GNS Science Consultancy Report 2006/0XX

1.0 INTRODUCTION
With the technological advances in the last 20 years the definition of an economically viable
geothermal resource has broadened to a resource that extracts heat from the rock or
circulating aquifer waters at temperatures ranging from as low as 4 o C (Lund and Freeston,
2001) to as high as sub-magmatic (400 o C) and from about 15m to >3500m depths. At
shallow depths and temperatures of 10-35 o C heat can be harnessed from the ground or
circulating water in boreholes or warm water in abandoned mines using ground source heat
pumps (Lund et al, 2005). In Central and Northern Europe, for example, all heat energy
stored below about 15m from the surface is considered geothermal energy (Rybach and
Sanner, 2000).
For some of the unconventional sources of geothermal energy, such as heat from
sedimentary basins and hot dry rock (HDR), permeability or the volume of circulating fluids
may be too low to economically extract heat. To increase the productivity and lifetime of
these marginal geothermal sources for power generation, enhanced or engineered
geothermal systems (EGS) technology may be required where, essentially, low permeability
hot rock at depth is artificially fractured and fluid is introduced into the newly-fractured rock
where it is heated by conduction. The heated fluids are recirculated to the surface by pumps
and the heat extracted by a heat exchanger (e.g. Smith, 1983; White, 1983).
1.1 GEOTHERMAL RESOURCES IN NEW ZEALAND
There are conventional and non-conventional sources of geothermal energy for power
generation and direct heat utilisation in New Zealand (Figure 1) ranging in temperature from
about 12 to 15 o C at about 15m to as high as >330 o C at >3,500m. Power can be generated
by steam turbines or by binary cycle systems using the Organic Rankine Cycle (ORC) where
the working organic fluid has a low boiling temperature such as isopentane, or the Kalina
Cycle which uses ammonia/water.
The newest system is called the NE (Natural Energy) Engine that operates on hot and cold
water and uses liquid CO 2 as a working fluid in the heat exchanger engine
(www.fossil.energy.gov).
Conventional sources include (1) high enthalpy hot spring systems in the Taupo Volcanic
Zone (TVZ) and Ngawha in Northland some of which are now harnessed for power
production, (2) waste-water from high enthalpy power-generating systems such as Wairakei,
Mokai, Ohaaki-Broadlands, Kawerau, Rotokawa and Ngawha and (3) hot spring systems
outside the Taupo Volcanic Zone and Ngawha and including springs in the North Island,
islands in the Bay of Plenty and Hauraki Gulf and in the South Island (Figure 2).
Nonconventional sources of geothermal energy include: (1) most of the TVZ outside high
enthalpy geothermal systems, where thermal gradients range from 40 o C/km to >50 o C/km but
permeability low, (2) 12 o C to 170 o C waters in abandoned oil and gas (hydrocarbon) wells, (3)
heated waters in abandoned flooded coal and mineral mines, (4) conductive heat from
shallow depths (70 mW/m 2 ) sedimentary basins, metamorphic terrain and rapidly
rising regions of the country such as the Raukumara Peninsula (Mazengarb and Speden,
2000) and the Southern Alps (Allis and Shi, 1995) and regions outside high heat flow
regions. The location of some of the nonconventional heat sources available in New Zealand
are shown in Figure 2.
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