Energy storage: the value of UPS systems for variable renewable resources

Power Control
20 Aug 2020

Variable Renewable Energy

The electricity industry is facing the biggest challenges seen in over a century with the development of a decarbonised electric power system and the transition to a ‘smart grid’ using variable renewable energy (VRE) sources such as wind and solar. Issues with reliability are also a concern when switching VRE on and off. As has happened recently in the UK with power cuts and blackouts during high demand.

The extent to which the VRE can be successfully deployed hangs on the future availability and cost of energy storage technologies.

Driving this evolution is the potential economic value that energy storage could bring to the industry. The amount of energy produced from VREs fluctuates daily and without the ability of firming the production in a cost effective way, the industry would not be able to compete with fossil fuels.

Energy storage options

Although the concept of storing energy is not new, batteries have been used to do this since the early 1800s and the way they are being used as storage devices has evolved greatly. They are starting to be used for storing energy from on-peak renewable sources, ready to be released when it is more needed in central, de-central and off-grid situations.

Lead acid batteries were among the first battery technologies to be used for storing energy. However, they are not able to provide high enough levels of energy density, a long enough cycle or working life to be used for grid storage.

With the rise in electric vehicles (EV) lithium-ion (Li-ion) battery prices have dropped significantly, 85% in the past decade and it is expected to continue to become more easily affordable in the future. Li-ion batteries provide a high energy density, increased numbers of maximum lifecycles and longer working life than their VRLA counterparts.

Moreover, because they can be easily sited, lithium-ion batteries are more likely to be used to solve curtailment issues, particularly behind the meter and on off grid sites.

5 Main Benefits of Energy Storage

There are more than 150,000 megawatts of storage system currently installed worldwide. With new technologies becoming available, this is set to increase rapidly.

  1. Maximise time of use rates – Batteries can store energy from low price periods like overnight when electricity is cheap and discharge it during high value periods at higher electricity rates. For example, peak demand periods shift to the evening when the sun is not shining or on days when the wind is not blowing.
  2. Improve reliability and resilience – Just as a company invests in back power on an individual basis, for specific devices or critical systems, the same concept applies for scaling up to an energy storage system at mains grid level. Batteries can be charged during low demand and discharged during high demand to help balance the voltage and frequency. Hence this reducing the risk of unplanned disconnection of VRE’s. Power outages can be costly for operators and so having measures in place to ensure reliability is key to business continuity.
  3. Integrate Diverse Resources – Energy storage systems not only smooth out the delivery of variable resources, but can also support the efficient delivery of electricity for inflexible baseload resources. When demand shifts and baseload resources cannot react quickly enough an energy storage system can inject or extract electricity to match.
  4. Environmental benefits – With the UK’s target to bring all greenhouse gas emissions to net-zero by 2050, government initiatives are in place to encourage businesses and consumers to switch to renewables. Energy storage sits at the heart of increasing the spread of renewable energy, it accelerates the broader adoption of renewable energy by improving the overall efficiency of the power grid. On a more local level, an energy storage system has no emissions, so it can be placed anywhere within a facility with no immediate impacts on the environment.
  5. Participation in demand response programmes – Energy storage makes participation in demand response programmes a more viable option. Unlike the traditional demand response, this emerging technology will allow customers to shift from an even-based demand response where utility requests the shedding of load, towards programmed utility price signals, a 24/7 demand response where consumers see incentives for controlling load at all times.

Demand response will also help the grid to maintain stability during periods of high supply and low demand giving financial incentives for grid operators too.

How this links to uninterruptible power supplies (UPS)

“As lithium-ion technology becomes more commonplace among UPS specialists, a UPS’s usage as an energy storage system will increase. Existing UPS topology can be modified effectively to grid tie and charge and discharge without the need for separate inverter and charger systems. UPS inherently have advanced battery management that can be used to ensure balanced charging and safety cut-outs in the event of thermal runaway.” – Graeme Tucker, Director at Power Control

As with typical energy storage systems, the modified UPS is connected to the grid and the batteries are charged during low electricity price periods and discharges power back on to the grid when necessary. This may be to smooth out the delivery of variable or intermittent resources (renewables) or to support the efficient delivery of electricity for inflexible, baseload resources, injecting electricity as and when required.

The amount of power that can be stored/pushed back on to the grid is dependent on several variables. One of which is the number of batteries used. It is possible to configure the bespoke energy storage system with a large UPS system and a small number of battery strings or a small UPS system and a large number of battery strings. The variations affect power availability and runtimes.