A trade-off between maxima in efficiency and specific work output of super- and trans-critical CO2 Brayton cycles

Abstract

a b s t r a c tSeveral operational aspects for thermal power plants in general are non-intuitive and involve simultaneous optimization of a number of operational parameters. In the case of solar operated power plants, it is even more difficult due to varying heat source temperatures induced by variability in insolation levels. This paper introduces a quantitative methodology for load regulation of a CO 2 based Brayton cycle power plant using the 'thermal efficiency and specific work output' coordinate system. The analysis shows that a transcritical CO 2 cycle offers more flexibility under part load performance than the supercritical cycle in case of non-solar power plants. However, for concentrated solar power, where efficiency is important, supercritical CO 2 cycle fares better than transcritical CO 2 cycle. A number of empirical equations relating heat source temperature, high side pressure with efficiency and specific work output are proposed which could assist in generating control algorithms.

Carbon dioxide based power generation in renewable energy systems

“…To realize a meaningful throughput Srinivasan et al [28] recommend that a minimum uptake efficiency of 40% must be targeted. Because of this limitation thermal compression ratio diminishes further and is found to remain fairly constant at about 2.3 [23] which is again comparable to the optimal expansion ratio for a sub-critical conventional Brayton cycle. The thermal efficiency of adsorption compression Brayton cycle against specific work per unit mass of activated carbon required is shown in Fig.…”

“…This is also one of the conclusions of Cayer et al [39] in a context of a free heat source like industrial waste gases where the focus should be on maximising the specific net output rather than the thermal efficiency. This aspect has been investigated in greater detail by Garg et al [23]. For the transcritical Rankine cycle the efficiency vs specific net work output plane is shown in Fig.…”

“…Garg et al [23] propose an option of operating with CO 2 in the Brayton cycle with a subcritical lower cycle pressure. This non-condensing cycle (TN-CO 2 ) is devoid of the consequences of high pressures at high temperatures because they are well below 150 bar which is due to a reduction in the optimum expansion ratio as well.…”

“…As this changes, the point of maximum efficiency also changes implying that the corresponding optimal expansion ratios also change. Garg et al [23] analyzed the performance of both TN-CO 2 and S-CO 2 cycles on efficiency-specific work output plane. This two path function coordinate system is shown to provide a lot of useful information on operational aspects such as load regulation and associated drop in efficiency.…”

Transcritical CO2 power cycle – Effects of regenerative heating using turbine bleed gas at intermediate pressure

“…Wang et al [27] conducted multi objective optimisation of geothermal power plant operating in transcritical CO 2 cycle and utilizing LNG as heat sink. Analysis conducted by Garg et al [28] revealed that though transcritical CO 2 cycle provided more flexibility under part load performance for non solar application, supercritical CO 2 cycle was a better option for the concentrated solar power system.…”