Practical
Solar Thermal Chilled Water
Bud Leavell
Sales Engineer
Yazaki Energy Systems Inc.
The Current State of Solar AC in the US
• As of January 2010, there are some 22
solar air conditioning systems installed in
the US.
• Very few perform any useful function.
• Most are there for a showcase or as a
“Proof of Concept”.
• The demand for Engineers to explore this
field is currently growing exponentially.
What’s Wrong with the Existing Systems?
• MOST are GROSSLY underpowered!
– They were typically designed based on
conditions which do NOT exist.
•
•
•
•
•
Many are over-engineered.
More are under-engineered.
Critical controls and safeties are missing.
Inadequate heat rejection.
There was no clearly defined purpose for
the system.
Where do we Go from Here?
• With this in mind, this Solar Chilled Water
Modeling Template was developed.
• It is based on the specific technical
characteristics of single-effect low temp
hot water fired absorption chillers and their
reaction to the energy sources and loads
applied.
• All modeling is based on “REAL WORLD”
conditions and empirical data.
Step 1. Explicitly define the expectation.
• Optimization by Design
• Our goal is to achieve an electrical energy
savings greater than the solar contribution.
Combined Cycle
Side-Stream Piping
VFD
Chilled
Water
Supply
44°F
Bypass
VFD
Water Fired Chiller
Using Solar Heat
52°F
Chilled Water
55°F
Chilled
Water
Return
Utilizing Solar Heat, this configuration provides additional capacity to the
system, when the need is the greatest, and the energy source for it has no
recurring cost.
Electric Cost Savings from Unloading
COP
4.88
Input KW elec
105.1
Output kW cooling
513.2
KW electric / Ton
0.720228
Cost per KW
$0.1266
Cost per Ton Hour
$0.0912
5.91
21%
0.595
$0.0753
-$0.0159
Typical Chiller Power Curve
Part Load Performance
For Chiller Type:
Screw
30%
50%
kWatts per Refrigerant Ton
0.800
0.700
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0%
10%
20%
40%
60%
Percent of Full Load
70%
80%
90%
100%
Optimization by Design
• Goal -- Keep the existing chiller at or
below 75% of full capacity during peak
periods.
• Goal – Improve plant COP by as much as
40% during hours of sunlight.
Step 2. How much sun do we have?
• Download hourly solar insolation and
meteorological data from the National Solar
Radiation Database.
• This hourly data is available for the United
States and its territories.
• Hourly data can be modeled from daily data.
• You will need to do some H.S. Trig to translate
the NSRDB data into actuals on the collector.
• Daily integrated values when used to size an
array for an Air Conditioning Application will in
most cases result in a grossly underpowered
chiller.
Step 3. What type of collector?
• This template recognizes two types of
non-tracking collectors:
– Flat Plate
– Evacuated Tube
• Let your location and application
determine the type of collector you select.
• Each has advantages and disadvantages.
Step 4. Choose optimum collector azimuth.
• Available solar energy and the air
conditioning load are only somewhat
coincident.
How much solar energy is available at my location?
Insolation adjusted for Time of Day and Solar Angle of Incidence
1.00
0.90
0.80
0.70
Apr
May
0.60
Jun
0.50
Jul
0.40
Aug
Sep
0.30
0.20
0.10
0.00
0900
1000
1100
1200
1300
1400
1500
1600
1700
1800
What time of day represents the peak load
on the building?
Average Temp in Fort Worth for 2006
100
Degrees F
95
May
90
June
85
July
August
80
September
75
70
800
900
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900
Central Standard Time
Compare Azimuth Options
Solar Heat Output in Watts, adjusted for Solar Angle of Incidence and
0
Degrees Azimuth
900
800
700
600
Apr
500
May
Jun
400
Jul
300
Aug
Sep
200
100
0
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
Compare Azimuth Options
Solar Heat Output in Watts, adjusted for Solar Angle of Incidence and
45 Degrees Azimuth
900
800
700
600
Apr
500
May
Jun
400
Jul
300
Aug
Sep
200
100
0
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
Step 4. Choose optimum collector azimuth.
• Available solar energy and the air
conditioning load are only somewhat
coincident.
• The best apparent time of day to size the
array is for 3:00PM standard time.
• Typically, a flat panel collector will give
your best 3:00PM performance with an
azimuth of 45° West of South where an
evac tube array is best at about 15°.
Step 5. Size the collector array.
• Using the solar energy available at
3:00PM and the desired “rated” chiller
output at that time, calculate the number of
collectors required.
– You can choose any time of day for sizing that
fits your application.
Step 6. Calculate a heat balance.
• The cooling tower is the single most critical
link in any absorption chilled water system.
• Don’t forget Bud’s Thermal Law of Goesins
and Goesouts.
– “For every goesin there must be a goesout”.
Step 7. Size the buffer tank.
• Notice I did NOT say STORAGE tank!
• Attempting to size a storage tank to
operate the chiller once the sun has gone
down will price the system out of the realm
of “PRACTICAL”.
Step 8. Define your control strategy.
• When do I turn on the water pumps?
• How do I manage the temp of the cooling
water?
• How do I control the flow of heat medium
through the chiller?
• How and when do I use a back-up energy
source.
• You will need a heat dump!
• Etc., etc.
Step 9. Evaluate modeled performance
• Did we meet our goal?
– Keep the electric chiller at or below 75% of full
capacity during peak periods.
– Increase the plant COP by 40% with a solar
contribution of 25%.
Evaluate modeled performance
Chiller Output in Refrigerant Tons
35
Jan
Feb
30
Mar
Apr
25
May
Jun
20
Jul
Aug
15
Sept
Oct
10
Nov
Dec
5
0
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
Did we keep the electric chiller at or below
75% of full load?
Optimization Effect on Existing Chiller Load
Typical Screw
Solar Application 1
August
100%
% of Full Load
80%
60%
40%
20%
0%
900
1000
1100
1200
1300
1400
1500
1600
Standard Time
Unoptimized Chiller Load
Optimized Chiller Load
1700
1800
Does this design achieve a 40%
improvement in plant efficiency?
Optimization Effect on Plant COP
Typical Screw
Solar Application 1
5.00
August
100%
4.50
3.50
60%
COP
3.00
40%
2.50
2.00
20%
1.50
1.00
0%
0.50
0.00
900
1000
1100
1200
1300
1400
1500
1600
1700
Standard Time
COP of Unoptimized Plant
COP of Optimized Plant
"COP Improvement"
-20%
1800
COP Improvement
80%
4.00
Step 10. Shade Avoidance
The Law of Sines
Conclusion
• Solar thermal chilled water can impact energy
use beyond the solar contribution.
• Don’t under-power the system.
• Size the collectors coincident with the load.
• Do NOT attempt to STORE heat for use after
the sun is gone.
• Always have a heat dump.
• For a practical system, combine sound
engineering principles with a healthy dose of
common sense.
• Don’t forget the heat balance!