The doubling of SESI’s chilled water capacity will help minimize the risks of energy curtailments at Stanford campus buildings and hospitals during heat waves.
The renewable energy system that powers Stanford University and its hospitals will undergo a major expansion over the next two years to allow it to keep pace with a growing campus and a warming climate while minimizing the risks of building cooling shutoffs like the ones that disrupted teaching and research in summers 2017 and 2019.
As part of a planned expansion, the Stanford Energy Systems Innovations (SESI) will nearly double its chilled water capacity. (Image credit: L.A. Cicero)
As part of the expansion, the Stanford Energy Systems Innovations (SESI) will nearly double its chilled water capacity by adding two new cooling towers that will allow its novel heat recovery chillers to operate when needed, as well as three new permanent chillers and cooling towers and an extra backup chiller in case one of the others is offline. The chillers and cooling towers work in tandem to extract waste heat from buildings. The upgrades will boost SESI’s maximum permanent chilled water capacity to 28,500 tons - or nearly double the summer 2019 capacity of 14,500 tons.
"There are three main drivers of this expansion: campus growth, curtailment risks and climate change. We feel very confident that with this upgrade, SESI will be able to handle predicted energy loads through 2028," said Jack Cleary, the associate vice president of Land, Buildings and Real Estate (LBRE) at Stanford.
The expansion is expected to cost $85 million and to finish by June 2022, although Cleary notes that delays related to COVID-19 could push that date back by a few months. Fortunately, temporary chilled water capacity installed in June 2020 nearly matches the planned expansion, so there should be no lapse in coverage in the interim.
The decision to expand SESI was based upon a detailed statistical analysis of future campus energy needs, conducted by an advisory committee that included Stanford researchers in disciplines ranging from physics to energy resources engineering. The team considered the possible impact of warming in local weather and developed a model that can be updated based on actual weather trends over the coming years.
"Instead of simply providing a binary choice of ’This will be enough capacity or it won’t be enough capacity,’ we were able to say through this analysis ’Here’s how bad we think curtailments could get at a given future date, and here’s what we think the chances of that happening are as a function of the capacity we put in place,’ " said the committee’s chair, David Goldhaber-Gordon , a professor of physics in Stanford’s School of Humanities and Sciences.
Designed for adaptabilityWhen SESI began operations in 2015, it helped make Stanford one of the most energy-efficient universities in the world. The novel renewable energy system, which combines solar power and electric heat recovery, replaced an aging natural-gas-fired power plant and allowed Stanford to slash its greenhouse gas emissions by 53 percent. It also put the university on a path toward accomplishing its goal of drastically reducing Scope 1 and Scope 2 emissions - emissions that come directly from creating energy and burning natural gas by campus buildings or vehicles.
The upgrades will include the addition of two new cooling towers and three new permanent chillers (like the ones shown here) as well as an extra backup chiller in case one of the others is offline. (Image credit: L.A. Cicero)
From the outset, SESI was designed to be adaptable. During its initial construction, space was set aside for a future expansion some 7 to 10 years down the road to keep pace with changing campus needs. A key factor behind that projection was the predicted number of times that cooling in campus buildings would need to be turned off due to extreme temperatures overwhelming SESI’s cooling capacity.
"We assumed that there would be an extreme curtailment event once every 10 years that could impact the campus," Cleary said.
But in 2017, just two years after SESI began operations, the campus experienced two multi-day curtailments in one summer, when temperatures climbed to 108 degrees Fahrenheit. "That set off alarm bells," Goldhaber-Gordon said. "We had an allocation of once in 10 years, and this was twice in one year."
Curtailments can have serious consequences for a research university like Stanford, where many labs rely on precise temperature control. "Basically, what you’re doing when you curtail is you’re shutting down the air conditioning and chilled water to the building systems but also to the research," Cleary said. Severe curtailments due to extreme heat events can damage equipment and disrupt or ruin carefully planned experiments.
The university considered moving up the timeline for SESI’s expansion after the 2017 curtailments but ultimately opted for a wait-and-see approach. And indeed, temperatures were cooler in summer 2018 summer and no curtailments were needed. But in 2019, in the midst of final exams and campus reunions, temperatures climbed once again, prompting the first of three curtailments that summer. (2019’s temperatures were actually lower than in 2017 and the higher cooling demands that year were due to campus growth.)
"The 2019 curtailments triggered a decision that something had to be done," Goldhaber-Gordon said.
Future planningAs a stopgap measure, the university installed temporary cooling towers to help handle the extra load. In October 2019, it also convened an energy supply advisory committee, headed by Goldhaber-Gordon, to advise LBRE and the university on why the curtailments occurred, and how to approach planning both the immediate and future expansion of SESI.
Early in the process, Goldhaber-Gordon invited Jacques de Chalendar, at the time a graduate student in the lab of Energy Resources Engineering Professor Sally Benson , to join the committee. de Chalendar had already gathered a wealth of data about Stanford’s energy system as part of his research, which studied how different buildings manage shared campus assets such as heating, cooling and electricity systems.
"One of the important components of Stanford’s central energy plant is it has the ability to store energy for heating and cooling for a period of time. That’s what gives us flexibility," de Chalendar said. "A lot of the work I was doing was around assessing how much flexibility we actually have, and what we can do with it as far as reducing our carbon footprint and cost and increasing our energy efficiency."
Decision frameworkUsing data from LBRE’s systems, de Chalendar, Aero/Astro postdoctoral scholar Ayan Mukhopadhyay, and others on the committee developed a statistical approach for predicting load on the cooling systems based on assumptions about campus growth, weather patterns and how the systems react to campus demand. This allowed them to quantify and visualize the degree of certainty that SESI would be able to meet the cooling loads in various future scenarios.
"This just goes to show that Stanford students can harness the full capability of their academic learning to provide real-time answers to real-world problems," said Benson, who is also the director of the Stanford Precourt Institute for Energy.
In addition to advising on SESI’s expansion, the committee also developed a framework for making similar decisions in the future that relies on a core principle of science: examining how well predictions line up with reality, potentially challenging previously held assumptions and then implementing changes to the model to generate new predictions.
According to this decision framework, the energy needs of the university and the capabilities of SESI will be reevaluated on a frequent basis to identify potential planning risks such as a faster-than-expected growth in campus programs or warming of the climate.
"Now that we made this decision to expand SESI, it’s very important that we revisit the decision on a very regular basis," de Chalendar said. "If we don’t, we’ll be in for surprises."