Efficient processes for power generation and energy storage
Finite fossil fuels reserves and unprecedented carbon dioxide levels warrant the need for efficient energy utilization and/or carbon free energy sources. This dissertation addresses the aforementioned issue and provides two solutions. 1) An efficient Natural Gas (NG) based Solid Oxide Fuel Cell (SOFC) power plant equipped with near 100% carbon dioxide capture. The power plant uses a unique refrigeration based process to capture and liquefy carbon dioxide from the SOFC exhaust. Here, carbon dioxide is captured and condensed at different pressure levels by contacting the gas stream with liquid carbon dioxide reflux generated at higher pressure. The uncondensed gas mixture, comprising of relatively high purity unconverted fuel, is recycled to the SOFC and found to boost up the power generation of the SOFC by 22%, when compared to a stand alone SOFC. If Liquefied Natural Gas (LNG) is available at the plant gate, then the refrigeration available from its evaporation is used for carbon dioxide Capture and Liquefaction (CO2CL). If NG is utilized, then a Mixed Refrigerant (MR) vapor compression cycle is utilized for CO2CL. Alternatively, the necessary refrigeration can be supplied by evaporating the captured liquid carbon dioxide at a lower pressure, which is then compressed to supercritical pressures for pipeline transportation. From rigorous simulations, the power generation efficiency of the proposed processes is found to be 70-76% on a lower heating value (LHV) basis. The benefit of the proposed designs is evident from the similar efficiency (73%) achieved by a conventional SOFC-Gas Turbine power plant without carbon dioxide capture. The refrigeration based process that capture and liquefy carbon dioxide is also found to be applicable for capturing and liquefying carbon dioxide from flue gases other than SOFC. An oxygen based Natural Gas Combined Cycle (oxy-fuel NGCC) process is developed and tested to implement the above mentioned capture process. The power generation efficiency here is estimated to be near 49% with almost 98% liquid carbon dioxide recovery. 2) Efficient means of using intermittent renewable energy such as solar for baseload applications with dense large-scale energy storage. Unique carbon recirculation cycles are developed for this purpose. Here, during the period of renewable energy availability, a suitable carbon molecule is synthesized from the stored liquid carbon dioxide and then stored in a liquid state. Subsequently, when renewable energy is unavailable, the carbon molecule is oxidized to deliver electricity and carbon dioxide is recovered and liquefied for storage. Eexergy based metrics are introduced to systematically identify candidate carbon molecules for the cycle. Such a search provides us the trade-off between the exergy stored per carbon atom, exergy used to synthesize the molecule and the exergy stored per unit volume. While no carbon molecule simultaneously has the most favorable values for all three metrics, favorable candidates identified include methane, methanol, propane, ethane and dimethyl ether. For cases where the molecule to be stored is gaseous under ambient conditions, we suggest synergistic integration between liquefaction and boilup of this gas and that of recirculating carbon dioxide. This unique feature allows for minimizing the energy penalty associated with the recovery, purification and liquefaction of carbon dioxide and storage of carbon molecules. Using process simulations we show that these cycles have a potential to provide GWh of electricity corresponding to an overall energy storage efficiency of 53-58% at much reduced storage volumes compared to other options.
Agrawal, Purdue University.
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