Pumped-well geothermal projects utilise the moderate temperature range of geothermal fluids between 100-190 C. Below this temperature, geothermal fluids are generally used for direct heating and above this the fluids are produced as steam and water with the steam providing the motive force in power generation. Pumped-well power projects maintain the geothermal fluid in single liquid phase throughout the production, power cycle and re-injection processes.
Open this Infographic for illustrations and a global overview.
Pumped-well geothermal projects utilise “binary” power plant technology which consists of Organic Rankine Cycle (ORC) heat exchangers. The geothermal fluid is pumped from the wells through the ORC units and re-injected back into the subsurface reservoir. The working fluids used in the heat exchangers are re-cycled and do not come in contact with the geothermal fluids. Binary units are also sometimes used to produce power from the separated water of high temperature geothermal resources.
This article is the first of a series of three installments that will cover a range of topics including a global overview of operating pumped-well projects and subsurface characteristics (this article), pump hydraulics and energy conversion (second in the series), and economics and emerging technologies.
Global overview of operating projects
Pumped-well geothermal power projects are primarily located in the Western U.S., but growth of these type of projects has occurred in recent years to include Europe, Turkey, and Honduras.
In this overview of operating pumped-well projects, installed capacities are given in Megawatts electric (MWe), sometimes referred to as nameplate capacity. Pumped-well projects consume 20-25% of their gross electrical generation for own use (mostly for the pumps). Net generation delivered to the power grid is also determined by on-line availability, which varies from 70-90% for most of these types of projects. Generation data is stated in Gigawatt hours (GWh), net delivered to the grid (if available).
Nevada and the Western U.S.
The state of Nevada, in the western U.S., has the largest concentration of geothermal pumped-well power projects in the world, with roughly 720 MWe of installed capacity, spread over some 15 fields. The author estimates approximately 3,400 GWh were delivered to the grid from pumped-well geothermal projects in Nevada in 2021.
Nevada contains the largest pumped-well project in the world – McGuiness Hills. As of 2021, the installed capacity at McGuiness Hills is estimated between 150-170 MWe (gross).
The Steamboat geothermal complex in Nevada is second in size with 80-150 MWe installed capacity (sources vary). Other major pumped-well projects in Nevada include Don Campbell, Blue Mountain, and Patua – each with over 40 MWe of installed capacity. Most of the Nevada projects produce fluids in the range of 120-180C, however Nevada also contains the lowest reported resource temperature (100C at Wabuska) of any pumped-well geothermal power project worldwide.
California
After Nevada, California has the second largest output for pumped-well geothermal projects with roughly 160 MWe of operating installed capacity. The author estimates approximately 800 GWh (net) were generated from pumped-well projects in California in 2021.
At one time the Ormesa complex in Southern California with four power plants and 80 MWe of installed capacity was the largest pumped-well geothermal project in the world (the adjacent East Mesa field utilises flash plant technology). In recent years the Ormesa fields have been reduced to 36-48 MWe capacity (sources vary) due to cooling in the reservoir as a result of both reinjection of the produced fluids and incursion of cooler water from surrounding aquifers.
The other major pumped-well projects in California are Heber and Mammoth. Heber, located near the border of the U.S. and Mexico, has both a flash plant facility and a pumped-well project. The pumped-well portion of Heber (known as SIGC) had an original design criteria of 34 MWe, and subsequent updates on the Heber project indicate that the project has performed as originally designed over a period of 30 years.
The Mammoth pumped-well project is located in the Eastern Sierra mountain region of California and consists of approximately 65 MWe (gross) installed capacity, including a recent 30 MWe expansion reported in 2022.
Other pumped-well projects in the western U.S. include Cove Fort, Utah (reported as 25 MWe installed capacity), Raft River, Idaho (10 MWe), Lighting Dock, New Mexico (14 MWe), and Neal Hot Springs, Oregon (8 MWe).
Germany
Most operating pumped-well geothermal power projects in Europe are concentrated in the province of Bavaria, Germany. This area has been developed for both power and direct heat geothermal projects, with individual power projects being relatively small. Public domain information indicates eight power projects in the Bavaria region between 3 to 5.5 MWe (gross) installed capacity each, and two projects in the Rhine Graben region of similar size – totaling 47 MWe of installed capacity in Germany as of 2021.
Although there is significant geothermal district heating activity in Europe, there are a limited number of geothermal power projects outside Germany. Exceptions are Velika Ciglena in Croatia (reported as 17 MWe) and Tura in Hungary (reported as 3 MWe).
Turkey
Turkey has approximately 300 MWe installed capacity for moderate and high temperature geothermal fields, many that utilise binary power plants. However, the author could only identify one operating project that uses pumped wells (the 15 MWe Buharkent project). The effect of high CO2 content on pumping operations for most of the geothermal fields in Turkey has precluded the use of pumping in that region.
