Yiğit Bulut, Partner, and CTO at EYP Mission Critical Facilities, also contributed to this article.

Globally, most data centers are designed and constructed with the primary goal of achieving highly reliable facilities that are engineered to remain online and operational in the face of any foreseeable or unforeseeable eventualities.

Data centers, by nature of the information technology (IT) equipment they support and the processes which run on them, are considered mission critical and must remain operational as continuously as possible.

The need for an emergency power supply system (EPSS) to run for 120 minutes (generally requiring onsite fuel storage), or longer for most data center operators, has resulted in the selection of diesel engine generators as the primary and most common choice of back-up energy for critical systems.

The typical topology consists of different combinations and levels of redundancy of UPS backed up by diesel generators with 12 to 72 hours or more of onsite fuel storage

But in terms of its future as a standby source of power for data centers, the road map for diesel looks like one of increasing restrictions on use, tougher tax regimes, permitting, lower emissions targets, improved air quality requirements and lower noise regulations.

This raises an intriguing question: Will data center primary or emergency power move to natural gas engines?

The case for gas

One of the main advantages of gas burning over diesel burning is lower emissions – less NOx and CO2.

Natural gas (NG) engines have better emission profiles compared to same size diesel engine generators. Depending on the type of ignition system utilized, NG generators in the higher power ratings range are typically either lean burning; the air to fuel ratio is higher than the stoichiometric air-to-fuel ratio (16:1) which results in a lower combustion temperature that minimizes NOx emissions - or rich burn, where the air-to-fuel ratio is lower.

The trade-off in the type of combustion selection nominally boils down to two competing considerations; The lean burn engine, while having a better emission profile and efficiency, does not have the block loading or load assumption capability of the rich burning engine. This explains why historically, rich burn engines have been preferred to lean burn engines for standby operation.

In comparison to US EPA Tier 2 diesel generators, both lean burn and rich burn NG generators produce less NOx and CO2 per kW - but at a higher initial cost.

For some, that alone might be seen as making a strong enough case for transitioning.

The case against gas

What are the challenges for using gas?

  • On site fuel storage
  • Poor utility connections
  • High initial capex – but TCO is narrowing
  • Design considerations

Onsite fuel storage has historically been one of the major stumbling blocks for the use of NG generators in emergency operations as it requires onsite fuel storage in an amount which is challenging with most codes. For a data center to be Tier certified a minimum of 12 hours standby operation of onsite storage of fuel is needed. Even if the facility data center is not to be Tier certified, most data center operators will not be comfortable with no onsite fuel storage.

In the past, the argument has been made that the natural gas distribution system is inherently reliable, principally because it is almost entirely underground and 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.

As stated above, when compared to EPA tier 2 diesel generators, NG units present a higher initial capex. Consider, however, that current US emission requirements for Tier 4 final certified engines adds significant cost to diesel generators. These will deliver lower emission levels which are closer to their NG counterparts. On the basis of emissions, this narrows the advantage of NG generators, but because of the added cost of diesel infrastructure it also makes NG generators more cost competitive.

Design considerations for natural gas engines

Given the advances in NG generator designs and capabilities as well as their favorable emissions profiles with respect to diesel generators, if the decision is made to incorporate them into the power plant for the data center there are design considerations that need be judged. NG generators cannot simply be substituted for their diesel counterparts in most designs.

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-times, some 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 evaluated to align with the load acceptance profile of the NG generator selected.

For a deeper perspective on standby power generation which provides a technical analysis of gas reciprocating engines read the i3 Solutions/EYP White Paper: “The Case for Natural Gas Generators," which compares emissions profiles and performance characteristics of diesel and gas generators, and outlines the case for, and the challenges to, adopting gas reciprocating engines for use in data centers.