Uninterruptible power supplies (UPS) are today very different in their design from those that started to appear in the early 1950s. In most areas there is less need of their ability to provide protection from mains borne power solution and a growing need for their primary role as an energy storage device and provider of uninterruptible power.
The Energy Storage Role of a UPS System
Modern switch mode power supplies of the type used in servers and high-density IT applications are today very robust devices with a wide input voltage window, built-in power factor correction and EMC filter. Modern servers can also be installed with dual/redundant configured A and B power supplies to provide further resilience. In a server room or datacentre environment, UPS system these devices are powered from, via a power distribution unit, are essentially there to bridge the gap between a mains power supply failure or power outage and the start-up of a local standby power generator.
The principle energy storage component within a UPS is a valve regulated lead acid (VRLA) battery. This type of battery is suited to standby and intermittent operations i.e. to provide a DC power supply to a UPS AC inverter during a mains power supply failure. A VRLA battery is not suited to the type of rapid charge/recharge cycles expected for energy storage applications. For this role lithium-ion batteries, of the type used within electric vehicles (EVs) are considered the standard. Some UPS also use super capacitors which are also capable of numerous and rapid charge/discharge cycles but do not have the same storage capacity. For super capacitors the energy discharge is within milliseconds where as with lithium batteries the period can be milliseconds to several hours including very long runtime periods.
Demand Side Response Programs and Incentives
In the UK, the National Grid and supply companies (District Network Operators aka DNOs) face the growing problem of how to maintain power uptime to avoid OFGEM penalties. More info: https://www.ofgem.gov.uk/.
There National Grid has launched several demand side response (DSR) programs including STOR, a Short-Term Operating Reserve program which rewards generator owners with an annual connection fee and revenue when they allow their generators to be used to support the grid. The connections are made via aggregating companies and the owners are effectively paid a Feed-in-Tariff (FIT) in much the same was as owners and operators of solar PV and wind farm installations. It should be noted that the FIT available for solar PV installations has been rapidly reduced in recent years and for wind farm operators also.
UPS Installations and Stored Energy
In the UK we can confidently say that there are almost 500,000 or more active UPS installations with some approaching 1MW or more. At an average size of say 25kW per UPS system this means the there is an active population of over 12.5MW of back-up power and stored energy to tap into.
So what is to stop these systems tapping into STOR and DSR-type programs? The simple answer is the existing battery set. Lead acid batteries are simply not suited to role of energy storage required within a modern server room or datacentre.
Lithium-ion versus Traditional Lead Acid UPS Batteries
Lithium-ion batteries have become the ‘go to’ energy source for many of today’s products including smart phones and mobile applications as well as road transportation in the form of hybrids and fully electric vehicles.
The downside of lithium-ion batteries is their higher cost than lead acid and the need for a more complex battery management system in order to prevent over charging and thermal runaway. Lithium-ion batteries are also more flammable and potentially dangerous when exposed to water or damage during a crash or explosion. Lithium releases hydrogen when exposed to water and their use within airplanes is restricted and transportation limited to cargo only flights under license.
The positives, aside from their rapid and virtually unlimited ability to charge and discharge is their power density which can reduce battery installation sizes by up to 60% compared to a traditional lead acid battery set. This can lead to more space for IT equipment within server racks more compute area within a server room or datacentre.
Lithium-ion batteries are also more temperature tolerant. Lead acid UPS batteries require an operating ambient of 20-25°C in order to meet 80-90% of their design life. A typical design life is 5 years or 10 years with replacement in years 3-4 or 7-8 years. Lithium batteries can work within higher ambient temperatures without derating up to 30°C or more. This has important energy efficiency implications, as lithium batteries place less demand on cooling systems and therefore potential aisle-containment installations.
Energy Storage Applications
Within a server room or datacentre, lithium UPS batteries will primarily perform the same role as that of traditional lead acid batteries i.e. to provide a source of DC power for the UPS inverter in order to maintain uninterruptible power.
A second application would be to allow the datacentre to operate off-the-grid as part of a load balancing DSR program. With enough stored energy (drawn from the grid or stored from local renewable power sources), the datacentre UPS can operate as a virtual power plant (VPP). In this scenario, the energy is stored during off-peak charge periods and is used to power the datacentre during peak price periods. The mains power supply acts as back-up power source in the same way as a generating set. This secondary application also helps to reduce load on the National Grid and can therefore be used to keep supply frequencies within tolerance.
A third application would be the discharge of the stored energy back into the mains power supply. This role is the same as that of a solar PV or wind turbine FIT program. For this role the UPS would need a bidirectional converter/inverter component, suitable metering and the type of electrical works required for renewable power generation and grid feed-in.
There is little doubt as to the interest in using lithium-ion batteries with UPS systems within datacentre environments. They do overcome the inability of lead acid batteries to recharge quickly and provide a better battery type in which to store energy from local renewable power sources. The issue for datacentres may be that as battery power is discharged to the grid, this could leave the datacentre with less resilience. The key here is a bit of oversizing to accommodate the start-up period of a local standby generator, the need to discharge to a specific charge capacity threshold into the grid and the maintenance of reserve power in case the generator does not start.