Review of low-temperature vapour power cycle engines with quasi-isothermal expansion

Abstract

External combustion heat cycle engines convert thermal energy into useful work. Thermal energy resources include solar, geothermal, bioenergy, and waste heat. To harness these and maximize work output, there has been a renaissance of interest in the investigation of vapour power cycles for quasiisothermal (near constant temperature) instead of adiabatic expansion. Quasi-isothermal expansion has the advantage of bringing the cycle efficiency closer to the ideal Carnot efficiency, but it requires heat to be transferred to the working fluid as it expands. This paper reviews various low-temperature vapour power cycle heat engines with quasi-isothermal expansion, including the methods employed to realize the heat transfer. The heat engines take the form of the Rankine cycle with continuous heat addition during the expansion process, or the Stirling cycle with a condensable vapour as working fluid. Compared to more standard Stirling engines using gas, the specific work output is higher. Cryogenic heat engines based on the Rankine cycle have also been enhanced with quasi-isothermal expansion. Liquid flooded expansion and expander surface heating are the two main heat transfer methods employed. Liquid flooded expansion has been applied mainly in rotary expanders, including scroll turbines; whereas surface heating has been applied mainly in reciprocating expanders.

Direct combustion of recyclable metal fuels for zero-carbon heat and power

“…For transportation applications, the efficiency of modern compact ECEs has been vastly improved in comparison to their coalfired steam predecessors of the industrial revolution era and now even surpass the efficiency and power density of ICEs [153][154][155]. ECEs also promise good performance as part of hybrid-electric powertrains, enabling range extension while maintaining the benefits of regenerative braking, elimination of engine idle, and operation of the engine at its optimum set-point [5].…”

Thermodynamic analysis of a liquid air energy storage system

“…An important consideration to be made is related to the very high pressure that the adoption of quasi-isothermal expanders, which have been lately the subject of several studies [23], could significantly increase work output and round-trip efficiency; indeed, some of these studies were focused on liquid nitrogen or liquid air as the working fluid in a quasi-isothermal expander [24][25][26]. However, these devices are volumetric expanders that are probably very difficult to scale up to the size required by a grid-scale energy storage system.…”

Optimised Liquid Flooded Gas Cycle for Heat Pump and External Heat Engine Applications

“…This is due to non-infinite time of the segment and insufficient heat exchange of the working fluid to the chamber wall. Many expander and compressor designs have been explored in attempts to create chambers with sufficient conduction surface, yet with increase in surface area, there is the corresponding increase in seal length, and thus friction (Igobo, 2014). The efficiency gains of approaching isothermal segments is countered by the increase in seal friction, making the concept of very large surface areas at best very challenging and cost prohibitive for any commercially viable system.…”