Icelanders have made effective use of their geothermal resources, extracting groundwater heated by molten rock in the earth below and using that hot water to heat the places they live and work. Otherwise, they’d have to choose between freezing in the winter or importing foreign fuels. Neither option is very attractive.
The groundwater used for heating homes isn’t especially hot: when it comes from the ground, it’s only about 85-130o C, or roughly 180-270o F. That’s enough to keep you comfortable in winter and to keep the roads from being overly encrusted with snow and ice; you could even use it to cook your food. But there’s a lot more heat lurking in the depths of the earth. Icelanders have put it to use.
A few weeks ago, I stopped off at Hellisheiði (HET-lis-hay-thi, with the final syllable pronounced like “this” but without the s), the largest geothermal power plant in Iceland. (I wrote about it in this post, if you missed it).
The plant is open to the public; there are no checkpoints, fences, or guards to keep people out. It’s a remarkable contrast to other places in the world, where you might well be arrested (or worse) simply for trying to approach such an important site.
Our class visited the plant as a group on a Friday, touring the main building and getting a look at the wells and at drilling operations underway. Basically, the power station is a tiny speck on the flank of a giant volcano, Hengill (HEN-gitl, with the l barely vocalized), whose peaks and craters cover 110 square kilometers of land about 30 minutes’ drive from Reykjavík. Hengill is believed to have erupted about 2,000 years ago, although there were no humans living here to observe it. It has been quietly steaming ever since.
Two miles below the surface of Hengill, the groundwater has an ambient temperature of 300-320o C, or about 575-600o F. At normal atmospheric pressures, water at that temperature could not exist in liquid form; it would instantly be converted to steam. But the pressures are enormous at that depth, and the water is still a fluid – although it’s a brackish liquid mixed with dissolved minerals and gases. The hottest liquid rises, bleeding off some of the heat below through the process of convection (described in my earlier post here).
This extremely hot liquid is called geothermal fluid. It’s accessed by drilling wells – called boreholes – to a depth of about 3-4 kilometers, or between 2 and 2 ½ miles.
The precise amount of fluid yielded by each borehole varies, because the layers of rock underground vary; but in general, each borehole yields enough high-temperature geothermal fluid to generate about 5-7.5 megawatts (MW) of electricity. Some, but not all, of the fluid turns to steam as it comes out of the borehole; the steam and the remaining geothermal fluid then flow to the plant in large, insulated pipes, losing only a few degrees of heat on a journey that sometimes covers several miles.
When it reaches the plant, the steam and fluid mixture flows through separators to isolate the pure, dry steam. Steam is used to turn the turbines that produce electricity, and it’s important that the steam be free from water droplets or corrosive minerals or chemicals because these can damage the turbines.
Fortunately, dry steam can be separated relatively easily from the liquid, and corrosive or otherwise undesireable minerals and gases can be removed as well.
The dry steam reaches the turbines at pressures of about 9 bars – nine times the pressure exerted by the Earth’s atmosphere at sea level, roughly 125 pounds per square inch (psi). The temperatures and pressures involved in power production at Hellisheiði would certainly be lethal to an unprotected human being,* but the steam remains contained within the various pipes and pieces of equipment, churning out power quietly without harming anyone. The pure dry steam drives six high-pressure turbines that produce a combined total of 270 MW of electricity.
The geothermal fluid left over after the initial separation that produced the dry steam is then used to generate additional steam. Although this steam is at a lower pressure — only about 2 bar — it still can produce electricity. A low-pressure turbine at the plant generates an additional 33 MW of electrical power.
A simple power plant design would vent the used steam into the air as shown here:
Hellisheiði, however, is not a simple plant:
The spent steam from the seven turbines is still quite hot, and using a heat exchanger warms pure water taken from the nearby ground at depths of a just a few hundred meters. This newly-heated water is infused with minute quantities of hydrogen sulfide, which absorbs free oxygen in the water and reduces corrosion as the water flows through a pipeline to Reykjavík, about 11 km (nearly 7 miles) away. The water there is distributed through the district heating system to provide space heating for homes and businesses throughout the capital, including Reykjavík University. Hellisheiði can produce an additional 400 MW of thermal power, although demand varies considerably depending on the time of year.
Because cooling magma naturally produces carbon dioxide, there are some carbon emissions from the plant – about 5 per cent of what would be produced by a fossil-fuel fired plant of equal output. Some of the remaining fluid is therefore used to inject carbon dioxide in solution back into the ground, where it can react with the basalts in volcanic rock to produce calcite. The rest of the spent geothermal fluid is pumped back deep into the earth, where it can flow back toward the hot spots near the magma chamber and be reheated and eventually extracted through the boreholes and used once more to generate power for Iceland.
As with the low-temperature fields near the capital, it’s possible to take too much fluid from the ground, essentially bringing the system to a halt. Not enough fluid means not enough steam. Orkuveita Reykjavíkur, as the utility responsible for the Hellisheiði plant, is currently drilling a new set of boreholes in a different field a few miles from the plant in order to balance the amount of fluid taken from each aquifer and to keep the consumption of geothermal fluid at a sustainable level. Building a geothermal plant requires careful calculations of the size of the fluid reservoir under the earth, and how fast it is replenished. Actually running the plant means constantly updating those figures.
Even where the land is arid, geothermal power can be generated by injecting water or other fluids into the hot rock layers beneath the surface and extracting the resulting steam. In general, this is a more expensive way of doing things than relying on groundwater, because it adds additional steps and materials. To make such a system economical, you would want to extract more energy per well. This may be feasible by digging deeper wells.
These maps show the difference in ground temperature in the US at a depth of 6 and 10 km (3 ½ and 6 miles):
You can see that there is more heat available, over larger areas, by drilling to six miles’ depth. By comparison, existing oil wells are up to 8 miles deep – so deeper geothermal wells are within reach of existing technology.
After touring Hellisheiði, we hiked a little less than five miles to the north, eventually reaching the Nesjavellir (NES-ya-vet-lir) power station.
Like Hellisheiði, Nesjavellir uses a multi-stage process to generate both electric and thermal power: about 120 MW of electric power and 290 MW of hot water, respectively. This means that Icelanders are currently extracting over a gigawatt of power from a volcano that hasn’t erupted in 20 centuries. Not bad, when you think about it. It was the addition of the hot water flow from Nesjavellir (discussed here) that allowed Icelanders to reduce the demand for hot water on the fields near Reykjavík, making it possible to operate the wells there at a steady and sustainable rate.
A final note: when we got to Nesjavellir, there was no one at the plant to greet us. That’s because the whole facility – which looks a little like the engine room from the rebooted Star Trek series – is run from an office in the capital city, more than an hour’s drive away. Geothermal power has reached a point where it is so reliable and easy to operate you can build a billion-dollar facility and walk away from it.
And speaking of walks – I’ll post some more pictures soon from our epic hike from the south side of Hengill to the north. It was a memorable stroll through some spectacular countryside!
* Mine-safety research indicates that an overpressure of 55-65 psi will kill about 99% of unprotected workers. Source: NIOSH, US Centers for Disease Control.