|Upgrade at the U of A: University of Arkansas saves energy with modular steam boilers|
Jason Smith, LEED AP, Director, Sustainability and Strategic Development, Miura North America Inc.
According to a 2005 survey by Energy & Environmental Analysis Inc. for Oak Ridge National Laboratory, nearly 50 percent of the commercial/industrial boilers in the United States - responsible for 8 quadrillion Btu/hr in energy consumption - are 40-plus years old, while as many as 80 percent are 30-plus years old. Clearly, this represents significant opportunities, with sizable impact, for improved efficiency, including on college and university campuses whose district energy systems are served by some of these aging units.
Recent economic conditions and climate change concerns have converged to bring unprecedented attention to the need for efficient, sustainable campus district energy systems. In response, many of the nation’s academic institutions are taking steps to update their aging boiler inventories. The University of Arkansas (U of A) at Fayetteville is among them. One of the first 100 signatories of the American College & University Presidents’ Climate Commitment, the U of A recently replaced three 55-year-old watertube steam boilers with six ‘on-demand’ modular units as part of a broader district energy plant upgrade.
The project showcases how modular steam boilers can support a phased approach to campus plant improvements and ultimately contribute to emissions reductions, reinforced by an impressive payback resulting from improved plant efficiency.
Modular boilers are increasing in popularity, in line with the increasing utilization of modularity in general - in modular chillers, modular fan-wall systems, etc. High-performance and scalable, they are capable of efficiently meeting both peak- and part-load heating demands.
Conventional boilers are designed to provide a fairly broad turndown ratio while still achieving acceptable levels of boiler efficiency. However, a conventional boiler operating at a level below its output rating suffers from some level of reduced efficiency because of its excess mass and surface area. Figure 1 illustrates the excess energy above the load curve required to operate the conventional boiler.
Modular on-demand boilers solve this problem by dividing total output capacity among several linked units and employing a computer controller to sequentially stage individual units to precisely match loads. In this way, turndown capability is expanded beyond just the burner to be comprehensive and systemwide, with modulation occurring at the scale of the boiler itself.
Modular boiler arrays divide total output capacity among multiple individual units, sequentially staged by a computer controller to precisely match loads.
As shown in figure 2, a typical modular boiler array consists of a master controller communicating with multiple ‘slave’ terminals at each boiler unit. The master control panel monitors steam demand via a steam pressure sensor at the header (reading high and low pressure differentials) and utilizes control algorithms to sequentially stage the boilers through off-low-high firing rates that match the load demand at any given time. Since each modular unit is completely isolated from the next and able to stage on and off very quickly - producing steam in under five minutes from a cold state - through-flow losses are eliminated and system turndown is maximized. The resulting modular district steam system is optimized for reduced energy consumption and emissions.
The benefits of modular boilers have been recognized by the U.S. Department of Energy’s Federal Energy Management Program (FEMP), which facilitates cost-effective energy management practices at federal agencies. FEMP recommends that multiple modular units be considered where building loads are highly variable, because they allow each boiler to operate at or close to full rated load most of the time, with reduced standby losses.
“A Good Complement”
The U of A placed its new modular boilers in service three winters ago as part of a $14.2 million energy savings performance contract (ESPC) with Johnson Controls. Previously, the campus central steam plant, built in 1956, housed three original 40,000-lb/hr watertube boilers plus two subsequently added 80,000- and 100,000-lb/hr units. These five natural gas-fired boilers produced steam for distribution at 100 psig to 68 buildings, for space heating and domestic hot water, via a 4-mile steam and condensate return network. (The 350-acre U of A campus is also served by a central chilled-water plant and three satellite cooling plants housing a total of nine electric chillers with 12,750 tons of production capacity.)
“Our three oldest boilers had fallen into disrepair and were the least efficient of the fleet,” recalls Scott Turley, the university’s director of utility operations and maintenance. “So as we started to put together the ESPC, one of our objectives was to replace those units.”
The project also called for installation of a 1,000-ton centrifugal heat pump at the main plant and a heating hot water distribution loop tying into 13 buildings served by central steam - enabling them to take service from either the existing steam or new heating hot water system.
“This was what led us to start thinking about the option of using modular, step-fired boilers,” explains Turley. “They have the ability to start up very quickly and get up to load fast to back up the heat pump, if need be. They are a good complement to the heat pump technology, and they fit very nicely into our summertime load profile, which would be tough to accommodate using our two remaining large boilers.”
Like many campuses, the U of A has significant variation in its steam loads, both seasonally and throughout the day. Peak steam demand swings from a high of 120,000 lb/hr in winter to a low of 15,000 lb/hr in summer, with a non-degree-day baseload of 24,000 lb/ hr. On days when classes are in session, the university hits high morning warmup loads as students are getting up and taking showers, then sees the load drop off as the day goes on.
The U of A also considered another advantage of a modular boiler system: If one unit were out of service for some reason, it would mean the loss of only a small portion of the plant’s production capacity, rather than the whole output of a large conventional boiler.
