ASHRAE JOURNAL ashrae.org AUGUST 2015 20 TECHNICAL FEATURE Frank Morrison is manager, global strategy at Baltimore Aircoil Company in Jessup, Md. He is past chair of ASHRAE TC 3.6, Water Treatment. BY FRANK MORRISON, MEMBER ASHRAE Saving Water With Cooling Towers Saving water with cooling towers. At first glance, this statement seems counterintuitive. Cooling towers save energy but aren’t they major users of water? This article will help readers understand the critical role evaporative heat transfer systems play in a sustainable environment, explore how water is consumed in such systems, and review the strategies that help minimize the use of both water and energy. The first water-cooled systems used potable water to provide heat rejection with the cooling water wasted to a drain. Cooling towers were developed to recycle more than 98% of this water, resulting in tremendous reduc- tions in both water and energy use as these systems grew in size and popularity. Evaporative heat rejection also enables higher system efficiencies, which conserves water at the power plant and reduces emissions of greenhouse gases and other pollutants. This is because thermoelec- tric power generation accounts for 38% of freshwater withdrawals in the United States—essentially equal to that withdrawn for irrigation. 1 By reducing the electrical energy consumed at the site, less power is required to be generated and less water is used at the power plant and in the extraction and pro- cessing of the plant’s fuel source. For example, in some climates, the total water use (source and site) between air- and water-cooled chillers is almost equal. 2 The lower energy use also enables a higher percentage of renew- able, clean power from solar and wind at a given facility. Energy is also required to treat and distribute water. The balance between the use of these two natural resources is often referred to as the “energy/water nexus.” Today, water supplies are challenged in many areas of the world, including Atlanta (recent drought) and California (current drought). Therefore, it is critical that all water consuming systems, no matter where they are located, optimize their use of this resource. Methods to conserve water include using low-flow bathroom fixtures, repairing leaks in water-distribu- tion systems, and taking advantage of relatively simple techniques to help ensure that evaporative heat rejec- tion systems use only the amount of water required to save energy, maintain optimized system performance, minimize system maintenance, and ensure a long sys- tem life. PHOTO COURTESY OF NCSA, UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN This article was published in ASHRAE Journal, August 2015. Copyright 2015 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.
Transcript
A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 1 52 0
TECHNICAL FEATURE
Frank Morrison is manager, global strategy at Baltimore Aircoil Company in Jessup, Md. He is past chair of ASHRAE TC 3.6, Water Treatment.
BY FRANK MORRISON, MEMBER ASHRAE
Saving Water With Cooling TowersSaving water with cooling towers. At first glance, this statement seems counterintuitive. Cooling towers save energy but aren’t they major users of water? This article will help readers understand the critical role evaporative heat transfer systems play in a sustainable environment, explore how water is consumed in such systems, and review the strategies that help minimize the use of both water and energy.
The first water-cooled systems used potable water to
provide heat rejection with the cooling water wasted to
a drain. Cooling towers were developed to recycle more
than 98% of this water, resulting in tremendous reduc-
tions in both water and energy use as these systems grew
in size and popularity. Evaporative heat rejection also
enables higher system efficiencies, which conserves water
at the power plant and reduces emissions of greenhouse
gases and other pollutants. This is because thermoelec-
tric power generation accounts for 38% of freshwater
withdrawals in the United States—essentially equal to that
withdrawn for irrigation.1
By reducing the electrical energy consumed at the site,
less power is required to be generated and less water is
used at the power plant and in the extraction and pro-
cessing of the plant’s fuel source. For example, in some
climates, the total water use (source and site) between
air- and water-cooled chillers is almost equal.2 The lower
energy use also enables a higher percentage of renew-
able, clean power from solar and wind at a given facility.
Energy is also required to treat and distribute water. The
balance between the use of these two natural resources is
often referred to as the “energy/water nexus.”
Today, water supplies are challenged in many areas
of the world, including Atlanta (recent drought) and
California (current drought). Therefore, it is critical
that all water consuming systems, no matter where
they are located, optimize their use of this resource.
