Drew Robb also writes:

Simple cycle gas turbines provide efficiency levels of around 35% to 40%. Combined cycle units boost efficiency to 60% and beyond. But another option is emerging that combines the attributes of simple and combined cycle designs. This hybrid configuration uses a single power turbine expander and recycles exhaust heat back to the combustor to combine the best of both worlds. Known as the VAST (Value Added Steam Technologies) Power Cycle, it promises efficiency levels above 50%. It holds the potential to become the most cost-effective backup option for grids where renewable energy dominates. Further, it achieves emissions levels that are cleaner than California’s stringent limits without catalysts.