Final answer:
The geothermal gradient is the rate of temperature increase with depth in the Earth's crust, typically 25-30 °C/km. It is a key factor in geothermal energy extraction, as deeper drilling is required to access higher temperatures for efficient heat and electricity generation. However, the practicalities of drilling make large-scale geothermal energy challenging to realize.
Step-by-step explanation:
The geothermal gradient is defined as the change in temperature with increase in depth within the Earth's crust. Typically, away from the edges of lithospheric plates, the gradient is about 25-30 °C/km. This means for every kilometer you go below the Earth’s surface, the temperature increases by approximately 25 to 30 degrees Celsius.
The geothermal gradient affects the efficiency of geothermal heating and electricity generation. For instance, if a geothermal plant aimed to achieve 60% of the theoretical maximum heat engine efficiency, it would need to drill deeply enough to access higher temperatures that can deliver sufficient heat to run the plant effectively. The depth required would depend on the geothermal gradient; with a gradient of 25 °C/km, drilling several kilometers down might be necessary to match the efficiency of standard fossil fuel plants, which is around 35%.
Moreover, the horizontal geothermal gradient, such as that measured along the Tana River, is crucial to understand the distribution of temperature across a region, which can differ from vertical gradients. Both gradients are important in estimating the potential for geothermal energy and in studying geological processes such as metamorphism, where typically a heat source like radioactive decay within the Earth contributes to significant increases in temperature.
Despite the potential for sustainable energy extraction through geothermal heating, covering significant land area with boreholes at least 1 km deep to capture the heat is tremendously challenging, making large-scale geothermal energy provisioning a difficult target to achieve.