A hybrid system using geothermal energy for both heating and electricity could reduce campus greenhouse emissions around 25% more than using it just for heating, potentially bringing Cornell close to its goal of carbon neutrality, according to new research.

A study led by Fengqi You, the Roxanne E. and Michael J. Zak Professor in Energy Systems Engineering in the Smith School of Chemical and Biomolecular Engineering, proposed two hybrid energy systems based on Cornell’s Ithaca campus, using geothermal energy in combination with other sources.

“Cornell has a huge campus. We have more than 600 buildings, and every building needs a lot of heating and cooling,” said You, senior author of “Carbon-Neutral Hybrid Energy Systems with Deep Water Source Cooling, Biomass Heating, and Geothermal Heat and Power,” which was published online May 9 in Applied Energy. “That’s the motivation of this study. We aim to redesign and optimize our existing energy systems, and find out how far we are away from being carbon neutral.”

First author of the paper is Xueyu Tian, M.S. ’19, who will begin his doctoral studies at Cornell next month and is a researcher in You’s lab.

You’s study was inspired by the Cornell Earth Source Heat project, in which faculty, students and staff are exploring geothermal energy as a sustainable method of heating the Ithaca campus. The project – part of Cornell’s goal of achieving carbon neutrality by 2035 – proposes to extract thermal energy from hot rocks several kilometers below the surface by circulating cooler water down a well and into contact with the hot rocks, and bringing the heated water back to the surface through a second well.

In the first case, the researchers conducted a rigorous life-cycle analysis to estimate that using geothermal wells combined with other sources for heat would reduce greenhouse emissions more than 90% compared with natural gas, which is currently used by many existing systems. In that model, the geothermal energy would heat campus, but cooling would require electricity produced by natural gas or renewable sources such as solar or wind, which are intermittent.

The second design – which considered using geothermal energy to produce electricity to cool the campus in addition to using it for heating, especially during the summer when heating demand is low – reduced greenhouse gas emissions by around 93%, compared with natural gas-based energy systems.

Both cases considered changes in energy demand from month to month, and took advantage of the unique cooling system in use by the university for nearly two decades.

“It’s a hybrid energy system,” You said. “The question is, how can we plan it in a very systematic way? The whole system must be integrated and it must be optimized so we can minimize the cost and reduce the carbon footprint.”

For a geothermal system, multiple technology options and dozens of potential working fluids could be used, and there are many possible ways to cool campus using electricity from different sources. The researchers developed a computational model and algorithm to screen all possible design options of the geothermal energy system to optimize its energy and cost efficiency.

The hybrid energy systems incorporate Lake Source Cooling, which creates a loop using the cold water of Cayuga Lake to cool Cornell and Ithaca High School, and which has been in place since July 2000. Both systems also include biomass energy – plant or animal material used for energy production – to provide enough additional heat for peak loads in the winter months.

The model using geothermal energy for both heat and electricity reduces greenhouse gases nearly a quarter from the heat-only model, but costs 5.6% more, the study found. “In the end it’s important to know that nothing is perfect, and that’s why we have to make sure we develop the most optimized system,” You said. “This is important not only for Cornell but for the broader public, because if we are successful this kind of model could be adopted by other universities or organizations.”

The research was partly supported by the National Science Foundation.