Tailored to the landscape

Marco Frassinetti, EXERGY,

Italy, discusses the rise of binary technology within the geothermal sphere and how it can be customised for a range of different projects.

According to the latest Renewable Energy Market Update published by the International Energy Agency (IEA) in May this year, the relentless growth of renewable energy demand was not stopped in 2020, despite the COVID-19 pandemic. Renewables were the only energy sources to expand and to set a new record with a capacity addition exceeding 45% from 2019, the largest year-on-year rise since 1999.

Geothermal energy contribution represented approximately 200 MWe and raised the total geothermal power generation capacity installed worldwide to 15.6 GWe. A modest growth for the sector that was influenced by the pandemic situation. Turkey, although in an unclear policy situation due to feed-in tariff renewal, was still confirmed as the leading country for geothermal development: 168 MWe was started up in 2020, the whole capacity with binary cycle power plants.

Binary technology has been the fastestgrowing technology in the last decade, featuring in more than 50% of new installations. This has been due to its advanced technology, being more responsive to environmental restrictions, and to the rising exploitation of low to medium temperature resources, for which binary systems reveal the best power generating solution in comparison to traditional dry steam power plants.

Italian EXERGY International, a provider of Organic Rankine Cycle (ORC) solutions owning the second largest geothermal binary fleet worldwide, has completed three of the new binary systems commissioned in Turkey in 2020: the 26 MWe power plant for Greeneco Enerji, the 12 MWe for RSC Enerji, and 10 MWe for Kiper Elektrick.

Greeneco Enerji geothermal installation represents the sixth unit of a multi-stage project, totalling 102 MWe, that involved the modular expansion of the power generation capacity following geothermal field development. In 2014, EXERGY was awarded the first contracts for the supply of two units, 12 MWe each, commissioned between 2015 and 2016. These were followed by four additional geothermal ORC units.

For each project, EXERGY designed a tailored solution equipped with the innovative and proprietary technology known as Radial Outflow Turbine (ROT) to grant high performance, customised solutions, and increased profitability

Figure 1. Greeneco Enerji geothermal power plants, Sarayköy, Turkey.

Figure 2. Akca Enerji’s 4 MW Tosunlar geothermal plant, Turkey.

of the power plants. For the first two 12 MWe units, EXERGY engineered and supplied two water-cooled pressure level cycles equipped with two ROT, one high pressure and one low pressure, coupled to a single, double-ended generator. For units three and four, the same design was employed but opting for an air-cooled condensing system in order to achieve zero water consumption and decrease the environmental footprint of the power plant.

The last two geothermal units supplied between 2018 and 2020 were the biggest projects, consisting of a 28 MWe and a 26 MWe ORC system where EXERGY designed a solution with one pressure level cycle equipped with two big turbines of 14 MWe each and employing an air-cooled condensing system. The last unit has been in operation since October 2020. The total installation of 102 MWe capacity of baseload renewable energy of the Greeneco project allows 478 000 tpy of CO2 emissions to be saved and avoids the consumption of 153 000 t of oil equivalent necessary to produce the same power with a conventional fossil fuel resource.

Turbine technology

Launched for the first time in the ORC market in 2009, the ROT is EXERGY’s innovation that proved to be the highest efficiency turbine on the market, with performance certified by third party tests on some plants in operation.

The key feature of the ROT configuration is its capacity to convert the energy contained in the organic fluid into mechanical power with high efficiency. In fact, when working with an organic fluid, the ROT configuration and characteristics entail several advantages: F Natural accommodation of the working fluid expansion allowing a broader range of applicable fluid conditions.

F Multiple pressure admissions possible on a single turbine disk giving a cost-effective solution to exploit low temperature geothermal resources.

F Minimum 3D effects and turbulence meaning maximum efficiency achieved with straight blades, much simpler to manufacture.

F Low speed turbine requiring no gearbox, meaning longer life of the bearings and higher reliability.

F Patented Mechanical Group is easy to extract, resulting in fast and easy maintenance with no necessity of fluid drainage and overhaul maintenance on the turbine.

F Standard mechanical design for each turbine frame.

The latter advantage is key, as it allowed EXERGY to offer optimised solutions and scale-up technology very rapidly,

reducing supply time, and guaranteeing maximum efficiency without need for mechanical redesign. In fact, the particular configuration of the turbine allows for the separation of ROT designing and manufacturing into two complementary processes: a first mechanical designing process that is extremely redundant and standardised to assure the highest turbine reliability, and a second fluid-dynamic designing process that is customised for the specific resource conditions in order to extract the maximum possible power from the geothermal fluid. Considering the geothermal market requests, EXERGY developed three standard mechanical frames for turbine sizes up to 5 MW, 10 MW, or 25 MW.

For every new turbine design, the correct mechanical frame can be selected according to the project details, and the machine reliability is assured even with a custom-made fluiddynamic design process. Today there are more than 40 ROT, of different sizes, successfully in operation in geothermal power plants worldwide, each with a unique and customised design in order to obtain the maximum efficiency from the available resource and thus guarantee the highest return of investment for the customer.

