Last year, multiple tornadoes touched down in Kentucky and tore a wide path of destruction across four states. Hurricane Ida made landfall as a Category 4 storm, destroying infrastructure across nine states. Several winter storms caused disastrous power failures in Texas. The number of extreme weather events has increased over the past few years, causing more and more widespread power outages in the U.S. and across the globe. The U.S. Department of Energy (DOE) indicated the number of reported electrical disturbances has more than doubled in the past three years from 150 disturbances in 2017 to 383 in 2020.

For critical facilities, like data centers, backup power is a necessity to maintain operation during utility outages. Facility personnel and operators consider the backup power system as the pacemaker of the electrical system. It keeps the power flowing and facility alive when the utility stops. Codes, such as National Electrical Code (NEC) 706, “Critical Operation Power Systems,” and ANSI-TIA 942, require backup power for certain critical facilities to maintain operation. Diesel generators are the most common source for backup power, as they are reliable, cost-effective, and have refueling capabilities that give them the ability to operate for extended run times. Because of this, the diesel generator market has increased rapidly, crossing the $25 billion mark in 2020. Global Market Insights is projecting a 5%-6% annual increase over the next seven years, mostly due to increasing weather severity, aging utility infrastructure, and the frequency of blackouts. But, are diesel generators the sustainable answer for providing backup power?

FIGURE 1: Exhaust from a diesel generator during startup. Image courtesy of Jacobs Engineering

In recent years, investors have shown interest in putting money where their core values stand, particularly when it comes to protecting the environment. Environmental, social, and governance (ESG) metrics have been developed to establish a set of standards for a company’s operations. Those metrics are used by investors to screen potential investments. One important ESG metric investors currently use is carbon dioxide (CO2) output or carbon footprint. Many environmentalists, scientists, and regulators believe the diesel generator is actually exacerbating the extreme weather conditions that are causing power outages by augmenting the CO2 concentration in the atmosphere.

Why Is CO2 Harmful?

COis a greenhouse gas. During the day, the Earth absorbs the solar energy from the sun, and, then, at night, the Earth releases that energy back into the atmosphere. Greenhouse gases, such as CO2, methane, and nitrous oxide, form an atmospheric barrier that keeps some of that energy or heat from escaping (greenhouse effect). This helps maintain a climate on Earth habitable for humans and other species. Most people associate greenhouse gas with CO2 because it makes up nearly two-thirds of greenhouse gas emissions.

CO2 is a naturally occurring gas through volcanic eruptions, wildfires, and human respiration. It can also be a human-generated gas via burning fossil fuels for power or transportation. When kept in balance, greenhouse gases, such as CO2, are important to keeping the planet at a temperature ideal for humans and other species to live. The problem is when the CO2 and greenhouse gas levels are out of balance. When the concentration of greenhouse gases increases, the Earth absorbs more of the sun’s energy and releases less, causing the planet to slowly warm.

Since the Industrial Revolution in the late 1800s, the global mean temperature has slowly increased more than 1°C. The fastest increase in temperature, about 0.6°, occurred over the last 40 years. During that time, the atmospheric CO2 concentration has also increased, substantially rising from 340 ppm to close to 420 ppm. As the atmospheric CO2 concentration increased, the global mean temperature increased.

FIGURE 2: Mean temperature change versus atmospheric CO2 concentration. Image courtesy of Jacobs Engineering

The expert consensus is that we must limit global warming to less than 1.5° (a 1.5° mean temperature change) to avoid catastrophic loss of life. The escalating temperatures are causing the ice caps to melt, sea levels to rise, and could make it difficult for some wildlife to survive. It could also cause extreme weather events, such as hurricanes, tornados, heat waves, floods, and wildfires. In fact, these types of extreme weather events are already happening more rapidly in the world today. According to the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information’s website, the U.S. experienced 20 separate weather and climate disasters in 2021 (events with losses exceeding $1 billion), just two events short of the record 22 weather disaster events that happened in 2020. The annual average between 1980-2020 was only 7.4 weather disaster events per year. More extreme weather events mean there is potential for more power outages, which increases the need for backup power.

Climate Action

Scientists predict that if the Earth continues down the current CO2 path, we could hit that 1.5°C mean temperature change threshold in 20-25 years, if not sooner.

Many global technology companies are leading the way toward climate action.

