed, and the impacts of the operating condition, installation position of the CLTs, and ambient temperature are investigated. The energy saving ratio of the novel refrigerator increases from 12.9% to 19.5% when the ambient temperature rises from 17.0 °C to 31.0 °C, the operating condition varies from 1 to 4, and the locations of CLTs from the inner wall of the freezer are changed from 2.1 to 4.9 mm. Results preliminarily prove the energy saving capability of the novel refrigerator.

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.

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.

A novel concept of energy harvesting method (PV-RC) based on the conventional photovoltaic (PV) panel, combining diurnal PV conversion and nocturnal radiative cooling (RC) method, was developed to generate electricity and obtain cooling energy. A PV-RC hybrid system was also designed, manufactured, and spectral characterized, which exhibit high solar absorption in PV conversion band (0.3–1.1 μm) and has strong thermal emission within thermal radiation band (4–25 μm). Experimental studies of the hybrid system were carried out in Hefei (117° E, 32° N), China. The measured results demonstrate that the hybrid system has an average electricity output of 75.7 W m−2 with an average electrical efficiency of 12.4% for diurnal PV conversion and exhibits an equilibrium temperature reduction of 12.7 °C below ambient temperature for nocturnal RC process. Parametric studies were also conducted by a validated simulation model, which discuss the effects of different meteorological parameters on the performance of the system. The simulated results imply that dry regions are more suitable for nocturnal RC mechanism.

In the present work, the expressions of volume exergy transfer (VET) coefficient and Nusselt number in vertical moving bed are derived by applying the non-equilibrium thermodynamics theory to the second law analysis. A steady state calculation model is established to investigate the exergy transfer performance of sinter cooling process in moving bed, and the verification of calculation model is performed by using the experimental data. The effects of gas inlet parameters, namely air mass flow rate and air inlet temperature on the exergy transfer performance in vertical tank are analyzed numerically. The results show that the pressure VET coefficient tends to increase first and then decrease with increasing the bed layer height. With the increase of air mass flow rate, the temperature VET coefficient decreases when the bed layer height is less than 5.5 m, while the VET coefficient also decreases when the bed layer height is less than 6.5 m, and vice versa. The increase of air inlet temperature only leads to the increase of VET coefficient. The VET Nusselt number decreases as the Reynolds number increases, and would be less than zero in the higher Reynolds number region, which is pointless for intensifying the sinter cooling process.

High collecting temperature in parabolic trough collectors (PTCs) induces considerable radiative heat loss of solar receivers, which causes significant negative effects on heat-collecting efficiency. Structural optimized solar receivers with inner radiation shield achieved superior thermal performance for reducing heat loss. Based on widely commercial EuroTrough and PTR70 solar receivers, the optimized solar receivers are numerically applied to a small thermal-collection field with 72 m loop using molten salt as heat transfer fluid to validate their enhanced overall performance. Mathematical models relying on spectrum parameter calculation and working fluid volume unit method are established to simulate the energetic and exergetic performances of the solar receivers. The influence of solar irradiance on parabolic trough collector system is studied, and all-day system efficiencies in different areas in China are investigated to validate the performance of the proposed solar receivers in real condition. Results show that the PTCs with novel solar receivers exhibit outstanding energetic and exergetic performances compared with conventional receivers. The heat loss reduction percentage of the novel receivers reaches approximately 24.0% when the absorber temperature is 600 °C. The heat-collecting efficiency and exergetic efficiency are effectively raised by 7.1% and 4.7%, respectively, at an inlet temperature of 580 °C.

