Liquid air/nitrogen energy storage and power generation system for micro-grid applications

The large increase in population growth, energy demand, CO2 emissions and the depletion of the fossil fuels pose a threat to the global energy security problem and present many challenges to the energy industry. This requires the development of efficient and cost-effective solutions like the development of micro-grid networks integrated with energy storage technologies to address the intermittency of renewable energy sources, provide localized electricity production, and smooth out power demand and supply curve. Among other energy storage systems, the cryogenic energy storage (CES) technology offers the advantages of relatively large volumetric energy density and ease of storage. This paper concerns the thermodynamic modeling and parametric analysis of a novel power cycle that integrates air liquefaction plant, cryogen storage systems and a combined direct expansion with closed Rankine power recovery system using two cryogens, liquid nitrogen, and liquid air. This cycle is part of a micro-grid system that provides electricity for a typical 50 unit residential building using either renewable energy sources or national grid off-peak electricity. This power cycle was modeled using MATLAB integrated with REFPROP software to investigate its performance at various operating conditions. Results showed that using liquid air as the working cryogen can significantly improve the cycle performance compared to that of liquid Nitrogen at all operating conditions, yielding maximum round trip efficiencies of 63.27% and 84.15% respectively. Also results showed that as the cryo-turbine efficiency and recovery expansion ratio are increasing the cycle round trip efficiency and network will increase, while as the compressor efficiency increases the round trip efficiency increases and the network decreases to reach the best value at 84% to produce round trip efficiency 80.62% and work 397 kJ/kg for the liquid air condition.