3E investigation of ORC and absorption chiller for repowering a Brazilian landfill biogas power plant

1 Introduction

The Government of the State of Espírito Santo (in Brazil) established the “Espírito Santo’s Decarbonization and GHG Emissions Neutralization Plan” (Governo do Estado do Espírito Santo, 2023b; Governo do Estado do Espírito Santo, 2023a) as part of its strategy to promote low-carbon development and accelerate the energy transition in the state. Among the proposed actions, the plan highlights the expansion of biomass, biogas, and waste-to-energy (WtE) thermal systems in the electricity matrix as a key strategy to reduce greenhouse gas (GHG) emissions while increasing electricity generation from renewable and dispatchable sources. In this context, regional industries have been developing initiatives focused on improving energy efficiency and reducing emissions (Gonçalves et al., 2025; Quintão et al., 2025).

Several studies have demonstrated the relevance of biomass and biogas technologies for sustainable energy systems. Biomass-based thermoelectric plants have been identified as viable alternatives for replacing coal-fired generation, contributing to significant emission reductions while ensuring continuous electricity supply (Bose and Saini, 2022). In addition, biomass has been recognized as a strategic resource for increasing energy system flexibility and supporting electricity generation in renewable-based scenarios (Colla et al., 2022; Korberg et al., 2021). Biogas utilization in combined heat and power (CHP) systems has also shown considerable environmental advantages, with impacts 35%–38% lower than conventional grid electricity (Mistretta et al., 2022), while hybrid systems incorporating biogas may reduce emissions by up to 96% and simultaneously increase electricity generation capacity (Anand et al., 2023). Likewise, integrated large-scale WtE plants contribute to reducing fossil fuel consumption (Chen et al., 2025) and may achieve energy efficiencies of up to 60%, providing higher electricity generation compared with conventional plants (Hsu et al., 2024).

Among WtE technologies using gaseous fuels, Internal Combustion Engines (ICE) are considered one of the most efficient and economically attractive alternatives for low- and medium-scale power generation applications (Carlos Escobar Palacio et al., 2019). However, despite their relatively high efficiencies, ICEs still reject a substantial amount of thermal energy through exhaust gases and cooling systems, creating opportunities for waste heat recovery (WHR) and efficiency improvement (Morawski, 2021).

Different Waste Heat Recovery (WHR) technologies have been investigated for engine applications, including the conventional Rankine cycle (CRC), Organic Rankine Cycle (ORC), and Kalina cycle.

Among these technologies, the ORC has emerged as one of the most promising solutions for recovering low- and medium-temperature waste heat because of its high efficiency, operational simplicity, reliability, and flexibility (He et al., 2012; Shengjun et al., 2011). Unlike the conventional steam Rankine cycle, whose performance is limited under low-temperature conditions, the ORC employs organic working fluids, making it particularly suitable for converting low-grade heat sources, such as solar energy, geothermal energy, and industrial waste heat, into useful power (Hung et al., 1997).

ORC technology has been extensively investigated from different perspectives, including working-fluid selection (Pirouzfar, Sheykhan, et al., 2026; Rowshanaie et al., 2022; Rowshanaie et al., 2026), thermodynamic performance analysis (Li et al., 2012; Pirouzfar, Aghajani, et al., 2026; Wei et al., 2007), system design (Clemente et al., 2012; Li et al., 2012), and optimization of operating parameters such as turbine inlet pressure, evaporation temperature, pinch-point temperature difference, and heat transfer area (Li et al., 2012; Roy et al., 2010). Its application has also been widely explored across several engineering fields. Bianchi and De Pascale (2011) identified the ORC as one of the most mature technologies for recovering waste heat from Internal Combustion Engines (ICEs), while Tchanche et al. (2011) and (Vélez et al. (2012) reviewed its technical maturity, applications, and economic potential in engine-related systems.

Comparative studies have further demonstrated the advantages of the ORC over alternative technologies. Bombarda et al. (2010) compared the ORC with the Kalina cycle for recovering waste heat from diesel-engine exhaust gases and concluded that, although both systems achieved similar power outputs, the ORC was more attractive because of its simpler configuration, lower equipment requirements, and reduced material constraints.

More recent studies have reinforced these findings, indicating that ORC systems have become one of the most technically and economically viable alternatives for recovering low- and medium-temperature waste heat from ICEs (de Araújo et al., 2022; Morawski et al., 2021; Ribeiro et al., 2017).

Landfill biogas power plants represent an important opportunity for sustainable engineering innovations focused on repowering strategies and energy efficiency improvements. Previous studies demonstrated that landfill biogas-fired combined cycle plants integrating ICEs with ORC systems can increase overall efficiencies to values between 40% and 50% (Saravia et al., 2012). Furthermore, landfills can serve as platforms for advanced multigeneration systems, including Combined Cooling, Heating and Power (CCHP), ORC–Kalina Cycle integrations, and electrolyzer couplings. Significant efficiency gains and environmental benefits have been reported in systems simultaneously producing hydrogen, heating/cooling, electricity, and natural gas, reaching energy efficiencies of up to 57% through the integration of Kalina cycles, ORC systems, and cold energy recovery technologies (Zhang et al., 2024). Similarly, advanced multigeneration systems combining CCHP, desalination, and LNG cold energy recovery have demonstrated efficiencies approaching 59%, while also contributing to CO₂ emission reductions (Xu et al., 2024). In addition, waste heat-driven biogas cogeneration systems highlight heat recovery as a key strategy for improving efficiency and mitigating environmental impacts (Wang and Li, 2025). In this context, landfills represent a strategic opportunity for sustainable innovation due to the continuous availability of biogas and their integration potential with renewable energy technologies. Consequently, landfill gas can be considered a lost energy opportunity when not recovered for electricity generation (Broun and Sattler, 2016). Some studies have evaluated the energy recovery potential from Brazilian municipal solid waste and demonstrated that it can represent a significant renewable energy resource (Guabiroba et al., 2023; Gutierrez-Gomez et al., 2021; Lino and Ismail, 2018).

In tropical countries such as Brazil, however, ICE performance is negatively affected by harsh climate conditions, particularly high ambient temperatures and excessive humidity, which reduce air density and consequently decrease electrical output, efficiency, and fuel conversion performance (Miotto, 2021; Miotto et al., 2022). To avoid knocking under such operating conditions, biogas engines commonly operate under derating conditions, limiting their nominal power output. In this sense, intake-air cooling and dehumidification emerge as viable repowering strategies capable of increasing engine performance and operational stability (Chun et al., 2021; Chun et al., 2023; Fonseca, 2024; Fonseca et al., 2024). Therefore, besides WHR technologies, intake-air conditioning represents another promising approach for improving ICE performance in landfill biogas applications.

Among WHR alternatives, mechanical chillers may be employed to cool intake air when integrated with power cycles, whereas absorption chillers can directly recover waste heat to produce chilled water for intake-air conditioning (Ryan, 2004). Beyond intake-air cooling and dehumidification, chilled water may also reduce engine coolant temperatures, enabling additional intercooler cooling and/or lower electricity consumption in radiator systems (Chun, 2020). Consequently, combining WHR technologies with intake-air cooling may simultaneously increase power generation, improve efficiency, and mitigate derating effects in ICE-based biogas power plants.

Within this context, the largest sanitary landfill in Espírito Santo (Brazil) recently underwent a major repowering process, replacing a steam-cycle thermal power plant that generated 1 MW at approximately 13% efficiency with three ICE units operating at nearly 40% electrical efficiency, increasing total electricity generation capacity to 3 MW. However, due to the simplicity and lower implementation cost of the current configuration, the plant was designed without waste heat recovery systems or intake-air cooling technologies. As a result, the engines still operate under derating conditions while rejecting significant amounts of thermal energy through exhaust gases and cooling water.

