Jökull - 01.12.1966, Síða 51

thermal area can, to a limited degree, be used
to meet tlie peak demand.
The problem of the peak demand can also
be met by fuel-fired booster plants which are
operated during the peak hours. This has been
the practice in the city of Reykjavik for 20
years.
Thus, we are able to meet the variable load
partially by using the storage capacity of the
geothermal area, and partially by operating
booster plants. Moreover, in the case of heat-
ing plants, it sliould be recognized that the
massive concrete houses, such as those built in
Iceland, have a considera'ble heat storage capa-
city, which also should be taken into account.
The geothermal engineer now must derive
the optimum operation of the geothermal area,
taking into consideration the storage capacity
of the area, the heat capacity of the houses,
and the possibility of using booster plants.
This optimization is mainly of importance
when a large market has to be supplied with
insufficient geothermal power.
The theoretical problem of deriving the
optimal policy is a rather formidable task. We
are here confronted with a problem which has
many aspects in common with the problem of
the optimal use of reservoir water in hydro-
electric operations. It is beyond the scope of
the present paper to present a detailed discus-
sion of this problem, which has not yet been
given much consideration.
According to an elementary investigation, the
optimum power output of geothermal lieating
plants in Iceland is close to the yearly average
of the power demand, which has to be met
when the air temperature is 4° C. This figure
is derived without regard to the storage capa-
city of the geothermal areas, and, moreover, on
the assumption of insufficient geothermal
power.
As a matter of fact, we are unable to tackle
this problem without knowing the capacitances
and resistances of the geothermal areas. As al-
ready stated, these quantities can only be de-
rived on the basis of an elaborate well-testing
program, in combination with the methods
discussed in section (2.3). The deriving of the
system function G-^s) is the principal purpose
of these studies. The geothermal areas have to
undergo a comprehensive pressure-flow testing,
and the continuous-fraction method in section
(2.3) should theoretically yield the system para-
meters. This rnethod has still to be tested. The
field testing will invariably involve test pump-
ing of borehofes for longer periods.
This procedure should afso yield data on the
maximum geothermal power available.
(3.5) The total energy available. At this very
end of the present discussion we will devote a
few words to the problem of the total energj’
capacity of geothermal areas.
It has been statecl that the total energy avail-
able depends on the heat content of the prim-
ary and the secondary source rock. From the
the theoretical point of view, it is not difficult
to define the total energy, and to point out
methods to measure this quantity. We have to
obtain the subsurface temperature distribution
below and around the thermal areas and sum
up the available lieat energy by measuring the
volume of the source rock.
But the energy recovery depends on the
hydrological conditions, and, in general, only
a fraction of the source energy can be brought
to the surface. The efficiency of the recovery,
that is, the recovery factor, depends on various
circumstances, mainly the geometry of the sub-
surface flow system. An elementary theoretical
study appears to indicate that only a relatively
small fraction of the energy content of flood
basalt sources can be recovered, perhaps 10%
at most. This is a relatively low figure, but it
must be considered that the volume of the
source rock is very substantial in many areas.
There are hundreds of cubic kilometres avail-
able.
Tlie studies of the total energy of geothermal
areas are still in the very beginning, and a
considerable amount of work needs to be done
in this field. The main difficulty is that sub-
surface temperature data are not easily obtain-
ed. Exploratory drilling is the only reliable
method, and this is a procedure which requires
a very considerable amount of capital.
In section (2.2) we have given a very element-
ary discussion of the heating mechanism in a
source rock composed of flood basalts. The
most interesting character of this type of source
is the dependence of the available energy on
the recovery time. The amount of thermal
water which can be produced with a tempera-
ture above a certain minimum is proportional
JÖKULL 205