**1. Introduction**

The technological advances developed in organic Rankine cycles (ORC) applied to waste heat recovery (WHR) systems could become a promising feature for the engine manufacturing industry due to its capacity to reduce fuel consumption, increase net power output, and reduce greenhouse gas emissions [1].

ORC is considered as a feasible tool to increase overall conversion efficiency in industrial processes due to its capacity to recover energy from alternative sources, such as exhaust gases, cooling water, or lubricating oil, by using organic working fluids [2]. Furthermore, ORC configuration can be modified to maximize overall engine–ORC system performance by optimizing net power output, first law, 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] proposed an integrated diesel internal combustion engine and simple ORD (SORC) with solids adsorption technology for waste heat recovery from the cooling system and the engine exhaust gases. Their results showed a maximum recoverable power from the cooling process and exhaust systems of 67.9 kW and 82.7 kW, respectively.

Vaja and Gambarotta [9] evaluated the performance of SORC and ORC with a recuperator (RORC) configurations for WHR in a stationary 2.9-MW ICE at a single operation point. The results showed an increase of 12% in the overall efficiency of the system. Kalina [10] investigated the performance of a biomass power generation plant. The plant consisted of two natural gas ICE coupled to a biomass gasifier and an ORC. The ORC system was used as a WHR system to produce power from the engine's exhaust line and cooling system. Mingshan et al. [11] analyzed a combined heavy-duty diesel engine–SORC system for WHR. This system achieved a heat recovery efficiency between 10–15% when the heat exchanger operation is optimized. This publication also evaluated the engine operation under partial load conditions with a medium–high power condition. They concluded that variations in engine rotational speed must be determined to evaluate the true performance of the combined system.

Regarding ORC and stationary compressed natural gas (CNG) engines integration, two approaches have been studied: the use of multiple temperature loops for WHR from the engine intercooler, cooling system, and exhaust gases, and the use of single-temperature loops for WHR from engine exhaust. Within the first approach, Yao et al. [12] were the first ones to propose an ORC system for WHR from the engine intercooler, exhaust gases, and cooling system by using a low and a high-temperature loop. Their ORC system used R245fa and achieved 10.8% thermal efficiency and 26.9 kW of power, which increased overall power production by 33.7%, keeping the same fuel consumption. Yang et al. [13] optimized Yao's ORC configuration by using a genetic algorithm, but their reported performance was lower than Yao's results: 8.8–10.2% thermal efficiency and 23.6-kW net power output. Yao's configuration was studied again by Yang et al. [14] and analyzed the thermodynamic performance of the ORC with double-pressure configuration (DORC) operating with six working fluid groups integrated into a compressed natural gas engine, but performance indicators such as the waste heat recovery efficiency and specific fuel consumption were not considered. Wang et al. [15] also studied a dual-loop ORC system for WHR in stationary CNG engines but using R1233zd and R1234yf as working fluids and two recovery heat exchangers to increase ORC efficiency. The results showed that ORC efficiency was 10–14% when R1233zd was used in the high-temperature loop, and R1234yf was used in the low-temperature loop.

Within the second approach, Han et al. [16] carried out a dynamic simulation of an ORC with a double piston expander and R245ca as a working fluid. Their system took advantage of the engine exhaust gases to produce power in the ORC piston expander and compress the engine natural gas. However, they focused on the component efficiencies and compression benefits, but their study did not report overall efficiency values. Song et al. [17] proposed an RORC working with R416A as the

ORC fluid. Their results showed that this approach increased the overall conversion efficiency by 6%. Liu et al. [18] examined the effect of engine load on the performance of an ORC cycle combined with a stationary motor and R245fa as the working fluid. Their results showed ORC efficiencies between 16.3–25.9% when the engine load varies between 40–100%, respectively.

The selection of ORC working fluid is a key design step because it defines the operation limits and the power production potential. However, publications have shown that working fluid selection depends on the heat source and temperature limits. Chacartegui et al. [19] studied a low-temperature SORC configuration in medium and large-scale combined cycle plants for WHR from commercial gas turbines. Their study considered R-113, R-245, isobutene, toluene, cyclohexane, and isopentane as working fluids. The results showed that toluene and cyclohexane achieved the highest combined cycle efficiency—up to 60%–which is high global efficiency in this process. Qiu et al. [20] examined the experimental integration of a small-scale thermoelectric generation with a dual ORC. Drescher and Brüggemann [21] performed a screening of suitable organic working fluids for biomass-fired applications. Their selection involved critical temperature and pressure, dryness, turbine and pump efficiencies, and autoignition temperature. However, they obtained a group of suitable fluids rather than an optimum working fluid. Mago et al. [22] studied the effect of working fluid selection on the ORC performance at different heat source temperatures, supporting Drescher and Brüggemann's conclusions.

Kosmadakis et al. [23] tested more than 30 organic fluids and stated that R245fa is a suitable working fluid for ORC applications with ICE in terms of net power and thermal efficiency, but its use is restricted as international standards promotes low global warming potential (GWP) organic substances. Regarding solar thermal energy conversion, Tchance et al. [24] established that R134a is a suitable fluid due to its low toxicity and flammability, in addition to the high-pressure ratio and efficiency that can be achieved. Tian et al. [25] performed a energy analysis of a combined diesel ICE–SORC system to evaluate 20 different working fluids in terms of net output power and thermal efficiency. The study showed that R-141b showed the highest net power output (60 kJ/kg), but R-123 achieved the highest thermal efficiency (16.60%). Hung et al. [26] investigated an SORC integrated into a solar ventilation system. Their results showed an overall efficiency increase of 6.2%. Zare [27] evaluated the revenue of three ORC geothermal plants. They showed that the RORC configuration improves thermal efficiency, but requires additional components, increasing the total exergy destroyed, and affecting the overall exergy efficiency. This trend was confirmed by Fontalvo et al. [28], who evaluated three ORC configurations for low-temperature WHR and showed that SORC had the highest revenue because the additional equipment increases the exergy destruction of the system.

Exergy analysis is an important tool to identify key design aspects that may improve overall conversion efficiency and maximize resource utilization. ORC research has shown that exergy analysis can be used to provide guidelines in ORC design for a wide range of heat sources. In addition, the calculation of exergoeconomic costs aims to set an economic value for materials and energy flows, providing a reasonable base for the allocation of economic resources [29]. Kerme and Orfi [30] evaluated the effect of the turbine inlet temperature on the exergy efficiency of an ORC driven by solar collectors. As a result, they showed that an increase in turbine inlet temperature increases exergy efficiency and reduces exergy destruction. The conservation of natural resources, the limited availability of spaces to generate energy through some renewable sources, cost savings, policies, and the national regulatory framework are some of the most important factors that encourage the research of a more efficient energy generation process for internal combustion engines [31].

The main contribution of this paper is to present the comparative analysis results of some energetic and exergetic performance indicators of a 2-MW natural gas engine integrated with waste heat recovery systems based on SORC, RORC, and DORC configurations with different organic working fluids such as toluene, cyclohexane, and acetone. The maximum net output power of the bottoming ORC cycle was studied for differents engine load percentages, which implied studying the exhaust gas thermodynamic properties and engine thermal performance in detail. Also, parametric studies are developed to identify the influence of evaporation pressure on exergy destruction for each component, and performance

indicator. These result help obtain the best ORC operational condition and configuration in terms of rational use of energy and environment preservation by increasing the overall energy and exergy efficiency of the Jenbacher JMS 612 GS-N. L natural gas engine of 2 MW with the bottoming ORC cycle.
