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The Renewable Energy Sources Power Grid, Part I
Submitted by Dan Sweeney on Sat, 2006-12-16 20:38.
The notion of clean hydrogen produced entirely from renewable sources is a vision explicitly or implicitly embraced by proponents of the hydrogen economy. Such a vision necessarily presupposes a hydrogen transition leading up to such an economy. Here we will examine the logistics of such a transition.
Interestingly, in all of our considerable research on this subject we have seen no attempt to construct a comprehensive "roadmap" from our present energy regime to a fully evolved hydrogen economy. While we have read countless white papers and positioning statements from various consortia projecting rapid growth for renewable energy sources, we have seen no detailed discussion of how such an all-encompassing renewable grid would come into being, what obstacles must be overcome to attain it and how they might be overcome, and precisely how such a grid would be structured and would operate.
The Notion of Hydricity
Hydrogen, as has often been noted, is a carrier of energy rather than a basic source since it is never abundantly available on earth in free form. This fact gives rise to the notion of a hydrogen energy currency or hydricity. According to this concept, renewable and largely intermittent resources such as wind and solar would be used to generate electricity or thermal energy, part of which would be used to generate hydrogen. The hydrogen, as well as being consumed in transportation, would also be used to generate energy through the agency of fuel cells or, alternately, hydrogen burning heat engines during periods when the renewable energy sources could not maintain the base load.
As an energy storage medium, hydrogen is intriguing because in theory a reserve of gaseous hydrogen has the capacity to store truly large values of energy. By contrast, storing energy in batteries or flywheels has distinct limitations. Currently, however, hydrogen storage in compressed or liquid form in tanks, or by means of other technologies such as hydrides, alanates, and other chemicals, has never been undertaken on more than a modest scale.
The cost of storing hydrogen today long term on a massive scale would appear to be prohibitive with existing technology, which is precisely why hydrogen storage has only taken place on a small scale. Some proponents of a hydrogen transition believe, however, that a number of nascent techniques are sufficiently promising to warrant the initiation of infrastructure projects today.
One suggestion, perhaps the boldest, is that hydrogen simply be stored in transit by being injected into a vast network of pipelines which would themselves communicate with a network of hydrogen powered generators. So long as hydrogen were being produced in sufficient volumes by renewable energy sources-based electrolysers or other hydrogen generators, and fairly promptly consumed subsequently, the lack of concentrated storage facilities would not be a problem. But the energy required to pump hydrogen through a pipeline network is considerable, as are the net losses in energy and in the gas itself, because 1.4% of hydrogen is consumed every 150 kilometers to energize compressors - this assuming that leakage of the gas is negligible. Thus transcontinental transmission of hydrogen gas would consume more than 50% of the total. It follows then that electricity itself would likely become the medium of energy transfer, and that hydrogen would have to be produced and consumed locally, simply because resistive energy losses during long range transmission of electricity are so much less than the energy losses attendant upon transporting hydrogen via pipeline. Therefore, pipeline storage, it appears, is not a very well founded notion.
Hydrogen could also be stored in underground reservoirs such as depleted salt mines, and pilot projects toward that end have achieved considerable success. The problem in a fully evolved hydrogen economy would be the ready availability of suitable sites and their distance from large user populations. Hydrogen, as we have seen, is expensive to transmit through pipelines, and if large volumes of hydrogen were to be stored in widely scattered reservoirs, then the demands upon the transmission system would be intense and probably insupportable.
We can only conclude, then, that the hydricity concept is fundamentally unworkable with current transmission techniques if it is to be imposed on a vast geographical scale, and that even the local storage of hydrogen is fraught with difficulties. We do not see any palliatives on the horizon that will change that perception.
Contemplating a Transition
Many individuals enamored of the concept of zero pollution power naively assume that renewable generators are straightforward replacements for existing fossil fuel or nuclear power plants. In only a few cases is this true.
If we except large scale hydroelectric power generation, which has little potential for further growth, and consider only the other dominant renewable sources, wind and solar, we find that neither is well suited to providing either base line power or premium power. In other words, an electrical utility attempting to serve a large geographical area with purely renewable sources with no supplementation of fossil fuel generators is facing formidable challenges.
This is not to say renewable energy alone cannot serve the electrical needs of a community. Countless off-grid communities, generally quite small, exist in various places around the world including the United States. The problem arises when one attempts to scale up the technologies used in such micro-grids and to create vast interconnections of localized grids on a national and international scale such as characterizes fossil fuel based electrical generation in developed countries today. Off grid renewable electrical power installations, when they are standalone and not supplemented by conventional generation facilities, normally serve individual residences or groups of residences, not industrial parks and never heavy industry. And to expand such renewable electricity generation plants to serve such purposes would be extremely difficult for a number of reasons.
