Environmental Engineering Reference
In-Depth Information
The sum of water that is lost from the tower must be replaced by make-up water:
Make-up water = Evaporation + Blowdown + Drift
A key parameter used to evaluate cooling tower operation is
cycles of concentra-
tion
. This is calculated as the ratio of the concentration of dissolved solids (or con-
ductivity) in the blowdown water compared to the make-up water. Because dissolved
solids enter the system in the make-up water and exit the system in the blowdown
water, the cycles of concentration are also approximately equal to the ratio of vol-
ume of make-up to blowdown water. From a water efficiency standpoint, cycles of
concentration should be maximized, which will minimize blowdown water quantity
and reduce make-up water demand. However, this can only be done within the con-
straints of make-up water availability and cooling tower chemistry. Dissolved solids
increase as cycles of concentration increase, which can cause scale and corrosion
problems unless carefully controlled.
Other cooling types (e.g., once-through and pond systems) may have different
water consumption and withdrawal rates, but these technologies are generally not
feasible in arid regions due to their higher withdrawal rates. Photovoltaics consume
little, if any, water during operation; some PV operators wash panels to maintain opti-
mal performance, whereas others do not. Concentrating solar technologies, including
concentrating photovoltaics (CPV) and CSP, require water for rinsing panels, mirrors,
and reflectors to ensure maximum energy production. Manufacturing solar technolo-
gies also consumes water. For a trough-based CSP facility with 6 hours of two-tank
indirect thermal energy storage (TES), Burkhardt et al. (2010) estimated that about
120 gal/MWh are consumed, mainly in the production of solar collector assemblies,
nitrate salts, and heat-transfer fluid (HTF). While water-consumption values for PV
manufacturing have not been established, Fthenakis and Kim (2010) provided some
information about water withdrawals related to PV manufacturing (i.e., water used
in the PV manufacturing process but not entirely consumed, with some of the water
processed and returned to the immediate water environment). Water consumed to
extract, process, and transport fuels can be significant for fossil fuel and nuclear tech-
The largest water consumption associated with solar-electricity production is for
cooling CSP trough and tower plants. The amount of water a CSP system consumes
for cooling depends on the technology, cooling system, location, climate, and water
availability. Three types of CSP cooling systems can be deployed: wet (once-through
or evaporative cooling using cooling towers), dry, or hybrid (combination wet/dry).
Wet cooling
(using cooling towers—evaporative water cooling) is the most common
cooling method for new power plants and currently offers the highest performance at
the lowest overall cost (Turchi et al., 2010), but it also consumes the largest amount
of water.
Dry cooling
cuts operational water consumption by as much as 97% com-
pared with wet cooling, but it increases capital costs and reduces efficiency on hot
days (Turchi, 2010). In addition, dry-cooling technology is significantly less effective
at temperatures above 100°F. The cost of electricity from a dry-cooled parabolic-
trough plant in the Mojave Desert is about 7% higher than from a similar wet-cooled
plant (Turchi, 2010; USDOE, 2009). Dish/engine CSP plants are dry cooled.
