Actual power output from gen-sets is found to be different from that stated by the manufacturer. Is there an empirical formula that can give this quantity in terms of those parameters that are found to play a role in this, like ambient temperature, pressure and humidity.....etc
As an initial direction, I would derate based on air density, specifically oxygen density, as compared with the manufacturer's stated conditions. High ambient temperatures will lower oxygen density, as will high humidity and low barometric pressure (for instance, operation at high altitude as compared with sea-level). After that, I would look at fuel quality.
For example, if the manufacturer rated the genset at sea-level and 25C, but you are at 400 meters altitude and 30C: The change in pressure from sea-level to 400 meters results in 95.5% oxygen availability as compared with initial conditions. When the change in temperature from 25C to 30C is factored in, it results in 93.9% oxygen availability as compared with initial conditions.
Thanks Ray, what you suggested sounds logical. Then can we say that the deratiion factor(DF) may be defined as :
DF = Actual Engine Power/Standard Power =Actual air density/Standard air density
There is no generic or universal answer to this question.
Older naturally aspirated mechanically controlled engines are affected by fuel volumetric energy density and fuel temperature (effects on engine-out power). When less air is available (high temperatures, high humidity), one needs to pay attention to cooling (both the engine and the generator) and to excessive smoke.
Newer turbocharged electronically controlled engines are well capable of compensating for many effects and there may even be no derating.
As a general rule, you should design an electrical system to have a 20-25% reserve, that is, you do not plan on loading the generator, the wires, or any other part of the supply system at more than 80% of its rated load.
The gensets that I had in mind for my answer are small, portable gensets. These are all mechanically-controlled, naturally-aspirated in my experience. Large gensets for supplying a hospital may be more sophisticated, but beyond my experience. Michal is correct that an electronically controlled, turbocharged genset would be able to compensate for altitude and may not need to be derated. However, Kamal directly stated, "Actual power output from gen-sets is found to be different from that stated by the manufacturer" so some derating is necessary. I assume that this is caused by a lack of turbocharging and electronic controls. Kamal, is your question universal? Or is it specific to gensets that you have? If universal, see Michal's answer.
Ray, Kamal - I would like to comment on the observation of the actual power being different from the rating.
The power output of a mechanically controlled diesel engine is given by a) the amount of fuel delivered into the engine and b) engine overall thermal efficiency. Lowering the oxygen content will cause the engine to smoke, but unless it is very dramatic, it should not affect the engine power.
If a mechanically controlled, non-turbocharged diesel engine runs at high altitude, or with humid air, it may lack oxygen, and without adjustments it will smoke horribly, but it will still provide its normal (rated) power. So it *should* be derated, and the maximum fuel delivery adjusted (lowered). But this does not explain the drop in power.
The only environmental factor is the temperature - hot diesel fuel has lower density and lower viscosity, both affecting the mass of the fuel delivered into the cylinder, and therefore the engine power. You can also have hot diesel fuel at normal temperature - it is returning hot from the engine, and if it is not cooled naturally (i.e., small or well insulated fuel tank), the fuel in the tank will warm up.
The engine can also be misadjusted or in bad mechanical shape - in such case it should be repaired, not derated.
Another explanation could be that a carefully selected specimen tested under controlled laboratory conditions works better than an average engine under real settings.
Hi, Kamal. I first use to vary the entering pressure according the International Standard Atmosphere using the barometric equation (P=P0*EXP(g/RT)*z or for SI P=101,325*EXP(9,806/287*298,15)*z) and recalculate the indicated power and then the brake power (assuming a typical mechanical efficiency) for a range of pressures. Then I repeat for a temperature variation. Finally I compare the results with the rated brake power computed for ISO8528 ambiente conditions. I use it for didactical purposes. Hope it interest you. Kindly.
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