In small communities one can either elect to tolerate the elevated possibility of outages and the limited capacity for load following that characterizes small renewable generation systems or else provide individual residences with expensive backup power systems or uninterruptible power supplies. But when one tries to extend the model to the macrocosm it quickly becomes highly problematic. And that is because of the almost entirely different operational characteristics of the traditional power grid and of those small local renewable energy generation and distribution networks that constitute today’s zero emissions microgrids.
How a Renewable Electrical Grid Would Differ from a Fossil Fuel Grid - How the Differences Affect a Hydrogen Economy
Let us begin with an indisputable fact to which we can return after any number of speculative flights. Eventually, sooner or later, the industrialized world will depend upon renewable energy just as it did before coal was first exploited on a large scale in the eighteenth century. True, there are those who believe that petroleum is replenished abiotically in the depths of the earth, including a very few credentialed oil geologists, but such individuals are in a tiny minority. The weight of scientific opinion is that fossil fuels are limited and are running out, and that therefore renewable resources must eventually replace them.
Having said that, we must still confront the equally weighty fact that fossil fuel is absolutely predominant today in the generation of electricity, and that the existing electrical transmission grid is highly reflective of certain characteristics of fossil fuel generators. For this reason, making the existing grid conform to a preponderance of renewable generators would involve changing its basic characteristics and possibly replacing a large part of the existing infrastructure.
These two types of grids, fossil fuel- based and renewable energy-based, are intrinsically different. To see why we need to look closely at the grid as it exists today.
The Legacy Grid
The term grid properly refers to the interconnections between local and regional electrical power distribution systems, not to the entire electrical system. A grid is synonymous with long distance transmission.
No grid existed in the earliest decades of the American electrical utility system, and it only gradually took shape from the 1930s onward. Extensive interconnection did not occur until the 1950s and 1960s when the electrification of the country was largely complete.
Before we discuss the specifics of how a grid operates, let us first let examine electrical generation, which lies at the core of all electrical utilities.
Since the late 1890s, almost all fossil fuel generators have utilized high speed turbines with their combination of good efficiency and high reliability. Steam turbines have always been predominant, but since the 1970s natural gas fired turbines resembling the turbojets used in aircraft have played an increasingly important role in electrical generation. Nuclear facilities, which are a special case and account for approximately 17% of total electrical power generated worldwide today, almost exclusively utilize steam turbines that are very similar in design to those used in coal plants, although next generation designs will move to helium gas and vaporized metals as the working fluids in lieu of steam.
All such turbine generators are designed spin at a constant rate, but can vary their power outputs simply by throttling their intake of combustible fuel and air. Thus they are very well suited to producing alternating current at a fixed frequency and maintaining line voltages in the face of fluctuating loads simply by increasing or decreasing their power outputs.
Hydroelectric generators can also maintain very stable rotational speeds but they cannot throttle output. In clear contradistinction, both wind and solar generators tend to be severely limited in their ability either to regulate power output or to produce alternating current at fixed frequencies.
Wind Power, Challenger or Challenged?
In assessing the very different characteristics of a renewable grid, let us first discuss wind power because apart from hydroelectric generation it is the major source for renewable energy generation today, and, unlike hydroelectric, its installed capacity is growing rapidly.
Wind energy is the only renewable energy source that could be readily utilized to support a hydrogen economy today, at least within most countries. The cost of wind generated electricity is now about 3 cents per kilowatt hour in the U.S., equivalent to that of coal or natural gas, and in clear distinction to fossil fuel electricity, wind power prices are apt to remain stable or even decline. Furthermore, the U.S. possesses sufficient wind resources on the Great Plains to match all of the present day electrical generating capacity, though whether such resources would be sufficient to meet the requirements of a full scale hydrogen economy where grid supplied electricity ultimately supports transportation as well as current industrial and residential uses of electrical power may be debated. Moreover, wind turbine design is mature, and numerous utilities successfully employ wind power today.
So what’s preventing a rapid transition to wind energy and the establishment of a hydrogen economy based on a foundation of wind powered electrical utilities?
The main problem in expanding wind power is the fact that most prime wind resources are located in remote areas and would require tremendous amounts of new transmission capacity and enormous capital expenditures to build it. Whether such a project could be undertaken purely with private funds is questionable, a topic we will explore at length in later pages.
