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2 Geothermal Resources Council Transactions, Vol 22, September 20-23, 1998 The Rotokawa Geothermal Project: A High Pressure, Sustainable And Environmentally Benign Power Plant Bruce Cole Generation Manager Power New Zealand Ltd., New Zealand Hilel l e~~ann Marketing Manager Ormat ndustries Ltd, srael ABSTRACT The Rotokawa Geothermal Project is one of a number of geothermal projects in the Rotorua-Taupo area being developed by electricity companies following deregulation of the New Zealand electricity industry. The 24MW development has been undertaken by a joint venture of the indigenous landowners and an Auckland based electricity company. The plant utilizes steam turbine and binary turbine technology to obtain the maximum energy from the resource with minimal ~ v ~ o impact. ~ e x ~ The 24 MW Rotokawa geothermal project is one of a number of new geothermal projects being developed in New Zealand by private sector interests following the deregulation of the electricity industry. mat makes the project unique are the ownership structure, which is based around a joint venture between the Maori (indigenous people) land owners and a power company, and the technology, which allows economic development of the high pressure resource. New Zealand lies on the South-west comer of the Pacific ring of fire ; the chain of volcanic activity which extends up through the Pacific slands, ndonesia, the Philippines, Japan, Alaska, the West coast of the US, and down to the tip of South America, The main geoth~~ area is centered around the towns of Taupo and Rotorua and the geothennal activity is of volcanic origin; with some of the volcanoes still active. Some of the world s early geothermal development was undertaken in New Zealand with the WairAei project (currently 156 MW) being the first large-scale devilopment of a water dominated geothermal field. Construction of the first station commenced in the mid 1950 s, with the original layout of the station allowing for the installation of a plant to produce heavy water for the British nuclear prog~e. This was dropped before station completion, but its influence is still present in the multiple steam pressures of the older station. The Rotorua-Taupo Geothermal Region There is geothexmal activity spread over both islands of New Zealand with the main high temperature fields associated with the volcanic activity in the Rotorua-Taupo area. Most of the other geothermal activity in the country is of tectonic origin and the heat flows and temperatures are not sufficient for commercial power generation. The original settlers of New Zealand, the Maoris, who arrived around 800 years ago &om the Central Pacific used the natural geothermal springs for bathing and the very hot springs and geysers for cooking food. There have been a number of major volcanic eruptions in the region; the most recent being the Tarawera eruption of 1886 which destroyed the world famous Pink and White Silica Tenaces and the largest being the eruption which formed Lake Taupo around 400 AD. The latter eruption was one of the largest known eruptions, with the ancient Chinese recording the impact of the ash emitted on their weather. The major geothermal fields of the Rotorua-Taupo area are iden~ied in Figure 1. The Wairakei, Rotokawa, Kawerau and Ohaaki fields have been developed for commercial power production as detailed in Table 1, and the Mokai field is currently being developed by a 53 MW plant. The Rotokawa Geothermal Resource The Rotokawa geothermal field is a deep high temperature field covering 25 sq. km and located appro~tely 12 km northeast of Taupo. (See figure

3 Cole and tegmann? \ L t t t.-y t / i Figure 1. Rotorua-Taupo Volcanic Zone. Table 1. New Zealand's Geothermal Resource. sioned Wairakei Taupo Wairakei , Tasman 1 lcawerau Kawerau The boundary of the field has been established by resistivity surveys which show that about 40% of the field area is on the north bank of the Waikato River and the remainder on the south side, but field exploration to date shows the usefbl resource to lie to the south of the river. The northern field area is mainly covered by a pine plantation with the southern area mainly in fannland with some new pine plantings. The dominant surface manifestation is Lake Rotokawa, a hot acidic lake which contains considerable deposits of elemental sulphur. The area around the lake has been considerably modified by sulphur mining, but still contains a number of important geothersnal ecosystems which have established on the hydrothermally altered ground. There are Fig 2. Rotokawa Geothermal Field also a number of small hot springs, steaming ground, steam vents and numerous hydrothermal explosion craters. The potential of &e Rotokawa field was recognised in the early 1950's and it was gazetted as a Geothml Steam Area in Eight wells were drilled during the period as part of the Government's programme to assess the region's geothermal resources. Most of these have since been capped and abandoned, either because they were not commercial producers or because of casing corrosion. Of the original eight wells only Rf(5 and El are being utilised in this development, for production and reinjection respectively. The field has an estimated capacity of MW, with the initial development being a 24 MW station to allow carem monitoring of the resource before any further development is undertaken. The main productive levels of the field are at around Om depth where the wells intersect productive faults. A numerical model of the field has been