Does the Efficiency of a Steam Boiler Generator Depend on Load?

[gentatsuki]

Hi, my friend and I were having discussion on the efficiency of a steam boiler in our thermodynamics lab (he has a model of a steam boiler in the thermodynamics lab). He told me that the efficiency of the steam boiler generator is dependent on the load. I thought it was absurd, saying that the efficiency of the generator has to be the same regardless of how much you load it. Apparently they vary the resistance value using a potentiometer for their experiment.

Which of us is right?

 

[russ_watters]

Welcome to PF.

Hi, my friend and I were having discussion on the efficiency of a steam boiler in our thermodynamics lab (he has a model of a steam boiler in the thermodynamics lab). He told me that the efficiency of the steam boiler generator is dependent on the load. I thought it was absurd, saying that the efficiency of the generator has to be the same regardless of how much you load it. Apparently they vary the resistance value using a potentiometer for their experiment.

Which of us is right?

The efficiency of both a steam engine and a generator will vary with load. Often the sources of inefficiency, such as heat loss through the boiler are not functions of load, so they are constant while the load changes. For example, a steam engine that puts out 100W while losing 100W through convection due to imperfect insulation and another 100W through heat rejection (total 100/300=33% efficient) will still lose 100W when it is putting out 50W (50/250=20% efficiency).

A generator will have mechanical friction losses that do not vary with load (for a constant speed generator anyway), so they make up a larger fraction of the work input when the load is lower.

 

[gentatsuki]

Thanks, that helped me a lot :D