You Can Do It Too

Proof of scalable innovation

Stanford Energy System Innovations (SESI)  supports the university’s academic mission, maintains economic viability, and leads sustainability by example. After four years of planning and three years of construction and implementation, the vision of SESI was swiftly realized in March 2015 when the Central Energy Facility became operational. SESI has reduced campus greenhouse gas emissions by 82%, campus potable water by an additional 18%, and saves the university $420 million over the business-as-usual case over the 35-year useful life of the facility.

Prior to concept approval in 2011, multiple energy options were considered, including on-site gas cogeneration with steam or modest heat recovery, grid power with heat recovery, and a combination of grid power and photovoltaics with heat recovery. Capital renewal and expansion had been already deferred for nearly a handful of years and most equipment was at or near the end of its useful life. Stretching the system at the time another five years would be very costly. Of the nine considered options, grid power plus heat recovery was the lowest cost option, including building a new cogeneration plant.  This option entailed modern, higher efficiency equipment, “free” heat via heat recovery innovation, 20%+ line-loss reduction by converting to hot water (steam line-loss averaged about 25% and hot water line-loss is 3.8% while chilled water is unchanged), long-term operations and maintenance savings of hot water as compared to steam distribution, and a transition away from reliance on a single fuel (natural gas).  

The elements of SESI include:

• A Central Energy Facility (CEF) that utilizes heat recovery and both thermal and electric storage to maximize efficiency in the university’s heating and cooling systems
•  Direct Access to the grid, meaning that we are our own energy service provider, plus an enlarged connection to the California grid to allow full electrification as well as enhancements to the local distribution system
•  A new hot water distribution system that replaced the steam system and involved installing more than 20 miles of pipes across campus
• Updates to the mechanical systems of 155 buildings to receive hot water instead of steam for heating and hot water services
• Model predictive control,  the software in of itself increases efficiency by 6%

Reasons for Heat Recovery

Heat recovery opportunity exists across all built environments regardless of climate. Heat pumping from ground, water, or air can augment heat recovery from existing processes.

In an ongoing pursuit of sustainability, the heat recovery design moved Stanford into a new energy era with a significantly lower reliance on fossil fuels, lower energy costs, reduced greenhouse gas emissions, and less water use. Just as Stanford’s move to cogeneration 25 years ago represented a major shift in campus energy supply technology for the better, so too does heat recovery represent a significant shift towards more efficient and sustainable technology for the future. 

Heat recovery is a central feature of the Central Energy Facility, which necessitated changing from the use of steam to hot water for campus heating. Prior to the Central Energy Facility and SESI, an intertwined system of steam and chilled water met the simultaneous demand for heating and cooling on Stanford’s campus. Steam from the Cardinal Cogen plant entered a given building, and, through heat exchangers, produced both warm air for space conditioning and hot water for restrooms, kitchens, and laboratories. Afterwards, the steam changed into condensate, and returned to Cardinal Cogen to be reheated back into steam and then sent out to buildings again. Simultaneous to the steam system, chilled water entered the building, and through a different heat exchanger, provided cold air to cool the building. After collecting waste heat, the chilled water then was piped back to the facility for re-cooling.

The Cardinal Cogen plant followed a Combined Heat and Power (CHP) system. Also known as cogeneration, CHP is an energy supply system that uses fossil fuels to produce electricity and then recovers waste heat from the combustion process for heating or other productive uses. Conversely, an energy system in which heat and power are produced separately, usually by on-site heat production equipment and off-site power plants respectively, is known as Separate Heat and Power (SHP).

Whether CHP or SHP is more energy efficient, economic, or environmentally preferable for a given site depends on many factors, including climate, relative heat, power loads, the energy efficiency of equipment used in each process (including off-site power production in the SHP option), and capital equipment cost.

At Stanford, these factors result in CHP and SHP being generally equal in expected overall efficiency when natural gas is used to fuel equipment in both cases. However, when heat recovery or alternative forms of renewable heat production (for example, ground source heat pumping or solar hot water production) are also applied, the SHP option becomes clearly superior economically and environmentally given the significant amount of heat recovery that is possible on campus.