1. Introduction
Due to the large consumption of coal, oil, natural gas, and other fossil fuels, energy production is increasingly unusual. Therefore, clean and efficient substitutes are needed to protect the environment and support sustainable development. Thus, with the increase in population and the global industry, energy-saving and emission reduction have been considered the main objective of the energy development strategy [
1]. The sustainability goal as well as the environmental and energy analysis had been conducted by some researchers in waste heat recovery systems based on ORC to convert the energy from renewable sources such as biofuels, biomass, solar energy, and geothermal values to electric energy [
2]. One way to improve energy efficiency and potentially reduce pollution is by applying ORC technology as waste heat recovery devices [
3,
4]. Organic Rankine cycles (ORC) are an effective way of converting medium-low temperature heat into electricity that cannot be used for conventional high-temperature Rankine cycles, even though many studies have been conducted in recent decades. The use of ORC as an alternative to waste heat recovery has received relatively little industrial attention [
5,
6,
7,
8,
9].
In recent years, research has been conducted to improve ORC performance based on the selection of different organic working fluids. Vivian et al. [
10] considered the selection of the most suitable working fluid and cycle configuration for a heat source to find the optimal ORC design. Therefore, they performed a design optimization of four different ORC configurations operating with 27 organic working fluids in the temperature heat source ranging from 120 °C to 180 °C, where the results showed that fluid temperature and heat source inlet temperature plays an important role in predicting the optimum performance of all system configurations. Yu et al. [
11] proposed a new predictive method for simultaneously determining the working fluid and operating conditions in an ORC system, concluding that the waste heat can be fully recovered, and a maximum output power is obtained when there is a positive temperature difference between the waste heat inlet temperature and the critical working fluid temperature. However, these studies do not consider, in their scope, the environmental impact’s assessment to have a sustainable solution that can be implemented worldwide in the industrial sector.
On the other hand, Invernizzi et al. [
12] investigated the possible substitution of HFC-134a in ORC applications by two low GWP cooling fluids such as HFO-1234yf and HFO-1234ze, adopting the Peng Robinson model available in Aspen Plus v7.3. The results showed a net power decrease of 13% and 1% for HFO-1234yf and HFO-1234ze, respectively. A decrease in turbine power of 20% was also obtained when operating with HFO-1234yf and 28% when operating with HFO-1234ze. Mavrou et al. [
13] studied the performance of various working fluid mixtures in the ORC with heat storage using FPC (flat plate collectors). For this, they developed a multi-criteria analysis, which revealed important tradeoffs between various system parameters such as the thermal efficiency of the ORC, the net power generated, the volume ratio across the turbine, the mass flow of the working fluid, the evaporator temperature, the temperature in the storage tank, and the total annual operating life of the ORC. Among the limitations of these works are that the studies were not performed for a characterized thermal source that would guarantee the results under real operational conditions. They consider fluids that do not comply with international standards of safety, thermal stability, and environmental criteria [
14].
Karellas and Braimakis [
15] developed a thermodynamic modeling and thermo-economic analysis of a micro-scale trigeneration system capable of combining heat and energy production and cooling based on the combined operation between an ORC and a Steam Compression Cycle. Only the effect of condensation and evaporation temperatures on system performance was evaluated. The results show an exergetic efficiency of the ORC of around 7%. Savings in fuel and electricity consumption represented an internal rate of return (IRR) of approximately 12% with a payback period of seven years. Despite being a very potential configuration, the results are very limited in scope since the environmental impacts have not been evaluated.
Another important parameter to consider in the design of the ORC is the Pinch Point and depends largely on the conditions of the source, sump, and selected working fluids [
16]. Wang et al. [
17] studied the selection of some organic working fluids to evaluate the correspondence between the evaporator Pinch Point and the condenser Pinch Point on the thermo-economic performance of the ORC system. The results show that the optimum working fluid among those present is R11, and the relationship between the evaporator pinch point and the most suitable condenser is 1.25–1.5. Other authors have carried out the analysis and optimization of new ORC systems by integrating them to other energetic processes.
