Heat Loss

 

Heat Loss - The heat that flows from the building interior, through the building envelope to the outside environment.

Heat - A form of thermal energy resulting from combustion, chemical reaction, friction, or movement. As a thermodynamic condition, heat, at a constant pressure, is equal to internal or intrinsic energy plus pressure times volume. 

Heat Transfer - The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space.

Conduction - The transfer of heat through a material by the transfer of kinetic energy from particle to particle; the flow of heat between two materials of different temperatures that are in direct physical contact.

Conductivity (Thermal) - This is a positive constant, k, that is a property of a substance and is used in the calculation of heat transfer rates for materials. It is the amount of heat that flows through a specified area and thickness of a material over a specified period of time when there is a temperature difference of one degree between the surfaces of the material.

Convection - The transfer of heat by means of air currents.

Go to the California Energy Commission's Website for a great glossary of energy definitions!

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Earth's Heat Loss at present is about:  74% from plate activity, 9% from  hot spots, and 17% from radiogenic heat lost from continental crust.	Heat Loss of Earth in terms of Energy:      Crust                 8x1012W        Mantle              31x1012W          Core                3x1012W         Total loss               42x1012W
99% of the Earth's mass is hotter than 1'000°C,  and less than 1% is cooler than 100°C.	The duration of typical hydrothermal systems  ranges upward from 5,000 to more than 1,000,000 years.

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Heat Loss Basic

As geothermal fluids rise to the surface, a transfer of heat from the fluid to the surrounding rocks takes place. So 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 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, or fumerole results.

 
Two techniques of determining the heat loss of a system are:

1)      Multiply the “system” or “reservoir” temperature x surface fluid flow rate, for known flow rate systems with accurate system temperatures.

2)      Sum the total heat lost in the thermal aureole, for where spring reservoir temperatures are not known or not sampled and little or no surface discharge occurs.

Chemical geothermometers are used to estimate the geothermal system (deep or reservoir) temperature of the hot springs.  However, fluids sampled at the surface (if any are available), are often not appropriate for determining the geothermal system source temperature with geothermometers because of shallow chemical changes in water chemistry.

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. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1 from the article The Nevada Story, Turning Loss into a Gain, GRC Bulletin May/June 2002.  Production capacity of existing geothermal areas versus their surface heat loss (after Wisian, 2001).  The square symbol is for geothermal plants in the Great Basin.  The circles are for all other geothermal plants.  The triangles represent the heat loss of undeveloped geothermal prospects in Nevada. (See Table 1 below for prospect names and estimated heat loss.

Table 1: Estimated Heat Loss in Nevada Geothermal Systems:  prospects with reasonable geothermal potential as depicted by heat loss 

 

Other article on Heat Loss from Geothermal Production: 

Wright, Phillip Michael, The sustainability of production from geothermal resources, Geo-Heat Center Bulletin Vol 19-2, 1998.