Geothermal - How it works

Geothermal Energy

Geothermal energy is energy generated from deep inside the earth’s core. We can use the steam and hot water produced inside the earth to generate electricity, to heat buildings, to help operate commercial industry and for leisure and therapeutic pools.

Geothermal energy is the only form of renewable electricity generation in New Zealand that doesn’t depend on the weather; our hydro dams need rain, wind farms need wind, but geothermal is always “on”. This provides reliable base load electricity 24 hours a day, seven days a week.

New Zealand is a pioneer in harnessing geothermal heat for conversion into electricity. In 1958 when Wairakei Power Station was commissioned, New Zealand became only the second country in the world to use geothermal energy for electricity. Lardarello, in Italy was first in 1904.

Perhaps it is not surprising that we are at the forefront of geothermal electricity generation: the Taupō region is one of the world’s most active and spectacular geothermal resources. Since Wairakei, many geothermal power stations have been developed in the volcanic regions around the world, most of them using technology developed here in Taupō, New Zealand.

Harnessing geothermal energy is among the world’s cleanest techniques for producing electricity, and advancements in geothermal technology are improving those processes all the time.

How does it work?

Photo courtesy of Contact.

Turning steam into power – the principle of making electricity from geothermal energy is simple!

Hot fluid from deep underground is discharged from a geothermal well, and it comes out either as pure steam or a mix of water and steam. It is the energy in the steam, piped under pressure to the power station , that is used to turn the turbine that’s generates the electricity.

The principle may be simple but getting there is not. Building, commissioning and operating a geothermal power station requires technical expertise from an extraordinary range of experts from many different disciplines.

Geothermal technology is a fascinating mix of sciences and engineering including geology, chemistry, physics, hydrology, maths, metallurgy, mechanical, electrical, drilling, civil and process engineering – and more. Following are some of the key steps along the way and who is involved.

What’s down there?

Way before you can start generating electricity you need to understand the geothermal resource in your location. It is not just one big “hot zone” under our feet, but a series of “fields” that have edges and limits.

Enter the geothermal and geological scientists and the reservoir engineers. These specialists “explore” the geothermal resource. They study, monitor and analyse things such as subsurface water and features, dissolved chemicals, rock temperatures, fault lines and more. They look for fractured zones and structures in the earth that act as channels for the upward flow of hot water and steam.

By doing this they build up a map of how big the resource is, how far it extends, and what sort of electricity generation can by supported.

Now that we know more or less what’s down there, we need to get down to it. It’s the over to the drillers.

Drilling

A typical geothermal well in New Zealand generally takes 30 to 50 days from start to finish. A drill may penetrate as little as one meter or as much as 30 – 40m an hour depending on the hardness of the rock.

Back in the early days , 1940s – 60s when geothermal drilling and generation was a new technology, very little was known about the reservoir and how to use it sustainably. There used to be lots of shallow wells, trying to get out as much heat as possible.

Today with so much more known about the science and make up of the geothermal reservoirs the wells are planned and drilled with sustainability in mind.

Production wells are drilled between 1000m and 3000m with the average depth closer to around 1500m deep.

The drilling rigs used on the Wairakei-Tauhara geothermal system are of the rotary type. At the tip of the drill is a Polycrystalline Diamond Compact (PDC) bit which uses synthetic diamonds to scrape away the rock as it turns.

Contact has had two drilling rigs working non-stop since 2005 on the Wairakei-Tauhara field and early in 2012 a major milestone of more than 100,000m had been drilled during the non-stop drilling programme.

Currently on the Wairakei-Tauhara fields there are more than 70 production wells, 15 reinjection wells and a number of monitoring wells.

With the wells drilled and geothermal fluid and steam flowing, now its time to turn it into electricity.

Generation

There are two types of production wells – those that produce steam and those that produce a mix of steam and water. Steam wells connect directly into a pipe network that feeds the steam at high speed to the power station turbines. The wells that produce the mix first feed the steam and water (known as two phase fluid) to equipment that separates the steam and water.

The technology for separating steam/water mixture did not exist before Wairakei. The separation process was developed and perfected by the Kiwi engineers in the 1950s and has since been widely used around the world.

From the wellhead the piped steam/water mix enters the separator where where it spins and the heavier water spins near the bottom of the separator and the lighter steam collects and rises to the top where it is piped to the station for generation.

Some of the hot water goes through another process in a flash plant where the pressure is reduced causing more steam to boil off before it is piped to the station.

In the power station the steam is sent to the steam turbine at high speed. It forces its way through several rows of turbine blades causing the shaft to spin at 3,000rpm. The turbine shaft is attached to the electrical generator and the rotor is spun by the energy from the turbine.

Now that the geothermal fluid and steam has done its work, what do we do with it? These days, we put it back where it came from.

Reinjection

Sometimes, geothermal fluid can contain minerals and chemicals from the earth that are best put back where they came from. Putting this fluid back also helps to sustain the resource, to ensure that we can continue to use the resource in a renewable way for generations to come.

Over the past 20 years engineers have trialled, developed and built systems to reinject geothermal water back into the ground. Adding small quantities of chemical tracers allows the reservoir team to track the fluid deep below the surface. This allows the cooler geothermal fluid to be disposed of in areas that it will not cool the hot production zones. It also helps to maintain underground pressures.

Managing environmental Effects

Construction of the new bioreactor at Wairakei. Photo courtesy of Contact.

The challenge for the geothermal industry, as well as all industry, is how best to manage environmental effects, safely dispose of waste products and ensure the ongoing sustainability of the geothermal resource.

Geothermal operators, technicians and environmental advisors monitor the chemistry of the reservoir, ground water levels, odour noise and discharges from geothermal operations to ensure the operation remains within its permitted limits.

Direct Heat Use

The use of geothermal resource for direct heat as well as electricity generation is providing value for energy intensive industry in the region.

Contact installed a new 27MW heat plant that supplies direct geothermal energy to timber-drying kilns used at the Tenon wood processing plant. Natural gas was previously used for the kilns and the change to geothermal resulted in a reduction of more than 25,000 tonnes of CO2 in the first 12 months of operation.

Other direct head use include the Wairakei Terraces which used the silica enriched geothermal fluid piped from the steamfield to create its silica terraces that look similar to the pink and white terraces destroyed in the Tarawera eruption of 1886.

Another direct heat user is the Huka Prawn Park, an aquaculture/tourism/restaurant that uses geothermal head to commercially breed and grow tropical giant Malaysian River Prawns.

More information on geothermal and how it works can be found in Power from the Earth on the Contact website.