Fuel Cells Enter the 2002 NEC
In the past, the National Electrical Code (NEC) has addressed issues of power quality by focusing on grounding, uninterruptible power supplies, generators and harmonics. The 2002 NEC has added some new elements, by addressing alternative "green" power devices, such as fuel cells and wind generation, as viable and realistic power sources.
In the past, the National Electrical Code (NEC) has addressed issues of power quality by focusing on grounding, uninterruptible power supplies, generators and harmonics. The 2002 NEC has added some new elements, by addressing alternative “green” power devices, such as fuel cells and wind generation, as viable and realistic power sources. These alternative power sources, however, while attractive, come with their own power quality problems and solutions.
Most of the information presented here is based on a particular fuel-cell power plant: a 200-kilowatt, 480-volt, 3-phase phosphoric acid fuel cell that is commercially available and has over 1,000,000 hours of operating experience. Most other types are still in Beta testing and are not yet commercially available.
The Measure of Quality
Power quality is best measured by the combination of mean time between failure (MTBF) and mean time to repair (MTTR) in the form of availability or unavailability (1 – availability) calculations. While most of distributed computer systems available today exhibit 99.999% (five 9’s) availability under perfect conditions, the traditional uninterruptible power supply/generator-utility design achieves a 99.999% availability under the best conditions. These availability measurements are made with respect to computer-grade power being present under all conditions. Computer-grade power means electricity that meets the requirements of the Information Technology Industry (formerly CBEMA) curve in Figure 1.
This standard sets time and voltage intervals that electronic equipment must tolerate without malfunction. Normal operation of utility systems cannot be counted on when addressing critical computer systems, as normal switching and fault-clearing conditions often fall outside the curve into the prohibited region.
Introduction of alternate power sources can increase the availability of the UPS/generator utility systems by 10—100 times their present capabilities. These power sources can be photovoltaic, natural gas generator sets or fuel cells.
Integrated fuel-cell/UPS/generator utility power-supply systems can achieve up to seven 9’s availability. In fact, an independent long-term availability analysis , conducted on just such a system at the First National Bank of Omaha Technology Center, concludes that the calculated availability is 99.999995%, 100 times greater availability than the traditional solution.
The techniques of probabilistic risk analysis were used to calculate the availability of the system. Probabilistic risk analysis was developed in conjunction with the space program to understand failures in rockets and complex military weapon systems. This type of analysis allows both qualitative and quantitative evaluation of reliability, availability and accident scenarios.
Fuel Cell Operation
The 2002 NEC defines a fuel cell as “an electrochemical system that consumes fuel to produce an electrical current.” While the main chemical reaction is not combustion, there may be sources of combustion used within the overall fuel-cell system such as reformers/fuel processors.
In simpler language, a fuel cell is an electrochemical device that converts hydrogen to DC electricity, with heat and water as byproducts. Fuel cells are combined into cell stacks to produce a useful level of DC power. Fuel cells are designated by their electrolytes, such as phosphoric acid, proton-exchange membrane or molten carbonate.
A phosphoric acid fuel cell has a phosphoric acid electrolyte separating its anode and cathode terminals. The electrolyte allows positive hydrogen ions to pass through, while blocking electron flow. The hydrogen ions combine with air at the cathode side in an exothermic reaction. This reaction produces heat and water. A load (inverter) is applied across the anode and cathode terminals to allow current flow to occur. The inverter changes the DC current to alternating current—the standard form of power used in electrical distribution systems (see Figure 2 on p.16).
Figure 3 on page 17 shows the operation and construction of this type of fuel-cell power plant. Natural gas, or some other hydrogen-rich fuel source, is brought into the fuel cell’s reformer, where the hydrogen-rich supply is produced. The hydrogen-rich fuel supply is then forced into the cell-stack assembly producing heat, water, DC power and minimal exhaust emissions. Steam is recovered from the cell stack assembly and used in the reformer process. The DC power is supplied to the inverter and 208 kilowatts (kW) of AC power is produced. Approximately 3% to 4% of this power is used to feed parasitic pumps and controls within the unit, leaving a net production of 200 kW of output power.
Fuel Cell Benefits
The simple fact that fuel cells produce electricity in a chemical reaction of hydrogen-based fuels, not by combustion, creates several advantages over internal-combustion machines. The operation of one fuel cell results in the reduction of 40,000 lbs. of priority air pollutants in comparison to a typical U.S. utility at equivalent electricity production. Fuel cells can often be sited in environmentally sensitive areas that combustion machines may be restricted from, since the exhaust contains no NO x or SO x components and very low carbon emissions if natural gas is used as a fuel source (see Table 1, p.17). If hydrogen is the fuel source, there are no emissions and only the byproduct of water and heat.
Fuel cells typically achieve 40% efficiency of input energy to output energy, and when waste heat is recovered they can reach 80% efficiency. A typical internal-combustion machine is 25% to 30% efficient. The recovered heat can be used to replace oil or coal-burning boilers, reducing harmful emissions even further.
In the case of the fuel-cell power plant described here, it has a proven availability, on natural gas, which surpasses the availability of most diesel generators.
Multiple fuel sources—propane and natural gas—can extend these advantages to remote sites and increase availability by allowing on-site storage of fuel.
