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Ngawha
Geothermal Project
Background Information
1. Introduction
The Ngawha power station development is located close to Ngawha Springs in Northland, New Zealand, approximately 7 km East of Kaikohe off State Highway 12. The power station platform is located adjacent to well NG12 and utilises wells drilled in the late seventies and early eighties by the then Ministry of Works with the intention of exploiting the Ngawha geothermal resource for electricity production purposes. The original development plans were subsequently abandoned. The present design is based around two identical 5.6 MWgross (4.65 MWnett) output binary cycle turbine generators.
The Ngawha project consists of:
- Two production wells, NG9 and NG12, with a back-up production well, NG4
- Two re injection wells, NG11 and NG18, with an alternative re injection well NG20
- The land within which the power station, wells and steam collection system are contained
- Two Ormat Energy Converter (OEC) units for converting geothermal energy into electrical energy, as well as all auxiliary systems including all plant and pipe work to collect and distribute steam, NCG and brine from the production wellheads to the re-injection wellhead necessary to operate the project.
- 33 kV switchyard
2. Construction
Construction activities commenced in late October 1997 the preparation of the laydown and power station platform areas as well as the clearing of the proposed pipe corridor to the re-injection wells.
Ormat Pacific Inc., the turnkey contractor commenced site activities in January 1998. Substantial Completion of the Works was achieved in June 1998. Final Acceptance is scheduled for mid August. Between Substantial Completion and Final Acceptance, the main contractor will be undertaking commissioning activities followed by performance testing.
3. Technical Detail
3.1 Ormat Energy Converter (OEC)
The geothermal steam and brine is supplied in two separate streams to the organic units. Major components of each organic unit consist of a working fluid recuperator, preheater and vaporiser, high pressure (HP) and low pressure (LP) turbines common generator, lubrication and seal oil systems, an air-cooled condenser, and working fluid cycle pumps. The module also includes automatic and manual control valves, level switches, pressure gauges, pressure controls, internal piping, and power and control boards. A simplified diagram of the power station is shown in Figure 1.
The organic unit system is based on the Rankine power cycle. The cycle uses pentane as the working fluid. Pentane is preheated and vaporized by the heat of the geothermal steam and brine flowing through the vaporizer and preheaters. The resulting pentane vapour expands as it passes through the HP and LP turbines, which are coupled to the double shaft end generator that produces 3-phase electrical power. The exhaust vapour flows to the recuperator where heat is recuperated to the liquid pentane coming from the condenser.
After passing through the recuperator, the working fluid vapours flow to the condenser. The condensed fluid accumulates at the hot well and then flows to the feed pump's suction inlet by gravity.
The cycle feed pumps, controlled by a level control device, pump the working fluid to the preheaters and vaporizer. The motor-driven pumps are multi-stage centrifugal pumps with mechanical seals.
The main components of the power station are:
- Turbo generator skid
- Lubricating and seal oil system skid
- Heat exchangers
- Air-cooled condenser
- Organic motive fluid (pentane) piping system
- Control and power system including governors
- 33 Kv Switchyard and main transformer
3.1.1. Organic Fluid Turbines of the OEC
The organic fluid turbines are designed to operate with organic fluid vapour at the design conditions. The turbine is a two-stage impulse unit with materials selected to work with the organic fluid. The vapours expand in series through the HP and LP turbines.
Sealing of the turbine is accomplished by a double mechanical shaft seal with oil as a buffer fluid. The mechanical seal is of the cartridge type.
3.1.2 OEC Generator
The generator is a synchronous type, brushless, weather protected, built to NEMA II specifications. The generator is cooled by a once through air cooling system. It has a dual shaft extension to enable it to be driven by two turbines, an LP turbine at one end and an HP turbine at the other end. The generator produces electricity at 11,000 volts (11kV). This is transformed up to 33 kV and injected into Top Energy's electricity distribution network.
3.1.3 Oil System for the OEC
The oil system is skid mounted and serves the following purposes:
- To supply lubrication oil for the bearings of the organic turbines and the generator.
- To supply sealing oil for the mechanical seals of the turbines.
Each system is equipped with an oil pump as well as oil filters, oil coolers (forced air-cooled type), solenoid operated control valves, relief valves and level switches.
As a contingency, the lubrication and seal oil systems are equipped with air operated emergency pumps and air accumulators to supply the lube and seal oil in case of loss of electrical supply to the main oil pumps.
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3.1.4 Electro-Pneumatic Control System
The electro-pneumatic control system operates the control valve actuators and provides air pressure to all pneumatic components. Included in the system are:
- Pneumatic control panels for the organic turbine with all the electro-pneumatic control solenoid valves
- Pneumatic lines and equipment will operate the control valves and auxiliary systems
- Actuators - diaphragm and piston type.
Electronic control signals for the system are provided from the central unit control in the form of digital or analog (4 to 20mA). The analog signals are converted by current to pressure (I/P) transducers to a proportional pneumatic output signal.
3.1.5Heat Exchangers
The heat exchanger contains the following items:
- Vaporiser
- Preheaters
- Recuperator
- Air cooled condenser
The vaporiser, preheaters and recuperator are a horizontal tube and shell heat design. The tubes are made of stainless steel with tubes ending in tubesheets and supported by support plates and baffles. The shell section is a cylindrical envelope made of carbon steel.
3.1.6 Air Cooled Condenser
The condenser is an induced draft air cooled heat exchanger. The motive fluid is cooled and condensed while flowing in the tubes by the air which flows outside of the finned tubes in a crossflow pattern. The condensed motive fluid accumulates in the outlet box from where it flows by gravity to the motive fluid filters and pumps.
3.1.7 OEC Control System
Each OEC control system is controlled by a programmable logic controller (PLC) and protection relay unit that controls the operation of the OEC as an independent part of the station.
The control system, when started, automatically accelerates the generator to the synchronous speed and then synchronises the generator to the bus. After being linked with the bus, the control system monitors and controls the operation of the OEC module. Pressures, temperatures, voltages, speed, kilowatts, kilovars, currents etc, are checked and monitored during each scan of the PLC and compared with preset values in the memory, to detect a warning or failure condition.
In a case of failure in the system, the unit is disconnected from the bus and shut down in a preprogrammed procedure.
3.1.8 Control and Monitoring System
The central station control system governs the power plant and steamfield operation under all operating conditions. The system controls and monitors the start-up procedure, normal operation, normal and emergency shut-off, protection, alarms and other functions. The controls provide for fully automatic operation including start-up of the OEC units.
The central station control system will control and monitor the steamfield, as part of the power plant, to allow start up, operating and shut down.
The central station control system is based on programmable controller (PLC) which can accept logic and analog signals coming from all elements of the station, process them according to a dedicated program (software) and send, as an output, logic or analog signals to control the equipment station.
The central station control includes a modem to enable remote monitoring of the station.
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