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Chilled Water Curtailment FAQ

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Past heat waves have prompted questions about the university’s need to curtail cooling on the main campus. This Q&A provides information about curtailment and the system that provides heating and cooling for campus. 

1. What is being done to improve the capacity of the system?

An extra 5,000 tons of cooling tower equipment has expanded capacity of the system by 35 percent. A permanent expansion has been approved to augment the Central Energy Facility and support expected growth over the next decade, which will nearly double plant capacity. The Board of Trustees gave concept approval for the project in its February meeting, with the permanent added capacity planned to come online in Fall 2022.

Because of the critical importance of preventing and mitigating disruptions to research as a result of cooling curtailment, Land, Buildings & Real Estate has convened an advisory committee of faculty members and facilities directors to make recommendations on the systems, including suggestions to further reduce loads in the early stage of a heat wave and reduce the likelihood of rapid escalation to higher levels of cooling curtailment.

A timeline of cooling capacity expansion since 2015. In June 2022, we have a total capacity of 774,000 ton-hours per day.

2. Why do we need to curtail chilled water in this relatively new energy system?

The campus heating and cooling system relies on a district level heat recovery process.  Heat recovery works by extracting excess heat from the chilled water system and repurposing it to the hot water system. At all times of the year, the university has a simultaneous need for both chilled and hot water service to most buildings. Chilled and hot water storage tanks add extra capacity to the system, functioning like large batteries that can be “charged” when campus demand is low and discharged when demand is high. This design meets our needed loads 99 percent of the time, given the current and expected temperature profile of the region. Of the approximately 1,600 days the Central Energy Facility (CEF) has been in service, curtailment has only occurred about 10 days, or less than one percent of the time.

However, for extreme heat events, when the temperature or humidity spikes to record or near record temperatures, there is so much excess heat that the system cannot  extract and expel it quickly enough to cool the chilled water to an optimal level.  When this happens, we curtail chilled water use though incremental stages (0-4) based on varied degrees of urgency to meet the load balance for a few days. With the expanded cooling equipment, this is not commonly expected. 

3. Can’t we tell when a heat spike is forecasted and avoid curtailment?

At the time of design and approval in 2011, the climate science and resiliency studies predicted increased heat waves in the region.  CEF models continually take temperature variation into account with a seven-day predictive modeling system.  During past heatwaves, the temperatures have been much higher than the weather forecast even hours before the actual temperature rise, and beyond what was reasonably predicted during design of the plant.  In addition, these high temperatures extend much longer into the afternoons and evenings. Curtailment preparation and implementation is a dynamic process that relies on effective load shedding (reduction) measures.  If loads do not shed as expected because of higher temperatures than forecasted or less load reduction in buildings, then additional curtailment may be necessary and we begin moving into more advanced stages of the curtailment plan.

The campus community and critical users receive communication as we go through a curtailment process, following protocol outlined in the Stanford Chilled Water Curtailment Guide.  During a curtailment event, we need the campus community’s help in implementing mitigation measures in their areas.  The Curtailment Guide provides actions for each stage that people can do to help ease the impact of a curtailment event. The new faculty and facility advisory committee will also help to refine these opportunities. 

4. Will there be more curtailment in the future?

Added capacity to the system helps to ameliorate the effects of these extreme heat events on the campus community.   We will provide advance notice to building managers, facility coordinators, and critical chilled water users when we see an approaching heat wave and continue to work and strengthen our communication to campus community about our plans.  You can always use the capacity dashboard to stay up-to-date with the current status and adequately prepare.

5. What can you do to help ease curtailments?

To reduce loads and minimize heat in buildings, it is always good practice to turn off lights in unused rooms, lower window shades, keep doors and windows closed, raise thermostat set points to 78F in office and common areas, shut off unused office equipment, close laboratory fume hood sashes when not in use, and, where possible, shut off lab equipment not currently being used. This will help to conserve energy and reduce chilled water loads.