Making the transition to solar power with energy storage

Michael Lippert

Island grids are increasingly turning to solar photovoltaic (PV) schemes to help reduce their reliance on diesel generators. Some practical examples demonstrate how megawatt-scale Li-ion Energy Storage Systems (ESSs) are supporting island grids and remote microgrids in making the transition to renewable energy. 

Enabling island grids to reduce their reliance on traditional generation

Island grids are turning to solar PV schemes to help reduce their reliance on expensive to run and environmentally unfriendly diesel generators. The challenge is that the output of a solar plant can be highly variable, even on a tropical island. Passing cloud cover can cause output to ramp up and down by 70% to 80% in less than a minute. Forecasting methods have improved, but they are not yet at a level to grant operators any certainty about the timing of their plant output. Island grids are continuing to increase their levels of renewables. As a result, unpredictability will continue to challenge grids over issues such as stability and substation congestion at peak demand periods.

Lithium-ion (Li-ion) battery energy storage systems (ESSs) are now playing a key role in mitigating the variability of PV on tropical islands. This is illustrated by a number of examples that demonstrate how energy storage can address issues such as peak shaving, frequency and voltage stability, grid compliance and power shaping.

Hawaii prepares for increased PV output

Natural sources of energy abound in the US state of Hawaii. Yet, historically it has had a high dependence on oil. That is changing, with a 2030 target set for 40% of its energy to come from renewables. This will make the state a US leader in the adoption of PV. In order to take its integration of renewables to the next level, Hawaii is now installing ESS technology.

Hawaii’s fourth largest island, Kauai, is aiming to improve its sustainability and reduce its reliance on imported fuel. The electric utility KIUC (Kauai Island Utility Cooperative) has an ambitious target to meet half of its power needs with renewable energy sources by 2023.

The Anahola PV array is a key element in KIUC’s plans. Its 59,000 panels provide a peak output of 12 MW - around 20% of the island’s daytime electricity needs.

KIUC has installed an ESS to react to the frequency fluctuations caused by the fast ramping up and down of PV resources.

When passing cloud causes a step change in output, the ESS will absorb or release energy. This ensures that the grid sees a smooth transition in output. Saft has applied its energy storage expertise for this challenging application, including modeling, systems design and engineering. The result has been an effective, reliable and financially viable ESS solution.

Saft’s Intensium Max 20 M ESS supplied for the Anahola project provides 6 MW power and 4.63 MWh energy capability. It delivers peak power up to the full 12 MW output of the plant. The ESS is housed in eight separate shipping containers, while two containers house the power conversion system.  

To maintain grid stability the ESS reacts to frequency disturbances in less than 50 milliseconds, helping to avoid load-shedding. When output exceeds demand the ESS stores the excess to reduce PV curtailment. This also helps to meet demand during the evening peak period.

Following its commissioning in October 2015 the Anahola Array is helping Kauai’s transition to a sustainable mix of renewable resources. The island now imports 1.7 million fewer gallons of oil each year, saving 35,000 tonnes of emissions annually.

The ESS has proved its capability to provide frequency response to events far beyond the variability of the Anahola array. In fact, the system prevented about half of the island being blacked out when the 28 MW Kapaia power station tripped.

Ensuring grid compliance for major new PV plant on Puerto Rico

In 2015, a new 10 MW PV plant was commissioned to feed into the grid operated by PREPA (Puerto Rico Electric Power Authority), the state-owned energy supplier. The plant’s output faced curtailment if it could not meet PREPA’s Minimum Technical Requirements (MTRs) that set out stringent interconnection regulations. Therefore, grid compliance was a critical issue.

The return on investment for this fully commercial, unsubsidized project is dictated by the number of PV kWh sold. So confidence that the plant would achieve compliance was essential for it to be an investable proposition.

A total of 13 MTRs apply for injecting energy into the PREPA grid. Two of these relate specifically to the use of an energy storage system (ESS). They include providing frequency response at up to 10% of the PV nameplate power, as well as limiting the ramp rate of the plant output to a 10% change per minute.

