Research On Drying Characteristics Of Kelp Knot With Solar-heat Pump Combined Drying System

Kelp is an important marine resource.China accounts for about 50%of the world kelp market.The fresh kelp must be dried and dehydrated during transportation or preservation because of its high moisture content and perishable characteristics.However,the drying process consumes a lot of energy,according to statistics,China’s current energy consumption of drying process accounts for about 15%of the country’s total energy consumption.So it is of great significance to save energy and reduce consumption and to study the new drying process of kelp.Based on the double evaporator normally closed heat pump dryer developed by the research group in the early stage,this paper constructs the solar-heat pump combined drying system by designing the heat utilization equipment of solar collector and air-water heat exchanger,and determines the drying mode and main control logic of the combined drying system.By selecting fresh kelp knot as drying material,this paper makes experimental and theoretical research on the system performance under different drying modes,the drying characteristics,influencing factors and drying model of kelp knot of combined drying system.Specifically,the main work and research results of this paper are as follows:（1）Based on the double evaporation normally closed heat pump dryer developed by the research group in the early stage,the effective area of solar collector and the selection of air-water heat exchanger in drying room are completed through design and calculation,and a solar-heat pump combined drying device is successfully designed and assembled.By using two sub-systems of solar energy and heat pump to work together,the device successfully solves the problem of discontinuity of solar energy drying alone and the problem of high energy consumption of heat pump drying alone.（2）Fresh kelp knots were selected as drying materials,and the drying performance of three different drying modes（solar drying alone,heat pump drying alone,and solar-heat pump combined drying）were comparatively studied.The experimental results show that:under the drying conditions in this paper,the solar-heat pump combined drying mode saves 26%of the energy consumption and about 10%of the drying time compared with the heat pump single drying mode;The solar energy single drying mode has the lowest energy consumption,which is only28.2%and 13.1%of the solar-heat pump combined drying mode and the heat pump single drying mode.Due to the discontinuity of solar energy,the total drying process takes up to 3 days,which is much longer than the solar-heat pump combined drying mode and the heat pump single drying mode.Under the dry conditions in this paper,the unit energy consumption and dehumidification of the solar energy independent operation mode,the solar heat pump combined operation mode,and the heat pump independent operation mode are 6.61,1.35 and 0.693 kg/k Wh,respectively,The dehumidification amount per unit time is 0.089,1.28 and 1.10kg/h respectively.（3）Based on the solar-heat pump combined drying mode,the drying characteristics of kelp knot under different drying temperature,wind speed,disc spacing and loading were discussed.The experimental results show that the higher the drying temperature,wind speed and disc spacing,the lower the loading,the shorter the drying time and the faster the drying rate.However,the drying rate of kelp knot will be reduced if the drying temperature,wind speed,disc spacing are too high and the loading capacity is too low.Through comprehensive comparative analysis,the comprehensive drying effect of kelp knot is the best under the drying conditions of drying temperature 45℃,wind speed 1.5m/s,disc spacing 150mm and loading capacity 3.6kg.（4）Based on the experimental results,10 drying kinetic mathematical models were selected,and the optimal drying model of kelp knot under the solar-heat pump combined drying mode is determined by using the evaluation indexes such as SSE,R-square,Adj R-sq and RMSE.The results show that the Logarithmic model,Page model and Verma et al model have higher R²and Adj R-sq mean values and smaller SSE and RMSE mean values,which are more suitable to describe the drying process of kelp knot under the solar-heat pump combined drying mode.Through further comparison and analysis of the predicted values and experimental values of the above three models,it is found that the fitting degree between the predicted values and experimental values of Verma et al model is the highest,indicating that Verma et al model is most suitable for predicting the kelp knot drying process under the solar-heat pump combined drying mode.Based on Fick’s second diffusion law and Arrhenius formula,the effective moisture diffusion coefficients under different drying temperature,wind speed,disc spacing and loading capacity are calculated to be in the range of 3.9251×10^-9～1.0012×10^-8、5.1118×10^-9～1.0028×10^-8、3.4253×10^-9～1.0001×10^-8and 2.1908×10^-9～9.1282×10^-9m²/s.respectively,the activation energy of kelp knot at different drying temperatures is 49.99k J/mol.