This section presents the results of the thermodynamic and exergoeconomic assessments for the compact cogeneration unit.
6.1. Thermodynamic Results
All thermodynamic states were determined for the points indicated in Figure 1
. These points represent the inlet and outlet conditions for each equipment within the system studied herein. Table 3
presents the results of the thermodynamic analysis.
shows that the highest temperature is found in the ICE exhaust gases, as expected (point #4). The high- and low-pressure levels of the refrigeration system can be seen when comparing the pressures of points 5–12 (2.63 bar and 13.51 bar).
It must be highlighted that temperature at #4 is extremely high—the heat of exhaust gases can be employed to drive an absorption chiller. A waste heat recovery scheme for a submarine was proposed in Reference [67
] to harness heat from exhaust gases and cooling jacket water and drive a mixed effect absorption chiller. Following the same concept, exhaust gases from a natural gas engine were used in a recovery boiler in Reference [68
], producing steam and hot water. The latter was used to drive an absorption chiller in a trigeneration system that met the energy demands of an ice cream factory. Nevertheless, excess heat could also be used to heat a secondary fluid (water or oil, for example) for another process. Waste heat recovery can speed up the warming of lubricant oil, with a significant reduction of fuel consumption and pollutant emissions experimentally demonstrated in Reference [69
]. Different waste heat recovery schemes for natural gas engines were presented by Reference [70
], based on an ORC coupled with a thermal oil circuit. Reference [71
] used natural gas to fuel an ORC + absorption chiller scheme, producing electricity, heat, and chilled water to satisfy the demands of a university building. Other waste heat recovery options have been reported by Reference [72
], which encompass direct uses (radiation/convection recuperator, passive air preheater, waste heat boiler, economizer, plate heat exchanger, to name a few) and indirect uses (heat-to-heat conversion, heat-to-cooling conversion, and heat-to-power conversion). The enthalpy and entropy values obtained herein are in accordance with scientific literature values for absorption-based refrigeration systems [73
6.2. Exergoeconomic Results
shows the results of the exergoeconomic evaluation via the SPECO method. The tariff considered for gasoline was USD 0.72/l.
As expected, the highest costs are associated with the entry of air and fuel at the unit (#3). The inlet and outlet flows of the steam generator (#10, #11) present high costs when compared with other flows of the refrigeration unit. The monetary costs associated with electricity and cooling are, respectively, 64.14 USD/GJ and 84.74 USD/GJ.
The specific exergy cost is presented in the far-right column of Table 4
. The heat transfer at the absorber (#17) presents the highest exergy cost, followed by the inlet of strong solution in the generator (#10). Two essential processes in the absorber can explain this behavior (higher exergy costs): heat transfer between the cooling stream and the ammonia–water solution, and mass transfer between ammonia vapor and water, which releases a high amount of energy (with an exergy cost). The strong ammonia–water solution also presents a high energy potential, which also contributes to increasing the exergy cost of this current.
presents more results of the exergoeconomic assessment: exergy destroyed (
), cost rate of equipment (
), exergy efficiency (ε), relative cost difference (rk
), and exergoeconomic factor (fk
The first column of Table 5
presents the destruction of exergy in each equipment. The highest exergy destruction occurs in the ICE, followed by the steam generator. The exergy destruction of the absorption refrigeration system is relatively small due to the low flow rate of the working fluid. Regarding the high destructions of exergy, the ICE and steam generator could benefit from reducing their heat losses. Higher efficiency heat exchangers could be employed to improve this situation and system performance. The highest exergy destruction rates and costs also occurred in the engine studied by Reference [77
], but as mentioned by References [78
], are inherent to the combustion process. Reference [80
] investigated the factors affecting exergy destruction and identified that the most sensitive parameters for ICE were the thermodynamic state before combustion and the fuel employed.
depicts the costs associated with exergy destruction (
The second column of Table 5
) accounts for capital costs, operation, and maintenance of equipment, which are the inputs of the simulation. The highest costs are associated with the ICE, followed by the steam generator. Regarding the costs related to destroyed exergy (
), shown in Figure 2
, again, the ICE presents the highest value, followed by the steam generator. When considering the combination of
(cost of destroyed exergy plus total cost rate of equipment), the highest value is associated with the ICE, which indicates the higher fraction of the total cost of the system. The second-highest value of
was for the steam generator. The other components have low values for
, which indicates that improvements should focus elsewhere. The same conclusions were obtained by Shokati et al. [81
]. As mentioned by Reference [33
], such a level of knowledge on the cost–benefit assessment of improvements is provided by exergoeconomics only.
