APES 6.9-6.10: Hydroelectric, Geothermal, and Energy Conservation

Hydroelectric power

Electricity generated by converting the energy of moving or elevated water into mechanical energy and then electrical energy. A turbine drives a generator.

How does a conventional dam-based hydroelectric plant generate electricity?

Water stored at an elevated height flows through a penstock, turning a turbine. The turbine drives a generator, which converts mechanical energy into electrical energy.

Hydraulic head

The vertical height difference, $Δh$, between the water source or reservoir and the turbine outflow. Greater head generally means more gravitational potential energy available per unit mass of water.

What factors determine the power available from falling water?

Power depends on the water density $ρ$, volumetric flow rate $ẏ$, gravitational acceleration $g$, and hydraulic head $Δh$. The ideal available power is $ρẏ gΔh$.

Hydroelectric power equation

The approximate electrical power output is $P=ηρẏ gΔh$, where $η$ is efficiency, $ρ$ is water density, $ẏ$ is volumetric flow rate, $g\approx9.8\ \text{m/s}^2$, and $Δh$ is the head.

How is volumetric flow rate related to mass flow rate in a hydroelectric system?

Mass flow rate is $ẏ_m=ρẏ$, where $ẏ_m$ is in $​\text{kg/s}$, $ρ$ is density in $​\text{kg/m}^3$, and $ẏ$ is volume flow rate in $​\text{m}^3/\text{s}$.

Hydroelectric efficiency

Efficiency $η$ is the fraction of the water's mechanical energy converted into electrical energy: $η=P_{\text{electric}}/P_{\text{available}}$. It is dimensionless and cannot exceed 1.

How does increasing the flow rate affect hydroelectric power output if all other variables remain constant?

Because $P=ηρẏ gΔh$, power is directly proportional to flow rate. Doubling the flow rate approximately doubles the electrical power output.

How does increasing the hydraulic head affect hydroelectric power output?

Power is directly proportional to head, according to $P=ηρẏ gΔh$. Doubling the height difference approximately doubles the power if flow rate and efficiency remain constant.

Why can a hydroelectric station produce little power during a drought?

Drought can reduce river flow and reservoir volume, decreasing the mass of water passing through the turbines. Since power is proportional to flow rate, $P$ decreases.

Pumped-storage hydroelectricity

A grid-storage method in which electricity is used during low-demand periods to pump water to a higher reservoir. During high demand, the water flows downward through turbines to generate electricity.

Why is pumped-storage hydroelectricity not an energy source?

It stores energy supplied by another source rather than creating net energy. Pumping requires more energy than is later recovered because of inefficiencies, so pumped storage is an energy-storage system.

How does pumped storage help balance an electrical grid?

It absorbs excess electricity when demand is low and releases stored gravitational potential energy when demand is high. This shifts generation to times when electricity has greater value.

Run-of-the-river hydroelectricity

A hydroelectric system with little or no reservoir storage that generates electricity from the water currently arriving from upstream. Excess flow generally passes through without being stored for later use.

Why is run-of-the-river generation more dependent on weather and seasonal flow than reservoir-based generation?

It has limited ability to store water, so its output closely follows the instantaneous river discharge. Seasonal rainfall, snowmelt, and drought therefore directly affect electricity production.

Small, micro, and pico hydropower

Small hydropower commonly refers to facilities up to about $10\ \text{MW}$, microhydro typically produces up to about $100\ \text{kW}$, and picohydro produces less than about $5\ \text{kW}$. Boundaries vary by country and organization.

Why can micro- and picohydropower be useful in isolated communities?

They can provide electricity without extending a national grid or purchasing fuel. Their small scale is suited to local streams and modest loads such as lighting and communication devices.

Why are hydroelectric plants considered dispatchable?

Operators can control water flow through turbines, allowing output to be increased or decreased in response to demand. Reservoir-based plants can often ramp from low output to full load within minutes.

Hydroelectricity as a source of peak power

Hydroelectric plants with reservoirs can store water and release it during periods of high electricity demand. This makes their electricity especially valuable compared with generation that is available only intermittently.

