Somebody linked above to a new closed loop turbine design which uses supercritical CO2. I know from CO2 refrigeration that CO2 has some insane volumetric expansion based on temp which makes it a good candidate for use in a closed loop turbine system. Plus, because they're running it through the turbine as a supercritical fluid, the density is higher than that of steam so it requires smaller turbines. The biggest issue is that because it's super critical CO2 youre talking about working pressures well over 1000PSI. That doesn't make it impossible to work with as we already know from CO2 refrigeration, but it does make it a bit more difficult than just boiling water.