Methods to conserve water include using low-flow
bathroom fixtures, repairing leaks in water-distribu-
tion systems, and taking advantage of relatively simple
techniques to help ensure that evaporative heat rejec-
tion systems use only the amount of water required to
save energy, maintain optimized system performance,
minimize system maintenance, and ensure a long sys-
tem life.
PHOT
O CO
URTE
SY O
F NC
SA, U
NIVE
RSIT
Y OF
ILLI
NOIS
AT
URBA
NA-C
HAM
PAIG
N
This article was published in ASHRAE Journal, August 2015. Copyright 2015 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.
A U G U S T 2 0 1 5 a s h r a e . o r g A S H R A E J O U R N A L 2 1
TECHNICAL FEATURE
Evaporative heat rejection systems encompass open
circuit cooling towers, closed circuit cooling towers, and
evaporative condensers. For the purposes of this article,
the term “cooling tower” will be used synonymously for all
of these devices, except as noted.
EvaporationThe primary consumption of water in a cooling tower
is through evaporation—a process that is also used by the
human body to help regulate its internal temperature.
In a cooling tower, the warm water from the system
comes into contact with the entering air, usually over
a heat transfer surface such as fill, where a small por-
tion of the recirculating water evaporates, cooling the
remaining flow. This process is very energy efficient as
approximately 1,000 Btu (1055 kJ) are required to evapo-
rate 1 lb (0.454 kg) of water at standard design condi-
tions (1,000 Btu/lb [2,326 kJ/kg]).
In contrast, air-cooled heat exchangers must move far
more air to reject the same heat, consuming additional
fan energy in the process, usually at a much higher system
temperature since the dry-bulb temperature is higher
than the wet-bulb temperature of the air. These higher
temperatures result in greater energy use by the cooling
system, often 30% or more as in the case of an air-cooled
versus a water-cooled chiller.
Note that the difference between the dry bulb and wet
bulb of the air is known as the wet-bulb depression. In
areas of high wet-bulb depression, such as the American
Southwest, evaporative heat rejection offers even greater
energy efficiency by enabling significantly lower system
temperatures.
While water is “consumed” by evaporation in the cool-
ing tower, the water is not “lost” or destroyed, unlike
what occurs when natural resources such as oil or natural
gas are consumed. This pure water, sometimes visible
as plume from the cooling tower discharge in cooler
weather, is returned to the environment as part of the
natural water cycle. Water in reservoirs, lakes, rivers, and
in cooling ponds used in some industrial applications also
evaporates; cooling towers simply put evaporation to work
to efficiently cool the buildings and processes that serve
our society.
Evaporation is a function of the heat rejection load and
the psychrometric properties of the air entering the cool-
ing tower. Many “rules of thumb” exist for calculating
peak evaporation, such as:
• 2.0 gpm of water evaporated per 1,000,000 Btu/h
(0.0004 L/s·kW) and
• 3.0 gpm of water evaporated per 100 tons
(0.0004 L/s·kW).
While these rules of thumb can be useful for calculat-
ing design makeup flow rates, sizing piping, and esti-
mating water treatment regimens, they significantly
overestimate the annual water use by a cooling tower
by not taking into account load profiles and the effect of
weather conditions throughout the year. Besides being
proportional to the heat load, the evaporation rate is
strongly influenced by both the entering air dry-bulb
and wet-bulb temperature. At off peak wet-bulb tem-
peratures, which occur the majority of the year, the
evaporation rate is reduced by up to 30% or more versus
design as can be seen in Figure 1.
Figure 1 also illustrates that water use increases in drier
climates and is reduced in more humid climates for the
same heat load. Furthermore, in cooler weather, the
heat load to be rejected is typically lower, especially on
HVAC applications, further reducing the evaporation.
Several water use calculators are available that can dem-
onstrate the variation in evaporation rate with varying
climate, load and design conditions.
BlowdownWhen water is evaporated in a cooling tower, pure water
vapor enters the air moving through the unit, leaving any
dissolved solids or minerals in the remaining water. Left
unchecked, the recirculating water will become increas-
ingly saturated with mineral content, scaling heat trans-
fer surfaces and increasing the corrosivity of the water.
FIGURE 1 Evaporation vs. wet bulb and relative humidity for a fixed, constant load; tower airflow at full speed for all cases.
