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February 10, 2009

Water and Energy


For any energy source, the cooling water can be waste-water or salt-water if the siting of the power plant has those non-fresh water sources available. Coal and solar-thermal can have more siting issues. Coal needs to be nearer to coal resources because millions of tons/year of coal is needed per plant. Solar thermal needs to be in sunny areas.

The main concerns with water that is used for energy relate to the fresh water budget of an area and rate of fresh water withdrawals. So if fresh water is turned into steam that leaves an area and falls as rain elsewhere it is no longer part of the current water budget for a particular area.

Understanding Water
Some water definitions


Watershed (Drainage Basin, Catchment Basin, River Basin)
(1) The total region or area above a given point on a stream or lake that contributes water to the flow at that point. It is designated by the dividing line of area from which surface streams flow in two different directions; the line separating two contiguous drainage areas. (2) United States usage. The total area above a given point on a stream that contributes water to the flow at that point. The entire region drained by a waterway or which drains into a lake or reservoir. (3) British usage. The topographic dividing line from which surface streams flow in two different directions; the line separating two contiguous drainage areas. (4) The area drained by a stream and its tributaries

Watershed Management
Managing all the natural resources of a watershed to protect, maintain, or improve its desired water budget, both quantity and quality, over time.
Water usage and biofuels.

A useful measure of performance from a water-efficiency standpoint is the net energy yield per unit of water withdrawn or consumed. Consumptive use of water is largely due to evaporation losses from cooling towers and evaporators during the distillation of ethanol following fermentation. Consumptive use of water is difficult to directly measure because it depends on relative humidity, wind speed, and temperature in addition to the process configuration. However, water permits are generally required from state authorities to withdraw well water or surface water for industrial use, and this water is more or less continually metered. For that reason, this report considers water withdrawals as the measure of water use. This includes both consumptive and non-consumptive use, but as biorefineries increasingly incorporate water recycling, the difference between consumptive and total water use is decreasing.


Water budget. A summation of inputs, outputs, and net changes to a particular water resource system over a fixed period. (Also, water balance model).

Wikipedia on water resources.

It is estimated that 69% of world-wide water use is for irrigation, with 15-35% of irrigation withdrawals being unsustainable.

It is estimated that 15% of world-wide water use is industrial. Major industrial users include power plants, which use water for cooling or as a power source (i.e. hydroelectric plants), ore and oil refineries, which use water in chemical processes, and manufacturing plants, which use water as a solvent.

It is estimated that 15% of world-wide water use is for household purposes. These include drinking water, bathing, cooking, sanitation, and gardening. Basic household water requirements have been estimated by Peter Gleick at around 50 liters per person per day, excluding water for gardens.

Recreational water use is usually a very small but growing percentage of total water use.

Explicit environmental water use is also a very small but growing percentage of total water use. Environmental water usage includes artificial wetlands, artificial lakes intended to create wildlife habitat, fish ladders around dams, and water releases from reservoirs timed to help fish spawn.


Nuclear Energy and Water

Nuclear reactors that are on the coast use salt water for cooling. About 40 of the 104 nuclear reactors in the United States are using salt water cooling.

Using seawater means that higher-grade materials must be used to prevent corrosion, but cooling is often more efficient. In a 2008 French government study, siting an EPR on a river instead of the coast would decrease its output by 0.9% and increase the kWh cost by 3%.

Apart from proximity to the main load centres, there is no reason to site nuclear power plants away from a coast, where they can use once-through seawater cooling. Coal plant locations need to have consideration for the logistics of fuel supply (and associated aesthetics), with over three million tonnes of coal being required per year for each 1000 MWe plant.

* The amount of cooling required by any steam-cycle power plant (of a given size) is determined by its thermal efficiency. It has nothing essentially to do with whether it is fuelled by coal, gas or uranium.
* However, currently operating nuclear plants often do have lower thermal efficiency than coal counterparts of similar age.
* Where availability of cooling water is limited, cooling does not need to be a constraint on new generating capacity. Alternative cooling options are available at slightly higher cost.






Apart from size, any differences between plants is due to thermal efficiency, ie how much heat has to be discharged into the environment, which in turn largely depends on the operating temperature in the steam generators.

A nuclear or coal plant running at 33% thermal efficiency will need to dump about 14% more heat than one at 36% efficiency. Nuclear plants currently being built have about 34-36% thermal efficiency, depending on site (especially water temperature). Older ones are often only 32-33% efficient. The relatively new Stanwell coal-fired plant in Queensland runs at 36%, but some new coal-fired plants approach 40% and one of the new nuclear reactors claims 39%.

USA 43% of thermal electric generating capacity uses once-through cooling, 42% wet recirculating cooling, 14% cooling ponds and 1% dry cooling (this being gas combined cycle only). The spread for coal and for nuclear is similar.

Recirculating wet cooling : Where a power plant does not have abundant water, it can discharge surplus heat to the air using recirculating water systems which mostly use the physics of evaporation.

Dry Cooling: A few power plants are cooled simply by air, without relying on the physics of evaporation. These work like a automobile radiator and employ high-flow forced draft past a system of finned tubes in the condenser through which the steam passes, simply transferring its heat to the ambient air directly. The whole power plant then uses less than 10% of the water required for a wet-cooled plant, but a lot of power (around one to 1.5 percent of power station's output) is consumed by the large fans required. This is direct dry cooling, using air-cooled condenser (ACC).

Generation III+ nuclear plants have high thermal efficiency relative to older ones, and should not be disadvantaged relative to coal by water use considerations.

Despite many coal and nuclear plants using wet cooling towers, in the USA electric power generation accounts for about 3% of all freshwater consumption, according to the US Geological Survey - some 15.2 gigalitres per day (5550 GL/yr).

Sandia Presentation on Water and Energy
Sandia has a 14 page water and energy overview








FURTHER READING
Production of High Purity Water From Seawater at Diablo Canyon Nuclear Plant
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