Study On The Performance Of ORC System With A Single-Screw Expander

With the development of economy in China, the consumption of primary energy is increasing. The consumption status of energy is characterized by huge quantity, low utilization rate and unreasonable structure, which is restrictedthe economy sustainable development seriously. Waste heat recovery is an effective technology to improve the utilization ratio of primary energy. In China, the technologies of waste heat recovery with high or high-media temperature are matureand have been widely used in different fields. Recently, waste heat recovery with low or low-media temperature has been obtained widly attentionin research and different technologies for utilization have been investigated. Among these technology ways, organic Rankine cycle(ORC) offers great opportunities.Expander is the core component in an ORC system. Single-screw expander is a new type of volume expanders, which has many advantages, such as long service life, balanced loading of the main screw, high volumetric efficiency, low noise, low leakage, low vibration, simple configuration, and so on. Single-screw expander can realize 1-200 k W range of power output, and it is more suitable for low temperature and small scale of ORC system for waste heat recovery. In this paper, the configuration and performant indexes of single-screw expander have been introduced. A parameter called tooth width-diameter ratio has been defined as the ratio of width of gatorator tooth to diameter of gatorator. Comprehensive consideration, the range of this ratio is 0.14 to 0.154. The expression of volume of basic unit can be simplified with the ratio.A testing platform has been set up for experimental study on a single-screw expander prototype with 195 mm screw diameter and testing experience of other single-screw expander prototypes has been used for reference. The performance of single-screw expander under different conditions has been obtained by analysis on four prototypes with 117 mm, 155 mm, 175 mm and 195 mm screw diameter, respectively. The results indicated that the relation between the performance of single-screw expander, such as inlet volume flow, power output and temperature drop of working fluid, and the screw diameter was not simple third-order relationship, meanwhileit was affected by other factors. Durning the rotational speed interval from 2500 rpm to 3000 rpm, the optimum power outputs of fore prototypes were gotten. The increase of inlet pressure and expansion ratio could improve the single-screw expander performance, but the increase of exhaust pressure wasnot good to the performance of single-screw expander. An experimental system for ORC system performance testingbased on single-screw expander has been developed and set up. The exhaust of diesel engine of WP10.336 supplied by Weichai Power Co.,Ltd was used as heat source and R123 was chosen as working fluid in ORC system. A single-screw expander prototype with 155 mm screw diameter, a spiral-tube type evaporator and an aluminum multi-channel parallel type condenser have been developed for this ORC system. In order to deduce the volume and weight of ORC system, titanium tubewas used in evaporator and high performance louvered fins and bending type construction were used in condenser.By changing the power output of diesel engine, torque and exhaust pressure of single-screw expander and mass flow of working fluid, the effects of different factors to single-screw expander and ORC system have been investigated, and the change lawshave been obtained. The special oil consumption and exhaust pressure of diesel engine became bigger after equipping ORC system, and the maximum increases were 0.7kg/h and 2.5kPa, respectively. However, the ratio of oil consumption to total power output of diesel engine and ORC system was decreased with the maximum decrease by 3.5%. The power output and shaft efficiency of single-screw expander and efficiency of ORC system were improved with the increase of power output of diesel engine. At the condition of 180 kW power output and 1900 rpm of diesel engine, the power output of single-screw expander and efficiency of ORC system were increased with the increase of organic working fluid vapor dryness, while they were steeply decreased with the increase of exhaust pressure of single-screw expander.Based on the performance of single-screw expander and evaporator obtained in experimental testing of ORC system, a model of ORC system has beenestablished. The maximum ratio of power output to whole volume of ORC system was used as objective function, and evaporating pressure, exhaust temperature of diesel engine, screw diameter and rotational speed of single screw expander were used as model variables.The performance of optimized ORC system has been investigated. The results indicated that with the increase of organic working fluid vapor dryness and power output of diesel engine, the power output of single-screw expander and ORC system and the efficiency of ORC system were increased. However, these indexes between 180 kW and 200 kW power outputs of diesel engine were little difference. Although power output of single-screw expander increased with the increase of evaporating pressure, the change laws of power output and efficiency of ORC system were increasing at the beginning, and soon decreased.