Supporting renewable energy developments in Tonga through TREP

Client: Asian Development Bank / Tonga Power Ltd (TPL)
Location: Tonga (Tongatapu, ‘Eua, Vava’u, and outer islands)
Date: 2017 - 2025

Supporting new solar generation and battery energy storage in the Pacific through the 'Tonga Renewable Energy Project' (TREP)

Background

Tonga comprises 171 South Pacific islands, 45 of which are inhabited, with a population of a little over 100,000. Like most island nations in the Pacific, it is threatened by rising sea levels and intense storms. Tonga aims to be powered by 70% renewable energy by 2030, reducing its reliance on imported diesel.

Entura has been supporting the ‘Tonga Renewable Energy Project’ (TREP) since 2017. The project was established to provide renewable energy generation and energy storage to increase electricity access, improve service quality and reliability, and support Tonga’s energy transition, climate resilience and environmental sustainability.

TREP was funded by the Asian Development Bank (ADB), Green Climate Fund (GCF) and Australia’s Department of Foreign Affairs and Trade (DFAT) as part of the Pacific Regional Development Partnership Plan.

The project included installing new renewable energy and battery storage systems (BESS) on Tongatapu, grid-connected renewable energy on ‘Eua and Vava’u islands, renewable-based mini-grids for electrification on 9 outer islands, as well as delivering capacity building and project management support.

The installed capacity for the Tongatapu BESS is 7.2 MW / 4.1 MWh and 7.2 MW / 20.88 MWh. The first BESS is for grid stability through frequency support, voltage control and renewable energy smoothing. The second BESS is primarily for load shifting excess solar generation to the evening peak load. The remaining total installed capacity across 11 island grids is approximately 1.3 MW solar, 3 MW / 12 MWh BESS and 20 km of distribution network.

Solution

Entura was contracted by the ADB to review feasibility for TREP, procure contractors, and subsequently to provide ongoing support during construction. We undertook technical, financial and economic due diligence on each of the projects, supporting Tonga’s applications for project finance. We then supported Tonga through the procurement of these projects, writing specifications and bidding documents for the EPC construction tenders, and supporting the tendering, evaluation and selection processes resulting in construction contract award.

Subsequently, Entura was engaged as construction supervision consultant for TREP. In this role, Entura provided a full Owner’s Engineer suite of services including design review, procurement verification, factory inspection test witnessing, inspecting site works, witnessing and reviewing completion and commissioning tests, reviewing EPC contractors’ management plans and quality assurance, as well as monitoring and reporting on environmental, social and gender safeguards.

The COVID-19 pandemic created a range of challenges for maintaining progress. We worked closely with TPL and the project management unit to manage risks and expedite tasks where possible. During the period of international travel restrictions, we adapted by facilitating aspects of the project remotely, such as detailed design reviews, remote witnessing of factory acceptance testing (FAT) for key equipment, and guiding community engagement and safeguards monitoring. Our team included a local engineer, providing construction supervision for the initial civil aspects of the works and presence on site.

Our specialists were instrumental in ensuring that the BESS could demonstrate the promised grid support functions, and that the SCADA and control systems developed for the project were fully integrated with those of TPL so that it could support the solar generation. Our longevity with the utility enabled us to support and fine tune the system over a period of years, responding to emerging challenges as new generation was brought online.

The capacity-building component of TREP included a range of training including how to review and update tariffs and develop operations and maintenance (O&M) plans. We also provided support to deliver a technical uplift in energy and power systems modelling, control systems and SCADA within Tonga Power Limited (TPL). This will enable TPL to have greater understanding and ownership of new projects moving forward.

Services provided

  • technical, financial and economic feasibility studies
  • technical due diligence
  • community engagement 
  • procurement and contract negotiation
  • design review
  • remote factory acceptance testing
  • construction supervision
  • capacity building

Outcome

Entura’s work has now concluded, and TREP was officially deemed complete at the end of June 2025.

All subprojects are now operational in Tonga. Both BESS in Tongatapu are operating, providing storage for excess solar and being controlled automatically by the new generation and distribution management system. ‘Eua and Vava’u main islands are running at higher contributions of renewable energy. Project savings enabled remediation of the pre-existing PV system in Vava’u.

For islands that did not previously have a power station or reliable power supply, the project has delivered a substantial change for local communities. As many as 494 households across 9 islands have been provided with 24/7 grid electricity for the first time, powered only by solar and battery systems. Overall, TREP has enabled greater access to modern, reliable energy services for around 94,000 people in Tonga. It is expected to deliver greater energy security by removing the reliance on imported diesel while delivering a major step towards achieving Tonga’s goal of 100% renewables by 2035.

The project has also created new employment opportunities for Tongans, including many women who have been trained as carpenters, electricians and line workers. Thanks to reliable electricity and new electric appliances, many community members have reported being able to better manage their work-life balance and studies, start micro-businesses that use electric appliances, earn more income, and eat fresher food.”

We are proud to have provided a range of expert support to this transformative project over an extended time, from inception to implementation. Our work to support the clean energy journeys of our Pacific neighbours is an excellent demonstration of the implementation of Entura’s vision statement: ‘A sustainable energy future for all’.

Entura has also provided specialist support for renewable energy solutions across the Indo-Pacific region, including energy roadmaps, hybrid renewable microgrids, and capacity building in the Federated States of Micronesia, Tuvalu, Cook Islands, Solomon Islands and Marshall Islands.

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