bodyguard – A Wolf in Sheep



Uri Kaplan – ORMAT Technologies Ltd.




The local Electric Company in the Azores Islands, has given the green light for the construction of a new  geothermal power plant at the Pico Vermelho site on the São Miguel Island. The new power plant is expected to contribute to the efforts of the local authorities to have the base load needs of the Island provided by dependable, indigenous and clean geothermal power. The new plant will optimize energy utilization by converting geothermal steam and brine from geothermal wells into electric power energy. The new plant will use air-cooled condensers which enable 100% geothermal fluid reinjection. The 100% reinjection serves both to sustain the reservoir and to produce electrical power with virtually no environmental impact. It will utilize the hi-performance, hi-efficiency organic turbines developed by ORMAT for geothermal and industrial recovered-energy applications.
Green Energy




             The Sociedade Geotermica dos Açores, S.A. (SOGEO) operates the Ribeira Grande and Pico Vermelho geothermal power plants, both located at the Lagoa do Fogo (Agua de Pau) volcano in the central part of the island of São Miguel, Azores, Portugal. The two plants are supplied by separate well fields that exploit different areas within a single extensive geothermal system on the Northern slope of the volcano.


             Exploration and development of the lower (Northern) part of the field, where the Pico Vermelho plant is located, took place during the late 1970’s and early 1980’s. Five deep wells (RG-1, RG‑22, PV-1, PV-2 and SB-1) were drilled during 1978-1981, and the 3 MW Pico Vermelho plant began operation in 1981. Well damage and scaling problems left PV-1 as the only well available for use by the plant, and as a result the plant output has typically not exceeded 0.7 MW.  remote power units


             During the late 1980’s, exploration of the southern part of the field (higher elevations on the volcano) was undertaken in an effort to identify a geothermal resource of higher temperature and lower scaling potential. Well CL-1 was drilled in 1988-89 and wells CL-2, CL-3 and CL-4 during 1992-94. The production from these wells was more than sufficient to supply Phase A of the Ribeira Grande power plant (5.08 MW) located in the upper part of the field. Constructed in 1993-94, this plant consists of two binary-cycle ORMAT® Energy Converters (OECs). Wells CL-1 and CL-2 served as production wells for the plant. Geothermal


             Phase B (9.4 MW) was installed at Ribeira Grande in 1997, bringing the total capacity of the plant to 14 MW (approximately 12 MW net to the grid). Well CL-3 and the excess capacities of wells CL-1 and CL-2 are used to supply this facility. CL-4 has also been used intermittently; however, following the assessment described below, this well has been converted to injection and a new production well is being drilled to replace it. CL-5 which was drilled a few years ago and CL-6, which  is currently being drilled will contribute to sustain the operation of the Ribeira Grande power plant.




             3.1      Two-Phase Flow with High Liquid Fraction


                        The geothermal resource of the Lagoa do Fogo Volcano is characterized by low enthalpy fluid (900 – 1100 kJ/kg) and as a result the separated fluid consists of a high portion of water (brine) and a low portion of steam. Typical figures are 78% water and 22% steam at a separation pressure of 5 bar a, and fluid enthalpy of 1100 KJ/kg.


                        ORMAT has developed the bi-phase type ORMAT® Energy Converter (OEC) to utilize this type of geothermal resource efficiently. The bi-phase OEC utilizes the separated steam to vaporize the organic fluid (pentane) in the vaporizer and the mixture of condensed steam and brine is used as the heating source for the preheater. Figure 1 is a schematic flow diagram of the bi-phase process. The ratio of heat quantities of steam and condensate/brine mixtures are similar to the heat quantities of the boiling and preheating of the pentane in the OEC. Figure 2 presents a TQ (temperature, heat quantity) diagram showing the heat quantities and temperatures of the heating fluid and working fluid in the OEC cycle. The figure shows the perfect match between the shape and quantities of the working fluid and heating fluid, indicating a very high efficiency in utilization of the heating source.


                        The bi-phase OEC concept is used in other low enthalpy geothermal resources in locations such as the Zunil project in Guatemala (24 MW), Ngawha project in New Zealand (12 MW), and the Olkaria III early generation project in Kenya (12 MW).

























Figure 1 – Schematic Flow Diagram of the Bi-Phase Process


































Figure 2 – TQ Diagram



3.2       Stages in the Construction of the São Miguel Geothermal Project


The construction of the Pico Vermelho project is the fourth stage in the development and construction of the geothermal plant on the São Miguel island.


