Going Underground - Geothermal Energy

December 2006

Pterra Consulting

Before the recent renewed interest in renewables, power systems already had two mainstays in the renewable category - hydro and geothermal.  So after taking a look at solar and wind in the previous two articles on Renewables. we now review, geothermally produced bulk electric power with our specific interest in transmission impacts.  

The world has about 8 GW of installed geothermal capacity.  Much of this is substantially derated due to the inherent difficulties with harnessing this energy source, including corrosive steam and shifting thermal pockets.  The geothermal plants built in the 60s and 70s were rated for 10 year life spans.  These were typically steam turbines operating on dry steam collected out of fractures in the ground or on flashed steam (take hot water, usually at temperatures over 350°F, out of the ground, and allow it to boil as it rises to the surface then separate the steam).  Like solar and wind, the term "field" applies to geothermal since the steam is harvested over a field and brought to collectors for conversion to electricity. 

Newer designs allow for harvesting geothermal energy:

• At higher efficiencies using combined cycle technology.  A third of the high temperature steam is used to drive a steam turbine, while the remainder is used to vaporize a secondary fluid which is then used to drive a second turbine. 
• At lower temperatures using binary methods.  In these "binary" systems, the low-temperature geothermal steam vaporizes a secondary fluid that is then used to drive a low-speed turbine. 

These newer designs permit geothermal plants to be implemented over a wider range of geothermal sources and fields. 

The largest dry steam field in the world is the Geysers in California with an installed capacity of 1360 MW, now derated to about 1000 MW.  Other installations exist around the Pacific "rim of fire" in the countries of Japan, Indonesia, the Philippines, Costa Rica, Mexico and New Zealand.  The rift area in Africa has some developed and potential sites, as well as in Europe, in Iceland, France and Italy.  It is not surprising that geothermal potential is closely related with volcanic activity. 

The potential for higher temperature geo sources is pending the development of technologies that can harness at extremely high temperatures, close to that of magma, or molten rock.

How would geo plants impact the grid?

Geothermal plants have the following characteristics:

• Their location is determined by the availability of natural fluid that is in the right temperature range (between 200 and 500 F) for conversion to to electricity. 
• The energy is collected over an area that may require collection and strategic location of collectors and power plants.  Hence, the geo plant is actually a system of plants that combine transport of steam and electricity.  To obtain power levels above 50 MW requires a farm or park of many individual collectors interlinked by power lines and cables.  Ultimately, the lines connect to substations which step up the voltage to transmission levels and interconnect with the power grid. 
• A unique feature of geothermal units in the same field is that the individual units have to balance their use of steam so as to maximize the available steam.  Drawing more steam to one unit, may reduce the available steam from another unit that may go below the minimum required to operate that unit.  
• On a daily basis, the energy is generally invariant, like a base load plant.  However, there are seasonal cycles as well, similar, but not coincident with hydro seasons.

System Impact Concerns

The technical concerns with integrating geo plants are:

• Steady-state - the internal network within the field must provide for sufficient thermal loading and voltage capacity to deliver the rated power at the point of interconnection.  This may require a local control center to coordinate the operation of the individual units, in particular, in the dispatch for steam use.  Reactive capability tends to be good for the standard steam units, and there is usually no issue with voltage response compliance, except where the area of interconnections is already experiencing voltage difficulties. 
• Fault levels - geo plants use steam units that, in general, contribute to short circuit levels.  Many units concentrated in one interconnection point may lead to fault levels above available installed interrupting capability.  Options to mitigate this include providing for higher capacity circuit breakers and fault-withstand equipment, designing for split systems, adding reactors, etc.   
• Stability - Mostly modeling issues.  Dry steam or flashed steam geo units can be modeled with typical data used for fossil-fired steam plants.  An aggregate model can be used when multiple small units are being modeled for a geo plant.  A binary design would require a special low-speed dynamic model, similar to low-flow hydro units.  A combined-cycle design does not yet exist in practice, and modeling one could be a challenge.  After addressing the modeling issues, the stability issues should generally tend to be transient and dynamic stability (as the individual geo units try to stay in synchronism with the rest of the grid) and inter-area oscillations. 
• Subsynchronous Resonance - the low speed design may be susceptible to SSR.  The location of geo plants, remote from main load centers, may also contribute to potential impact from SSR.  Torsional analysis may be needed to demonstrate potential impacts from torsional stresses.  

Modeling

Power Flow Modeling

For power flow modeling, an aggregate may be used to represent the combined real and reactive power capability of several individual units.  The actual reactive capability of the aggregate unit will not necessarily match the sum of reactive capabilities of the individual units, except if all are dispatched to the percentage of rated MW.  This is because there is an internal dispatch to optimize the use of the available steam that may result in the individual units not being dispatched equally.

Some part of the internal network needs to be represented to capture impacts of internal contingencies.  

Modeling for Short Circuit and Switching Analyses

Model the equivalent unit by the subtransient (X"d) or transient (X'd) reactance of one unit but on the MVA base of all the individual units combined.  More details on the internal network may need to be modeled if fault levels are close to withstand ratings of existing equipment.

Modeling for Dynamic Simulation

For dry steam and flashed steam geo plants, most standard steam unit models will apply.  The inertia constant is typically in the 4-5 sec range.

For binary and combined cycle plants, a user-model would be required.  (For further information on special model development, see "Developing My Dynamic Model," by M. Gutierrez, Techblog of March, 2006.)  

Conclusions

Like many of the new set of Renewables that are being implemented or proposed for today's power systems, geo plants, especially newer designs that optimize energy conversion and allow for a wider range of steam temperature, pose some challenges to transmission analysis.  Binary and combined-cycle designs will require specific dynamic and steady-state models that may not yet exist.  Low-speed designs may be susceptible to subsynchronous resonance. 

There has been significant experience with operating, modeling and analyzing the existing types of geo plants.  Some of this experience can be extended to future geo plants. 

For questions, comments and further discussion, contact us at mailto:info@pterra.us

Notes

1. When the temperature of the hydrothermal liquids is over 350°F, flash-steam technology is generally employed. In these systems, most of the liquid is flashed to steam. The steam is separated from the remaining liquid and used to drive a turbine generator. While the water is returned to the geothermal reservoir, the economics of most hydrothermal flash plants are improved by using a dual-flash cycle, which separates the steam at two different pressures. The dual-flash cycle produces 20% to 30% more power than a single-flash system at the same fluid flow.
2. A binary cycle system can be used with liquids at temperatures less than 350°F. In such a system, the hot geothermal liquid vaporizes a secondary working fluid, which then drives a turbine.

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