Researchers from the Kermanshah University of Technology in Iran and Kuwait University have proposed a system for generating electricity and hydrogen by utilizing the exhaust heat of an internal combustion engine. In the basic design mode, the system’s net electrical power reaches 1,012 kW, with hydrogen production totaling 6.5 kg per hour.
As is known, a major portion of the heat generated during the operation of internal combustion engines and industrial plants is lost to the environment along with exhaust and flue gases. This heat can be used to generate additional electricity, although the process is not always successful: as the gases cool down, their temperature becomes too low for the conventional Rankine vapor cycle, in which water acts the working fluid. To recover this low- and medium-temperature heat, one can use the Kalina cycle, which utilizes a mixture of water and ammonia, or the organic Rankine cycle (ORC), which uses working fluids with lower boiling points.
The Iranian and Kuwaiti researchers have decided to combine the two cycles into a single cascade system. They used the exhaust gases of a methane-fueled internal combustion engine as the source of heat. At the system inlet, the exhaust gases had a temperature of 246°C and a flow rate of 1,102 kg per second. Initially, the hottest portion of this heat was used in the Kalina cycle: the exhaust gases heated the ammonia-water mixture, which activated the first turbine, after which some of the remaining heat was transferred to the organic Rankine cycle, where it heated another working fluid, activating the second turbine. This way, the same heat source was used sequentially at different temperature levels.
This combination proved much more efficient than using only the Kalina cycle. Calculated energy efficiency stood at 6.48%, rising to 12.9% with the addition of the ORC and an electrolyzer. The first turbine of the combined system generated 396.8 kW, and the second one produced 1,093 kW. A portion of the generated electricity, a total of 433.9 kW, was sent to a proton-exchange membrane (PEM) electrolyzer, which splits water into hydrogen and oxygen. As a result, the unit produced 6,506 kg of hydrogen per hour, maintaining 1,012 kW of net electrical power.
In order to select the most efficient operating mode, the scientists used a genetic algorithm. It varied six key parameters, including turbine inlet pressure and temperature, ammonia concentration in the mixture and temperature in the separator, while also taking into account three criteria: energy efficiency, net energy losses and cost. This resulted in 70 optimal choices with an energy efficiency ranging from 11.62% to 28.04% and estimated expenditures ranging from $31.03 to $54.30 per hour. However, the overall cost increased along with efficiency.
As the most balanced option, the scientists chose one with an energy efficiency of 13.1%, estimated expenditures of $36.15 per hour and irreversible net energy losses of 3,136 kW. For the purposes of practical engineering, the scientists also determined the preferred ranges for the key parameters: 70–75% ammonia concentration in the mixture, first turbine inlet pressure of 22–23 bar, and temperature of 92–94°C.
The primary source of losses turned out to be the first evaporator, in which exhaust heat is transferred to the water-ammonia mixture: it accounts for about 77.4% of all irreversible losses. For comparison, losses in the first and second turbines totaled 116.6 kW and 180.3 kW, respectively, versus 2,392 kW in the first evaporator. This is why improving heat exchange at the initial stage could become one of the primary ways to further increase the system’s efficiency.
As of yet, the proposed system exists as a mathematical model and has not been tested as a single industrial unit. Later on, the scientists will evaluate the possibilities of its practical application. In addition, the authors propose looking into combining the system with solar and wind generators, as well as heat and hydrogen accumulators.
