Tri-County Electric, a rural Minnesota electric cooperative investigated the 144,000 square foot Caledonia High School, in Caledonia, Minnesota, for energy saving opportunities in response to the District’s concerns regarding energy costs and a pending rate increase. The facility was approximately 4 years old and includes a ground source heat pump system. Tri-County identified potential savings with the water circulating system and referred the District to Michaels for further assistance.
Opportunity
Michaels conducted a walk-through investigation to identify potential opportunities and gather information for proposing the next steps. The pumping system indeed provided opportunity, but so did makeup air units, and ventilation controls. Michaels conducted the feasibility study and developed three primary energy saving measures:
Convert the primary/secondary pumping system, a parallel pumping system, to series only. This included removing one 100 hp circulating pump that operated at 60Hz altogether, installing isolation valves on heat pumps and interlocking them with the heat pump compressors, and controlling the remaining pump with circulating loop static pressure.
Revise the control sequence for makeup air units to allow the discharge temperature to float rather than continuously providing 70F air to downstream heat pumps. This provided an opportunity to eliminate heating at the unit and then cooling downstream – simultaneous heating and cooling, which is virtually ubiquitous when air handlers serve multiple zones.
Revise the control sequence on CO2 demand controlled ventilation to 700 parts per million above ambient CO2 levels rather than 700 ppm, absolute, which is only about 300 ppm above ambient. This resulted in a substantial reduction in ventilation volume and associated conditioning energy.
Michaels provided control sequences and implementation documents for the project and later provided functional performance testing of the measures.
Results
Energy savings for these measures was predicted to total 30% of the building’s energy consumption with a 3.6 year payback. Projects were implemented in the summer of 2007. Comparison of twelve months pre and post installation indicates a 40% reduction in energy consumption for a 2.7 year actual payback. In addition, a billing error was identified at the outset of the feasibility study resulting in an instant $5,000 refund from the utility.
As part of the project, the building earned the ENERGY STAR®, increasing its ENERGY STAR Portfolio Manager score from 37 to 93.