Many of the studies also focus on improving the exergetic and/or exergo-economic performance of the ORC system [
3]. Mahmoudi and Ghavimi [
18] conducted a thermo-economic analysis of an integrated carbon dioxide and the ORC energy system using liquefied natural gas as a heat sink. For the proposed system, the authors performed multi-objective optimization using a genetic algorithm and selecting the unit cost of the product as objective functions and the exergetic efficiency. The results showed that the highest and second-highest exergy destruction rates occur in a catalytic burner and fuel cell. Zhang et al. [
19] presented a new energy cycle for cascading the ORC of an offshore gas turbine. As for performance indicators, they considered net energy production and leveled energy cost. The results showed that the proposed waste heat recovery system could increase net energy production by 30.1% compared to the gas turbine alone.
To evaluate the impact on the environment with the use of ORC technology, different environmental methodologies, and indicators have been used [
2,
20], which have been defined within the object of study in various analyses through a Life Cycle Analysis (LCA). Cioccolanti et al. [
21] investigated a plant with combined cooling, heating, and energy systems, which includes an ORC plant. The authors performed a sensitivity analysis of the environmental and energy impact of the plant by varying the organic fluid in the ORC unit. Their results confirmed that the LCA was of paramount importance for appropriate selections of component specifications and operating conditions of the integrated system. Ding et al. [
22] established an environmental model of the ORC system and determined that fabrication and working fluid leakage are important for an LCA. Therefore, the environmental impact of working fluids for the ORC cannot be neglected. Heberle et al. [
23] performed a Life Cycle Analysis (LCA) for geothermal power generation in binary power plants with ORC facilities, based on scenarios with representative conditions from Germany, to evaluate potential plant concepts considering fluid work losses and the associated environmental impact. This study found that substituting working fluids such as R245fa and R134a with low Global Warming Potentials (GWP) fluids such as R1233zd and R1234yf leads to 2% higher exergetic efficiency and an 83% reduction of global warming impact.
Several relevant works have been oriented for the development of theoretical models of waste heat recovery systems with organic Rankine cycles [
24,
25,
26]. In this order of ideas, research into new configurations of the organic Rankine cycle to recover residual heat in the exhaust gases in an internal combustion engine, is benefited by these. Shi et al. [
27] used the traditional design method, which does not adequately adapt to internal combustion engine waste heat recovery because of the specific large-gradient temperature drop characteristic of the residual heat in combustion engines. This led to a large number of investigations to implement this system optimally. Therefore, the research covered a review of the various models proposed to determine the cycle with the ideal modifications. However, its integration with industrial natural gas generation engines has not been widely evaluated [
28]. The implementation of these generation solutions from waste gases, in addition to minimizing the environmental impacts in each of the phases of the life cycle of these processes, pursue the sustainability of these systems [
22,
29].
The sustainability of waste heat recovery systems based on ORC can be promoted through the integration of the energy method and environmental impact assessment through life cycle analysis [
2]. Thus, it is concluded that the sustainability of waste heat recovery systems using ORC is more favorable than generation systems using petroleum-based fuels for their operation [
2].
The environmental impacts of a heat recovery system change when using different working fluids and materials in its components [
30]. Therefore, a life cycle analysis was developed in detail to evaluate the environmental impacts of an organic Rankine cycle, where the main objective was to evaluate seven different organic fluids in both the construction phase, operation, and decommissioning. It is concluded that the greatest potential impacts in the global warming category occur in the construction phase [
20].
Among the benefits of proposing sustainable waste heat recovery systems is that they demand less consumption of resources for energy production [
31]. They make it possible to achieve a sustainable energy supply by adopting environmentally-friendly organic fluids, in terms of low ozone depletion potential and global warming potential [
30,
32]. This allows the amount of reusable material to be increased at the end of the product life cycle, which significantly reduces environmental impacts throughout the life of the thermal system [
33].
