Understanding backup power system fuel choices

 While diesel remains popular as a fuel supply, engineers have several additional fuel options from which to choose.

Diesel fuel
As previously mentioned, diesel fuel has been the traditional fuel of choice for commercial and industrial backup power applications (see Table 1). Among the diesel-engine advantages is its high thermal efficiency, which can yield a low capital cost per kW in large-kW applications—typically 150 kW or more. Because diesel fuel must be stored on-site, diesel-fueled generators can also provide backup power in remote areas that do not have the benefit of a natural gas infrastructure. For the same reason, market segments with mission critical applications, such as hospitals and 911 call centers, often choose diesel-fueled generators because on-site fuel helps to ensure reliability. Finally, because diesel fuel has been used for so long in backup power applications, there is a perception within the market that diesel engines are the most reliable prime movers for backup power systems.
Despite its widespread acceptance, diesel fuel does have its drawbacks. For example, the U.S. Environmental Protection Agency requires the use of ultra-low sulfur diesel (ULSD) in all standby generator applications. ULSD goes through additional refining processes, which makes it less stable than traditional diesel fuel. If not maintained, diesel fuel will degrade over time. Within the first year of storage, it will suffer from oxidation, which occurs when the hydrocarbons react with oxygen to form a fine sediment and gum. If pulled into the engine, these contaminants could clog the fuel filter and fuel injectors. Microorganisms can similarly contaminate the fuel. Water, which can enter the fuel management system as condensation, promotes bacteria and fungi growth. These microorganisms actually feed on the fuel itself. If allowed to grow, they can form gelatinous colonies that can also clog fuel management systems. Additionally, their waste is acidic in nature, which can lead to fuel tank corrosion.

These are significant concerns in backup power applications. A diesel-fueled generator with a tank sized for 72 hr of full-load operation could easily take about 20 years to burn a single tank of fuel, assuming a 60% typical load level, weekly no-load exercising, and average power outages of only 4 hr per year. However, these issues can be mitigated by instituting an ongoing fuel testing and maintenance plan that regularly removes both water and sediment from the fuel tank. For emergency applications, fuel maintenance is required by code within NFPA 110: Standard for Emergency and Standby Power Systems. This type of maintenance program adds to a genset’s total cost of ownership, which also must be considered. Automatic fuel polishers, which consist of a pump and filtration system, add to the upfront cost of a backup power system, but they reduce ongoing fuel maintenance costs. Manual maintenance plans are more costly over the long term.