Geothermal Overview

Geothermal electric power production begins thousands of feet below the earth's surface in large underground reservoirs (Fig.1). Water in the reservoir is heated naturally by the earth's core and pumped through a well to a power plant on the surface. After thermal energy is extracted by turbines and converted to electricity, the cooled well water returns to the reservoir via a separate injection well to repeat the cycle again.


Figure 1: Geothermal Plant

There are many advantages of geothermal energy as an alternative to traditional fossil fuel and nuclear power plants. Foremost, geothermal is renewable and emission free, providing 5% of all renewable energy produced in the U.S. today (Fig. 2). And the “fuel” of geothermal energy is free. Also, unlike wind or solar technologies, it supplies continuous base load power regardless of whether the wind is blowing or the sun is shining.


Figure 2: Geothermal Contribution


Low Plant Efficiency:

Three challenges limit rapid growth and investment in this industry: 1) high drilling cost and risk associated with resource discovery; 2) low plant conversion efficiency; 3) high cost of plant equipment. Helidyne technology addresses the latter two.

Geothermal plant efficiencies are typically less than 20% due largely to the fundamental processes intrinsic to their operation, namely: “flashing” in a water-based flash plant; boiling in a refrigerant-based binary plant.


Flash Plant:

Pressurized dry steam driving the plant turbine is separated from well water in a flash tank (Fig. 3). The flashing process alone introduces an immediate 50% efficiency penalty due to dissipation of expansive energy in the flash tank rather than performing work in the turbine.

In addition, low well temperatures produce relatively low pressure steam that cannot be efficiently utilized by steam turbines, especially for well temperatures below 300°F.


Figure 3: Flash Plant Cycle


Binary Plant:

The industry solution to low temperature wells has been to replace water with a low boiling-point refrigerant to take advantage of higher pressure created by the refrigerant (organic) fluids at low temperature (Fig.4). This “binary” approach comprises essentially a large air conditioning unit operating in reverse.


Figure 4: UTC Pure Cycle

Instead of flashing well water directly, a binary system requires a heat exchanger to boil the refrigerant and produce vapor for the turbine (Fig.5). An advantage of this design is turbine isolation from the harsh well water chemistry. The two fluids never mix, being fully contained within their respective closed loop heat exchangers (Fig. 5).

The boiling process introduces the same degree of inefficiency as the flashing process discussed above, specifically a 50% loss of available thermal energy.

Although costly to implement, the binary approach nevertheless enables electric power production from geothermal temperatures too low for efficient utilization by a steam-driven flash plant.


Figure 5: Binary Plant


Helidyne TFC Solution:

The Helidyne positive displacement expander represents a novel, high flow, purely rotary design that mitigates the tremendous energy losses associated with flashing and boiling processes.

The Trilateral Flash Cycle (TFC) represents the ideal thermodynamic cycle for a 2-phase (liquid/vapor) system. Also known as the “total-flow” process, TFC is equivalent to the Carnot cycle of a 1-phase (gas) system.

High Volume-Ratio of the expander is critical for effectively implementing the total flow TFC process. Expansion ratios exceeding 200:1 are necessary to maximize conversion efficiency. Prior-art machines, notably the twin-screw expander, are inherently limited to ratios of about 5:1.

The unique Helidyne design exposes a single rotor cavity directly to the intake port once per cycle. A metered quantity of water is injected directly into the cavity. This technique achieves very high volume ratios (up to 500:1) for adequate steam expansion and efficient energy conversion.

The Helidyne expander permits isentropic flashing of pressurized hot well water within the expander cavity, thereby eliminating the tremendous losses associated with external flashing or boiling.

Implementing the total-flow TFC approach requires replacing the turbine with a positive displacement expander (Fig.6), a feat previously attempted but never commercialized due to inherent design limitations of prior-art devices, particularly the problems of excessive scaling and restricted expansion ratio.

According to extensive research conducted by experts in the field over a period spanning more than 35 years, the total-flow approach potentially doubles plant output by raising the current efficiencies of 20% to over 40%. In addition, the same researchers have concluded that a total-flow TFC process would significantly decrease plant capital cost.


Figure 6: Total-Flow Plant

The total-flow process is visualized conceptually in the animation below. The red shapes represent hot well water in the liquid state entering the expander intake. Blue shapes represent expander exhaust in the liquid/vapor (2-phase) state.

Scale Formation (Fig.7) accumulating on total-flow expanders can cause moving parts to bind and eventually seize, sometimes failing catastrophically. Turbine systems avoid this problem by separating steam from liquid water by flashing, essentially the distillation process described earlier.


Figure 7: Hard Water Scale

The sweeping action of four unidirectional rotors (Fig. 8) prevents scale deposits on rotor surfaces, a feature not possible with counter-rotating twin-screw rotors. Automatic scale removal assures continuous maintenance-free operation. Helidyne rotors are coated with a proprietary non-corrosive, hard carbide material that withstands exposure to the harshest operating conditions.


Figure 8: Rotor Sweeping Action


Summary:

Helidyne LLC offers a novel TFC expander, which resolves many of the issues historically plaguing total-flow expander technologies. Geothermal, solar thermal, and waste heat recovery may realize a doubling of electrical output relative to conventional binary or turbine systems.