Yesterday, two colleagues and I had the chance to visit a geothermal plant at The Geysers in Lake County, California. For those of you not familiar with the area, it’s near Middletown on Highway 29 about 32km (20m) north of Calistoga. There is a very narrow valley between two ~1,000m (~3,000′) elevation ridgelines. On the west ridgeline about 8km (5m) north-northwest of Middletown is an old mining road that winds up onto the ridge. Along the way, one sees well after well all connected by large insulated pipelines headed to one of the 21 active power plants scattered across the landscape.

The major operators on the field now are Calpine (19 plants,) Northern California Power Association (2 plants,) and US Renewables is reworking the Bottle Rock plant due to come online in 2007. Another decomissioned 65MW plant is rumored to be part of a repower project at lower output (~25MW.)

The geothermal resource at The Geysers is relatively rare, it’s dry steam that is produced from the wells (vs. a standard hot fluid that either “flashes” to steam or is used for heat transfer to a working unit in a binary system.) The area being utilized is approximately 11km (7m) long and 8km (5m) wide in the rough shape of a football located some 2,000m (6500′) below the surface. The reservoir rock is a graywacke, a type of sand stone with fractures that capture water and transfer heat from a suspected magma dome below. There are other layers of rock acting as a capstone (trapping the steam in the reservoir) such that it can only escape to the surface through faults and wells.

The resource has been known for some time and has been exploited in some way for more than 40 years, specifically with a large buildout of power plants in the 1980’s. The peak output for the field was 2,100MW during the late 1980’s the output of steam declined some 17-20% per year thereafter to a current production level of around 750MW. Over production aided in the precipitous decline which was largely stemmed by taking the measure of injecting treated wastewater from nearby communities back into the reservoir. The facility we visited yesterday produces some 132MW (120MW net minus the parasitic load of the plant) from 70 production wells and 5 injection wells. While the decline of steam yield continues, it has slowed considerably and is predicted to reach an equilibrium in the next few years with the injection program replacing the mass extracted on a daily basis.

I believe (aside from overproduction) that the major culprit in decline of steam production is due to the condensation of post-turbine steam which is cooled through cooling towers where 70% of the water is lost to evaporation. A closed system would preserve much more of the mass and enable a more effective injection regime (not to mention the opportunity to boost production by using the cooling steam to power binary systems down to 90C.) One other side effect of the steam emission is the requirement to cleanse the steam of sulphur, mercury, and vanadium before it is released into the atmosphere which adds more cost and negative impact from this power generation method.

The physical aspects of the plant we visited were impressive. There were two enormous steam turbines (the “spare” blade waiting nearby weighed 25,000kg – or 27 tons and when functioning spins at 3,600 rpm.) Each turbine was connected to a generator sending electricity to a substation and then onto the grid through a 230kva line in the area. Each generator had a separate gigantic stainless steel condenser box, separator for non-condensable gases (about 0.2% of the steam,) and cooling tower infrastructure. There is a separate Stretford process aspect to the plant that separates the sulphur, mercury, and vanadium from the gas for atmospheric emission. One thing that struck us was the overall cleanliness of the entire operation. I’ve been to coal fired power plants before and they are absolutely filthy despite the efforts by the maintenance staff to keep them clean. These plants are relatively small and very clean by comparison.

We’re grateful to have had the opportunity to visit one of the oldest and largest geothermal power production areas on the planet. We had a great host in a friendly plant manager who had worked out at the site since the late 1970’s and was very knowledgeable and helpful in answering our many queries. We learned a ton and it will be helpful as we embark upon our mission to acquire and operate a renewable energy portfolio. I’m certainly hopeful that another opportunity for a visit will arise.


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