Wet Bulb (°F)
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
Evap
orat
ion (g
pm o
r L/s
)(B
ase
= 1.0
at 7
8°F
[25.6
°C] W
B an
d 50
% R
H)
Base = 1.0
40 45 50 55 60 65 70 75 80 85 90
30% RH50% RH70% RH
A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 1 52 2
To keep dissolved solids to an acceptable level, a small
amount of water must be bled from the system in propor-
tion to the evaporation rate to achieve the desired “cycles
of concentration.” Cycles of concentration (COC) mea-
sures the ratio of the dissolved solids, such as calcium,
chlorides, or magnesium, in the recirculating water to
the concentration found in the incoming makeup water
supplied to the cooling tower. The COC can be calculated
according to the following formula:
COC = (Evaporation/Blowdown) – 1 (1)
Evaporation and blowdown are measured in gpm (L/s).
Conversely, the formula for calculating blowdown, also
known as the bleed rate, based on the desired COC is as
follows:
Blowdown = Evaporation/(COC – 1) (2)
The COC that can be achieved is dependent on the qual-
ity of the incoming makeup water, the water treatment
program, and the system’s construction materials (and
not just those of the cooling tower). While four or five
cycles is often used as a target value for a typical system,
this value can range from two cycles on a system with very
poor supply water quality up to 30 or more when very
soft water, such as air conditioning condensate, is used
as makeup water. Thus, a regulation requiring a single,
fixed minimum COC can be misguided, as the COC that
is achievable is dependent on site-specific factors, which
can vary with each installation and over time. Even in
cases where the water quality is generally good, high lev-
els of a single constituent, such as chloride or silica, can
exist, which can limit the maximum achievable COC.
Several calculators, including one from the California
Energy Commission, are available to assist in determining
the proper COC value for a given facility. Additionally, a
water treatment professional, either internal or external
to the facility, should be consulted to analyze the site and
help to establish an optimized, balanced water treatment
program designed to control scale, fouling, corrosion, and
biological growth while conserving water.
In the past, a fixed bleed rate often was employed, but
this method should not be used. Instead, the desired COC
setting can be maintained in several ways, two of which
are described below.
The most common method is the use of a conductivity
controlled blowdown system, consisting of a conductiv-
ity probe, a controller, and a motorized blowdown valve.
The conductivity controller signals the blowdown valve
to open as necessary to maintain the desired COC. Water
treatment chemicals can affect the conductivity of water,
and this effect needs to be considered when adjusting set-
tings on the controller.
Another technique involves the use of flow meters on
the makeup and blowdown lines. In this method, the
blowdown valve is opened in proper proportion to the
makeup flow to maintain the desired COC. However,
drift, leaks, filtration backwash, and other uncontrolled
water losses can result in a lower COC than desired, in
turn leading to excessive water loss, so these factors must
be properly accounted for when using this method.
For either system, the blowdown valve should be located
before the introduction of any chemical treatment to
allow the chemicals to mix thoroughly in the tower,
while reducing the loss of treatment chemicals.3 Regular
inspection, calibration, and maintenance of the motor-
ized blowdown valve and the conductivity probe or flow
meters will help to sustain the desired setting over time.
A filter or strainer ahead of the flow meters and valves is
also good practice to protect these devices.3
Maintaining the proper COC is important to system
water and energy efficiency. Too high a COC setting for
the site-specific conditions can lead to scaling and corro-
sion of the system, leading to poor heat transfer, higher
energy use, and shortened equipment life. Too low a COC
setting will generally result in better recirculating water
quality, but at the price of higher than necessary water use
and loss of associated treatment chemicals.
Reducing EvaporationAs previously stated evaporation is the largest use of
water in a cooling tower and is primarily dependent
on the load and the psychrometric properties of the air
entering the cooling tower. So reducing the load that must
be handled will not only save energy but also directly
reduce the evaporation required along with the associated
blowdown. Load reduction techniques include, but are
not limited to, optimal building orientation, using better
insulation, more energy-efficient production processes,
and heat recovery.
