Two bottoming units turn geothermal brine already being pumped into 45 megawatts

PT PLN Indonesia Power and PT Pertamina Geothermal Energy signed a letter of intent covering the purchase of electricity from two geothermal projects totalling 45 megawatts, concluded during the twelfth Indonesia International Geothermal Convention and Exhibition at the Jakarta International Convention Center. Reports place the signing between 19 and 21 August, and the letter formalises tariff terms that the parties had already settled earlier in 2026.

The capacity splits into two units. The Ulubelu Binary Bottoming Unit in Lampung accounts for 30 megawatts. The Lahendong Binary Bottoming Unit in North Sulawesi accounts for the remaining 15 megawatts. Both sit at fields that are already in production.

What bottoming technology actually does

The engineering here is the reason the announcement matters more than its size suggests.

A conventional geothermal plant takes hot fluid from a reservoir, uses the steam to drive a turbine, and is left with brine that still carries heat but not enough pressure or temperature to run that same turbine. Historically that brine goes back into the ground. A binary bottoming unit takes the brine before reinjection and passes its heat to a secondary working fluid with a lower boiling point, which vaporises and drives a second turbine.

The result is additional electricity from fluid that is already being extracted, already being pumped, and already being handled by existing surface equipment. No new wells are required to supply it. No new reservoir is being drawn down.

Expressed in the language of efficiency, the change raises how much electricity the field yields from each unit of fluid it produces. The reservoir is unchanged and the extraction rate is unchanged. What improves is the proportion of the available heat that ends up as power rather than returning underground unused. This is the same principle that governs waste heat recovery in industrial plants, applied to a power station rather than a furnace, and it belongs to the category of measures that raise output without raising input.

Utilising bottoming technology is one way to optimise geothermal resources in the working areas we already hold.

That description came from Bernadus Sudarmanta, President Director of PLN Indonesia Power, and it states the case precisely. He framed the Lahendong and Ulubelu work as turning existing potential into reliable clean energy while strengthening national energy independence. Ahmad Yani, Director of PGE, represents the resource side of the arrangement.

Capacity without a new working area

The phrase that deserves attention in the official language is wilayah kerja panas bumi, the geothermal working area, usually shortened to WKP. Securing a new WKP is among the slowest steps in Indonesian geothermal development. It involves exploration risk, drilling capital committed before resource certainty, permitting across multiple authorities, and negotiation with communities and land users.

A bottoming unit avoids all of it. The resource is proven because it is already producing. The wells exist. The access roads, transmission connection, and operating staff exist. What is added is a heat exchanger, a secondary turbine and the associated balance of plant. Against greenfield geothermal, the risk profile is close to incomparable.

That is why 45 megawatts of this kind is not equivalent to 45 megawatts of new field development. It arrives faster, it carries far less exploration risk, and it improves the economics of assets already on the balance sheet. The letter of intent is described as an initial phase of a broader framework between PLN Indonesia Power, PT PLN and PGE, with potential development reaching 230 megawatts.

A centenary year for Indonesian geothermal

The timing carries a symbolic weight that officials noted at the convention. Indonesian geothermal traces its origin to drilling at Kamojang in 1926, which makes 2026 the hundredth year. Speakers also drew attention to a property that distinguishes geothermal from the variable renewables dominating current investment, which is that it generates around the clock.

Bahlil Lahadalia, Minister of Energy and Mineral Resources, put the ambition in plain terms, saying that geothermal energy must not remain stored in the earth or exist only as a number, and must instead become electricity that benefits society, moves the economy and strengthens national energy independence. He tied delivery to collaboration among regulators, PLN, developers and other stakeholders. Eniya Listiani Dewi, Director General of New and Renewable Energy, was also among the officials present.

The framing is worth recording because it points at the gap between resource estimates and installed capacity, which has been the persistent feature of Indonesian geothermal. Bottoming units do not close that gap on their own. They do show a route to progress that does not depend on resolving it first.

That distinction matters for how a centenary is best marked. A hundred years of drilling has produced a substantial operating fleet, and every plant in it was designed to the standards of its own era. Fields commissioned decades ago were built when lower temperature conversion was less mature and less economic than it is now. Revisiting those assets with current technology is a different exercise from exploring for new ones, and it is one where a century of accumulated operating history is an advantage rather than a legacy cost.

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Three points follow for policy and planning.

The first is that optimisation of existing assets deserves a defined place in capacity planning. National targets are typically expressed as new capacity, which directs attention toward new sites. Yet output recovered from producing fields arrives sooner and at lower risk than the equivalent from greenfield development. Planning documents and procurement processes that treat brownfield optimisation as a distinct category, with its own pipeline and its own targets, would surface opportunities that currently compete unfavourably against larger headline projects.

The second is that the tariff was settled before the letter of intent, not after. Commercial terms agreed early are what allow a technically straightforward project to move without prolonged negotiation. For public institutions acting as offtakers or regulators, establishing predictable tariff treatment for incremental capacity at existing fields would let a class of similar projects proceed on a standard basis rather than case by case.

The third is replication. Indonesia operates numerous geothermal fields where brine is reinjected at temperatures that still hold usable energy. If bottoming units are viable at Ulubelu and Lahendong, the same assessment is worth running systematically across the operating fleet. A national inventory of recoverable heat at producing fields would be inexpensive to compile relative to the capacity it could identify.

What to watch next is the conversion of the letter of intent into a binding power purchase agreement, the construction schedule for both units, and whether the 230 megawatt framework is broken into further tranches that follow the same brownfield logic.