Liquid piston compression efficiency with droplet heat transfer

Wind turbines and other unsteady power producing systems can benefit substantially by integrating inexpensive and easily sited Compressed Air Energy Storage (CAES) to provide a more continuous energy supply. However, traditionally CAES has not been combined with off-shore wind turbines. In addition, typical CAES systems are inefficient since the compression work also generates thermal energy, which is generally lost to the ambient when pressurized air is stored over a long period of time. To enable off-shore wind energy storage, a droplet spray heat transfer concept is investigated to establish a near-isothermal high-efficiency compression process. In particular, the use of small water droplets and high mass loading can allow for a large interfacial surface area for heat transfer. Issues associated with this liquid introduction can be mitigated with a liquid piston, which also allows variable piston cross-sections to further improve efficiency. To investigate the droplet spray heat transfer concept, a detailed multiphase thermodynamic model was developed and validated with experimental data. Based on this approach, one-dimensional simulations were performed using a sinusoidally driven piston in a 5 kW first-stage cylinder with various compression ratios, as well as both pre-mixed and direct injection scenarios. The results show that the total surface of aloft droplets is critical to achieve high performance in a liquid piston. This is best achieved with small droplets and high mass loadings combined with direct injection. For example, the compression efficiency (defined as a ratio of isothermal stored energy to real compression work) increased from 71% for adiabatic compression to as much as 98% with spray injection, for a tenfold pressure. However, the effects of droplet collision (with other droplets and the chamber walls), three-dimensionality, injector dynamics, and the wall heat transfer should next be considered to help improve design and understanding of such systems.