An innovative solar thermal power generation system using cascade steam-organic Rankine cycle (SORC) and two-stage accumulators has recently been proposed. This system offers a significantly higher heat storage capacity than conventional direct steam generation (DSG) solar power plants. The steam condensation temperature ( T 2 ) in the proposed system is a crucial parameter because it affects the SORC efficiency ( η SORC ) in normal operations and the power conversion of the bottoming organic Rankine cycle (ORC) in the unique heat discharge process. The present study develops a methodology for the design of T 2 with respect to a new indicator, that is, the equivalent heat-to-power efficiency ( η eq ). η eq is a compromise between the efficiencies in different operation modes. The effects of main steam temperature ( T 1 ), Baumann factor (a), mass of storage water ( M w ), and ORC working fluid on T 2 are investigated. Results show that η eq is a better indicator than η SORC . The optimum steam condensation temperature ( T 2 , o p t ) that corresponds to the maximum η eq ( η eq , m a x ) is generally higher than that based on the maximum η SORC . T 2 , o p t reduces as T 1 , a, and M w decrease. η eq , m a x rises with the increment of T 1 and the decrement of a and M w . Pentane is a more preferable ORC fluid than benzene and R245fa. The T 2 , o p t and η eq , m a x of pentane are, respectively, 139–190 °C and 20.93%-24.24%, provided that T 1 ranges between 250 °C and 270 °C, a varies from 0.5 to 1.5, and M w changes from 500 ton to 1500 ton.

Radiative cooling is a natural phenomenon in which earthbound objects emit energy into outer space through thermal radiation. Emitters should have high spectral reflectivity in the solar radiation band and high spectral absorptivity (emissivity) in the infrared band, especially the atmospheric window band (i.e., 0.3–3 and 8–13 µm), to efficiently utilize radiative cooling. In this work, we proposed two types of surfaces to achieve such spectral properties based on the analysis of selective and broadband radiator cooling performance. One type is a specular surface made by ultra-white glass-plated silver (Ag). The other type is a diffuse surface made by spraying zinc phosphate sodium (NaZnPO4) particles on aluminum (Al) substrate. Sample tests showed that both surfaces can effectively reflect sunlight and strongly exchange heat with outer space via thermal radiation. A radiative cooling apparatus with simple structure was built, and the equilibrium temperatures of the surfaces were tested. Both surfaces achieved consecutive 24 h sub-ambient radiative cooling effect. The equilibrium temperatures of the specular and diffuse surfaces at nighttime were lower than those of ambient air by approximately 9.8 °C and 7.3 °C, respectively. When solar irradiance exceeded 430 W m−2 at noontime, the two surfaces reached equilibrium temperatures of 2.5 °C and 1.5 °C lower than those of the surroundings. Both surfaces showed daytime radiative cooling characteristics and were superior to existing radiative coolers in terms of cost and fabrication.

As a passive, effective, and renewable way of decreasing cooling energy requirements without power input, radiative cooling has attracted considerable attention in the field of energy-saving applications. Historically, radiative cooling was limited at nighttime because radiators with strong thermal radiation lack high reflectivity in the solar radiation band. With the recent technological advancements in radiators, such as the development of photonic radiators and metamaterials, the advantages of diurnal radiative cooling has been demonstrated. In this paper, the current state of the art in passive radiative cooling technology is reviewed and updated. First, the fundamental principles of radiative cooling, which comprise different mathematical and physical descriptions, are introduced. Then, the advanced materials and structures of various radiators, which are popular topics in radiative cooling, are presented. Furthermore, application developments in radiative cooling are also summarized and its prospects are preliminarily analyzed. This study provides a detailed introduction and analysis of radiative cooling technology, thereby serving as a key reference for promoting the development of radiative cooling utilization.

The experimental investigations on the pressure drop behavior of air flow in packed bed with sinter particles were performed for different air superficial velocity and particle diameters. Based on a homemade gas flow experimental platform, the pressure drop of air flow through sinter bed layer was measured under different experimental cases. Considering the influence of wall effect on the pressure drop behavior, the experimental correlation of air flow pressure drop through sinter bed layer was redefined through the experimental data fitting, and the applicability of experimental correlation was also checked. The results show that, the increase in reduced pressure drop, ΔP/Hu, shows a first power relationship with the air superficial velocity for a given particle diameter. The curve slope of frictional pressure drop of wall correction, fw, is basically unchanged with the increase of particle Reynolds number of wall correction, Rew, and the frictional pressure drop of wall correction also increases as a first power relationship with the particle Reynolds number of wall correction. In addition, the modified viscous and inertia coefficients, k1 and k2, both decrease as an exponential relationship with the increase of bed geometric factor, D/dp. The redefined experimental correlation of air flow pressure drop in sinter bed layer predicts the whole experimental data best, and the mean deviation of redefined correlation is 4.44%.

