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Our preoccupation with soaring energy costs and declining supplies of oil have focused our attention on possible alternate sources of energy. The media are continually calling our attention to undeveloped resources such as solar and geothermal energy. The extent of this leads us to wonder just why it is that more is not being done to tap these seemingly limitless reservoirs of power.
For those of us interested in old steam power, perhaps it would be a useful project to examine geothermal steam. At least it has an intriguing name and that might excite our curiosity. First, though, let us examine the very word itself. Geo comes from the Greek meaning earth and, of course, we know that termal means heat. So, there we have 'geothermal' meaning 'earth heat.'
Throughout the world there are many areas of geothermal activity manifest at the surface in such forms as hot springs and geysers.. There are even more widespread areas of subterreanean heat below the surface. We are not necessarily aware that they exist, but they could be tapped through deep well drilling. Also, there is the hot magma beneath the earth's crust with energy beyond imagination ... if we could just get at it. But, let us not befog ourselves with such esoteric sources as the earth's magma and simply grasp that which we can see and feel as we cross the width and breadth of this land. Then let us examine the problems and possibilities of generating steam power from the earth's heat.
There are a few places around the world where one can drill a well not greatly different from an oil well and produce dry steam (Umm, well, nearly dry) for direct use in a steam engine or steam turbine. Probably the oldest commercial installation of this type is in Larderello, Italy, where a 250 kilowatt generator (235 horsepower) went on line in 1913. The largest is undoubtly at the Geysers area in California where presently some 800,000 kilowatts of capacity are in service and more is being added. Up until around 1975 the New Zealanders had operated 192,000 kw at Wairakei Power Station but that is being phased out due to technical and economic problems. In 1970, the author witnessed some field tests of steam producing wells near Reyjavik, Iceland, where power generation was being studied. Around the beginning of 1978 the Icelanders started up a 30,000 kw unit of a planned 60,000 geothermal development. There have been some start-up problems associated with insufficient steam production from the wells and there have been some steam quality problems. But, Unit #1 is pumping power into the national system.
So far, fairly extensive geophysical exploration around the world has not revealed additional reservoirs of dry steam. Generally, geothermal heat is in the form of hot water which can be flashed into steam. And, in order to be hot enough to be useful in generating steam power it must be very deep in the earth. Let us see why this is true. One of the sources that we will want to discuss in more detail as an active power project receives the water at 550cF. In order for water to exist as a liquid at this temperature it must be held at a pressure in excess of 1000 psig. Therefore, it must occur at a depth below the surface in excess of 2400 feet (0.43 psig per foot of water column). Drilling wells to this depth is no great problem, technically, but doing so into a formation at 550°F is. Some of the very hottest oil wells such as in Alaska are not over 180°F. This presents a real challenge.
Many of the geothermal fields of interest are at lower temperatures and pressures. One in California is a 325°F or about 80 psig. And, there are some at not more than 180°F.
Thus it is that we can conclude that for us there are three categories of geothermal energy. One is dry steam itself. Another is very hot water which can be flashed into steam. And, we should not overlook those situations where we really do not necessarily have to make steam and turn it into something like electricity for the ultimate consumers use to conserve our dwindling oil and gas supplies. There are a number of direct heat applications in which lower temperatures can be used.
In this latter case there is a very interesting application in Fernley, Nevada, where some 27 million pounds of onions are dehydrated each year using hot water from the Brady Hot Springs formation. This is estimated to save 117 million cubic feet of natural gas annually that had been used formerly in the drying operation. Similarly, the city of Reyjavick, Iceland, is 75% heated through a geothermal hot water distribution system operated at about 200°F. But, for we steam engine folks we will stay with steam power applications.
Steam well demonstration at Reyjavik, Iceland. Note vertical separator vessel on left. Mixture from well enters in the middle with rejected water and silt going out the bottom connection and product steam off the top. Well is being 'blown down' before start of test.
However, for the record, there is one other possibility. That one is called 'hot dry rocks.' There are areas where the hot magma from the earth's core has intruded into the outer crust. Studies are being made on the speculation of very deep wells drilled into the formation with water being injected to pick-up the heat. That turns out to be a modification of our dry steam method.
Since the Geysers area in California represents the most outstanding application of the dry steam concept, that might be the best one to examine. In 1847 William Elliott was hunting grizzly bear in what is now Sonoma County, north of San Francisco, when he discovered what he thought were 'the gates of hell' as he witnessed fumaroles venting live steam. Later in 1956 a system started out using 11,000 kw generators and has grown over the years. The latest machine is a 135,000 kw unit and in a few years the area will contain 1,100,000 kw of capacity. This, at first, looks like a large amount. But, it is only a very small percentage of the total power generation in the area by more conventional means. And, it now looks as if the developers have about reached the capability of the steam field. Most new steam plants built in this country today are supplied with turbines for use with high pressure high temperature (3500 psig 1000°F) steam. These are around 600,000 kw each as an average. They are 4 times larger than the Geysers largest machine. It does not follow that a plant 4 times larger costs 4 times as much. No, it costs only about 2 times as much. So geothermal power has the laws of 'economy of size' working against it. Furthermore, the heat or the steam is not free. Even a 50,000 kw turbine will require many wells to support it and the cost of these is comparable to the boiler of a conventional plant and the fuel that it would burn. Someone once said, 'There is no such thing as a free lunch.'
