Transcritical Carbon Dioxide Based Heat Pumps: Process Heat Applications

Ozone layer depletion has caused most of the conventional refrigerants used in high temperature heat pumps to be phased-out recently, and a suitable synthetic refrigerant is yet to be found. Several natural refrigerants such as hydrocarbons, water, and carbon dioxide have been presented as alternatives. In this context, environmentally benign transcritical CO2 cycle based heat pumps offer widespread possibilities in process heat applications due to the large temperature glide present in the gas cooler and can be extensively used in food and dairy, chemical, textile, paper, sugar and other industries where process heat is a critical input. A detailed performance analyses of heat pump systems have been carried out for several types of process heat applications with various possible heat sources. Important performance characteristics of the transcritical CO2 process have been reviewed along with their optimization aspects. Performance based comparisons have been made between the conventional heat pump systems and CO2 based ones. Suitability of the CO2 system in specific process application has been reported; this arises because a particular process application imposes specific temperature and pressure requirements. Results show that although the CO2 heat pumps give marginally lower COP (5% to 25%) compared to other conventional refrigerants in industrial heat pumps, they offer considerably lower pressure ratios (3 to 4 times) and higher volumetric capacity. It is observed that CO2 systems have strong potential in process heating and cooling (refrigeration / water cooling / air conditioning / process cooling). It is expected that CO2 heat pumps will be viable alternatives for process heat application due to its eco-friendliness, higher volumetric capacity and good heat transfer properties.