By Rob McAtee, P.E., LEED AP, M.SAME
In 2009, a major ice storm downed trees over a wide area and shut down operations at Fort Knox, Ky. This extreme weather event is one example of many recent disruptions to military installations arising from natural and man-made disasters—and it highlights the importance of on-site generation for maintaining missions for extended durations.
Even though renewable energy and battery storage continue to improve and decrease in cost, a continuous source of on-site power generation is still needed for most locations to supplement these technologies. Installations with ready access to natural gas can benefit from fuel cells serving as base-load generation throughout the year and during periods of prolonged power utility outage. Fuel cells can be lifecycle cost effective and result in significantly lower greenhouse gas emissions when compared to typical generating utilities.
Fuel cells have been used in aerospace and communications for more than 60 years and are available in many forms. Their basic mechanism takes hydrogen from a fuel source and passes it through a catalyst that strips off electrons. The positively charged hydrogen ions then make their way through an electrolyte to the other side of the fuel cell, where they are combined with wayward electrons and oxygen (typically from the air) to produce heat and water. The circuitous path taken by the electrons is the electricity produced.
Hydrocarbon-based fuels can be used by many stationary fuel cells instead of hydrogen, but they can produce unnecessary elements, including carbon, and lead to unwanted emissions. Overall emissions are typically lower than in combustion processes. Net carbon emissions from fuel cells are much lower than a typical electrical utility that is heavily reliant on coal. Particulate emissions from fuel cells are also extremely low.
Compared to coal-fired utilities, fuel cells are 15 to 30 percent more fuel conversion efficient, with 58 to 70 percent less carbon dioxide emissions. These savings values include reduced distribution losses and substation transformer losses since the electricity is produced very near the point of use. Fuel cells generally have higher run times and require less overall maintenance than combustion generators.
All fuel cells produce heat as a byproduct to the generation process. Some manufacturers use this waste heat to pre-condition, or re-form, the incoming fuel to achieve higher efficiencies. For those that do not, the waste heat may be used for space heating, for cooling with absorption chillers, or for process loads. It is therefore important to analyze the total benefits achievable from a given unit of fuel.
IMPROVING ENERGY SECURITY
Installations with ready access to natural gas can benefit from fuel cells serving as base-load generation throughout the year and during periods of prolonged power utility outage. Fuel cells can be lifecycle cost effective and result in significantly lower greenhouse gas emissions when compared to typical generating utilities.
Although fuel cells can be used in many applications, their energy security, economic returns, and environmental benefits are dependent on several factors. Chief among these are the cost and reliability of natural or renewable gas; the cost, reliability, and fuel source of the local electrical utility; the profile of electrical loads to be served by the fuel cell; the ability to use fuel cell waste heat for heating or process loads; and the local electrical distribution architecture.
To explore how each of these factors affects fuel cell attractiveness, it is instructive to consider their application at an existing military installation. After Fort Knox was impacted by extended disruptions in electrical utility service, it installed significant on-site generating capacity in the form of multiple generating plants near substations. These plants have a robust electrical distribution architecture with sophisticated microgrid controls that allow them to share loads among substations. The base also has 2-MW of ground-mounted solar photovoltaic arrays.
Even with this system implemented, all of Fort Knox is not able to run continuously with on-site generation. Additionally, much of the generation capacity is based on diesel generators that require periodic refueling, limiting the duration of back-up power. To overcome this, Fort Knox considered natural gas fuel cells as a way to further improve energy security and reduce costs and emissions. Extensive technical and economic analyses were prepared based on a number of factors.
Natural Gas. Fort Knox can purchase gas at wholesale prices from the multiple pipelines running across its property. There also are natural gas wells on its land and the ability to store gas in underground geological formations. With an abundant supply of inexpensive gas from two sources, it is unlikely that disruptions in supply will occur. This factor is very positive for fuel cell applicability.
Local Electric Utility. Fort Knox is served by Louisville Gas & Electric. Although the utility is seeking to increase generation from natural gas and renewables, it is still heavily reliant on coal for power production. As with most local utilities, generation and transmission are typically reliable, although distribution (the last few miles from the primary substations to the point of use) is susceptible to disruptions, primarily from downed trees as lines travel through heavily wooded areas. Louisville Gas & Electric has a fairly complex rate structure, but it is still on the low to moderate side for total cost for delivered electricity when compared to the rest of the United States. This factor is neutral for economic effectiveness, but it is very favorable when considering reductions in source emissions.
Electrical Load Profiles. Most fuel cells are not able to vary output quickly. For those that can modulate, their economic benefits are maximized when running continuously at or near full output. Fuel cells are best used for base-load applications, with a utility, local generators, or energy storage assets handling short-duration peaks. Substation trending data from Fort Knox are fairly typical in that they reveal daily and seasonal variability. However, electrical demand is rarely below 40 percent of peak, and Fort Knox has interconnected substations. This factor is attractive to fuel cell applicability as well.
Ability to Use Waste Heat. Fuel cells produce significant heat in their conversion process. To maximize the overall usefulness of the fuel source, heat can be used for process loads, for space heating, or to produce cooling through absorption chillers. A significant number of facilities at Fort Knox are heated and cooled by ground-source heat pump systems, which are highly efficient and would not benefit from additional heating or chilled water. There are a few large buildings that can still take advantage of the heat source, but combined heat and power will likely not be achievable at every substation application. This factor is only moderately favorable to fuel cell applicability.
Distribution Architecture. Fort Knox has a robust electrical distribution and microgrid control system in which local substations are continuously monitored and loads can be shared among them. The base has multiple, centralized generating sources, including diesel and natural gas reciprocating generators and photovoltaic arrays. Fuel cells can be sited to supply energy to medium-voltage substations. This is very favorable to fuel cell applicability.
THE RIGHT FACTORS
Fort Knox has an overall favorable rating for most of the considerations relating to fuel cells, making it a good candidate for the application of the technology. Despite relatively inexpensive local electricity rates, the low cost of natural gas and lower operating and maintenance costs make fuel cells an economically competitive source of power.
Fort Knox has an overall favorable rating for most of the considerations relating to fuel cells, making it a good candidate for the application of the technology. Despite relatively inexpensive local electricity rates, the low cost of natural gas and lower operating and maintenance costs make fuel cells an economically competitive source of power. If the fuel cells were provided by a third party that could take advantage of available investment or production tax credits, the application could be even more attractive.
Perhaps most importantly, the evaluation shows that leveraging fuel cells—combined with existing spinning generation and possibly storage—could allow Fort Knox to run nearly indefinitely if disconnected from the larger electrical utility. Finally, the application could reduce source emissions related with electricity use by as much as 70 percent.
Rob McAtee, P.E., LEED AP, M.SAME, is Director of Energy & Sustainability Services, Mason & Hanger; firstname.lastname@example.org.
[This article first published in the March-April 2021 issue of The Military Engineer.]