Heat Flow and Heat Loss

Heat Flow

Heat flow is the movement of heat (energy) from the interior of Earth to the surface.  Heat flow is discussed in much greater detail here.  

Heat Loss

Heat loss is the transfer of heat from thermal fluids to the surrounding rocks, water, and air.  There are three methods of heat transfer within the Earth: radiation, conduction and convection.  Radiation is heat transfer by the emission and absorption of thermal photons.   Conduction is the transfer of energy between atoms of a material.  Convection is the movement of warm mass towards a cooler region. 

Heat Loss Basics

As geothermal fluids rise to the surface, a transfer of heat from the fluid to the surrounding rocks takes place. By the time a geothermal fluid reaches the Earth’s surface it has usually cooled by some amount, perhaps even to the mean surface temperature of the area.  The lost heat is transferred either to the ground conductively or to the air/surface convectively. When the fluid moves very slowly upward or even horizontally, the source heat is partially or completely transferred to the surrounding rock by conductive heat loss, lowering the starting initial fluid temperature. The conductive heat loss produces the thermal aureole that is mapped by thermal gradient drilling. If fluids rise quickly to the surface, before complete heat transfer can take place, then a hot spring, geyser, fumerole or geothermal system results.

 Techniques to determine the heat loss of a system include:

1. Multiply the “system” or “reservoir” temperature by the surface fluid flow rate.  This works well for systems with known flow rate and accurate temperature measurements.
2. Sum the total heat lost in the thermal aureole based on the heat flow (mW/m²) multiplied by the surface area (m²).  This technique is used when hot spring reservoir temperatures are not known or not sampled and little or no surface discharge occurs.

Chemical geothermometers can be used to estimate the reservoir temperature of a hot spring.  However, fluids sampled at the surface (if any are available) are often not appropriate for determining the geothermal system source temperature due to changes in water chemistry that occur near the surface.

A complete method of calculating heat loss needs to combine the conductive and convective components.  To measure the conductive loss a contour map of the elevated heat flow- the amount above the regional background level- is integrated.  Second, the convective thermal loss occurring from the area springs is determined (flow rate x surface temperature), relative to their maximum geochemical temperature and the flow rate.  Sum the two parts together to estimate the area heat loss.  Where possible, it is important to compare the complete method of heat loss estimation to technique 1 above.  If there is lack of agreement, it is often due to an inaccurate “geothermal system” temperature from the geothermometer calculation since this has greater uncertainty.