Additionally, this approach also complements the identification of exergetic improvement opportunities that can be determined with dissipative energy balances, as exergy studies have been conducted for this purpose [
4,
34]. However, the results obtained by complementing the environmental analysis with the exergy analysis for the processes of energy recovery in the exhaust gases through ORC, allow proposing sustainable energy solutions that favor their application and inclusion in the industrial market.
Despite many studies on performance improvements for different ORC configurations through the selection of working fluids, optimization of operating parameters and equipment characteristics, and environmental impact assessment, very little information is available in the literature on multi-objective optimizations involving thermal, economic, and environmental criteria. In most cases, no systematic approach is offered that correlates these different variables for the different ORC applications. The objective and scientific novelty of this study is to propose an exergetic, economic, and environmental evaluation through indicators such as net power, exergy destroyed by components, LCOE, and the environmental impacts of a residual heat recovery system using ORC in an industrial power generation engine to natural gas, by changing operational parameters such as turbine efficiency, evaporation pressure, evaporator, and condenser pinch point. This study provides guidelines and recommendations for exergetic, economic, and environmental improvement for the configuration of the heat recovery system based on ORC to design generation systems for cleaner production of electricity with lower environmental impacts.
4. Conclusions
This research evaluated, from an exergy, economic, and environmental impact assessment, the possibility of waste heat recovery to generate electricity from an organic Rankine cycle integrated into a natural gas engine Jenbacher JMS 612 GS-N, which is in operation in the city of Barranquilla-Colombia.
The main contribution of this study was to identify the operational and design variables that most contribute to the exergetic, economic, and environmental performance of the system. It identified the equipment that presents greater exergy destruction varying the different operation parameters, which require us to focus efforts to find greater use of the energy, which contributes to the technical and economic viability of the implementation of this solution in real operational scenarios.
From the parametric results, it was found that the evaporation pressure is the one that has the greatest influence on the exergy destruction. Additionally, it was established that the highest fraction of exergy destroyed was obtained for the ITC1, which is the device with greater opportunities for improvement since it represents approximately more than one-third (38%) of the total exergy destruction of the system. In addition, the high value of the heat transfer area increases its acquisition costs and the LCOE of the thermal system, which, in some operating conditions, exceeds the leveled costs of generating other energy. Therefore, the greatest effort should be aimed to reduce the exergy destroyed cost in the heat exchanger equipment. In addition, the optimal sizes of these types of equipment under the different organic fluid evaluated in this configuration should be studied from the commercial information on the geometric characteristics of the plate heat exchanger, and shell and tube heat exchanger, to reduce the purchase equipment cost, LCOE, and the environmental impact of the system.
The thermal oil pump (P1) is the equipment that has the lowest exergy efficiency from all the system components, Cyclohexane (15.85%), Toluene (15.60%), and Acetone (15.43%). However, among the heat exchangers equipment, the condenser (ITC3), which has the lowest exergetic efficiency for the different fluids evaluated (Cyclohexane (44.02%), Toluene (46.53%), and Acetone (59.27%)), also stands out because the organic fluid cannot reach the temperatures at which the gases are in the engine exhaust line since there are limitations to thermal stability. Therefore, these alternatives present better results with a low and medium temperature of the waste gases. It is necessary to explore the use of zeotropic mixtures with lower environmental impacts as organic fluid.
From the exergetic and thermo-economic results, it can be concluded that these systems must have a turbine technology with efficiency not greater than 90% since, from this value, the LCOE of the system exceeds the LCOE of the gas turbine, which affects the implementation of these systems in the Colombian industrial sector. Thus, the component with the greatest environmental impact was the turbine, which reached a maximum value of 3013.65 Pts when the material is aluminum, and the organic working fluid is Acetone. This result was obtained when selecting aluminum as a material. The environmental impacts of the components were greater since the Eco 99 coefficient is greater for aluminum than steel.