Pico Vermelho Phase A


           The first stage was a 3 MW back pressure steam turbine utilizing the steam of PV-1 but generating only about 600 kW.


Ribeira Grande Phase A

           The first ORMAT project in the Azores was the Ribeira Grande Phase A of 5.08 MW gross, utilizing the steam and brine of wells CL-1 and CL-2. The working parameters of the project were as follows (design point):



Steam flow


43.7 t/h

Brine flow


74.6 t/h

Steam pressure at inlet to plant


5.16 bar a

Brine temperature at inlet to plant



NCG in steam



Design ambient temperature



Power generation (gross at generator terminals)


5,080 kW

Power generation (net at sub station inlet)


4,400 kW


The generation unit consists of two bi-phase OEC units, each with two turbines connected to a common generator.


Ribeira Grande Phase B


           The second phase of the Ribeira Grande project was the construction of an additional two OEC units on the same site, utilizing the same plant facilities and utilizing the geothermal fluid from wells CL-3, CL-4 and CL-5. As mentioned above, well CL-4 was later converted to an injection well.


Phase B parameters are as follows:


Steam flow


71.1 t/h

Brine flow


263 t/h

Steam pressure


5.11 bar a

Brine temperature



NCG in steam



Design ambient temperature



Power generation (gross)


9,400 kW

Power generation (net)


8,000 kW


The figures given above are at  “design point”, as presented in the project agreement and are based on the best information on the well production at the time of the project conceptual phase. During the acceptance tests of the two phases the well production performance was slightly different from the design point figures, however, the performance of the generating equipment – the OEC units was better than the expected performance at the new conditions, when applying the correction curves.


                        Pico Vermehlo Phase B


In mid-2004 SOGEO issued a bid for the design, manufacture and construction of a 10 MW (net) geothermal project using the steam and brine from four production wells –  PV-1, PV-2, PV-3 and PV-4. The spent fluid would be injected to two injection wells – PV-5 and PV-6. Wells PV-4, PV-5 and PV-6 have not yet been drilled and will be drilled in parallel to the construction of the power plant.


Based on the experience of the previous phases, a resource analysis was conducted by GeothermEx to determine the design separation pressure which would take into consideration future reservoir depletion and would result in best overall reservoir sustainability and maximum plant energy production. The result was a relatively low design working pressure, as shown the following working parameters:


Steam flow


76.8 t/h

Brine flow


346.7 t/h

Steam pressure at inlet to plant


5 bar a

Brine temperature at inlet to plant



NCG in steam



Design ambient temperature



Gross power at design point


12,000 kW

Gross power at 13°C ambient temperature


13,000 kW

Net power at design point


10,500 kW












4.         Continuous Improvement


The three phases of ORMAT equipment in the Azores are a good example of the continuing improvement of the energy conversion equipment (OEC unit and the auxiliary component) resulting in higher efficiency and better reliability and lifetime.


The first phase of the Ribeira Grande project included two dual OEC units, each consisting of two 3,000 rpm turbines and two speed reduction units connected to a 1,500 rpm generator.


The second phase included two improved OEC units with 1,500 rpm turbines direct-coupled to the generator without the need for reduction gears and with much higher expansion efficiency.


The third phase – the Pico Vermehlo plant, will have one of the new generation OECs each with a capacity of up to 18 MW.


Table 1 presents the improvement in turbine expansion efficiency since ORMAT’s first geothermal projects in 1984, resulting from an improvement in the nozzle and blade design, better wheel geometry which came with the reduction of the turbine speed from 300/3600 rpm to 1500/1800 rpm (for 50 and 60 Hz grids respectively).


The reduction in the turbine speed was also one of the main reasons for the significant improvement in the reliability of the mechanical components such as bearings and seals, and of the extremely high availability and lifetime of the plants.


Table 1 – Turbine Efficiency


Year of First Use

Representative Projects

Turbine Efficiency %














Upper Mahaio/Rotokawa









The development of geothermal power generation projects in the Azores island started in the late 70’s and today include four power plants with an accumulated capacity of close to 25 MW.


The 10 MW Pico Vermehlo power plant is the third ORMAT plant in the Azores and represents the successful cooperation between the Azores electric company – SOGEO and ORMAT to develop high reliability, efficient power plants utilizing the available geothermal energy.


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