Showing papers on "Thermodynamic cycle published in 2018"

A review of thermodynamic cycles used in low temperature recovery systems over the last two years

Abstract: This review explores the potential of low and medium grade heat in different thermodynamic cycles used to transform wasted heat into mechanical work. The aim of this review is to study the state of the art of the thermodynamic cycles used to recover low-grade heat. The relevance of researching low grade heat or waste heat applications is that a vast amount of heat energy is available at negligible cost within the range of medium and low temperatures, with the drawback that existing thermal cycles cannot make efficient use of such available low temperature heat due to their low efficiency. The different types of Organic Rankine Cycles have been reviewed, highlighting their relevant characteristics where Simple Organic Rankine Cycle, Regenerative Organic Rankine Cycle, Cascade Organic Rankine Cycle, Organic Flash Cycles, Other Rankine Configurations and Trilateral Cycles are included. Reviews were conducted of specific applications of the low-grade heat recovery. In contrast, there are no actual publications which summarise the current state of the art of the thermodynamic cycles used to convert wasted heat into mechanical power. This paper offers a different approach and analyses low-grade heat recovery from a thermodynamic point of view and compares their efficiency. The analysis shows that cycles using closed processes are by far the most efficient published thermal cycles for low-grade heat recovery. Rankine cycles reviewed show similar low efficiencies. In contrast, closed process cycles have a configuration, which allows efficient exploitation of low-grade heat.

Thermodynamic analysis and optimisation of a combined liquid air and pumped thermal energy storage cycle

Abstract: Pumped thermal energy storage (PTES) and liquid air energy storage (LAES) are two large-scale electricity storage technologies that store energy in the form of thermal exergy. This is achieved by operating mechanically-driven thermodynamic cycles between thermally insulated storage tanks. Both technologies are free from geographic restrictions that apply to pumped hydro and most compressed air storage. The present paper describes a novel, combined system in which PTES operates as a topping cycle and LAES as a bottoming cycle. The fundamental advantage is that the cold thermal reservoirs that would be required by the two separate cycles are replaced by a single heat exchanger that acts between them, thereby saving significant amounts of storage media per unit of energy stored. In order to reach cryogenic temperatures, the PTES cycle employs helium as the working fluid, while the LAES cycle uses supercritical air (at around 150 bar) which is cooled sufficiently to be fully liquefied upon expansion, thus avoiding recirculation of leftover vapour. A thermodynamic study of a baseline configuration of the combined cycle is presented and results are compared with those of the separate systems. These indicate that the new cycle has a similar round-trip efficiency to that of the separate systems while providing a significantly larger energy density. Furthermore, three adaptations of the base-case combined cycle are proposed and optimised. The best of these adaptations achieves an increase in thermodynamic efficiency of about 10 percent points (from 60% to 70%), therefore significantly exceeding the individual cycles in both energy density and efficiency.

Reversible Heat Pump–Organic Rankine Cycle Systems for the Storage of Renewable Electricity

Abstract: Storage of electricity from fluctuating renewable energy sources has become one of the predominant challenges in future energy systems. A novel system comprises the combination of a heat pump and an Organic Rankine Cycle (ORC) with a simple hot water storage tank. The heat pump upgrades low temperature heat with excess power. The upgraded heat can drive an Organic Rankine Process using the heat pump in reverse operation mode. This approach allows a comparably efficient storage of excess electricity. Waste heat sources usually do not qualify for electricity production even with ORC processes due to low temperatures. Upgrading the temperature of the waste heat by means of excess electricity makes the use of an ORC feasible in order to recover the electricity input. Thermodynamic cycle simulations with IPSEpro software outline that the process provides power-to-power efficiencies in a range of 50% for small-scale applications based on commercially available heat pump components. The isentropic efficiency of compressors/expanders plays a crucial role on the system performance. Applications of the proposed cycle in the megawatt range with more efficient turbines and dynamic compressors will therefore increase the power-to-power efficiency to above 70%.

Comprehensive Thermodynamic Analysis of the Humphrey Cycle for Gas Turbines with Pressure Gain Combustion

Abstract: Conventional gas turbines are approaching their efficiency limits and performance gains are becoming increasingly difficult to achieve. Pressure Gain Combustion (PGC) has emerged as a very promising technology in this respect, due to the higher thermal efficiency of the respective ideal gas turbine thermodynamic cycles. Up to date, only very simplified models of open cycle gas turbines with pressure gain combustion have been considered. However, the integration of a fundamentally different combustion technology will be inherently connected with additional losses. Entropy generation in the combustion process, combustor inlet pressure loss (a central issue for pressure gain combustors), and the impact of PGC on the secondary air system (especially blade cooling) are all very important parameters that have been neglected. The current work uses the Humphrey cycle in an attempt to address all these issues in order to provide gas turbine component designers with benchmark efficiency values for individual components of gas turbines with PGC. The analysis concludes with some recommendations for the best strategy to integrate turbine expanders with PGC combustors. This is done from a purely thermodynamic point of view, again with the goal to deliver design benchmark values for a more realistic interpretation of the cycle.

Coherent Josephson thermodynamic cycles

Abstract: A superconductor/normal metal/superconductor Josephson junction is a coherent electron system where the thermodynamic entropy depends on temperature and phase difference across the weak-link. Here, exploiting the phase-temperature thermodynamic diagram of a thermally isolated system, we argue that a cooling effect can be achieved when the phase drop across the junction is brought from 0 to $\pi$ in a iso-entropic process. We show that iso-entropic cooling can be enhanced with proper choice of geometrical and electrical parameters of the junction, i.e. by increasing the ratio between supercurrent and total junction volume. We present extensive numerical calculations using quasi-classical Green function methods for a short junction and we compare them with analytical results. Interestingly, we demonstrate that phase-coherent thermodynamic cycles can be implemented by combining iso-entropic and iso-phasic processes acting on the weak-link, thereby engineering the coherent version of thermal machines such as engines and cooling systems. We therefore evaluate their performances and the minimum temperature achievable in a cooling cycle.

Critical-point behavior of a measurement-based quantum heat engine

Abstract: We study how a quantum heat engine performs across the critical value of an external parameter, pertaining to the quantum phase transition. Considering a two-ion system subjected to a magnetic field, we show that the system performs in a quantum Otto cycle above a critical value of the magnetic field, while below such critical point, it does not operate in a heat cycle at all. Moreover, at the critical point, its interaction with an ancillary ion deteriorates the performance of the system as a heat engine. We further show that a strong interaction between the constituent ions of an ion-based system is crucial for it to work in a heat-work cycle, while the coupling to the ancillary system must be minimized.