This scenario motivated an R&D Project aimed at investigating WHR alternatives for further repowering of the landfill biogas power plant through the integration of ORC systems and absorption chillers, based on a combined energy, exergy, and economic (3E) assessment. Therefore, the main objective of this work is to evaluate the integration of waste heat recovery and intake-air cooling technologies as strategies to improve the efficiency and repowering potential of landfill biogas ICE systems, thereby increasing the contribution of biomass-derived energy to the electricity matrix in accordance with the decarbonization goals established by the Government of Espírito Santo, which has committed to the United Nations “Race to Zero” and “Race to Resilience” initiatives, aiming to achieve net-zero emissions by 2050.

Using design and nominal operating data, mass, energy, and exergy balances were performed to evaluate the thermodynamic limits of waste heat recovery using two ORC configurations, in Engineering Equation Solver EES (F-Chart Software, 2017). Cycle simulations were conducted using IPSEpro 8.0 (Simtechnology, 2024) software. The same tools were also applied to simulate intake-air and biogas conditioning systems employing absorption chillers. The economic assessment considered both operational expenditures and capital investments, the latter estimated through parametric equations and reference market data for equipment sizing and costing. Finally, using EES software, economic feasibility indicators, including payback period, net present value (NPV), and internal rate of return (IRR), were applied to assess the viability of the proposed repowering alternatives.

Although several studies have investigated waste heat recovery in landfill biogas power plants, most have focused separately on ORC systems for exhaust gas recovery or on intake-air conditioning to mitigate engine derating. Consequently, there remains a lack of integrated assessments capable of determining the most appropriate recovery pathway for waste heat streams with different thermodynamic qualities, particularly under the high-temperature and high-humidity conditions found in tropical regions. To address this problem, the present study evaluates, through a combined energy, exergy, and economic assessment, an integrated repowering strategy that uses exhaust gas heat in an ORC system and high-temperature cooling-water heat in an absorption chiller for intake-air cooling and biogas conditioning. The novelty of the work lies in combining these different recovery pathways within a real industrial landfill biogas power plant and in demonstrating that effective repowering requires matching each waste heat source with the technology most compatible with its thermodynamic quality and the required energy service.

2 Case study and power plant description

The case study investigates a landfill biogas power plant operated by Marca Ambiental, a municipal solid waste treatment and recovery company located in the metropolitan region of Grande Vitória, Brazil. The facility has been receiving and treating municipal solid waste since 1997 and represents a relevant waste-to-energy application in an urbanized and industrialized regional context. The investigated plant was selected because it integrates continuous municipal solid waste treatment, landfill biogas recovery, and distributed electricity generation within an industrial infrastructure, providing a suitable framework for evaluating repowering alternatives.

At the landfill, municipal solid waste is deposited and subsequently covered with soil layers, creating confined zones in which the organic fraction of the waste undergoes microbial degradation under predominantly anaerobic conditions. This process generates landfill biogas and leachate, both of which are collected and directed to dedicated treatment systems. The recovered biogas is captured through a dedicated collection network and transported to the thermal power plant, where it is used as fuel for electricity generation.

The average volumetric chemical composition of the treated landfill biogas, together with the representative environmental conditions adopted in this study, is presented in Table 1. These environmental parameters are particularly relevant because they directly influence the fuel heating value, combustion behavior, engine performance, and the thermodynamic characteristics of the waste heat streams evaluated in the subsequent analyses. As expected for treated landfill biogas, methane (CH₄) and carbon dioxide (CO₂) are the predominant components, with minor fractions of inert gases and oxygen.

TABLE 1. Average landfill biogas characterization and representative environmental conditions adopted in this study.

2.1 Previous power plant and background

The energy recovery system at the facility has undergone a significant technological transition over time. The plant initially operated with a steam-cycle power generation system, producing approximately 1 MW of electrical power with an efficiency of around 13%, using a comparable landfill biogas flow rate.

The thermal scheme of the project when the power plant operated with a boiler producing steam at 320 °C and 22 bar, with the capacity to burn slightly more than 0.70 kg/s of biogas. However, the system operated with steam condensation at atmospheric pressure, which limited its overall efficiency. One advantage of this configuration was that the landfill biogas did not require prior treatment, and the system was also capable of burning dry waste. Nevertheless, dry waste was not used in practice because its availability was very limited.

Several simulations aimed at evaluating alternatives for repowering and improving plant efficiency were conducted (Passos and Bonelle, 2011). The results indicated that, by adopting vacuum condensation, the plant could achieve an electrical output exceeding 2 MW and an efficiency greater than 18%. However, from an energy-performance perspective, the most attractive alternative was the adoption of internal combustion engine technology, which could more than triple both power generation and overall efficiency. Nevertheless, this alternative would prevent the use of dry waste as fuel and would require the installation of a dedicated biogas treatment system.

2.2 Actual landfill biogas power plant

Figure 1 presents an overview of the landfill biogas capture and power generation facility investigated in this study, highlighting the integration between the landfill gas collection network and the electricity generation system. In 2020, the current power plant began operation with three internal combustion engine generator sets. There are plans to install two additional units if landfill biogas production increases in the future.

As shown in Figure 2, before reaching the power generation plant, the biogas passes through filters and heat exchangers to remove compounds that may damage the equipment, such as water vapor and hydrogen sulfide (H₂S). To remove condensable compounds, a vapor-compression chiller is employed to cool the biogas to 3 °C through a counterflow heat exchanger arrangement. The compression chiller used for biogas cooling has a cooling capacity of 105,000 kcal/h, equivalent to 122.11 kW or 34.72 TR, with a coefficient of performance (COP) of 4.2 (Carrier, 2023).

Due to the high ambient temperature and humidity conditions at the site, the three generator sets operate under derated conditions to prevent engine knocking, producing approximately 1 MW of electrical power each, for a total installed output of approximately 3 MW. The units are Jenbacher J416 GS engine-generator sets (Figure 3), consisting of four-stroke Otto-cycle reciprocating engines equipped with turbochargers and specifically designed for biogas applications. Under nominal design conditions, however, each engine is rated at 1,141 kW, with an electrical efficiency of 41.3%.

Figure 4 presents details of the facilities at Marca Ambiental. Figure 4A) illustrates the position of the engines, indicated by letter A, inside the green container, as well as the air filter (B), the radiators (C), and the chimney (D). Figure 4B) shows the chiller (E) and the flare (F).

Figure 5 presents the thermal scheme of a single engine-generator set, including the main mass and energy streams under nominal design conditions. The engine cooling system operates with demineralized water in two closed thermal circuits: a high-temperature (HT) circuit and a low-temperature (LT) circuit. The HT loop removes heat primarily from the engine block and lubrication-related thermal loads, whereas the LT loop is responsible for lower-temperature thermal management functions, including charge-air aftercooling processes. Heat rejection from both circuits is currently performed through radiator banks, without any useful energy recovery for electricity generation or cogeneration applications.

The main technical characteristics and representative operating conditions of the engine-generator units considered in this study are summarized in Table 2. LT and HT stand for low-temperature and high-temperature, respectively.

TABLE 2. Main technical specifications in nominal operating conditions of the engine-generator sets.

