The facility, operated by the waste treatment company Marca Ambiental, has already undergone one dramatic transformation. For years it ran on a modest steam cycle that converted landfill biogas into about one megawatt of electricity at a meager 13 percent efficiency. Simulations showed that switching to internal combustion engines would more than triple both power output and efficiency, and in 2020 the plant was rebuilt around three Jenbacher J416 GS engine-generator sets, each designed to produce 1,141 kilowatts at 41.3 percent electrical efficiency. In practice, however, each unit delivers only about one megawatt. The culprit is derating: in Espírito Santo’s tropical climate, high ambient temperatures and humidity thin the intake air, raising the risk of engine knocking and forcing operators to hold the engines back from their nominal output.

That operational constraint created an unusual opportunity, one that the research team from the Federal University of Espírito Santo, the Federal Institute of Espírito Santo and the University of Coimbra set out to quantify. Each Jenbacher engine consumes about 0.22 kilograms of biogas per second, a fuel blend of roughly 48 percent methane and 47 percent carbon dioxide with a lower heating value of about 12,750 kilojoules per kilogram. Of the 2,760 kilowatts of fuel energy entering a single engine, only 1,141 kilowatts emerge as electricity. The rest is split between 751.6 kilowatts of exhaust gas losses and 755.6 kilowatts of cooling water losses, with the remainder lost through generator inefficiencies, irreversibilities and minor streams. Two waste heat reservoirs of nearly identical size, yet with radically different quality, sit waiting at the plant’s periphery.

This is where exergy analysis, the second E of the study’s so-called 3E framework, becomes decisive. Exergy measures not just how much energy a stream contains, but how much useful work it could theoretically deliver. The energy balance alone paints a seductive picture: both the exhaust gases and the cooling water carry about 750 kilowatts per engine, each roughly 27 percent of the fuel input. But the exergy balance tells a very different story. The exhaust gases, leaving the engine at a scorching 457 degrees Celsius, retain more than 410 kilowatts of exergy, about 14 percent of the fuel’s work potential. The high-temperature cooling water, which exits the engine block at just 93 degrees Celsius, carries less than 120 kilowatts of exergy, a little over 4 percent. In thermodynamic terms, one stream is a capable workhorse and the other is nearly spent.

To test what those numbers mean in practice, the team simulated two Organic Rankine Cycle systems, one driven by exhaust gases using toluene as the working fluid, the other driven by the high-temperature cooling water using refrigerant R141b. Unlike conventional steam cycles, which lose efficiency at low source temperatures, ORC systems use organic working fluids that vaporize readily from modest heat, making them the technology of choice for recovering low- and medium-grade waste heat from engines, solar collectors and geothermal wells. Running the simulations in the IPSEpro 8.0 process simulator with mass, energy and exergy balances handled in Engineering Equation Solver, the researchers found that the exhaust-driven cycle achieved a thermal efficiency of 24.26 percent and delivered a net electrical output of 422.83 kilowatts across the three engines. The cooling-water-driven cycle managed only 5.83 percent efficiency and a paltry 94.08 kilowatts.

The economics amplified the contrast dramatically. Using the Module Costing Technique with parametric cost correlations, corrected to 2022 values through the Chemical Engineering Plant Cost Index, the team estimated that the exhaust-gas ORC would require about US$1.99 million in capital, working out to a specific investment of 4,701 US dollars per kilowatt, with an internal rate of return of 21.7 percent and a net present value of over US$1.3 million. The cooling-water ORC, despite its smaller absolute cost of about US$1.12 million, demanded sprawling heat exchangers, an evaporator of nearly 301 square meters and a condenser approaching 892 square meters, because small temperature differences require large surfaces. That pushed its specific investment to a punishing 11,870 dollars per kilowatt, drove its net present value negative, and left its discounted payback unachievable within the project’s 20-year lifetime. Thermodynamically viable, economically dead.

