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Directly Controlling the Winter Peak

Learning lessons from PSE’s residential demand response pilot.

Fortnightly Magazine - August 2012

that the impact of curtailing a heat pump isn’t linear in outdoor temperature. Or, to put it simply, the demand reduction impact isn’t necessarily proportional to the change in temperature. The incremental impact that would be observed when the temperature decreases from 32 to 30 degrees F, for example, is much greater than the incremental impact that would be observed when the temperature decreases from 35 to 33 degrees.

Further investigation revealed that the apparent step-change in heat pump curtailment impact is inherent to the design of the heat pump. A heat pump extracts heat from the air and then recirculates it. As a result, the heat pump’s efficiency diminishes as temperatures fall. Once the ambient temperature falls below a certain point, a heat pump typically will engage auxiliary resistance heat to supply the home’s load. This auxiliary resistance heat uses more electricity per Btu of home heating than the heat pump does, hence the step-change in curtailment impact between the morning, when temperatures were just a touch below freezing (when auxiliary resistance heat was required), and the afternoon, when temperatures were on average about 37 degrees F (when no auxiliary heat was required).

Baseboard Heaters

After some initial exploratory regressions, it became clear that baseboard curtailment was having little apparent effect on demand during control events, a matter of some consternation among the analysts involved. As noted before, the data-set included only those devices for which the receipt of the curtailment signal had been confirmed. As a result, installation data and interval demand data were investigated in greater depth.

A thorough examination—on a participant-by-participant basis—of individual interval data during the events showed that in many cases there appeared to be no impact on demand whatsoever as a result of baseboard curtailment. In other cases, it appeared as though baseboard curtailment had a very small, almost negligible impact. In a very few cases, however, it was apparent that baseboard curtailment was working as intended and delivering significant demand reductions.

While several theories have attempted to explain this phenomenon, the most plausible emerged from a review of installation data and discussions with installation personnel. The installation protocol established by PSE required that the largest-load baseboards in primary living areas and adjacent rooms be connected to the control switch. Through discussions with installation personnel, it became apparent that these key baseboard units couldn’t always be connected—either due to participant concern for their own comfort, or because the location of the electrical breaker panel precluded the connection of such key baseboards.

It also became apparent that in many cases, not all of the participants’ baseboard units were connected to the control switch. Baseboards not connected to a control switch simply worked harder to replace the controlled baseboards, leading to only a very small net reduction in household demand.

Winter Peaking Lessons

The results of the PSE pilot provide three significant lessons for winter-peaking utilities. The first—and most obvious—is that the direct load control of water heaters and electric furnaces can provide significant and reliable demand reductions during periods of peak winter demand. The second lesson