This configuration provides a representative industrial case for evaluating waste heat recovery opportunities under practical operating conditions. From a thermodynamic perspective, the main recoverable heat sources are the engine exhaust gases and the cooling water circuits associated with the thermal management of the internal combustion engines. These streams differ substantially in temperature level, thermal quality, and practical recoverability, making the investigated facility particularly suitable for comparative repowering assessments. Among the available sources, the exhaust gases constitute the highest-temperature thermal stream in the system, representing a thermodynamically attractive source for bottoming-cycle integration. However, practical recovery of exhaust gas heat is constrained by limitations associated with the potential condensation of sulfur-containing acidic compounds.

Table 3 summarizes the main recoverable heat streams considered in the preliminary thermodynamic assessment. At first inspection, the HT cooling circuit appears particularly attractive because of its comparatively high recoverable heat rate, which even exceeds the thermal availability of the exhaust gases. However, thermal magnitude alone is insufficient to evaluate practical repowering feasibility, since useful power generation fundamentally depends on temperature level and heat quality.

TABLE 3. Nominal recoverable heat streams considered in the preliminary repowering assessment.

The recoverable heat available in the exhaust gases (464 kW) was determined by difference, based on the heat dissipated through the radiators and on manufacturer data indicating a total recoverable heat capacity of 1,220 kW, although without specification of the final exhaust gas temperature after complete heat recovery.

Assuming complete combustion and based on the data presented in Table 1, 2, the molar composition of the exhaust gases was estimated as 12.85% CO₂, 14.49% H₂O, 67.57% N₂, and 5.09% O₂. These characteristics complete the dataset required for the energy and exergy balance assessments of the waste heat recovery systems investigated in this study.

3 Methodology

This work presents and discusses the procedures and results of an R&D Project, here called Marca Ambiental R&D, that incorporated multiple methodologies and analytical approaches, including both those developed in a master’s thesis (Loyola, 2023) and an undergraduate thesis (Piski and Zani, 2023). The exergy balance used to evaluate the repowering potential through waste heat utilization was performed using Engineering Equation Solver (EES). The mass and energy balances for the simulation and design of the organic Rankine cycles and an absorption chiller were carried out using IPSEpro 8.0.

The evaluation of biogas and intake air cooling and dehumidification system was based on design data obtained from a similar experimental project, the CRATOS R&D (Fonseca, 2024). For the economic assessment, actual operational and economic data were employed, while the equipment costs were estimated using parametric cost correlations. For the organic Rankine cycles, the methodology proposed by Turton et al. (2012) was adopted. The costs of the remaining equipment were estimated according to the methodologies of Boehm (1987) and Bejan et al. (1996), using reference data from the CRATOS R&D Project. Conventional economic indicators were employed, including net present value (NPV), internal rate of return (IRR), and both simple and discounted payback periods.

3.1 ICE energy and exergy balance

The first step of the methodology consisted of performing energy and exergy balances for the existing internal combustion engine-generator units. This assessment was necessary to quantify the distribution of biogas energy and exergy among useful electricity generation, exhaust gas losses, cooling water losses, and other irreversibilities. The same control-volume approach was applied to each Jenbacher J416 GS unit, assuming steady-state operation and neglecting variations in kinetic and potential energy.

The energy input supplied by the landfill biogas was calculated from the biogas mass flow rate and its lower heating value, according to Equation 1, where Q ˙ b is the biogas energy input rate, m ˙ b is the biogas mass flow rate, and LHV is the lower heating value of the biogas.

Q ˙ b = m ˙ b · L H V (1)

The chemical exergy rate of the biogas mixture was determined from the molar composition of the fuel gas, according to Equation 2 (Moran et al., 2018), where E x ˙ b is the biogas exergy rate, n ˙ b is the molar flow rate of the biogas, y i is the molar fraction of component i, e i c h is the standard chemical exergy of component i, R ¯ is the universal gas constant, and T 0 is the reference environment temperature. This formulation follows the energy and exergy balance methodology adopted in the previous thermodynamic assessment of the plant.

E x ˙ b = n ˙ b ∑ i = 1 j y i e i c h + R ¯ T 0 ∑ i = 1 j y i ⁡ ln ⁡ y i (2)

For each cooling water circuit, the recoverable heat rate was calculated from the water mass flow rate and the enthalpy difference between outlet and inlet states, according to Equation 3, where Q ˙ c w , i is the recoverable heat rate of cooling circuit i, m ˙ c w , i is the cooling water mass flow rate, and h o u t , i and h i n , i are the specific enthalpies at the outlet and inlet of the considered circuit.

Q ˙ c w , i = m ˙ c w , i h o u t , i − h i n , i (3)

The total cooling water heat rejection was obtained by summing the contributions of the high-temperature and low-temperature circuits, according to Equation 4.

Q ˙ c w = ∑ i Q ˙ c w , i (4)

The corresponding exergy rate associated with each cooling water stream was calculated according to Equation 5, where s o u t , i and s i n , i are the specific entropies of the cooling water at the outlet and inlet of circuit i. The total cooling water exergy rate was obtained by summing the exergy rates of the individual cooling circuits.

E x ˙ c w , i = m ˙ c w , i h o u t , i − h i n , i − T 0 s o u t , i − s i n , i (5)

The energy rate associated with the exhaust gases was calculated using the molar flow rate and the enthalpy variation of each component between the reference state and the exhaust gas state, according to Equation 6, where Q ˙ e g is the exhaust gas energy rate, n ˙ e g is the molar flow rate of the exhaust gases, y i is the molar fraction of component i, and Δ h ¯ i is the molar specific enthalpy variation of the component.

Q ˙ e g = n ˙ e g ∑ i = 1 j y i Δ h ¯ i (6)

The exergy of the exhaust gases was evaluated as the sum of physical and chemical contributions, according to Equation 7 and the physical exergy component was calculated according to Equation 8, where Δ s ¯ i is the molar specific entropy variation of component i.

E x ˙ e g = E x ˙ e g p h + E x ˙ e g c h (7)

E x ˙ e g p h = n ˙ e g ∑ i = 1 j y i Δ h ¯ i − T 0 Δ s ¯ i (8)

The chemical exergy component of the exhaust gas mixture was calculated from the deviation of the exhaust gas composition from the environmental reference composition, according to Equation 9, where y i e is the molar fraction of component i in the reference environment (Moran et al., 2018). The exhaust gas molar composition was obtained from a complete combustion model with wet air excess, as adopted in the previous plant assessment.

E x ˙ e g c h = n ˙ e g R ¯ T 0 ∑ i = 1 j y i ⁡ ln ⁡ y i y i e (9)

The overall energy balance of each engine-generator unit was expressed according to Equation 10, where W ˙ e l is the useful electrical power output, Q ˙ e g is the exhaust gas heat rate, Q ˙ c w is the cooling water heat rejection, and Q ˙ l o s s accounts for other heat losses not explicitly recovered.

Q ˙ b = W ˙ e l + Q ˙ e g + Q ˙ c w + Q ˙ l o s s (10)

The corresponding exergy balance was written according to Equation 11, where E x ˙ d represents the exergy destruction associated with combustion, heat transfer irreversibilities, internal engine losses, and other non-recovered exergy losses. Since electrical power is pure exergy, the useful power output appears directly in the exergy balance.

E x ˙ b = W ˙ e l + E x ˙ e g + E x ˙ c w + E x ˙ d (11)

This energy–exergy framework enabled the identification of the recoverable waste heat streams and their thermodynamic quality. The comparison between Q ˙ e g, Q ˙ c w, E x ˙ e g and E x ˙ c w was used to distinguish energetic availability from actual work potential, providing the basis for selecting the thermal sources subsequently evaluated in the ORC repowering models.