“This generation owes it to the next generation to address climate change,” said Sundar Pichai, CEO, Google. “The time to act is very narrow and shrinking as we go.”

Satya Nadella, CEO, Microsoft, has shared a similar sentiment: “The world’s climate experts agree that the world must take urgent action to bring down emissions. Ultimately, we must reach ‘net zero’ emissions, meaning that humanity must remove as much carbon as it emits each year.”

These and other leaders are setting aggressive net-zero targets. Such targets have included:

  • 100% renewable energy;
  • Net-zero emissions by 2030;
  • Net-zero carbon by 2030;
  • Diesel-free data centers by 2030; and
  • Removal of historical carbon footprint by 2050.

The climate action goal for most technology companies is a net-zero or net-zero carbon-energy facility. Over time, they are continuously driving to be more efficient, use less energy, and consume less fuel, while, at the same time, purchasing and utilizing more green, carbon-free energy. Net-zero energy is reached when all the energy being utilized is carbon-free.

FIGURE 3: Achieving net-zero energy. Image courtesy of Jacobs Engineering

The Diesel Generator

So, how does the diesel generator affect net-zero carbon? The diesel generator is a machine that combines an electric alternator with a diesel engine to convert mechanical energy into electrical energy. During combustion, the generator engine burns fossil fuels to convert the chemical energy contained in the fuel to mechanical energy. The mechanical energy is used to rotate a crankshaft that drives the alternator and converts the mechanical energy into electrical energy.

The burning of fossil fuels creates exhaust gases and emissions, such as CO2, nitrogen oxides (NOx is a combination of NO and NO2), hydrocarbons (HCs), and particulate matter (solid and liquid particles). During the operation of the generator, exhaust gas and emissions are released into the atmosphere, adding to the concentration of greenhouse gases and reducing the air quality in the surrounding area. These emissions are making it harder and harder to get permits, and without permits the facility can’t operate.

The U.S. Environmental Protection Agency (EPA) does regulate emissions from diesel-powered equipment per the Clean Air Act. Most of the regulations on diesel engines are focused on removing nitrogen oxides that cause smog and acid rain. Cleaner burning fuel and after-treatments products placed on the generator, such as selective catalytic reduction (SCR) and particulate filters, will remove nitrogen oxide and particulate matter, but they will not remove the CO2.

As stated previously, technology companies and data centers are expected to maintain a high level of availability and reliability. They rely on backup power sources, such as generators, to get them through utility outages. Large hyperscale and cloud-type data center facilities can have 30-100 diesel generators on-site, making it very difficult to be carbon neutral.

Net-Zero Carbon Options

How can companies achieve carbon neutrality if they still need diesel generators? One option being investigated is the use of a CO2 scrubber that can absorb and store CO2 right at the source. The following are three scrubber technologies that are currently being used to capture CO2 in other applications:

  • Amine scrubbing, or amine gas treating, uses a solution of alkylamines to absorb CO2 through a chemical reaction. Due to the high cost, it is mostly used on large coal and gas-fired power plants and refineries.
  • Regenerative CO2 removal system (RCRS) removes CO2 by using a solid amine chemical in the form of porous ion resin beads that are heated. RCRS requires very high temperatures, making it not efficient for large quantities. It is mostly utilized in low-flow systems, like those on a space shuttle.
  • Metal organic framework (MOF) is a porous material that bonds and captures the CO2. It is probably the most promising technology, but, unfortunately, it is not readily available in large scales due to the limited resource availability and difficulties in production.

Unfortunately, until better technology is developed, the use of CO2 scrubbers to eliminate CO2 from a single diesel generator has not proven to be economically feasible for many data center facilities to undertake.

Alternate fuels have also been utilized to reduce emissions. Natural gas generators have much lower carbon monoxide and nitrogen oxide emissions than diesel generators, making them easier to permit. The CO2 emissions for natural gas generators, however, are only approximately 10% lower than diesel: 1,395 pounds per megawatt hour compared to 1,555 pounds per megawatt hour1.

Renewable diesel fuel, or green diesel, is an option being investigated by many large technology companies for reducing carbon, specifically in California. Renewable diesel is an HC biofuel produced through biological, thermal, and chemical processes from a biomass source, such as agricultural crop residue and municipal waste. Renewable diesels carbon intensity is approximately 20 gCO2e/MJ compared to 102 gCO2e/MJ for ultralow sulfur diesel2. The main issue with renewable diesel is that there is not enough of the fuel available.

