Nearly every building at the University of Rochester’s main campus is heated, cooled, and powered by the operations of the Central Utilities Plant. Now, a new optimization project by University Facilities and Services’ energy team is set to cut the University’s greenhouse gas emissions by at least 6,000 metric tons of carbon dioxide equivalent while saving approximately $850,000 annually—enough natural gas savings to heat over 1,700 average American houses for a year.
These improvements target the Plant’s cogeneration system, which simultaneously generates electricity and hot water. Cooperative efforts across Energy Services staff over the past three years have led to immense strides in improving the operational efficiency and cost effectiveness of this system. Now, Energy Services is addressing the system’s physical limitations to unlock even greater benefits.
How does the cogeneration system work?
The cogeneration system works largely through the production and utilization of steam. First, the system’s pumps push water into boilers, which are heated by natural gas, to produce steam. Steam leaves the boiler and enters a turbine, which leads to a generator and produces electricity. This steam can then go back into the system and be used to make hot water, which is run through pipes to heat campus buildings. If not being used to produce hot water, the heat from the steam is rejected into the atmosphere via cooling towers – the water vapor produced by this heat rejection can often be seen exiting the Central Utilities Plant on Elmwood Avenue.
While independently generating electricity and heat has a total efficiency of 56%, the cogeneration of the two has an 80% total efficiency, making it a much more efficient system.
Limitations of the existing equipment
Despite being more efficient than individual heat and electricity production, the cogeneration system has certain limitations that prevent it from running at an ideal efficiency. One major limitation is that the pumps that move water into the boiler must be constantly run at a suboptimal water flow rate.
“Since the water going to the boiler can only go down to 50% of its maximum flow rate, that means the boiler has to make at least 50% of its maximum amount of steam,” says Tim Vann, Energy Engineer at the Central Utilities Plant. “That steam then goes to the turbine, which generates electricity. The waste heat from the steam that’s going to the turbine should go to making hot water—and some of it does—but there’s only so much hot water that the campus needs.”
This means that some of the steam produced must currently be rejected into the atmosphere, providing limited benefit to the University. Additionally, the pumps generate too much pressure for the system, sometimes requiring the boilers to open safety valves to relieve that pressure. In turn, the system needs to be shut down, and hot water must be made through a different set of heat exchangers, which is less cost effective.
What changes will the project make?
“In any system, there’s waste energy. We’re trying to reduce that,” says Vann.
The project will rework the pumps to allow them to run down to 18% minimum of their maximum flow rate. This will be achieved by trimming the pumps’ impellers (the component of the pump that spins to move water through it), cleaning the pumps, testing them, and then recertifying them. New piping and valves will also be put in.
The combination of these changes will improve flow control and allow the cooling tower to be turned off during winter months (October to May) to save money, water, and energy. Additionally, after these changes are made, the boiler’s minimum flow rate will be cut down from about 80 klb/h (thousands of pounds per hour) of steam to about 25 klb/h, a 69% reduction. This means that the boiler’s steam production will be able to better match the University’s need for hot water.
Benefits of the project
The project is expected to take 6-9 months in total, providing a myriad of benefits once completed. One such benefit is a lowered carbon footprint.
“If you’re looking to cut our energy use and our CO2 emissions, we need to focus on reducing our natural gas, which is what this project is doing,” Vann says. About 87% of the University of Rochester’s greenhouse gas emissions come from natural gas, according to Vann.
Once the pumps are able to run at a lower flow rate, the amount of water the system moves can be better matched proportionally to the actual campus needs for hot water. This will cut down the natural gas needed to heat the boilers by about 100,000 mmBtu (one million British thermal units) in the October to May period, enough to heat over 1,700 average American houses for a year. Electricity usage will be decreased by about 4,500,000 kWh, enough electricity to power one average American house for 425 years.
These gas and electricity reductions amount to a greenhouse gas emissions reduction of at least 6,000 metric tons of carbon dioxide equivalent, with approximately $850,000 expected to be saved between October and May each year.
A cultural shift
Making operational changes and planning for physical improvements to the cogeneration system has not always been easy. It has required the challenging of cultural norms and tradition to find new ways of doing things.
“None of this would have been possible without people being willing to work together and accept change,” says Vann.
It was not until everyone came together to make the system operate more efficiently that the physical limitations of the system became apparent. A continued collaborative spirit allowed the group to diagnose and document the physical changes that were needed, resulting in the upcoming optimization project for both environmental and financial benefits. This is just one of many examples of how innovation in one area can serve many different efforts for improvement.
The cogeneration optimization project will contribute to a greener campus and make strides for a future that values environmental consciousness at the intersection of engineering and sustainability.
Written by Raelen Green, ‘28
