Analysis and Perspective on Heat Pump for Industrial Steam Generation

Steam (water vapor) is an important heat carrier with a latent heat value over 2000 kJ kg , which has advantages such as uniform heating, convenient temperature regulation, clean and nontoxic, convenient transportation. Therefore, it has been widely used in industrial production and is mainly provided by industrial boilers. Currently, the research on steam generation mainly focuses on supply energy and efficiency optimization. Fossil fuel, biomass, solar heat, and electric (direct heating) are the input energy source of most boilers. Fossil-fuel boiler is the most widely used boiler in industrial process. The existing boilers are mainly based on the combustion of coal or natural gas. Recently in China, the coal-fired boiler has been restricted to use due to its severe environmental pollution impacts. In this case, natural gas is regarded as a cleaner energy source; however, it is indicated that the gas-fired boilers will also emit gaseous pollutants such as NOX; [3] the high cost of natural gas has increased the cost of production process. Biomass is regarded as carbon-neutral fuel because the CO2 it produced during the combustion process is equal to the amount which was taken from the atmosphere during the growing stage. However, the biomass will also emit PM2.5 and CO during the combustion process, and its resources are not possible to meet the demand for industry heating. Solar heating could also serve as an energy source for steam generation, and current research can be mainly divided into solar heating for industrial processes (SHIP) and small-scale solar-driven steam generation. SHIP with solar collector converts solar irradiation hitting on its surface into thermal energy by heating a suitable heat-transport medium. However, the unavailability at night and the large collector area make this technology difficult for stable industrial steam demand and also the high initial cost may deter the customer. Electricity can be directly used for steam generation in electric boiler, which is quite flexible and easy to use if low-cost electricity is accessible. However, in terms of thermodynamics, electrical boilers have a lower efficiency of its first energy resources (fossil fuel and biomass with efficiency typically between 30% and 50%), whereas electrical-driven heat pump could increase electric heating performance as it absorbs heat from low-temperature heat sources (waste heat, solar heat, or even ambient heat, etc.). There are several heat-pump methods of steam generation appeared to meet the high-temperature industrial steam requirement. Kobe Steel. Ltd. developed two heatpump-based steam supply systems: the high-efficiency steamsupply system with a steam temperature of 120 C and the standard steam-supply system with a steam temperature of 165 C. The heat pump system, which is equipped with a newly developed screw compressor and a high-efficiency motor resistant to high temperatures, uses a refrigerant suitable for high temperature supply. Chamoun et al. proposed a new R718 high-temperature heat-pump system to realize the high temperature output of 145 C. The water vapor heat pump is investigated to satisfy different temperature demands for industrial use with high performance. Wu et al. studied a water vapor high-temperature heat-pump system for waste heat recovery H. Z. Yan, Prof. R. Z. Wang, Dr. S. Du, Dr. B. Hu, Prof. Z. Y. Xu Institution of Refrigeration and Cryogenics MOE Engineering Research Center of Solar Energy Shanghai Jiao Tong University Shanghai 200240, China E-mail: rzwang@sjtu.edu.cn