"By introducing more geothermal into the grid it is going to take a much longer period to get the emissions out of the New Zealand energy sector."
The geothermal industry sees itself in the vanguard of decarbonising the energy sector, but NZ Geothermal Association president Paul Siratovich says geothermal fields would emit carbon dioxide anyway.
"One thing that we have to consider is that any geothermal field, whether you put a power station on it or if it lives on its own, emits carbon dioxide," says Siratovich.
"When we extract power from them we do bring up a little bit more CO2, but over the life of those geothermal fields we actually have modelling that shows it is more or less neutral from what it would do on its own."
He says large-scale surveys from Rotorua show a lot of natural CO2 comes from the ground.
At the Wairakei field, first harvested in the 1950s, carbon emissions have fallen over time, likely because a pocket of gas has vented.
The emissions gap narrows further between geothermal and hydro, solar and wind when whole-of-life emissions are taken into account instead of just operating emissions.
But two geothermal fields measure alarmingly high. The worst is Ohaaki power station north of Taupō, recording carbon emissions almost as high as some fossil fuel electricity plants.
Built in the 1980s, Contact Energy's Ohaaki power station is a landmark with its large cooling tower.
"I think it's best to look at geothermal as a spectrum rather than picking just the outliers," says Contact Energy head of geothermal generation John Clark.
"As an industry we are relatively low carbon emissions.
"We are not zero - we do acknowledge our operations release carbon.
"CO2 occurs naturally in the reservoir and our processes help release that.
"We are looking to how we can reduce our carbon footprint overtime with our emissions.
"We are looking to do a trial at our Te Huka station on the other side of Taupō this year, where we will be trying to reinject that CO2 back into the reservoir."
Clark says for the trial to be rolled out it must first assess the risk to the geothermal field.
"There are some challenges with the pressures you need to maintain during the process flow to keep the gases in the water stream - not exsolving out or bubbling out.
"We have got to make sure we are not corroding equipment too quickly, that we don't introduce scaling risks.
"Scaling risk is when these mineral deposits deposit-out underground and can then block the fractures that we're actually putting the fluid back into.
"We want to avoid things like the CO2 coming out of the fluid partly down the well, which can cause an air block.
"These are the kind of technical challenges we want to make sure that we address and are comfortable with, before we go to every single one of our stations."
He says the success of Contact's Wairakei field's transition to a new power station shows geothermal will continue to be an inter-generational provider of electricity.
"Geothermal is a great low-emission way to provide electricity into the homes of New Zealand."
"It provides baseline generation that is completely independent of weather.
"So while other renewables require rain, sunshine, the wind to be blowing - we are always there and we are always available to provide onto the generation network for NZ."
The increasing use of geothermal's leftover heat by surrounding industries further improves its green credentials.
A recent project has seen Contact Energy providing heat by pipe to wood pellet manufacturer Nature's Flame.
Thanks to the Emissions Trading Scheme there's also a financial incentive for the industry to become carbon neutral, but it is already full of geologists and engineers proud to be displacing fossil fuels worldwide.
Siratovich says there is even a chance for geothermal energy to become carbon negative.
"There's an awesome group within the industry who is looking at how to put those gases underground to abate those carbon emissions.
"If we look overseas to our colleagues in Iceland, they are actually capturing CO2, putting it in fluid, putting it underground and turning that CO2 into rock in human time scales.
"They've actually drilled wells where you can see that CO2 has actually locked itself away in the subsurface.
"We could do it here in New Zealand, it is just a matter of putting our brains into it and actually doing it."
But until geothermal energy has zero carbon emissions, Ecotricity won't be interested.
Ecotricity is majority-owned by Genesis Energy, which owns the coal and gas-burning Huntly power station.
Yates says as Ecotricity grows, Genesis is forced to add to its green-energy sources.
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