MEMORANDUM

Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone	703-385-6000

fax	703-385-6007

DATE: 		January 16, 2009

TO:			Paul Shriner and Jan Matuszko, EPA

	

FROM:		John Sunda, SAIC and Kelly Meadows, Tetra Tech

SUBJECT:	Helper Cooling Towers

Cooling towers are one technology used to discharge waste heat from
electric generators and, depending on their configuration, can also
reduce the volume of cooling water withdrawn from the source waterbody. 
EPA tasked Tetra Tech and SAIC with describing the prevalence and
characteristics of “helper” cooling towers, which serve a different
purpose at power plants.

Cooling System Configuration

Cooling water systems at power plants that utilize cooling towers may be
capable of operating in one or more of the following flow
configurations: 

Once-through mode (non-recirculating) – A facility may elect to bypass
its cooling towers and operate in once-through mode as a cost saving
measure.  There is no reduction in intake flow or heat rejection while
operating in this mode.

Closed-cycle mode (recirculating) – Warm water exiting the steam
condensers is passed through cooling towers and is then returned to the
cool water inlet of the steam condensers.  This configuration minimizes
the volume of cooling water that must be withdrawn and rejects most of
the waste heat to the atmosphere.

Helper mode (non-recirculating) – Cooling water flows through a
once-through cooling system with warm water effluent from the steam
condensers passing through cooling towers prior to being discharged. 
There is no reduction in intake flow, but there is a reduction in the
waste heat discharged to the receiving waterbody.

Combination mode – Portions of the cooling water flow in more than one
of the above configurations.

As described above, cooling towers can be operated in either the
closed-cycle or helper modes (or sometimes not at all).  At most plants,
cooling towers and the associated piping are configured to operate in
either the closed-cycle mode or helper mode but not both.

Helper Tower Use in the Phase II Industry

Of the 220 Phase II facilities that reported information on cooling
system configuration, only 12 (approximately 5.5%) plants indicated that
they used a cooling system that was “Once-Through With
Non-Recirculating Cooling Towers,” another name for helper towers.  By
comparison, 81 of the 526 Phase II facilities (15%) reported using
recirculating cooling towers either in closed cycle or combination mode.
 

Helper Mode

Typical Usage and Design

Cooling towers operating in the helper mode are intended to serve only
one purpose: removing enough waste heat to ensure that the heat rejected
to the receiving water meets NPDES discharge limits.  These limits
typically include a maximum increase in water temperature and a maximum
discharge temperature; in some cases, they may also include a maximum
discharge heat load (typically measured in BTU).  In other words, helper
cooling towers generally only function to help facilities meet discharge
requirements associated with Section 316(a).  Helper towers are most
commonly used during warmer summer periods when discharge temperature
limits are more difficult to meet due to higher ambient water
temperatures.  Helper towers also allow a plant to avoid reducing its
power generation (derate) to meet heat load-related discharge limits.

Since helper towers serve a different purpose, they are often sized
differently than cooling towers used for closed cycle cooling.  Helper
towers are typically sized to transfer only enough waste heat to the
atmosphere to ensure that the discharge limits are met.  Thus, they may
be designed to cool only a portion of the total cooling water flow
volume; typically, the helper tower cool water discharge is mixed with
the remaining once-through cooling water prior to final discharge.

Typical Operation

Because the cooling towers require energy to operate, facilities usually
minimize helper tower operation, with the periods of tower operation and
pumping rates being adjusted to meet the NPDES temperature limits.  For
example, Georgia Power’s Harllee Branch Power Plant installed a helper
cooling tower in 2001 that was capable of cooling only about half of the
total cooling water flow.  The tower operation starts and stops when the
plant's flow-weighted discharge temperature exceeds or falls below 94
oF.  The tower is capable of reducing the temperature of the water
passing through it by about 20oF (Blankenship 2001).

While they may be sized differently, there is nothing inherent in the
design of helper cooling towers that would prevent them from being used
in a cooling system that was converted to operate in a closed cycle or
mixed mode configuration.  However, depending on the flow and heat
rejection capacity, they may not be capable of providing full
recirculation without construction of additional towers.  Issues
associated with converting helper cooling towers to operate in closed
cycle or mixed mode are discussed in greater detail below.

Effects on Biota

Cooling towers operating in the helper mode may provide little benefit
with regard to impingement mortality or entrainment reduction and may
possibly be detrimental with regard to a reduction in survival rate of
entrained organisms due to an increase in mechanical injury resulting
from the additional passage of the cooling water through the tower. 
However, they may also provide some benefits with regard to reduced heat
stress and improved dissolved oxygen for entrained organisms in the
cooling water that has passed through the towers and, of course, for
organisms in the receiving water.

Mixed Mode Systems

As described above, the pumping and piping system for cooling systems
utilizing cooling towers can be configured such that the system can
alternate between once-through, closed cycle, helper, and combination
modes.  Such mixed mode systems offer greater flexibility for meeting
316(a) and 316(b) permit conditions while optimizing power generation. 
Such flexibility requires the installation of a flow and temperature
monitoring and control system and the establishment of decision rules
that guide the timing and selection of specific operating modes.  Such
decisions will be guided by the environmental standard requirements that
must be met and the overall design of the cooling system. 

In the past, the environmental standards of concern were primarily
associated with 316(a), as discussed in a case study of TVA's Browns
Ferry Nuclear Plant (Stolzenbach et al 1979).  In 1979, the existing
cooling towers at the Browns Ferry Nuclear Plant were built with the
capability to operate in both the helper and closed-cycle mode and this
capability was unique among power stations at that time.  The case study
provides an analysis of the multimode operation at the Browns Ferry
Nuclear plant, evaluating mixed mode operation and the capability of
computer models to predict river temperatures.  The goal was to optimize
power generation while meeting 316(a) limits.  The study did not address
flow reduction.  