The 10 MW PV plant requires frequency response to be supplied at ±1 MW and controlling the ramp rate to 1 MW per minute. A 70% drop in output in only a minute is typical for a facility of this size. This requires the ESS to discharge so that the grid sees this output reduction over seven minutes. The reduced ramp rate is slow enough to allow other generation on the island to respond and maintain grid frequency. Other compliance metrics call for the ESS to ramp up and provide peak power of 4.5 MW (45% of the plant output) for one minute, followed by a controlled ramp-down.

Saft utilized advanced modeling to identify the optimum size for an ESS capable of delivering the required energy and power reliably over the life of the installation. This iterative process 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.

Modeling is used to determine the ESS cycling operation profile depending on the location specific PV generation and compliance with MTR conditions. It then calculates the lifetime costs and operating revenue for a particular size of ESS. By repeating the process with a range of different sizes, it is possible to identify the sweet spot, where the operator finds the optimum balance between revenues and costs during the whole life of the installation.

The heart of the modeling process is the algorithm that is used by battery management systems in the field.  This mimics the performance of the ESS right down to the individual cell level, taking account of electrical and thermal performance and electrochemical aging.

The cost profile of the battery varies according to the specified size and technical capabilities. A smaller ESS will have a lower capital cost. But this could lead to lower revenues and more penalties, lower compliance with the grid code, or more curtailment losses. It will also reduce the system’s calendar life.

By plotting the total cost of ownership (TCO) against the specification, it is possible to tailor the size of the ESS to meet the customer’s business objectives and operating environment.

PREPA’s ESS solution 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.

The optimum solution for this 10 MW PV plant was determined to be three containers providing a total of 5 MW power with just  1.3 MWh energy storage capacity. This enables the PV plant to be controlled to ensure a smooth output while keeping grid frequency stable around 60 Hz.

The ESS enables the PV plant to operate in full compliance with PREPA’s 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.

Shaping up on La Réunion

Power shaping to deliver steady and predictable power is the key role for the ESS installed at the 9 MW PV plant at Bardzour on La Reunion in the Indian Ocean. The system injects power into the grid at a constant 40% of the plant’s peak power capacity. Effectively, it makes intermittent renewable resources perform like baseline generation.

The operational pattern for the Bardzour ESS is to deliver a large discharge in the morning as long as PV power is below the 40% threshold. Then, it charges up during peak daylight hours in the middle of the day by storing any excess PV, before  discharging again in the evening. It must also provide primary reserve for 15 minutes and provide voltage support.

Saft modeled the optimum size of the ESS as 9 MWh energy storage capacity. A 9 MWh Intensium Max system housed in nine individual shipping containers has been in operation since the end of 2014.

Integrating renewables into off-grid microgrids

Communities, industrial facilities and military bases in locations that are remote, usually with no grid connection, often adopt  microgrid solutions in order to meet their growing power demands. In particular, they want to ensure  the reliability and autonomy of their electricity supplies and to optimize  their operating costs.

Historically, microgrids have relied on diesel generators. But operators are now turning to renewables and especially PV installations to reduce their reliance on diesel fuel. Hybrid schemes in which diesel generation and PV plant complement each other offer considerable savings in terms of the costs of fuel purchase, transport and handling as well as maintenance, since the diesel genset runtime is reduced. There are also environmental benefits from reduced greenhouse gas emissions. 

Using standard power electronics, PV can contribute up to 20-30% of the power that would be generated by the diesel genset during daytime hours. Adding dedicated software can increase the penetration of PV to 50%. For example,  a 1 MW microgrid might accept up to 300 kW, but this could be raised up to 500 kW of PV in the best case. Taking into account that PV generation is limited to sunlight hours, is highly variable and does not necessarily meet the required consumption profiles, its contribution to the overall energy mix is naturally limited.

When an energy storage system (ESS) is added, an operator can maximize the contribution of renewables, increasing the penetration and harvesting all of the PV power. It is possible to realise fuel savings of 50-75%. Two recent examples show how energy storage is now making an important contribution to the development of remote microgrids.

