When the method for converting heat into mechanical energy using the heat of the environment ("free heat") will have to "pay" for cooling. Nature will heated liquid working substance having a boiling point lower than ambient temperature (e.g. ammonia - 240K BP). So working substance will be under pressure to evaporate in the unit. In this must have a closed cycle of working substance to our "remain profitable" mechanical energy. "Profit" - mechanical energy will be possible when we will "pay" less for cooling than of all the energy of the a given amount of working substance. For each configuration of my unit exists flow on pumps (and it will not be zero), where cooling is enough "cheap" for us to remain "profit" mechanical energy.