A discrepancy seems to exist between the cost of water to the
average consumer and the actual value of that water to the society and
environment. Because it is such a vital component to life and functions
in society, municipal water systems strive to provide water to customers
at the lowest costs. This natural resource is available for such a low
cost that we purify the water to make it suitable for drinking, pump it
through distribution networks over considerable distances, and then
flush it down the toilet.
Guz (1) in 2005 brought attention to successful condensate
collection in San Antonio that led to requirements for new construction
to incorporate condensate collection. That article provided suggestions
for how to build condensate collection into new project designs. This
article focuses on the more difficult task of retrofitting condensate
collection into existing buildings. Since existing buildings comprise
approximately 98% of the building stock (the other 2% being new
construction), they represent a significantly greater immediate benefit
to society in terms of energy or water consumption savings. In the right
situation, retrofitting existing buildings can potentially provide
significant water consumption savings.
ASHRAE has recognized the importance of water to society and the
consumption of water done by buildings and building systems. ASHRAE
Standard 189.1-2009, Standard for the Design of High Performance Green
Buildings Except Low-Rise Residential Buildings, and proposed Standard
191, Standard for the Efficient Use of Water in Building, Site and
Mechanical Systems, provide requirements for water-using systems and
condensate collection.
Some consider water concerns to be as important as energy. For
example, in an article published in 2009 concerning a survey by
McGraw-Hill, 69% of those surveyed indicated that water was a critical
issue for a sustainable building and that number is anticipated to rise
to 85% by 2013. (2)
It takes energy to produce potable water, and water is used to
produce energy. For example, in California it has been estimated that
water-related energy use consumes 19% of the total electricity, 30% of
the natural gas and 88 billion gallons (333 billion L) of diesel fuel
annually. (3) While that may be an extreme, we don't see the hidden
cost in terms of energy needed to obtain, cleanse and distribute water.
Nationwide, the number is about 4% of total electrical consumption, (4)
with some estimates higher. Conversely, we often forget that a large
amount of water is used generating electricity in thermoelectric and
hydroelectric power plants (some proportion is lost to evaporation in
the process while some is lost due to additional evaporation due to
higher downstream temperatures in the waterways). Studies are being done
on this issue. (5)
Condensate Collection
Factors that determine whether condensate collection should be
considered include location (climate), building type (particularly
relating to amount of outdoor air required), the size, number, and
accessibility of air handlers that condition outdoor air, location of
potential uses for the condensate, etc. Location determines the
potential to collect a significant amount of condensate, as well as the
value of the water to the local community. In periods of drought, the
actual value of a unit of water to the local society and economy may be
worth more than the rate paid to the local utility. A similar situation
can occur in areas where water cost is artificially kept low through
subsidies or other means. As water demand continues to grow due to
population pressures and lifestyle changes, even regions that might have
been considered moist or water rich will experience tight supplies of
fresh, clean water.
[ILLUSTRATION OMITTED]
Building or space occupancy type determines the amount of outdoor
air required and the amount of moisture in the incoming air. As the air
passes across cold cooling coils, if the coil surface temperature is
less than the dew point of the airstream passing by, then the potential
exists for water condensation on the coils. (See sidebar on How Much
Water Will Collect at Design Conditions?) Water that condenses on the
coils collects and drops to the drain pan below. The actual amount of
water collected depends on parameters such as the absolute humidity
level, total airflow and coil bypass ratio. A major source of this
condensed moisture is from outdoor air brought in through outdoor air
intakes or through infiltration exchange with the outdoors (other
sources include the people in the building, cooking, etc.).
A building or space that requires a large quantity of outdoor air
on an ongoing basis (e.g., laboratories) is suitable for condensate
collection. Other obvious candidates include spaces with indoor water
features, natatoriums, gymnasiums and shower rooms, although these may
face special challenges. Any building that has a cooling tower located
on site is another prime candidate as this offers an easy means for
reuse of the collected water. Dedicated outdoor air systems are another
candidate.
The amount of water collected at cooling design condition may not
fully represent the total amount of condensate collected over the course
of a year. This depends on the range of conditions that exist throughout
the course of the year. Similar calculations could be done for any
location using a more detailed, hour-by-hour analysis. We have taken
hourly weather data (6) and constructed a spreadsheet for doing such
analysis on our university campus buildings for use when evaluating a
situation for potential retrofit. The spreadsheet estimate is based on
the amount of outdoor air conditioned by a particular air-handling unit,
and can be adjusted to account for situations when economizing cooling
is expected, as well as variable outdoor air requirements, and operation
schedules, etc.
