Total-flow Geothermal Energy Technology
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Latest update:
20 February, 2012
What is Geothermal Energy?
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1) Dry steam power plants. |
Dry steam plants are the simplest and oldest design. These are the most favorable wells, like the geysers that Iceland is known for. They directly use geothermal steam of 150°C or more and at high pressure directly from the well, to turn turbines, but those have a typical efficiency of around 25% only. The reason is that all the steam has to be condensed after the turbine and because steam has a very high enthalpy (heat energy content), also at room temperature (ca 2500 kJ/kg), around 75% of the input heat has to be cooled off in the condenser, at an as low temperature as possible. The condenser is needed to create a very low pressure, way below atmospheric pressure, on the exhaust side of the turbine (typically between 0.1 and 0.05 bar). It is pressure ratio over the turbine that determines how much it gives, NOT pressure difference! |
Flash steam plants (as in the video above) pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines. They require fluid temperatures of at least 180°C, usually more. This is the most common type of plant in operation today.
Depending on the well's temperature and the flash pressure in the separation tank, between 10 and 20, at the most 30 mass% of the source brine flashes off to usable steam. Of course, this reduces the pressure ratio over the turbine and so it gives far less power output than a dry steam plant, thus larger (or more) turbines and more mass-flow from the well are needed for a given production capacity.
The efficiency, as calculated on the used steam, is of course the same as for all steam turbines, but calculated in percent of the specific heat energy content coming out of the well, far lower than for a dry steam plant - between 10 and 15 % only.
However, if one would superheat the steam, prior to entering the turbine, with heat from an external source (like fuel in a burner), the output would become much larger and also the overall efficiency would increase considerably. This is due to that the specific energy for superheating is much lower than that of evaporation, which came from geothermal heat, and so the major part of the energy converted in the turbine, still comes from the brine. As this is obvious, it appears that the "psychology of renewable energy" has a greater impact than economy and available technolgy - amazing in that case! For the same reason, heat pumps are not seen as renewable energy sources, though 75% of the output comes from ambient heat = solar energy!.
3) Binary cycle power plants
Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 57°C (these, below 100°C, are the most abundant ones, especially in the USA). The moderately 'hot' geothermal water is passed by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor, which then drives the turbines.
This is the most common type of geothermal electricity plant being built today. Both Organic Rankine and Kalina cycles are used. The thermal efficiency of this type plant is typically about 10-13%
Of course, also here aditional superheat would increase the efficiency considerably. In fact, using a heat pump for that would do far better, extracting between two and three times more energy from the brine than to drive the heat pump with (from the generator output - no fuels needed), but we want "renewable energy"... so?
Give me an assignment and I will work it out for you!
The Total Flow Energy Converter - better than nuclear power!
I found a method for a total flow energy converter that gives a much higher efficiency for geothermal heat conversion to electricity, than the direct use of steam. Instead it uses the hot water (or the source steam turned into hot water), to partially flash off inside an expander.
The result is that only a minor fraction of the water turns to steam that has to be condensed, taking only ca 0.5% of the power output to waste. In this way, 40% or more of the source well's specific enthalpy (ideally 60%) can be converted to mechanical power on the shaft, thus driving an electrical generator. Applied on a global scale, this would give more than nuclear power today (forget solar and wind... waste of money!).
If the temperature of the source well is higher than 100°C, expansion can be done in two stages (not shown here), where the atmosphere does work in the second stage, increasing the output with between 20 and 40%. The exhaust gives 100 °C brine, which will be fed to a low-temp converter. The total efficiency on the brine alone would be around 80% then, plus the atmospheric work added, giving a COP of one or larger (depending on machine-efficiency)!
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