3.2 Thermo-economic modeling for ORC repowering

To evaluate the technical and economic feasibility of repowering the landfill biogas power plant, organic Rankine cycle (ORC) systems were thermodynamically modeled for each of the main waste heat sources identified in the previous assessment: one associated with the exhaust gases and another with the high-temperature (HT) engine cooling circuit. The low-temperature (LT) cooling circuit heat, 62 kW only, was neglected. The objective of the ORC modeling was to quantify the additional power that could be generated from the recoverable thermal energy under realistic thermodynamic and selected design constraints. Figure 6 illustrates the ORC configurations evaluated in this study.

Separate ORC configurations were modelled for each waste heat source due to their substantially different thermal levels and thermodynamic characteristics. Based on the optimized decision variables in the works of Morawski et al. (2021) and de Araújo et al. (2022), as well as in the work of Benato and Macor (2017) who optimized ORC for ICE using landfill biogas, toluene was selected as the working fluid for the exhaust-gas-driven ORC, whereas R141b was adopted for the cooling-water-driven ORC, and the main ORC design parameters used in the simulations are summarized in Table 4.

TABLE 4. Principal ORC modeling parameters adopted for the repowering scenarios.

For each ORC configuration, steady-state operation was assumed, with negligible kinetic and potential energy variations. Pressure losses in cycle heat exchangers were neglected, and all equipment were considered adiabatic. In addition, no superheating or subcooling was assumed at the heat exchanger outlets, with saturated vapor entering the turbines and also saturated liquid leaving the condensers.

Capital investment was estimated using the Module Costing Technique (MCT) combined with parametric equipment cost correlations, a methodology widely adopted for preliminary thermo-economic assessments of thermal systems. This approach enables realistic estimation of installed capital cost by incorporating both direct and indirect expenses, including equipment acquisition, installation, engineering, contingencies, transportation, and auxiliary infrastructure.

The principal ORC components considered in the economic assessment included the turbine, working-fluid pump, evaporator, condenser, cooling tower, and associated auxiliary equipment.

The economic analysis of equipment in chemical plants frequently follows the methodology proposed by Turton et al. (2012), which provides cost estimates ranging from equipment acquisition to installation. Morawski (2021) validated the parametric cost correlations proposed by (Turton et al., 2012) using commercial investment data for ORC systems.

The economic methodology proposed by Turton et al. (2012) requires the estimation of the heat transfer areas of the heat exchangers. Therefore, the following procedure was adopted to determine these areas: the heat transfer areas were calculated using the Logarithmic Mean Temperature Difference (LMTD) method, assuming a counterflow configuration. The LMTD is determined according to Equation 12.where Δ T 1 and Δ T 2 represent the maximum and minimum temperature differences in the evaporator and condenser heat exchangers, respectively. The corresponding heat transfer area A is calculated according to Equation 13, where U denotes the overall heat transfer coefficient.

Δ T L M T D = Δ T 1 − Δ T 2 ln Δ T 1 Δ T 2 (12)

A = Q U · Δ T L M T D (13)

The base equation used to estimate the purchased equipment cost C P 0, assuming carbon steel construction and operation at ambient pressure, is given by Equation 14.where (A) denotes the equipment capacity or characteristic size parameter. Different types of equipment, including pumps, turbines, compressors, and heat exchangers, have specific coefficients K i provided by Turton et al. (2012). To account for variations in construction materials and operating pressures, appropriate correction factors are introduced according to Equation 15.where (p) represents the operating pressure of the equipment. Typical values of the coefficients C i, as well as the corresponding correction factors, are tabulated for different equipment categories in (Turton et al., 2012).

C P 0 = K 1 + K 2 ⁡ log 10 A + K 3 log 10 A 2 (14)

log 10 ⁡ F P = C 1 + C 2 ⁡ log 10 p + C 3 log 10 p 2 (15)

In addition to the capital expenditure associated with equipment acquisition, other costs must be considered, including installation, auxiliary materials, instrumentation, structural components, piping, and labor. Indirect expenses, such as transportation, insurance, and engineering services, are also incorporated. To account for these contributions, the bare module factor F B M is applied, aggregating both direct and indirect investment costs and multiplying the purchased equipment cost (Turton et al., 2012).

When the equipment is not manufactured from carbon steel, a material factor F M must also be included. This coefficient, obtained from reference tables, varies according to both equipment type and construction material. The resulting bare module cost C B M therefore represents the total direct and indirect expenditures associated with the equipment, as expressed in Equations 16, 17: where the coefficients (B _*i*) depend on the equipment type (Turton et al., 2012). For existing thermal power plants, additional fees and contingency costs are typically included, corresponding to 3% and 15% of the bare module cost, respectively. The total investment cost is then calculated according to Equation 18.

C B M = C p 0 . F B M (16)

F B M = B 1 + B 2 + F M . F p (17)

C T M = 1.18 ∑ i = 1 n C B M , i (18)

The parameters required for estimating the acquisition cost of cooling towers are not addressed by (Turton et al., 2012). Therefore, the cooling tower costs were evaluated using the methodologies proposed by Boehm (1987) and Bejan et al. (1996). This approach determines the purchased equipment cost (C P , i) based on the reference cost (C r e f , i), reference parameter (X r e f , i), equipment-type exponent (m i), and calculated parameter (X i), according to Equation 19.

C P , i = C r e f , i X i X r e f , i m i (19)

In addition, Boehm (1987) provides two reference costs, together with the corresponding tower-type exponent, reference cooling-water flow rate, and operating range, as presented in Table 5. The selection of the appropriate cooling tower reference depends on the calculated cooling-water flow rate.

TABLE 5. Reference parameters for cooling towers for the year 2001.

Finally, the total cost must be adjusted to the year of project implementation. In the methodology adopted, as proposed by Turton et al. (2012), the equipment costs were originally estimated based on 2001 values and therefore require updating. For this purpose, the costs are corrected using inflation-based indices, and for thermal system projects, the Chemical Engineering Plant Cost Index (CEPCI) (Chemical Engineering, 2023) is recommended. Finally, the corrected cost using the CEPCI index is calculated according to Equation 20.with CEPCI ₂₀₀₁ = 397 and CEPCI ₂₀₂₂ = 821.3.

C T M 2022 = C T M 2001 C E P C I 2022 C E P C I 2001 (20)

Economic viability was evaluated through conventional investment indicators, namely, simple payback period, discounted payback period, net present value (NPV), and internal rate of return (IRR).

The simple payback period was calculated using Equation 21, where C F a n n u a l is the annual net cash flow generated by additional electricity production.

P B = C t o t C F a n n u a l (21)

To account for the time value of money, discounted cash flow analysis was performed using Equation 22, where C F n is the net cash flow in year n, i is the discount rate, and N is the project lifetime.

N P V = − C t o t + ∑ n = 1 N C F n 1 + i n (22)

The internal rate of return was obtained from Equation 23.

0 = − C t o t + ∑ n = 1 N C F n 1 + I R R n (23)

A project was considered economically attractive when presenting a positive NPV, an IRR exceeding the minimum attractive rate of return, and a discounted payback period shorter than the project lifetime.

Annual cash flow was estimated from the revenue associated with the additional electricity generated by the ORC systems, based on the electricity sale tariff adopted in the reference case and the corresponding plant dispatch conditions. Operation and maintenance costs (6%) were also incorporated into the economic assessment.

The baseline economic assumptions adopted in this study included a minimum attractive rate of return of 12% per year, a project lifetime of 20 years, and an electricity price of USD 0.130/kWh.

Because the exhaust-gas-driven and cooling-water-driven ORC systems exhibit significantly different thermodynamic performance and capital requirements, the economic assessment was conducted independently for each repowering scenario.

