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 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. 52°F

Electric Cost Savings from Unloading

Typical Chiller Power Curve 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?

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

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

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

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

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!