Lowering Peak Power Demand in Radiant-Cooling Systems

Lowering Peak Power Demand in Radiant-Cooling Systems

By Greg Cunniff, Applications Engineering Manager, Taco, Inc.

If the chilled-water flow of a radiant-cooling system can be reduced to that of a conventional system, peak power demand can be reduced even further. Injection pumping can achieve this goal. Injection pumping has been used in radiant-heating systems for a number of years, lowering 180°F boiler water to the 100°F-120°F needed for a radiant floor panel. The same principal can be applied to a radiant-cooling system in reverse by raising 40°F chilled water to the 55°F-60°F required by a chilled ceiling panel or beam.

The primary chiller flow in an injection-piping-system is lower than that in a conventional system. Primary chilled-water temperatures in an injection-piping system is 16°F – or 1.5 gpm per ton – while a conventional system ranges from 8°F-12°F – or 2-3 gpm per ton.  An injection-piping system’s flow rate is 50-percent less than that of a conventional system and 75-percent less than that of a typical radiant-cooling system. This results in a corresponding decrease in pump horsepower and materials for smaller pipe. An injection-piping system keeps the transport horsepower used to move a building’s heating and cooling energy to a minimum.

The use of low-temperature chilled water also allows spot dehumidification. A 100-percent DOAS pressurizes a building, negating infiltration of outside air. Natural infiltration can temporarily overwhelm the amount of outside conditioned air delivered by a DOAS when a building’s outside door is opened, especially in humid climates.

The amount of fresh air supplied to a building can be controlled by a differential pressure sensor to measure the difference in static pressure between the building and the outside. This sensor and an accompanying DDC controller will then control the speed of the DOAS unit fan to maintain a slight positive pressure. Fan coils can be used at the building’s entrances to overcome the inrush of humid air – or cold air in winter -that can overwhelm the slight building positive pressure when a door is opened.

A fan coil requires chilled water (50°F max.) to achieve adequate dehumidification, which cannot be supplied by a distribution system using 55°F-60°F chilled water for radiant panels and  chilled beams. With a proper low-flow piping layout, an injection-pumping system can deliver chilled water to building entrances for adequate dehumidification. Additionally, lower chiller operating temperatures (e.g., 40°F-45°F) allow a DOAS, rather than a direct-expansion unit, to use chilled water.

Next up… More on Passive Vs. Active Chilled Beams

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