Meeting peak demand

Two massive new energy projects are bringing energy consumers closer to optimal use of New Zealand’s renewable energy sources, while enhancing electricity generation flexibility and security. Jenny Baker reports.
Contact Energy recently opened a new 200 megawatt gas-fired peaker plant near Stratford. On the same occasion it also opened an underground storage facility for natural gas near Ahuroa.
Contact chief executive Dennis Barnes says the projects, at a combined cost of $420 million, are significant additions to Contact’s portfolio and important assets for both Contact Energy and the country. “The projects are also representative of Contact’s dedication and commitment to enhance New Zealand’s energy security in an increasingly volatile energy market,” he said during the opening ceremony.
Although gas storage facilities are common across the globe, Ahuroa is the first natural gas storage facility constructed in New Zealand and as such posed a significant geological, engineering, and construction challenge for the company. But, says Barnes, Contact was fortunate “to be able to draw on the expertise of our majority shareholder, Origin Energy, to develop Ahuroa from a concept to a working asset.”
A Contact Energy spokesperson explained to me the purpose of the gas storage and peaker plants and the challenging work that went into commissioning the two projects.

What is the purpose of peaker and gas storage plants?
There are two types of electricity plants, base load and peaker. Base load plants operate continuously, stopping only for maintenance or unexpected outages. They generate electricity from geothermal, hydro, and wind energy (renewable sources) and from energy released by burning fossil fuels such as oil, coal, or natural gas (thermal sources). They take a long time to start and reach full operational power.
The demand for electricity, however, varies widely, depending on time of day, weather and other factors. Peaker plants are generally run only when there is a high, or peak, demand for electricity. It ensures extra energy generation in times of high energy demand.
Thermal energy, in particular from burning natural gas, is the only energy source for a peaker plant.
The Stratford peaker plant is capable of going from a cold start to full-power and producing enough electricity to power 200,000 homes in ten minutes, offering outstanding flexibility in its ability to meet spikes in demand.
Gas production is a 24 hour enterprise, but demand for gas-fired base load and peaker electricity generation varies a lot, too, depending on how much electricity is being produced from renewable energy sources. A gas storage plant is a facility that provides storage of gas, so the gas can be injected into it during periods when it is not needed and withdrawn when it is.
The spokesperson says approximately 70 percent of New Zealand’s electricity is generated from renewable energy sources: “This means the majority of our electricity generation depends on the weather. This weather-dependent generation needs to be backed up by reliable electricity generation, such as thermal, which can be turned on when it is most needed.”
Peakers in New Zealand therefore work not only during peak daily and seasonal demand periods, but also fill in the gap between what renewables can produce and what the country needs at any moment in time. “It is this thermal generation that keeps the lights on if the weather refuses to play its part,” the spokesperson says.
In addition, meeting increases in demand with peaker plants means that larger combined cycle thermal plants can be used at a more constant, and therefore more efficient, rate to produce base load electricity. But even if the weather pulls its weight, renewable energy is abundant and demands for natural gas thermal power are lower, the Ahuroa storage plant now provides a way to better use natural gas.
“The Ahuroa gas storage facility bridges the gap between the way gas fields would like to produce and the way in which the energy market likes to use gas,” the spokesperson says. Gas can now be stored during off peak times such as summer and used during times when it is most needed, like winter, when demand peaks, or when fair weather impacts negatively on lake and wind levels.