prepared to help with developing the reinjection strategy, and this shows that shallow reinjection near the field centre will- not cause rec~c~ation of geothermal fluid at the initial level of development. Once data fiom the first few years of operation and a high temperature tracer study are available the reinjection strategy will be reviewed before any fixher field developments. The Project Structure The project is unique in its structure and technology; in its structure which brings the indigenous Maori people into the development as a joint venture partner with Auckland power company Power New Zealan& and in its technology which allows effective use of the high pressure steam from this resource. The Tauhara North No 2 Trust owns the land over the middle of the geothermal field, including the land around the well RK5 which is the best of the Rotokawa wells drilled - 510

4 Cole and Legmann by the Government and RKl. The Trust has also lodged a claim with the Waitangi Tribunal for ownership of the geothermal resource under the provisions of the Treaty of Waitangc the founding treaty between the British Crown and the Maori people. Utilising their position as land owner, the Trust purchased the existing wells RK5 and RK1 fiom the Government and has made these wells available to the project. There have been several attempts to develop a geothermal project at Rotokawa. The last attempt was a joint venture similar to the present one but with Merent partners, and reached the stage of selecting contractors for the plant co~~ction before t e ~ a ~ The g. development needed a do er, and Power New Zealand, who recognised the business o p p o ~ took ~, the initiative and negotiated a new joint venture agreement with the Trust in May This new joint venture took over the unsigned contracts and the planning consents of the earlier project. The plant selected at that tirne was a conventional condensing steam turbine of 20 h4w output. On becoming a partner in the project Power New Zealand undertook a thorough review of the plant conf$pration and the technology options, and decided to Her investigate the Geothermal Combined Cycle c o ~ i ~ tusing i o the ~ steam and brine components of the geothermal fluid. The plant c o ~ ~ a fmally ~ o n selected was the more flexible and modular Geothermal Combined Cycle techno lo^ offered by O mt which, uses a steam turbine and binary plant to capture the best features of each technology. The turn key contractor and supplier of the equipment for the 24 MW net output plant is the Ormat Group of Companies. An additional production well and two reinjection wells were required for the project and a contract for well drilling and testing was awarded to Downer Energy Serviees of Taupo. Project Design The project has two p~ducti~n wells of around 2000 metres depth producing two phase fluid which is piped to a separator at the station. Steam is separated fiom the brine at 23 bar and both the steam and the brine are used for electricity generation. The condensed steam is pumped up to the brine pressure, combined with the high pressure brine, and reinjected with no further pumping. There are three reinjection wells of around 500 metre depth, one of which is one of the original field exploratory wells. To maximise the benefits of the high steam pressure a General Electric back pressure turbine of 14 MW output is utilised to drop the steam pressure to approximately 1.5 bar. This low pressure steam is condensed in two binary units of 5 MW output each. This configuraton, called by Ormat a Geothermal Combined Cycle Unit, has the advantage of the low capital cost of a simple back pressure turbine and of condensing the steam in a tube and shell heat exchanger where steam wetness is not a problem. There is a third binary unit, also of 5 MW output, uti~sing the hot brine flow and cooling it fiom 219 deg C to 150 deg C. The motive fluid in the binary units is pentane and the cooling is by air cooled condensers, The plant confguration is shown in Figure Zlb Vh RiS PRODUCTON WELL RK9 PR0WCK)N WELL t 6 m RK1 RKll RK12 Flaws of steavwater and tsmperrmrsl M appmxlmrte value5 Only. RENJECTON RENJEClON RENJECTON WELL wut mu Figure 3. Process diagram. 51 1

5 Cole and Legmann Station Parameters Steam turbine output Binary unit output Net output Annual energy output Heat And Mass Balance 14 MW 3x5MW 24MW 189 GWh The geothermal steam exiting fiom the separator and the brine into and out of the brine binary unit have the following average conditions: Steam flow rate NCG flow rate Steam wetness ex separator Steam pressure separator outlet Steam temperature separator outlet Brine flow rate Brine inlet temperature Brine outlet temperature Design ambient air temperature Main Equipment Description 130 t/hr 3.1 t/hr 99.98% 23.2 bara "C 303 t/hr "C 150 "C 12 "C The steam turbine is a back pressure multi-stage reactiontype turbine. The turbine housing, shaft assembly and nozzle ring were designed to Onnat's specification for operation with geothermal steam. Level Steam Turbine & Generator Steam turbine type Steam inlet pressure Steam outlet pressure Speed Construction Multi stage, single cylinder reaction 22.3 bara 1.5 bara 3000 rpm Horizontal split casing Generator rated output 14 MW Voltage Power factor 1 lkv, 3 phase, 50 Hz 0.85 (lagging) Efficiency 97.5% Level 1 Ormat Energy Converter Organic vapour turbine type mpulse Speed 1500 rpm Construction Number of stages 2 Motive fluid Pentane Horizontal (overhung) vertical split casing Generator rated output 5 MW Voltage 11kV 3 phase, 50 Hz Speed 1500 rpm Efficiency 