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Performance comparison of two low-CO2 emission solar/methanol hybrid combined cycle power systems
Author: Li Yuanyuan | Print | Close | Text Size: A A A | 2016-10-29

Two novel hybrid combined cycle power systems that use solar heat and methanol, and integrate CO2 capture, are proposed and analyzed, one based on solar-driven methanol decomposition and the other on solar-driven methanol reforming. The high methanol conversion rates at relatively low temperatures offer the advantage of using the solar heat at only 200–300 ℃ to drive the syngas production byendothermic methanol conversions and its conversion to chemical energy. Pre-combustion decarbonization is employed to produce CO2-free fuel from the fully converted syngas, which is then burned to produce heat at the high temperature for power generation in the proposed advanced combined cycle systems. To improve efficiency, the systems’ configurations were based on the principle of cascade use of multiple heat sources of different temperatures. The thermodynamic performance of the hybrid power systems at its design point is simulated and evaluated. The results show that the hybrid systems can attain an exergy efficiency of about 55%, and specific CO2 emissions as low as 34 g/kW h. Compared to a gas/steam combined cycle with flue gas CO2 capture, the proposed solar-assisted system CO2 emissions are 36.8% lower, and a fossil fuel saving ratio of 30% is achievable with a solar thermal share of 20%. The system integration predicts high efficiency conversion of solar heat and low-energy-penalty CO2 capture, with the additional advantage that solar heat is at relatively low temperature where its collection is cheaper and simpler. The systems’ components are robust and in common use, and the proposed hybridization approach can be also used with similar benefits by replacing the solar heat input with other low heat sources, and the system integration achieves the dual-purpose of clean use of fossil fuel and high-efficiency conversion of solar heat at the same time.

Taking advantage of the high conversion ratio of methanol conversion at relatively low temperature of 200–300 ℃, the authors propose the use of low/mid temperature solar heat be integrated and upgraded thermo-chemically in a way that contributes to the overall energy input, increases power generation efficiency, offers the energy storage potential of the produced syngas, and integrates low-energy-penalty pre-combustion decarbonisation. All these advantages reduce the use of fossil fuel and the associated undesirable emissions.

Two such novel system configurations have been proposed, based on solar heat methanol decomposition and reforming, respectively. The main components of the systems are power generation, solar-driven methanol thermochemical reactors, and CO2 sequestration subsystems. They are simulated and compared with a conventional gas-fired gas–steam combined cycle system with CO2 separation from the exhaust gas (CC-Post). The system performance analysis results show that with the same methanol input and a chosen 91% CO2 capture ratio, the specific CO2 emission of the proposed hybrid systems is about 33 g/kW h, 36% lower than that in the reference conventional CC-Post cycle. Solar heat input contributes to the augmentation in system power output and, by replacement of some of hydrocarbon fuel a reduction of CO2 emission. A 30% fossil fuel saving ratio is achievable with a solar thermal share of about 20%, and the net solar-to-electricity efficiency, based on the gross solar radiation incident on the collector, is more than 45% higher than that of a CC-Post system with the same fuel input, which is much higher than can be attained in the solar-alone thermal power system operating at the same or even higher solar heat temperatures. Taking into account the methanol production energy consumption and the conversion efficiency of solar radiation to heat, the system primary thermal efficiency is found to be 28% for the RFM system and 28.8% for the DCP system, which is lower by 22–23% points than the system direct thermal efficiency. Potential exists for system performance improvement along with technology advancement for methanol production and solar heat collection. Summarizing, the proposed systems’ thermochemical upgrading of methanol by using solar heat, and integration of cascade use of multiple heat sources was shown to accomplish much cleaner use of fossil fuel, and high efficiency conversion of low/mid temperature solar heat.

The results have been published on APPLIED ENERGY Volume:155 Pages:740-752.

 
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