Day 38: getting into hot water, part one

Iceland is one of the world’s northernmost countries.  While most of its land mass lies below the Arctic Circle, it’s not very far below.  Though the Gulf Stream waters that keep Europe warm also flow around Iceland, actually living here would be uncomfortable without some means to heat homes and workplaces during the cooler months.

Ég er hér. So are a lot of other people.
Ég er hér. So are a lot of other people.

To stay warm, Icelanders use more than 20 petajoules of energy per year for space heating.  That’s roughly equivalent to the energy release of a 5 megaton thermonuclear bomb.

If this accidentally-dropped bomb had gone off, Goldsboro NC would have received Iceland's annual heating budget in about 30 nanoseconds.
If this had gone off, Goldsboro NC would have received Iceland’s annual heating budget in about 30 nanoseconds.  Image credit: Wikimedia.

Early home heating in Iceland involved burning wood.  This was a reasonably good strategy until the forests were gone.  Burning peat also worked, although harvesting the fuel was difficult and tended not to yield as much heat as other alternatives.  As the industrial era got underway, Icelanders turned to oil – but there are no local petroleum deposits here, and no coal or natural gas.  Imagine a country entirely dependent on foreign energy.  Now imagine a bankrupt nation.  They pretty much look the same.    All the while, Icelanders were sitting on top of a veritable sea of molten rock.

As we discussed before, volcanic eruptions can heat things up – but violently and generally only for a little while.  If only there was a way to access all that heat lurking beneath the surface …

Occasionally, nature hints at a solution.  Sometimes she’s not very subtle.

Strokkur

It turns out that water is the key.

When a magma chamber is formed beneath the earth’s surface, the heat it contains is gradually transferred to the surrounding rock by a process called conduction.  Heat can be carried to the surface through a process called convection.

Conduction is the transfer of thermal energy that occurs when a warmer object comes into direct contact with a cooler one.  When you hold snowflake in your hand, heat is transferred from your body to the snowflake via conduction.  The snowflake melts.  Convection is the transfer of thermal energy that occurs when a liquid or a gas (both of which are known as “fluids” in engineering terms) carries heat from one object to another.  When you breathe on a snowflake that landed on your window, heat is transferred from your body to the snowflake via convection.  The snowflake melts, even though it never touched the heat source (you).

An important feature of convection is that the liquid or gas changes density as it absorbs heat, and this tends to make it move.  If you hold your hand over a lit candle, you are more likely to be burned than if you hold your hand the same distance to the side of the same candle.  This is because the heat is being transferred to air, which when it becomes warm tends to rise.  Heat streams away from the flame in many directions, but more of it flows upward with the air as its convection medium.

Groundwater that comes into contact with the heated rock around the magma pocket heats up and rises too, carrying thermal energy toward the surface via convection.  The closer it gets to the surface, the easier it is to harness.  Icelanders use this energy in many ways: one is to heat their homes and businesses.

Image credit: Iceland School of Energy.
Image credit:  Iceland School of Energy.

Orkuveita Reykjavíkur (OR) is the utility responsible for providing heat to the nation’s capital region.  It operates wells in and around the city, some in places you might not expect.  This well, for example, is located in a parking lot next to the Japanese Embassy.

About the size of a single-car garage.
About the size of a single-car garage.

The well was drilled in 1963.  It produces up to 50 liters per second of hot water at 125o C – just under 260o F.  Water at that temperature is difficult to work with: metal corrodes, rubber breaks down.  The first pumps at this well each lasted only about a week before the bearings would give way, requiring a rebuild or replacement.  Eventually OR hit on the notion of using bearings made from Teflon – not coated with Teflon, but actually made from it.  This proved to be a winning approach, and pumps now last 30 years or more.  There are 10 active wells in the Laugarnes geothermal field in downtown Reykjavik.

The Laugarnes geothermal field. Wells are circled in blue; the pumping station in red. Image credit: Orkuveita Reykjavíkur.
The Laugarnes geothermal field. Wells are circled in light blue; the pumping station in red. Image credit: Orkuveita Reykjavíkur.

There are other such fields around the city, with just over 50 wells supplying hot water.  The well water flows to pumping stations, where it’s mixed with cooler groundwater to supply each building in Reykjavík with access to a steady supply of water at 80C, or about 175F.

Pumping station interior. Well water flows in through purple pipes; cool water through blue. The mixture flows out to homes and businesses through red pipes.
Pumping station interior.
Water flow diagram. Image credit: Orkuveita Reykjavíkur.
Water flow diagram. Image credit: Orkuveita Reykjavíkur.

Demand for space heating is highest in winter, but because OR also supplies hot tap water, the system remains constantly in use.  During the winter, the water used for heating – which cools after it flows through a building and surrendering most of its heat – is drained through pipes under the streets and sidewalks to keep them free from snow and ice.

braedslukerfi12000
Water pipes under the asphalt keep this parking lot ice free even in winter. Image credit: Orukustofnun.

The net effect has been to reduce CO2 emissions due to space heating from over a quarter million tons per year in 1961 – when Reykjavík had a population just over half the number it has now – to zero.

Icelanders don’t import the power they need to heat their homes (a fact that was extraordinarily helpful during the financial crisis here, when the Icelandic currency was sharply devalued), and heating bills are by all accounts ludicrously low.

There’s just one potential problem: although the supply of heat from magma is essentially limitless, the supply of groundwater that carries it to the surface is not.

Image credit: Orkuveita Reykjavíkur.
Image credit: Orkuveita Reykjavíkur.

This chart shows how overuse began draining water from the aquifer around Reykjavík faster than it was replenished by rainwater and glacial melting, until the supply of hot water was supplemented with water sourced from a different aquifer.  Once the demand on the original aquifers was relaxed, they recovered to a sustainable level.

Even though it’s not really possible to use up all the energy available under the earth, it is possible to render it effectively out of reach and useless.  Fortunately, Icelanders realized the problem before it became a crisis, and took steps to deal with it.  But the incident serves as a reminder that even seemingly-limitless resources have to be protected: it’s always possible to kill the goose that lays the golden eggs.

One thought on “Day 38: getting into hot water, part one

  1. […] About two miles below the surface of Hengill, the groundwater has an ambient temperature of about 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 fluid mixed with dissolved minerals and gases.  The hottest fluid rises, bleeding off some of the heat below through convection (described in my earlier post here). […]

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