Fuel Cell Power Generation Primer - Part 2
The Decline and Fall of the Coal Fired Steam Turbine.
Why is it that no more of these extremely large sizes are being built?
First, it became the consensus that sizes larger than 500,000 or 600,000 kw made it difficult for the system operator to run his system. There were just too many eggs in one basket. When the unit was down for maintenance or with a forced outage, it was just too burdensome on the system, even when it had reserve sharing arrangements. Those arrangments are a sort of mutual insurance system to obtain emergency energy from eighboring utilities. The supply was from available surplus capacity. No capacity charge was made. The neighbor chared only for the incremental cost of the energy. Of course you must promise to help the neighbor when he is in need and you have some surplus capacity available. It was unlikely that both utilities would have forced outages at the same time.
The next step toward the obsolesence of the large coal fired steam turbine was the development of the aeroderivative gas turbine.
For many years there had been heavy industrial gas turbines with efficiencies of 25% to 28% used by electric utilities for peaking. In the 1970s, the airlines had pressed the turbine manufacturers to develop more efficient gas turbine jet engines for the larger and larger jet planes going into commercial service. When the manufacturers were successful, they commenced using this technology derived from aircraft jet engines or "aeroderivative" technology to manufacture more efficient gas turbine generators for electric utilities.
These aeroderivative gas turbines with efficiencies of up to 42% could be combined with an HRSG or heat recovery steam generator which recaptured the waste heat of the gas turbine and used it for additional generation in a in a steam turbine "bottoming cycle" . The combined cycle gas and steam system increased the efficiency by an additional 12 to 15% to over a 50% efficiency. The greatly enhanced efficiency made it feasible to fuel electric generation with natural gas, even though natural gas was slightly more expensive per mmbtu (million BTU's) than coal. As of 1999, coal averaged $1.65 per mmbtu and natural gas sold for boiler fuel averaged $2.30 per MCF which is little more or a litle less than a mmbtu.
A system with a load more than 500,000 kw wasn't necessary to load up the aeroderivative gas turbine. A simple cycle gas turbine as small as GE's 28,000 kw machine could achieve a 42% efficiency. A 50% efficiency could be obtained with a combined cycle system as small as 60,000 kw and just recently, GE announced its "H" technology combined cycle which in sizes of 400,000 kw could generate at the amazing efficiency of 60% . That one will be available commercially next year. It should be noted, however, that gas turbine output does fall off significantly at in hotter weather and higher altitudes. And the small gas turbine or microturbine with an output sized to meet a single family load or a small commercial loadstill had a low efficiency of 25% or 28% and is also very noisy.
The Load Center.
A load center is an area that can be supplied by primary distribution lines. The higher the voltage of the primary distribution, the larger the load center that can be served. Prior to the integration movement starting in 1910, each load center had its own generating station. If energy were generated and distributed at the 115 - 230 volts used by most consumers, a load center could only have a radius of about one half mile because larger load centers would required much heavier and far more expensive copper conductors or else the voltage at the periphery of the system would be too low.
Adoption of the polyphase alternating current system invented by Tesla and sold by Westinghouse permitted inexpensive change in voltage and permitted higher "primary distribution" voltages and larger load centers. A typical primary distribution voltage of a nominal 12,000 volts will permit serving a load center some 5 miles in radius; a few utilities have adopted 34 ,000 volts as their primary distribution voltage and can serve load centers with a radius of some 25 miles.
With the integration of more than one load center, the local generating station dedicated to just that load center was replaced by a distribution substation obtaining its energy over transmission from one or more larger regional generators of far greater efficiency. Primary distribution integrates the load at all of the sites in the load center; transmission integrates the load of many load centers into a regional bulk power supply system.
Integration of the load centers was crucial to the feasiblity of obtaining the benefits of the larger generating units with their lower unit capacity and operating costs, but this feasibility came at a price. Although the integrated generator was more efficient at its busbar, its energy had to travel many miles over transmission, subtransmission and distribution to reach the customer. For the typical residential customer, this meant losses of 13% to 16%.
According to Detroit Edison, the current average historical investment cost of all those wires from the integrated generation to the residential customer, i.e. the transmission, subtransmission, distribution and the transformers, is about $400 to $500 per kw of capacity, but if you had to install them now they would cost from $650 to $850 per kw. Another study, nationwide in scope, suggests that the current average cost of installing transmssion, subtransmission, and distributon, and substations, is almost $1,500 per kW.
If you have a typical one hundred amp electric service, which at 110 volts will provide the typical household with up to 11 kilowatts of service, installing new wires to serve your house could cost your. electric utility up to almost $15,000 just for the wires and transformers, without regard to the cost of the generator. If a generator is just as efficient, or more efficient than the larger one, and it can be located on site or very near by, all or most of this wires and transformers cost can be eliminated.
Another cost of integration was the electrical losses of transmitting electrical energy as many as several hundred miles from the integrated regional generator to the residential load. These are typically 13% to 16%.
That amazing 60% efficiency of GE's "H" technology can be only 50% by the time the electric energy wends its way down transmission, through transformers, down subtransmission, through more transformers, down distribution, through a pole top transformer to secondary distribution levels and through the service drop to the customer's meter. The simple cycle areoderivative gas turbine of 28,000 kw may be small enough to serve a single load center or two or three of them in a city, but it will need backup from either another 28,000 kw gas turbine or else from the grid -- which brings you back to the full wires cost and at least half of the losses.
For many years the standard for firm power was a "deterministic" standard. The system must be able to carry its estimated annual peak load with its single largest generating unit down. After World War II when digital computers became available, one could model a system using so called "Loss of Load" and "Loss of Capacity" mathematical models to determine, based on unit experience, either the percentage of time a generating unit would not be available, or the frequency and duration that system outages would exeed a constant risk reserve. These were "probabalistic" measures of reliability.
With a single largest unit of 500,000 kw to 1,400,000 kw, the costs of maintaining reserves was not inconsiderable. It is easy to see that with units sizes of 10 kw or 250 kw one can more inexpensively maintain two or even three units in reserve and be able to sustain several forced outages before the system loses power. With small in your cellar or your backyard, one can avoid the 30 momentary outages or blinks per year coming from transmission line circuit breaker operations which are so disliked by computers and digital clocks, and two or three sustained distribution outages per year on average, totaling about one hour. In comparison with the 3 nines (99.9%) reliability of the grid, now one can have 6 nines, 8 nines or more, by adding some extra very small but amazingly efficient fuel cells.
Wasted Thermal Energy
Integration wastes thermal energy. Thanks to Nikola Tesla the inventor of the polyphase alternating current system and George Westinghouse its promotor (and no thanks to Thomas Edison who bitterly opposed it), electric energy can travel two hundred or three hundred miles to a load with not unaceptable losses. The same isn't true for the thermal energy that inevitably is a byproduct of electric generation. It will only travel up to half a mile. Integrated generation had to collect load from many load centers. Most of the load was at least half a mile or more from the generator. The heat from generation heated up the atmosphere or nearby streams.
Small generators can be used for cogeneration. With small generators on site, just a few feet from the thermal load, the heat can be used for domestic hot water, space heating. The high grade heat of SOFCs and MCFCs can also be used for air conditioning. The CHP or combined heat and power efficiencies of these generators are likely to reach 80% or more.
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