Rather than abandon the cooling water’s thermal content, the researchers matched it with a technology better suited to its humble temperature: a single-effect absorption chiller driven by the 93 degree Celsius cooling water, capable of producing chilled water near zero degrees Celsius. The simulation showed the cooling-water circuit could supply about 2.08 megawatts of thermal power to the chiller’s generator, yielding roughly 800 kilowatts of cooling capacity at a coefficient of performance of 0.385, typical for absorption machines at low driving temperatures. About 122 kilowatts of that cooling would replace the existing electric vapor-compression chiller that currently cools the biogas to three degrees Celsius to strip out water vapor and corrosive hydrogen sulfide, while 158 kilowatts would chill and dehumidify the engines’ intake air, directly attacking the derating problem. The remaining 520 kilowatts would pre-cool radiator circuits, easing the heat rejection load.

The payoffs of that cooling pathway compound in ways a simple kilowatt tally would miss. Eliminating derating restores each engine’s full rated capacity, and replacing the compression chiller cuts auxiliary electricity consumption to zero. All told, the absorption chiller route delivered an equivalent gain of about 452 kilowatts of electrical output, roughly 15 percent additional repowering, from a total investment of just US$488,558, or 1,081 dollars per kilowatt. The financial indicators were extraordinary: a simple payback of about one year, a discounted payback of 1.2 years, and an internal rate of return of 94.43 percent per year. The specific investment also landed within five percent of the value measured in the earlier CRATOS experimental project, lending real-world credibility to the costing methodology.

Combining both viable pathways, the exhaust-gas ORC plus the absorption chiller, yields the study’s headline result: roughly 875 kilowatts of additional effective capacity, a 29 percent repowering of the plant, for a total investment of US$2.48 million. The integrated configuration returns a net present value of US$4.44 million, an internal rate of return of 36 percent, and a discounted payback of 3.57 years. Sensitivity analyses showed the combined system stays profitable across electricity prices ranging from US$0.08 to US$0.18 per kilowatt-hour, with only the standalone ORC failing under the most pessimistic 40 percent price cut. The team also calculated that capital costs could more than double, to nearly US$7.07 million, before the integrated project’s net present value turned negative, a substantial margin of economic robustness.

Beyond the numbers, the study delivers a broader engineering lesson: waste heat recovery works best when the recovery technology matches the thermodynamic quality of the source and the energy service actually needed. High-temperature exhaust deserves a bottoming power cycle; lukewarm cooling water is better spent producing chill. That principle, demonstrated on a real industrial plant rather than a paper design, could guide waste-to-energy operators across tropical regions where derating and rejected heat are chronic problems. For Espírito Santo, which has committed to the United Nations Race to Zero initiative and targets net-zero emissions by 2050, the findings suggest that its landfills are not merely disposal sites but underexploited power stations, capable of producing significantly more clean, dispatchable electricity from the same garbage, same fuel and same engines, simply by refusing to let their heat vanish into the air.

Subject of Research: Energy, exergy and economic assessment of waste heat recovery with an Organic Rankine Cycle and absorption chiller for repowering a Brazilian landfill biogas power plant

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

Article References: Loyola, L. R., Piski, G. B., Fonseca, F. M., Guedes dos Santos, R., Rosseto de Faria, P., Conceição Soares Santos, J. J., Baranda Moreira da Silva Ribeiro, J. M., & Santos, M. (2026). 3E investigation of ORC and absorption chiller for repowering a Brazilian landfill biogas power plant. Frontiers in Chemical Engineering, 8, Article 1895157. https://doi.org/10.3389/fceng.2026.1895157

Image Credits: AI Generated

DOI: 10.3389/fceng.2026.1895157

Keywords: landfill biogas, waste heat recovery, Organic Rankine Cycle, absorption chiller, exergy analysis, internal combustion engine, derating, intake-air cooling, waste-to-energy, techno-economic assessment, decarbonization, Brazil