Jökull - 01.12.1961, Blaðsíða 65

(7) COMBINED SCHEMES.
There are no reports on combined schemes
involving the generation of power combined
with the utilization of waste heat for space or
industrial heating. The combination of power
generation and space heating is, as a matter of
course, an obvious possibility. As of now there
are no thermal areas where this combination
appears economical.
The chemical industry at Larderello, men-
tioned above, furnishes an example of the re-
covery of chemicals in combination with the
generation of power.
(8) FUTURE PROSPECTS, GEOTHERMAL
RESOURCES WITH NO SURFAC.E DISPLAY
This report has stressed the low transporta-
bility of geothermal energy. The resources are
available for local uses only. Moreover, it is un-
fortunate that many of the known geothermal
resources are locatecl in areas of low population
density and low economic activity. Also, a
number of resources are located in areas of an
inadequate supply of raw materials for low-
temperature processing.
These circumstances pose the rather impor-
tant and intriguing question whether there may
exist other exploitable geothermal resources
than those indicated by such conspicuous phe-
nomena as thermal springs. Moreover, are
such hidden resources available in areas favour-
able for a large scale exploitation.
This problems has not been discussed in the
literature. The present writer is therefore of
the opinion that this question should receive
some attention in this report, which deals with
rather general aspects of the utilization of low-
temperature resources. Of course, the cliscussion
presented below lias to be restricted to sorne
general remarks and speculations.
This subject matter is recorded in passing
but will not of itself be appropriate for detailed
cliscussion uncler the present agenda item.
(a) Thermal activity and volcanism.
All known high-temperature geothermal re-
sources are located in areas of recent volcanic
activity and are no doubt closely related to the
volcanism. There is little doubt that magma
transported by the volcanic processes is in some
way or other ultimate source of the heat caus-
ing the thermal activity. The geothermal areas
are probably convective systems drawing on the
heat content of recent intrusives.
Volcanism is a relatively common pheno-
menon. Volcanoes are distributed all along the
Circum-Pacific Belt, on the Mid-Atlantic Ridge
and in other locations in Asia, Africa and
Europe.
Tliermal activity, on the other hand, is not
nearly as common as volcanism. There are, of
course, the few large thermal areas that are dis-
cussed in many papers presented at the Con-
ference. But their number stands in contrast to
the relatively great number of volcanoes.
It should also be remarked that the volume
of rock affected by thermal metamorphism is
really enormous. Hydrothermal phenomena are
of much greater importance than actually indi-
catecl by the surface display.
It is by no means inconceivable that one of
the reasons for the relatively few areas with sur-
face display is to be founcl in the phenomenon
of selective transport of materials in thermal
areas as discussed above. The surface outlets
are possibly closed relatively rapidly by the
precipitation of silica and calcium carbonate at
and near to the surface. The deeper processes
may remain relatively unaffected.
Such areas, with closed surface outlets but
some convection at depth, would be character-
ized by a relatively large concluction flow of
heat in the surface layers. It should be possible
to locate them by means of a study of the tem-
perature gradient in shallow boreholes.
At his juncture this is only a possibility. Un-
fortunately, very little is known about the varia-
tions of the temperature gradient. There are
practically no data at hand from most volcanic
areas, probably becauses these areas are regar-
ded as abnormal. But the degree of abnormality
is the point of present interest.
Summing up, it can be stated that there are
reasons for expecting hidden geothermal re-
sources in volcanic areas and there is as yet no
observational evidence contrary to the hypot-
hesis.
(b) Low-temperature resources in non-
volcanic areas.
As a matter of course, there are rather small
possibilities for geothermal resource in non
volcanic areas. The average temperature gra-
dient in such areas varies in general from 10
to 50° C/km. However, the upper limit is not
63