Just over 20,000 hectares of pine forest would be enough to provide New Zealand with the fuel reserves needed to fire a power station over a dry hydro year.
Energy industry consultant Dr Lindsay Robertson has outlined the assorted options for providing dry-year base load electricity cover that have been considered by the Ministry for Business Innovation and Employment (MBIE).
The options have become even more pertinent now the new government has decided to can the proposed Lake Onslow hydro battery project, estimated to come with a $15 billion-plus price tag.
At present Genesis’s Huntly power station is NZ’s main base load supply, but it is under pressure to reduce its coal-fired emissions, accounting for about 4 million tonnes of CO2 a year, about 5% of NZ’s total carbon release.
In a webinar highlighting the potential of bioenergy as one of the remaining preferred base load options, Robertson said MBIE had very specific and stringent requirements for alternative base load supply proposals. They need to be sustainable, fully renewable and fully dry-year focused in application.
Alongside bioenergy sources, the other two options are geothermal and hydrogen generation.
In terms of capacity, bioenergy has the greatest potential, capable of providing between one and four terra-watt hours (TWh) of power.
As a supply to generate that sort of energy, “K” grade debarked logs currently sold overseas represent the most energy dense source of supply.
These could be converted to pellets and fed into a typical Rankin-type power generator or converted to a liquid biofuel option to feed a gas turbine plant.
“For large-scale fuel source, we decided at an early stage we were talking log wood and not much else,” he said.
One of the key fuel options is turning the logs into torrefied pellets that can be stored outdoors. Such pellets were trialled by Genesis over a week in early 2023 in the Huntly power station, and were hailed as a successful replacement for coal.
The pellets offer similar energy value to coal and produce less than 10% of coal’s emissions.
Robertson cited several benefits of pine logs, including being a relatively “clean” fuel source, having open market access for acquisition, and their ability to stockpile the requisite 1TWh of energy in the event of a dry year.
Estimates were a forest area of about 22,000ha would provide sufficient volume of timber to be sustainably harvested to provide the base load dry-year energy required.
NZ has about 1.8 million hectares in exotic tree plantations. The Bay of Plenty-Waikato area alone can supply 1.7 million to 1.8 million tonnes of K grade logs a year.
But Robertson also cautioned about the energy required to produce the fuel pellets, which could become significant economically.
“We also need to ask, ‘Where is the energy for processing coming from?’”
But he said a pine log supply is well aligned with NZ’s needs in that the tree grows so well here, often close to major demand areas, and a shift to this supply would use existing forestry infrastructure.
With a sustainable fuel Huntly power station remains a good solution to the base load issue, with potential to push its lifespan out for another 15 years if that fuel can be sourced.
Next steps would be to identify possible locations for growing and processing pine-based biofuel and confirm transport options.
Robertson said such a scheme could provide long-term employment advantages and is proven to be competitive with pumped hydro.
