Central Energy Facility
As part of Stanford Energy System Innovations (SESI), the Central Energy Facility (CEF) includes three large water tanks for thermal energy storage, a high-voltage substation that receives electricity from the grid, and an innovative heat recovery system that takes advantage of Stanford’s overlap in heating and cooling needs. Unlike the previous fossil-fuel-fired combined heat and power plant, the CEF is powered completely by electricity, which Stanford has committed to procure from renewable sources. In addition to the mechanical operations, the facility also includes administrative, classroom and meeting space that contribute to the educational component of the plant. As is true throughout the facility, these spaces also feature the latest in efficient design. Sustainable highlights include:
- LED lighting throughout
- An open-air floor plan, with high ceilings, fans and windows on each side of the building to facilitate cross breezes
- Flooring that utilizes radiant heat and chilled beam systems for heating and cooling
- Ceiling panels with energy absorbing filler that absorbs heat, and as the room cools off it releases it for heating purposes
View the architect's brochure for an overview of the design intention behind these spaces.
Heat Recovery Chillers
The Heat Recovery Chillers are the star of the facility.
Stanford’s many varied pursuits, from cutting-edge research to olympic-level athletic facilities, result in the campus having a 70% real-time overlap of heating and cooling demands. This presents the opportunity for heat recovery—using waste heat collected by the chilled-water system to meet the university’s concurrent heating need. The new heat recovery system collects waste heat from buildings via a chilled water loop and captures it at the Central Energy Facility (CEF) for reuse, eliminating the use of cooling towers to discharge the heat. Instead, heat recovery chillers move waste heat collected from the chilled water loop to a new hot water loop that distributes heat to the buildings. The heat recovery system meets 88% of the heating load on campus with waste heat and reduces total campus water consumption by 18%.
The CEF's heat recovery chillers (HRC) are the first of their kind of this size. Each HRC has a 2,500-ton cooling capacity for chilled water and can simultaneously produce 40 million BTUs of heat per hour, enough to cool and heat approximately 1000 houses simultaneously. The HRCs send out chilled water to campus at 42°F, which returns at 56-60°F. The heat removed from the chilled water to cool it back down to 42°F is used by the HRC to reheat spent hot water from 130°F back up to 160-170°F to supply for buildings that need heat. The CEF actively operates three HRCs and has the space to install another in case needed in the future.
Thermal Storage Tanks
While the recovery of heat and new efficient equipment is the key to the system’s energy efficiency, thermal storage is the key to its economic efficiency. Thermal storage tanks allow flexibility to operate the heat recovery chillers and other equipment at times with the lowest energy pricing and then store the hot and cold water for later use when the buildings need it. Thermal tanks cost much less than an equivalent capacity of additional heat pumps, chillers, and hot water generators and also contribute to the system’s energy efficiency by allowing the equipment to run at optimal load settings, and, in the case of regular chillers—which are incorporated into the facility as backup for peak-load days—when outside air temperatures and humidity favor evaporative cooling. The capacity of each chilled water tank is 5 million gallons, and the hot water tank holds 2.3 million gallons.
The substation runs on two different transmission feeds from both the north and south to power all of the core Stanford campus, not including faculty housing, which is on the external utility (PG&E) system. About 1/3 of the electricity consumed at Stanford will be used to operate the new thermal energy plant, while the rest will supply power to buildings for lighting, machinery, and electronics such as computers. The substation is designed to handle about twice Stanford’s current load or about 100 MW, which is enough to power about 100,000 homes. This is both for redundancy and to allow room for growth in the future. The facility also has an emergency generator for powering emergency lighting, elevators, safety systems, and even provides enough power to operate the thermal energy storage tank pumps to provide hot and cold water for the hospital during emergencies.
To assure optimal operation of the CEF for both service reliability and economic performance, Stanford invented a new central energy plant optimization program.
Stanford collaborated with Johnson Controls to transform a new patented plant optimization model into industrial-grade software, known as Enterprise Optimization Solution (EOS), and hardwire it into the energy facility controls system. EOS is an modeling and dispatch system using over 1220 variables including building occupancy, ambient conditions, time of year, projected energy prices, weather forecast, current system conditions, etc. to develop 15-minute dispatches that show the optimal way to run the plant—essentially an “autopilot" for the plant. The system predicts the university’s background electrical profile (electricity used by the buildings) for the next seven days and schedules HRC operation in hours each day so as to minimize the overall electrical footprint of the university on the grid. EOS will perform this forward-looking analysis and recalibrate the HRC operating schedules as needed every 15 minutes on a continual basis. It can be used be used in either advisory or fully automated modes, and the control room is staffed 24/7 to monitor operations. Each HRC uses about 5% of the total electricity used by Stanford, so the university must be adept at how and when they are used to minimize electrical impact on the grid and the corresponding ‘demand’ charges paid for its use. EOS provides the most efficient method for operating these complex systems. View a video about the EOS system, produced by Johnson Controls