97% The power plant consists of the geothermal combined cycle unit, the brine driven OEC unit plus following main systems: J Power plant geothermal fluid gathering system Auxiliary systems Electrical systems Main station control Fire fighting systems Auxiliary buildings The generator circuit breaker, control and auxiliary electrical equipment for each binary unit is housed in a container which is fully wired and pretested. This reduces construction time and speeds commissioning on site. The overall station control is from a control room attached to the steam turbine building. Station control is by.computer utilising sohare and graphics developed by Ormat and by GE Fanuc programmable logic controllers programmed by Ormat. Construction Programme The modular nature of this plant allows a very short construction period on site. All the binary turbines are mounted on simple low level foundations. As the steam turbine has no attached condenser it too is mounted on a low level foundation, allowing a simple turbine building of modest size. The binary plant components are designed to be shipped in packages of standard container size and within days of the shipment arriving the main components were bolted down and the air cooled condenser erection was under way. The overall programme for the plant development was as follows: Decision to proceed July 1995 Contract award. December 1995 Notice to Proceed March 1996 Delivery of turbine-generator April 1997 Commissioned December 1997 Environmental mpact The station is designed to have minimal environmental impact. Under normal operating conditions the geothermal fluid is completely contained from production to reinjection with the only emissions being small quantities of steam emitted by the steam traps and the non-condensable gases emitted above the air coolers. Only during start-up and shut down and in emergencies steam is emitted from the rock muffler. The plant has a relatively large footprint, but a much lower profile than a conventional condensing steam turbine with underslung condenser. The air cooler structures are also of much lower profde than wet cooling towers, and have the advantage of never producing a visible plume. n addition to its low profile, the plant has no water or chemicals consumption and no blowdown of contaminated cooling tower water. The power generation technology implemented at Rotokawa complies with the resource consents and is dedicated to the needs of a sustainable, environmentally benign and reliable geothermal power plant. 512

6 Cole and Legmann Because the development is small relative to the ultimate capacity of the very deep resource it is expected that there will be little impact on the surface features. A comprehksive baseline monitoring programme was undertaken prior to project operation and an ongoing programme will monitor any changes against this baseline. erati ti on and Maintenance Operation of the station is contracted to Power New Zealand Contracting who supply six operators to provide 24 hour cover on a 12 hour shift basis. Two operators are on duty during the day shift and one at night. Operators are required to undertake routine and emergency minor maintenance work as well as operations duties. The station is supervised by an Operations Manager who is also involved in other geothermal development work. The possibility of moving to manual local start and shutdown of the station and remote control at other times is being investigated. There are a n-ber of manned geothermal stations nearby which could form a suitable operating base. All maintenance work is contracted in from selected contractors, Apart fkom the operators, some of whom are qualified trades people, there are no permanent maintenance staff. t is too early to determine if this contracting in of mainte~nce staff will affect station annual output, but it certainly reduces operating costs. The frst major plant inspection is planned for November 1998, around a year after initial operation, This will include opening of the steam turbine to check on scaling and general wear. The Rotokawa field has been assessed as having a development potential of greater than 1OOMW. This initial conservative development of 24 MW is not expected to have any significant effect on the resource, neither fiom a temperaturelpressure perspective nor from a sufface environment perspective. However, before the joint venture partners embark on any Mer developm~t a p r o g ~ of e environmental and reservoir monitoring is being undertaken to monitor reservoir and environmental changes. Subject to these impacts being within acceptable limits, the partners will look at M er stages of development. Summary The Rotokawa Geothermal Project is a working example of the indigenous landowners developing their resources through a p~ership with a power company. n addition to the scientific resource modelling and resource behaviour projections there, were two essential comfort factors which were taken into ons side ration by the project developers: A power generation technology that avoids the drainage of the geothermal resource and assures long term ~~tenance of the fluid level and characteristics of the field, and A power plant configuration which xnaximises the use of the geothermal energy, minimises the risk factors for the equity owners and generates the highest possible income. Exactly on the day of passing it s first six months of operation, the Rotokawa power plant achieved its first loogwh of generated energy, an excellent result when compared with its 189 GWh per annum energy goal. Ct 3