New heat-recovery system makes Stanford one of world’s most energy-efficient uni'sView gallery - 7 images
At Stanford University in California, it’s normally the Nobel-winning researchers who make the news. But with the commissioning of a novel renewable energy system, the campus’s humble heating and cooling system has grabbed some headlines. Using a first-of-its-kind heat recovery system, and drawing a substantial percentage of its electricity from solar, the university is greening up its operations in a move that will see greenhouse gas emissions cut by 68 percent and fossil fuel use cut by 65 percent.
For a campus that’s more akin to a small city, comprising of 8,000 acres and over 1,000 buildings totaling more than 15 million square feet (1.39 million sq m), CO2 emissions can add up to a sizeable environmental impact of about 150,000 tons annually. The new system replaces what was once a state-of-the-art natural gas-powered cogeneration plant when it was commissioned in 1987, which heated buildings through a network of underground steam pipes, while cooling buildings with chilled water pipes. Buildings often require both heat and cooling simultaneously depending on the room temp needs (computer rooms and labs versus offices and classrooms).
“Basically if you think of air conditioning or cooling not as the delivery of cold, but rather as the collection of heat, things become more clear,” said Joe Stagner, executive director at Stanford’s Sustainability and Energy Management office.
After completing its route, the steam was then returned to the plant in the form of very hot water, known as condensate, along with chilled water which collected waste heat from the buildings. Once back at the plant, that excess heat was simply vented out into the atmosphere via evaporative cooling towers.
But campus growth had pushed the old system to its limits, and intermittent failures forced the university to buy relatively expensive energy from the grid. At the same time, plant engineers noticed that heat being collected from the campus by the chilled water loop overlapped with heat being delivered to the campus by the steam loop, which occurred about 75 percent of the time. With that, the idea for the heat recovery system was born.
As part of the new system, known as SESI (Stanford Energy Systems Innovations), heat that was previously discharged is now collected from the chilled water loop by a new heat recovery chiller that then moves it to a new hot water loop. The university replaced its steam pipes with 22 miles of hot water pipes, while retrofitting 155 buildings’ steam connections to hot water.
"What SESI does is use electrically powered heat pumps to take that waste heat from the cooling system to make hot water for campus heating instead of wasting it, thereby greatly increasing efficiency," says Stagner. "And by using electricity to power this system instead of natural gas, we can use renewable power and not burn gas and create air pollution."
Operated by patented software specifically designed for the system, SESI is claimed to be 70 percent more efficient than the previous cogeneration plant, while reducing heat loss that was an issue in the previous distribution system. It was also built with an additional 25 percent capacity, to cover the inevitable campus growth through 2050. And because steam will no longer be thrown away, the new system will save about 70 percent of the water used at the central plant, which translates into a 15-18 percent saving in the total amount of water used on campus.
Another major green aspect of SESI is a 68-megawatt peak solar farm being built on 300 acres (121 hectares) in California, along with 5 megawatts of rooftop solar panels to be installed on campus, all of which will provide about 53 percent of Stanford’s electricity. The rest will be bought from California’s energy grid, of which about 25 percent is from renewable sources (and growing), meaning at least 65 percent of the university’s power will be green.
"We know of no other system like this in the world, especially at this scale, with both hot and cold thermal energy storage, powered by clean electricity and run by newly invented 'model predictive control' software that continuously directs efficient system operations," says Stagner.
The video below gives an overview of the project.
Source: Stanford University