Harnessing earth’s heat to warm our homes

Press, 11 June 1977, Page 13

Harnessing earth’s heat to warm our homes

ALAN DRUMMOND

reports

Can heat and hot water from the earth’s hot core be used to supply “cheaply” these two very necessary services in our homes? According to research and experiments in several countries, the answer is a positive “yes.” Indeed, even the production of electricity by vapour from hot centres underground has proved to be both possible and practical. The source of this energy, known as geothermal heat, is in the rocky depths beneath the earth’s crust. As engineers drill into the earth, the temperature rises about 3deg. Centigrade for every 100 m. In volcanic areas, such as Rotorua, this increase is usually much faster, causing spontaneous eruptions of hot water, called geysers. In the forefront of successful research into the uses of geothermal energy for space and industrial heating are the United States, New Zealand and France. In France, for instance, by mid-1975, more than 2000 houses in Melun (near Paris) were being heated by hot water — 71 degrees — coming from what is ’known as a porous Jurassic layer. Furthermore, surveys by the French Geological and Mining Research Office have led experts "to believe that the geographical basin in which Paris is situated possesses a reserve of 10 billion billion of calories, or enough to heat 30M inhabitants for 1000 years.” This belief, if substantiated, would mean that new towns springing up around Paris are likely to be heated from this source in a few years time. A significant factor is that the cost of supplying space heating and hot water for households would be much less than that, at present. For instance, it has been expertly estimated that the use of hot water to heat all French apartments, offices, and factories would save about 36M tons of fuel oil a year — about 31 per cent of French imports of oil, or about one-third of the scheduled capacity of France’s nuclear power stations in the 1980 s. All the heat envisaged in such a programme would come from many hundreds of

wells that probed deep into the rocky depths be* neath the crust of French soil. Some have asked whether the supply of hot water in these wells will be exhausted eventually. To eliminate this possi-

bility, the French method is to re-inject cold water after hot water has been drawn off. The superheated rocks quickly heat up this water, which can then be used.

That this harnessing of the earth’s heat is practical and worth while is indicated by the fact that, by mid-1976, many schemes had been undertaken. Geothermal energy is being used in homeheating systems in a number of French towns, including Creil, Houilles, Blagnac, Villeneuve-1 a-Gar-enne and Mont-de-Marsan. Moreover, when others still in the planning stage (such as Strasbourg) are included, some 25,000

dwellings will be heated by this energy-saving method. Inexpensive production of electricity through the direct use of natural vapour from the earth’s depths may sound unlikely, yet the way to do

this has been successfully accomplished in several countries. It was pioneered by a French engineer, Francois Larderel, whose work resulted in the building of the world’s first thermal power station, at Larderello, in Tuscany. Until the 1950 s this power station was the only one to produce geothermal electricity. Since then other nations, particularly New Zealand and the United States, as well as France, have built such stations. In France’s Massif Central area, for instance, the price of electricity produced by the use of the earth’s natural vapour is one-third cheaper than that pro-

duced by conventional powerhouses and nuclear power stations. For all that, however, the necessary conditions that must exist for geothermal electricity to be produced successfully are not present in every country. An official French report describes these conditions thus: “The first natural condition needed for the tapping of vapour is the existence of a heat centre that is usually to be found between 3000 and 10,000 m under the surface. Its temperature must be very high — 500 to 600 degrees. Above this centre, the rocks must present sub-vertical systems of cracks or flaws that will permit hot water to rise to the surface. "Closer to the surface must be a ‘reservoir,’ a permeable layer located between 300 and 1500 metres, where water loses some of its heat in beginning the downward movement in the cycle of convection. Above this reservoir there must be a rela-

tively watertight layer of rocks to prevent the loss of thermal energy in the atmosphere. "When drills reach the reservoir they cause the boiling of water through a drop in hydrostatic pressure. The actual operation of a power house using natural vapour is quite simple. Vapour reaches the surface at a speed of nearly 1000 kilometres an hour, and if it is dry’ it is carried directly into turboalternators by insulated pipes. If it is moist, the water must first be estracted.” These stringent conditions explain why tapping for this kind of energy is not widespread, even in those regions where heat centres of 350 degrees exist at the right depths. In the United States, the Ploughshare project envisages the use of underground atomic blasts to crack the rocks, to overc o m e impermeability where conditions are otherwise suitable.