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Solar Thermal Retrofit

 

©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 9, September 2012.

By Barry Jim On, P.E., Member ASHRAE; and Min Zhong, P.E., Member ASHRAE

About the Author
Barry Jim On, P.E., is vice president and Min Zhong, P.E., is project engineer with InSynergy Engineering in Honolulu. Jim On is an ASHRAE-certified Healthcare Facility Design Professional. The authors are members of the Hawaii chapter.

SSFM International, a civil and structural engineering firm in Honolulu, is located in a former warehouse built in 1919. The firm needed to improve thermal comfort in the two-story building constructed of corrugated metal walls and asbestos transite panels. Additionally, it wanted to show its commitment to sustainable building. Therefore, SSFM decided to do a solar thermal retrofit of its five-year-old air-conditioning system to achieve both goals. The project resulted in energy savings of nearly 65,000 kWh/year.

The building’s total floor area is 40,589 ft2 (3771 m2), and it has 17,897 ft2 (1663 m2) of air-conditioned office space. The building recently received Gold certification under LEED for Existing Buildings: Operations and Maintenance, 2009.

The existing AC system is served by three 15 ton (53 kW) packaged self-contained, air-cooled, air-conditioning units that are manifolded together to provide conditioned air throughout the office areas. The older system operated as a large constant volume system that staged each 15 ton (53 kW) AC unit to match the loading in each space, which ranges between 20 and 40 tons (70 and 141 kW) throughout the year.

The existing 15 ton (53 kW) units have an EER of less than 9.5, and because the system operated as a large single-zone, constant-volume system, thermal comfort within the building was not satisfactory to many occupants. To exacerbate the problem, the roof was not insulated, and heat often radiated down into the occupied area, causing uncomfortable temperature gradients. As a result, SSFM decided improve its air-conditioning system to provide better comfort, and increase energy savings, which also helped it to achieve its LEED certification goals.

Due to the large roof area, SSFM evaluated photovoltaic panels and solar thermal air conditioning. Based on the upfront capital cost, including available state and federal tax credits, the solar thermal air conditioning provided the most “bang for the buck.” Therefore, it was determined that a solar thermal AC system would be the most appropriate system to demonstrate renewable technology, yet still use its existing packaged air-conditioning system.

The retrofit’s concept incorporates a solar thermal chilled water AC system, which operates parallel to the existing direct expansion (DX) packaged system. With the completion of this project in March 2010, the solar thermal system operates as the primary system and the DX system is the backup for cooling the building on cloudy days. The solar thermal air-conditioning system unloads the existing DX system to save electricity.

The solar AC system consists of an absorption chiller driven by 180°F (82°C) water generated by standard domestic flat plate solar collector panels. To further optimize the system, one of the manifolded AC units was converted to a 100% outside air unit, and the existing air-distribution system was converted to a dedicated outdoor air system (DOAS). The converted outdoor air-conditioning unit was also modified with a desuperheater heat exchanger that uses refrigerant superheat from the AC compressor to preheat the solar hot water heating system in the morning at system startup. 

Citation: ASHRAE Journal, vol. 54, no. 9, September 2012

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