Why can hydropower complement wind and solar power?

Hydropower can rapidly increase or decrease output when wind or solar generation changes. Reservoirs and pumped storage can also shift energy from times of excess renewable generation to times of high demand.

Lifecycle greenhouse-gas emissions of hydropower

Hydropower generally has low lifecycle greenhouse-gas emissions because it does not burn fuel during operation. Construction produces emissions, and some reservoirs—especially flooded tropical forests—can release methane.

Why can a tropical hydroelectric reservoir emit methane?

Flooded vegetation and soil may decompose under oxygen-poor conditions, producing methane. Methane is a potent greenhouse gas, so emissions can be significant in some lowland rainforest reservoirs.

What direct air pollutants are largely absent from normal hydroelectric operation?

Because hydroelectric plants do not combust fuel, normal operation produces essentially no direct emissions of carbon dioxide, sulfur dioxide, nitrogen oxides, or particulate matter from combustion.

Environmental effects of dam reservoirs

Reservoirs can inundate forests, wetlands, grasslands, farmland, and river valleys. They may fragment habitats, displace people and wildlife, and alter the physical and chemical conditions of aquatic ecosystems.

How does a dam alter a river ecosystem downstream?

It changes the timing and amount of water released and can disrupt natural flow patterns. Water released through turbines may contain little suspended sediment, promoting downstream bed and bank erosion.

Siltation in a hydroelectric reservoir

Siltation is the accumulation of transported sediment in a reservoir. It reduces storage capacity, can impair flood-control function, and may eventually reduce the amount of water available for power generation.

Why does sediment trapping behind a dam cause downstream erosion?

Water leaving the reservoir has reduced sediment load but still has flow energy. It may therefore erode the riverbed and banks downstream as it gains sediment.

How can evaporation reduce hydroelectric output?

Water lost from a reservoir by evaporation lowers the volume available to pass through turbines. The effect is particularly important in hot, dry climates or in reservoirs with large surface areas.

How can climate change affect hydroelectric generation?

Changes in precipitation, temperature, snowmelt, evaporation, and drought frequency can alter river discharge and reservoir levels. Reduced flow lowers available power because $P$ is proportional to $ẏ$.

Why can dam construction cause human displacement?

The reservoir may flood homes, farms, communities, and culturally important sites. People living in the inundated area may need to relocate, creating social and economic impacts.

How can a hydroelectric dam provide benefits besides electricity?

Its reservoir may support irrigation, municipal water supplies, flood control, navigation, aquaculture, recreation, and tourism. Managing these uses can create trade-offs because water released for one purpose may not be available for another.

Why does hydroelectric generation not always operate at its maximum nameplate capacity?

Nameplate capacity is the maximum instantaneous output under specified conditions, whereas actual generation depends on water availability, maintenance, environmental releases, and demand. Seasonal variation in flow can be substantial.

Hydroelectric plant nameplate capacity

The maximum electrical power a facility is designed to produce under specified operating conditions, usually expressed in megawatts. It is a power rating, not the total amount of energy generated over time.

Distinguish power from energy in a hydroelectric context.

Power is the rate of energy production, measured in watts, while energy is the accumulated amount produced, measured in joules or watt-hours. A plant's energy generation depends on its power output and operating time.

Why does a larger, modern turbine often have greater efficiency?

Improved turbine and generator design can reduce mechanical, hydraulic, and electrical losses. In the equation $P=ηρẏ gΔh$, a higher $η$ increases the fraction of water energy converted to electricity.

What is the main energy transformation in conventional hydroelectric generation?

Gravitational potential energy of elevated water becomes kinetic energy as the water flows, then mechanical rotational energy in the turbine, and finally electrical energy in the generator.

Why is dam failure considered a severe hazard even though dams can reduce flooding?

A functioning dam can regulate downstream flow and reduce some flood risks, but sudden structural failure can release a large volume of water rapidly. The resulting flood can cause catastrophic damage downstream.