Science Spin March 2008

Hot water below ground can be tapped

By Tom Kennedy

As we know from volcanic eruptions, the Earth, below the crust is extremely hot. The deeper we go the hotter it gets. For every kilometer down into the Earth's crust, the temperature goes up by 20ºC. Mines are often deep enough to be oppressively hot, and if we could drill down 150 kilometres the temperature would be about 1,300ºC. Sometimes, when the Earth's crust is disturbed, water can penetrate down into the depths, and in volcanically active areas, such as Iceland, it circulates back up to the surface in springs, hot enough to generate electrical power.

Ireland is far removed from any earthquake zone, but even so, we do have a number of not so much hot, as tepid, springs. In north Leinster alone there are eighteen wells or springs where the average temperature can be as high as 24ºC. The best known of these, but not, at 17ºC, the hottest, is the old spa at Louisa Bridge in Leixlip. In the 19th century, this spa was so popular, that a tramline, running out from Dublin to Lucan, was kept busy bringing bathers out to 'take the waters'.

From data gathered over the years, the GSI has concluded that most of our warm springs involve relatively rapid circulation of water from some depth. High rainfall speeds up the cycle, so water comes back up to the surface with minimum loss of heat.

Sources

Compared to Iceland, Ireland's springs are cool, yet, if we go down a few hundred metres, there is a virtually inexhaustable source of heat, and this has made geologists wonder why we have been so slow to tap into geoenergy. In one SEI study, Róisin Goodman, Gareth Jones, John Kelly, Ed Slowey, and Nick O'Neill looked at the potential, and they concluded that "Ireland is particulary well suited for the extraction of heat from ground sources."

When people talk about geothermal energy, they often refer to near surface extraction of heat, and heat exchangers, which work like fridges in reverse, that have become quite a common feature in large energy managed buildings. Often, the heat is extracted from just below the surface, and strictly speaking this has more to do with energy recovery rather than tapping into geothermal sources. Although, at one or two metres depth, the temperature gain is small, scale can make extraction worthwhile.

The authors of the report pointed out that we are letting an enormous amount of energy go to waste in cities, and they calculated that heat from buildings, traffic and other activities can contribute as much as one third of the solar gain in city areas. Ground temperatures in urban areas, they found, can be up to 4ºC higher than in open countryside. In Cork and Dublin, buried gravel and eskers underlying some of the most built up areas have the potential to act as large, readily accessible reservoirs for open loop heat systems.

Of course, there can be a price for an over-enthuiastic campaign of heat recovery, and there have been reports of staff in a large Dublin institution coming back to chilly offices, and as one architect commented about small domestic systems, don't be surprised if the daffodils in the garden fail to flower.

The ultimate potential for tapping into geothermal energy could well lie in drilling deep, and in general the deeper the better. The problem is that drilling is very expensive, over €1million per 1,000 metres, and that's just the start. There is nothing certain about actual performance, and so far, investors would prefer to speculate on the old, but also not always reliable, georesource, oil.

Drilling

Many geologists believe that geothermal energy is well worth developing, and within Ireland the underlying structures give us a good idea where to drill. The north Leinster and Munster springs are warm because they are adjacent to deep faulting, and going north, we see a marked increase in temperature where basalt welling up from the interior thinned the Earth's crust. At 500 metres depth there are some hot spots in west Clare, northwest Cavan, north Antrim and east Tyrone, where temperatures range from 25ºC to 27ºC. Modelling from the available data suggests that these hot spots continue well beyond this depth. At 5,000 metres depth is believed that the background temperatures in the south range from 60ºC to 75ºC, but in the north, temperatures at the same depth could be as high as 180ºC. The ancient Iapetus Suture, running from Dublin to Limerick, along which the two sides of Ireland came together about 350 million years ago, closing off the ancient Iapetus Ocean, has been a big influence at these deeper levels, and in the north, temperatures are also high because of the underlying geology.

The north, compared to the south, is quite hot, and recently the Geological Survey of Northern Ireland commissioned a study to examine the possibility of exploiting this natural advantage. While stopping short of advocating deep drilling, the study concluded that drawing heat from near the surface is now an attractive option for many homes and offices in Northern Ireland.

Nineteen deep boreholes were sunk in Northern Ireland, and the recorded temperatures, of up to 180ºC at 5,000 metres on Rathlin, suggests that it could be worth going deeper in launching a heat mining programme. The temperature range occurring in Northern Ireland compares very well to some of the international test sites chosen for their potential to yield geothermal energy.

Tapping into ground heat is on the increase, and while the authors of the SEI report stated that "it is unlikely that Ireland will ever generate electricity from geothermal energy" they thought that with drilling there is "good potential for temperatures between 50ºC and 100ºC."

Examples

This potential, while not in the power plant league, is quite significant, and in one example, the swimming pool in Mallow is already being heated from warm water rising from a 75 metre deep borehole. The water coming up from the depths along a natural rock fracture emerges at just over 19ºC. Instead of having to pump heat into the pool from conventional supplies, a top up is enough to keep it up to temperature.In Cork the Lewis Glucksman Gallery at UCC draws heat from water in gravel beds 20 metres below the city, but these schemes are relatively modest compared to some of those from abroad.

In the UK, when one of the local authorities, Southampton, opted for an integrated energy scheme in the 1980s, geothermal became an important part of the mix. The city council drilled down into Triassic sandstone, over 1,700 metres below the city. The porus sandstone contained a 24-metre deep aquifer with salt water at 76ºC, and, although this supply has a limited life, the energy has been used continuously for district heating since the scheme began.

In Paris, which perches above a sedimentary basin, more than 40 geothermal systems are reported to be tapping into water at 80ºC. At about 30ºC water is extracted before the water, which is saline, is pumped back into the basin.

The cost of drilling remains one of the biggest obstacles to the development of geothermal systems, but the possibility of high yields is encouraging engineers to come up with more efficient solutions, such as plans is to fracture deep rocks with explosives. In pumping water down into the hot cracks, the engineers hope to mimic on a grander scale the rain driven cycle that gave us the Spa in Leixlip.