Tidal Current Energy

212 J. L. Renner
temperature with depth is complex. The change in heat generation and, hence,
heat flow with depth can be ignored for most general heat flow studies in conductive
areas.
Temperature ( T ) at depth ( D ) is given by T � T surface � D Γ , where Γ (temperature
gradient) is related to heat flow ( q ) and rock conductivity ( K ) by Q � K Γ .
Diment et al. [2] provide a general review of temperatures and heat flow, with
particular emphasis on heat content in the USA. Temperature at a given depth
can then be calculated using the relationship, T � T surface � D Γ , where T surface is
the average surface temperature. Tester et al. [3] , Wisian et al. [4] , and Blackwell
and Richards [5] provide detailed discussions of the relationship between rock
conductivity and heat flow. Tester et al. [3, Chapter 2] also provide insight into
projecting temperature gradients to depths beyond those reached by drilling
when heat flow and subsurface geology are reasonably well known.
In older areas of continents, such as much of North America east of the Rocky
Mountains, heat flow is generally 40–60 mW � m � 2 . This heat flow, coupled with
the thermal conductivity of average rocks in the upper 4 km of the crust, yields
a gradient of 20°C� km � 1 and subsurface temperatures of 90–110°C at 4 km depth
if the average surface temperature is 20°C. Heat flow within younger areas is
generally 70–90 mW� m � 2 and temperatures are about 150°C at 4 km. Although
direct-use applications of geothermal energy can utilize temperatures as low as
about 35°C, the minimum temperature suitable for electrical generation in most
instances is about 150°C. Therefore, areas of somewhat above average temperature
with depth require wells about 4 km deep for production of electricity and
geothermal explorationists must seek areas of much higher than average temperatures
for economic electrical production. As will be discussed later, where
energy prices are high and environmental constraints limit greenhouse gas
emissions, deeper drilling can be economic.
The previous discussion assumes that heat transfer in the earth is conductive.
Fortunately for geothermal developers, spatial variations of the thermal energy
within the mantle of the earth can drive convective transfer of mass and energy
within the mantle and crust of the earth. This convective transfer gives rise to
concentrations of thermal energy near the surface of the earth that can provide
an energy resource.
2 .
Tectonic Controls
The unifying geologic concept of plate tectonics provides a generalized view of
geologic processes that move concentrations of heat from deep within the earth
to drillable depths. The reader is directed to Kearey and Vine [6] , for example, for
a discussion of global tectonics. The heat can be related to movement of magma
within the crust, particularly when associated with recent volcanism, or deep circulation
of water in active zones of faulting. Much of the geothermal exploration
occurring worldwide is focused on major plate boundaries ( Figure 12.1 ), since
most of the current volcanic activity of the earth is located near plate boundaries
associated with spreading centers and subduction zones.