Power plants – Utilizing natural heat – Geothermal
Reexamination Certificate
1999-05-20
2001-06-19
Nguyen, Hoang (Department: 3748)
Power plants
Utilizing natural heat
Geothermal
C060S641500
Reexamination Certificate
active
06247313
ABSTRACT:
The present invention relates to a plant for exploiting geothermal energy by circulating water through a geological formation at least 1000 m below the earth surface, comprising at least one supply hole leading from the surface down to said formation, at least one return hole for the transport of heated water from said formation to the surface, and a heat absorbing arrangement connecting the supply and return holes, said arrangement comprising a heat transfer surface across which heat is transferred from said formation to said water.
An example of such a plant is disclosed in U.S. Pat. Nos. 3,863,709 and 4,223,729, the latter and other patents mentioned therein being included by reference. U.S. Pat. No. 4,223,729 relates to the exploitation of geothermal energy from hot dry rock (HDR) formations. Due to the low thermal conductivities of such formations, it has been the general belief that thermal energy could not be extracted at a useful rate without a very large heat transfer surface being available in the geological formation.
Up to now, in almost all known plants in HDR one has tried to obtain such very large heat transfer surface areas by creating fracture zones between the supply and return holes, either by expanding existing fracture zones, by blasting the rock between the holes using explosives, or by establishing a fracture system through the use of hydrostatic pressure and/or heat. Even if such fracture zones could be established, they provide rather unpredictable flow conditions for the water since the water tends to take the path of least resistance and therefore not flow through the narrower fissures of the fracture zone.
Quite contrary to the common belief, the present inventors have surprisingly realized that if a geothermal plant of the type in question is to have any useful life, the magnitude of the heat transfer surface area is not a critical factor. Instead, the decisive factor is the availability of a large volume of rock in close proximity to the heat transfer surface. Thus, the inventors believe that a geothermal plant, e.g. designed for heating and hot water purposes, should have at least 15,000 m
3
of rock located within 50 m of the heat transfer surface for every kW the plant is to deliver. For smaller plants in unfavourable rock conditions, this volume may be more like 60,000 m
3
/kW.
Consequently, in one aspect of the invention, it provides a plant for exploiting geothermal energy of the kind defined in the introductory paragraph above, the plant being characterized in that it has a given nominal power in MW defined as the heat to be absorbed by said arrangement after one year of operation, in that said heat transfer surface comprises at least one drilled heat absorbing hole, and in that the volume of said formation located within 50 m of said heat absorbing hole, is at least about 10×10
6
m
3
, preferably at least 20×10
6
m
3
, multiplied by said nominal power.
These numbers represent a much larger mass of rock than contemplated by any prior art plant with an economically viable output.
The inventors have found that the most efficient way of establishing a sufficiently large volume of rock in close proximity to the heat transfer surface would be to use a drilled hole for said surface. However, such a hole would have to be quite long to penetrate the required volume of rock, and the drilling costs would appear prohibitive since one still had to assume that a substantial heat transfer surface, i.e. large diameter hole, would be necessary to provide the required heat flux from the rock to the water circulating through the hole.
Nevertheless, the inventors set out to calculate the heat transfer from a large cylinder of rock into water flowing in a central hole of the cylinder using the differential equation presented by H. S. Carslaw and J. C. Jaeger in “Conduction of Heat in Solids”, Second Edition, Oxford, which is hereby included by reference, a task that to their knowledge nobody had done before. Surprisingly, they found that in the course of 30-40 years, the temperature in the rock at a distance of 100 m from the hole would hardly change at all. Even more surprisingly, they found that the available energy could be extracted over this time period with a hole which, from a heat transfer point of view, had a diameter as small as 10 cm and even less. Further analysis showed that increasing the hole diameter to 1 m, which would increase the heat transfer surface tenfold, would less than double the possible heat extraction rate, other conditions being held equal. However, the cross-sectional area of such a hole, and therefore the mass having to be removed to make the hole, would increase 100 times. Consequently, the most economical solution seemed to be to use the smallest hole diameter that could be drilled over long distances, which with the current technology is limited to about 10 cm.
However, the length of such a heat absorbing hole would normally exceed 5 km even for the smallest practical economical plant and, in addition, the supply and return holes could extend for much the same distance. Besides, the pressure drop and consequent pump losses could be too large for very long slender holes. To solve this problem, the inventors have suggested to divide the heat absorbing hole into a plurality of passes connected in parallel and being spaced sufficiently apart to have available a sufficient volume of rock to supply the desired heat through the required lifetime of the plant.
Thus, according to a second aspect, the invention provides a plant for exploiting geothermal energy by circulating water through a geological formation at least 1000 m below the earth surface, comprising at least one supply hole leading from the surface down to said formation, at least one return hole for the transport of heated water from said formation to the surface, and a heat absorbing arrangement connecting the supply and return holes, the plant being characterized in that said heat absorbing arrangement comprises a plurality of drilled heat absorbing holes connecting the supply and return holes in a parallel flow manner, a substantial part of each heat absorbing hole lying at least 50 m, preferably at least 100 m, from the nearest heat absorbing hole, the total length of the heat absorbing holes preferably exceeding 5000 m.
According to a further aspect of the present invention, a plant for exploiting geothermal energy of the type recited in the classifying part of the above paragraph is characterized in that said heat absorbing arrangement comprises a plurality of drilled heat absorbing holes arranged in parallel flow relationship, said heat absorbing holes extending at an angle downwards from the supply hole to the return hole.
The sloping of the heat absorbing holes makes them simpler to drill, for instance by using a water driven percussion hammer and coiled tubing. The weight on the drill bit can more easily be controlled since the friction of the coiled tubing against the hole wall can substantially support the weight of the tubing. This may considerably increase drill bit life and reduce drilling costs.
Since the rock temperature increases with increasing depth, letting the water flow direction be downward through the sloping holes will permit the temperature increase in the water to follow the temperature increase in the surrounding rock, thus keeping the temperature difference between the rock and water substantially constant. This may be likened to a countercurrent flow heat transfer arrangement and will allow for a shorter length of heat absorbing hole and an optimum absorption of heat from the rock. A countercurrent flow arrangement would be expected to produce heat from the same rock volume for 2-3 times longer than an equivalent co-current heat transfer arrangement.
The magnitude of the sloping angle will depend on several factors, for instance the temperature gradient in the rock, the length of the heat absorbing hole and the water flow rate. Calculating the angle will be well within the capabilities of the skilled person and will therefore
Moe Per H.
Rabben Kjell M.
Frommer William S.
Frommer & Lawrence & Haug LLP
Nguyen Hoang
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