Enbridge’s hybrid fuel cell breaks new ground
At its company headquarters on Consumers Road in Toronto, Enbridge has installed a world first: an Energy Recovery Turbine (ERT) paired with a large stationary fuel cell to produce ultra-clean electricity. To take advantage of the ERT, the hybrid fuel cell is installed at one of Enbridge’s natural gas pressure reduction stations (PRS). Between them, they generate 2.2 MW of electrical power, at over 60% fuel to electrical efficiency, and the power is fed directly into the local grid. There are built-in synergies between the two technologies, and the fuel cell looks to be especially desirable for taking advantage of distributed generation opportunities where there are severe restrictions on noise or emissions – for example, say, Hospital Row in downtown Toronto.
Enbridge has identified some forty to sixty megawatts worth of potentially commercially-ready development in a comparable power range, 2 MW plus. This could include installations at about a dozen or so PRS over the next few years. Enbridge has over 80 PRS that are as large, or larger, than the 2.2 MW pilot plant – and Union Gas will likely have a comparable number in its part of the province – suitable for this kind of hybrid fuel cell installation. It works like this: where high-pressure natural gas off the main pipe has to be transferred to a lower pressure distribution line, conventionally a standard pressure reduction valve takes care of it. That pressure drop is a potential source of power, just like a drop in hydro head that a water power installation could take advantage of. Why not put in some device that can take advantage of it to produce electrical power? That’s the energy recovery turbine, a mature technology in service in many parts of the world.
At the same time, and in a way similar to your refrigerator, a drop in pressure means a drop in temperature of the gas. A source of heat is needed to prevent the installation from turning into a block of ice. Normally some gas is bled off and burned to provide the needed heat. Enbridge’s innovation is to pair the ERT with a fuel cell. The waste heat from the fuel cell keeps the installation at operating temperature and the fuel cell more than doubles the electricity generation. Dave Teichroeb, Business Development, Fuel Cell Markets at Enbridge , explains that the installation is engineered according to the size of the expansion turbine it plans to install (the size of the ERT is based on typical average gas flow – any extra natural gas flow goes through a parallel standard pressure reduction valve).The size of the ERT dictates how much heat is needed to keep the pressure reduction station from freezing. The size of the companion fuel cell is then set according to the heat output needed.
These stationary fuel cells operate with readily available natural gas or renewable biogas. The high efficiencies reduce the carbon dioxide emissions, and in the case of renewable biogas more electricity is produced from the same quantity of gas. The fuel cells are both silent and produce no emissions other than water and carbon dioxide. For the hybrid plant the PRT can easily be silenced with a suitable enclosure, and the combination is ideally suited for distributed generation in urban locations where there is little tolerance for noise or emissions that reduce local air quality.
As technology cost comes down, including the ERT, opportunities open up in the sub-MW class, using for example a 600 kW fuel cell and a 300 kW ERT. This then could fundamentally change the way the gas utility does its business, Teichroeb says: Enbridge has over 2000 of these smaller pressure reduction locations across province (though naturally not all of them would be technically suitable).
Fuel cells come in a number of different flavours, of course, each of which has experienced a different development history. Ontario Hydro's former research arm at Kinectrics in the west of Toronto worked on a solid oxide fuel cell, reported on in IPPSO FACTO September 2003, that suffered from fouling problems related to sulphur compounds in the fuel stream. Ballard Power in Vancouver is known for its work on proton exchange membrane fuel cells in both passenger vehicles and stationary applications that are part of a residential micro-CHP market in Japan. The molten carbonate type of cell that Enbridge uses seems to be enjoying good results. The unit at Enbridge is made by FuelCell Energy in Connecticut, which counts over 73 installations worldwide of its DFC® model, and over 275 million kWh of power using a variety of fuels including wastewater gas, biogas from beer and food processing, as well as natural gas and other hydrocarbon fuels. The basic module produces 300 or so kilowatts, and is typically bundled into a cluster of four. The total output has recently climbed from 1.2 to 1.4 MW. A standard installation is expected to last 15 to 25 years, with individual fuel cell stacks rotated in and out every five years.
There is Canadian content in Enbridge’s installation, with the inverter made by Satcon Power Systems in Burlington Ontario. Contour Terminals in Mississauga makes the weather-resistant enclosures for water treatment and control systems.
At present, Enbridge is limiting the power output from its installation, as the electricity is being delivered into the grid at HOEP prices. Like many others, they are looking for a more attractive pricing regime to take over under the new Green Energy Act that is now in place, one much like that announced under the Clean Energy Standard Offer Program that would have provided a suitable price for this kind of power. At the largest size, 10 MW, the hybrid fuel cell package would likely be competitive in 12.5 cent range for power, Teichroeb says, comparable to the bottom tier of biomass under the province’s new feed-in-tariff. For smaller plants, like the at 2.2 MW pilot at its headquarters, 14 cents is more like the price Enbridge would need if it were to replicate the pilot plant sized installation.
“The hybrid fuel cell technology grew out of the desire to make the fuel cell competitive with other types of technology, like biomass or biogas; but, without the air pollutants typically emitted from burning a fuel,” Teichroeb says. “We wanted to accelerate adoption of the technology by leading it, so that potential hosts, like hospitals, as well as local utilities, will be more ready to take it up in the future. A fuel cell by itself without the expander would need 16 cents plus to be competitive as a zero emission CHP plant.”
High-temperature fuel cells like the molten carbonate type run at an electrical efficiency of 47%. It’s the addition of the energy recovery turbine that raises the efficiency over 60% and brings the power price into a more competitive range. Lower-temperature fuel cells, like Ballard’s PEM, are less efficient. What the high-temperature type gains in efficiency, however, it loses in dispatchability, precisely because it needs to maintain that high temperature, so for the present it’s suitable only for baseload power. In an interesting twist, the power output from the ERT increases as natural gas flow through the PRS increases throughout the day, so in a way the hybrid fuel cell is tracking the baseload curve. But Teichroeb anticipates that, with further development, the fuel cell may add a degree of dispatchability as well.
Direct-current devices like fuel cells have an additional advantage in getting around many of the short-circuit limitations, such as Toronto Hydro has in a number of locations downtown. The short circuit problem has restricted how much distributed generation has been possible in the city of Toronto. However, generation that produces DC power and uses inverters to mate with the grid may reduce or eliminate short circuit obstacles, Teichroeb points out. Solar photovoltaics have the same advantage as fuel cells in that respect, and these inverter-based technologies can help pave the way for a smart-grid.
Funding for the pilot project came from Enbridge Gas Distribution and its corporate parent Enbridge Inc. In total, Enbridge put up about 70% of the $10M cost, with Natural Resources Canada and the Ontario Ministry of Research and Innovation providing about $2.8 million in funding. First-time engineering issues involved in putting the two technologies together generated some extra cost.
There are a fair number of potential biogas resources in its service territory for which Enbridge also wants to employ fuel cells combined with systems for converting the renewable fuel into a form suitable for use in fuel cells. Using more efficient and cleaner technology isn’t just for fossil energy supplies, it can also help maximize finite quantities of renewable fuels.