• Energy Research

A solar air collector (SAC) is a main device of a solar-thermal air system, which can absorb solar radiation and transfer the absorbed thermal energy to the air. This paper presents a systematic review of three basic types of SAC, namely, the flat-plate SAC (FPSAC), the evacuated tube SAC (EVTSAC), and the concentrated SAC. High efficiency, low heat loss and simple structure have always been the goals of SACs development and research. Enhancement methods applied in the SACs, including enhanced-surface technology, absorber materials, airflow reorganization, internal heat transfer component and integrated systems of energy storage and PV/T, are reviewed. Performance comparisons among the different SACs of each type are conducted. The analysis indicates that the FPSACs, with the groove absorber and half conical, enable a performance evaluation criterion of about 2. An evacuated tube SAC with micro-heat pipe arrays can provide a thermal efficiency up to 73 %. Among the concentrated SACs, the linear concentrating SAC with an open Brayton cycle can achieve a highest outlettemperature of 350 ◦C, corresponding to 75% thermal efficiency. It is recommended to consider the use of high-efficiency heat transfer component in the optimal design of SACs. Some suggestions for future research are proposed.

A solar air collector (SAC) is a main device of a solar-thermal air system, which can absorb solar radiation and transfer the absorbed thermal energy to the air. This paper presents a systematic review of three basic types of SAC, namely, the flat-plate SAC (FPSAC), the evacuated tube SAC (EVTSAC), and the concentrated SAC. High efficiency, low heat loss and simple structure have always been the goals of SACs development and research. Enhancement methods applied in the SACs, including enhanced-surface technology, absorber materials, airflow reorganization, internal heat transfer component and integrated systems of energy storage and PV/T, are reviewed. Performance comparisons among the different SACs of each type are conducted. The analysis indicates that the FPSACs, with the groove absorber and half conical, enable a performance evaluation criterion of about 2. An evacuated tube SAC with micro-heat pipe arrays can provide a thermal efficiency up to 73 %. Among the concentrated SACs, the linear concentrating SAC with an open Brayton cycle can achieve a highest outlettemperature of 350 ◦C, corresponding to 75% thermal efficiency. It is recommended to consider the use of high-efficiency heat transfer component in the optimal design of SACs. Some suggestions for future research are proposed.

Abstract This study investigated the performance of an integrated collector storage solar air heater based on latent heat storage and flat micro-heat pipe arrays. The structure and working principle of the device were explained in detail. The effective area of the collector is 0.93 m 2 , and the mass of the phase change material (PCM) is 45.8 kg. An integrated collector storage experimental system was set up, and outdoor experiments were conducted to demonstrate the feasibility of the device for energy storage and air heating. The temperature distribution of the PCM was recorded during charging and discharging to monitor the propagation of the melting and freezing front. Efficiency and power during charging and discharging were also calculated. The integrative device exhibited 59% and 91.6% efficiencies for solar air charging and discharging at 393 and 344 W power, respectively.

Abstract This study investigated the performance of an integrated collector storage solar air heater based on latent heat storage and flat micro-heat pipe arrays. The structure and working principle of the device were explained in detail. The effective area of the collector is 0.93 m 2 , and the mass of the phase change material (PCM) is 45.8 kg. An integrated collector storage experimental system was set up, and outdoor experiments were conducted to demonstrate the feasibility of the device for energy storage and air heating. The temperature distribution of the PCM was recorded during charging and discharging to monitor the propagation of the melting and freezing front. Efficiency and power during charging and discharging were also calculated. The integrative device exhibited 59% and 91.6% efficiencies for solar air charging and discharging at 393 and 344 W power, respectively.

The pursuit of sustainable district heating solutions has driven a growing interest in ultra-low temperature district heating (ULTDH) systems, where booster heat pumps (BHPs) play a pivotal role despite challenges posed by their efficiency limitations under large temperature glide conditions. This paper investigates the potential of drop-in R-1234yf/R-32 zeotropic mixtures in BHPs compared to a baseline R-134a system, within the context of a ULTDH framework. This study focused on the viability of the mixtures of R-1234yf/R-32 with the composition ratio of 80 %/20 % and 90 %/10 %. The investigation reveals disparities in compressor efficiency and heat exchanger pressure drop at the component level. Device-level analysis unveils increased COP for R-1234yf/R-32 mixtures, alongside with maximum second-law efficiencies reaching 0.32. A remarkable enhancement in heating capacity up to 58 % was found. System-level analysis demonstrated exergetic efficiencies and identified preferable district heating temperatures. Exergetic efficiencies of 0.47, 0.55, and 0.59 were achieved for domestic hot water preparation at district heating supply temperatures of 30 °C, 35 °C, and 40 °C, with a subsequent shift in optimal district heating temperatures as central heating station efficiency decreased. Temperature profile analysis underscored challenges stemming from excessive subcooling, highlighting the need for configuration refinements.

