PracticalSolar 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 CycleSide-Stream Piping

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.

BypassVFD

44°F

55°F

VFD

Chilled Water

ChilledWater Supply

ChilledWater Return

Water Fired ChillerUsing Solar Heat

52°F

Electric Cost Savings from Unloading

COP 4.88 5.91 21%

Input KW elec 105.1

Output kW cooling 513.2

KW electric / Ton 0.720228 0.595

Cost per KW $0.1266

Cost per Ton Hour $0.0912 $0.0753 -$0.0159

Typical Chiller Power Curve

Part Load Performance

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Percent of Full Load

kWat

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For Chiller Type: Screw

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

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What time of day represents the peak load on the building?

Average Temp in Fort Worth for 2006

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Central Standard Time

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0Solar Heat Output in Watts, adjusted for Solar Angle of Incidence and Degrees Azimuth

Compare Azimuth Options

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45Solar Heat Output in Watts, adjusted for Solar Angle of Incidence and Degrees Azimuth

Compare Azimuth Options

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

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Did we keep the electric chiller at or below 75% of full load?

Optimization Effect on Existing Chiller Load

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Standard Time

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Unoptimized Chiller Load Optimized Chiller Load

Solar Application 1 AugustTypical Screw

Does this design achieve a 40% improvement in plant efficiency?

Optimization Effect on Plant COP

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COP of Unoptimized Plant COP of Optimized Plant "COP Improvement"

Solar Application 1 Typical Screw August

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!