Fuel Cell Power Generation Primer
By now it is pretty clear that distributed generation is the wave of the future in electric power supply.
Are you already familiar with the concept of "distributed generation" ? It is small scale generation based on new technology. It is technology that is twice as efficient in turning hydrocarbons into electrical energy as the coal fired steam turbine. These are in much smaller generating unit sizes than the giant generator sizes currently in use. They are small enough to place in the cellar of your home and dedicate to serving just your residence needs.
Some bigger models can serve an apartment building, an office building or a small subdivision. The idea is to place the generation close to the load rather than concentrating it in giant plants which could be as much as one or two hundred miles away.
We choose the coal fired steam turbine for an initial comparison of the feasibility of this new form of generation because it now comprises a little less than half of the generating capacity of the grid. However a new form of gas combustion turbine generator developed in the seventies called an "aeroderivative gas turbine" has a higher efficiency.
Distributed generation is small in scale. It can include small gas turbines called "microturbines". It can include the diesel engine-generator set. However the revolutionary new star of distributed generation, which will likely be commercial as of the end of this year, is the fuel cell.
These tiny generators, 5 kW to 300 kw to 3,000 kilowatt in size are far smaller than the giant 600,000 kW to 1,000,000 kW to 1,400,000 kW coal fired steam turbines, gas combustion turbines and combined cycle generators currently in use that they will likely replace. They are small enough so they can be dedicated to serve only a single family residence, a large office building, a large apartment house.
Slightly larger fuel cells can supply a single load center or a discrete area of the grid containing one or two small load centers, or can supplement grid power by supplying an island in the grid created by limitations in transmission. In contrast, existing generators add to an interstate pool of power from which energy serving all loads on the grid is drawn.
Fuel Cell Basics
Fuel cells can be classified by their operating temperature, and their electrolyte. The low temperature fuel cells are the phosphoric acid cells (PAFC) and the proton exchange membrane fuel cells (PEMFC). These have, respectively, phosphoric acid and plastic as their electrolyte.
In your car battery, for example, sulphuric acid is the electrolyte..
In some fuel cell electrolytes a hydrogen proton passes through the exchange membrane but the electron can't get through so it has to go around through a wire to meet up again with its proton companions.
If you put a light bulb in the circuit, it will, if there are enough fuel cells, light it up. There have to be quite a few for a 110 volt light bulb because each cell only provides about one volt or less.
There are also high temperature fuel cells. These operate at 650 degrees Centigrade or even 1000 degrees Centigrade. The FCEL fuel cell is a molten carbonate fuel cell (MCFC, carbonate electrolyte) and operates at 650 degrees Centigrade. The classic solid oxide fuel cell (SOFC) operates at 1000 degrees Centigrade and has a ceramic electrolyte.
There is a variation on the theme, however, which is receiving a lot of attention. That is the RTESP SOFC. Westinghouse, now Siemens Westinghouse, had difficulty in sealing the planar fuel cells because there was so much expansion between the cool phase and the hot operating phase. So it went to a tubular configuration which solved the sealing problem.
However a German scientifc group, Julich, found a way to make one with a very thin ceramic electrolyte which it supported with one of the electrodes and found this could operate at lower temperatures. Gobal Thermoelectric of Calgary, Canada is makeing those RTESP SOFCs which refers to Reduced operating Temperature, Electrode Supported, Planar (flat rather than tubular) solid oxide fuel cells.
PEMFC are favored for providing the propulsion for cars. Ballard of Vancouver is leading in that field. However the Internal Combustion Engine is a much tougher competitor than the stationary generator. The price of the fuel cell must be much lower to compete with it.
Electrical efficiency refers to the ratio of electrical energy output to fuel input. The high temperature fuel cells have a greater electrical efficiency than the low temperature cells. In the high temperature fuel cells, when hydrocarbons such as natural gas or diesel fuel are used to obtain hydrogen for the fuel cells, the hydrogen can be disassociated from the remainder of the fuel in the stack. In the low temperature fuel cells, there must be an external reformer which uses about one third of the energy available from the fuel.
The PEMFC has an electrical efficiency, therefore, of only 40% at best, as low level operations, and its efficiency falls off as one reaches its continuous rating. In contrast, the MCFC is slightly off its highest efficiency at below 20% operation and above that loading (approximately) its efficiency goes up to 54% - 57% and stays there until it reaches its continuous rating. Currently, Global Thermoelectric's RTESP SOFC has a 45% efficiency.
To put this in contrast, coal fired steam turbines have an efficiency at best of 38% at the busbar. . US average efficiency of coal fired steam turbines, which is slightly less than one half of the bulk power supply, is only 33% at the busbar (near the point of output from the generator.) Electrical losses from having to travel so far down transmission, subtransmission and distribution, to get to residential customers and small commercial loads, bring their average efficiency down some 13% to 16%, down to 27.5% at the customers meter.
Advances in turbine technology in the mid 70s in gas combustion turbines raised the simple cycle gas turbine efficiency from about 25% to 42%. When the waste heat is collected and used to make steam for a steam generator (bottoming cycle) the efficiency can go up to 50% to 60%; about 16% less at the residential customer's meter. At low loading their efficiency falls off considerably. Formerly gas combustion turbines were only used for peakers. Since the 70s they have been used for base load as well.
