Michael Lippert, marketing and business development manager for energy storage at Saft outlines how megawatt-scale Li-ion Energy Storage Systems (ESSs) are being deployed to support large solar photovoltaic (PV) installations on tropical islands
Solar PV schemes are playing an increasingly important role for island grids, especially in helping them reduce their reliance on expensive to run and environmentally unfriendly diesel generators. However, even on tropical islands the output of a solar plant can be highly variable, with passing cloud cover causing it to ramp up and down by 70-80% in less than a minute.
So while forecasting has improved, operators cannot be certain about the level of their plant output. As the penetration of renewables within island grids continues to grow, this unpredictability can result in issues related to grid stability and substation congestion at periods of peak demand.
Energy storage in the form of lithium-ion (Li-ion) battery energy storage systems (ESSs) can mitigate the variability of PV and help both plant and grid operators on tropical islands as we can see in a number of examples.
ESS delivers peak shaving and frequency regulation as Hawaii prepares for more PV
While the US state of Hawaii is well placed to generate energy from natural sources, it historically has a high dependence on oil. It is now targeting 40% of its energy to come from renewables by 2030, making it a US leader in the adoption of PV as well as industrial-scale wind farms producing megawatts of power. Hawaii is now installing ESS technology to increase its capability to integrate an even greater level of renewables.
Saft’s first installations for the Hawaii Electric Light Company (HELCO) in 2012, were on Big Island. They comprise two Intensium® Max 20E containers providing 248 kWH of energy storage per container, coupled with a 100 kW power conversion system. Each ESS has a two-hour runtime, can be charged overnight during off-peak times and has an expected operating life of at least 15 years.
One of the main roles of the first HECO ESS installation is for peak shaving to reduce congestion on the Hawaii grid. It provides effective energy storage to time-shift PV power production to support high afternoon loads caused by increased use of air-conditioning systems. The ESS charges when output from the PV plant power exceeds a set limit and releases energy into the grid later in the day once the peak has subsided.
The second HECO installation, located adjacent to a bottling plant, provides frequency regulation, which is a service normally provided by bringing primary and secondary reserve generating assets online at short notice. It ensures stability of the grid by injecting or absorbing active power to keep the frequency inside its set limits. Typically, the deviations experienced are of short duration and only infrequently at full amplitude and the Saft Intensium Max 20E is designed specifically to cope with this type of duty.
ESS makes Kauai’s new Anahola array a consistent and reliable source of power
KIUC (Kauai Island Utility Cooperative) is the electric utility on Hawaii’s fourth largest island. To improve sustainability and reduce its reliance on imported fuel, KIUC has an ambitious target to meet half of its power needs with renewable energy sources such as hydropower, photovoltaic, biofuel and biomass by 2023.
A key element in KIUC’s sustainability plans is the Anahola array. With 59,000 panels and a peak output of 12 MW, this PV installation is now supplying around 20% of KIUC’s daytime electricity needs.
To make PV a reliable and consistent element in its energy mix, KIUC has installed an ESS to react to the frequency fluctuations caused by this fast ramping up and down of PV resources.
The ESS effectively limits the rate of change at which power is injected into the grid. So when passing cloud causes a step change in output, the ESS will absorb or release energy to ensure that the grid sees a smooth transition in output instead of the step change. This is a challenging application that called for Saft’s expertise in renewable energy application modelling, systems design and engineering to deliver an effective, reliable and financially viable ESS solution.
At Anahola, Saft has supplied an Intensium Max 20 M ESS providing 6 MW power and 4.63 MWh energy capability. The system delivers peak power up to the full 12 MW output of the plant. It is housed in eight separate shipping containers plus two containers housing a power conversion system that stabilizes the grid.
The Anahola array was commissioned in October 2015 and is now helping KIUC’s transition to a sustainable mix of renewable resources – it now imports 1.7 million fewer gallons of oil each year, saving 35,000 tons of emissions annually.
The Saft ESS is playing a vital role in the Annahola success story by mitigating the variability of its output during rapidly changing weather conditions. To maintain grid stability the ESS reacts to frequency disturbances in less than 50 milliseconds, helping to avoid load-shedding. At times when output exceeds demand the ESS can store it to reduce PV curtailment and also helping to meet demand during the evening peak period.
Recently, the ESS proved its capability to provide frequency response to events far beyond the variability of the Anahola array. When the 28 MW Kapaia power station tripped Saft’s system prevented about half of the island being blacked out.