A world first

In 2015 in Turkey, EXERGY’s turbine design allowed the execution of a world-first geothermal project: the first example of a binary plant equipped with two pressure-level cycles on a single turbine disk.

The project was developed for Akca Enerji and it allowed the customer to profitably exploit a low enthalpy geothermal resource with fluid temperature at 105˚C to generate 4 MWe of electricity. This configuration with two pressure levels on a single expander offered a higher plant performance, producing up to 20% more power than using a single pressure level system. This plant was awarded the European Geothermal Innovation Award in 2016.

Geothermal brine recovery

Binary technology is an effective choice, not only for the development of greenfield low-medium temperature geothermal resources but also to better harness existing geothermal power plants by combining binary plants to traditional flash technology.

One application of flash and binary technology is for brine recovery. Although efficient and reliable, dry steam power plants lose a significant part of the total thermal capacity in the liquid phase (brine). Retrofitting an existing flash plant with a bottoming binary cycle is an effective way to recover the thermal energy of the brine to generate additional electricity, thus increasing the power output of the plant and the productivity of the geothermal site without further environmental impact.

EXERGY International has recently started the development of a geothermal brine recovery project in the Philippines for Energy Development Corporation (EDC), the world’s largest vertically integrated geothermal energy producer. Located on the Mindanao island, the new Mindanao 3 unit will be EDC’s first geothermal brine recovery plant to go online in 2022. It will help generate an additional 3.6 MWe of power

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Figure 3. Pico Alto geothermal power plant, the Azores.

Figure 4. 3D section of EXERGY’s Radial Outflow Turbine.

utilising the waste geothermal brine available from Mindanao 1 and 2 steam power plants in an existing geothermal field.

EXERGY’s activities involve the engineering, design, and supply of the ORC system and technical equipment, including the cooling towers and the brine dosing system. Moreover, EXERGY’s task encompasses a substantial technical advisory service to EDC for the project execution. The engineered solution consists of a single pressure level cycle equipped with ROT.

The overall project has a very tight time schedule, and the start-up of the plant is expected by 1H22. Once in operation, the ORC systems will help save approximately 16 000 tpy of CO2 emissions and 5000 tpy of oil equivalent consumed.

Taking on a challenge

Other interesting geothermal applications of EXERGY technology are again located in Turkey, where in 2017 EXERGY installed one of the biggest ORC turbines on the market – 18 MWe in size – at the Kuyucak power plant for Turcas, and a second one at the Pico Alto project in the Azores islands. The latter was a particularly challenging task.

Developed for EDA RENOVAVEIS, the utility of the Azores island, it involved the supply of a 4 MWe geothermal plant to harness a high temperature geothermal resource. The aim was to provide renewable electricity for the Terceira island and decrease the island’s dependence on diesel power generation. The high temperature resource would fit with the choice of a flash power plant but the binary solution was preferred as it is more flexible in adapting to the resource characteristic and to the environmental constraints. In this geothermal field the production wells were characterised by a cycling behaviour, causing flowrates and pressures oscillations, hence making the exploitation of the reservoir and the operations of the power plant fairly challenging. Other environmental and logistical constraints that needed to be taken into account were the high levels of iodine and salt effect on plant components, severe weather conditions during construction and site operations, and the remote location at 1500 km off the mainland – making the project even more difficult to execute.

Due to the high enthalpy resource, EXERGY designed a first of its kind turbine in the company’s geothermal application, with a single level saturated cycle employing a type of working fluid typically used for heat recovery applications. A fast delivery time was possible for the characteristics of the ROT manufacturing process. The plant entered into operation in August 2017 and helped to decarbonise power generation on the Terceira island, reducing the electricity production from fossil fuels by 20% in eight months and increasing the share of renewable energy from geothermal by 13%.

Reliable renewable energy

The remaining geothermal projects in EXERGY’s portfolio are located in Italy and Turkey, with a total 27 power plants and approximately 440 MWe installed or under delivery capacity.

These power plants, as with many other projects around the world, demonstrate how geothermal energy is an effective, reliable renewable energy source that can be efficiently harnessed by employing the most advanced available technologies. It also has the advantage of being abundantly present in many parts of the world, being a valuable choice for remote locations with difficult access to electricity. Moreover, in contrast to all other renewable energy sources, geothermal is the sole renewable that can satisfy baseload power generation without jeopardising the grid stability with unpredictable behaviour.

Nevertheless, it continues to be disregarded, representing a very small share of the total renewable energy capacity addition year-on-year. As the IEA’s Executive Director Fatith Birol recently remarked in one of his interventions, geothermal “is one of the renewable resources that [is] being treated unfairly” and “is not on the agenda worldwide enough”. To develop geothermal activities further, it is fundamental that all the risks and the costs connected to the exploration phase are correctly addressed by policy makers, with adequate financing that can encourage investments during the early stage.

With the international geothermal associations at the forefront to bring this theme to the attention of institutions and with oil and gas majors starting to show their interest and make investments in this sector, the geothermal community can maybe hope for a brighter future and a stronger development of geothermal worldwide.