At the time, the total cost of running diesel generators as the only power source was a wash with the per-kW/hr utility power. The real expense that kept them on generator full time, was the significant amount of time and cost to bury power lines blasted into solid rock up the mountain, then paying to maintain those lines over time.Wow, thank you for that information, Kelly! I never realized that it was possible to run a transmitter on generator power for an indefinite period of time. Using your example with the Three Sister's site, what was the cost differential between AC power versus the cost of diesel fuel? Obviously, I am not factoring in inflation of fuel prices, or the cost of potentially running power lines to the site. I previously worked with a radio group that utilized a single Kohler generator at their transmitter site (likely 100kW, like in your example) to power an older Continental Electronics system for a period of time. I never found out what the logistics were of trucking fuel to the transmitter site, or how long the generator could operate on a tank of fuel. Looking back, that would have been interesting information to know. What is the longevity of the fuel like in these large holding tanks? I'm far from an expert on diesel fuel, but I imagine that it degrades over time like regular unleaded gasoline. Many of us may be acquainted with "Stabil" as a fuel stabilizer in our gasoline powered appliances, but this fluid is not a permanent solution and gasoline still will go bad after long periods without use. With that being said, how do radio groups decide when to fuel up these large tanks, and how long will that fuel typically sit around before new fuel needs to be brought in?
Regarding the longevity of fuel; modern diesel can sit in the tank for at least two years before needing some sort of additive to refresh the fuel. The ketones of the diesel start to break down over time, especially if water from condensation gets into the fuel. There are several types of fuel additives to replenish ketones and help disperse water. At Three Sister's Mt., fuel was never in a tank longer than one year, so there were no need for additives. The more important maintenance to a diesel generator that sits around, is weekly exercising to warm up the engine and evaporate moisture and regular oil and filter changes. At Three Sister's, the engine oil and filter were changed on the generator which had been running the prior week. So each generator got an oil and filter change every two weeks.
As David mentioned, some sites use propane (LP) or LNG (Liquefied Natural Gas). The advantage with those fuels is they never go bad. The downsides are; neither fuel produces the same amount of energy as diesel or even gasoline, so the engine must be larger to produce the same amount of generated watts. Larger engines are more thirsty. The second disadvantage with LP or LNG is getting fuel delivered to a hilltop or mountain site in the Winter if needed during an extended outage. You've no doubt seen propane delivery trucks on the road. Most LP/LNG fuel companies won't deliver fuel if the road is icy. Ask me about the KPLZ incident someday.
During a typical year, I've always made it a point to top-off my generator tanks yearly, usually in the Fall. Assuming there were no utility outages, the amount of fuel burned would just be from automatic weekly exercising of the generator. Obviously, if there was an outage of more than a few hours, the fuel truck would be called for an earlier top-off.