Another option for obtaining zero carbon while still using a generator for backup power is carbon credits or carbon offsets. Carbon credits allow a company to financially invest in carbon mitigation projects, such as forest and wetland conservation or direct air capture (DAC) projects that offset the company’s carbon emissions. Photosynthesis removes CO2 naturally. Forest projects, wetland projects, and even planting cover crops on farms can increase and extend photosynthesis and remove more CO2. DAC is a process being used to chemically scrub CO2 directly from the ambient air. It uses a technology like carbon scrubbing to remove and capture CO2 directly from the atmosphere instead of trying to capture it at the source of emission.

On average, one carbon credit equals 1 metric ton of CO2. A 2-MW diesel generator produces about 3,110 pounds or 1.4 metric tons of CO2 per hour at full load. By purchasing credits proportional to their emissions, companies can meet their ESG and net-zero carbon goals while still maintaining the reliability of the diesel generator for backup power.

Backup Power Alternatives

Photovoltaic solar panels and wind are used for providing green energy; however, due to intermittency issues and the amount of space required per MW, they are not a reliable or feasible on-site energy source for critical facilities. Engineers for facilities that consume large amounts of energy, like data centers, have investigated alternate types of electrical energy storage systems that save energy and are more efficient and sustainable.

  • Lithium-ion batteries are electrochemical batteries that do not use a chemical reaction. During discharge, positively charged lithium-ions and negatively charged electrons are formed in the anode. The positive lithium ions move from anode to the cathode through electrolyte. The negative electrodes can’t pass though the electrolyte and flow from the anode to the cathode through an external circuit. The lithium-ions and electrodes then recombine in the cathode material, completing the power circuit. Lithium-ion batteries are often used in conjunction with solar or wind to ride through the intermittency periods. Lithium-ion battery storage systems are probably the most widely used stationary energy storage options besides diesel generators. This is mostly due to the decreased cost, making them a very efficient solution. Other advantages include minimal space requirements, low maintenance costs, and quicker recharge times. Their disadvantage is that their run time is limited by the battery’s size.
  • Flow batteries are a fully rechargeable energy storage device where fluids containing the active material are pumped through a cell. This promotes oxidation on both sides of an ion-exchange membrane, resulting in an electrical potential and current flow. Advantages include low costs of ownership and longer discharge durations. Disadvantages include the high cost of the fluids and parasitic loads that make it less efficient.
  • Pumped hydroelectric storage (PHS) is where water is pumped to a reservoir at a higher elevation during off-peak hours and then released using gravity back to a lower reservoir when power is needed. As the water flows down, it is run through a turbine that generates electricity. Advantages include efficiency and long run times. The disadvantages of PHS include the large amount of space required, a long permitting process, and high capital costs.
  • Compressed air energy storage (CAES) is where compressed air is stored in underground caverns. During off-peak hours, electrical energy is converted into high-pressure compressed air. The compressed air is stored in underground cavity that can help maintain hydrostatic pressure. The hydrostatic pressure forces the air to the surface, where it is heated, expanded, and run through a turbine that generates electricity. Advantages include low maintenance costs, long storage time, and net-zero emissions. One disadvantage is that it is only about 55% efficient and, thus, expensive to operate.

The key metrics to review when comparing the different energy storage systems include recharge time, power duration, cost per MW, and space required per MW. It is also important to remember that energy storage can be used in different ways. A 240-MWh system can be used at 60 MW for four hours, 30 MW for eight hours, or 15 MW for 16 hours. Lithium-ion and flow batteries are typically good for applications looking for one to four hours of backup operation. Pumped hydroelectric storage and compressed air energy storage are typically better for providing power over an extended period of time greater than eight hours.

FIGURE 4: A hydrogen fuel cell. Image courtesy of Plug Power

Backup Power in the Near Future

The difficulty of meeting net-zero carbon climate goals while maintaining a high level of availability and reliability has forced the industry to look at new energy storage technologies. The following are three energy storage technologies that are trending in the data center industry.