Controllable loop thermosyphon (CLT) can be used as a significant temperature management component in solar- and electric-powered cool-storage refrigerators. However, the behaviours of CLT with non-condensable gas (NCG) under active control require further investigation. In this study, air is mixed in working fluid R134a as NCG to evaluate the steady-state and start-stop performances of CLT for selected control modes. CLT with 0–0.62% NCG is tested. The larger the amount of NCG is, the lower the heat transfer rate is. When the mass ratio of NCG reaches 0.62%, the steady-state heat transfer rate varies from 245.0 W to the minimum 118.6 W for different heat sink temperatures. This finding means that CLT loses efficacy due to the excess NCG. In addition, the start-up performances of the two modes decrease as the mass ratio of NCG increases to 0.31% and become entirely unacceptable when NCG reaches 0.47%. By contrast, the stopping time of CLT remains less than 100 s in various conditions. Results indicate that the mass ratio of NCG should be less than 0.47%, and CLT with NCG is suggested to be controlled by the valve in the vapour line.

Phase change materials used for the storage of thermal energy can play a critical role in the efficient use and conservation of solar energy. The effect of the different types of phase change materials on the thermodynamic performance of a direct vapor generation solar organic Rankine cycle system is evaluated in this study. The system consists of an array of evacuated flat plate collectors, phase change material based thermal storage, expander, condenser, and organic fluid pump. The thermodynamic cycle model of the ORC system is integrated with phase change material heat storage tank that is modeled using the finite difference method in MATLAB. The thermodynamic performance of the system is analyzed by using 12 different phase change materials. Effect of phase change materials on the thermodynamic performance of organic Rankine cycle including the net power output, rise and fall in the working fluid temperature, and on the amount of energy stored and released are evaluated and compared for charging and discharging mode. The results indicate that MgCl2·6H2O has shown the highest overall system’s efficiency. However, KNO2–NaNO3 and Acetamide have resulted in maximum ORC and collector efficiency, respectively. Moreover, Acetamide, KNO2–NaNO3 and Mg(NO3)2·6H2O have shown maximum rise and fall in organic fluid temperature, maximum net power and maximum amount of energy stored and released during charging and discharging mode. Salt hydrates have shown overall better performance among the selected PCMs in terms of overall system efficiencies and the amount of energy stored and released.

This study presents a scroll expander modelling methodology for small scale power generation systems by combining scroll geometry and semi-empirical model. Although the semi-empirical model is quite popular, its dependence on several experimentally-determined scroll geometrical and operational parameters makes this approach inflexible for different capacities and operating conditions. Some studies have sought to improve its flexibility in terms of using different working fluids and more accurate empirical parameters, however, those improved models still depend on a considerable number of experimentally-obtained scroll parameters. Therefore, in this study, a practical methodology for a simpler semi-empirical model combined with the operational flexibility of the scroll geometry is presented. Firstly, the flow rates of mainstream and leakage flows are analysed, where a correlation between scroll clearance and pressure ratio is determined. Secondly, a simpler approach to the semi-empirical model of scroll expander is proposed, whereby dependent parameters have been reduced to two parameters by using scroll geometrical calculations. The model is further improved to predict the rotational speed and electricity output by considering the overall friction coefficient of the coupled expander-generator unit. The findings are then compared with the results of an experimental study. The results show that the effective clearance values between scrolls vary according to pressure ratios, increasing from 20 μ m to 34 μ m . Mass flow rate can be predicted within 10% deviation from the experimental results for the same inlet conditions and rotational speed at a transient state. Additionally, considering steady state conditions, modelling results show that the rotational speed and electricity output can also be predicted within 8% and 7.5% of deviation, respectively.

