By John Franceschina, P.E., Vice President, FuelCell Energy, Inc.
The majority of electrical power is produced by centralized power stationsprimarily natural gas-, oil- and coal-fired plants. Unfortunately, these plants are disadvantaged to varying degrees by emissions and environmental concerns. Combined cycle plants use residual heat to improve overall power generation efficiency. However, the distance to consumers makes it difficult to otherwise utilize combined heat and power (CHP) effectively.
Fortunately, advancements in distributed generation technologiesin particular fuel cellsoffer solutions for primary (baseload) power that can augment the grid in ways that improve efficiency, reliability and environmental impact.
Today, most of the electricity produced in the United States is provided by regional utilities and supplied to customers via the grid. Slowly, however, the landscape is changing, as distributed power generation becomes more practical. Commercial businesses and institutionshotels, universities and government facilities, to name a feware moving toward energy independence and reduced reliance on the grid. Doing so provides a degree of flexibility not otherwise possible and reduces grid congestion and power transmission issues associated with centralized generation. There is also another advantage of distributed generation: proximity to the consumer provides an opportunity to use CHP effectively, thus reducing overall energy costs.
Fuel cells can offer the unique advantage of compactness, competitive operating costs and low emissions. They also operate 24/7 and so are easily managed in concert with grid power.
Today, systems known as direct fuel cells or DFCs are reaching their potential as being among the cleanest and most reliable sources of distributed generation, with efficiencies unmatched by most other types of power plants. The Environmental Protection Agency (EPA) recently reported that a new DFC at the State University of New York (SUNY) operated at a 48.4 percent electrical efficiency while maintaining a 99.9 percent power availability rating. Similar efficiencies for this type of fuel cell have been demonstrated at dozens of sites, presenting a viable alternative to combustion-based plants for growing numbers of baseload power applications.
Historically, fuel cells have been limited in practicality because of the need for a supply of hydrogen to operate. DFCs are unaffected by such a limitation in that they operate on natural gas, biofuels (for example, gases from food processing and wastewater treatment), ethanol and propane. They have even been shown to generate clean power from diesel fuel and coal gas; fuels traditionally associated with higher levels of emissions.
The systems do this by internally reforming hydrogen from the source fuel. Whatever the fuel source, DFCs emit reduced CO2 greenhouse gas compared with combustion alternatives and negligible amounts of many pollutants. The EPA estimated that the SUNY installation will provide an annual emissions reduction of 1.97 tons per year (tpy) of NOX and 588 tpy of CO2 when compared to all similar regional power systems in New York State. And compared to nationwide power system averages, the DFC returns annual savings of 3.52 tpy of NOX and 1,020 tpy of CO2 .
How They Work
In essence, fuel cells are electrochemical devices that combine fuel with oxygen from the ambient air to produce electricity and heat, as well as water. The non-combustion process is a direct form of fuel-to-energy conversion and is more efficient than conventional heat engine approaches. CO2 is reduced due to the fuel cell’s high efficiency. The absence of combustion negates the production of NOX and SOX pollutants.
1 MW fuel cell at Sierra Nevada Brewing Co. provides baseload electrical power and heat for the brewing process.
Fuel cells incorporate an anode and a cathode with an electrolyte in between, similar to a battery. The material used for the electrolyte and the design of the supporting structure determine the fuel cell’s type and performance. Fuel and air reactions for the molten carbonate fuel cell occur at the anode and cathode, which are porous nickel (Ni) catalysts. The cathode side receives oxygen from the surrounding air.
As can be seen in Figure 1 (page 70), hydrogen is supplied to the anode through a hydrogen recovery unit (HRU), which reforms the fuel into H2. The gas is then consumed electrochemically. The O2 supplied to the cathode, along with CO2 recycled from the anode side, reacts with the carbonate salt electrolyte to produce carbonate ions that pass through the electrolyte to the anode. There they combine with the H2 to produce water, CO2 and electrons. The electrons flow through an external circuit to the cathode and produce electricity.
Fuel Cells and CHP
DFC power plants have an exhaust temperature ranging from 650 F to 750 F. This heat energy can be captured to provide heat for buildings, swimming pools and other facility needs. In fact, the already high efficiency of fuel cells can be increased from around 47 percent to as much as 80 percent. Alternatively, the heat can be used with a turbine generator to convert the heat to electrical energy. This process can increase overall electrical power generation efficiency by 10 percent to 15 percent.