Solid state power centers combine a MV transformer with an integrated battery storage unit. It connects to a MV AC utility feed, transforms to DC with an integrated battery, and outputs clean LV AC power into the building. The power center not only advances on-site sustainability by replacing generators with zero-emission backup power but also by increasing efficiency. Vendor analysis showed an improvement from 95.8% to 96.5% overall efficiency along the utility to IT load path. A total of 89% of the indoor gear moves outdoors and 2.5 MW of data center power delivery fits into less than 40 outdoor square feet. Comparing the system to a traditional one-line, the generator and automatic transfer switch (ATS) are eliminated; less switchgear is needed; and the transformer, DC bus, and power conversion fits in a small, pad-mounted enclosure next to a battery bank.

One disadvantage is that the run time is limited to the amount of storage. The added benefits are simple: zero emissions, reduced supply chain risk, and streamlined construction. There is also the potential to earn more than $400,000 per MW from frequency regulation and demand response programs in some U.S. locations, resulting in a seven-year simple payback period.

FIGURE 5: A micro nuclear reactor. Image courtesy of U-Battery

Hydrogen fuel cells produce electricity using an electrochemical process similar to a battery, but they do not need to be recharged. Comparable to a generator, the fuel cell will continue to operate as long as there is fuel. The fuel cell works by passing stored hydrogen through the anode and oxygen through the cathode. Hydrogen molecules are split into protons and electrons in the anode. The positive protons move from anode to the cathode through electrolyte. The negative electrons can’t pass through the electrolyte and flow from the anode to the cathode through an external circuit. The protons, electrons, and oxygen then recombine in the cathode to produce water and heat. The waste heat can be coupled with a combined heat and power (CHP) system for heating or cooling applications. Fuel cells that utilize pure green hydrogen (hydrogen produced using renewable energy) are completely carbon-free. Hydrogen fuel cells are highly efficient, reliable, scalable, and operate quietly. Limited large-scale green hydrogen production, bulk storage, and distribution of hydrogen are some of the current barriers for the wide-scale deployment of hydrogen fuel cells. That issue is being addressed with publicly announced investments in large-scale green hydrogen production and distribution networks. Another option to address that issue would be to colocate a large energy-consuming facility, like a data center, with a renewable energy source and a green hydrogen producer. There are 56 large-scale hydrogen fuel cell-generating units (greater than 1 MW) currently operating in the U.S. and the International Energy Agency (IEA) believes hydrogen will play a big role in the world’s energy mix, especially when it comes to decarbonized energy.

Micro nuclear reactors that convert thermal energy into electric power are also being developed to produce clean, zero-carbon energy. The U.N. Economic Commission for Europe found nuclear energy has the lowest life-cycle carbon emissions of all energy sources. The micro nuclear reactor utilizes small amounts of low-enriched uranium fuel, and a passive cooling system, such as molten salts, liquid metal, and inert gases, makes them safer and much smaller than traditional nuclear plants. The reactor is a highly efficient system that ranges in capacity from a few 10s of kWs to 30 MWs. Designs vary; however, all aim to maximize factory fabrication and minimize the amount of on-site construction. Micro nuclear will allow the facility to go off grid and self-produce zero-carbon primary power. Critical facilities can install N+1 micro nuclear reactors for reliability and concurrent maintenance. There is potential to offset the cost of the micro nuclear by selling power from the redundant unit back to the utility. The heat byproduct of micro nuclear could be used to drive evaporative cooling to reduce heating electrical loads or a DAC system, making the data center carbon-negative.

Regulators, public perception, waste, and determining the requirements for safety and security are four of the biggest hurdles facing micro nuclear. Although the technology is a few years from being readily available, engineering firms, like Jacobs, are in the early stages of feasibility and concept planning for the use of its energy for data centers.

As global technology companies set aggressive climate goals and strive to be carbon-free, providing sustainable, zero-carbon backup power to maintain availability and reliability becomes progressively more challenging. Operators and developers of large data centers need to satisfy investors using ESG metrics. Although diesel generators are highly reliable and cost-effective, they burn fossil fuels that score low on the ESG metrics. Finding a way to reduce the carbon emissions from diesel generators or finding carbon-free alternates means for storing energy and providing reliable backup power must be investigated if we are going to help limit global warming.


1 Enchanted Rock LLC - Back-up Generator Emissions: Fact vs Fiction

2 U.S. Energy Information Administration - Renewable diesel is increasingly used to meet California’s Low Carbon Fuel Standard, November13, 2018