Solar-to-thermal conversion efficiency of the parabolic trough collector significantly degrades at high operating temperatures, which exerts seriously negative effects on the development of parabolic trough collectors. To solve this knotty problem, based on the theory of the negative thermal-flux region, a novel parabolic trough solar receiver with a radiation shield was proposed, manufactured, and tested. In this framework, the proposed solar receiver is employed in the concentrated solar power plants using solar salt as the heat transfer fluid to analyze its feasibility in real solar power plants. In this study, the mathematical models of heat collection and economic assessment are established, and the simulation results yield a good agreement with the experimental data. The techno-economic performances of the solar power plants installing the proposed solar receivers in three typical areas under different installed capacities and thermal storage capacities are comprehensively investigated. The results demonstrate that the proposed solar receiver has a great potential for significant enhancement of the techno-economic performance of the solar power plant. The solar power plant with the proposed solar receivers located in Dunhuang can effectively improve the annual net electrical energy production by 9.77%, reduce the levelized cost of energy by 8.67%.

The efficiency improvement of household refrigerators is of significance to electricity consumption reduction of residential buildings. The cold loss recovery in household refrigerators is a promising development direction. In this study, a refrigerator improved by heat pipes is designed to reduce the cold loss at freezer walls to enhance the overall efficiency of the refrigerator. A complete steady-state mathematical model is built for characterizing its overall energy saving behaviors by introducing the real structural parameters of a Midea BCD-111 refrigerator and measured temperature boundary conditions. The performances of the improved refrigerator are investigated with typical operating states, variable ambient temperature, and heat pipe design. The simulative results indicate that the cold loss of the freezer of the improved refrigerator can be reduced by 8.3&ndash, 16.5%, and the energy saving capability is relatively reliable. Results verify that the cold loss recovery of the improved refrigerator is feasible in various operating conditions.

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 &mu, m, 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 &deg, 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.

Several passive cooling design techniques are known for reducing solar heat gain through building envelope in summer season. These include the use of phase change materials (PCM), which has received an increased attention over the last years, and the strategy of increasing the above-sheathing ventilation (ASV) in ventilated roofs. However, few studies combine both technologies to maximise the building resilience in hot season. The effect of including a PCM layer into a ventilated roof is numerically analysed here in two different configurations: firstly, laid on the roof deck (PCM1 case) and, secondly, suspended in the middle of the ASV channel (PCM2 case). A computational fluid dynamics model was implemented to simulate airflow and heat transfer around and through the building envelope, under 3 days of extreme hot conditions in summer with high temperatures and low wind speed. Results showed slight differences in terms of mean temperatures at the different roof layers, although temperature fluctuations at deck in the PCM1 case were smaller than half of those estimated for the benchmark case. However, PCM2 configuration achieved a daily reduction of about 10 Wh/m2 (18%) in building energy load with respect to the benchmark case, whilst PCM1 got only 4% due to the lower ventilation at night time. Therefore, a suspended PCM layer in the ASV channel would be a better measure in terms of energy performance than laid on the deck surface, although this last option significantly decreases thermal stress of the insulation layer.

Radiative cooling (RC) is a passive and green cooling technique dissipating heat to outer space, which is a natural heat sink. Yet most available radiative coolers are encountered with the challenges of low power density and daytime operating ability. In the meantime, solar photovoltaic/thermal (PV/T) collectors cannot work at night. To overcome this limitation, the present work proposed, designed and manufactured a practical-scale photovoltaic-photothermic-radiative cooling (PV-PT-RC) collector. The hybrid PV-PT-RC collector is capable of generating electricity and heat during the day and providing cooling energy at night. Subsequently, to study the performance of the collector, we developed an outdoor experimental system and performed experiments at various operation modes. As results suggest, the average electrical efficiency of the collector around noon was 10.3% and showed thermal efficiency at zero-reduced temperature of 55.3% in a diurnal collector mode test. The net RC powers of the collector in a clear night and overcast night reached 72.0 and 30.8 W/m2, respectively. Multi-day daily system mode tests were performed, and the results suggest that the overall electrical/thermal efficiency of the PV-PT-RC system was ranged from 40.4% to 56.9%. The overall cooling energy gain of 2.90 MJ was obtained by the system at a typical clear night. The tri-functional collector is expected to provide indispensable electricity, heat as well as cooling energy in building and agriculture fields.