As an example, more efficient chillers can reduce the
heat load on the cooling tower. A chiller with a full load
efficiency rating of 0.55 kW/ton (0.16 kW/kW) (6.39 COP)
will evaporate 2.4% less water per peak ton than a chiller
operating at 0.65 kW/ton (0.18 kW/kW) (5.41 COP). This
TECHNICAL FEATURE
A U G U S T 2 0 1 5 a s h r a e . o r g A S H R A E J O U R N A L 2 3
is due to the reduced amount of non-productive, waste
compressor work (heat) that must be rejected to the atmo-
sphere, in this case, 0.10 kW/ton (0.03 kW/kW) (35 COP).
Furthermore, as the amount of water evaporated is
directly proportional to the load and the blowdown is
proportional to the evaporation, there will be a corre-
sponding reduction in the amount of blowdown required.
Assuming four cycles of concentration, for every gallon
(3.79 L) of evaporation avoided there will be an additional
TABLE 1 Effect of reduced condenser water temperature and cooling tower fan speed on energy and water use in a water-cooled chiller system.
OPERATING MODE BASE TOWER PRIORITY REDUCTION VERSUS BASE
A U G U S T 2 0 1 5 a s h r a e . o r g A S H R A E J O U R N A L 2 7
even with the most efficient elimina-
tors, is typically not a benefit. Besides
being a very small volume com-
pared to evaporation and bleed, any
drift leaving the cooling tower only
serves to reduce the amount of water
required to be bled from the unit
when using a conductivity controlled
blowdown to maintain the desired
cycles.
Two exceptions to this would be
where makeup and blowdown flow
meters are used to maintain the
water quality, or where the blowdown
is captured and used for another
purpose such as toilet flushing. Thus,
claims of “water saving” drift elimi-
nators can often be misleading. Low
drift rates can also be very difficult,
expensive, and time consuming to
accurately verify.
Splashout, Leaks, and OverflowsSplashout can occur at the air inlet
faces of the cooling tower where the
falling water can “splash out” of the
unit itself. Splashout must be mini-
mized with good air inlet louvers that
capture any water from the fill or
plenum and return it to the basin.
As with eliminators, the louver sec-
tions, whether slat type or cellular,
must be maintained properly and
operators must ensure they are tight
fitting with the proper spacing. Spray
nozzles should also be kept clean and
free flowing over the heat transfer
surfaces, not only to minimize scaling
and maximize thermal performance
but also to reduce splashout. Clogged
nozzles can also cause water to over-
flow hot water-distribution basins
or spray out through the top of the
PHOTO 1 Proper inspection and maintenance of drift eliminators is critical.
cooling tower, resulting in unneces-
sary water losses.
Leaks can occur in a variety of areas,
but most often from the cold water
basin seams. Obviously, any leaks in
TECHNICAL FEATURE
Advertisement formerly in this space.
A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 1 52 8
the cooling tower or system pipework should be corrected
immediately. The use of welded stainless steel or poly-
urethane lined cold water basins can also help minimize
leaks by eliminating seams in the basin. Loss of treated
water can be especially costly as it includes both the cost of
the water and the water treatment chemicals.
Another method to reduce water use and lower treat-
ment costs is by minimizing the volume in a water-cooled
system, including the design of the system pipework.
These savings take place from the initial fill to every time
the system is drained for cleaning. Closed circuit cooling
towers keep the process flow in a clean, closed loop and
the volume of the open spray water loop is limited to the
internal volume of the unit, which can be considerably
less than a system using an open circuit cooling tower.
Note that upon shutdown of a cooling tower, all the
water above the operating level will flow back into the
cold water basin, which must have volume available to
accommodate this water without overflowing the cold
water basin. Such overflows can be a significant, yet
often hidden, water use, occurring every time the system
pump shuts down. Designers must ensure that the model
selected has an adequately sized cold water basin or
remote sump for the project.
Once installed, the water levels in the cold water basin
of the cooling tower must be properly set for unit startup
and to avoid wasting water upon shutdown through the
overflow connection. Using the makeup valve arrange-
ment, whether mechanical or electronic, the operating
level of the cold water basin should be set close to the
lowest possible level, such that air is not drawn into the
cooling tower pump to avoid system flow issues and noise,
while allowing the maximum basin volume to accommo-
date the shutdown water.
