Hot water systems are the most common geothermal systems suitable for power generation. In a hot water system, both the liquid and steam phases appear at the wellhead because the water flashes as it ascends the wells. The dryness fraction at the wellheads is normally less than 0.3. Depending on the wellhead pressures, the power cycle selected can be a singlepressure steam system (Fig. 3) or a dual-pressure one (Fig. 4). It is common to flash the water phase to produce more steam in a low-pressure flasher or separator. In practice, there is no real distinction between a flasher and a separator. Both are pressure
FIGURE 5 Power cycle for hot water or two-phase geothermal systems with binary ORC. B, barometric leg; C, condenser; G, generator; H, hot well; HX, heat exchanger; OT, organic vapor turbine; P, pump; PW, production well; RW, reinjection well; S, separator; ST, steam turbine.
vessels, and both have the function of separating the liquid from the steam phases. The flashing is normally done by a throttling orifice or valve installed upstream of the flash vessel. However, to maintain the large volume of low-pressure flashed steam, a flasher is normally larger than a high – pressure separator. Wairakei originally had a three – pressure steam system, but it was down-rated to a two-pressure system in 1983 because the steamfield drawdown could not sustain the high-pressure steam.
Nowadays, it is common to use the separated water for an ORC (Fig. 5). Exhaust steam from a back pressure turbine can also be used for a separate ORC (Fig. 6). For a hot water system that has a low dryness fraction, high gas content, or corrosive fluids, the steam Rankine cycle is unsuitable. A total flow or binary cycle (ORC, KCS, or TFC) may be used.
In a total flow cycle, the entire geothermal fluids from the wells are fed to a total flow machine that is robust enough to accept two-phase fluids (Fig. 7). The materials of construction can be selected to withstand the corrosive fluids. The helical screw expander is the first total flow machine to be developed and is now also used to generate electricity from waste heat in other industries. Other total flow machines are the biphase turbine and the Robertson engine.
In an ORC, the geothermal fluids are used to vaporize a low-boiling point (30-40°C) fluid, such as isopentane or isobutane, to very high pressure (15-20 bars), and this fluid is then used to provide the motive force for a vapor turbine. The advantage
FIGURE 6 Power cycle for hot water systems with back pressure steam turbine (BPST) and binary ORC. See Fig. 5 caption for abbreviations.
FIGURE 7 Total flow power cycle for hot water geothermal systems. G, generator; HSE, helical screw expander; PW, production well; RW, reinjection well.
of the binary ORC is that geothermal fluids with a temperature as low as 80°C can be used to generate electricity (Fig. 8).
A Kalina power cycle makes use of a mixture of two fluids in the secondary loop of the power cycle. It is considered a family of power cycle systems because not only can the ratio of the mixture of fluids vary with the temperature of the heat source, but different fluids also can be used for the mixture. An ammonia/ water mixture appears to be the most suitable for geothermal applications. A pilot Kalina cycle geothermal power plant was commissioned in Iceland in 2000 and tested successfully using a geothermal fluid temperature of 121 °C.
TFC, developed in 1993, is the latest binary power cycle. The concept of TFC is to improve the heat transfer process between the geothermal fluid and the secondary working fluid by keeping the working fluid
FIGURE 8 Binary ORC power cycle for hot water geothermal systems. See Fig. 5 caption for abbreviations.
as pressurized liquid, which is then used to power a helical screw expander. The best working fluid is still being researched, but low hydrocarbons appear to be most suitable.
HDR geothermal technology is still in the research stage. The concept is to drill two wells into hot rocks at a depth of 5 to 10 km, where the temperature gradient is high (70-90°C/km). Cold water is pumped into one well and pressurized to cause fractures in the hot rocks so that pressurized hot water flows up the other well. This is equivalent to an artificial liquid-dominated geothermal system. Power cycles suitable for natural hot water systems would be suitable for the HDR system.
A geopressured system of 1000 bar abs and 100°C is equivalent to an average hot water system in its ability to produce electricity. Because this is still at an early research stage, no power cycles have been proposed for a geopressured system, although the total flow power cycle using a helical screw expander appears to be most suitable.