POWER September 2014 - 53

WATER
3. Hourly efficiency penalties from water temperature. Generating units often
operate less efficiently with closed-cycle cooling, leading to increases in electricity production
costs and emissions. Source: Veritas Economic Consulting LLC
■ Closed-cycle effect ■ Once-through effect
9%
8%
7%
6%
5%
4%
3%
2%
1%
0%
1,000
2,000
3,000
4,000
Hour
reduce their efficiency.
This effect is specific to the atmospheric,
water temperature, and operating characteristics
of affected units. As Figure 3 illustrates,
these efficiency impacts vary over the year.
The red line represents efficiency reductions
using once-through source water over the
8,760 hours in a year. The unit begins the
year at maximum efficiency, becomes less
efficient as water warms in the summer, and
more efficient as it cools down in the fall and
winter. The maximum efficiency loss that occurs
around hour 5,000 is about 4%.
The blue line represents the impact from
using closed-cycle cooling water. As the
figure indicates, when using this warmer
water, the unit is almost never at maximum
efficiency. At the worst point, the unit loses
about 8% of capacity. Whereas once-through
efficiencies relate to source water temperatures,
closed-cycle efficiencies are related to
atmospheric heat and humidity (that is, wet
bulb temperatures). This leads to the more
variable hourly effect evident in blue.
In the context of system dispatch, this is important;
high electrical loads often coincide with
5,000
6,000
7,000
8,000
high wet bulb temperatures because hot humid
air increases air conditioning load. As a result,
this effect is at its worst when system generation
efficiencies are low, leading to relatively high
production cost increases in these hours.
The externality implications of the energy
penalty effect depend on fuel type. Because
nuclear plants operate on a fixed amount of
fuel, efficiency reductions become losses in
capacity and emission-free nuclear generation
must be replaced with generation from
marginal (load-following or peaking) thermal
sources elsewhere on the grid. This leads to
higher system-level fuel consumption, emissions,
and costs. Conversely, fossil-fueled units
typically can use more fuel. In this case, efficiency
is reduced as described; however, capacity
is not. Rather than system level changes, the
efficiency effect translates into increased fuel
consumption and air emissions at the site.
Many Unknowns
Observing externalities at economic equilibrium
is helpful in identifying what social costs
would occur with cooling tower retrofits. Ideally,
these costs will be measured and quantified.
Although no specific guidance is yet available,
some of these costs have been studied.
For example, the Electric Power Research
Institute (EPRI) Closed Cycle Cooling program
has studied many of the issues described
in this article. EPRI Report 1022760 (2011)
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September 2014 | POWER
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53
VEO HORIZON Gas-Turbine HRSG Series
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Hourly efficiency penalty (%)

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POWER September 2014

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