The DOE published a feasibility study on harnessing geopressure to produce electricity and found no commercial off-the-shelf equipment available to meet the need. The market situation remains the same 20 years later despite attempts of various traditional designs for the technical reasons listed in Fig.1.
Figure 1: Comparison Chart
The Pelton wheel was recommended by the DOE as the most promising solution. However the study cited issues not only with shaft seals, but with the stress limitations of the buckets themselves. Maximum operating pressure was restricted to 2,000 psi. Higher pressure accelerated geofluid to velocities that ripped the buckets off the wheel. The design is further problematic due to excessive wear sustained by the buckets from particles suspended in the brine -a scenario similar to waterjet cutting.
Figure 2: Pelton Wheel
Twin screw expanders are also a poor choice for reasons delineated under the Geothermal Technical tab. Specific to geopressure, the twin screw machine requires shaft seals capable of withstanding extreme fluid pressure to prevent bearing contamination. Shaft seal ratings peak at 1000psi, while typical well pressures exceed 4,000psi. As a result, only a fraction of the potential hydraulic energy can be harnessed by a twin screw device. In addition to shaft seal problems, the outer casing (housing) must be sufficiently strong to contain the high radial forces caused by fluid pressure in the external rotor working cavity.
Figure 3: Twin Screw Cross-section
Piston-cylinder reciprocating designs would appear as good candidates for a number of reasons. First, they are very robust and easily handle the range of pressures under consideration. Second, they are fairly efficient with little leakage across the piston rings. Third, geofluid pressure is not exposed to the shaft seal making it a non-issue. And lastly, they can be directly coupled to a generator, keeping costs down and the overall design simple.
Figure 4: Piston Cylinder Design
Unfortunately there are drawbacks that prevent it from becoming a successful solution. All reciprocating designs require lubrication of the cylinder wall, piston rings, connecting rod, and crankshaft journal bearings. Without lubrication the device eventually seizes, especially if corrosive brine is present in the cylinder head, block, or crankcase. Scale build-up presents another issue. Over time, scale forms at the top of the piston, decreasing the amount of dead head volume, a particular concern for piston-cylinder machines operating on liquids verses gases such as air or steam.
The intake and exhaust valves also present difficulties by introducing water hammer as a consequence of their hard open/close function. The valves must perform this cycle continuously in a corrosive environment while withstanding the opposing forces exerted by the fluid pressure. Lastly, the flow-rate for a given size machine is quite low relative to rotary designs, one reason why gas turbines replaced reciprocating engines in the early 1900's for power generation. Large, low rpm reciprocating engines -with their numerous moving parts- can be replaced with much smaller, high flow-rate, rotary turbines with fewer moving parts and less maintenance requirements. Costs are further reduced by purchasing a generator of equivalent output yet smaller in size by running at the higher speeds achieved with rotary designs.
Helidyne (PRG) Pressure Reduction Generator
Helidyne's PRG using the tri-rotor format is unique in that it satisfies all the conditions necessary for a practical machine. The whole process occurs without exposing the shaft seals or housing to high pressure. Rotors are self-cleaning, require no lubrication and perform in a soft start/stop manner to reduce inertial impulse (water hammer).
A brief overview of the Helidyne cycle: High pressure fluid enters the rotor cavity without a mechanical valve mechanism. Geofluid immediately exerts force upon all three exposed rotor surfaces causing them to rotate and convert hydraulic energy to mechanical energy without high velocity fluid acceleration. The torque cycle follows a sinusoidal wave function making for a smooth on/off process. Geofluid is then exhausted while simultaneously receiving new fluid at the intake to begin the cycle anew. See concept animation below.
Figure 5: Tri-rotor animation
Torque is transmitted to a directly coupled generator via a proprietary drive train by a single output shaft. The rigid drive train is backlash free, essential for maintaining perfect rotor synchronization and constant running clearances of .002" under full load. Non-contact rotor meshing permits lubrication free or "open" operation. Any build up of scale on the rotor surface is swept away by the adjoining rotors. In fact, scaling is preferred to further decrease the inter-rotor gap and fill-in any pits or grooves that may develop over long periods of operation. To some extent, scaling enables self healing and reduction of leakage, improving efficiency and rotor longevity.
Figure 6: Rotor Sweeping Action
Because the design is rotary it has an inherently high flow-rate, with 6 flow cycles per revolution on a dual rotor configuration. It is also ideally suited to driving a conventional AC generator at speeds of 3600 or 1800 rpm. Fixed generator speed matches the grid frequency of 60 Hz while avoiding the use of gear boxes or high power inverters.
The inherent volume ratio of 1:1 is perfectly matched to incompressible fluid handling in a pressure let down scenario. It keeps machine size small for a given flow rate and maintains liquid output pressure above saturation for downstream processing before re-injection. As usually occurs in many geopressure installations, the developer will extract methane (natural gas) and thermal energy that coexist with the hydraulic energy in the brine. With geothermal power production, maintaining brine in the liquid state is preferred for both ORC and TFC -see Geothermal Technical tab. The following chart portrays extraction of all three forms of energy.
Figure 7: Plant Diagram
Potential issues with the tri-rotor design include rotor wear from suspended particulates and corrosion. Filtering geofluids of sand or grit particles will extend rotor life. A silicon carbide coating or diamond composite layer reduces wear and increases rotor life. Such “hard facing” is well understood and can be used repeatedly to recondition worn rotors. Like all oil & gas and geothermal plant equipment and plumbing, Helidyne equipment will adopt appropriate materials and coatings to ensure durability and resistance to corrosion.
A third source of energy and revenue from geopressured wells is natural gas dissolved in the brine. The DOE estimates 4 trillion standard cubic feet of natural gas remains to be harvested in the abandoned wells of Texas. The gas is released as part of the pressure let down process. It can be extracted from a separator tank (see Fig. 7 above) downstream from the PRG and further captured from the condenser if geothermal power is generated.
Brine Well Injection
A concern is often raised over disposal of the brine following the production of electricity from hydraulic energy. A tremendous amount of water -an average of 25 billion barrels annually- is currently co-produced from oil & gas wells. The industry has been managing this issue for decades largely by pumping the now separated fluids back safely into the ground via an injection well. A typical abandoned oil field will have many such wells accessible for brine disposal.
Renewable geopressure power production will likely be a companion to geothermal energy in certain regions of the country as developers utilize the heat and pressure of abandoned wells. There are 37,000 possible sites for realizing this approach with a potential of generating 7,800 MW in Texas alone. But success in this new market will require innovative technologies like those offered by Helidyne.
DOE. (2010, July). Geothermal Energy Production with Co-produced and Geopressured Resources. Retrieved November 30, 2011, from NREL: www.nrel.gov/docs/fy10osti/47523.pdf
Thurston, G., & Plum, M. (1991). The Feasibility of Hydraulic Energy Recovery from Geopressure-Geothermal Resources. Idaho: U.S. Department of Energy.