A model ESS ensures grid compliance for a major new Puerto Rico PV plant
In 2015, Sonnedix, the solar power plant developer and operator, commissioned a new 10 MW PV plant that feeds into the grid. Grid compliance was a critical issue, as the plant’s output faced curtailment if it could not meet the Minimum Technical Requirements (MTRs) that set out stringent interconnection regulations. As a fully commercial, unsubsidised project, with its return on investment dictated by the number of kWh sold, then confidence that the plant would achieve compliance was essential for it to be an investable proposition.
The island has a total of 13 MTRs for injecting energy into its grid. Two of these relate specifically to the use of an energy storage system (ESS), including delivering frequency response at up to 10% of the nameplate power and limiting the ramp rate of the plant output to a 10% change per minute.
The 10 MW capacity of the new plant requires frequency response to be supplied at ±1 MW and the ramp rate must be controlled to 1 MW per minute. A facility of this size can see a 70% drop in output in about a minute, so the ESS must discharge so that the grid sees this output reduction over seven minutes, this is slow enough to allow other generation on the island to respond and maintain grid frequency. To meet the other compliance metrics the ESS must also be able to ramp up and provide peak power of 4.5 MW (45 percent of the plant output) for one minute, followed by a controlled ramp down.
Saft was called in to supply an ESS that would meet the technical requirements while optimising the plant’s Total Cost of Ownership (TCO).
Advanced modelling helped Saft to identify the optimum size for the unit to deliver the required energy and power characteristics reliably over the life of the installation. It is interesting to explore the modelling process in some detail.
Modelling is an iterative process that starts with a first estimate of the battery specification that is combined with a range of other inputs to the overall EMS (Energy Management Strategy) to deliver a cost profile.
It calculates the lifetime costs and operating revenue for a particular size of ESS. By repeating the process with a range of different sizes, it’s possible to identify the sweet spot, where the operator finds the optimum balance between revenues and costs during the whole life of the installation.
At the heart of modelling is the algorithm that is used by battery management systems in the field. It mimics the performance of the ESS down to the level of individual cells, taking account of electrical and thermal performance and electrochemical aging.
Varying the size and specification of the battery changes the cost profile. A smaller ESS will have a lower capital cost but could lead to lower revenues, more penalties, lower compliance with the grid code, or more curtailment losses. It will also alter the system’s calendar life.
Plotting total cost of ownership (TCO) against specification, it is possible to tailor the size of the ESS to meet the customer’s business objectives and operating environment.
The ESS solution identified is based on the Intensium Max 20 P high power system housed in a standard sized container that incorporates the Li-ion batteries as well as battery management, active cooling, monitoring and power and communication interfaces.
Three containers have been installed that together provide 5 MW power and 1.3 MWh energy storage capacity. The system is capable of controlling the plant to ensure smooth ramped output and keeping grid frequency stable around 60 Hz.
Thanks to the Saft ESS, Sonnedix is able to operate the the plant in full compliance with the MTRs, avoiding any risk of curtailment of its output. The cost of the ESS has been factored into a 25-year Power Purchase Agreement (PPA) price based on the cost per kWh of energy fed onto the grid.
The capability to make effective use of its solar power resources is helping Puerto Rico in reducing its dependence on imported oil and improving its environmental credentials.
ESS shapes up on La Réunion
At the 9 MW PV plant at Bardzour on La Reunion in the Indian Ocean, the role of the ESS is power shaping to inject power into the grid at a constant 40% of the plant’s peak power capacity.
This means the ESS is used to shape the power output of a plant to deliver steady and predictable power. In effect it makes intermittent renewable resources perform like baseline generation.
The Bardzour ESS is required to deliver a large discharge in the morning, before charging up during peak daylight hours in the middle of the day and discharging again in the evening. It must also provide primary reserve for 15 minutes and provide voltage support.
Modelling identified the optimum size of the ESS as 9 MWh energy storage capacity and Saft has delivered a 9 MWh Intensium Max, housed in nine individual shipping containers.
As tropical islands make the transition to renewable energy resources the intermittent nature of solar power is a significant challenge for owners and operators of PV installations. Megawattscale Li-ion energy storage is now being deployed successfully on a commercial basis to address the key grid integration issues of ramp rate, curtailment and frequency regulation.
