P14
HEAT EXTRACTION FROM THE IRRIGATION SYSTEM Just as Stanford’s irrigation water system appears suitable for heat rejection in the summer, it may be suitable for heat extraction in the winter when the district energy system does not have enough heat to meet campus demands. Most of the water the university uses for summer irrigation is collected from a large creek on the campus during high flow events in the winter rainy season. This water is pumped from the creek to a large open-air man-made reservoir on the campus where it is stored for summer use. The quantity of this water flow is of course similar to the summer irrigation flow, and the water temperature ranges from 50°F to 60°F. This temperature range is a good match for the large heat pumps SESI uses for heat recovery, most of which occurs in the spring, summer, and fall seasons, leaving high heat pump availability in winter for extracting heat from winter irrigation water collection flows. Furthermore, the same heat exchanger, which would be used for heat injection to the irrigation flows out of the reservoir in summer, could be used for heat extraction from the irrigation flows in to the reservoir in winter. Reducing the temperature of the irrigation water collected in winter from 60°F down to about 42°F may also help reduce algae and other undesirable biological growth in the reservoir and improve campus irrigation water quality.
Courtesy ZGF Architects LLP; © Robert Canfield
SUMMARY SESI achieves significant increases in energy efficiency, cost reduction, greenhouse gas reduction, and water conservation for the Stanford district energy system through electrification of building heating and cooling functions via heat recovery. Hot and cold thermal energy storage, a patented advanced model predictive control energy management system, and new large-scale on- and off-site renewable electricity generation are also key features of what the United Nations refers to as a ‘4th Generation’ district energy system that opens a path to a fully sustainable building energy supply system. While SESI has propelled Stanford decades ahead in the important race to sustainability, there are still enhancements which could further reduce cost, GHG, and water use to complete the university’s transition to full sustainability. These include thermal exchange with campus water and wastewater systems and ground source heat exchange, electrification of campus transportation and emergency power fleets, and moving the last third of its electricity supply to renewable sources. Work at the university is underway to develop sound business options for these remaining steps to sustainability.
Courtesy ZGF Architects LLP; © Robert Canfield
HEAT EXTRACTION OR REJECTION TO THE DRINKING WATER SYSTEM
Campus drinking water supply originates as rainfall and snow melt runoff from the Sierra Nevada Mountains several hundred miles east of the university. Water temperatures arriving on campus from the system are in the 50°F to 60°F range and flows occur all year round. Just as with irrigation water flows these flows appear highly suitable for heat extraction and rejection from the district energy system. However, potential impacts to public health, research, and other university activities must be carefully considered to determine how much, if any, thermal exchange with this system might be feasible. Campus utility and water quality engineers and scientists are now reviewing the constituents of concern in the drinking water supply and the acceptable temperature ranges for each to advance this option. Notwithstanding the technical and regulatory feasibility of thermal exchange with the drinking water system, the human perception and concern of doing so must also be addressed. For all these reasons, the university’s current first focus is on thermal exchange with the non-potable irrigation water system, however use of the drinking water system for this will also be fully explored.
For further information please contact:
Stanford University Sustainability and Energy Management Executive Office Att.: Joseph Stagner Stanford California 94305, USA
Phone: (650) 721-1888 jstagner@stanford.edu
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