For all these reasons, the university chose to complement its heat pump with a modular boiler array from Miura consisting of six 10,000-lb/hr natural gas-fired units, which began operation in February 2008. Other equipment was added to the plant as well: a deaerator, feed water pumps, softeners, a chemical feed system and a condensate polisher. The upgrade also involved installation of a digital control system that will eventually incorporate more of Miura’s existing control technology for monitoring and optimizing boiler operation. The total investment for these boiler house improvements was just over $2.9 million.
The six boilers are installed in two banks of three units each, headered together as a single point of discharge into the steam system. Each boiler is breached to an individual stack. To measure performance and efficiency, each bank is equipped with a steam meter and a standalone natural gas meter.
Besides helping the U of A better meet its variable heating demands, the modular boilers support the university’s climate action plan - a comprehensive set of strategies for developing cleaner, green energy systems and deploying other sustainability measures to achieve carbon neutrality by 2040. With their energy-efficient design, the Miura LX-300 units produce from 25 percent to 75 percent lower greenhouse gas emissions than conventional boilers, depending on fuel type. They also more than meet current and proposed regulations for nitrogen oxide emissions levels (i.e., currently rated as low as 9 ppm).
From mid-April through mid-October, the U of A district heating system runs only on the on-demand boilers and heat pump. (At full load, the pump makes 15,000 MMBtu/hr of hot water at 155 degrees F, distributed via 2.2 miles of hot water supply and return piping; it also produces approximately 1,000 tons of chilled water.) During the primary heating months, the university baseloads one of its large boilers and allows the modular boilers to handle the swing production.
Besides helping the U of A better meet its variable heating demands, the modular boilers support the university’s climate action plan.
At the U of A steam plant, the retrofit six-boiler configuration takes up more floor space than would a single central station boiler with comparable steam output. That’s due to the geometry of the retrofit, Turley explains. Designed to fit through a mechanical room doorway, most modular boilers (with the exception of the larger module sizes) can be stacked in a multilevel arrangement where vertical space permits, keeping their overall footprint compact. “In our case, the plant layout wasn’t able to accommodate that,” he says. “If we were designing a modular plant from scratch, the boiler configuration could have been done differently and been very space-efficient.”
While the U of A now has hot water heating capability, it has no plans to abandon its steam system anytime soon. Says Turley: “Steam is hard to beat in terms of moving a lot of energy over long distance, and our system has historically done that well. We also have a tremendous capital investment in our steam infrastructure.” Instead, the university is taking an ‘evolutionary’ approach, developed out of the original ESPC project, to extend steam service from the existing district system to a number of heating hot water production nodes or ‘mini plants’ around the rapidly growing campus.
As called for in the ESPC, a new nanotechnology building recently joined the first 13 campus buildings served by the new hot water loop. Unlike those 13 buildings, however, the nanotechnology facility was designed without a connection to steam service. To provide a backup heating source for this building, the U of A added steam-to-hot-water converters in parallel with the heat pump loop, enabling the plant to put hot water into the loop using either steam or hot water from the pump. The heating hot water converters also provide a summertime fuel-switching capability if plant operators want to take the heat pump down to reduce peak electrical demand. Again, this operating strategy is complemented by the rapid starting capability of the modular boilers.
The plan not to supply steam to the nanotechnology building got the U of A thinking about developing hot water nodes to serve future campus facilities. By summer 2013, six more newly constructed or renovated buildings will be added to the original hot water loop (receiving heating hot water only). A second hot water node is already commissioned and serving two buildings; it will eventually serve five. Design of a third node is now under way. Following completion of this third plant by summer 2013, a total of 71 campus buildings will ultimately be supplied with district steam and/or hot water, with 11 of those operating off of heating hot water only.
“We take advantage of the steam system to move energy over distance to a single central point, then use hot water to distribute it further to clusters of buildings,” Turley explains. “This allows us to make the best use of the thermal capabilities of steam while reducing the distribution losses from the individual steam runs to each building. Another advantage is that it reduces the maintenance costs associated with steam-to-hot-water conversion at each building.”
Further system improvements are also under consideration. In fact, the U of A is in the middle of a district energy optimization study that, among other things, will consider how to improve the efficiency of the steam plant’s two older watertube boilers. The study may also recommend incorporating combined heat and power as a greenhouse gas mitigation strategy.
Energy Saved, Emissions Reduced
Fiscal Year 2011 performance measurements indicate that the six-boiler modular array is operating at 84.6 percent efficiency (not including blowdown losses) in comparison with the two large watertube units (boilers No. 7 and 8), operating at 72 percent and 77 percent, respectively. For the most part, the modular array has taken the place of boiler No. 7, relegating it to an emergency backup unit that rarely if ever operates. Annual energy savings guaranteed under the ESPC contract from the boiler upgrade are $203,000 per year with reductions in carbon dioxide emissions estimated at 550 metric tons annually. Actual energy savings for the boiler array have met or exceeded these targets over the past two fiscal years.
By utilizing a modular boiler system, the University of Arkansas has the flexibility to efficiently accommodate its variable loads. It also has the flexibility to leverage the resulting energy savings to eventually phase out the remaining conventional boilers to support base-load operation with additional modular units. However the U of A’s district systems may evolve, the advantages of employing multiple on-demand boilers will continue to benefit the Fayetteville campus - and the environment - for decades to come.
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