Investigation of an Ammonia−water Based Power/cooling Cogeneration System Using Sensible Waste Heat

In this study, the problem of a parallel connected power and absorption cooling cogeneration system was analyzed and then an improved configuration based on this system was proposed. This system can make cascade utilization of the turbine exhaust heat. By varying turbine outlet pressure, different turbine exhaust vapor temperatures can be obtained, resulting in different amounts of heat that can be recovered in the refrigeration subsystem. Simulation results show that when the turbine outlet pressure in the improved system is 3 bar, 119.6 kW of heat can be recovered in the refrigeration subsystem, and the total equivalent power output of the improved system is 17.6% higher than that of the original system. The corresponding equivalent heat-to-power efficiency and thermal energy saving ratio are increased from 16.1% to 18.9% and 6.6% to 26.5%, respectively. When the turbine outlet pressure increases from 1.6 bar to 4 bar, the cooling output increases rapidly, resulting in better performance, and the ratio of power to cooling is in a wide range of 0.19 to 1. When the turbine outlet pressure is higher than 4 bar, the favorable effect on refrigeration cannot compensate for the adverse effect on power generation, leading to the performance reduction. This study provides a new method for high efficiency utilization of waste heat. INTRODUCTION A large amount of mid/low-temperature waste heat is discharged into the environment from industrial plants and processes, resulting to both energy waste and thermal pollution. The recovery and utilization of waste heat is thus of great interest to energy saving and environmental protection. Refrigeration, power generation and of course direct use of the heat are the main mature methods to recover waste heat. The absorption refrigeration system has been widely used in combined cooling, heating, and power systems to utilize the heat of the engine or gas turbine flue gas, whose operating temperature is usually higher than 300 °C [1, 2]. The highest temperature of working fluid in the refrigeration systems is usually below 150 °C. The large temperature difference between the flue gas and the working fluid in the refrigeration system leads to large exergy destruction in the heat recovery process. Rankine power cycles can use steam, organic materials and mixture as working fluids. Steam Rankine Cycles suit large-scale and high-temperature waste heat source. However, Organic Rankine Cycles (ORCs) are more suitable for lowtemperature sensible heat source because of the low boiling temperature and small latent heat of organic fluids [3]. Considering that organic working fluid may decompose at a high temperature [4], the ORCs are suitable for heat sources with temperatures lower than 250 °C. The power cycle with ammonia–water mixture as the working fluid was first investigated in detail by Maloney and Robertson in the 1950s [5], and later ammonia–water based power cycles were studied 1 Copyright © 2013 by ASME Proceedings of the ASME 2013 International Mechanical Engineering Congress and Exposition IMECE2013 November 15-21, 2013, San Diego, California, USA