A Trans-critical CO2 Heat Pump System for Waste Heat Utilization in Warm Weather Condition Applied to a Milk Refrigeration Plant

Abstract

Based on field data from a medium scale ammonia based milk refrigeration plant located in northern part of India, a trans-critical CO2 based heat pump system is conceptualized to utilize the waste heat from the plant to improve overall coefficient of performance. Year round plant operating data is collected. The plant handles between 0.1 to 0.15 million liters of milk and milk products daily. It maintains two cooling chambers, one at 4°C and the other at -10°C. Every ten days it is required to replenish a 5000 liter tank with ground water for evaporative cooling of the system. Up-stream and down-stream processes in the milk processing plant utilizes a coal fired boiler system which also uses ground water. In this study, the condenser of the ammonia based refrigeration system is coupled with the evaporator of the proposed CO2 trans-critical heat pump system which is maintained at 25°C round the year. The heat pump delivers heat at about 70°C to pre heat the boiler feed water available in temperature range 22-25°C round the year. Augmentation of the proposed CO2 system is able to reduce coal consumption by utilizing the waste heat from the ammonia system at the same time reduce ground water utilized by eliminating evaporative cooling. The proposed heat pump system has encouraging COP and is designed to be largely independent of variation in ambient temperature. Based on field data, a comparative study is carried out using thermodynamic & thermo-economic analysis for a duration of one year to establish feasibility of the proposed system