ASHRAE Journal -November 2012 - B12

Role of Variable Speed Compressors
The new variable speed compressor motors provide new opportunities.** Slowing down variable speed compressors can better maintain the comfort level of consumers while saving money. Variable speed compressors, fans and blowers significantly reduce energy use by precisely matching the system’s level of operation to that of cooling demand, which can vary throughout the day based on a building or home’s individual cooling needs. They can maintain the occupant’s setpoint without resorting to inefficient full-on, full-off system cycles. This on/off cycle, which is a common function of traditional fixed speed air-conditioning systems means that the system is always running at full load for the given ambient condition, whereas an air-conditioning system using variable speed technology is capable of operating under much more efficient part-load conditions. Variable speed compressors are one example of HVAC capabilities that can be used to create value. For example, a fixed speed residential air-conditioning system with a seasonal energy efficiency rating (SEER) of 13 might achieve an energy-efficiency ratio (EER) of 11.5 at 95°F (35°C). Even if it is unloaded 25% by compressor cycling, as many utility load controlled systems are, it still performs at 11.5 EER at 95°F (35°C) while it is running. A variable speed system, however, improves SEER by up to 30% and can also operate at a greater EER rating—up to 3.5 points higher—when unloaded by the same 25%. Reducing a fixed speed system’s output, through cycling, by 25% at fullload conditions will result in an equal power reduction of 25%. Reducing a variable speed system’s output by the same amount under similar conditions will result in a power reduction of approximately 43%—the result of improving efficiency.3 The value stream for variable speed air conditioning is robust and two-fold, providing short-term and long-term benefits to consumers (building or home owners) and electric utilities. For the home or building owner, a variable speed system provides significant energy savings by improving the system’s SEER rating. By avoiding full-on, full-off cycles, space comfort is also improved, including tighter temperature control, enhanced dehumidification, lower noise levels and smoother heating ability. In heat pump applications, variable speed systems provide another benefit: more heat. With a higher heating seasonal performance factor (HSPF) index, variable speed heat pumps enable effective heat pump use in more northern climates. For the electric utility, variable speed not only improves system energy use, but is smart grid-ready—better able to facilitate peak load reductions through demand response or price response capabilities. By providing a continuously adjustable output, the local utility would be able to adjust the system’s speed to better accommodate periods of peak load. Therefore, demand is reduced without much noticeable impact on the occupant’s level of comfort. The use of variable frequency drives

(VFDs) to achieve variable speed also improves the utility’s distribution power factor. Power quality is improved by virtually eliminating compressor in-rush current, which in part creates conspicuous light flickers, and limits load control deployment on utility distribution circuits.

Role of Thermal Energy Storage
Thermal energy storage systems for both heat (bricks) and cold (ice) deliver significant savings to utilities and their consumers. For the utility, these systems reduce coincident peak demand and shift costly on-peak energy consumption to the lowest cost off-peak period. Similarly for consumers, the use of ice storage reduces the peak demand charge and shifts energy consumption to lower cost off peak rates. In extreme hot and dry climate zones where diurnal temperature differentials average 25°F (14°C), ice storage can improve overall building cooling energy efficiency. The electrical system efficiency gains associated with peak load shifting are dramatic; meaning a 50% improvement in generator source fuel efficiency associated with cooling buildings is typical (Figure 2). These gains are achieved because each asset in the electricity supply chain operates more efficiently at night. Central generation plant efficiency is better at night; this is expressed as average heat rate (Btu/kWh), which is the amount of fuel burned to produce a kWh. Electrical transmission and distribution system line losses are relatively lower at night, so fewer kilowatt hours are lost between the generator and the HVAC unit. Finally, HVAC system efficiencies improve as evening temperatures drop. Conversely, the amount of generator source fuel consumed to cool buildings increases as heat rates, congestion, and temperatures rise. The overall electrical system efficiency, from the burning of generator source fuel through the production of building cooling during the day or the production of ice at night, is the product of the efficiencies along the path. Using the example below, during the day, roughly 18% of generation source fuel is delivered into the building as cold air (82% losses or 0.28 × 0.95 × 0.92 × 0.72 = 18%) (Figure 2). However, at night, due to higher efficiencies along the path, 27% of the source fuel is delivered into the building and stored as ice for use the next day (73% losses). This equates to roughly a 50% improvement in generator source fuel efficiency (18% × 1.5 = 27%).

Role of HVAC in Regulation Service
Another benefit of integrating utilities and thermal energy storage is the ability of thermal energy storage to keep up with the requirements of utility and other grid operators. For example, the Midwest Independent System Operator (MISO) is charged with keeping supply and demand in balance over an 11 state region in the Midwest. The grid operators dispatch generation for each hour based upon the locational marginal pricing (LMP) in real time. Those power plants that bid the lowest price are dispatched first.

**This variable speed technology is also being built into other appliances that can be controlled by utilities such as pool pumps.

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November 2012

ASHRAE Journal -November 2012

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