Search

• An improved cascade method for compression-absorption heat pump is proposed. • The improved cascade heat pump can output temperature 110 °C at inlet source 10 °C. • The energy performance of improved cascade method is 16.2% higher than existing mode. • The exergy performance of novel cascade method is raised by 19.2% than existing mode. • The novel cascade method can increase the available heat capacity by 238.4%. Air is a readily available and renewable source, but its efficient utilization for industrial heating is hindered by the significant temperature mismatch between the low-grade air source and the high-temperature industrial demand. Heat pumps can elevate the source temperature from low to high levels. However, there is still a significant knowledge gap in achieving output temperature exceeding 90 °C when the input source is below 30 °C. While there are some preliminary studies on air-source heat pumps operating in the mentioned temperature range, they have yet to distinguish the respective coupling temperatures required for different cascaded components. Therefore, the optimal cascade configuration between various sub-cycles has not been established, and the full efficiency potential remains untapped. To address the identified limitations, in this work, an advanced cascade method is proposed and applied to the compression-absorption heat pump (CAHP). The novel cascade method allows independent control of the coupling temperature in different branches as needed, facilitating an optimal cascade configuration between various subloops. Consequently, the operating efficiency of CAHP is optimized. Simulation results show that the proposed cascade method reduces the exergy destruction of CAHP by 6.3% compared to the existing mode. Moreover, it always achieves higher efficiencies in the studied conditions, with a maximum increase in the coefficient of performance and exergy coefficient of performance by 16.2% and 19.2%, respectively. Under ambient temperature of 10 °C to –30 °C and an output temperature of 100 °C, the improved cascade CAHP exhibits an increase in heating capacity, ranging from 57.2% to 238.4% higher than the existing mode. These advantageous features contribute to achieving cleaner industrial heating with air source heat pumps. [ABSTRACT FROM AUTHOR]

• The work and efficiency of advanced gas turbine cycles are analytically optimized. • Polytropic efficiencies are used for the compression and expansion processes. • The overall and intermediate pressure ratios which maximise the net-work are given. • The ranges of the pressure ratios yielding a benefit in the efficiency are found. • The maximum-efficiency point is analytically given for the intercooled & reheat cycle. The paper presents a theoretical analysis of advanced gas-turbine cycles. Specifically, three cycles are investigated, that is the intercooled, the reheat, and the intercooled and reheat cycles. The internal irreversibilities, which characterise the compression and expansion processes, are taken into account through the polytropic efficiencies of the compressors and turbines. New analytical formulations for the overall and intermediate pressure-ratios which maximise the net work of the three aforementioned cycles are proposed along with an order relation between these optimum pressure-ratios. Moreover, the thermal efficiency of these cycles is also analysed providing, among other findings, the ranges of the intermediate pressure-ratios returning a benefit in the thermal efficiency in comparison with the simple cycle. Finally, for the sole intercooled and reheat cycle, a novel analytical expression for the maximum point of the thermal efficiency is given. It is also shown that, for the intercooled and reheat cycle, there is a unique value of the overall pressure-ratio which simultaneously maximises the net work and the thermal efficiency. To give some quantitative information, consider a maximum to minimum cycle-temperature ratio equal to 1573/300 and a compressor (resp. turbine) polytropic-efficiency equal to 0.8 (resp. 0.88). The net work and the thermal efficiency are maximised by a set of overall pressure-ratios obeying an order relation. The simple, the reheat, the intercooled, and the intercooled and reheat cycles reach the maximum net-work (resp. thermal efficiency) for increasing values of the overall pressure-ratio, that is 8.550 (resp. 18.260), 14.950 (resp. 25.039), 20.846 (resp. 39.984), and 73.109 (resp. 73.109). The reheat cycle achieves a 34.899% (resp. 6.077%) gain in the net work (resp. thermal efficiency), while the intercooled cycle returns a 31.477% (resp. 15.970%) increment. The maximum net-work of the intercooled and reheat cycle exactly doubles that of the simple cycle. Finally, the maximum thermal efficiency of the intercooled and reheat cycle yields a 24.966% improvement in comparison with the simple one. [ABSTRACT FROM AUTHOR]