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One AHU coil design winter performance schematic. Image: Konvekta USA Inc.

The recently designed Univ. of Colorado Boulder Sustainability, Energy and Environment Complex (SEEC) implemented a Konvekta intelligent high-efficiency heat-recovery system with MeeFog direct evaporative cooling.

Labs typically implement one of four systems including run-around loops, energy-recovery wheels, refrigerant heat pipes or plate heat exchangers. Run-around loops are the most flexible by allowing the supply air-handling units to be remote from the exhaust fans. However, they’re the least efficient among energy-recovery systems. Energy-recovery wheels have a high thermal efficiency. However, they require the supply and exhaust airstreams to be adjacent and bypass some of the exhaust air to the supply. Heat pipes also have a high thermal efficiency. However, they too require supply and exhaust airstreams to be adjacent. One lesson learned is heat pipes may contain too much refrigerant, which voids the enhanced refrigerant points in LEED. Air-to-air plate heat exchangers also require the supply and exhaust airstreams to be adjacent, have a high pressure drop and are difficult to keep clean.

An intelligent high-efficiency energy-recovery system is similar to a run-around loop, however, it uses strategies which make it more efficient than the other systems listed above. First, this system utilizes specialized coils with a high water pressure drop to allow turbulent flow through a wide range of flow rates, allowing a variable flow system with control valves at each coil. This high water pressure drop allows lower flow rates compared to a conventional run-around loop, resulting in similar pump horsepower. A single coil in the supply air handler performs energy recovery, preheat and cooling by using one piping system to supply the correct water temperature needed to achieve the required air temperature. The pumping skid implements heat exchangers to transfer energy from the heating and cooling systems to change the temperature of the recovery loop. By eliminating the heating and cooling coils, there’s an air pressure drop reduction up to 1 in water column. The pumping skid has an intelligent control system that monitors all air temperatures in the supply and exhaust; all water temperatures in the energy-recovery, chilled water and heating water loop to perform optimization of the energy-recovery water temperature; and pump speed and control valve operation. This control system performs a numerical simulation once per second, using the aforementioned inputs, inputs from the BMS and 3-D performance maps of the coils, pumps and valves. Using direct evaporative cooling in the exhaust increases the summer time efficiencies in Colorado by another 40%. The primary manufacturer of this energy-recovery system, Konvekta, offers a financial guarantee on the annual energy savings of this system.

Since this system is much more expensive than a conventional energy-recovery system, it’s imperative to review the potential reductions in system cost to bring this system into cost neutrality for projects that can’t afford increased cost. The preheat and cooling coils are removed. This allows the air-handling units to be reduced in size and removes the controls, coil piping hook-up costs, coil pumps and two piping networks to these air-handling units. The need for glycol in the chilled and heating water is also eliminated, along with the glycol feeders for these systems. For the Univ. of Colorado Boulder SEEC project, there was a glycol preheat system that was removed, as well as a heat-recovery pump. Another large cost-savings item is the reduction in pipe size for the energy-recovery loop piping. Lastly, if allowed by the owner, reductions in boiler and chiller plants can be accomplished due to the peak load reduction from the amount of energy recovered from this system.

In conclusion, we were able to find ways to save $1,200,000 from a conventional system in order to implement an intelligent high-efficiency energy-recovery system.

Sean T. Convery, PE, is a Mechanical Principal at Cator, Ruma & Associates in Denver, Colo. His 19 years of mechanical design experience include energy-efficient mechanical systems for higher education campuses and research labs.