technical report

Supplementing Electric Utility Power With Fuel Cells in Brazil

• by Henry Oman
• Consulting Engineer
  Seattle, WA

International authors came to the 37th Intersociety Energy Conversion Engineering Conference, July 29-31 in Washington, D.C., to discuss fuel-cell technology and opportunities. They brought many new ideas for solving problems like California’s power shortage that necessitated roving blackouts to avoid collapse of the U.S. West Coast power transmission network.

Growing electric loads and a dearth of new power plants have created severe worldwide power shortages and a crisis in Brazil. Luciane Neves Canha described the analysis and modeling showing that the crisis could be relieved in a fast and practical manner by placing fuel cells at optimum locations and carefully scheduling their operation (1). Acquiring the data for the model and optimizing the model took a lot of effort. For example, the peak load time varies among power consumers. In some residential neighborhoods of Brazil the peak load occurs when people shower with electrically heated water. There the fuel cells can be best located where their heat losses can be used to heat water.

The growing electric-power demand forces construction of large generation systems, transmission lines, and big substations, plus ever-greater power-transmitting capacity of the distribution networks. On the other hand, the world appeal for environment protections, and reduction in pollutant-gas emissions, have been factors that limit the construction of large thermal and hydroelectric power plants. Canha observed that the solution to this impasse is the incentive use of renewable energy, and generating power with efficient alternatives. Having huge power-generating plants running at part-load, on-line, and waiting for an expected peak load may not reduce overall pollutant emissions.

Advantages of Fuel Cell Power

For carrying load peaks the hydrogen-consuming fuel cell has many advantages. The hydrogen can be produced by electrolysis of water during an electric utility’s non-peak-load periods. At late night an otherwise unloaded hydro plant could generate power for the water-electrolyzer in a hydrogen-producing station. During peak-load periods a reserve power-generating capacity, often 5% of the load, has to be on line. This extra capacity could be routed to hydrogen-producing electrolyzers, which could be turned off in a fraction of a second when a load crisis develops. Also, fuel cells can start generating power within a few seconds after they receive a power-up command. The efficiency of the electrolysis process is high, and depending on the current cost of electric energy, the cost of produced hydrogen can become viable.

Factors such as temperature, pressure, and concentration of the reactants have a large influence on the output voltage, and consequently the efficiency of the fuel cells. To investigate these effects, Canha’s team developed a special electrochemical model of a fuel cell.

Modeling the Electric Loads of a Utility

The peak-period load curve in an electric-power-consuming district was establishing by analyzing the period where the largest daily demand could be verified. This peak-load period had to be correlated with the Brazilian electrical system as a whole. Some utility-distribution companies have their daily demand peak during the area’s normal peak-load period. Other companies and entities present their peaks at varying times during a day or night. Such companies should have available, at any period of the day or year a supply of supplementary power at a fixed cost per kWh. It was evident that power consuming customers, as well as the power supplying utilities, had identical interests in load reduction during the peak periods.

The general load curve of a utility distribution company (UDC) is composed from the individual demand curves of each consuming unit. The reduction of the peaking in a large consumer’s load curve produces a substantial decrease in the UDC’s demand peak. On the other hand, residential consumers must be considered individually as small loads. However, as a motivated group they can significantly contribute to the reduction of the peak demand, for example, by the elimination of electrically heated showers, a significant part of peak load in Brazil.

An algorithm was developed for estimating the best location and size of fuel-cell power plants. The operation team of the UDC should identify the possible locations of distribution substations where fuel cells could be installed, considering their security and the simultaneous use of thermal and electrical energy. For the selected locations, load curves were constructed, taking into account operational information such as the typical load curves and data on the monthly energy consumption for each group of consumers. The fuel cell size could then be calculated.