Honduras
Honduras has one operating geothermal field, the Platanares pumped-well project, reported as commissioned in 2017 at 35 MWe (gross) capacity.
Subsurface characteristics
Pumped-well geothermal projects can occur in a wide range of subsurface geological environments, including sandstone, volcanic, crystalline basement, and limestone. The common subsurface characteristic that enables these systems to be productive is very high permeability, generally associated with faulting and fracture zones, although sufficiently high permeability can also be present in sandstone and dolomitised limestone.
Basin and Range Province
The prolific Basin and Range geothermal province in Nevada is attributed to the high heat flow and extensional tectonic setting of the region. Most of the geothermal activity in the region occurs along faults and cannot be attributed to any recent shallow magma sources. Extensional faulting is required to generate productive geothermal systems, with the fault zones enabling both the upflow of hot fluids and the high permeability characteristics of these systems.
Much of the geothermal production in Nevada comes from Miocene age volcanic or sedimentary rocks, although production ranges from Pliocene alluvium to Mesozoic crystalline basement. In general, the geothermal production zones are between 500-1500 meters in depth.
Hot Sedimentary Basin – Southern California
The Ormesa and Heber pumped-well geothermal fields both occur in the Salton Trough, a prominent continental rift basin that straddles Southern California and Northern Mexico. This prolific geothermal area also features the high temperature Salton Sea and Cerro Prieto geothermal fields.
The Salton Trough is a Pliocene age sediment-filled basin that contains high poro-perm sandstones in the regions of Ormesa and Heber. These sandstones are capable of supporting the high flow rates typical of pumped geothermal wells. Though faulting is known in these areas, porosities of 20-30% and permeability-thickness of 10-60 Darcy-meters (D-m) are characteristic of the sandstone matrix in these fields.
Mammoth – California
The Mammoth geothermal project consists of pumped wells completed in recent volcanic formations of the Bishop Tuff and overlying lavas. These formations are very recent (< 1 million years) and are associated with resurgent dome activity within the Long Valley Caldera.
Molasse Basin – Germany
The Molasse Basin is located in Southern Germany and contains the Jurassic age Malm limestone, overlain by Oligocene and Miocene sediments. The Malm limestone is the primary target of the geothermal pumped-wells projects in this region, and can often contain fracture or dolomitised zones that enhance permeability.
Rhine Graben – Germany
The Rhine graben geothermal area is located on western border of Germany with France. The primary target of the Rhine Graben pumped-well projects is the Triassic Bundenstein Formation, which consists of interbedded sandstones and claystones with permeability controlled by faulting.
Geothermal wells for power production in the Molasse Basin and Rhine Graben are generally drilled to depths between 3,500-4,500 meters, significantly deeper than those of pumped-well projects in the western U.S., Turkey or Honduras.
Productivity and Permeability
Individual pumped geothermal wells produce on the order of 1,500-2,500 gallons per minute (GPM) – equivalent to 90-150 liters per second (L/s). The productivity of a pumped well is often expressed as a volumetric index, usually in GPM/psi (or L/s-bar). By application of the diffusivity flow equation, the productivity index (PI) can be related to the permeability-thickness (kh) of the formation that the pumped well is completed in. A simplified but useful conversion of PI to kh can be derived and written as:
Where is the fluid viscosity in centipoise (cp). For example, for a PI of 10 GPM/psi and a viscosity of 0.17 cp (for 160C fluid), the estimated kh is about 20 D-m. Volumetric PI values for pumped wells are typically in the range of 5-30 GPM/psi. This is equivalent to kh in the range of 10-60 D-m, although wells producing from fault zones can have much higher kh and PI (e.g., McGuiness Hills and Mammoth).
These concepts of kh and productivity will play an important role in the exciting world of pump hydraulics and energy conversion … coming soon in the second of this series on pumped geothermal wells!
Elliot Yearsley – enyearsley@gmail.com
Selected references
The author draws on his own experience and public data bases including Nevada Bureau of Mines and the California Energy Commission. Other selected references include:
- Hulen, J.B.: Geology and Conceptual Model of the Silver Peak Geothermal Prospect, Esmerelda County, Nevada. A Report for Sierra Geothermal Power (2008).
- Nordquist, J., Delwiche B: The McGuiness Hills Geothermal Project. GRC Transactions (2013).
- Weber, J., et al.: Geothermal Energy Use in Germany, Country Update 2019-2021. European Geothermal Congress (2022).
- Yearsley, E.N.: Common Characteristics of Pumped Wells in Geothermal Power Projects. Proc. of 41st New Zealand Geothermal Workshop (2019).