shows that the condenser of the absorption system presents the highest exergy efficiency (ε) (90.91%), followed by the intermediate heat exchanger of the refrigeration unit (70.09%). The pump presents the lowest exergy efficiency, followed by the evaporator and then the ICE. Table 5
also presents two essential parameters for the exergoeconomic evaluation: the relative cost difference (rk
) and the exergoeconomic factor (fk
) of each component. Figure 3
shows the graphic relationship between rk
for each piece of equipment. For visualization purposes, Figure 3
In Figure 3
, attention should focus on the combination of high values for the blue columns and low values for orange columns (high rk
and low fk
), which will indicate the optimization priorities. This combined analysis of rk
shows where optimization efforts should focus: steam generator, evaporator, and absorber. The values obtained for the ICE indicate that there are margins for improvement; however, this piece of equipment is commercially available and mass-produced, preventing interventions, and therefore, substituting the engine for another with higher efficiency has limited applicability. This approach was also followed by the authors of Reference [33
], who identified that the absorber heat exchanger, steam generator, and heat recovery unit could benefit from improvements.
] applied the same methodology to an integrated solar combined gas/steam cycle system and verified that the condenser presented the lowest exergoeconomic factor. In Cavalcanti et al. [84
], a cogeneration system producing steam and electricity was analyzed, for which the lowest exergoeconomic factor was found for the combustor. These low fk
values indicate the importance of decreasing irreversibilities in the equipment. Wu et al. [85
] studied a combined supercritical CO2
recompression Brayton/absorption refrigeration cycle and obtained high fk
values for the reactor and turbine, indicating that a decrease in their capital costs could be obtained at the expense of their efficiency. Rashidi and Yoo [86
] studied power-cooling cogeneration systems and verified that optimization efforts for the Kalina-based system should focus on the absorber (highest rk
and lowest fk
) and then on the superheater.
Shokati et al. [81
] analyzed an ammonia–water double effect absorption refrigeration/Kalina cogeneration cycle, and obtained very low fk
values for the condenser, high-pressure steam generator, and boiler, and concluded that selection of these components with higher quality and price can improve the overall performance of the cogeneration cycle. Mousavi and Mehrpooya [87
] reported on a cascade absorption-compression refrigeration system, and after evaluation of rk
, decided that the compressor and gas heat exchanger presented the best potential for optimization. Wang et al. [88
] explored a novel cooling and power cycle and identified the vapor generator #1 and solution heat exchanger as possibilities for optimization due to the high rk
and low fk
. Souza et al. [71
] proposed a cogeneration system based on the coupling of ORC and an absorption refrigeration system for a university building in Northeast Brazil and verified that further efficiency enhancement actions could be considered for the ORC steam generator.
Although exergoeconomics is as diagnosis tool and provides indications on where to concentrate improvement efforts, the decision-making process itself is beyond the scope of the method. Suggestions to improve the performance of the compact cogeneration unit include the consideration of a different heat exchanger (different materials and geometry, improving the heat exchange area) and pre-heating of input air. Also, other refrigerant fluids can be studied.
Regarding the design of the heat exchanger, investigations encompass the number and orientation of tube passes in the shell, longitudinal fins’ length and thickness, and materials for shell, tube, and fins. Passive intensification of heat transfer in the form of baffles was studied by Andrzejczyk et al. [89
], who obtained higher energy efficiency of the heat exchanger. Yan et al. [90
] carried out numerical simulations with twisted tapes on the shell side of a shell and tube heat exchanger, verifying that the heat transfer coefficient increased with decreasing twist ratio and that the geometric and structural modifications improved design optimization. Considering that a significant component of a shell-tube heat exchanger is the tubes, Tahery et al. [91
] studied different tube count, tube layout, and tube diameter at different baffle sections and obtained better heat transfer and lower exergy destruction rate for shell-tube heat exchangers with segmental baffles. Regarding materials, Khan et al. [92
] demonstrated a significant effect on the thermal performance and observed that copper, aluminum, and aluminum 6063 presented a better thermal performance than steel AISI 4340. Because of the wide range of applicability of these heat exchangers, these improvements enhance domestic and commercial heat storage applications.
