A Reader’s Assessment of Microturbine Efficiency

Some comments for the microturbines:
1) The numbers you state are biased:how did you find the reduction of the efficiency from 28% to 18%? Do you have any information, studies etc?
2) You dont say something about the air pollution, which is one of the advantages of microturbines.
3) You have to understand that the history repeated: the electricity started as distributed generation. Microturbines will help to achive the distirbuted generation in our days.
4) Microturbines and the leading producer (CAPSTONE) are the TOYOTA PRIUS of energy production. The first generation of Prius was not so successful and had to prove their value. But the second generation of Prius is very successful, make TOYOTA number car manufacturer world wide and the European and american car industries are still looking for solutions!!!

Felipe's remarks remind me of the economist who spots a twenty dollar bill on the sidewalk, but passes right by it. His friend, who sees this from across the street, asks: "Why didn't you pick up the $20?"

"Obviously that was not a real $20," replies the economist, "because if it were, someone else would have picked it up before me."

Your comments regarding MicroTurbine effiency are warranted. The Capstone 30kW and 60 kW MicroTurbines are about 28% efficient under the best of circumstances. This efficiency is based on sea-level standard day conditions, which is 59 degF. On hot day conditions where the grid is taxed, the efficiency falls off considerably. This is a function of the maximum turbine exit temperature, which must be held as a constant regardless of ambient temperature.

At altitude, where the air is less dense, the turbine efficiency is also degraded.

Next, take into account the natural gas supply. Most facility gas supply pressures are about 5 psia and the gas pressure must exceed the combustion inlet pressure, which is similar to the compressor exit temperature. The electrical loses for running a gas compressor are significant.

Also, load following may require running the gas turbine at less then maximum power. Part power operation is significantly less efficient then maximum power operation.

Thus, a 28% microturbine may only be 18% efficient under these circumstances (ambient temperature, pressure, gas pressure, and load).

Next, let's consider co-generation. The 90% efficiency is only achieved under perfect circumstances where 100% of the heat load and 100% of the power load is used, which is rare. This is referred to as "heat balance."

In most cases, a portion of the heat or power is used, dropping the efficiency significantly. This 90% efficiency is also quoted at sea-level standard day conditions.

Considering a standard natural gas water heater is 80% efficient, it is comparable with the expected efficiency of a co-generation microturbine application where the heat and power load are not perfect. Thus, a low cost natural gas hot water heater is as or more efficient then a microturbine co-generation system at a fraction of the cost.

Agreed, free fuel is the best application for a microturbine, but this a tiny niche market and the free "dirty" fuel must be cleaned by scrubbers, which is costly and difficult to maintain. Thus, it may be cheaper to simply flare the gas.

It is true that if MicroTurbines were more efficient then the grid or recips, they would be everywhere.