Investigation of thermal performance of a solar-assisted heat pump drying system with underground thermal energy storage tank and heat recovery unit

The conventional drying systems in the industrial applications have been used for decades, and consumed a great amount of energy. Since these systems have not been operated efficiently, they have lost great deal of energy, and cause environmental pollution. Therefore, innovative, efficient and environment friendly drying systems need to be developed and operated in these days. In this study, theoretical models for a Solar-Assisted Heat Pump (SAHP) drying system including underground Thermal Energy Storage (TES) tank with and without using Heat Recovery Unit (HRU) are developed, and thermal performance parameters for the drying systems are investigated. Two different analytical models are developed to find the thermal performance parameters for the drying system. In the first model, transient heat transfer problem around the TES tank is solved by using Duhamel's superposition and similarity transformation techniques to find hourly temperature variation of water in the TES tank and the other performance parameters. In the second model, the unsteady heat transfer problem for the TES tank is solved applying Complex Finite Fourier Transform (CFFT) Technique to find monthly periodic store temperature and the other performance parameters. These parameters are Coefficients of Performances for the heat pump (COP) and system (COPs), Specific Moisture Evaporation Rate (SMER), annual variation of water temperature in the TES tank, energy fractions for energy charging, and extraction from the system. MATLAB programs have been generated to build new models for the drying system. For the SAHP drying system with TES using the first model, when wheat mass flow rate and Carnot efficiency are selected as 50 kg/h and 40% respectively, the values of collector area, TES tank volume, COP, COPs and SMER are obtained as 70 m2, 300 m3, 4.2, 4.1 and 6 respectively when the system attains periodic operation time in fifth year. For SAHP drying system with TES tank and HRU using the first model, the results for COP, COPs and SMER are 5.55, 5.28 and 9.25 respectively, by using wheat mass flow rate of 100 kg/h, Carnot Efficiency (CE) for heat pump of 40%, collector area of 100 m2 and TES tank volume of 300 m3 when the system attains periodic operation duration from the fifth year onwards for ten selected years of operation. Annual energy saving is calculated to be 21.4% in comparison with the same system without using HRU for the same input data. For SAHP drying system including TES with and without HRU using the second model, the results reveal that moisture contents in wheat at initial and final states, and wheat mass flow rate have great effects on system performance parameters. Results also indicate that using of HRU increases the system performance parameters.