California. Archived data allows us to note any trends in system performance over time. That is how Roy first noticed deterioration of battery capacity after only a year of operation, and brought it to my attention. Anyone with Internet access can take a look at how the system is performing. The Web address is included at the end of this article.
NPS has used similar generator modules elsewhere, including the Arctic and Antarctica. The units have generally served well, but the bottom line is that the design requirements exceeded the actual performance of the unit.
First and foremost is the fuel efficiency issue. The unit was designed to run unattended for a full year at a 200-watt rated output. The actual power requirement ended up at an average of 240 watts continuous. This doesn’t sound like much, but add it up. That comes to 5,760 WH a day, or 2,100 KWH a year. That is a fair bit of energy, and it takes a large volume of diesel fuel to produce it. Also, the thermal inefficiency of the container resulted in a tremendous number of run cycles on the Espar heaters, which are run on diesel fuel to keep the electronics and generators warm enough to operate. We could only run the system for a bit longer than eight months before the two 175-gallon (660 l) fuel tanks were critically low.
Another problem that rather quickly became evident was a serious loss of battery capacity. The unit autonomy (period of time between charge cycles) became less and less, while the amount of fuel used became more and more. To understand why this happened, you have to take a look at the conflicting requirements of the generators and the battery bank.
Diesel generators like to run under full load. This is where their efficiency is greatest (still only about 30% conversion efficiency), and where the longevity of the engine is greatest. If a diesel generator is run at low load for extended periods of time, it is far less efficient in burning fuel. Incomplete combustion and dramatically increased emissions result in heavy deposits in the exhaust system, and often, premature cylinder wear, sticking piston rings, and a host of other problems. Cold weather exacerbates the condition markedly.
Batteries, on the other hand, demand long periods of low-current input at the end of a charge cycle. Generators are often run at maximum efficiency for a relatively brief period of time, and then are shut down to avoid excessive fuel consumption and possible engine damage.
A longer run cycle would be better for the batteries, but bad for the engine. A shorter run cycle would be better for the engine, but bad for the batteries. The compromise ultimately settled on in Ivotuk was fairly bad for both components. The fact that the battery bank was rather undersized—720 AH at 24 VDC—only made the problem worse. The generators had to cycle more frequently, and thus ran a greater percentage of the time under low load. Here was a system crying out for some renewable