Waste heat recovery (WHR) from exhaust gases in natural gas engines improves the overall conversion efficiency. The organic Rankine cycle (ORC) has emerged as a promising technology to convert medium and low-grade waste heat into mechanical power and electricity. This paper presents the energy and exergy analyses of three ORC-WHR configurations that use a coupling thermal oil circuit. A simple ORC (SORC), an ORC with a recuperator (RORC), and an ORC with double-pressure (DORC) configuration are considered; cyclohexane, toluene, and acetone are simulated as ORC working fluids. Energy and exergy thermodynamic balances are employed to evaluate each configuration performance, while the available exhaust thermal energy variation under different engine loads is determined through an experimentally validated mathematical model. In addition, the effect of evaporating pressure on the net power output, thermal efficiency increase, specific fuel consumption, overall energy conversion efficiency, and exergy destruction is also investigated. The comparative analysis of natural gas engine performance indicators integrated with ORC configurations present evidence that RORC with toluene improves the operational performance by achieving a net power output of 146.25 kW, an overall conversion efficiency of 11.58%, an ORC thermal efficiency of 28.4%, and a specific fuel consumption reduction of 7.67% at a 1482 rpm engine speed, a 120.2 L/min natural gas flow, 1.784 lambda, and 1758.77 kW of mechanical engine power.Energies 2019, 12, 2378 2 of 22 and exergy efficiencies and minimizing exergy destruction [3]. Nevertheless, ORC-engine coupling must be carefully designed to avoid safety, performance, and revenue issues such as gas-fluid contact, as well as weight, complexity, and backpressure increase [4].ORC-WHR research has addressed the integration between ORC and combustion engines. Plenty of studies have established that ORC improves the overall conversion efficiency by increasing net power production without penalizing fuel consumption. Patel and Doyle [5] presented a first attempt for WHR from diesel engines by using ORC. Their ORC system achieved an overall power increase of 13% in a Mack 676 diesel vehicle engine without increasing fuel consumption. Peris et al. [6] simulated six ORC configurations for WHR from cooling water in internal combustion engines (ICE) by using 10 non-flammable fluids. Their study showed that ICE electric efficiency could be increased by 4.9-5.3%, by achieving overall conversion efficiencies up to 7.15% at a relatively low-temperature cooling water (90 • C). Yu et al.[7] simulated a diesel engine-ORC integration for WHR from the engine exhaust gases and cooling system by using R245fa as the ORC working fluid. Their results showed that 75% of exhaust gases energy and 9.5% of cooling water energy could be recovered if ORC operating conditions are optimized and controlled to maintain the power output. However, these results are limited to an exergetic analysis of a single ORC configuration. Lu et al. [8] ...

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Low-grade heat sources such as solar thermal, geothermal, exhaust gases and industrial waste heat are suitable alternatives for power generation which can be exploited by means of small-scale Organic Rankine Cycle (ORC). This paper combines thermodynamic optimization and economic analysis to assess the performance of single and dual pressure ORC operating with different organic fluids and targeting small-scale applications. Maximum power output is lower than 45 KW while the temperature of the heat source varies in the range 100-200°C. The studied working fluids, namely R1234yf, R1234ze(E) and R1234ze(Z), are selected based on environmental, safety and thermal performance criteria. Levelized Cost of Electricity (LCOE) and Specific Investment Cost (SIC) for two operation conditions are presented: maximum power output and maximum thermal efficiency. Results showed that R1234ze(Z) achieves the highest net power output (up to 44 kW) when net power output is optimized. Regenerative ORC achieves the highest performance when thermal efficiency is optimized (up to 18%). Simple ORC is the most cost-effective among the studied cycle configurations, requiring a selling price of energy of 0.3 USD/kWh to obtain a payback period of 8 years. According to SIC results, the working fluid R1234ze(Z) exhibits great potential for simple ORC when compared to conventional R245fa.

The application of equilibrium thermodynamics in the study of thermal plant performance under real operating conditions is a constant challenge. In this paper, an analysis of a reservoir pressure piston working between two linear flow resistances is performed by considering the friction of the piston cylinder system on the walls. The proposed model is developed to obtain the optimum power output and speed of the piston in terms of first law efficiency. If the friction on the piston–cylinder assembly is neglected, the expressions obtained are consistent with those presented in the literature under laminar regime. It was also demonstrated that for both laminar and turbulent regimes with overall size constraints, the power delivered can be maximized by balancing the upstream and downstream flow resistances of the piston. This paper also evaluated the influence of the overall size constraints and flow regime on the performance of the piston cylinder. This analysis is equivalent to evaluate the irreversibilities in an endo-irreversible Carnot heat engine with heat loss resistance between the engine and its heat reservoirs. The proposed model introduced some modifications to the results obtained from the recent literature and led to important conclusions. Finally, the proposed model was applied to calculate the lost available work in a turbine operating at steady flow conditions with an ideal gas as working fluid.

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