• Energy Research

Abstract An indirect active solar-biomass hybrid dryer (SBHD) has been developed to study the drying kinetics, mathematical modeling, and quality of dried small cardamom. The experiments were carried out in hybrid (solar with biomass) and biomass mode to assess the performance of the dryer. Freshly harvested small cardamom with a moisture content of 82.4 % on a wet basis (w.b) was dried to 9.1 % (w.b) in hybrid mode. In biomass mode, moisture content was reduced from 83.2 to 9.5 % (w.b). The drying period of 19 h was sufficient in both modes of drying to attain the required final moisture content. The drying rate curve depicted that the small cardamom drying occurred under the falling-rate drying period in both SBHD and Biomass mode. In this study, non-linear regression analysis was performed to determine the drying behavior of small cardamom. Eleven empirical models were used to determine the coefficients of determination (R2) and model constant for drying characteristics of small cardamom. Midilli model recorded the highest R2 value of 0.992 and 0.988, least chi-square of 0.0001 and 0.0005, and RMSE of 0.0002 and 0.0002 for SBHD and Biomass mode, respectively. The average drying efficiency of 28.63% was observed for the drying of small cardamom in the SBHD while 23.98% was obtained from Biomass mode. The colour analysis showed that the retention of green colour was better in SBHD than biomass mode of drying. The biochemical analysis revealed that the small cardamom dried under SBHD retained the maximum percentage of oleoresin (3.30 %) and volatile oil (7.33 %) compared to biomass mode. The microbial quality of dried small cardamom was within the acceptable limit for both SBHD and Biomass mode.

The present study details the development of a solar dryer with double pass flat plate solar collector having carbon nanodots (CNDs) coated absorber surface. Among the various concentrations of CNDs (0.1, 0.2, 0.5, 1, and 2%), the 0.5% CNDs coated absorber surface recorded the highest absorptance and lowest reflectance with higher spectral selectivity of 0.933. Hence, the 0.5% coating was selected as the optimum concentration and coated over the absorber surface of flat plate solar air collector. SEM image of black painted surface is clear and smooth and CNDs coated absorber surface is having dispersed particle with rough surface. FTIR absorption peak values revealed that the presence of black paint and CNDs in the 0.5% CNDs coated aluminium sheet. The efficiency of collector coated with 0.5% CNDs was calculated at various air flow rate of 0.008, 0.016, 0.018, and 0.021 kg/s and results revealed that the rise in air flow rate from 0.008 to 0.021 kg/s increased the efficiency from 39.22 to 82.99%. The solar dryer connected with the developed collector was tested for the performance and the drying studies revealed that shrimp and false white sardine required 10 h and 11 h drying time during the experimental studies.

The application of nanoparticle coating in a collector could resolve the issues of lesser heat transfer rate and lower thermal efficiency associated with the flat plate collector (FPC). In this study, carbon nanodots (CNDs) synthesized from fish scales wwere coated over the black paint coating to enhance the thermal efficiency of FPC. The effect of various concentrations (0, 0.1, 0.2, and 0.5%w/v) of CND coatings and mass flow rates of air (0.011 kg/s (natural convection) and 0.015, 0.029, and 0.044 kg/s (forced convection)) on the collector outlet air temperature and efficiency of the FPC were evaluated with a control (black paint). Solar radiation ranged from 382 to 913 W/m2 on the experimental days, and the atmospheric temperature and relative humidity values varied from 26.2 to 34.8°C and 50–72%, respectively. Fourier transform infrared spectroscopic studies of absorber plate coating revealed that the CNDs interacted with the black paint surface through phenolic OH, C=O, and NH mainly by hydrogen bonding. It was observed that the exit air temperature of the collector varied from 55 to 65°C. The thermal efficiency of 59.97%, 62.72%, 71.87%, and 67.75% was obtained for control (black-paint), 0.1%, 0.2%, and 0.5% CNDs coating, respectively, at the airflow rate of 0.044 kg/s. The coating of 0.2% CNDs recorded a maximum thermal efficiency of 71.87% at 0.044 kg/s. Overall, the CNDs coating on the absorber surface enhanced the thermal efficiency of the collector by 19.84% over the black paint-coated surface (control). It can be concluded from the study that CNDs coating resulted in enhanced outlet air temperature and improved thermal efficiency, which can be utilized for low-temperature applications like the drying of food materials.

Abstract The study was aimed at design and development of an energy efficient solar dryer suitable for continuous drying operation. In this dryer, water was used as a thermal energy storage and heat transfer medium, and air as an intermediate fluid. The major parts of the dryer were flat-plate solar water collector, water storage tank, drying chamber, heat exchanger, and liquefied petroleum gas (LPG) water heater. The dryer was designed to work mostly on solar energy during peak sunshine hours and LPG water heater as auxiliary heat source during low sunshine hours. The performance of the developed solar-LPG hybrid dryer was evaluated using shrimps (Metapenaeus dobsoni). The moisture content of fresh shrimp was reduced from 76.71% (w.b) to 15.38% (w.b) within 6 h of drying. The drying rate curve showed that shrimp drying occurred under falling-rate drying period. Results revealed that the water was capable of capturing maximum heat energy during peak sunshine hours. The maximum collector outlet temperature of 73.5 °C was obtained during drying. Overall, solar system supplied 73.93% of heat energy and LPG water heater assisted rest of the energy requirement due to lower incident solar radiation during start and end of drying. The maximum collector and drying efficiency obtained for shrimp drying were 42.37% and 37.09%, respectively.

AbstractFour hot-gas bypass defrosting configurations for CO2-NH3 cascade blast freezer for application in fish processing firm are numerically investigated. Due to the high moisture content of fish, defrosting is necessary after every 4 to 5 h of batch operation. A thermodynamic model for the cascade system and defrosting was developed to study various defrosting configurations formulated by rearranging the existing compressor to operate as a defrosting compressor and with the addition of an external defrosting compressor. From the simulation findings, it can be summarized that the conventional hot-gas bypass defrosting without defrost compressor is suitable for a high-capacity cascade refrigeration system with more than three evaporators. For low cooling capacity refrigeration systems, a defrosting compressor is necessary to elevate the temperature above the cascade condensing temperature. A dedicated defrosting compressor with a power consumption of 3.1 kW and a modified refrigeration/defrosting compressor with a power consumption of 6.8 kW can deliver 33.3 kW of heating at a temperature of +10 °C (45 bar). Incorporating a desuperheater between the main and defrosting compressors reduces compressor temperature and maintains the lubricating oil stability, without change in defrosting energy consumption and less exergy loss. The defrosting efficiency is obtained in the range of 39.7–42% which is in agreement with published literature.