A focus of Engineers has been the extensive research of numerous heat engine cycles. The goal is to enhance the amount of usable work from a given power source. Engineers have deliberated to work around the Carnot Cycle limit of the gas-based cycle. There has been at a few ways developed to possibly get around that limit to increase the efficiency. One way is to increase the temperature difference in the heat engine by increasing the hot reservoir temperature. This method is used in combined cycle gas turbines but environmental concerns of nitrogen oxides and physical limits of materials restricting maximum temperatures on the feasible heat engines. With these concerns in mind, modern turbines run at the maximum temperatures for physical properties such as melting point to not be reached and keeping the nitrogen oxides output within acceptable range.
Another method used to increase efficiency is to decrease the output temperature by use of mixed chemical working fluids and utilizing the shifting performance of the blends. An example of this is the Kalina Cycle which uses 70/30 mix of ammonia and water which permits the generation of valuable power.
(Figure 1 shows a simplified diagram of the Kalina cycle which is used to increase efficiency) (2)
(Figure 2 shows a temperature vs entropy plot of the Kalina Cycle) (3)
A third example engineers use to increase efficiency is to use the physical properties of the working fluid to the advantage of the heat engine. The most common way is to use water above the critical point (supercritical steam). The activities of fluids above this point changes dramatically and the behaviors can be used to extract better thermodynamic efficiency. The Chemical properties of the working fluids can also be used as an advantage for efficiency. Temperature can be increased denaturing a molecule into smaller units. This lowers the molecular weight of the fluid, in turn significantly increasing the efficiency of the heat engine. The fluid of lower molecular weight moves through the engine and is cooled by a heat sink which causes the molecules to reform into their original state and can be recycled back into the engine for reuse.
Heat Engine Processes (1)
(Table 1 shows various heat engine processes. Each process is isothermal (constant temperature), isobaric (constant pressure), isochoric (constant volume), adiabatic (no heat added or removed), or isentropic (reversible adiabatic).) (1)
- “Heat Engine.” Wikipedia, Wikimedia Foundation, 4 Dec. 2018, en.wikipedia.org/wiki/Heat_engine#Heat_engine_processes.
- “Otec Explained.” What Is Biodiesel, Engineering Department University of Strathclyde in Glasgow, www.esru.strath.ac.uk/EandE/Web_sites/02-03/ocean_thermal_energy/group project/exports/otecex.html.
- Chiranjit Maji. “Kalina Cycle.” Research Gate, Research Gate 2018, 2016, www.researchgate.net/figure/T-S-diagram-of-Kalina-Cycle-modified-after-13_fig6_319293219.