System Efficiency

I’ve always been fascinated by system efficiency, in effect the output divided by the input. For most systems this is a ratio of the energy output divided by the energy input. Fundamentally though this describes the impact we have on the world, hence we should look at the system efficiency very carefully and especially at how it is used and operated. This also means we need to be careful around defining the “system”, this will become more apparent as we look at them.

Hence I thought it would be good to list a number of systems from lowest to highest.

0.05 to 2.5% Model Steam Engine

Model Trains – LittleLec 2019 gave measured efficiency numbers up to around 0.43%

From a calculation for methylated spirit burner, boiler and single cylinder double acting engine I get a number around 2.5%.

4.5% Single Cylinder Traction Engine

Output rotational energy / input coal higher heating value

6% Compound Traction Engine

Output rotational energy / input coal higher heating value

12.4% Compressed Air

From a mechanical input to an air compressor, to a storage tank and then to an air motor and out to a mechanical output. For the mechanical input and output I’m thinking of a rotating shaft at a given rpm and torque, hence power input/output.

My simple calculations around compressed air system efficiency give me numbers from 12.4% to 34%. A lot depends on the heat recovery during compression and then reuse during expansion.

50 to 60% Fuel Cell

A Proton Exchange Membrane fuel cell can achieve 50 to 60% efficiency at 25% load dropping to 30 to 55% at 100% load.

Note that at low loads (~5%) the efficiency can drop to below 30%.

Note though that this is just a sub-system and that we need to include the losses around creating, purifying and compressing the hydrogen in the first place.