Improving Electric Grid Stability through HVAC Chillers

By Leo Su – Former Fraunhofer CSE Fellow; Master’s Candidate, Massachusetts Institute of Technology Building Technology Lab

Despite its enormous size, the North American electric power grid is, in fact, a finely tuned machine that must constantly be kept in balance to maximize efficiency and minimize disturbances for users – ranging from variations in frequency and voltage that disrupt machines and power electronics, to the worst-case scenario of widespread blackouts that can occur within seconds following instability from power imbalances. These issues gain importance as the grid continues to absorb a wide variety of intermittent generation resources, including wind and solar technologies.

Regulation service provides grid stability by adjusting power to continuously compensate for short-term imbalances. Traditionally, generators have provided regulation; however, in recent years, a number of North American jurisdictions have allowed demand resources to compete with generators to provide regulation. Demand resources leverage existing equipment and could potentially be more energy efficient and cost effective than generators.

The majority of Demand Response (DR) is event-based, and is deployed a few hours a year during critical peaks in the grid. In contrast, Regulation DR can contribute to the grid during all hours.

Comparison between regulation service and the traditional use of DR to meet peak demand.

Figure 1: Comparison between regulation service and the traditional use of DR to meet peak demand.

Commercial HVAC chillers present an opportunity for Regulation DR. Chillers account for a significant portion of North American electrical demand and can vary power consumption on short-time intervals without compromising cooling requirements. Fundamentally, enough thermal mass exists in the building and the HVAC system that short-term variations in chiller cooling output do not impact occupant comfort.

Testing this theory, researchers at the Fraunhofer Center for Sustainable Energy Systems CSE and MIT’s Building Technology Lab collaborated on a joint project to demonstrate how existing chillers can be used to provide regulation service. Control algorithms were retrofitted onto Fraunhofer CSE’s chiller using an external controller. Experiment results showed that the chiller is able to vary power to compensate for grid imbalances with speed and accuracy that exceeds some generators currently used on the grid.

Chiller power demand while controlled to follow target power signal from the grid system operator (ISO) to compensate for short-term grid imbalances and provide regulation service.

Figure 2: Chiller power demand while controlled to follow target power signal from the grid system operator (ISO) to compensate for short-term grid imbalances and provide regulation service.

Although chillers are small compared to power plants, there is significant potential for this technology when applied in aggregate to create a distributed resource on a smart grid. Ultimately, using demand resources for regulation will allow generators to operate more efficiently with less variation in output, as well as defer capital investment in generation and transmission capacity.

Research funded by NSF and NSERC.

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