Building the peaker plant
The Stratford peaker plant is situated three kilometres east of Stratford on the site of Contact’s former Stratford power station, adjacent to the company's existing Taranaki Combined Cycle (TCC) power station, and nine kilometres southwest of the Ahuroa gas storage plant.
The peaker is a 200MW gas-fired, fast-start power plant. The stars of the plant are its two US-made 100MW General Electric LMS100 gas turbines, the first of their type to be installed in Australasia. The gas turbines are conventional aero-derivative gas units but with three important enhancements – a low pressure compressor, an intercooler, and a power turbine – that work together to increase the efficiency and power output of the plant. The total combined output of the two power stations at this site (the Stratford peaker plant and TCC) is 580MW.
In March 2008, Contact signed a contract with United Group (NZ) Limited to design the plant, project manage, engineer and install the two peaking units, and procure and install the balance of plant required to complete the project. General Electric and Contact engineers played a strong role in many of the design and build aspects. In September 2008, the foundations and subsurface work of the decommissioned power station were demolished.
In January 2009 work began on driving 191 piles to an average depth of 11 metres. In June, the main foundations were poured by Fletcher Construction in an eight hour continuous pour with 180 concrete truck movements.
In May the two Brush generators arrived from the Czech Republic at Port Taranaki on the Tampa, at 67 thousand tonnes the largest vessel ever to visit the port. Other equipment started to arrive on site from various locations around the world, notably Kobelco gas compressors from Japan, and Pauwel transformers from Indonesia.
In July 2009, the gas turbines were positioned on their foundations. By June 2010, construction was completed and commissioning the plant began. In November the first unit was fired, and following a test period, the plant was opened on May 31 this year.

The plant is designed specifically to operate within local compliance limits with minimal impact on the local community, and Contact spent more than $4 million to ensure this outcome. The model cooling tower the Stratford station uses is a hybrid wet-dry design. Its operation minimises the formation of visible moisture plumes. A mixture of natural gas and high purity water are injected into the combustors. The combustion process produces the hot high pressure gases that drive the turbines. The addition of the high purity water reduces the formation of nitrous oxide emissions.
The power turbine that drives the generator is not mechanically connected to the other turbine shafts and can consequently deliver superior performance even should there be disturbances in the electricity grid.

Building the gas storage facility
The Ahuroa gas storage facility is a near depleted fossil fuel reservoir modified to receive gas for storage. Oil and gas were extracted from this field from 1995 until 2007. Contact Energy acquired rights to the field from American energy company, Swift Energy, in 2008 through the sale of their assets to Origin Energy.
It is in a body of porous sandstone rock sealed in impervious layers of clay stone. To accomplish the conversion, a large injection compressor, gas processing equipment, and four injection-extraction wells had to be installed.
Contact plans to store gas in it from a number of sources. The system is fully pressurised with sealed pipes. All four wells can be used for both injection and extraction, although Contact at present prefers to use two wells for injection and two for extraction. Isolation valves are installed at key points throughout the system so sections can be locked out if necessary.
Extracted gas flows from the reservoir via a treatment plant into the transmission network. The treatment plant is similar to that used for conventional oil and gas production plants and removes water and oil from
the gas.
Contact contracted out the drilling, design, project, and operational management work to Origin Energy. Work to reconfigure the plant for injection started in June 2008. In May 2009, the new 2,000HP V16 Walker Shaw quad-turbo injection compressor, capable of 230 barg maximum discharge pressure, arrived in Port Taranaki. By the end of September, installation was completed, and in October 2009 the new compressor was commissioned.
In November 2009, drilling of the three new injection-extraction wells started, By March 2010 the job was completed and in July the coiled tubing rig was set up and well testing operations commenced.
Transfield Worley Hawkins was engaged to engineer, procure and construct the gas extraction facility using a wide variety of subcontractors.
In June 2010, site works commenced for the installation of an extraction train that enabled gas to be extracted at up to 45 terajoules per day – enough to supply the two fast-start gas turbine peaking units at Stratford power station – in addition to the existing injection capacity.
In August 2010, the gas-to-gas exchanger and low temperature separator coalescer vessel were installed. Fitzroy Engineering from New Plymouth built the gas exchanger; Robert Stone from Auckland built the low temperature separator coalescer vessel, and ITL, also from New Plymouth, provided the design services.
By February 2011, the facility was fully operational, able to extract up to 45 terajoules/day of gas. The facility can be expanded with the addition of a second new injection- extraction train, and ultimately, could inject or extract up to 160 TJ a day if needed.
The Ahuroa project won the award for ‘Innovation in Petroleum’ at the 2011 Deloitte Energy Excellence Awards, held on August 17 in Auckland. The Deloitte Energy Excellence Awards provide an annual opportunity to recognise excellence and achievement across the electricity, oil, gas and petroleum industries.

Jenny Baker is an Auckland-based freelance writer.




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