Jungle Fuel Cell Safari
By Bill Moore
A menagerie of exotic and rare tropical animals and plants depend on a state-of-the-art fuel cell to keep their "jungle" habitable. In fact, the Lied Jungle at the Henry Doorly Zoo in Omaha, Nebraska may be the very first zoo complex in the world to power one of its major attractions with a fuel cell.
The 200kW UTC (Onsi) phosphoric acid fuel cell provides 50 to 60 percent of the jungle's electric power needs, which Omaha Public Power District project engineer Doug Roberts, believes can be improved significantly as the utility gains experience with the technology and learns to take full advantage of its potential efficiency gains.
Roberts was waiting for me on a sub-freezing but sunny afternoon outside the entrance to the zoo complex, which is situated adjacent to Rosenblatt baseball park, the site of the annual College World Series. He and OPPD communications specialist Jeff Hansen took me on a tour of the installation, beginning with a walk into the hidden bowels of the jungle.
With the outside air temperature in the low teens (Fahrenheit), a blast of humid air instantly glazed our glasses with an opaque layer of ice as we entered the jungle complex. We unzipped our heavy coats and made our way down to the jungle floor level. Several different tropical ecosystems coexist under an expansive glass canopy, the roof of which is held up by a pair of massive artificial tropical "trees." Birds fly freely though the eight story, 60,000 square feet living canopy, while monkeys swing easily from one tree to the next. Some ninety species call the Lied Jungle home, including pygmy hippos, tapirs and tree sloths.
Roberts stopped by a glass-enclosed model of the UTC fuel cell to explain its operation and a little of its history. OPPD -- which serves utility customers in eastern Nebraska -- embarked on its visionary program several years ago to gain practical experience with various distributed power generation technologies. Besides the fuel cell, the public utility also operates a Capstone Microturbine installation in Blair, has partnered with Valmont Manufacturing on a wind turbine in Valley and installed a methane landfill gas-powered diesel backup generator outside of Elkhorn; all in Nebraska and most with some level of federal support.
Leaving the model behind, the three of us walked down one of the jungle paths, past a waterfall and over a bridge. We left the well-worn trail and stepped through a door into the mechanical part of the building. As we walked, Roberts explained some of the lessons the utility has learned over the last 30 months of operation.
The original plan was to tie the fuel cell to the Jungle independent of the of the local power grid but letting it synchronize with the grid, and follow the peaks and troughs of demand in the Jungle . For five months OPPD struggled with frequent shut downs because the unit simply couldn't respond fast enough to unexpected line spikes such as a pump turning on at a nearby water treatment plant. The fuel cell would take itself off-line to protect itself, forcing continual restarts that had to be done on-site.
Eventually Roberts and OPPD engineers came up with a solution of running the fuel cell completely grid independent using its own internal systems to produce 480V 60 cycle power. A $28,000 static transfer switch between the fuel cell and the grid allows switching the power back to the grid if the fuel cell shuts down for any reason. Since it can transfer in 1/4 cycle the switch from the fuel cell back to the grid was seamless with no power outage at the Jungle. Since running the fuel cell completely grid independent , the unit has run almost continually, except for a brief shut down for normal maintenance.
But for six months, the unit's sporadic operation and reduced output drove up its operating costs. Roberts estimates that the current bus bar electricity cost is 13.5 cents a kilowatt hour, significantly higher than the 1-3 cents of a conventional coal-fired, central power plant. He calculates that the unit was available 55 percent of the time, with a capacity factor of 30 percent for a total efficiency of 30 percent.
Once the operating problems were solved , the unit began operating closer to its designed parameters. Availability shot up to 88 percent, the capacity factor improved to 52 percent and total efficiency rose to 38 percent.
Roberts led us outside the building to an area zoo visitors never see. Here sat the PC 25 fuel cell, occupying a space nearly identical to that of half a semi-trailer. It measures 10 feet high, 10 feet wide and is 18 feet long. It's interior is divided into four major compartments: the reformer that strips hydrogen from natural gas, the phosphoric acid fuel cell stack, the electric power conditioner, and the water processing unit. He patiently opened each compartment so I could peer inside, though I only vaguely understood what I was seeing. I noticed that each heavy steel cabinet door was locked and secured with multiple bolts.
The process of converting natural gas into electric power is pretty straight forward. It actually begins in a small steel shed a few feet away that is similar to the kind in which people store their lawn equipment and old bicycles. Here a pair of Membrane Technology Research scrubbers passively remove 50% of the nitrogen from the natural gas stream. Because there are no moving parts in the system, maintenance and operating costs are virtually zero.
OPPD takes the excess nitrogen and re-injects it into the Zoo's natural gas line. Roberts explained that it is important to remove the nitrogen because it can react with hydrogen, creating ammonia, which can poison the fuel cell stack. The re-injected nitrogen has little effect on the effective BTU content of the Zoo's gas feed, he added.
