Thermal power plant includes conventional coal-fired, oil-fired, gas-fired, nuclear, solar thermal and geothermal power stations. Although design will vary, they all use water to drive the steam system that generates electricity. They also all use heat engines and heat exchangers which require cooling, usually through water, unless the heat can be used for other purposes (e.g.: for district heating or process steam supply). Water is also sometimes used to reduce emissions.
In a thermal power plant water is heated thanks to the fuel chosen: coal, oil, gas, nuclear, biomass, sun or hot rocks. The water turns into steam and spins a steam turbine which drives an electrical generator. After passing through the turbine the steam is condensed in a condenser and then re-used in the turbine. Thisinternal steam cycleis a closed-loop system: there is no heat loss to the atmosphere through evaporation.
Upon eventually leaving the turbine, the after-turbine steam is condensed and cooled.
The cooling system can operate according to different technologies:
- Direct or once-through cooling systems: a large amount of water is taken from the sea, a large river or a lake and runs through the condenser before being released back into the relevant body of water. The loss of water is negligible but the released water is slightly warmer.
- Recirculating or indirect wet cooling systems: in these systems, cooling water is recirculated within the plant, with a fraction evaporating through cooling towers. In this evaporative cooling the waste heat is transferred from the water to the air.
The amount of water necessary is influenced by a number of factors: fuel type, plant design and age, electrical efficiency, operational mode, local conditions, etc. Water released from the cooling systems is still available for subsequent use.
While dry cooling using air is an option, it is less efficient than using water: the cooling fans require a lot of power and the temperature differential is smaller. Efficiency is higher with larger temperature differentials, whether by high internal temperature, low external temperature or both. There are also combined (hybrid) cooling systems in which the hybrid cooling tower can be operated either as a pure wet cooling tower or as a combined wet/dry cooling tower, depending on the ambient air temperature.
The first figure below shows a nuclear power plant with a cooling tower and the second figure illustrates a coal power plant with a once-through cooling system.
In power plants that use fossil fuels or biomass to generate electricity, emissions abatement technologies also require water. For instance, flue gas desulfurization (FGD) systems - also called wet scrubbers - use a mix of water and lime or limestone to absorb up to 95-99% of a plant's SO₂ emissions. At some gas turbines water or steam injection is applied to reduce the formation of thermally induced NOx emissions.
Hydropower converts the energy of falling water into electricity. In all hydropower plants water flows through turbines which drive a generator to produce electricity. The electricity is produced in a power house which can be located next to the dam, further downstream or in a parallel canal. The amount of electricity that can be produced is determined by the strength of the flow (the amount of water running) and the height of the 'head' (the difference between the upstream and the downstream water levels).
A broad distinction can be made between run-of-river plants and reservoir/storage plants, depending on whether the plant is able to store water and thus potential energy. Hydropower plants without storage capacity are climate dependant: for instance, their output may be diminished during hot summers with lower water levels. By contrast, hydropower plants with reservoirs can store water and thereby provide energy supply security, even in times of climate change. Generally, the larger the reservoir, the higher the storage capacity.
Run-of-riverplants use the energy from flowing water in rivers without any substantial storage. A certain upstream water level is maintained by an adjustable weir as barrage; surplus water is discharged over the weir.
Run-of-river plants can either use a large flow rate with small head on large rivers with gentle gradient as in the picture above, or a small flow rate with high head in mountain areas.
Storage plants use water from reservoirs. For this kind of hydropower production, altitude difference is important. Reservoirs make the plant less dependent on variable natural inflow of water and allow adjustments to the amount of power produced according to demand. These plants are commonly used for intense load following and to meet peak demand. There are two types of storage hydropower plants: hydropower plants with a reservoir and pumped storage hydropower plants.
Hydropower plants with a reservoir
Hydropower plants with a reservoir are often built in mountainous areas. In these reservoirs water is collected from rainfall, rivers or melting snow and glaciers. They provide a reserve of water and energy to satisfy electricity demand during dry seasons and/or periods of high demand. Large reservoirs allow for long-term storage (monthly, seasonal and multi-annual).
Pumped storageplants operate with two reservoirs: a lower and an upper one. A river, a lake or an existing reservoir can serve as either reservoir. Pumped storage plants are used to store electricity for short periods of time. Whenever there is a surplus of electricity in the grid (e.g. because of strong wind or during the night when demand is low), this electricity is used to pump up water from the lower to the upper-level reservoir. In times of peak demand, the water can then be released to generate electricity.
A tidal power station uses turbines placed under water to capture the energy generated by moving water due to tides. A dam creates a difference in the height levels of the water, which can pass through the turbines in both directions (during the incoming and outgoing tidal flows). On the ebb (outgoing) tide the barrage traps water behind it and releases it slowly as it generates electricity.
Alternatively, energy from tidal currents can also be converted by turbines similar to wind turbines. They are submerged offshore, generally in areas with strong currents and, whenever possible, close to the coastline. Turbine designs vary, for instance turbines can have a horizontal or a vertical axis.
Wave energy refers to capturing the energy of ocean waves, driven by the wind blowing onto the water surface. There are different techniques to capture wave energy:
- Float or buoy systems: the rising and falling waves drive hydraulic pumps, which can be fixed on a floating raft or to a device on the ocean floor. Buoys can be either vertical or horizontal.
- Oscillating water column: the rise and fall of water into a column, following the motions of the waves, moves air or fluid at the top of the column in order to spin a generator.
- Tapered channel or "tapchan" systems: structures near the shore are used to channel and concentrate waves into an elevated reservoir. The water flowing out of the reservoir is used to generate electricity through turbines, as in traditional hydropower technologies.