Chilled Water Curtailment FAQ

Summer heat waves prompted many questions about the university’s need to curtail cooling on the main campus. We understand the concerns, particularly regarding the prevention of research disruption. This Q&A provides information about the situation and next steps.

1. 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 we have had to curtail about 10 days, or less than one percent of the time. 

Performance Over Time

This chart showcases how the CEF has met chilled water capacity in 2019.

However, for extreme heat events, when the temperature spikes to record or near record tempartures, 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 (0-6) based on varied degrees of urgency to meet the load balance for a few days.  Usually, we can expect one or two mild to moderate curtailment events during a season.  The events we have experienced in 2017 and again this year have involved record temperatures above normal for this time of year.  Some years, such as 2018, we do not have to curtail at all during the summer.

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

We have installed an extra 5,000 tons of cooling tower capacity on a temporary basis that allows us to use reserve equipment during extreme heat events.  This expands the cooling capacity of the system by 35 percent. We are working on plans to expand chilled water capacity at the Central Energy Facility permanently, which will also support expected growth over the next decade.

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. This committee will make recommendations, including on the question of how we can further reduce loads in the early stage of a heat wave, to reduce the likelihood of rapid escalation to higher levels of cooling curtailment.

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.  SESI models continually take temperature variation into account with a seven-day predictive modeling system.  During recent 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 is necessary and we begin moving into more advanced stages of our curtailment plan.
 
The campus community and critical users are receiving communicationas 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?

We feel that our plans for added capacity to the system will help 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.  

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.