New heat-recovery system makes Stanford one of world’s most energy-efficient uni's

 

Stanford's heat-recovery system, or SESI, will cut greenhouse gas emissions by 68 percent and fossil fuel use cut by 65 percent Image 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.   Source: Stanford University

Electricity use slashed with efficiency controls for heating, cooling

 

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Commercial buildings could cut their heating and cooling electricity use by an average of 57 percent with advanced energy-efficiency controls, according to a year-long trial of the controls at malls, grocery stores and other buildings across the country. The study demonstrated higher energy savings than what was predicted in earlier computer simulations by the same researchers.

"We've long known that heating and cooling are among the biggest energy consumers in buildings, largely because most buildings don't use sophisticated controls," said the study's lead researcher, engineer Srinivas Katipamula of the Department of Energy's Pacific Northwest National Laboratory. "But our tests of controls installed at real, working commercial buildings clearly demonstrate how much more energy efficient air-conditioning systems can be."

This research was supported by DOE's Office of Energy Efficiency & Renewable Energy and the Bonneville Power Administration.

Sitting on the roofs of many commercial buildings are shiny metal boxes containing heating, cooling, ventilation and air conditioning (also known as HVAC) units. These are pre-made in a factory and have all their components inside a box, leading the industry to call them "packaged" HVAC units. Another kind of commercial HVAC, called air handling units, have long used sophisticated controls to ensure they work as efficiently as possible. But packaged units are often allowed to run for hours on end, even if they aren't needed, and receive little maintenance.

Packaged HVAC units consume the same amount of electricity each year as 8 million U.S. residents, or about 2,600 trillion British thermal units of energy. All those ignored and often-inefficient HVAC systems add up, creating higher power bills and contributing to the nation's greenhouse gas emissions.

Putting an idea to the test

Katipamula and his PNNL colleagues have spent their careers thinking up ways to reduce energy use in buildings. In 2011, they set out to adapt the controls already found in air handling HVAC units for use in packaged rooftop HVAC units. The goal was to enable packaged units to automatically adjust their operations based on conditions inside and outside a building. Using sensors and variable-speed motors, the controls decide when and how fast ventilation fans should run, and if the units can use naturally cold air from the outside instead of mechanically cooling indoor air.

While the PNNL team was evaluating how these controls could work, they learned a few companies were simultaneously and independently in the process of developing such advanced controls. During the summer of 2012, the team installed one of the commercially available control kits on 66 rooftop HVAC units at eight volunteer commercial buildings in Washington state, Ohio, California and Pennsylvania. The buildings included shopping malls, grocery stores, big-box stores and a medical clinic. The installed devices, manufactured by Transformative Wave of Kent, Wash., were chosen because they most closely resembled the advanced controls PNNL had envisioned.

Real energy savings

Katipamula and his colleagues found that, compared to standard operations, the HVAC units using advanced controls cut their energy use by an average of 57 percent. The actual energy savings ranged from 20 to 90 percent. Larger buildings such as malls, which need bigger HVAC units, saved more energy than smaller buildings. And buildings that ran ventilation fans more, such as stores open long hours, tended to save more energy.

Translating the energy savings into dollars saved depended on local power costs. Nationwide, energy costs an average of 10 cents per kilowatt-hour, though areas with abundant and inexpensive power supplies often pay less and large cities with greater energy needs generally pay more. When using the national average, researchers found all the field-tested HVAC units would have saved an average of $1,489 annually per unit. The team calculated it would take a building owner three years to recoup the cost of buying and installing advanced controls with that average cost savings. Commercial buildings often have multiple rooftop HVAC units, so actual savings per building would depend on the number of units used.

But the exact payback period depends on several factors. To help building owners weigh the costs, the research team developed a table that lays out which specific combinations of an HVAC unit's size, the number of hours its fan runs daily and the local energy rate would result in a three-year or less payback period. The team concluded installing advanced controls in smaller units with a capacity of 15 tons or less could achieve a three-year payback in areas where energy costs 12 cents per kilowatt-hour or more, or where sufficient utility incentives were available.

"I'm proud to see the advanced controls my colleagues and I evaluated not only work in the real world, but produce significant energy savings," Katipamula said. "We hope commercial building owners will be inspired by these tangible savings and install advanced controls in their rooftop HVAC units."

The reports are available at:

S. Katipamula, W. Wang and M. Vowles, "Improving Operating Efficiency of Packaged Air Conditioners & Heat Pumps," ASHRAE Journal, March 2014, http://buildingsystems.pnnl.gov/documents/036-043_Katipamula.pdf.

W. Wang, S. Katipamula, H. Ngo, R. Underhill, D. Taasevigen and R. Lutes, "Advanced Rooftop Control (ARC) Retrofit: Field-Test Results," report for DOE, July 2013, http://www.pnl.gov/main/publications/external/technical_reports/PNNL-22656.pdf