Primary Innovation: Heat Recovery
A heat recovery scheme is the most innovative component of the Energy and Climate Plan. During the planning exercise, analysis of real time energy use revealed that Stanford has a significant and simultaneous need for both heating and cooling in its buildings throughout the year. Therefore, the opportunity exists to recover heat discharged from the cooling system to the atmosphere via cooling towers to meet simultaneous campus heating loads. Analysis shows that the campus can recover up to 70 percent of the heat now discharged from the cooling system to meet at least 80 percent of simultaneous campus heating demands, significantly reducing fossil fuel and water use in the process. Based on this finding, Stanford will replace the current natural-gas powered cogeneration plant with an electricity powered Heat Recovery plant. Such a change also requires conversion of the campus steam distribution system to a hot water system because available heat recovery equipment can produce only very hot water but not high temperature and high pressure steam. Due to the significant heat recovery and lower line losses of hot water, compared to steam, the new energy system is 70 percent more efficient that the combined heat and power process provided by the current cogeneration plant.
District level heat recovery is the most innovative component of Stanford’s Energy and Climate Plan. Opportunity exists to recover heat normally discharged from the cooling system to meet simultaneous campus heating loads instead.
New Heat Recovery Design
In the new heat recovery design, the waste heat collected from buildings via the chilled water loop will be captured by new heat recovery chillers and moved to a new hot water loop. The new hot water loop will replace the current steam system for delivering heat to the campus. Due to the significant heat recovery and lower line losses of hot water, compared to steam, the new system is 70 percent more efficient that the combined heat and power process provided by the current cogeneration plant.
Cogeneration vs. 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 for direct comparison. 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.
In an ongoing pursuit of sustainability, the heat recovery design will move 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.
SESI will result in immense benefits for Stanford University in the years to come. The benefits of Stanford's new energy supply system are substantial.
- As the Replacement Central Energy Facility comes online, the campus will reduce its carbon emissions, ultimately 50 percent below 1990 levels by 2050 . This reduction in carbon emissions will be accompanied by a flexible and electricity-dependent energy supply system, offering higher reliability, lower cost, and greater flexibility for greener power procurement. Having recently achieved Direct Access to the California electricity market, Stanford is now exploring opportunities for a more economic and environmentally sound power portfolio.
- Due to the significant heat recovery and lower line losses of hot water, compared to steam, the new energy system is 70 percent more efficient that the existing combined heat and power process provided by the current cogeneration plant.
- A great feature of the heat recovery design is the significant reduction in water use. Since the majority of the waste heat from the chilled water loop is reused instead of being discharged out the evaporative cooling towers, potable water use is reduced by 18 percent.
- Converting the campus to heat recovery and hot water distribution, along with other system improvements, will require an estimated $438 million investment. While the heat recovery approach has a modestly higher upfront cost than a conventional approach, it will pay Stanford back $600 million dollars over the next 40 years.
Stanford adopted a long term approach in developing climate action strategies because decisions regarding building design, energy infrastructure, and energy supply have lasting impact and should be based on a planning horizon equivalent to the lifecycle of these investments. The individual components of the plan have been consolidated into an overall energy and climate plan that provides an adept balance of investment to optimize capital management and operating costs, as well as GHG emissions.