3.3 Thermo-economic assessment of absorption chiller pathway

Loyola (2023), using the methodology described in the previous section and considering the generator sets operating under nominal design conditions, concluded that recovering heat from the exhaust gases through an ORC system was economically viable, whereas the use of ORC for heat recovery from the high-temperature cooling water circuit was not economically feasible. Therefore, the author proposed evaluating the use of cooling water heat through an absorption chiller to produce chilled water for engine intake air conditioning, as well as to replace the vapor-compression chiller employed in the biogas conditioning system. Based on this proposal, Piski and Zani (2023) defined the layout shown in Figure 7, which incorporates an absorption chiller and auxiliary equipment for this purpose.

Both the layout presented in Figure 7 and the entire methodology adopted for the thermo-economic evaluation of this repowering route were developed and assessed using real data from another similar experimental project, the CRATOS R&D Project, as the technological and economic reference, as reported by Chun (2020); Fonseca (2024) and B. M. de F. Miotto (2021). The simulation of the absorption chiller was carried out using IPSEpro 8.0 software and the overall system simulation in EES software.

Figure 8 presents the thermal scheme of the absorption chiller simulation developed in IPSEpro 8.0, while Table 6 summarizes the main input data used in the simulation. Based on the results obtained from IPSEpro 8.0, the overall mass and energy balances of the integrated layout were performed using EES software. For the equipment cost estimation, the purchased equipment cost C P , i from the reference equipment cost C r e f , i and the corresponding reference parameter (X r e f , i were obtained from the CRATOS R&D Project, according to Equation 19, together with Equations 12, 13.

TABLE 6. Input data used in the IPSEPro 8.0 for simulation of the absorption chiller.

Figure 8 shows that the decision was made to utilize all the heat from the high-temperature cooling water circuit to produce as much chilled water as possible, using an absorption chiller (AC). This chilled water is used for two main purposes: to replace the existing compression chiller in the biogas conditioning (Biogas heat exchanger) and also to condition the intake air for the engines in the cooling and dehumidification coils. In addition, the remaining chilled water is used in an auxiliary heat exchanger to reduce the cooling tower load.

Table 7 presents the reference costs and capacities, used for equipment costing in this work, together with the corresponding scaling exponents, based on real data from the CRATOS Project. The economic viability was evaluated using conventional economic indicators, including payback period, net present value, and internal rate of return, as presented in Equations 21–23, for ORC economic analysis.

TABLE 7. Equipment reference data from the CRATOS Project and their respectively exponent.

Similarly, the baseline economic assumptions adopted in this study included a minimum attractive rate of return of 12% per year, a project lifetime of 20 years, and an electricity price of USD 0.130/kWh.

3.4 Final economic analysis

After the independent thermo-economic assessment of the ORC and absorption chiller pathways, a final integrated economic analysis was performed considering the combination of the two recovery strategies identified as viable in the previous evaluations. Therefore, the final repowering configuration consists of an exhaust-gas-driven ORC system combined with the absorption chiller arrangement for both biogas and ICE intake-air conditioning, using cooling and dehumidification processes. The high-temperature engine cooling-water circuit heat was thermal driving source for the absorption chiller.

The objective of this assessment was to evaluate the overall economic attractiveness of the integrated repowering strategy from the perspective of the landfill biogas power plant operator. The annual economic benefit was determined as the sum of two contributions: (i) the revenue associated with the additional electricity generated by the exhaust-gas-driven ORC and (ii) the economic gains resulting from the absorption chiller system, mainly avoiding all engine derating losses and the electric power due to the total replacement of the existing vapor-compression chiller in the biogas conditioning.

The total investment cost of the integrated configuration was obtained by combining the capital expenditures estimated for the ORC system and for the absorption cooling pathway. The operation and maintenance costs associated with both systems were incorporated into the economic evaluation too.

The overall economic feasibility of the integrated repowering strategy was assessed using the same indicators adopted throughout this work, namely, simple payback period, discounted payback period, net present value (NPV), and internal rate of return (IRR).

4 Results and discussion

The first step of the repowering assessment consists of evaluating the thermodynamic characteristics of the principal waste heat streams available in the landfill biogas power plant. Although exhaust gases and cooling water circuits exhibit comparable thermal magnitudes from an energy perspective, their actual potential for useful work recovery differs substantially due to the temperature level and thermodynamic quality of each stream. Therefore, the present analysis is based not only on energetic availability, but also on exergy considerations, aiming to distinguish the theoretical thermal potential from the practical repowering capability associated with each waste heat source.

4.1 Thermodynamic limits for ORC repowering

The thermodynamic limits for ORC repowering were first evaluated from the energy and exergy balances of one representative engine-generator unit. This preliminary assessment is essential because waste heat recovery potential cannot be interpreted solely from the amount of rejected heat. Although energy balances quantify the magnitude of the thermal streams available in the system, exergy balances indicate their thermodynamic quality and, consequently, their maximum theoretical potential to be converted into useful power.

Table 8 summarizes the energy and exergy balances for one engine-generator unit, while Figures 9, 10 presents the corresponding Sankey and Grassmann diagrams, respectively. These results provide the basis for distinguishing the apparent thermal availability of each waste heat source from its actual potential for repowering.

TABLE 8. Energy and exergy balances for one representative engine-generator unit.

Although the mechanical and electrical power outputs remain exactly the same in both the energy and exergy balances, the corresponding exergy efficiencies of the engine-generator unit are lower than the energetic efficiencies because the exergy of the landfill biogas is inherently greater than its energy evaluated from the lower heating value. This distinction becomes even more relevant when analyzing the waste heat streams available for repowering.

As expected, the exergy associated with the residual heat streams is always lower than their corresponding energy content, and this difference becomes more pronounced as the temperature level decreases. In this sense, while the energy balance indicates the magnitude of the available heat, the exergy balance reflects its thermodynamic quality and, consequently, its maximum theoretical potential for useful power generation.

The exhaust gases and the cooling-water circuit present nearly the same energetic magnitude, corresponding to approximately 27% of the fuel energy input, or nearly 750 kW per engine-generator unit. However, the exergy analysis reveals a substantially different repowering potential between these two waste heat sources. While the exhaust gases retain more than 410 kW of exergy per engine, corresponding to approximately 14% of the fuel exergy, the cooling-water stream presents less than 120 kW, equivalent to slightly more than 4% of the fuel exergy.

From a purely energetic perspective, the complete recovery of all residual heat streams could theoretically increase the overall efficiency to approximately 98.8%. However, the exergy balance imposes a much more realistic thermodynamic limit for repowering applications, restricting the maximum achievable efficiency to approximately 60.1% even under ideal reversible conditions.

Considering the three installed engine-generator units, the exergy balance indicates that the exhaust gases alone could theoretically provide more than 1230 kW of additional power generation, corresponding to approximately 36% of repowering under ideal thermodynamic conditions. Similarly, the cooling-water circuit could theoretically provide nearly 360 kW of additional power, representing more than 10% of repowering potential. These results demonstrate that the thermodynamic feasibility of waste heat recovery is strongly dependent not only on the amount of residual heat available, but also on its temperature level and exergy content.

It is important to emphasize, however, that the exergy balance represents only the thermodynamic limit of waste heat recovery. The actual technological limit is substantially lower because bottoming cycles, such as ORC systems, are irreversible and therefore cannot achieve exergy efficiencies approaching 100%. Consequently, a more realistic assessment of the practical repowering potential requires the detailed thermo-economic simulation of the proposed waste heat recovery systems.

