Understanding Geothermal Energy and Electricity Production

The question asks what happens to underground water when it comes into contact with hot rocks deep inside the Earth, and how this process helps generate electricity by turning turbines.

How Hot Rocks Heat Underground Water

Deep within the Earth, there is significant heat from the Earth's core and the decay of radioactive elements. This heat warms the surrounding rocks. When underground water, also known as groundwater, seeps into areas where these rocks are very hot, the water absorbs this heat.

The State Change: From Water to Steam

When water is heated to a high enough temperature under sufficient pressure (due to the depth underground), it changes its state from liquid water to steam. Because this process often occurs at great depths, the pressure is high, resulting in high-pressure steam.

This high-pressure steam is the key to generating electricity in many geothermal power plants.

Driving Turbines with High-Pressure Steam

Geothermal power plants are designed to capture this high-pressure steam (or sometimes superheated water) and direct it towards a turbine. The force of the expanding steam causes the turbine blades to spin rapidly. The turbine is connected to a generator, which is a device that converts mechanical energy (the spinning turbine) into electrical energy. Thus, the turning of the turbines by the high-pressure steam leads to the production of electricity.

Analyzing the Options

• Option 1: Turns to ice. This is incorrect. Hot rocks would heat the water, not cool it to ice.
• Option 2: Turns to steam at high pressure. This aligns with the principles of geothermal energy where heated underground water becomes high-pressure steam, used to drive turbines.
• Option 3: Cools down. This is incorrect. The presence of hot rocks indicates the water is being heated, not cooled.
• Option 4: Flows out. While water does flow underground, simply flowing out doesn't explain how it would turn turbines to generate electricity in this context. The critical aspect is the transformation into high-pressure steam (or hot fluid) due to the heat from the rocks.

Therefore, the correct description of what happens to underground water due to hot rocks that can turn turbines to produce electricity is that it turns to steam at high pressure.

Revision Table: Key Concepts

Additional Information: Types of Geothermal Power Plants

There are different types of geothermal power plants based on the nature of the geothermal resource:

• Dry Steam Plants: Use directly tapped high-pressure steam from underground reservoirs. This steam is sent directly to a turbine.
• Flash Steam Plants: Use high-pressure hot water from underground. When this hot water is brought to the surface, the pressure drops, causing some of the water to "flash" into steam, which then drives a turbine.
• Binary Cycle Plants: Use geothermal hot water (which might not be hot enough to flash to steam) to heat a secondary working fluid (with a lower boiling point). This working fluid turns into vapor, which drives a turbine. This method uses the geothermal heat less directly but can utilize lower-temperature resources.

All these methods rely on the heat from underground rocks to create a force (steam or vapor pressure) capable of rotating a turbine connected to a generator for electricity production.