At a depth of 6000km, the temperature in the innermost core of the Earth has been predicted to be over 6000°C, roughly the same temperature as the surface of the sun.
At the edge of the earths crust (at 35km), temperatures are still a staggering 2000°c.
This has made drilling to depths over 12km incredibly hard as drill bits have to cope with temperatures above 200°c.
Geothermal power generation takes advantage of temperature at depth by using the heat from the earths core to heat liquid and bring it back to the surface.
In volcanic regions such as the ring of fire around the pacific basin and Iceland’s location on the mid atlantic ridge, heat sources involving magma trapped in permeable rock exist relatively close to the surface. This makes geothermal power in areas such as Iceland a key renewable resource.
As Australia is not a volcanic region, sources are at lower depths and produce heat through natural intrinsic radiogenic measures.
This process heats up the rock and where the rock is saturated, creates a heat reservoir. This usually occurs at depth below regions where hydrocarbons are generally found.
This provides a massive, environmentally friendly heat resource.
DEPTH AND TEMPERATURES | EARTH’S CORE
Traditional Geothermal Systems
The traditional twin well method of extracting heat, which is then used to generate electricity, is outlined in the picture opposite.
In reality, most re-injection wells are drilled much shallower and re-injection occurs in shallower formations. Here, we only consider the non volcanic geothermal geologies we would expect to see in Australia.
Using the conventional twin well system, pumps are installed in production wells and the geothermal groundwater (often brine) is drawn to the surface and kept under pressure.
This superheated water is passed through a heat exchanger and most of the heat (thermal energy) is extracted for electric power generation.
Due to the corrosive nature of the geothermal water, the piping and heat exchangers of the twin well system have a very short life span.
Closed Loop Geothermal Systems
GWE propose closed loop geothermal system that circulates non-corrosive fluid (the working fluid) down a fully lined and isolated well by thermal syphoning effect, to be heated by the ground heat source (the geothermal heat reservoir).
The well systems are classified as a coaxial design, closed loop downhole heat exchanger to allow maximum heat transfer from the geothermal heat reservoir to the cooler descending working fluid.
With 400°C to 700°C granite temperatures easily accessible with the GWE patented drilling system, thermal energy delivered at pressure from the wellheads will have a temperature of 250 to 300°C after being pushed by thermal syphoning effect from the bottom of the wells.
The surface flow and temperature will be sufficient for the use of direct flash steam turbines for the generation of electricity and this is our preferred option over ORC power generation systems that are less efficient.
Efficiently utilising excess heat
A large portion of established geothermal power plants use an air-cooling system to cool and condense (re-liquify) the steam or vapor after it is exhausted from the generator turbines.
This cooling system normally consists of a large number of radiators where the binary fluid is cooled by electric fans. Much the same as a car fan cools the hot water generated from an engine.
This cooling process uses from 15% up to 40% of the electricity generated and is normally the largest footprint and most noticeable component of a geothermal power plant.
The efficiency of the cooling system is dependent on the ambient air temperature becoming far less efficient in hot climates.
GWE proposes that rather than using energy to cool the steam from the turbines, we use the steam energy to power compressors and pumps and to produce other products such as fresh water through desalination.
Multi-effect distillation (MED) as a means to desalinate sea water requires heat. By linking the power generation system to the MED system we have the required heat source for the MED and the required heat extraction to cool the turbine output vapour.
This results in a highly efficient use of available heat energy. After desalination of seawater has used up the excess heat from the power plant, the cooled fluid is returned to the geothermal well to be re-heated.
A “farm” of single wells can be used to produce large scale, constant and renewable electric power.