Fuel cells are tiny compared with conventional generators but are far more efficient than conventional generators of the same size. They can be sized to match the size of a load at a single site, and therefore are usually located on the site of the load. That is close enough so their heat can be utilized usefully as well as their electricity. Use of the thermal energy inevitably produced by a generator, as well as its electrical energy is referred to as "co-generation". The combined heat and power efficiency or CHP efficiency may be as much as 75% to 85%.
In contrast, conventional coal fired steam turbine generators and gas combustion turbines and combined cycles must be large to reach needed efficiencies. Therefore their heat, which can't travel great distances to the load, must be thrown away except to the extent it is used for a bottoming cycle for a gas turbine combined cycle unit.
The high temperature fuel cells can also provide heat for a turbine bottoming cycle. These are called "hybrids" and have electrical efficiencies as high as 78% or more.
PEM's are currently being made by Ballard in about 50 kW sizes which is equivalent to 75 horsepower. Plugpower is making PEM fuel cells but apparently may be having difficulty in getting the bugs out of its reformers or its electrolyte water management system since its introduction has been delayed..
Plugpower is also making a PEMFC for home use. It will be sized from 5 kW to 7 kW with maybe some flywheel storage that will give you up to one half hour at as much as 10kW. It may go up to 35 kW or more for small commercial sales. Vaillant, its European partner is pushing cogeneration with its first model. It will provide both electricity and heat for its customers.
Global Thermoelectric's RTESP SOFC will likely be made in sizes of 5 kW to 175 kW. It can be used for small remote power supplies, and also for an auxilliary power unit for cars which want to keep their interal combustion engine for propulsion.
FCEL's mature commercial stacks will have modules of 300 kW which it will also provide in systems of 1,500 kW and 3,000 kW. These can be used by apartment houses, office buildings, small subdivisions, and can be used by electric utilities to add to the capacity of existing distribution substatios so they can add the capacity in small increments.
At the present the fuel cells are being produced in small volumes and cost quite a lot. The manufacturers expect the costs to diminish significantly at mass production volumes.
But which form of generation will win out? And will distributed resouces really be able to displace the integrated electric power systems, replace giant scale generators operating in networks of transmission lines, that have been the bulwark of electric power supply for over 100 years? Place your bets now.
Here are the alternatives.
Will it be a 30,000 kW simple cycle aeroderivative gas turbine developed in the mid 70's with an efficiency of up to 42%, a combined cycle steam and gas turbine with efficiencies up to 50% or 60% but in sizes of 60,000 kw to 500,000 kW? Will it be a 250 or 300 kw molten carbonate fuel cell with simple cycle electrical efficiencies of 55%, a 10 MW to 40 MW combined cycle (or hybrid) fuel cell/gas turbine with electrical efficiency of up to 78%?
Or will it be small 5 to 10 kW on site proton exchange membrane (PEMFC) with a peak efficiency of 40% or or a 3 kW to 5 kW solid oxide (SOFC) fuel cells of 45% efficiency, able to use their thermal energy for cogeneration with efficiencies up to 85% or more? How about a 30 kw or 75 kw microturbine or a 20 kW to 10,000 kw diesel generator?
My money is on the small but very efficient fuel cell. Here's why.
The key parameters, are size, efficiency, reliability, cost per kw and per kwh, and clean air.
Starting in 1910 or 1920, when polyphase alternating current proved its superiority over direct current, the owners of power systems tied more and more load centers together into a single integrated system using high voltage and then extra high voltage transmission. Why did they do it? Principally so they could use ever larger coal fired steam turbines.
As the generating unit size increased, the investment cost per kilowatt decreased drastically and so did the operating cost per kilowatt hour. A 200,000 kw steam turbine generator cost only 150% of the cost of a 100,000 kw generator. A 400,000 kw steam turbine generator cost only 150% of the cost of a 200,000 kw generator. A 20,000 kw steam turbine generator might have a heat rate of 14,000 BTU's per kwh equivalent to an efficiency of about 24%. But a 500,000 kw to 600,000 kw steam turbine generator could achieve a heat rate of 10,000 BTU's per kwh or lower, equivalent to efficiencies up to 38%.
The large integrated systems also obtained significant benefits from "load diversity". Since not everyone would be using electricity at the same time, if everyone drew from the same power supply, fewer generators would do the job. Also, with integration, other large generators could be called on to supply emergency service when the primary generator was unavailable as they were during forced outages of 5% to 10% of the time, and for several weeks a year during scheduled maintenance.
By the 1960s, 500,000 kw and 600,000 kw coal fired steam turbines were being built. The "Big Allis" generator of Con Ed broke the 1,000,000 kw barrier and was in service by 1965, the time of the big East Coast blackout. The Federal Power Commission's National Power Survey in the 1960s in a quest for ever lower cost and greater efficiency stimulated even larger power systems by urging utilities to engage in programs of coordinated development of base load generating units and reserve sharing. These programs made possible ever larger units sizes without increasing reserves as they otherwise might have.
These would be programs undertaken by the dominant regional electric utility systems in each region which had already obtained the benefits of large scale integration. Such institutional power pooling arrangements made it feasible for the dominant systems in an area to install even larger sizes than they had been able to install as a result of their own integration. With coordination, it became economically feasible to install generating unit sizes of up to 1,400,000 kw.
Next Week.... Part 2 "The Decline and Fall of the Steam Turbine"
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