First: You should use a safety valve on your system. If the pressure gets to high, you can be in for some nasty damage to your body!
Second: You have a point when you talk about using the energy in the exhaust (exhaust = CO2). Using the exhaust to run the turbine will not get you anywhere. There won't be nearly enough pressure in the exhaust to do that. If you want to use the energy in the exhaust you should use the heat, not the pressure. Using the heat in the exhaust to help heat the water will increase the effeiciency of your system. You can do this by letting the exhaust go through pipes which run through the "steam chamber" on its way out. Ideally, the temperature of the exhaust should be equal to the temperature in the steam when you release it into the atmosphere. I do not think you will be able to do this in your small system, but that is what you would do in a full scale system.
Good luck!