Neither solar collectors nor nocturnal radiative cooling units could work all day. A novel combined diurnal solar heating and nocturnal radiative cooling (SH-RC) collector was proposed and investigated in this study. The collector can obtain heat at daytime and gather cooling energy at nighttime, therefore offering multi-functionality and enhancing its time utilization ratio. A spatial distributed parameter mathematic model that considers the spectral radiant distribution was established to evaluate the performance of the collector. Besides, the precipitable water vapor amount was introduced to predict the spectral emissivity of atmosphere. Experiments were conducted to validate results obtained from the numerical simulation. The root-mean-square deviations between the experimental and simulation results are only 4.59% for thermal efficiency and 4.90% for cooling power. Based on the validated models, the thermal performance of the collector was investigated under different insulation thicknesses, wind velocities, ambient and inlet temperatures, water flow rates, precipitable water vapor amounts and solar irradiance. Annual performances of four Chinese cities were also conducted. The annual heat gains of the SH-RC collector in Beijing, Hefei, Fuzhou and Urumqi are 3328.76, 2423.53, 2543.31 and 3313.46 MJ/m2, respectively. The annual net radiative cooling gains of the collector in the four cities are 862.44, 831.54, 741.63 and 775.34 MJ/m2, respectively.

Given the disadvantaged seasonal adaptability of solar air heaters and radiative air coolers, this study proposed an air-based integrated solar heating (SH) and radiative cooling (RC) collector (SH–RC collector). Such dual-function collector is capable of obtaining heat in SH mode and gaining cooling energy in RC mode. Accordingly, an air-based SH–RC collector can offer hot air during cold and harvest seasons and provide cool air during hot seasons. A spectrally selective plate termed as TPET collecting plate was trial-manufactured, serving as the panel of the air-based SH–RC collector. Also, this study developed a mathematical model and investigated the SH and RC performances of the proposed collector on a fine summer day. As the results suggest, the average thermal efficiency of the collector in SH mode reaches 45.88%, and the average cooling power of the collector is 36.61 W/m2. Parametric studies on the effect of various ambient temperatures, inlet air temperatures, air flow rates, relative humidity values, and wind velocities on the SH and RC performances of the collector have been conducted. In addition, the monthly energy gains of the air-based SH–RC collector were calculated. The annual heat and cooling gains of the SH–RC collector reached 2328.45 and 980.43 MJ, respectively.

Recent research on cooling solar cells through the radiative cooling method has elicited much interest. Compared with bare silicon solar cells, modified cells with enhanced radiative cooling can reduce temperature by over 10 °C, thereby improving the photovoltaic conversion efficiency of solar cells. At present, the commercial silicon photovoltaic (PV) module is the mainstream product for PV application. Thus, investigating the effect of enhanced radiative cooling on cooling solar cells based on a commercial PV module is imperative. In this study, the effect of enhanced radiative cooling on solar cells based on a commercial PV module was explored, and the application prospect was analyzed preliminarily. A sample of PV module (referred to as “commercial structure” hereinafter) was fabricated by encapsulating a bare crystalline silicon cell with glass. The spectral property of the commercial structure was modified by adding a polydimethylsiloxane (PDMS) film on its top surface; this structure is referred to as “modified structure” hereinafter. A comparative experiment was conducted, and results showed that the solar cell temperatures in the commercial and modified structures were almost consistent. The effects of enhanced radiative cooling on the PV module under different conditions were also analyzed theoretically through a universal mathematical model. Simulation results revealed that the solar cell temperature could only be reduced by 1.75 K even in the ideal case. Experimental and simulation results indicated that enhanced radiative cooling based on a PV module shows no specific potential for cooling solar cells in actual settings. The performance of enhanced radiative cooling for solar cells in an extraterrestrial environment was also discussed preliminarily, and the results showed that radiative cooling can be applied as an alternative method for the thermal management of solar cells in the extraterrestrial environment.