With much of the nitrogen stripped out, 1,900 SCF/hr of natural gas is piped a short distance to the steam reformer section of the fuel cell unit where its hydrogen atoms are separated from their carbon and nitrogen companions. Roberts explained that the UTC unit is rated at 200kW peak output, but its actually a bit higher than that because an additional 80kW are needed to run various ancillary processes like the steam reformer and water processor
The hydrogen stream flows into 34 sub stacks, each made up of eight individual cells measuring 32 inches by 32 inches and 7/16th inch thick. Here the proton and electron that make up the hydrogen atom diverge, the proton passing through the silica carbide, phosphoric acid electrolyte, while the electron flows around it creating a direct current electrical circuit. Each substack generates a total of 5.8 volts. The stack runs at a moderate 350 degrees F and kicks out both high grade heat (above 180 F) and low grade heat (below 180 F). It's this heat that is the real key to making fuel cells economical, as Roberts would explain to me later.
As our ears started to turn red and tingle with the cold, we moved around from the fuel cell stack assembly, swathed in an insulating blanket, to the steam reformer and from there to the water processor. The water -- which is used to control the internal temperatures of the stack -- has to be de-mineralized, so there is a separate subassembly for doing this. The recombination of hydrogen atoms with oxygen atoms on the cathode side of the cell also generates water and a lot of potentially valuable waste heat.
The fourth compartment is the power conditioner (inverter) that converts the stack's 200 VDC voltage to 480 volts alternating current (AC). It is this very clean electric energy generated with virtually no pollution that is the attraction of the technology. OPPD estimates that the unit produces just 1 ppm of NOx, 5 ppm CO and negligible SOx and particulates. It is also extraordinarily quiet, producing just 62 decibels at 30 feet, about the level of a someone practicing the piano or having a conversation. We could easily talk to each other, while Doug Roberts locked up the last cabinet door and explained that the utility was absorbing the extra cost of the electric power produced by the fuel cell rather than passing it on to the zoo. OPPD also gives the zoo the excess heat generated by the unit at a rate of between 700,000 to 1 million BTUs per hour, using the hot water to preheat the Jungle's water.
As we ducked back inside the warm interior of the jungle's mechanical section, Roberts showed me where the hot water from the fuel cell is dumped into a large, insulated water tank where it co-mingles directly with the cold water that is pumped into the complex's hot water heater. There is no need for a heat exchanger, Roberts explained. What water isn't sent to the conventional gas-fired boiler is recirculated back to the fuel cell. In the summer, when heating loads are low, excess fuel cell heat is dissipated through an outside dry cooler.
As we wrapped up the tour, Roberts explained some of the economics of the system to me. The project originally cost $1.229 million dollars including $840,000 for the PC 25, $252,000 for installation costs, $85,000 for engineering -- which was performed by HDR Engineering, a local firm. Added to this was $36,000 for project management and $16,000 for financing. Of the $1.2 million, the US Department of Energy contributed a $200,000 grant. The cost per kilowatt is $5,145.
One of the more interesting revelations to come out of this grand experiment is the fact that OPPD spent more on natural gas than it earned from the electricity the unit produced. In the first 24 months of operation, including the sporadic first six months, the utility bought $98,031 of natural gas ($0.054/kWh) and sold $87,196 ($0.048/kWh) worth of electricity to the zoo.
Roberts gave me a copy of a Powerpoint presentation that includes many of the numbers found in this report, but one of the more interesting graphs is a bar chart showing the relative thermal efficiencies of a conventional gas-fired electric generator (20%), a microturbine (24%), a diesel-electric generator (30%), and the PC 25 (40%). If you added to this heat recovery from the fuel cell -- and Roberts estimates he's only capturing about 10% of the potential heat available -- the overall thermal efficiency of the fuel cell would be closer to 80%.
The OPPD project engineer confided that for the unit to make the most economic sense, it needs to be running at full capacity and as much of the waste heat as possible needs to be utilized. Under these conditions, he estimates the bus bar costs can be brought down to about $0.10/kWh. Based on his experience, Roberts thinks that the ideal installation for units like the PC 25 would be a hospital or hotel where there's much greater demand for hot water than the zoo.
OPPD's experience strongly suggests to me that fuel cells are going to make a lot more sense economically and efficiency-wise as stationary power units where their waste heat, hot water and clean power generation capabilities can be combined to wring out the maximum energy potential of the source fuel, in this case, natural gas. In an automotive application, much of this advantage is thrown away. You have but to look at the size of the radiators on the Honda FCX fuel cell car to appreciate this conclusion.
The tour concluded, I thanked my hosts and parted company, Jeff headed back to the office and Doug left to finish locking up the fuel cell. I took the opportunity to tour the Desert Dome -- a relatively new attraction I'd not had the chance to visit. Housed under one of the largest geodesic domes in the Midwest are recreations of some of the world's great desert ecosystems. The temperature of these mini-deserts is kept at a steady 72-75 degrees Fahrenheit year round. Imagine, within a few hundred feet I had moved from the Amazon Jungle in South America to the Namib Desert in southern Africa, courtesy -- in part -- of a 200kW fuel cell.
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