The pursuit of sustainable district heating solutions has driven a growing interest in ultra-low temperature district heating (ULTDH) systems, where booster heat pumps (BHPs) play a pivotal role despite challenges posed by their efficiency limitations under large temperature glide conditions. This paper investigates the potential of drop-in R-1234yf/R-32 zeotropic mixtures in BHPs compared to a baseline R-134a system, within the context of a ULTDH framework. This study focused on the viability of the mixtures of R-1234yf/R-32 with the composition ratio of 80 %/20 % and 90 %/10 %. The investigation reveals disparities in compressor efficiency and heat exchanger pressure drop at the component level. Device-level analysis unveils increased COP for R-1234yf/R-32 mixtures, alongside with maximum second-law efficiencies reaching 0.32. A remarkable enhancement in heating capacity up to 58 % was found. System-level analysis demonstrated exergetic efficiencies and identified preferable district heating temperatures. Exergetic efficiencies of 0.47, 0.55, and 0.59 were achieved for domestic hot water preparation at district heating supply temperatures of 30 °C, 35 °C, and 40 °C, with a subsequent shift in optimal district heating temperatures as central heating station efficiency decreased. Temperature profile analysis underscored challenges stemming from excessive subcooling, highlighting the need for configuration refinements.

Latent heat storage technology is a valuable research direction to alleviate the imbalance of energy supply and demand, but its application is limited by the low thermal conductivity of phase change material. In recent years, the fin–foam structure is proven to alleviate this problem. The solidification behavior of the phase change material in this structure should be predicted to promote its engineering application. In this study, a cold thermal energy storage unit with metal foam and straight fins was constructed. On the basis of dimensionless analysis, experimental and numerical methods were used to investigate the structural parameters of straight fin and metal foam on the liquid fraction and effective Nusselt number(Nu∗). Results showed that the height and spacing of fin are the main factors affecting the solidification rate of phase change material, compared with the porosity, pore density and material of metal foam. When fin height increases from 17 to 57 mm and spacing reduces from 120 to 3 mm, the Nu∗ of the phase change material increases by 1.5 and 12.61 times, respectively. The transient correlation between liquid fraction and Nu∗ of phase change materials is obtained to guide related design.

Latent heat storage technology is a valuable research direction to alleviate the imbalance of energy supply and demand, but its application is limited by the low thermal conductivity of phase change material. In recent years, the fin–foam structure is proven to alleviate this problem. The solidification behavior of the phase change material in this structure should be predicted to promote its engineering application. In this study, a cold thermal energy storage unit with metal foam and straight fins was constructed. On the basis of dimensionless analysis, experimental and numerical methods were used to investigate the structural parameters of straight fin and metal foam on the liquid fraction and effective Nusselt number(Nu∗). Results showed that the height and spacing of fin are the main factors affecting the solidification rate of phase change material, compared with the porosity, pore density and material of metal foam. When fin height increases from 17 to 57 mm and spacing reduces from 120 to 3 mm, the Nu∗ of the phase change material increases by 1.5 and 12.61 times, respectively. The transient correlation between liquid fraction and Nu∗ of phase change materials is obtained to guide related design.

Abstract Understanding of droplet dynamic characteristics on gas diffusion layer (GDL) of polymer electrolyte membrane (PEM) fuel cell is of great importance for cell performance improvement. This study comprehensively investigated droplet dynamic characteristics on fuel cell cathode GDL with gradient pore size distribution (GPSD). The influence of different pore sizes, GPSD and wettabilities of GDL on droplet dynamic characteristics is numerically evaluated by using the volume of fluid (VOF) method through the analysis of the interaction among the forces over droplet. Results indicate that droplet has a relatively short detachment time and a smaller detachment radius on superhydrophobic and superhydrophilic GDLs compared with on a moderately hydrophobic GDL regardless of pore size. Moreover, large pores can facilitate droplet detachment, but increase droplet detachment radius and pressure drop. Compared with uniform (U)PSD, for transverse (T)-GPSDs with a small pore distance at a relatively low airflow velocity or a large pore distance at a high airflow velocity, or for the longitudinal (L)-GPSD with a large pore in the upstream and a small pore in the downstream at a large pore distance, the droplet detachment time decreases by 34.3% and 25.2% with a reduction of pressure drop, respectively.