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.