4.2 ORC repowering thermo-economic performance

After establishing the thermodynamic limits associated with the available waste heat streams, the next step consists of evaluating the realistic repowering potential achievable through ORC-based bottoming cycles. Unlike the idealized limits indicated by the exergy balance, the actual performance of ORC systems is strongly affected by technological constraints such as finite temperature differences, component irreversibilities, operating conditions, and economic limitations. Therefore, the thermo-economic assessment presented in this section provides a more realistic evaluation of the practical feasibility of ORC repowering for the different waste heat recovery pathways considered in this work.

Based on the thermodynamic assessment presented in the previous section, ORC systems were evaluated for the two principal waste heat recovery pathways considered in this work: the exhaust gases and the high-temperature cooling-water circuit of the three engine-generator units. Figures 11, 12 present the thermodynamic states design parameters and T-s diagrams for both ORC configurations. The legend in the lower-left corner of Figures 11, 12 shows the thermodynamic properties (mass flow rate, pressure (p), specific enthalpy (h), and temperature (t)) at each state point of the working fluid.

According to Neves (2015), the efficiencies of the electric generator and electric motor were assumed to be 94% and 90%, respectively. Taking into account these efficiencies for the electric generator and for the two electric motors of each ORC System, considering the combined recovery of the waste heat streams from the three installed engine-generator units, the exhaust-gas-driven ORC delivered a net electrical power output of 422.83 kW, whereas the cooling-water-driven ORC generated only 94.08 kW. The corresponding energetic efficiencies of the cycles were 24.26% and 5.83%, respectively.

This substantial difference confirms the thermodynamic interpretation established in the previous section: despite the comparable energetic magnitudes of the two waste heat sources, their practical repowering potential differs significantly due to the difference in thermodynamic quality associated with each stream. The superior performance of the exhaust-gas-driven ORC is primarily associated with the higher temperature level of the available heat source. The elevated exhaust gas temperature enables a substantially larger enthalpy drop across the turbine and, consequently, greater useful work generation. In contrast, the lower temperature of the cooling-water restricts the evaporation conditions and limits the achievable expansion work, substantially reducing the overall cycle performance.

From a plant-level perspective, the exhaust-gas-driven ORC corresponds to an additional electricity generation equivalent to approximately 12.35% of the nominal power plant capacity. Considering the actual operating conditions currently observed in the plant, this additional generation corresponds to approximately 14.09% of the present electrical output. In contrast, the cooling-water-driven ORC provides only a limited incremental contribution, corresponding to 2.75% and 3.14%, respectively.

These results reinforce that repowering feasibility is governed not only by the magnitude of the recoverable heat, but fundamentally by the thermodynamic quality of the heat source. A purely energy-based assessment would suggest comparable recovery potential between the exhaust gases and cooling-water streams; however, the ORC simulations demonstrate that the practical electricity generation capability differs substantially between the two recovery pathways, considering ORC technologies.

To complement the thermo-economic assessment, Table 9 summarizes the main equipment dimensions and bare module costs obtained for both ORC repowering configurations. These results provide an important perspective on the relationship between heat-source quality, equipment sizing, and overall economic performance for the two simulated ORC systems.

TABLE 9. Principal equipment capacities and bare module costs for the ORC repowering configurations.

The economic results reveal important differences between the two ORC repowering pathways. Although the cooling-water-driven ORC operates with substantially lower turbine power generation, its evaporator and condenser require significantly larger heat-transfer areas due to the lower temperature gradients involved in the thermal exchange processes. Consequently, the reduction in electrical power generation is not accompanied by a proportional reduction in equipment costs.

This behavior directly affects the economic feasibility of the systems. While the exhaust-gas-driven ORC combines relatively high-power generation with moderate equipment dimensions, the cooling-water-driven configuration demands large heat-exchange surfaces to recover comparatively low-grade thermal energy, substantially penalizing the economic performance of the cycle.

To provide a consolidated economic comparison between the two ORC repowering pathways, Table 10 summarizes the principal economic performance indicators obtained for each configuration.

TABLE 10. Economic performance indicators for the evaluated ORC repowering configurations.

To provide an additional economic indicator, the specific investment cost (US$/kW) was calculated by dividing the total capital investment by the corresponding net electrical power output. The exhaust-gas-driven ORC presented a specific investment cost of 4,701 US$/kW, whereas the cooling-water-driven ORC reached 11,870 US$/kW. The substantially higher value obtained for the cooling-water-driven configuration reflects its limited net power generation (94 kW), which significantly increases the investment required per unit of electricity produced. For comparison, Morawski (2021) reported optimized ORC specific investment costs of approximately 2,700–3,000 US$/kW for waste heat recovery systems integrated with internal combustion engines. Although the exhaust-gas-driven ORC presented a higher specific investment than the optimized values reported in the literature, this difference is expected because the present study evaluates the integration of commercially available equipment into an existing industrial landfill biogas power plant, considering practical installation constraints and real operating conditions rather than an unconstrained thermoeconomic optimization.

The economic results reveal a clear divergence between the two ORC repowering alternatives. The exhaust-gas-driven configuration demonstrated favorable investment performance, combining relatively high additional electricity generation with economically attractive return indicators. In contrast, the cooling-water-driven ORC proved economically unfeasible under the adopted assumptions, despite its technically feasible thermodynamic operation.

From an engineering design perspective, this result is particularly relevant because low-grade heat recovery systems may initially appear attractive from an energy availability standpoint while still failing to provide sufficient electrical generation to justify the required capital investment. The substantially larger heat-exchange surfaces required for the cooling-water-driven ORC directly penalize the economic performance of the system, leading to a negative net present value and an internal rate of return below the adopted discount rate.

These results reinforce that the thermodynamic quality of the waste heat source strongly governs not only the energetic performance of ORC repowering systems, but also their practical economic viability.

Overall, the thermo-economic assessment demonstrates that ORC repowering constitutes a technically and economically viable strategy only when applied to waste heat sources with sufficiently high thermodynamic quality, such as the exhaust gases of the internal combustion engines. In contrast, although the cooling-water stream still presents considerable energetic magnitude, its low temperature level substantially limits the practical feasibility of electricity generation through ORC technology.

These limitations motivate the investigation of alternative recovery pathways for the cooling-water circuit, particularly those capable of directly utilizing low-grade thermal energy with reduced thermodynamic penalties.

4.3 Repowering with intake-air and biogas conditioning assessment

The results presented in the previous section demonstrated that, although the high-temperature cooling-water circuit contains a considerable amount of recoverable thermal energy, its utilization through an ORC system is not economically attractive due to the low thermodynamic quality of the heat source. Consequently, an alternative recovery pathway was investigated, in which the cooling-water heat is employed to drive an absorption chiller for biogas and intake-air cooling and dehumidification.

This approach is particularly important to complement the previous results, since they were obtained under nominal operating conditions of the power plant, without considering the impact of engine derating. Moreover, the proposed configuration matches the low-grade thermal energy available in the cooling-water circuit with a cooling demand that naturally operates at similar temperature levels, thereby avoiding the thermodynamic penalties associated with electricity generation from low-temperature heat sources. In addition, and perhaps more importantly, the proposed system simultaneously addresses two operational limitations of the power plant: the derating of the engine-generator units caused by adverse ambient conditions and the significant electricity consumption associated with the existing biogas conditioning system.

Figure 13 presents the main thermodynamic states obtained for the absorption chiller system driven by the high-temperature cooling-water circuit of the engine-generator units. The recovered heat is supplied to the steam generator, cooling the water stream from 93 °C to 74 °C, where it thermally drives the absorption refrigeration cycle and enables the production of chilled water at approximately 0 °C. The simulation results indicate that the available thermal energy from the cooling-water circuit is sufficient to sustain the operation of the absorption chiller without requiring additional external thermal energy input. Consequently, a cooling capacity becomes available for applications that can directly benefit from low-temperature thermal energy recovery, such as intake-air cooling and dehumidification, as well as biogas conditioning.

