Náttúrufræðingurinn - 2012, Blaðsíða 70

Náttúrufræðingurinn
70
lekt er góð yfir heitum varmagjafa
og hiti við botn hræringar lítillega
undir krítískum hita vatns (374°C).
Hræringu af þessu tagi fylgir öflugt
varmaflæði frá varmagjafanum.
Vitað er að mörg lághitakerfi tengj-
ast virkum tektónískum sprungum.
Eins liggja flest háhitasvæði lands-
ins í virkum sprungureinum. Lekt í
sprungum er meiri en í þeim berg-
grunni sem þær skera. Almennt
grunnvatnslíkan af bæði háhita- og
lághitakerfum felur því í sér að
írennsli sé mest um sprungur. Upp-
streymi er líka mest um sprungur
þar sem þær eru yfir varmagjafanum.
Mikið kalt grunnvatnsstreymi á litlu
dýpi, aðallega í gosbeltunum, gæti
drekkt uppstreymi jarðhitavökva,
a.m.k. að hluta, sbr. t.d. efra kerfið
í Kröflu.
Sá möguleiki virðist vera fyrir
hendi að háhitakerfi með kviku-
varmagjafa geti staðið undir tiltölu-
lega miklu vinnsluálagi í alllangan
tíma ef vinnslan leiðir til aukins
írennslis vatns niður í rætur þessara
kerfa og örvi með því storknun og
kælingu kvikunnar, en eigi það sér
stað gæti svo farið að varmanáman
yrði þurrausin.
Summary
The nature and renewability of
geothermal systems
Energy resources that are renewed at a
rate equal to or higher than they are
consumed are regarded as renewable.
By this definition, energy in the form of
heat in the Earth's crust and in individ-
ual geothermal systems are not renew-
able. Here it is concluded that it is a
good approximation to regard all geo-
thermal systems as mines of heat be-
cause the rate at which heat is replen-
ished in individual geothermal systems
anywhere on Earth, certainly for exten-
sive exploitation, is so slow that it is
hardly relevant. This conclusion is in
line with the volume method that has
been generally adapted for geothermal
energy resources estimates.67,83 Also,
most of Earth's heat is not a resource, at
least according to the definition of “re-
source” given by the United Geological
Survey and the Bureau of Mines of the
United States (Mineral Commodity
Summaries, 2011, pp 193–195). Only hy-
drothermal systems which are charac-
terized by good permeability and geo-
pressurized systems in sedimentary
basins can be considered to be an en-
ergy resource because present-day tech-
nology allows their exploitation.
Opinion is divided whether or not to
classifiy individual geothermal systems
as renewable. Duffield and Sass84 clearly
define hot-dry rock systems as non-re-
newable (mines of heat). Gudmundur
Pálmason6 considers hydrothermal sys-
tems also to be non-renewable. By con-
trast, Valgardur Stefánsson13 concludes
that geothermal energy should be re-
garded as a renewable energy resource.
According to Sanyal17 and O'Sullivan et
al.,16 the recovery time of a geothermal
system after a period of exploitation is
proportional to the amount of thermal
energy extracted from the system rela-
tive to the natural heat flow. For exploi-
tation to be economically feasible heat
extraction from individual hydrother-
mal systems is usually much in excess of
natural heat loss, or frequently by a fac-
tor of ~10 for areas presently under ex-
ploitation.17 Using this number the extra
energy withdrawn is 9 times higher
than natural heat flow. Accordingly, the
recovery time for a 50-year production
period is 450 years. For hot-dry rock
systems recovery times may exceed
100,000 years,16 depending on the ther-
mal gradient. The method of Sanyal17
and O'Sullivan et al.16 to estimate recov-
ery times likely is minimum time be-
cause some the natural heat flow from
the heat source during the recovery pe-
riod may be transported to the surface
of the geothermal field and thus lost. In
tectonic convection systems, such as
many low-temperature systems in
Iceland, there may not be any recovery.
The heat source to these systems is hot
rock in their roots and mining of heat
from the rock occurs under natural con-
ditions. After production from such
fields is stopped, mining of heat from
the rock may continue through density
driven convection.
The European Union and the U.S.
Ministry of Energy classify geothermal
energy as a renewable energy resource.
Seemingly, this classification is not
based on the nature of this resource but
a reflection of environmental policy.
Due to the adverse effects of fossil fuel
combustion on global climate as well as
seawater acidification, every measure
should be taken to reduce fossil fuel us-
age. It is for that reason logical to em-
phasize development of geothermal en-
ergy as this energy source is generally
environmentally benign. Also, the
amount of heat stored in the uppermost
parts of the crust is enormous making it
attractive to develop technology to ex-
ploit this heat source.
Hydrothermal energy is important
to many countries although its use is
very small on the world scale. To the
non-expert, the public and politicians,
the present official definition of geo-
thermal energy as a renewable resource
can be misleading and possibibly have
a negative impact in the long term on
sustainable use of this energy source.