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Solar Air Conditioning

Model Overview

Industrial solar heating facilities have been built at various times and places in response to economic and to some extent, environmental concerns. Historically, uncertainty in future upside fuel prices has been a strong motivating force for developing these plants.

An example of a solar heated thermal process is solar heated absorption chillers. These machines make chilled water for use in air conditioning systems. While technically feasible, whether or not this makes practical economic sense depends on the specific circumstances.  THERMOFLEX can help with this analysis.

THERMOFLEX was used to analyze one technical aspect of this issue: how much real estate is needed to install a solar field to run the amount of chillers needed to air condition a building?  The building is located in a hot climate with reasonable solar conditions, such as the US southwest where a significant amount of energy is used for air conditioning for seven to nine months per year.

Absorption chillers consume low to moderate grade heat and produce chilled water suitable for air conditioning. When connected to “waste heat” streams, absorption chillers can be used to save energy.

Single stage (effect) absorption chillers make use of hot water streams in the 185 to 195 F range, and have nameplate COPs in the 0.65 to 0.75 range.  Two-stage (double-effect) chillers get heat input at higher temperatures, typically in the 325 to 350 F range, but are more efficient with COPs in the 1.25 to 1.4 range.

In hot locations, the rule of thumb for sizing commercial building air conditioning systems is to install one ton of refrigeration for every 280 square feet, on average.  Variations in building construction details and space usage mean the actual value for a particular building and location can range from 360 to 190 ft2/ton.

A THERMOFLEX model of a small collector model servicing an absorption chiller circuit was built to estimate solar field size for a given A/C load. The model is shown below with values established for a single story commercial building with 4000 ft2 of conditioned space, requiring a nominal 15 tons of refrigeration capacity.

Solar design conditions are 781 W/m2 DNI on August 15 at 35˚ North, zenith angle of 21.22 degrees and azimuth angle of 180 degrees.

Assumed solar collector nominal optical efficiency is 70%.  The collector aperture is 5 ft, the concentration ratio is 48, the focal length is 1.5 ft, and the row pitch is 2.5.  Relatively small flows require use of smaller receiver tubes; 1.25 inch nominal OD is used here.

Model results for a single stage absorption chiller with COP of 0.7 , heated by 195 F water, indicate 455 ft2 of solar field land area is required per nominal ton of refrigeration. This means the field would be 162.5% the size of the single-story building’s footprint.

The more efficient double effect chiller, with COP=1.4, requires water at 350 F, but yields a smaller field for the required heat input.  In this case, land area for the solar field is 270 ft2 per nominal ton of refrigeration.  This is about the same as the building's specific cooling load.  That result implies a solar field with the same footprint as the building could provide the required heat input to run the chiller under these conditions.