How Much Backup Power Is Sufficient?

Within a datacentre environment, any break in electrical power, whether momentary or a complete mains power supply failure can be a disaster. The consequences being downtime, system crashes and service loss. For some datacentres, especially those providing colocation services, any break in the critical power supply can lead to service level agreement penalties and potentials claims on professional indemnity insurance.

The most common power protection solution is the installation of an uninterruptible power supply but how long a battery backup time is required? Few take the time to really look into this aspect of the solution or examine the quality of the energy storage solution that installed with the UPS system itself.

A UPS system is in its basic form is a battery management system with an inverter. When the mains power supply fails, the inverter draws on the connected alternative energy storage system. In almost 90% or more of installations this is a lead-acid battery system. Alternative energy storage solutions exist and can provide sufficient backup time to ride through momentary power failures or a standby power generator to start.

A well maintained generator will automatically start within 30 seconds or less and be capable of providing a complete electrical supply to the connected UPS system and other loads (air conditioners, lighting and so on). All the battery within a UPS system does is provide ‘fill-in’ power during the period it takes the generator to ‘spin-up’ to a sufficient level of voltage and current. Alternative energy storage solutions that offer an alternative to battery power include super capacitors and flywheels. Why are they not used more? The reason is a perceived fear that the backup period is not long enough should the standby generator fail to power-up. For a well-maintained generator this is extremely rare. In addition, if there is a generator fault, unless it is something obvious like an open-breaker or air within the fuel supply, the battery period will not be nearly sufficient to prevent the IT system crashing.

All a modern generator-UPS system configuration requires is an energy ride-through capability. Super capacitors can provide this. For most applications they already provide a back-up power service in terms of seconds but not the minutes and hours that a lead acid battery can provide. Cost and capacity are further drawbacks and mean that most UPS manufacturers will only consider super-caps for smaller UPS systems around 10kVA or less. Some sites will actually already be using capacitors within their critical power paths within Power Factor Correction systems. These are most commonly found on sites with half-hourly metering.

Flywheels offer another form of energy storage and we are slowly seeing flywheel technology becoming more common within transportation and lifting apparatus. Compared to super capacitors, flywheels offer far greater energy storage and can support large kilowatt rated UPS installations. In the US, some datacentres are using flywheel enabled UPS and removed their battery sets. Whilst flywheel costs are high compared to lead-acid batteries, they have a longer working life (20 years or more) and require minimal maintenance. Their long working life gives them greater ‘eco power’ credentials that neither a 5 nor 10 year design life battery can achieve.

So if a battery set is to remain the standard energy storage source for UPS systems, is there an alternative to lead-acid technologies? The answer is lithium-ion. Elon Musk the CEO of Tesla has launched his company’s Powerwall systems for domestic and grid scale energy storage using lithium-ion batteries. Already a convert to lithium-ion (as used in the Tesla electric cars) Musk is thinking on a grand scale whereby lithium-ion could be used to move an economy away from carbon based fuels. Tesla’s energy storage systems could help facilities become less reliant on the mains and build-in uninterruptible power using the Powerwall devices, which come in 10kW and 100kW sizes. The Powerwall can store energy generated from solar PV installations or from lower rate night time mains power supplies. The devices then discharge this into the building during the day to meet local demand.

Does lithium-ion offer an alternative battery technology for use in uninterruptible power supplies? Lithium-ion batteries are more suited to frequent charge/discharge cycles than lead-acid batteries but this only provides an advantage if the UPS system is used within a widely fluctuating and/or unstable mains power supply. Costs are also high but Tesla Giga factories could lead a rapidly closing price gap. So what about environmental advantages? Well, today’s lead acid batteries are designed to allow as much material to be reclaimed at their end of life (plastic cases, lead plates, gel/liquid electrolytes) for use within new batteries. The same level of recycling experience does not yet exist for lithium-ion.

Will Tesla’s Powerwall spark a revolution? It may is the simple answer but it is not the first company to have launched a lithium-ion based energy storage system. Others including manufacturers like Samsung have also launched similar systems for use with solar PV installations. For lithium-ion to become the adopted norm for UPS systems will require a rapid price reduction, improvement in recycling facilities and change in how a UPS is perceived and used. If the UPS is used within a Smart-grid to discharge energy into the grid then lithium-ion is the only choice. If datacentre managers can change their backup power time needs, super capacitors and flywheels offer more cost-effective and greener ride-through backup power.