A cursory review of the model output graphs provides the following rough
estimates of the relative magnitude of energy penalties for operating
cooling towers in the helper and closed cycle mode in comparison to
once-through mode.  The analysis estimates that while operating cooling
towers in full helper mode, total plant output would drop roughly 60 to
85 MW or 1.8% to 2.6% of the 3300 MW capacity.  And that while operating
cooling towers in closed cycle mode, total plant output would drop
roughly 110 to 150 MW or 3.3% to 4.5% of the 3300 MW capacity.  Thus,
the case study estimates that the parasitic energy penalty associated
with cooling towers operated in either the helper or closed cycle mode
would range from 1.8% to 2.6% and that the turbine efficiency penalty
(i.e., difference between helper and closed cycle) would range from 1.5%
to 2.0%. 

The case study’s model predicted the net power generation for each
mode of operation throughout the year and selected the optimum mode of
operation for optimizing power generation while meeting 316(a)
limitations.  Closed-cycle operation was selected by the model as
optimum power generation mode for less than 10 days of the year. 
Depending on the selected scenario, helper mode operation was selected
for roughly 30 to 40 days of the year. 

Converting Helper Cooling Towers to Recirculating or Mixed Mode Cooling
Towers

As noted above, only about 5.5% of Phase II facilities reported using
helper cooling towers in 2000.  Thus, the availability of this
technology for conversion to closed-cycle or mixed mode operation is
somewhat limited.  In existing helper cooling towers, warm water that
exits the condensers is typically pumped from either the condenser
outlet conduit or the discharge channel up to the top of the helper
cooling tower and then flows by gravity back into the discharge conduit
or channel in a location downstream of the withdrawal point or is
discharged through a separate discharge conduit.  In other systems, the
intake pumps, pipes, and condenser cooling system are hydraulically
designed such that the intake pumps are capable of pumping water through
the condensers and then up to the top of the cooling towers.  Either
system could be converted to one that operates in a multi-mode fashion
capable of recirculating the volume that the tower is designed to
handle, which may involve only partial closed cycle (combination) mode
if helper tower flow capacity is limited. 

To convert helper cooling towers to closed-cycle mode, facilities would
need to return the cooled water from the tower basins back to the intake
canal/channel or the condenser cooling water circulation pump well. 
Depending on the location of the cooling tower basins and/or discharge
channel in relation to the intake, various engineering solutions may be
available to construct return flow pipes or channels.  In many cases,
the elevation of the cooling tower basins will be high enough to allow
return flow to occur by gravity, or installation of control structures
and a return channel connecting the discharge channel to the intake
channel could provide for return flow.  Either type of system would not
require an additional set of pumps.  

The costs of converting a helper tower system to a closed-cycle or mixed
mode system will include the costs of additional piping, channels,
valves and control system upgrades.  The magnitude of these construction
costs may vary greatly depending on site-specific conditions.  While
site-specific challenges may exist with regard to the placement and
design of return piping or channels, in most cases engineering solutions
should be available and the difficulties and distances involved will be
major drivers in determining the magnitude of the project capital costs.
 No matter the solution, such a system modification would provide for an
intake flow reduction associated with closed cycle cooling while
avoiding the substantial cost component associated with construction of
new cooling towers.

References

Blankinship, Steve. “Georgia Power's New Cooling Tower Design Reduces
Environmental Impact.” Power Engineering. September 2001. Article
accessed on December 19, 2008 at website:

  HYPERLINK
"http://pepei.pennnet.com/articles/article_display.cfm?article_id=121621
" 
http://pepei.pennnet.com/articles/article_display.cfm?article_id=121621 

Stolzenbach, Keith D. Freudberg, Stuart A. Ostrowski, Peter and Rhodes,
John A. 

Operational Issues Involving Use of Supplementary Cooling Towers to Meet
Stream Temperature Standards with Application to the Browns Ferry
Nuclear Plant.  MIT Energy Laboratory Report No. MIT-EL 79-036.  January
1979.

Accessed on December 19, 2008 at website:

  HYPERLINK
"http://dspace.mit.edu/bitstream/handle/1721.1/35204/MIT-EL-79-036-09510
127.pdf?sequence=1" 
http://dspace.mit.edu/bitstream/handle/1721.1/35204/MIT-EL-79-036-095101
27.pdf?sequence=1 

Sunda, J., SAIC.  Memo: Lost Generation - Construction Downtime and
Energy Penalty. December 5, 2008.

 Additionally, there may be two sets of circulation pumps with one each
for the condenser and cooling tower portions of the circuit.  Or there
may be a single set of circulation pumps that pump water through the
condensers and then through the cooling towers or that pump water to the
top of the cooling tower which is set at an elevation such that tower
effluent flows by gravity through the condenser.

 Cooling systems can be designed such that the cooling water flow can be
alternated between all of the modes described above.  Such systems are
referred to as mixed mode or multimode cooling systems.

 Data extracted from the 2000 detailed industry questionnaire; Question
1d in Part 2.

 Note that this data does not reflect changes to cooling systems
installed after 2000, such as the helper tower system at Harllee Branch
described below.

 The energy penalty associated with helper towers includes only the
parasitic component for pumping water to the top of the towers and for
operating the fans but does not include the turbine efficiency loss
associated with closed cycle cooling towers. 

 The thermal discharge limits were an increase in downstream river
temperature of no more than 5 oF and a maximum downstream river
temperature of 86 oF

 These values compare favorably to EPA’s estimates used in the
economic analysis of an average parasitic penalty of 1.7%, an average
turbine efficiency penalty of 2.0%, and an average total closed cycle
penalty of 3.7% (Sunda, 2008).

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