The project entails the revamping of the Ribeira Grande geothermal plant, which has been in operation since 1994. Two of the four existing Organic Rankine Cycle (ORC) units, each rated at 2.5 MWe, will be replaced with a new 5 MWe unit.

A binary geothermal power plant is a type of geothermal power plant that uses a secondary working fluid to generate electricity. Unlike traditional geothermal plants that directly use hot water or steam from the earth to turn turbines, binary plants transfer the heat from geothermal fluids to a secondary fluid with a lower boiling point. This secondary fluid then vaporises and drives a turbine, producing electricity.

The Ribeira Grande project is the second contract awarded to Exergy by EDAR, the renewable energy division of Azorean utility company EDA. The first was for the supply of the 4 MWe Pico Alto plant on Terceira Island, which has been in operation since 2017 and meets 10% of the island’s energy needs.

Exergy’s ORC system is based on a closed thermodynamic cycle that is designed to produce power from low-to-medium-temperature heat sources ranging from 80–400°C and for small-to-medium applications at any temperature level. ‘It allows for efficient exploitation of low-grade heat that otherwise would be wasted,’ explains the company.

The ORC units feature Exergy’s radial outflow turbine (ROT) technology. ‘The ROT configuration makes it possible to convert the energy contained in the organic fluid into mechanical power with higher isentropic efficiency than traditional axial or radial inflow turbines,’ reports the company. Isentropic efficiency is a measure of how effectively a thermodynamic process converts input energy into useful work.

‘The ROT diverts the fluid radially through stages, simplifying blade geometry and enabling multiple stages on a single disc. In radial inflow turbines, the fluid enters perpendicular to the rotation axis, transitioning to the axial direction as it expands within the rotor. This configuration results in limited flow rates and complexity in achieving multistage configurations. Axial turbines handle fluid parallel to the rotation axis, facilitating high flow rates and are commonly used in large-scale power generation.’

The Ribeira Grande ORC units will harness a geothermal resource at approximately 150°C in both the brine and steam phases. They will feature a single-pressure-level cycle designed to utilise all available heat content of the steam, minimising geothermal fluid loss. An air-cooled condensing system will help avoid water consumption, and the geothermal steam and brine will be reinjected into the well at the ORC outlet.

Exergy’s scope of work on the Ribeira Grande repowering project includes the full engineering procurement and construction (EPC) of the ORC plant and associated gathering systems. The plant is planned to be fully operational before the end of 2026.

The Azores is working towards a goal of carbon neutrality by 2050.

Around 89% of the electricity currently fed into the Azores grid from renewable or indigenous resources is the responsibility of EDAR, according to Managing Director António Félix Rodrigues. He adds that the revamping of the Ribeira Grande geothermal power plant, along with the expansion of the Pico Vermelho geothermal plant – both located on the island of São Miguel – will help increase the share of renewable energy on the island to around 60% by the end of 2026.

Geothermal is the most important renewable resource in the Azores, meeting around 21% of total regional electricity demand in 2024, despite current exploration being limited to the islands of São Miguel and Terceira. The Azores hosts three geothermal plants at present: two in São Miguel (10 and 12 MWe), and one 4 MWe plant in Terceira. There are thermal plants on all nine islands running on combustion of heavy fuel oil.

The Azores is planning to increase the share of electricity produced by renewables (primarily geothermal, also wind and hydro) from 34% in 2024 to ‘above 70% in the near future’, according to Exergy. The region is targeting full carbon neutrality by 2050.

In order to help achieve this goal, EDAR is also currently renovating and expanding its existing wind farms. At Figueiral, on Santa Maria Island, the five installed wind turbines with a total power of 1,500 kW will be replaced by three new wind turbines totalling 2,700 kW. At Pico da Urze, on São Jorge Island, six wind turbines with a total power of 1,800 kW will be replaced by five new wind turbines totalling 4,500 kW. On Flores Island, at Boca da Vereda, two wind turbines totalling 600 kW will be replaced by a new 900 kW turbine. Work is due to complete by year-end and will see wind accounting for 30% of electricity generation on these islands, up from 12%.