The simple spreadsheet model predicts condensate to occur when the
outdoor air absolute humidity levels are above those for the assumed
supply air conditions. Based on typical supply air conditions, this
spreadsheet model estimates approximately 12.5 gallons per cfm (100 L
per L/s) of outdoor air if supplied around the clock throughout the
entire year for a unit located in Athens, Ga.
We designed and installed a condensate collection system on an AHU
that conditions 100% outdoor air and supplies one portion of a research
laboratory building. Although there is a variable-speed drive installed,
the actual fan speed varies by only [+ or -]5%, with a typical outdoor
air intake of about 19,400 cfm (9150 L/s). Since this unit conditions
100% outdoor air and runs 24/7/365, there is no need to make adjustments
in the condensate collection predictions for items such as economizer
operation, unit operating schedule or adjustment of outdoor ventilation
air through demand-controlled ventilation. Based on typical supply air
conditions, an estimated 240,000 gallons (908 500 L) of annual
condensate could theoretically be collected for this unit.
We collected daily condensate production data from this
air-handling unit during the 2009 cooling season while using dataloggers
to monitor fan speed, as well as temperature and humidity of the outdoor
and supply air. We will use this to help validate and refine the model
for future use and application to other climate regions. How detailed a
model and prediction of water collected is needed depends on the
situation. Certainly, if a project requires justification strictly based
on short-term economic analysis, then an accurate prediction is probably
necessary. In other cases, such as with our university during a time of
extreme drought, it is sufficient to know within a general range what
the collected water amount would be.
Predicted Annual Collection At Five Locations
The same spreadsheet model was modified to estimate the condensate
collected at the other representative cities:
* Athens, Ga. =12.5 gallons per cfm or 100.3 L/(L x [s.sup.-1]) of
continuous annual outdoor airflow
* Houston = 22.4 gallons per cfm or 180.5 L/(L x [s.sup.-1]) of
continuous annual outdoor airflow
* Boston = 4.5 gallons per cfm or 35.7 L/(L x [s.sup.-1]) of
continuous annual outdoor airflow
* Sacramento, Calif. = 1.3 gallons per cfm 10.4 L/(L x [s.sup.-1])
of continuous annual outdoor airflow
* Denver = 0.5 gallons per cfm or 4 L/(L x [s.sup.-1]) of
continuous annual outdoor airflow
[ILLUSTRATION OMITTED]
The total annual rainfall is only a partial indicator of how much
condensate might be collected, as shown in Table 1. Some U.S. cities
have similar rainfall totals, but vary in the total amount of condensate
collection potential.
What to Do With Collected Water
The best end use for the collected water depends on the particular
circumstances of the location. Drought in Athens, Ga., led to the
realization that the primary use of city water at the University of
Georgia (UGA) is for makeup water in campus cooling towers. In this
situation, the most logical choice was to route a building's
collected condensate to its cooling tower sump. In most cases, peak
condensate production will occur at the same times as peak makeup water
demands, creating an elegant feedback loop. This is also the simplest
retrofit, involving reasonably inexpensive equipment and piping. Water
can be routed directly to the tower with no need for treatment.
Complications arise when dealing with district cooling systems with
satellite chillers, because it is possible to produce condensate in an
air-handling unit while the chiller and cooling tower for that
particular building are idle. While it is no tragedy that condensate
sent to the cooling tower will simply overflow to the sewer, where it
would have gone prior to retrofit, there is the risk that treatment
chemicals in the sump will be diluted and needlessly washed away. In
this scenario care should be taken to prioritize condensate retrofits in
buildings with baseline chiller plants.
Condensate collection also can be integrated into a rainwater
collection system, a scheme sometimes referred to as "rainwater
plus." This will usually involve a storage tank or cistern, and can
require considerably more expense and engineering than using the
condensate in a cooling tower. Depending on the intended use, such as
irrigation, fountains, toilet flushing, or potable water, different
amounts of further treatment will be required. In all cases, local
building codes must be followed.
Condensate Collection Retrofit Program
The second severe drought in 10 years brought down water levels in
local lakes and rivers to critical levels. The university and the
greater Athens community worked together to prevent water shortages by
imposing watering bans and shutting off fountains. However, it
didn't take long to realize that better, long-term solutions were
needed.
The Paul D. Coverdell Center for Biomedical and Health Sciences was
constructed in 2006 on the UGA campus. It represented the first
large-scale implementation of water conservation practices in a newly
constructed building at the university. The design included condensate
and rainwater collection systems, as well as sumps around the perimeter
of the building. The successful implementation of this project provided
some direction for retrofitting existing buildings on campus.
