Thermal efficiency

In thermodynamics, the thermal efficiency (

η

t

h

{\displaystyle \eta _{\rm {th}}}

) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.

For a heat engine, thermal efficiency is the ratio of the net work output to the heat input; in the case of a heat pump, thermal efficiency (known as the coefficient of performance or COP) is the ratio of net heat output (for heating), or the net heat removed (for cooling) to the energy input (external work). The efficiency of a heat engine is fractional as the output is always less than the input while the COP of a heat pump is more than 1. These values are further restricted by the Carnot theorem.

Overview

In general, energy conversion efficiency is the ratio between the useful output of a device and the input, in energy terms. For thermal efficiency, the input,

Q

i

n

{\displaystyle Q_{\rm {in}}}

, to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work,

W

o

u

t

{\displaystyle W_{\rm {out}}}

, or heat,

Q

o

u

t

{\displaystyle Q_{\rm {out}}}

, or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is

η

t

h

≡

benefit

cost

.

{\displaystyle \eta _{\rm {th}}\equiv {\frac {\text{benefit}}{\text{cost}}}.}

From the first law of thermodynamics, the energy output cannot exceed the input, and by the second law of thermodynamics it cannot be equal in a non-ideal process, so

0

≤

η

t

h

<

1

{\displaystyle 0\leq \eta _{\rm {th}}<1}

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Efficiency must be less than 100% because there are inefficiencies such as friction and heat loss that convert the energy into alternative forms.