Unlike the cooling-water-driven ORC evaluated in the previous section, the absorption chiller does not seek to convert low-grade thermal energy into electricity through a bottoming cycle. Instead, it utilizes the available heat to satisfy refrigeration demands, thereby reducing the operational penalties associated with the ICEs and improving the overall utilization of the waste heat stream.

Table 11 summarizes the heat transfer rates obtained for the main components of the absorption refrigeration system. The results indicate that the high-temperature cooling-water circuit provides sufficient thermal energy to drive the absorption cycle, supplying approximately 2.08 MW of thermal power to the steam generator while delivering nearly 800 kW of cooling capacity at the evaporator.

TABLE 11. Heat transfer rates and coefficient of performance (COP).

The COP of 0.385 is consistent with the expected performance range of single-effect absorption chillers operating under relatively low driving heat-source temperatures. Although substantially lower than the COP typically achieved by vapor-compression refrigeration systems, the absorption chiller utilizes waste heat that would otherwise be rejected to the environment, resulting in improved overall energy utilization.

In the present work, this cooling capacity was allocated to two operational demands of the landfill biogas power plant: intake-air conditioning for the engine-generator units and biogas cooling and dehumidification. Table 12 presents the distribution of the chiller cooling capacity.

TABLE 12. Distribution of the thermal capacity of the absorption chiller.

As shown in Table 12, only a fraction of the refrigeration capacity produced by the absorption chiller is required to satisfy the cooling demands associated with biogas conditioning and engine intake-air cooling and dehumidification. Approximately 122 kW are allocated to biogas treatment, while nearly 158 kW are used for intake-air conditioning of the engine-generator units. Together, these applications account for approximately 35% of the total cooling capacity available from the absorption cycle.

The remaining refrigeration capacity, corresponding to more than 520 kW, is directed to the auxiliary heat exchanger, thereby reducing the cooling tower load. This result indicates that the ICE cooling-water circuit contains sufficient thermal potential not only to fully satisfy the refrigeration demands currently present in the plant, but also to accommodate future cooling applications associated with plant operational expansion.

From a thermo-economic perspective, this result is particularly relevant because it demonstrates that the cooling-water waste heat stream, although unsuitable for economically viable electricity generation through an ORC system, remains capable of supporting substantial refrigeration loads when matched with an appropriate recovery technology. Consequently, the limiting factor is not the quantity of recoverable energy, but rather the suitability of the recovery pathway selected for its utilization.

After evaluating the thermodynamic performance of the absorption cooling system and the distribution of the available refrigeration capacity, the associated equipment costs were estimated to assess the economic implications of implementing this recovery pathway. Table 13 presents the main equipment sizes and purchase costs considered in the economic analysis, obtained from real reference data.

TABLE 13. Equipment sizes and purchase costs for the absorption cooling system.

The absorption chiller represents the largest individual investment within the cooling pathway, accounting for a substantial portion of the total equipment cost. Nevertheless, the remaining components exhibit relatively modest costs compared with the heat-exchange equipment typically required for low-temperature ORC systems. This characteristic contributes positively to the economic attractiveness of the cooling pathway and further supports the use of refrigeration as an alternative utilization route for the low-grade thermal energy available in the engine cooling-water circuit. Considering that is necessary one cooling coil for each ICE, the total cost is USD 186,684.41.

However, the purchased equipment cost alone does not represent the total capital expenditure required for project implementation. Therefore, installation, instrumentation, piping, electrical systems, engineering services, and contingency costs were incorporated, based on real data from CRATOS R&D Project. Consequently, using a correction factor of 2.62, total investment was USD 488,557.94.

The economic indicators, in Table 14, demonstrate that the absorption cooling pathway is highly attractive under the operating conditions considered in this study. Despite utilizing the same cooling-water heat source that proved unsuitable for economically viable electricity generation through an ORC system, the absorption chiller configuration achieved a simple payback period close to 1 year, a discounted payback period of only 1.2 years, and internal rate of return higher than the discount rate.

TABLE 14. Economic performance indicators for the evaluated chiller repowering configurations.

To complement the economic assessment, the specific investment cost of the absorption chiller was calculated as 1,080.88 US$/kW. This value is in close agreement with the specific investment obtained in the CRATOS R&D Project (1,028.04 US$/kW) (Fonseca et al., 2024), differing by approximately 5%. This agreement provides additional confidence in the adopted economic methodology and reinforces the competitiveness of the proposed absorption chiller configuration for recovering low-grade waste heat from engine cooling water.

These results reinforce the conclusion that the viability of waste heat recovery depends not only on the amount of thermal energy available, but also on the compatibility between the thermodynamic quality of the heat source and the final energy service delivered. In the present case, the utilization of low-grade thermal energy for refrigeration proved to be more advantageous than its conversion into electricity.

4.4 Final economic feasibility analysis

The previous sections demonstrated that different waste heat recovery pathways should be adopted according to the thermodynamic quality of the available heat sources. While the exhaust gases proved suitable for electricity generation through an ORC system, the high-temperature cooling-water circuit exhibited superior thermo-economic performance when utilized in an absorption chiller for intake-air cooling and dehumidification, as well as biogas conditioning. Therefore, a final economic assessment was conducted considering the simultaneous implementation of both waste heat recovery strategies.

The exhaust-gas-driven ORC provides an additional net electrical generation of approximately 423 kW, whereas the absorption cooling pathway contributes an equivalent gain of approximately 452 kW through the reduction of engine derating and the replacement of the existing vapor-compression refrigeration system. Together, these recovery strategies result in an overall repowering capacity of approximately 875 kW.

The total investment associated with the integrated repowering configuration was estimated at USD 2,477,153.37. Table 15 summarizes the main economic indicators obtained for the combined system.

TABLE 15. Final economic feasibility results of the integrated repowering system.

The results indicate that the integrated repowering strategy is economically attractive under the adopted assumptions. Although the ORC and absorption cooling systems exhibit distinct thermodynamic characteristics and recovery objectives, their combination provides a complementary utilization of the available waste heat streams. The positive net present value, short payback periods, and internal rate of return substantially higher than the adopted discount rate confirm the economic viability of the proposed configuration.

Perhaps more importantly, the results demonstrate that maximizing waste heat utilization is not necessarily achieved through a single recovery technology. Instead, the most effective repowering strategy is obtained by matching each thermal source with the application most compatible with its thermodynamic quality. In the present case, high-grade waste heat is more effectively converted into electricity through an ORC system, whereas low-grade heat is more advantageously utilized for refrigeration purposes. This combination enables a significant increase in the overall plant efficiency.

Obviously, the final thermo-economic indicators of a project can only be fully validated after implementation and commissioning. Nevertheless, the results obtained are consistent and reliable when compared with the studies conducted by Morawski (2021) and Fonseca (2024), among others. The former validated ORC methodologies for heat recovery in internal combustion engines (ICEs) using commercial-scale data, while the latter evaluated a real repowering application involving absorption chillers for engine intake-air conditioning.

The integrated repowering system presented a specific investment cost of 2,831.03 US$/kW. As a reference for large-scale industrial power expansion, the TEVISA (Termelétrica Viana S.A.) power plant presented a specific investment of approximately 1,430 US$/kW (Fonseca et al., 2024). Although the proposed repowering strategy requires a higher investment per unit of additional installed capacity, it is based on recovering waste heat from an existing facility rather than installing new prime movers. Consequently, the additional power is obtained with substantially lower fuel consumption than would be required for conventional capacity expansion, reinforcing the attractiveness of waste heat recovery as an alternative strategy for increasing generating capacity.