The UGA Main Library was the prototype building for the retrofit
effort. The library has two penthouses, each with its own AHU, on either
side of a central, roof-mounted cooling tower. For each unit, the
original drain pipe from the cooling coil's condensate collection
pan ran to a floor drain (in this case the AHUs are so large that the
floor drain is inside the unit), which ultimately runs to the city sewer
system. A sump basin was installed so that the drain pipe runs into it,
with an emergency overflow completing the original pipe route to the
floor drain. A sump pump with a diaphragm-type float switch pumps the
condensate out of the basin and up about 8 ft (2 m), where it then runs
horizontally through the exterior wall and drains into the cooling tower
sump. The prototype was successful with one major lesson learned: the
water meter for recording the total amount of collected condensate
should be installed either in the vertical section of pipe above the
pump, or in a U-trap in the horizontal run, because, otherwise, the pipe
is never totally full of water, and an impeller-type meter could misread
the flow volume.
[ILLUSTRATION OMITTED]
With the knowledge and confidence gained from the project at the
Main Library, UGA was able to move forward and expand the retrofit
program into other buildings on campus. The focus was on buildings with
laboratories and buildings that operated on a 24/7 schedule due to the
high condensation potential mentioned previously. Highly visible
facilities on campus also were targeted for campus education efforts,
such as the Miller Student Learning Center and a new building for the
Lamar Dodd School of Art.
The basic system design has not changed much since the prototype,
but we have continued to evolve the design with improvements such as a
cover on the basin to prevent debris accumulation and insulation on the
basin and on indoor pipe runs to prevent sweating. In most applications,
so far at UGA, the condensate is ultimately delivered to a cooling tower
sump or a collection cistern that is used for cooling tower makeup water
or building internal use. However, one system has been built and two
more have been designed in which the condensate will be routed to
rainwater collection cisterns for irrigation purposes.
Cost Savings Analysis
As discussed earlier, dire conditions may lead to less emphasis on
economic payback when it comes to water. However, it is possible to
demonstrate good economic performance with conden sate collection. A
system designed and installed in UGA's College of Veterinary
Medicine is predicted to produce 450,000 gallons (1.7 million L) of
condensate per year. Using the combined supply and sewer utility rate
from the local utility of $0.0075/gallon ($0.002/L) as the cost of
water, installing this system will save UGA $3,375 per year. The system
cost was $3,200 installed, and the simple payback is expected to be less
than a year. This admittedly was one of the more favorable installations
we have done so far, but it indicates the potential that exists.
Another AHU retrofit that we did during the winter of 2008-09 was
monitored daily for condensate collection through the 2009 cooling
season. The installation had a similar installed total cost of
approximately $3,000 and about 200,000 gallons (750 000 L) were
collected during the 2009 cooling season. This translates to $ 1,500 in
water savings and a simple payback period of two years. Drought
conditions that existed during June through August probably reduced the
amount of condensate for this cooling season, possibly making this
retrofit even more economically viable.
Lessons Learned
The near future may bring a surge of interest in retrofitting
buildings with condensate collection systems, particularly in the
eastern United States where water concerns continue to build. We are
confident and hopeful that these type projects will continue to become
more effective as more is learned about the possibilities. For our part
in furthering the development, we'd like to share some of the
lessons we learned and suggestions for the future.
A significant concern in these projects is the cleanliness of the
water and the system components. The external collection pan should be
covered to prevent any foreign particles from getting into the system.
In these pans, as with condensate pans within air handlers, there may be
a potential for biological growth and contamination. Also, there may be
an increase in corrosion potential in the cooling tower loop, if that is
where the condensate is sent. We are closely monitoring the particular
cooling towers where this is being done to verify our thinking that this
is not a major concern.
Sweating on the outside of the condensate piping can be an issue,
particularly in semi-conditioned mechanical rooms, so all lines (as well
as the collection basin itself) should be insulated. If the condensate
line is tied into rain downspouts, sweating on the downspouts can cause
potential problems such as discoloration and damage to the exterior of
the building. One possible solution is to run a smaller pipe or tube
inside of the downspout.
The dimensions of the U-trap in the existing condensate drain pipe
between the AHU and the floor drain should be maintained when connecting
the drain pipe to the external collection pan. It is also highly
recommended that a meter be installed, and that it be located to
facilitate easy reading. The additional cost of the meter is worthwhile
because of the feedback on functionality and the education potential it
provides, as well as a means to verify water and cost savings.
We recommend also that future study be done on improving methods to
predict the amount of condensate collected, as well as the potential
impacts that might exist with routing this water to cooling tower
condenser water systems.
[ILLUSTRATION OMITTED]
Conclusion
As the value to society of fresh water supply is recognized, the
role of condensate collection in new designs, as well as retrofitting
existing buildings will continue to grow. The experience we have seen
with several cases of air-handling unit retrofits within our university
campus indicates that these retrofits can be done with manageable
up-front costs and favorable simple payback periods.
