what is hot spring?
How much energy is carried by a single wave depends on what?
HOW DOES THE PRSCENCE OF MAGMA CLOSE TO SURFACE AFFECT THE POTENTIAL OF GEOTHERMAL POWER.
What is hot spring?
The most important practical factors involved in assessing an area's geothermal potential are:
• the presence of magmas close to the surface in volcanically active regions;
• high heat flow in near-surface solid rocks;
• the ability to transfer geothermal heat to power plants and other energy consuming technologies efficiently, usually as hot water or steam.
The most favourable areas for geothermal exploitation might seem to be those in volcanically active regions, at destructive and constructive plate margins. The distribution of geothermal electricity generation plants confirms this.
What is less obvious is that the geothermal potential of some areas that are remote from volcanically active regions is also worth exploiting. The geothermal energy exploited directly by two of the major users, China and Turkey, is not related to active volcanism, but to abnormally high heat flow.
The local potential of geothermal energy is fundamentally a function of the enthalpy of the area, i.e. the total energy content of the geothermal system that lies below it. The concept of enthalpy is
H = U + PV
Both the heat content (U) and the pressure (P) of steam in a geothermal system play a role in converting geothermal energy into electricity.
Enthalpy cannot be measured directly for a geothermal energy source. However, it can be estimated from the heat flow through the crust, which is measurable at the surface. Heat flow is expressed as the geothermal power associated with an area of the surface, measured in mW m-2 .
The diagram shows that large areas of the Earth's surface have much the same heat flow. However, the temperature at depth in the crust depends on how conductive different rocks are, and this in turn depends on their physical properties. This subsurface temperature is crucial in evaluating whether or not an area constitutes a geothermal resource.
The diagram shows that on a global scale, the main areas where heat flow is much greater than the global average of 60 mW m-2 are close to constructive plate margins beneath the oceans.
In volcanically active continental areas that lie above subduction zones, as around the Pacific, regional heat flow can be above the average, but only up to 120 mW m-2 .
However, values on the continents can be as high as 300 mW m-2 where magma is being generated locally. Such continental occurrences and volcanically active oceanic islands are the most favourable for geothermal exploitation, for obvious reasons.
Specialists recognize three kinds of area with geothermal potential - those with high, medium and low enthalpy.
High-enthalpy areas are those where subterranean water and steam are at temperatures greater than 180 °C,
Medium-enthalpy areas those where temperatures range between the boiling temperature of water (100 °C) and 180 °C, and
Low-enthalpy areas where temperatures are lower than 100 °C.
A mass of pressurized steam has an energy content that includes the pressure-volume (PV) term in the equation, and also contains the heat involved in converting liquid water into gas (the latent heat of vaporization).
To allow their successful exploitation, geothermal fields usually need three characteristics:
1. An energy source to heat the fluids;
2. An aquifer, either natural or artificially created , to act as a reservoir for fluids that can transport heat energy to the surface;
3. A cap rock (mudstones and unfractured lavas are idea) with low permeability to seal in these geothermal fluids - in a similar way to the trapping of petroleum.
If hot fluids removed through one borehole are replaced by cold water pumped in through another to recharge the geothermal field, the cold water heats up again. Carefully managed recharge enables the enthalpy of an area to be exploited continuously.