Guopeng Yu

Simulations of a Bottoming Organic Rankine Cycle (ORC) Driven by Waste Heat in a Diesel Engine (DE)

A bottoming waste-heat-recovery (WHR) model based on the Organic Rankine Cycle (ORC) is proposed to recover waste heat from exhaust gas and jacket water of a typical diesel engine (DE). The ORC model is detailed built based upon real structural and functional parameters of each component, and is able to precisely reflect the working process of the experimental ORC system constructed in lab. The DE is firstly tested to reveal its energy balance and the features of waste heat. The bottoming ORC is then simulated based on experimental data from the DE bench test using R245fa and R601a as working fluid. Thermodynamic evaluations are done on key parameters like waste heat recovered, expansion power, pump power loss and system efficiency. Results indicate that maximum expansion power and efficiency of the ORC are up to 18.8kW and 9.6%. Influences of engine condition, fluid mass flow and evaporating pressure on system performance are analyzed and meaningful regularities are revealed. The combined system of DE and bottoming ORC (DE-ORC) is also investigated. The results showed that the integration of the bottoming ORC greatly changed energy distribution of the DE, and the DE thermal efficiency is up to 47.2%, increasing by 9.0%.

Theoretical Analysis of Engine Waste Heat Recovery by the Combined Thermo-Generator and Organic Rankine Cycle System

The combined thermo-generator and organic rankine cycle (TEG-ORC) used in exhaust heat recovery of internal combustion engine (ICE) is analyzed theoretically. Only about one third of the total energy released from fuel combustion is converted into useful work in engines, while the remaining energy goes into ambient environment, among which exhaust gas possesses high-grade thermal energy. Most of previous studies on energy recovery from engines have focused on exhaust heat recovery by ORC. However, if the heat is exchanged directly with high-temperature exhaust gas, organic working fluid would resolve with its lower decomposition temperature, and this is extremely harmful to ORC system. To avoid this phenomenon and utilize waste heat, preliminary thermoelectric modules are used to lower exhaust temperature and to generate electricity simultaneously. The heat rejected by TEG is used to preheat working fluid, and more energy is recovered to improve gross output power and thermal efficiency. A theoretical numerical model has been established in this paper to study the performances in both supercritical and subcritical combined TEG-ORC systems. The results suggest that, this model could efficiently identify the optimal performance parameters of both TEG and ORC systems. The utilization of TEG can extend the range of choosing working fluids if the temperature of waste heat source is high, so combined TEG-ORC system is suitable to recover waste heat from automotive vehicle engines, and thereby to improve the fuel economy of a passenger vehicle.

Thermodynamic Analysis of a Novel Combined Power and Cooling Cycle Driven by the Exhaust Heat Form a Diesel Engine

A novel combined power and cooling cycle based on the Organic Rankine Cycle (ORC) and the Compression Refrigeration Cycle (CRC) is proposed. The cycle can be driven by the exhaust heat from a diesel engine. In this combined cycle, ORC will translate the exhaust heat into power, and drive the compressor of CRC. The prime advantage of the combined cycle is that both the ORC and CRC are trans-critical cycles, and using CO₂ as working fluid. Natural, cheap, environmentally friendly, nontoxic and good heat transfer properties are some advantages of CO₂ as working fluid. In this paper, besides the basic combined cycle (ORC-CRC), another three novel cycles: ORC-CRC with an expander (ORC-CRCE), ORC with an internal heat exchanger as heat accumulator combined with CRC (ORCI-CRC), ORCI-CRCE, are analyzed and compared. The cycle parameters, including the coefficient of performance (cop), the cooling capacity (Qro) and expansion power of CRC (We) have been analyzed and optimized as the variation of the high pressure of ORC, the high pressure and the outlet temperature of gas cooler of CRC, and temperature drop of heat source in heat accumulator of ORC. The results indicate that there is an optimal high pressure of CRC (about 8.6 MPa to 8.8 MPa) for the combined cycles, at which the combined cycles achieve the optimal performance. The results also show that both the expander and heat accumulator could improve the system performance. The higher ΔTi could improve the system performance, but also resulting the more insufficiency of waste heat recovery.