References
(1.) Guz, K. 2005. "Condensate water recovery." ASHRAE
Journal 47(6):54-56.
(2.) Greener Buildings. 2009. "Water to Rival Energy
Efficiency as Key Concern in Greening Buildings: Report." www.
green erbuil dings. com/news/2009/06/04/water-efficiency.
(3.) California Energy Commission, 2005. "California's
Water-Energy Relationship." Final Staff Report CEC-700-2005-011-SF.
(4.) EPRI. 2002. "Water & Sustainability (Volume 4): U.S.
Electricity Consumption for Water Supply & Treatment--The Next Half
Century." Electric Power Research Institute Report 1006787.
(5.) Torcellini, P.A., M. Long, and R. Judkoff. 2004.
"Consumptive water use for U.S. power production." ASHRAE
Transactions 110(1):96-100.
(6.) National Solar Radiation Data Base, 1961-1990. Typical
Meteorological Year 2, National Renewable Energy Laboratory,
http://rredc.nrel.gov/solar/old_data/nsrdb/tmy2/.
By Tom Lawrence, Ph.D., P.E., Member ASHRAE; Jason Perry, Associate
Member ASHRAE; Peter Dempsey, Student Member ASHRAE
About the Authors
Tom Lawrence, Ph.D., P.E., is public service associate, Faculty of
Engineering, Jason Perry is a research engineer at Engineering Outreach
Services, and Peter Dempsey is a student at the University of Georgia in
Athens, Ga.
How Much Water Collects At Design Conditions?
[FIGURE 1 OMITTED]
For simplicity, consider the process of a unit conditioning 100%
outdoor air (such as with a dedicated outdoor air system or DOAS).
The psychrometric chart shown in Figure 1 represents a path of
outdoor air as it passes across the cooling coil for the 0.4%
cooling design condition in Athens, Ga. Assuming a supply air
condition of 55[degrees]F (12.8[degrees]C) and 85% relative
humidity (wet bulb temperature = 52.5[degrees]F [1 1.4[degrees]C]),
the humidity ratio changes across the coil from 0.0141 to 0.0078
[lb.sub.water]/[lb.sub.dryair] (kg/[kg.sub.air]). The difference in
absolute humidity ([omega]) between the incoming outdoor air and
supply air leaving the unit represents the amount of condensation
that occurs. Thus, for every pound of air supplied by the unit,
0.0141-0.0078 or 0.0063 lbs (kg) of water are condensed.
The total amount of condensate expected is determined by the
equation below:
condensate collected = airflow x density x 60 min/h x
[delta][omega]
Assuming 1,000 cfm (472 L/s) of outdoor air is being conditioned,
the total amount of condensate expected would be:
condensate = 1,000 [ft.sup.3]/min x lb/13.133[ft.sup.3] x 60 min/h
x (0.0141- 0.0078) [lb.sub.water]/ [lb.sub.dry air] = 28.8 lb/h
(13.1 kg/h)
This is approximately 3.5 gallons (13.1 L) per hour at the cooling
design condition.
Similar calculations can be run for any locality, and the result
can vary widely depending on the climate. For example, when the
calculation is run for other representative cities that range from
hot and humid to dry climate types, the condensate yields are:
* Houston (95.1 [degrees]F [33[degrees]C] dry bulb/77.6[degrees]F
[25[degrees]C] MCWB)
= 7.0 gallons (26.5 L) per hour
* Boston (90.8[degrees]F [33[degrees]C] dry bulb/73.3[degrees]F
[23[degrees]C] MCWB)
= 3.2 gallons (12 L) per hour
* Sacramento, Calif. (I00.4[degrees]F [38[degrees]C] dry
bulb/70.7[degrees]F
[22[degrees]C] MCWB) = 0.8 gallons (3 L) per hour
* Denver (94.3[degrees]F [35[degrees]C] dry bulb/60.3[degrees]F
[16[degrees]C] MCWB)
= no condensate collected at design cooling load
Table 1: Annual condensate collection compared to total annual
rainfall.
Annual Condensate Average Annual
For 1 cfm of Continuous Outdoor Rainfall
Air gal/cfm [L/(L x [s.sup.-1])] in. [m]
Athens, Ga. 12.5 (100.3) 47.8 (1.21)
Houston 22.4 (189.5) 54.0 (1.37)
Boston 4.5 (35.7) 42.5 (1.08)
Sacramento, Calif. 1.3 (10.4) 17.9 (0.45)
Denver 0.5 (4) 15.4 (0.39)