Cogeneration to Heat Recovery
An energy supply system that uses fossil fuel to produce electricity and then recovers waste heat from the combustion process for heating or other productive uses is known as Combined Heat and Power (CHP), or cogeneration. 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 over the long term, if 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 at Stanford, an energy supply system featuring SHP with heat recovery is more economically and environmentally viable than CHP over the long term.
Heat recovery is a central feature of the new Central Energy Facility, and required changing from the use of steam to hot water for heating the campus. This transition increases safety, reduces energy lost in the heat distribution system, and reduces system operating and maintenance costs. Making the switch from steam to hot water required the addition of 22 miles of new hot water piping, the subsequent abandonment in place of the old steam distribution system, and conversion of 155 building steam connections to hot water. In addition to this new thermal energy facility and major changes to the campus heating and cooling distribution system, a new campus high voltage substation with new interconnections to the campus grid in ten different locations was also installed to increase the capacity and reliability of the campus electrical system.
In an ongoing pursuit of sustainability, the heat recovery design moves Stanford into a new energy era with a significantly lower reliance on fossil fuel, lower energy costs, reduced GHG 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 of the campus energy supply to a more efficient and sustainable technology for the future.
Prior to the new Central Energy Facility developed through SESI, an intertwined system of steam and chilled water met the simultaneous demand for heating and cooling of campus buildings. Steam from the Cardinal Cogen plant entered the 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.
Plan Approval and Report
In December 2011, Stanford’s Board of Trustees gave concept approval to the Stanford Energy System Innovations (SESI) project, which is a collection of operationally distinct initiatives designed to meet the university’s energy demand while reducing greenhouse gas emissions and water consumption. SESI represents a significant transformation of the university from 100% fossil-fuel-based cogeneration to a more efficient electric heat recovery system and will result in immense benefits for Stanford in the years to come. The SESI project has reduced campus greenhouse gas emissions by 68% from peak levels, and saved 18% of campus potable water, in its first year of operation. These savings will continue to increase as the project opens up the energy supply to future technologies and enables the campus to better manage its power portfolio by incorporating renewables.
2015 Energy and Climate Plan - PDF
Implementation of the SESI program involved significant work throughout the campus between 2012 and 2015. The Department of Project Management managed design and construction of 22 miles of hot-water pipe, conversion of 155 buildings to receive hot water instead of steam, and installation of the Central Energy Facility (CEF) and a new campus high-voltage substation. View a timelapse video detailing construction of the new CEF.
This work was carefully sequenced in multiple phases to minimize disruption to campus life. As each phase of piping and building conversion was completed, that section of campus moved off steam and transitioned to hot water via a regional heat exchanger that converts steam from the existing cogeneration plant to hot water at a district level. A full transition from the cogeneration plant to the new CEF took place in April 2015, the regional heat exchange stations were removed, and the cogeneration plant decommissioned and removed to make way for new academic buildings within the campus core.