To complement the economic assessment, Table 16 presents the Total Capital Investment (TCI) threshold for the ORC system, the absorption chiller, and the integrated configuration. The reported values represent the maximum capital investment that preserves the economic feasibility of each system. These values were obtained by progressively increasing the initial investment until the Net Present Value (NPV) became zero, while maintaining all other economic assumptions unchanged, including the project lifetime, discount rate, electricity sales revenue, and the O&M rate of 6% of the capital investment.

TABLE 16. Total capital investment - TIC.

The results demonstrate the economic robustness of the proposed systems. The ORC configuration could tolerate an increase in capital investment from USD 1.99 million to USD 3.42 million, whereas the absorption chiller could support a substantially larger increase, reaching a maximum allowable TCI of USD 3.65 million despite its relatively low initial investment. The integrated configuration exhibited the highest absolute investment threshold, with a maximum allowable TCI of USD 7.07 million. These findings indicate that all configurations remain economically viable even under significant increases in capital expenditure while maintaining the same economic assumptions and an O&M rate of 6% of the capital investment.

In addition, a sensitivity analysis was performed to evaluate the impact of electricity price on the economic performance of the proposed systems, since electricity sales constitute the project’s primary source of revenue. The base-case electricity price adopted in this study was USD 0.130/kWh, while variations of −40%, −20%, +20%, and +40% were considered to account for potential market fluctuations over the project lifetime.

Table 17 summarizes the effects of these electricity price variations on the annual net revenue and the discounted payback period (DPB) of the absorption chiller, ORC, and integrated ORC–absorption chiller system. As expected, higher electricity prices increase the annual net revenue, leading to shorter DPB. Conversely, lower electricity prices reduce project revenues and increase the time required to recover the initial investment.

TABLE 17. Sensitivity analysis of the economic performance with respect to electricity price.

Among the evaluated scenarios, only the 40% reduction in electricity price rendered the standalone ORC system economically unattractive, as its discounted payback period exceeded the assumed project lifetime of 20 years. Nevertheless, the absorption chiller and the integrated configuration remained economically feasible under all investigated electricity price scenarios.

No additional sensitivity analyses were performed for other economic parameters because they were considered unrealistic or inappropriate for the case study. For instance, extending the project lifetime beyond 20 years would not be representative of the expected service life of the equipment, while varying the discount rate would not be meaningful because it corresponds to the company’s minimum attractive rate of return adopted for investment appraisal. Consequently, electricity price was identified as the most relevant uncertain parameter affecting the project’s economic performance.

Figure 14 illustrates the sensitivity analysis results presented in Table 17. As shown in Figure 14, net revenue increases with increasing electricity prices, while the discounted payback period (DPB) decreases accordingly. These trends confirm the direct impact of electricity price on the project’s economic performance, as higher electricity prices increase revenues and shorten the time required to recover the initial investment.

5 Conclusions and final remarks

This work presented the procedure and discussed the results of the R&D Project that carried out a 3E (energetic, exergetic and economic) analysis on waste heat recovery for a landfill biogas power plant repowering in the state of Espírito Santo, Brazil, considering both Organic Rankine Cycle (ORC) and absorption chiller technologies. The results proved that both pathways can be combined on waste heat recovery (WHR) for repowering of internal combustion engine (ICE) landfill biogas power plants.

The results demonstrated thermo-economic feasibility for the ORC system using exhaust gas heat recovery, generating an additional 423 kW of net electric power, which represents approximately 14% of repowering. Likewise, the recovery of cooling-water heat through absorption chillers for intake-air and biogas conditioning avoided engine derating and provided an equivalent gain of more than 452 kW of electric power, corresponding to an additional 15% of repowering. Although the cooling-water-driven ORC system was not economically viable, the proposed integrated solution requires a total investment of US$ 2,477,153.37 and achieves a discounted payback period of 3.57 years, resulting in an overall increase of approximately 29% in plant repowering capacity.

The thermodynamic and thermo-economic analyses demonstrated that the selection of waste heat recovery technologies should not be based solely on the amount of recoverable thermal energy, but primarily on the thermodynamic quality of the available heat source. Although the exhaust gases and cooling-water circuits exhibit comparable energetic magnitudes, the exergy analysis revealed substantial differences in their work potential, leading to fundamentally different recovery pathways. While the exhaust gases proved suitable for electricity generation through an ORC system, the cooling-water circuit was shown to be considerably more effective when employed in an absorption chiller for refrigeration purposes.

The main novelty of this work lies in demonstrating, through a real industrial landfill biogas power plant, that maximizing repowering is achieved by matching each waste heat source with the recovery technology most compatible with its thermodynamic quality and the required energy service. Rather than applying a single recovery technology to all available waste heat streams, the proposed integrated strategy combines ORC technology for high-temperature exhaust gases with absorption cooling for low-temperature cooling water, simultaneously increasing electricity generation, mitigating engine derating, and improving the overall utilization of the available thermal resources.

In addition to the base-case economic assessment, robustness analyses further confirmed the feasibility of the proposed solutions. The maximum allowable Total Capital Investment (TCI) demonstrated that all economically viable configurations can tolerate substantial increases in capital costs while maintaining a non-negative Net Present Value (NPV). Likewise, the sensitivity analysis identified electricity price as the most influential economic parameter, with higher electricity prices increasing annual net revenue and reducing the discounted payback period. Only the standalone exhaust-gas-driven ORC configuration became economically unattractive under the most conservative scenario, corresponding to a 40% reduction in electricity price.

From a practical perspective, the proposed integrated repowering configuration constitutes a technically mature and economically attractive solution that can be replicated in landfill biogas power plants operating under tropical climate conditions. The proposed methodology also provides a systematic framework for selecting waste heat recovery technologies according to the thermodynamic characteristics of each heat source, making it applicable to other industrial waste-to-energy systems where multiple waste heat streams with different temperature levels are available.

Finally, this work contributes to the advancement of sustainable waste-to-energy technologies by demonstrating that significant improvements in electricity generation and overall plant performance can be achieved without additional fuel consumption. By integrating thermodynamic analysis, process simulation, and techno-economic evaluation into a unified decision-making framework, the proposed approach supports the development of more efficient, economically viable, and sustainable strategies for landfill biogas utilization, contributing to energy efficiency improvement and decarbonization goals.

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Footnotes

Edited by: Adeniyi Jide Isafiade, University of Cape Town, South Africa

Reviewed by: Omid Rowshanaie, Farhangian University, Iran

Kaouther Kerboua, Ecole Preparatoire aux Sciences et Techniques Annaba, Algeria

Footnotes

3E Energy, Exergy, and Economic analysis; AC, Air chiller; CCHP, Combined Cooling, Heating and Power; CEPCI, Chemical Engineering Plant Cost Index; CHP, Combined heat and power; COP, Coefficient of Performance; CRC, Conventional Rankine cycles; DPB, Discounted payback; EES, Engineering Equation Solver; EG, Electric generator; EM, Electric motor; GHG, Greenhouse gas; HHV, Higher heating value; HT, High-temperature; ICE, Internal Combustion Engine; IRR, Internal rate of return; LHV, Lower heating value; LMTD, Logarithmic Mean Temperature Difference; LNG, Liquefied natural gas; LT, Low-temperature; MCT, Module Costing Technique; NPV, Net present value; ORC, Organic Rankine cycle; R&D, Research and Development; TIC, Total Capital investment; WHR, Waste heat recovery; WtE, Waste-to-energy.