Fuel cell plants are typically located within or near the facility where the electricity is to be used. The amount of waste heat produced is often suited to provide supplemental heat or hot water requirements. Together, this proximity and efficiency can be an advantage over conventional centralized power plants, which are challenged with moving larger amounts of waste heat over much longer distances.
1.5 MW fuel cell at Sheraton San Diego sits among tennis courts and provides power, domestic hot water and heat for three swimming pools.
Other CHP considerations regarding the tradeoff between heat and electricity help highlight the benefits of fuel cells over turbine and other combustion generators. Electricity generated during a cogeneration process has a greater economic value than that of the associated waste heat. Thus, the generation of electricity is paramount in the economic efficiency equation, since the more electricity that can be produced by the power plant, the less of this relatively high-priced electricity that must be purchased from the grid.
With traditional sources of distributed power generationfor example, reciprocating engines, microturbines and so onCHP can mask the power source’s underlying electrical power generation efficiency. Whatever CHP adds to the overall efficiency, there is no getting around the plant’s actual electrical power-generating efficiency. Thus, in the case of a gas turbine (operating at 25 percent electric power generation efficiency) and a reciprocating engine (operating at 35 percent electrical power generation efficiency) considerably less of the overall output of the system (percentage wise) is in the form of electricity. In contrast, the fuel cell operates at 47 percent electrical power generation efficiency.
Baseload Power Applications
Fuel cell power plants can be used in a variety of distributed generation applications. In particular, hotels, food and beverage processing plants and wastewater treatment plants are discussed below. DFC power plants also operate at manufacturing plants, universities, hospitals, correctional institutions, government facilities and even as pure grid support applications.
Take, for example, food and beverage processing. Digestor gases are produced by the fermentation of organic matter. Fuel cells can use the methane-containing digestor gas to produce electricity and heat, which is used in the digestors. This avoids the need to flare unused gas and provides the power much more cleanly than if the gas were used in a combustion-based generator.
600 kW fuel cell at Dublin San Ramon uses anaerobic digester gas to power the wastewater treatment process.
The Sierra Nevada Brewing Co. in Chico, Calif., installed a 1 MW DFC power plant. The system is fueled by digester gases given off in the beer production process and augmented with natural gas. The power plant provides virtually 100 percent of Sierra Nevada’s baseload power requirement, of which about 40 percent is produced using digester gas. Waste heat is used to produce steam for the brewing process. Overall energy efficiency for the plant is twice that of power supplied from the electrical grid.
Hospitality represents another application. The Sheraton San Diego Hotel and Marina has made one of the world’s largest commitments to clean power by a hospitality establishment, installing 1.5 MW of electrical capacity provided entirely by fuel cells. The system has demonstrated a 98 percent reliability rating and provides 60 percent and 80 percent of the Sheraton’s baseload power (East Tower and West Tower, respectively). Water from the power plant provides a majority of the hotel’s hot water and heats three of the facility’s swimming pools.
Wastewater treatment plants naturally produce biogases that historically were let off into the atmosphere. For the past 30 years, combustion generators have been used to produce electrical power from this resource. However, pollutant and greenhouse gas emissions are produced in the process. Fuel cells take advantage of the biogas by reforming it into usable hydrogen, which provides a source of energy for the fuel cell power plant.
The Dublin San Ramon wastewater treatment plant in Pleasanton, Calif., installed a 600 kW fuel cell power plant on site. Sludge from the conventional treatment process is fermented and uses anaerobic processes to convert organic manner into digester gas. The fuel cell receives the treated digester gas and converts it into electricity. Electrical power produced in the process provides roughly 75 percent of the plant’s total electrical needs. Heat produced by the fuel cell is recovered and used to warm the sludge, thus optimizing the anaerobic digestion process.
Opportunities for on-site power include grid congestion relief, higher efficiency through reduced line losses and CHP and emissions reductions. While distributed generation traditionally has been viewed primarily as backup power, fuel cells offer the opportunity to produce continuous baseload power with negligible pollutant emissions and reductions in greenhouse gases. Fuel cell power plants are beginning to take center stage for distributed generation of baseload power in a variety of commercial markets.
Author: John Franceschina is Vice President of Business Development for FuelCell Energy. He has more than 18 years of experience in the energy industry. Prior to working at FuelCell Energy, he was the Director of Business Development for KeySpan Corp.’s enterprise account program. Mr. Franceschina is a licensed New York State professional engineer.