Results from Modeling

The results from modeling a fuel cell plant for reducing peak-power demand in a typical installation were presented by Canha. This fuel cell plant would be located in a building where the temperature is 30EC. The fuel cells run at 43% electric-generation efficiency between 6 and 9 p.m. An electrolyzer operating in the off-peak hours of midnight to 8 a.m. produces the hydrogen needed to feed the fuel-cell stack. The assumed electrolyzer efficiency was 80%. The data used in the variable-power and constant-power fuel-cell output operations are summarized in Figure 1.

The building’s daily load plotted in Figure 2 assumes that a 51.21kW fuel-cell stack operates at a variable output during the three-hour load peak. During this peak the fuel cell produces 911.44 liters of hot water. This assumes that the fuel-cell’s output heat can supply the hot water demand of the 17 showers installed in the building. Assuming that the shower power was 6.5kW, the reduction of the demand peak was 27.63kWh.

The plot in Figure 3 was based on the assumption that a 13.94kW fuel-cell stack operates at constant power during the three hours of peak load. During these three hours 948.7 liters of hot water was produced. It was assumed that the fuel-cell output was sufficient to feed the 18 showers. Assuming a shower-power of 6.5kW, the reduction in peak demand is 29.25kWh. Partial reduction of demand was achieved by not requiring the utility to supply shower-water heating during the peak-load period.

Canha concluded that fuel cells are the route to innovative and clean types of distributed generation of energy for reducing the load-curve peaks. Location and sizing of the fuel cells for peak-power generation can be accomplished with careful analysis and validated modeling. Algorithms are provided in the paper for making good estimates of the fuel-cell output power and the electrolyzer power consumption for installation by industrial, commercial, and residential consumers.

Acoustic Fields Enrich Hydrogen Production by Reformers

Fuel-cell powered electric vehicles need hydrogen refueling, which can be provided by specially constructed “service stations” in cities. These stations can be supplied hydrogen from underground piping or from tank trucks. The stations might even manufacture hydrogen with electrolyzers. However, these hydrogen resources might not be available in remote areas. Also, a hydrogen-powered electric vehicle might be stranded in a remote area, with no way of getting hydrogen. Paul Erickson had evaluated for these applications the possibility of a practical steam reformer for generating hydrogen (2).

Reforming hydrocarbons is today’s principal process for producing the commercial hydrogen used to cool big generators in power plants or propel the boosters that launch spacecraft. Preheated hydrocarbons are mixed with steam and passed over a nickel catalyst at temperatures ranging from 649EC to 982EC, producing carbon dioxide and hydrogen. In the steam-iron process, hydrogen is produced by reacting steam at high temperature over reduced iron oxide to give hydrogen. Then a reducing gas, such as water gas or producer gas, is used to re-reduce the iron oxide, so it can be used again.

Erickson investigated the possibility of enhancing reformation processes through superposition of an acoustic field in the catalyst bed of a steam reformer. Proven acoustic enhancement of various processes was reviewed, and the theory of steam-reforming process was developed. Relevant parameters of the acoustic field were quantified.

Although the facility used has not been optimized for utilizing acoustic waves, significant acoustic enhancement of the steam-reformation process was demonstrated, and acoustic enhancements showed a positive effect on the steam reforming. Results included increased reactor capacity for a given size and mass, smoothing the temperature profile, and better control of the temperatures in the catalyst bed. Erickson expected similar results for other fuels and reforming methods.

References

The following papers are published in the “Proceedings of the 37th Intersociety Energy Conversion Engineering Conference,” held July 29-31, 2002, which carries IEEE Catalog Number 02CH37298, and Library of Congress Number 2001096634. Copies of the Proceedings or of individual papers can be procured from the Institute of Electrical and Electronics Engineers, IEEE Operations Center, P. O.Box 1331, Piscataway, NJ 08855-1331.

1. Canha, Luciane Neves and Associates, “Optimal Characteristics of Fuel Cell Generating Systems for Utility Distribution Networks,” IECEC 2002 paper 20060.

2. Erickson, Paul A., “Enhancing Hydrogen Production for Fuel Cell Vehicles by Superposition of Acoustic Fields on the Reformer. A Preliminary Study” (Not in Proceedings of 37th IECEC.)