Nothing can beat the efficiency of the Carnot engine. That's one of the equivalent ways of stating the Second Law of Thermodynamics [1]. Just like you can't create a perpetual motion machine, you can't extract more energy from warm water than a Carnot engine.
And the efficiency of the Carnot engine is 1-T_cold/T_hot, both temperatures expresssed in Kelvin. Room temperature water is 300K (that's by convention, this translates in 27 C or 80 F). Let's say warm water is at 333K (that's more like scalding hot, at 60 C or 140 F). The Carnot engine efficiency is then going to be 1 - 300/333 = 10%.
So, not engine can exceed 10% efficiency when using scalding hot water, for merely warm water it's probably going to be 5% or less.
And the efficiency of the Carnot engine is 1-T_cold/T_hot, both temperatures expresssed in Kelvin. Room temperature water is 300K (that's by convention, this translates in 27 C or 80 F). Let's say warm water is at 333K (that's more like scalding hot, at 60 C or 140 F). The Carnot engine efficiency is then going to be 1 - 300/333 = 10%.
So, not engine can exceed 10% efficiency when using scalding hot water, for merely warm water it's probably going to be 5% or less.
[1] https://en.wikipedia.org/wiki/Second_law_of_thermodynamics#C...