Harnessing the midnight sun for an Arctic Circle community

NTPC (Northwest Territories Power Corporation) is the power utility serving 43,000 people spread across 33 communities and 1.1 million square km in northern Canada. The corporation has an ambitious strategic plan to transform the region’s power supply to be cheaper, cleaner and more reliable by replacing expensive diesel generation with renewable energy, especially for its remote off-grid communities.

One of the most remote communities served by NTPC is Colville Lake, which lies 50 km north of the Arctic Circle. This small community of about 160 is only accessible by air or by ice roads during a six-week window in February and March.

For some years, the 150 kW peak and 30 kW base load was met by two 100 kW diesel generators. It is one of the most expensive diesel generation communities in the Northwest Territories. But since 2015 a microgrid has been deployed that combines 136 kWp of PV with a further 150 kW of diesel generators and an ESS. The goal was to reduce the runtime of the diesel generators, especially in the summer when the sunlight is available for virtually 24 hours per day. 

To operate the microgrid at maximum efficiency and save on fuel, NTPC needed an ESS that would withstand the harsh temperature variations from -50˚C to 35˚C. NTPC also wanted to ensure maximum value for money with an ESS of the optimum size to balance ESS capacity and cost vs the size of PV panels and fuel savings. Saft’s response was to provide an Intensium Max 20M Medium Power containerized system with 232 kWh energy storage capacity to work with a 200 kW power conditioning system. The Saft team used advanced modeling to identify both the optimum size of the ESS and the solar array.

The Colville Lake ESS is a special cold temperature package that combines layers of high-tech insulation with a hydronic heating coil that makes use of the same hot glycol that maintains the diesel gensets at operating temperature. This minimises the cost of keeping the battery in its optimum temperature range.

The ESS is designed to support the network frequency and voltage. It also  allows the diesel generators to operate at their point of maximum efficiency and to be shut down whenever possible. The generator runtime has been reduced to around 50% to provide significant savings in diesel consumption – this is particularly important for this remote community as delivering diesel fuel via the ice road is both expensive and logistically challenging.

Operating the diesel generators in combination with the ESS also leads to reduced maintenance expenses as there is lower wear and tear on plant when it is run at a steady set point, rather than ramping up and down to meet short-term load variations.

In addition to their new autonomy from total dependence on diesel, the Colville Lake community now benefits from noise reduction and elimination of emissions that greatly improve their quality of life.

Ensuring reliable output from the Cobija PV-diesel hybrid power plant

Cobija, the capital city of Bolivia’s Pando Province, is not connected to the country’s national electricity network. Previously, diesel generators met its 37 GWh demand. A new hybrid plant combines a 5 MWp PV array with 15 MW diesel generation and 1.2 MWh energy storage. This supports around half the energy needs of some 50,000 people, a total peak load of around 9 MW.

At Cobija, the contribution of PV is around double that of traditional PV-diesel hybrid systems. This will save between 1.5 and 2 million litres of diesel per year – helping reduce annual CO2 emissions by at least 5,000 tonnes.

To enable the Cobija plant to achieve the highest possible contribution of PV to the energy mix it requires effective energy storage that ensures continuity of supply, system stability and the smoothing of short-term variations in output from the PV array.

The Saft ESS solution, in operation since 2015, includes two  Intensium Max 20M medium power containerized systems, each with a nominal storage capacity of 580 kWh and 1.1 MW peak power output. The batteries operate in combination with inverters and intelligent control systems to ensure system stability and smooth control of the diesel gensets.

A key role for energy storage

Achieving an effective transition to solar PV is a major challenge for tropical islands due to its intermittent nature. Megawatt scale 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.

ESS also has a key role to play in the development of hybrid diesel and PV microgrids serving remote off-grid communities. It is proving its capability to work as an integral element of the microgrid to deliver a reliable and autonomous supply of electricity. A key benefit is that energy storage can maximize the contribution of PV. This results in a significant reduction in the runtime of diesel generators, saving on fuel and maintenance costs and reducing emissions.

Michael Lippert is marketing and business development manager for energy storage at Saft. 

 

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