Although Riffat et al. [93
] mention that volatile fluids will continue to be employed in cooling and power generation, new refrigerant fluids are required to optimize energy efficiency, increase safety, and decrease environmental impacts. The experimental results on the exergy behavior of R513A vs. R134a were discussed by Mota-Babiloni et al. [94
], with higher exergy efficiency verified for R513A with the advantage that the system does not require retrofitting. Employing a mixture of nanomaterials with pure conventional working fluids (such as refrigerants) presents significant benefits, such as lower global warming potentials, zero ozone depletion potential, higher energy efficiency, lower power consumption, non-flammability, non-toxicity, heat transfer enhancement, and better tribological and rheological behavior [95
]. Most research is still primarily focused on the use of R141b and R134a as working fluids [95
], but the possibilities of using non-standard refrigerants must be considered (such as hydrofluoroolefins, either pure or mixed). Investigation and analysis of R463A as an alternative refrigerant to R404A was carried out successfully by Saengsikhiao et al. [96
], while Życzkowski et al. [97
] focused on R1234ze(E) due to the restrictions against the use of many refrigerants in the European Union since 2015.
6.3. Finals Remarks
A detailed guide has been presented herein, which can be applied to other energy systems (even industrial processes). Special attention must be focused on the adequate selection of control volumes, establishment of the input and output locations for the energy flows, data collection (temperature, pressure, and flow), determination of other thermodynamic properties, correct application of first and second laws of thermodynamics, and finally, correct application of the SPECO method to define the costing equations.
Regarding the choice of prime mover, Reference [33
] has already made a case for the utilization of ICE as the prime mover. ICE has several advantages related to drivability, durability, availability of equipment in different sizes, and easy maintenance. Also, ICE can operate with other fuels, such as biodiesel; however, the economic and environmental advantages are not straightforward. The emissions obtained by Reference [98
] for biomass syngas were much lower than fossil diesel, and confirmed the potential to mitigate climate change, but Reference [99
] obtained much higher emissions for soybean biodiesel than for fossil diesel.
Concerning the depletion of fossil fuels and the consequent uncertainties regarding the security of supply, the development and analysis of combined cooling and power systems has been attracting increasing interest in recent years [100
]. The overarching purpose of combined cycles is the improvement of overall energy conversion efficiency in comparison with separate production. These combined systems can operate with low-grade heat sources and are very promising to supply power and refrigeration simultaneously. Traditional applications of electricity–cooling cogeneration systems include beverage and food industries, medical research facilities (storage of medicines and sensitive products), and marine transportation (marine engines are the largest category of ICE). Some of the advantages of compact electricity–cooling cogeneration units include: (1) possibility of meeting smaller-scale energy demands, such as for the tertiary sector (shopping centers, hospitals, hotels, supermarkets), (2) wide utilization in distributed generation schemes and outside large urban centers, (3) use of internal combustion engines, which are cheaper and easier to maintain than turbines, and (4) enables the use of renewable energy resources.
The advantages of utilizing exergoeconomics for the diagnosis of energy systems have been the overarching aim of this study, and demonstrated herein, taking into account the energy and exergy balances of system components. In this case, exergoeconomics enables the diagnosis of thermodynamic inefficiencies and identification of where exergy is destroyed within the system. Thus, it is clear which equipment needs to be prioritized regarding improvements in the project or even in the process as a whole. This demonstration is followed by a discussion on the method and productive structure.
Finally, when considering energy efficiency strategies and levels of greenhouse gases, electricity–cooling cogeneration systems are interesting options for the industrial sector but can also meet the cooling and electricity requirements of a district or a city. As of August 2020, the crisis precipitated by the COVID-19 pandemic has cascaded across socio-economic sectors [103
]. The energy sector has been affected, which could potentially be averted by implementing stimulus plans to boost clean energy technologies [103
] and energy efficiency schemes [104
]. Pina et al. [105
] mention that in the wake of the pandemic, countries must kickstart their economies while consolidating climate change commitments, and energy efficiency is a crucial pillar of the energy transition (along with renewable energy and energy storage). Due to the deceleration of most energy transition programs and considering the scarcity of studies on the impacts of COVID-19 on the electric sector, cogeneration systems can and must be part of energy efficiency solutions to enhance economic competitiveness, providing more affordable energy services, and reducing environmental impacts.