Energy storage plays a vital role in balancing capacity and demand of the power grid as well as mitigating intermittency issues of renewable energy. According to the International Energy Agency (IEA) market analysis, renewables will have the fastest growth in the electricity sector from 2018 to 2023, providing almost 30% of power demand in 2023 — up from 24% in 2017. However, the increased deployment of renewable energy systems is leading to greater grid instability, the need for additional grid services, and local energy storage. Data center owners and operators have an exciting opportunity to reinforce grid stability, add new revenue models for internal and external clients, and ensure greater resiliency for their operations by utilizing energy storage systems that expand on existing proven systems with new approaches and innovative chemistries.
Considerations and Options
To start, let’s explore the different types of energy storage systems available? First, we need to describe the power profile of the storage system, which includes power (kW) and duration (kWh), acceleration, and speed. The duration of a power profile may range from a 10- to 30-second burst fulfilled by an ultracapacitor to sustained power for weeks or months from a pumped hydropower site. The rate of onset of power delivered, or the “acceleration” of a storage technology, limits the types of load profiles it can respond to. For instance, a site that requires an immediate sustained 100% power load will typically rely on batteries. Load profiles that include highly variable peak loads require an energy storage system that provides repeated bursts of power. From a storage perspective, speed refers to the rate at which the storage system reaches full power capacity — instantaneous in the case of ultracapacitors, and 10 minutes or more for gravitational/compressed storage systems. In the data center world, managers prefer to control their destiny, hence the longstanding reliance on batteries that are closely coupled to on-site generators. However, the combination of new, nonflammable energy storage systems of modular energy capacities, paired with alternative energy resources that can be dispatched in seconds (fuel cells) or minutes (turbines), may change the standby generator status quo.
From a technology perspective, there is renewed interest and innovation in gravity and pumped/compressed media systems. This is in large part due to their respective abilities to store a large quantity of energy (high capacity) but without a high density. In the case of pumped hydro (gravity for generation) this could be in the gigawatts of pumped storage. On a global basis, 95% of today’s stored energy is comprised of pumped hydro electric storage (PHES). Unfortunately, broader deployment of pumped hydro is unlikely considering the geographic, environmental, and political challenges associated with developing new sites.
Meanwhile, large inertia and compressed-gas systems have drawn extraordinary media attention despite a limited track record for successful deployments. Check out the Energy Vault and the Hydrostor Australia project for some examples. On paper, these systems can scale and have the potential to store enough energy for 4 to 24 hours of operation. However, for most load profiles, batteries must be used to temporarily bridge the load. An energy storage tower in the desert (provided it doesn’t shade the solar field or block the wind turbines) or compressed gas in abandoned mines can add value in a blended, smart-grid eco-system. Just don’t plan on one near your data center any time soon.
Batteries
Bringing our tech trend update back down to Earth, the vast majority of venture capital and government funding (grants to companies and academic research) support research and development (R&D) associated with batteries. This tends to be clustered around device chemistry (with lithium still at the top of the list), anode/cathode chemistry/material science, break-through approaches (thermal batteries), and emerging applications such as EV Fast Charging.
Overall, batteries continue to receive the highest amount of investment and R&D for energy storage systems. Most of the focus is on lithium-based batteries with new chemistries that reduce the use of rare-earth and conflict minerals and the potential for fire and explosions, while they improve the rate of electron flow through the cathode and anode, manufacturability, and supply chain.
So, why all this interest in batteries? Well, they work. Don’t think for a minute that the tried-and-true, lead-acid battery is going away any time soon. The lead-acid battery is well-known, the characteristics and life cycle are understood, it can be relatively inexpensive, and it is easy to recycle. On a global basis, lead-acid batteries are already providing somewhere on the order of 15 minutes of backup time for 4% to 8% of the grid.
Lead-acid batteries do come with certain operational requirements, though, including dedicated cooling. They also require periodic maintenance and some form of remote monitoring. However, if a site manager abides by these requirements, lead-acid batteries should be ready, willing, and able to provide their full-rated performance when needed. With proper operation and maintenance (O&M) practices, a quality lead-acid battery will typically perform to specifications for two to five years before requiring replacement. Anticipated life cycle may vary by model, ambient temperature, and the load profile.
Nickel-cadmium (Nicad) and Nickel-zinc both have robust chemistries. Nicad battery packs were known to provide great performance in early electric vehicles (EVs) where their frequent discharge/charge, wide operating temperature range, and energy density were key.
Lithium-ion Battery Adoption
Over the last five years, the mission critical industry has begun to seriously evaluate and commercially deploy lithium-ion batteries. They promise higher power density than lead-acid, no special cooling requirements, and slightly higher power capacity (kW/kWh). Furthermore, their relatively long service life and cycle count are added benefits, especially in markets where grid power quality is poor and lights-out edge deployments are standard practice. This combination of characteristics makes lithium-ion a compelling alternative to lead-acid.
Unfortunately, as some early adopters have found, no battery is perfect. The lithium-ion battery is not without its own unique challenges.
The amount of data center floor/rack space allocated for energy storage can be reduced by using a high-power density lithium battery. Doing so requires careful planning for thermal runaway (regardless of lithium chemistry), long recharge times, and a safety-critical battery management system (BMS) that is always on: active during float, discharge, and recharge. Moreover, site managers interested in lithium-ion should prepare to work with the local authority having jurisdiction (AHJ) or fire marshal when deploying any sizeable amount of lithium-ion batteries. The regulatory environment for lithium-ion batteries in data centers is becoming increasingly murky, with pending changes anticipated to both the National Fire Protection Association (NFPA) 855 and National Electric Code (NEC). Insurance underwriters are also increasingly aware of the risk of lithium battery fires and explosions. For these reasons, successfully deploying lithium-ion batteries at scale requires extensive planning. But these risks and regulatory barriers are likely to limit the rate at which lithium-ion is adopted for mission critical applications.
A Safer Energy Storage Chemistry
Enter sodium-ion, the new (seven years in development) battery for mission critical, high-power, high-cycle, long-life applications. Again — no battery is perfect. But when conducting a thorough evaluation of performance versus risk, many mission critical users may find a sodium-ion chemistry that is right for high-value applications and business models.
Fortunately, research continues for other promising chemistries, including sodium-ion cells based on Prussian blue electrodes from Natron Energy and ceramic/carbon chemistry from U.K.-based Faradion. Of these, the Prussian blue electrode sodium-ion battery (Figure 1) delivers many of the characteristics modern data centers demand. It is nonflammable; has no thermal runaway; provides high-power capacity; can operate over a wide temperature range (minus 4° to 113°F), negating the need for special battery room cooling; offers a fast recharge (0% to 99% in eight minutes); and can operate for tens of thousands of deep discharge cycles.
Prussian blue pigment, commonly used in blueprints and medical treatments, stores and releases energy in the form of sodium ions. Prussian blue has a unique open framework structure, analogous to a porous sponge, which allows it to store energy rapidly and reversibly. For this reason, batteries based on Prussian blue electrodes can be fully charged and discharged tens of thousands of times at very high power with round-trip energy efficiencies greater than 96%.
When deploying any battery technology, safety considerations are of paramount importance (Figure 2).
Going forward, the energy storage market will continue to see significant financial investment, announcements of innovative engineering projects, and news of new battery plants. In the meantime, data center operators will continue to deploy the proven combination of practical energy storage systems based upon lead-acid, lithium-ion, and sodium-ion batteries backed by diesel generators. The time-tested battery/gen-set solution is here to stay.
