Thermodynamics defines efficiency as " work energy out / heat energy in "
maximum efficiency for an ideal gas engine is " 1 - (Tmax / Tmin )"
For a given chemical reaction heat energy is evolved or absorbed.
Dealing with exothermic reactions, is there a rule as to how much of this energy can be absorbed by a heat engine ?
Biological systems produce work directly from chemical energy, their efficiency will be " work energy out / chemical energy in ", how is this efficiency limited ?
This is the "thermal efficiency", yes.
... for an ideal, cyclic, heat engine operating between two thermal reservoirs with constant temperatures. The term "cyclic" is important, and if the source or sink temperature vary, you have to integrate the formula.
The theoretical limit is that all heat can be absorbed (Though, in practice you need finite temperature difference, finite area of the heat exchanger, and you will have to use a flowing media with some friction).
The work output is limited by the exergy difference between the input energy carrier (reactants) and output reaction products of the system. Your "chemical energy in" is (probably) the enthalpy difference between the same reactant and product flows. The "chemical energy" is a heating value, which may be higher or lower than the corresponding exergy change.
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