How to optimize the sizing of the battery energy storage system for a hybrid grid

The EVN PECC4 engineering team used ETAP software to model and calculate the optimal BESS capacity required to support Phu Quy Island’s hybrid renewable system. The study evaluates the island’s 2025 power system scenario - characterized by a strong penetration of wind and solar - and determines the most efficient and economical BESS configuration to improve energy stability, minimize diesel generation, and reduce curtailment.
Huỳnh Kim Long, Power System Calculation Engineer, EVN PECC4

The integration of Battery Energy Storage Systems improves system reliability and performance. This case study demonstrates how the EVNPECC4 company utilized ETAP to size the Battery Energy Storage System (BESS) for the Phu Quy Island electrical distribution network. Plans involve installing additional wind and solar power plants to meet increasing load demands on Phu Quy Island. However, due to renewable energy's dependence on weather conditions, the diesel power plant remains crucial, prompting a study on the optimal size of Battery Energy Storage Systems (BESS) to support the integration of renewable energy and optimize costs.

Power Engineering Consulting Joint Stock Company 4 (EVNPECC4), formerly known as Southern Power Planning and Designing Sub-Institute, was founded in 1976 with the mission of renovating, restoring and reconstructing the power system for Southern provinces in order to meet people’s living needs and restore the post-war economy. Nowadays, EVNPECC4 has become one of the leading power engineering consulting companies in Vietnam.

Location: Nha Trang City, Khanh Hoa Province, Vietnam

Year: 2023

Choose the optimized size of the Battery Energy Storage System

Challenges

Phu Quy Island, located approximately 100 km off the coast of Vietnam, relies on a hybrid system combining diesel generators, solar PV, and wind turbines. With increasing renewable penetration, operational challenges arise regarding stability, curtailment, and the high cost of diesel backup, such as:

  • Modeling and validating the existing Phu Quy power grid, including diesel generators, solar PV, and wind farms.
  • Forecasting the load demand expected to rise from 6.8 MW (2023) to 8.4 MW (2025).
  • Managing high levels of excess wind generation, leading to significant curtailment without storage.
  • Sizing a BESS capable of reducing diesel dependency, while maintaining grid stability under variable renewable conditions.
  • Evaluating the economic feasibility, including investment cost, fuel savings, and payback period.

Which solutions did they choose?

Selected applications

EVN PECC4 collaborated with ETAP to analyze the island’s future 2025 system configuration and determine the optimal BESS sizing to ensure reliability, reduce emissions, and maximize renewable energy utilization. EVN PECC4 engineers used several ETAP tools, including:

  • Load Flow Analysis for 2023 and 2025 grid scenarios
  • Time-Domain Simulation to evaluate dynamic behavior with BESS integration
  • ETAP DER & Renewable Energy Modeling to simulate wind and solar penetration
  • ETAP BESS Modeling to optimize sizing based on diesel operation, wind excess energy, and reliability needs
These tools enabled precise analysis of renewable variability, diesel performance, and storage requirements.

Why do they use ETAP?

Main customer benefits

1. Accurate BESS sizing methodology

Using ETAP simulations, engineers identified:

  • Maximum wind power containment requirement: 7.85 MW
  • Maximum diesel generation requirement: 6.8 MW
  • Recommended BESS power rating: 7 MW
  • Required energy capacity: 12.28 MWh, based on the longest diesel-generation period (≈4 hours)
2. Significant reduction in diesel consumption

Annual diesel generation decreases from 8447 MWh to 1410.76 MWh, dramatically cutting costs and emissions.

3. Improved renewable utilization
ETAP analysis shows 35,391 MWh of excess wind energy available in 2025.

The optimized BESS greatly reduces curtailment and supports grid stability.

4. Clear economic justification
  • Total BESS investment (Li-ion technology): USD 10.12 million
  • Annual diesel cost savings: USD 2.392 million
  • Payback period: 4.23 years, for a 15-year project lifespan
5. Enhanced operational reliability
BESS mitigates fluctuations in renewable output and reduces reliance on expensive diesel generation.

What do they think about ETAP?

Customer perspectives

Thanks to ETAP software, which calculates load flow results for each time step directly, we can determine the real power system in 2025.
— Huỳnh Kim Long, Power System Calculation Engineer, EVN PECC4

Using ETAP’s modeling features, the BESS model enables engineers to define an accurate method to size the system and verify its economic efficiency.
— Huỳnh Kim Long, EVN PECC4

 



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Optimization of Battery Energy Storage System Sizing for Hybrid System in PHU QUY Island

To meet increasing load demands on Phu Quy island, plans involve installing additional wind and solar power plants, yet due to renewable energy's dependence on weather conditions, the diesel power plant remains crucial, prompting a study on the optimal size of Battery Energy Storage Systems (BESS) to support renewable energy integration and optimize costs.


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