An electric water level control can also be used to pro-
vide finer control of the level in the cold water basin. To
assist operators, a low level alarm can be used to help
protect the tower pump, and a high level alarm can be
added to alert the operator of possible overflow condi-
tions (which is now required by certain codes, such as
California Title 24). These sensors can be separate devices
or incorporated into the electric water level control
TECHNICAL FEATURE
Advertisement formerly in this space. Advertisement formerly in this space.
Advertisement formerly in this space.
A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 1 53 0
device. Finally, to avoid overflowing the hot water basins
on cross-flow designs, the gravity flow spray nozzles must
be properly sized for all expected flows and kept clean.
Alternative Sources of WaterAlternative sources of water can also be used to reduce
the use of potable water in cooling towers. Any water that
can meet the cooling tower manufacturer’s guidelines for
the construction material can be used. The most common
is reclaim, or recycled water, which is water that has pro-
cessed through a treatment plant. Rather than return this
water to a lake or river, the reclaim water is used for other
uses, including irrigation or makeup water for cooling
towers. “Purple” pipe, along with appropriate signage, is
used to distinguish such distribution systems from pota-
ble lines. This water is often good quality, although the
concentration of minerals is usually higher than potable
water, having been “cycled” through the system at least
once. As a result, usually the COC cannot be set as high
when using recycled water.
Rainwater can also be harvested from roof surfaces or
parking lots, filtered, disinfected, and stored for use in
cooling towers. Another popular alternative source of
water is air-conditioning condensate. As both rainwater
and condensate are “pure” water with few minerals, they
often must be blended with other water sources so they
are not overly aggressive. Such systems are frequently
operated at higher cycles of concentration since the water
is “soft,” even after blending.
The most critical aspect of using any of these alternative
sources of water is to monitor biological activity and mini-
mize the introduction into the cooling tower of possible
nutrient sources for biological growth. Such contami-
nants increase the risk in operating the cooling tower and
increase the water treatment requirements. For instance,
the use of untreated sink water would not be suitable,
as soap would serve as a nutrient source for biological
growth as well as foul heat transfer surfaces.
Hybrid Wet/Dry DesignsClosed circuit cooling towers and evaporative condens-
ers use a coil to contain the heat transfer medium in a
closed system. As such, the spray pump can be shut off
and these units operated in an air-cooled or “dry” mode
in colder weather. The amount of water saved with this
technique will depend on the sensible heat transfer sur-
face area available, the local climate, and the load profile.
However, any water savings is usually more than offset by
the increase in unit fan energy in the dry mode, especially
when considering the reduced loads and water consump-
tion typically experienced in colder weather (Figure 1).
By definition, the switch point for dry operation is at
100% fan speed and power draw, while the fan speed for
the unit operating in wet mode at this same point is only
a small fraction of the design speed and power draw.
Because of their higher first cost and the energy penalty,
standard closed circuit cooling towers and evaporative
condensers are not used to replace open circuit cooling
towers on the basis of cold weather water savings.
For areas where water is in short supply and/or expen-
sive, cooling tower manufacturers have developed hybrid,
wet/dry designs that can save significant amounts of
water, yet still offer the low system temperatures critical
to efficient operation. These units incorporate a wet heat
exchange section along with a dry cooling section (Photo
2). Such systems are equipped with controls that balance
the use of water and energy to provide the desired level of
system efficiency. By handling a portion of the load dry,
typically ranging from a minimum of 20% all the way up
to 100% in colder weather, evaporation and the associ-
ated blowdown can be reduced proportionally while also
reducing or eliminating plume, which can be an advan-
tage on certain projects. Many of these hybrid units are
also closed circuit cooling towers, which have the added
benefit of keeping the process fluid in a clean, closed
loop that is separate from the external spray water loop.
Keeping the process fluid isolated helps maintain system
efficiency and reduce equipment cleaning and mainte-
nance over time.
Adiabatic designs precool the air with wetted pads
before the air enters a dry finned heat exchanger
enabling reduced condensing or fluid temperatures and
lower system energy use compared to air-cooled heat
PHOTO 2 Hybrid closed circuit cooling towers with dry and wet heat exchange sections.
