ENERGY SHARING

To understand the energy-sharing benefits of a TET system, one must understand the interaction of loads in the core and perimeter zones of a building during occupied periods with internal gains and unoccupied periods with temperature setback/setup and few or no internal gains throughout the year. For example, following are the main energy-consuming operating modes of an office building in a temperate climate:

• Summer, occupied: Typically, all zones require cooling and are rejecting heat into the water loop. The heat rejector maintains the maximum water-loop temperature according to a predetermined set point (fixed or outdoor reset). The boiler is off.

• Winter warm-up: During recovery from night setback, most zones extract heat from the water loop. The boiler maintains the minimum water-loop temperature according to a predetermined set point. The heat rejector is off. The warm-up period typically is one hour or less per day.

• Winter, occupied: Because of internal heat gain, most core zones require cooling. Meanwhile, most perimeter zones require heating. Because heat is rejected into and extracted from the water loop simultaneously, both the boiler and heat rejector are off much of the time. This is the essence of TET: The sharing of energy within the water loop minimizes boiler and heat-rejector operation and provides maximum system efficiency.

ADVANTAGES

Advantages of water-to-water systems include:

• Year-round individual control. Each zone heat pump provides individual temperature control. This means occupants can control heating and cooling around the clock, regardless of the season and what is happening in other zones. Zones served by a single heat pump can be as small as 200 sq ft or as large as many thousands of square feet.

• Energy savings. WSHPs provide zone heating and cooling at the highest rated levels of efficiency. The water loop recovers much of the energy needed to heat a building, minimizing boiler use. WSHP systems operate efficiently even at part-load conditions. Also, they eliminate the double energy waste of zone reheat (cooling with subsequent reheating), which is common in many HVAC systems.

• Tenant metering. The majority of the cost of operating a TET system is incurred at the zone-heat-pump level, where tenant metering can be implemented. Thus, each tenant pays only for what he or she uses.

• Quiet operation. Modern WSHPs operate with great stealth, even when located in occupied spaces. The latest models utilize new compressor technology, variable-speed fan motors, and sound-isolating features.

• Low initial cost. WSHPs are factory-assembled and tested and usually incorporate all zone-level controls and hydronic accessories. They use basic low-pressure duct systems or, in some configurations, no duct system at all. The water loop is uninsulated and requires no more than two pipes — a supply and a return. The pump, boiler, and heat rejector, meanwhile, require a minimum of temperature controls and valves. All of this leads to one of the lowest initial costs among HVAC systems.

TET systems often cost 10- to 15-percent less to install than traditional two-pipe — and up to 20- to 30-percent less to install than traditional four-pipe — central, chiller-based systems.

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