July 29, 2020

Team Profile:
What is Supercritical?

photo credit:
Geothermal: The Next Generation

We throw around a lot of technical terms when we talk about our scientific research. Here are some explanations for a few key concepts to help you navigate the world of supercritical geothermal energy resources.

What is a supercritical fluid?

Commonly observable “states” of matter in everyday life with which we are familiar are solid, liquid and gas. For water, these states are ice, liquid water and steam. However other states of matter also exist, and some of these only exist under extreme conditions.

Supercritical fluid is one of these “other states” where the fluid is neither a liquid nor a gas, has properties between these two states, which become more liquid-like or more gas-like depending on the pressure and temperature conditions of the fluid.

What conditions create a supercritical fluid?

The saturation line is the line on the phase diagram which separates distinct liquid and vapour phases. The saturation line is also known as the ‘boiling curve’.

The critical point is found at the end point of the saturation line, where separate liquid and vapour phases cease to co-exist. At this point the substance becomes a single phase supercritical fluid.

The image below is a phase diagram for pure water. This image shows the liquid, gas (vapour) and supercritical states as a function of pressure and temperature. The blue line is the boiling curve (or saturation line): above the blue boiling point line, water is in the gaseous state (vapour), below the blue line water is in the liquid state…until the critical point is reached.

For pure water, the critical point means a temperature of 373.9°C and a pressure of 22.06 MPa.  At conditions of higher temperature or pressure the fluid is in the supercritical state.  

Pure water phase diagram. The area in yellow defines supercritical for the GNG programme.

In reality, a supercritical fluid will not be pure, and will contain impurities and other dissolved materials. These will influence the critical point, and change the supercritical fluids physical and chemical properties.

Why supercritical fluids?  

Supercritical geothermal fluids offer a possible source of “unconventional” geothermal fluid and heat energy. The GNG programme is exploring how supercritical geothermal resources might play a role in New Zealand’s energy sector in a future low carbon world.

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