TECHNICAL FEATURE
Advertisement formerly in this space.
A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 1 53 2
rejection, with a lower volume of water evaporated com-
pared to a typical evaporative condenser or closed circuit
cooling tower. To minimize water treatment needs, the
sumps are typically drained once per day, but the sump
is designed to keep the volume low to minimize this
water loss.
When considering these hybrid designs, it is important
to weigh the potential savings in water, water treatment,
sewage, energy, and maintenance costs, along with such
factors as the availability of water on the site, against the
higher initial cost of such equipment. Systems with high,
constant year-round loads typically benefit most from
these technologies. An accurate assessment of potential
water savings must be made, taking into account both
load profiles and ambient conditions, to properly calcu-
late the potential payback for such investments. Other
drivers may also influence the decision to use hybrid tech-
nologies, such as reduced water availability to a facility, a
desire to limit plume in colder weather, or a critical need
to be able to operate dry in the event of a loss of the water
supply to the site.
Tracking Water UseTracking cooling tower water use through makeup
and/or blowdown flow meters can be a wise invest-
ment. This data provides useful information on
how well the system and water treatment program
are operating over time so system parameters and
maintenance schedules can be adjusted for peak
overall performance. For instance, by metering the
makeup and blowdown flows, the COC for the system
can be tracked and the savings and benefits of water
conservation efforts can be more easily established,
as well as helping earn a sewage credit for evapora-
tion. Several codes and standards, such as ASHRAE
Standard 189.1-2013 and California Title 24-2013,
have incorporated requirements for such flow meters.
Some water utilities also offer rebates on energy and
water saving equipment, which can offset the cost of
such monitoring. Finally, efforts to reduce water and
energy use can help earn LEED points through direct
energy and water savings and possibly innovation
credits.
TECHNICAL FEATURE
Advertisement formerly in this space.
A U G U S T 2 0 1 5 a s h r a e . o r g A S H R A E J O U R N A L 3 3
Summary Cooling towers play an integral role in the conserva-
tion of both energy and water and as such are a key
part of a sustainable future. This article describes
many relatively simple techniques and best practices
necessary to reduce the use of water in cooling tow-
ers, along with consideration of alternative makeup
water sources and hybrid wet/dry designs. No matter
what the choice of equipment or techniques used, the
manufacturers’ operating and maintenance guidelines
must be followed to ensure optimal performance and
service life.
The items listed in the Bibliography, including “Saving
Energy with Cooling Towers,” the companion piece to
this article, can also be consulted for further guidance.
Finally, the services of a water treatment professional
should always be used when treating evaporative heat
rejection equipment to maximize thermal performance,
conserve water, and extend equipment lifetimes, while
effectively controlling scale, corrosion, and biological
growth.
References1. Maupin, Molly A., et al. 2014. “Estimated Use of Water in the
United States in 2010.” Circular 1405, U.S. Department of the Interior. U.S. Geographical Survey.
2. Hydeman, Mark. 2008. “A Comprehensive Comparison of Air- and Water-Cooled Chillers Over a Range of Climates.” Seminar 48 at the ASHRAE Annual Conference.
3. Aherne, A.J. 2015. Best Practice Guidelines: Water Conservation In Cooling Towers. Australian Institute of Refrigeration, Air Condition-ing, and Heating.
BibliographyANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard for Buildings
Except Low-Rise Residential Buildings.
ANSI/ASHRAE/IES/USGBC Standard 189.1-2014, Standard for the Design of High-Performance Green Buildings.
California Title 24–2013, Building Energy Efficiency Standards for Residen-tial and Nonresidential Buildings.
Hamilton, J., T. Bugler, J. Lane. 2010. “Water/Energy nexus, com-paring the relative value of water versus energy resources.” Cooling Technology Institute Technical Paper. TP10-16.
Morrison, F. 2014. “Saving Energy with Cooling Towers.” ASHRAE Journal (1).
Torcellini, P., N. Long, R. Judkoff, D. 2003. “Consumptive Water Use for U.S. power production.” National Renewable Energy Lab. NREL/TP-550-33905.
