While Stanford Energy System Innovations (SESI) is fully operational and has allowed Stanford to exceed state, national, and international goals for greenhouse gas reduction by several decades, the university continues to explore additional enhancements that can further advance efficiency and sustainability, while reducing cost.
Thermal Exchange with Campus Water Systems
The centerpiece of SESI is the electrification of building heating and cooling though district-wide heat recovery, coupled with a renewable electricity supply. However, since this combined heating and cooling process only covers about 75% of campus thermal loads, Stanford is investigating ground source heat exchange to meet the balance of its loads and advance the system to full sustainability. Feasibility has been tested through a closed loop geothermal well, with largely successful results. That said, given the pattern of thermal loads, engineers have determined that an open-source ground source heat exchange system would be better. Testing for this system is in the planning and design phase, along with considerations for how existing campus water and wastewater systems might also support the thermal exchange.
Heat Rejection and Extraction to the Irrigation System
The advantages of using the campus irrigation system for heat rejection include greatly reduced water use; switching the water used for heat rejection from high-quality drinking supplies to non-potable irrigation sources; and gaining cooling tower capacity.
Initial data indicate that the campus non-potable irrigation system averages flows of about 1,500 gallons per minute in summer and that the temperature of the water in the system ranges between 60°F and 70°F. Rejecting 20°F into the irrigation water flow would provide 1,500 to 3,000 tons of cooling capacity, depending on the rate of irrigation flow and heat rejection used. Most of the irrigation flow currently occurs overnight, which coincides with the optimal time to perform heat rejection so as to minimize grid electricity demand and costs. Campus irrigation also occurs primarily in summer when the university has excess waste heat to reject, making the potential of using the campus irrigation system for heat rejection very promising. If successful, this would reduce existing evaporative cooling tower use through SESI by 25% to 40% with corresponding water savings and heat rejection capacity increase. To determine feasibility, the university plans to explore possible effects on campus landscape vegetation, if any. Plans for a landscape test plot next to the Central Energy Facility would test these impacts under scientific control.
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 irrigation is collected during the winter rainy season from a large creek on the campus and pumped to a large open-air reservoir. The water temperature ranges from 50°F to 60°F, which is a good match for the pumps SESI uses for heat recovery. Furthermore, the same heat exchanger that 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 into the reservoir in winter. Reducing the temperature of the irrigation water collected in winter from down to about 42°F may also help reduce algae and other undesirable biological growth in the reservoir, improving campus irrigation water quality.
Heat Rejection and Extraction 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 year round. Just as with irrigation water flows, these 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 reviewing the constituents of concern 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 of these reasons the university’s first focus is on thermal exchange with the non-potable irrigation water system, however use of the drinking water system will also be fully explored.
SESI incorporates substantial energy storage in the form of hot and cold water thermal energy storage tanks and employs advanced model predictive control software programs to optimize its use. Thermal energy storage provides some of the same benefits that electricity storage does, namely the ability to increase load factor and control when power is taken off the grid so as to lower peak electrical demands and minimize commodity cost.
Additional benefits of electricity storage include: instantaneous mitigation of campus electrical demand fluctuations caused by variations in behind-the-meter solar power generation or plug-in electrical vehicle charging; voltage frequency regulation to the grid; and supply for fast-demand response programs in reaction to grid power generation curtailments. To determine the best electricity storage configuration for Stanford, the energy system optimization program is being enhanced to incorporate electricity storage in its algorithms. Following this, the system will be used to test the feasibility and value of adding electricity storage to SESI and if results so indicate the university may add it to the system.
Centralized Emergency Generation and Distributed Electricity Storage (CEGDES)
Should the addition of electricity storage to SESI appear favorable, the conventional approach may be to install it at the campus electrical substation interface to the regional electrical grid. However, a better opportunity for electricity storage may be to deploy it in smaller increments located at campus buildings, in place of diesel fueled emergency power generators. Unlike stationary building emergency power generators, which due to noise and air pollution regulations cannot be operated except for monthly testing and actual emergencies, batteries can serve as emergency building power supply while also providing services such as grid frequency regulation, unplanned demand response, or planned demand management. If a building battery configuration can be developed that balances emergency power supply with these other valuable services, it may be a desirable replacement for the essentially useless and expensive stationary building diesel generator fleet.
Under the CEGDES concept, if power to a building were interrupted a distributed electrical storage device (e.g. battery) would provide electricity for critical loads instead of an emergency generator. Battery capacity would be sized to provide enough time (one to two hours) for a mobile refueling device such as a truck mounted capacitor, battery or diesel generator to be dispatched to recharge it before it is depleted so as to assure continuous uninterrupted emergency power to the facility for as long as required. With the advent of electric busses and cars, employment of vehicle-to-grid strategies to allow the campus electric vehicle fleet to recharge distributed electrical storage may also become possible.