33 1. Overview of the different geothermal systems: role of geophysics Variation in measured heat flow at the surface of the Earth (Figure 1.2) highlights significant lateral differences across regions, suggesting a strong influence of subsurface characteristics. Indeed, the subsurface is far from being homogenous. This heat is stored in rocks and reservoirs of water deep underground. Different rock types, fractures, water presence, and other geological features, particularly related to tectonic plate activity, play a role in how heat is stored and transferred underground. Figure 1.2 From Davies (2013). Map of heat flow measurement points. Geothermal energy is all about harnessing the natural heat generated beneath the Earth’s surface. However, harvesting geothermal heat directly from the Earth’s surface is challenging because on average, the natural geothermal heat flux reaching the Earth’s surface is only about 0.06 watts per square meter, which is a tiny amount compared to solar power, which delivers around 200 watts per square meter on a sunny day. While the Earth’s surface temperature is highly influenced by atmospheric conditions, fluctuating with daily and seasonal changes, this effect diminishes rapidly just a few meters below the ground. After descending around 10 to 20 meters, the Earth’s temperature becomes nearly constant throughout the year, insulated from surface weather variations. This stable temperature zone is primarily influenced by the geothermal gradient. This stable subsurface temperature zone is crucial for surface geothermal applications, as it provides a reliable, year-round source of heat for geothermal heat pumps. To access deep geothermal energy for more energy-intensive applications like direct heating or power generation, we must drill deeper into the Earth’s crust, where temperatures are significantly higher. For direct heating applications, temperatures typically need to reach between 50 °C and above. At these depths, geothermal fluids can be used directly for district heating, greenhouse heating, aquaculture, and industrial processes. For electricity generation, however, much higher temperatures,
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