Solar evacuated receiver as a key part of parabolic trough collector (PTC) suffers considerable heat loss at high operating temperature, which exerts significantly negative effects on the overall performance of PTC system. Based on the fact of maldistributed solar irradiation around the inner absorber tube, a novel solar evacuated receiver with an inner transparent radiation shield (TRS) is proposed and designed. The heat loss of the proposed solar evacuated receiver is numerically studied by the established heat transfer model based on the spectral parameters. The heat-collecting efficiency of a commercial UrssaTrough solar collector installed with PTR 70 receivers using therminol VP-1 as heat transfer fluid is investigated to validate the performance of the proposed solar receiver. Moreover, the influences of the property parameters of films on the two sides of the TRS on the solar receiver are also studied. Comparisons between simulated and experimental results show the differences of their heat-collecting efficiencies are lower than 1%, which demonstrates that the model can yield satisfactory consistency with the experimental results. The simulation results show that the novel receiver exhibits dramatically superior thermal performance to that of the traditional receiver. The heat loss reduction percentages of the novel receiver can reach approximately 15.7% and 14.9% when the absorber temperatures are 400 °C and 600 °C, and the thermal efficiency can be enhanced by 0.93% and 4.42% at inlet temperatures of 400 °C and 580 °C, respectively.

A spectral splitting photovoltaic-methane-steam-reforming hybrid system for heat and power cogeneration has been proposed. In the system, sunlight with wavelengths shorter than 870 nm is assigned to photovoltaic cells for direct power and heat cogeneration, while the rest of the solar spectrum is utilized by a methane-steam-reforming reactor via the route of solar thermal energy – chemical energy – heat and power. Analysis reveals that the hybrid system is characterized by high net solar-to-exergy efficiency (40%), high energy storage capability and low-emission energy supply (63% fossil energy saving and 77% carbon-dioxide emission reduction). Primary reasons underlying the excellent performance are full-spectrum optimized utilization of solar energy and comprehensive utilization among solar energy, thermal energy and chemical energy.

The spectral selectivity of solar selective absorbing coatings enhances coating performance in diurnal heating collection but also limits the potential application of these materials in nocturnal radiative cooling. A radiative cooling surface shows poor solar heating performance due to the same reason. The present study proposed a novel surface that combines solar heating and radiative cooling (SH-RC) considering the spectral selectivity of photo-thermic conversion and radiative cooling. A hypothetical SH-RC surface was also proposed. This hypothetical surface had an absorptivity of 0.92 in the solar radiation band, emissivity of 0.70 in the “atmospheric window” band, and absorptivity (emissivity) of 0.05 in other bands. The thermal performance of this spectrally selective SH-RC surface (SH-RCs surface) was numerically investigated by comparing it with three surfaces, namely, solar selective absorbing coating surface (SH surface), spectrally selective radiative cooling surface (RC surface), and spectrally non-selective black surface (SH-RCblack surface). Results indicated that the SH-RCs surface is most suitable for achieving integrated SH and RC. In a typical summer day, the heat gains of the SH, RC, SH-RCblack, and SH-RCs surfaces are 17.14, 0, 15.57, and 13.22 MJ/m2, respectively. The cooling energy gains of the four surfaces are 0, 1.02, 0.95, and 1.01 MJ/m2, respectively.

Shallow ground heat exchangers are increasingly studied due to their advantages in cost and long-term energy performance stability when coupled with heat pumps for space heating and cooling. As for borehole heat exchangers, the backfilling material affects significantly the operating efficiency of the whole system, mainly driven by the low thermal diffusivity of the soil. To enhance the heat transfer, the mixing of the backfilling material with phase change materials (PCMs) is a novel strategy still partially investigated, especially with regards of the heat pump on/off cycling. This study presents the results of experimental tests carried out at lab-scale to analyse the performance of a shallow Flat-Panel ground heat exchanger (FGHE) coupled with water-sand mixture. Firstly, the comparison between FGHEs coupled with dry sand and water-sand mixture is performed; then, the impact of latent heat resulting from freezing is further studied in three on/off operating modes. A maximum of 31.6% increment in heat transfer efficiency is observed in wet conditions and for the highest on/off frequency. Therefore, coupling FGHE with water-sand mixture enhances the heat transfer, especially in icing interval and when combined with a suitable on/off operating frequency.