Question

In areas of enhanced heat flow, geothermal heating can be used to supply heating to homes, either by using ground water directly or by placing a water reservoir at depth. In Iceland, where the average temperature over the year is 6.1ºC, the geothermal flux can be as high as 300 mW m-2.

(a) If you wanted to be able to supply a home with water at a temperature of 50ºC, how deep would your reservoir need to be?

(b) Why might Iceland have such a high geothermal flux compared to other areas?

Answer 1

2(a) Temperature inside the Earth increases with depth and the rate of increasing temperature with respect to increasing depth in the Earth's interior is known as geothermal flux or geothermal gradient. This heat from Earth's interior can be used as an energy source, known as geothermal energy. The geothermal gradient varies with location and is typically measured by determining the bottom open-hole temperature after borehole drilling. It averages 25 to 30 °C/km [15 °F/1000 ft] which means for every 1km/1000 ft of depth, temperature increases by 25° C/15° F but temperature gradients sometimes increase dramatically around volcanic areas. In Iceland, the flux is said to be as high as 300mW/m^2. Thus,for every 1m² of depth, the temperature increases by 300milliwatt or 0.3Watt.

It is said that the surface temperature is 6.1° C, and for tectonically active areas for every 1km depth, the temperature may increase upto 200° C. Therefore in Iceland, which is also tectonically active, if we want to be able to supply a home with water at a temperature of 50° C, we need to dig a reservoir of around 250-300metres deep.

(b) The geothermal flux or heat flow is much more concentrated in areas where thermal energy is transported towards the crust by convection such as along mid-ocean ridges and mantle plumes. Geologically, Iceland is a part of the Mid-Atlantic Ridge, a ridge along which the oceanic crust spreads and forms new oceanic crust This part of the mid-ocean ridge is located above a mantle plume, causing Iceland to be subaerial (above the surface of the sea). The ridge marks the boundary between the Eurasian and North American Plates, and Iceland was created by rifting and accretion through volcanism along the ridge and that is why it has a high geothermal flux compared to the other areas. The high rates are due to molten volcanic rock (magma) rising to the surface. Iceland represents the largest portion of the mid-Atlantic ridge exposed above sea level.