Incidentally, it should be theoretically possible to build geothermal steam turbines up to the 600,000 kw (800,000 horsepower) size. The steam conditions would be about the same as those of nuclear plants. These machines would be multiple casing designs much like those at Three Mile Island or at Oyster Creek Nuclear Power Station. The well field, however, to support such a gigantic machine would be enormous and the problems of transport of the high pressure water or steam would be extremely expensive to solve. There is an economic balance between machine size and well field size. We tend to forget just how concentrated an energy package we have in a pound of coal or a gallon of oil.
High temperature water in the 550°F range is fairly widespread throughout the western part of the United States. This temperature level permits nearly the highest geothermal theoretical cycle efficiency. Thus, it presents the best economic possibility in the field of geothermal heat recovery.
Any good steam man will tell you that if you reduce the pressure on a body of hot water some of it will flash into steam. That is just the process that is used in the system that we will now examine. First, a deep well, say 2400 feet, is drilled onto a hot water formation. The well casing is perforated. The hot water can now flow across these holes into the well tubing and some of the water will flash into steam. This steam acts just like it does in a coffee perculator. It lifts a slug of hot water up the casing to the surface where it flows into a pressure vessel through a valve where the pressure is again reduced to the operating pressure and the useful steam is formed.
This steam is really quite dirty. There is silt and salts and noxious gasses in it. It becomes necessary to separate the silt and entrained water. The flow enters the separator vessel tangentially and much of the undesirable matter is thrown out by centrifugal force. Steam scrubbers in the top of the vessel further clean the steam before it goes to the engine or turbine. Do you remember the crinkled copper mesh scrubbing pads that were used in the kitchen? Chore Girl was a trade name for one. Make a mat of these for the steam to pass through and you have an effective steam scrubber. Naturally, the noxious gasses pass on through with the steam, but except for environmental problems in having these ultimately released into the atmosphere they can be forgotten.
Perhaps the best known installation of this type is in old Mexico at Cerro Prieto, south of Mexacali on the border with California. In this installation the temperature is only 360°F and it is of the flash type.
Geothermal fluids can be contaminated to the point where steam is not suitable for turbines. Then the heat is used to boil a second fluid such as butane or LPG. This then brings us to the 'binary cycle.' In these types the water is always kept under high enough pressure that steam is never formed. The water has its heat extracted in a heat exchanger system and this heat goes into a secondary working field. There are a number of working fields that could be used. Among them are the flurocarbons such as Freon as used in refrigerators. Amonia could also be used. Thermodynamically the fluid that seems most attractive is benzene. But, taking into consideration such things as safety, both health and fire, and considering costs as well, it would seem that butane wins.
It is interesting to note that probably the most outstanding man in the binary cycle development is Mr. J. Hilbert Anderson of York, Pennsylvania. A man of great creativity and vision from the very heart of IMA country.
Magma Power Company has built a 11,000 kw demonstration plant using this binary cycle at East Mesa, California. It is currently undergoing completion and initial operation. There have been many paper studies to determine whether or not at these lower temperatures of about 350°F it would be better to use a low temperature flash system or a binary system. The arguments are complex and involved far beyond the scope of the present discussion. Suffice it to say that for the moment the final decision making data is not in and it could still be considered to be a stand-off with specific local conditions at a specific site being the final determining factors.
It would seem then that we do have the technology to extract geo-thermal energy once we have either the steam or the hot water at the earth's surface. There would appear to be very little incentive for the government to promote research in the area of equipment and machinery. On the other hand, we do need to learn how to drill for hot high pressure water. And, the DOE is sponsoring work in the drilling technology field. Also, we do need to delineate the areas of our nation where significant geothermal fields are located for possible future use. Here again, there is exploratory drilling going on in Texas and along the East Coast. At first one might think, 'Why aren't we more aggressive in developing geothermal energy?' The answer to that question is both economic and geographic.
For our economic evaluation let us look at one of the flash systems now under government sponsorship in its development. It is a 50,000 kw plant projected for New Mexico. The total project is set at about $100 million with perhaps a quarter of this tied up in the wells. Using the usual privately owned public utility type economic evaluation system we find that power produced from this plant will cost about 5.5/kwh. By the time transmission and other overheads are added we are looking at 7 power. Just drilling the wells alone represents about 2/kwh. It could be marginally attractive in the 1990s if nuclear power is further delayed in its development. It shares an economic similarity with nuclear power in that it too has high investment costs but low fuel cost. We might call the 2/kwh geothermal well cost its 'fuel' cost. The nuclear equivalent in the 90s would be something like 1/kwh.
As we presently understand the existence of high temperature water, the geography is presenting a limitation. In general, the occurrence of exploitable geothermal energy is west of the Continental Divide. Much of it is either remote from areas of need or shares its geography with relatively low cost western coal deposits with which it must compete. The fuel cost of a coal-burning plant built in the same area for operation in the 90s is expected to be around 1.8/kwh.
With this kind of an economic outlook for geothermal power it cannot have a high position in our list of energy priorities. It takes its place along with the other alternates. So, as long as we have some coal in the ground to dig then dig we must to dig ourselves out from under the economic cloud of high-priced energy. Geothermal is something for the longer term. Let us not be misled into thinking that the massive injection of tax dollars into research will materially change the outlook. It is today a technically viable alternate but is not economically competitive with either coal or nuclear fueled plants. There will continue to be isolated unique applications such as the onion drying case or some district heating perhaps. Beyond that it is not likely to be an alternative that will supply any significant amount of our needs in the immediate future.