The last stage of power production in a fuel-cell power system is to convert the DC power out of the cell stack assembly into AC power. This requires an inverter similar to those found in high-end UPS systems, and therefore, the fuel cell typically provides CBEMA-grade power.
Design and Installation
Fuel cell systems can be designed to be independent of all electrical utility sources, grid independent, or paralleled with the electrical utility source—grid connected.
When providing a system that is not interconnected to the electrical utility, adherence to NEC Article 692—which covers fuel cell requirements—becomes relatively easy. Design considerations, however, require some additional investigation into the fuel cell design.
While the fuel cell may not be interconnected with the electrical utility, it may require an alternate power source to start up the power plant. Operating temperatures shown (see “Proton Exchange Membrane Fuel Cells,” p. 15, and “Phosphoric Acid Fuel Cells,” p. 16) are also the temperatures required for startup of the fuel-cell power plant. The fuel-cell system needs to achieve these operating temperatures to allow chemical reactions and reformer operations to occur. This is usually accomplished by an electric heating source within the fuel-cell system, powered from an exterior power supply.
Fuel cell systems that are interconnected with the utility may use the “point of common coupling” as a bi-directional power source. During start up conditions, the fuel-cell power plant will draw power from the electric utility and during normal operation will export power to the utility. Article 692 defines the “point of common coupling” as “the point at which the power production and distribution network and the customer interface occurs in an interactive system. Typically this is the load side of the power network meter.” When connecting to the utility, additional utility requirements and NEC Article 705—covering the interconnection of power supplies—must be applied, as both importing and exporting of power may occur.
Additional design items include load profiling and overcurrent protection. An inherent drawback of fuel-cell operation, due to the chemical reaction process, is their inability to instantaneously react to step loads. Most fuel cells are limited to a 40% step load, unless advance warning is given to handle a predetermined load at a specific moment. This function is significant in the electrical system design with respect to motor starting and load transfers via automatic transfer switches and tie breakers. System design may require variable frequency drives on larger motors, time delays on motor starts and other load management schemes. Use of supplemental power sources such as batteries or flywheels may be incorporated to assist the fuel cell during these step load conditions.
Overcurrent protection must be carefully addressed with the fuel cell manufacturer and local code officials. Some fuel cell manufacturers may rely on inverter current limiting to open output switches, disconnecting the output power source under overload or fault conditions. These switches may physically look like circuit breakers, but they do not contain any overcurrent protection. This may necessitate the installation of an overcurrent protective device at the output of the fuel cell. An additional overcurrent protective device may be required for an external heating circuit if required by grid independent operation.
Fuel selection must also be examined. High nitrogen levels in natural gas supplies may cause accelerated deterioration of the fuel cell stack assembly. Sulfur and carbon monoxide may also poison the fuel cell, causing malfunction or accelerated deterioration.
Fuel Cell Applications
Fuel cells are still very expensive, averaging about $5,000 per kW installed, though there are some applications where they are economically viable. These include:
Sites with free fuel byproduct such as digester gas from wastewater treatment plants, methane gas from cattle rendering plants and hydrogen from plastics and petroleum operations.
Environmentally challenged areas, where air quality restrictions limit use of combustion machines, access for overhead power lines is prohibited or sites where noise abatement prohibits combustion machines.
High availability sites where downtime is a major factor, such as wafer fabrication plants, mission critical data centers or telecom exchange buildings.
Typical Emissions Levels and Sound-Pressure Level
|Emissions||Emission level at 200 kW (ppmV, 15%, O2, dry)|
|Noise||60 dBA at 30 ft.|
From Pure Power, Spring 2002.
Proton Exchange Membrane Fuel Cells (PEM)
176°F operating temperature.
Fluorinated sulfonic acid electrolyte.
Primary applications include residential and automotive markets.
Low temperature allows faster startup.
High current densities (automotive applications)
Interest from automobile manufacturers will increase research and production.
Fuel-cell power plant required for an automobile is also capable of powering residential sites.
CO is a poison (>10 ppm).
Low temperature heat is not preferable for cogeneration.
Heat and water management issues limit electrical production size.
Prominent Manufacturers: Ballard, H Power, Plug Power
Molten Carbonate Fuel Cells
Sodium and potassium combinations in electrolyte
Uses common metals due to high temperature operation
Internally reforming (large gains), so no extermal reformer is required.
Higher operating temperature can power steam turbines/cogeneration.
Operates well with bio-fuels, due to high temperature.
Stainless steel hardware.
Higher operating temperature promotes material problems.
Fuel Cell Energy
Phosphoric Acid Fuel Cells
H 3 PO 4 matrix electrolyte.
Commercially available (200-kW units).
Proven operation on natural gas, hydrogen, land fill gas and methane.
Units qualify for $200,000 federal rebate.
Demonstrated fuel sources include: natural gas, propane, hydrogen.
Quiet operation (>62 dBA at 30 ft.)
Commercial fleet reliability greater than 96%.
High cost of platinum catalyst component.
Needs external reformer to separate hydrogen from fuel supply.
CO, NH 3 and S poison reformer.
Units currently cost $850,000 for base 200-kW units.
Cell-stack assembly costs about $300,000 for a complete change out. Depending on nitrogen levels in natural gas supply, this occurs every two to five years.
Prominent Manufacturer: International Fuel Cells