Given the changing landscape of the electrical utility grid with the requisite battery energy storage systems being incorporated to account for the varying degrees of renewable energy supplies (RES) as well as the inherent energy storage on the data center side in the form of UPS, the need for quick load acceptance of the generator plant should be re-evaluated. Further, given that all modern UPS typically have an adjustable walk-in time and that all mechanical loads are typically variable frequency drive (VFD) controlled, there is little to no load that requires block load acceptance capability.

Finally, if the data center itself is part of a microgrid which incorporates RES of some form, there is also likely to be large-scale energy storage of some form that could be used to provide ride-through capability to the data center in the event of a utility outage. This would also alleviate the need for rapid start-up and load acceptance of the data center generator plant.

 

 

 

6. Design considerations for natural gas-based backup power

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Given the advances in NG generator designs and capabilities as well as their favorable emissions profiles when compared to diesel generators, the decision to incorporate them into the power plant for the data center demands further design considerations to be taken into account.  NG generators cannot simply be substituted for their diesel counterparts in most designs.

 

As discussed above, the load acceptance profile of NG generators requires a careful evaluation of the design topology and the interaction of the various loads with the generators. Given that over the last few years most data center UPS battery plants have been optimized for shorter and shorter run-time, sometimes as low as 1 to 3 minutes, it is necessary to evaluate this based on the decision to incorporate lean or rich burn engines. UPS walk-in times, as well as an evaluation of the mechanical load start-up profile, should be aligned with the load acceptance profile of the NG generator selected.

 

Then there is the question of onsite fuel storage. This has historically been one of the major stumbling blocks for the use of NG generators in emergency operations, since it requires onsite fuel storage in an amount which is challenging with most codes. Even if the generators are not required by regulation to have onsite fuel storage, there is the Uptime Institute’s continuous operation requirements for their Tier 3 and Tier 4 data centers, that nominally requires onsite storage of fuel for a minimum of 12 hours standby operation if the data center is to gain Tier certification.

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Even if a facility is not to be Tier certified, most operators will not be comfortable with no onsite fuel storage. This is another issue that will require careful analysis of the supply source of both utilities, the substantial costs associated with multiple utility connections and their relative reliability.

 

The argument has been made that the natural gas distribution system is inherently reliable, principally because it is almost entirely underground and also because it is a mesh-based system. However, this is not always so, as evidenced with the failure of the natural gas supply during the Texas power outage of the winter of 2021. Although the general consensus was that this was a rare anomaly, the perception of the reliability of the natural gas distribution system has been damaged to some degree.

 

In a report on the comparison of fuel sources for generators, the authors state “…We estimate that the higher reliability of the natural gas fuel supply compared to that of diesel fuel for long outages makes natural gas generators more reliable options than diesel generators…”[2] Also, in comparison to the United States, the natural gas supply networks of Europe and many Nordic states are extremely reliable. In any case, best practice suggests an evaluation of the reliability of both utilities to the site should be performed and considered in the design. In addition, the risk of a concurrent failure of both utility supplies should be assessed, and the inherent “storage” provided by the volume of the gas held in the supply pipe system to the site calculated.

It’s important to note that onsite diesel storage has challenges associated with maintaining the quality of the fuel as well, which typically requires fuel polishing. Further, in the event of a major event disrupting the electrical grid, reliable replenishment of diesel fuel can also prove to be highly susceptible, as observed in numerous natural disasters such as hurricane Sandy. [3]

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7. Looking ahead, dual fuel engines

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In the context of minimizing GHG emissions, the use of fossil fuel-based generators cannot be considered viable as the ultimate solution.  However, one aspect of NG generators not discussed up to this point is the potential of dual fuel generators. Currently, most NG generators with dual fuel capability include units that are optimized for emergency use with rapid start-up or onsite fuel storage solutions. They include diesel NG units, or NG with alternative onsite fuel storage such as propane.

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What is more promising and offers the potential for meaningful change is the availability of dual fuel generators that utilize NG and hydrogen. It is widely considered that hydrogen holds the promise to be the ultimate source of sustainable energy - subject to the development of cost-effective, sustainable and efficient means of production for a multitude of technologies from fuel cells to generators. Whilst natural gas and hydrogen dual fuel engines have to be derated, meaning the engine has to be oversized to achieve the desired power output, the reduction in emissions they achieve means that this approach should still be evaluated.

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As methods are developed for the sustainable, carbon neutral, production and storage of hydrogen, it will fundamentally change the way that energy is sourced and used globally. In the near to midterm, dual fuel generators could be stepping-stones to a more sustainable, hydrogen-based future.  Overcoming the immediate hurdles for the implementation of NG generators that lead the way to dual fuel generators and ultimately to hydrogen generators, could be a road map to a near net zero future.

 

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8. Additional Considerations

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In the context of GHG abatement, there are additional considerations that also need to be weighed, and associated with them, additional options to evaluate. For example, for the purposes of discussion, this white paper is focused on generators for standby operation, but natural gas engines can also be utilized in combined heat and power (CHP) arrangements as a prime source of electrical power for the site. This option will be analyzed in more detail in a separate article, but it might be a particularly applicable solution in instances where natural gas supply is more readily available than electrical utility power, or if the electrical utility cannot meet the demand for the data center for any number of reasons. 

A particular concern and potential benefit of onsite prime power generation would be the GHGe of the utility company, and whether the data center generator plant could contribute to the reduction of GHG emissions in a meaningful way. Onsite natural gas generation would be counterproductive in a location where, for example, the utility has a fuel mix that is mainly hydroelectric with minimal fossil fuel generation. In principle, the generation of power in close proximity to the load could be of benefit in terms of the sheer value of the losses avoided in the transmission of the same amount of useful energy from a utility grid with remote generation, however beneficial for the reduction in GHGe this is it is very difficult, if not impossible to quantify any economic benefit for the data center operator.

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9. Conclusion

 

Data centers are energy dense consumers of resources. While they are not the largest contributors to GHG emissions globally, they present an opportunity to be significant players in its mitigation. Design approaches and technologies that allow for the transition of data centers to use more sustainable, GHG neutral solutions can be a significant step in the right direction for this industry. As more data centers are designed and constructed with these goals in mind, utilizing forward-looking technologies, their contribution to a near net zero future will be invaluable.