Furthermore, the fact that wind resources are so diffuse and so difficult to tap efficiently makes them highly problematic when the level of usage grows sufficiently great. Advocates of electrolytic hydrogen produced with renewable electricity are fond of citing the total extent of wind resources in the U.S., which is undeniably impressive and more than adequate for all of our energy needs for at least several decades of come. Still such claims, while technically true, are somewhat disingenuous for a number of reasons.
First of all, typical Danish type utility grade wind turbines are only about 30% efficient in capturing the energy of the wind impinging directly upon them. Further losses occur when that energy is transduced into electrical energy, and yet more losses occur when the high frequency alternating current is down converted to 60 cycle or converted to DC and then reconverted to 60 cycle AC. Significant mechanical losses also occur in the turbine’s transmission unless it utilizes a direct drive variable speed generator. Overall conversion efficiency is well under 20% in most cases.
Interestingly, the maximum raw efficiency of a wind turbine for converting aerodynamic lift into mechanical motion is 59%, the Betz limit discovered by a German physicist of the same name, but, although some experimental designs have approached that figure, there is currently no significant infrastructure employing such advanced designs. Nor do we expect such advanced designs to predominate in the foreseeable future.
Yet another problem with conventional designs is the requirement that they be separated by distances of several multiples of the span of the rotor blades. Because of this requirement, most of the wind energy impinging directly on a wind farm simply can’t be captured. Then too, turbines are limited to rotor blade lengths of less than a hundred feet due to geometrical increases in mass and mechanical stresses with length, which means that a moving air mass extending thousands of feet above the surface of the earth is sampled at a single point, as it were, and most of its energy is not harvested.
Furthermore, most utility grade wind turbines have maximum outputs of only a few megawatts or a few thousand horsepower, the output of one fair sized diesel generator. A cluster of large coal and nuclear turbines, on the other hand, can exceed 10,000 megawatts, in other words, more than three orders of magnitude more. Thus a wind farm with several hundred turbines, each costing over a million dollars, will produce at maximum wind speed less than a tenth of the output of a few giant steam turbine used in coal or nuclear facilities. And since most of the time the wind turbines are producing but a fraction of their maximum outputs, the actual disparity is much greater. It follows that a tremendous number of wind turbines and a large amount of supporting infrastructure would have to be constructed to replace the existing fossil fuel grid, and at a capital cost far exceeding the total assessed value of existing infrastructure.
Yet another limitation in wind turbines is operating life and ongoing maintenance requirements—in other words, the operational expenses associated with them. Those turbines utilizing gearboxes, and they are the majority, require regular tear downs and rebuilds. Generally, the operating life of a wind turbine does not exceed twenty years. In contrast, a steam turbine will normally last for twenty-five years.
Currently wind accounts for less than 1% of total electrical generation in the U.S. in spite of extensive deployments in the Great Plains region. Solar, as a matter of interest, accounts for far less than that and may not be viable for utility applications within the time span of this study. A hydrogen economy based upon electrolytic hydrogen generated by wind energy may indeed be possible in the U.S., but it would require a series of engineering projects of far greater scope than the construction of the railroads, the Interstate Highway System, or the current electrical system—in fact much greater than all of these put together. We estimate that two to three million large wind turbines minimally would have to be installed to replace the current fossil fuel and nuclear energy generation system in the U.S., with each turbine costing about 1.5 million dollars at current pricing, making for a multi-trillion dollar project. Further augmenting the system to handle transportation energy needs would mean trillions more. Setting up a distribution system for hydrogen, or, alternately, distributed generation, would, by most estimates, cost additional hundreds of billions if not trillions. The cost of new transmission capacity defies back of the envelope estimates, but probably amounts to millions of dollars per mile, and might ultimately equal or surpass the cost of the turbine generators. As is, transmission accounts for approximately 18% of the total asset value of all American utilities combined, and a system suited for renewable hydrogen energy regime might well be more expensive. Altogether, we’re talking about allocating a major portion of American productive capacity to the changeover.
True, such costs could be distributed over a period of many years, but they are still very, very substantial, and, unlike the original electrical system which brought about a second industrial revolution and the twentieth century age of affluence, in other words, created vast material wealth, this replacement grid would probably require heavy subsidization and would likely not revolutionize manufacturing as did prior energy revolutions, though it would certainly stimulate employment.
Here, it must be remembered we are only considering the sheer extent of the resource, not the difficulties involved in utilizing wind energy as the primary source of electrical power. Those, as it happens, are quite formidable, and it is to those that we turn our attention next.
Stabilizing a Renewable Grid
Wind turbines are generally sited where strong prevailing winds occur, but plus or minus wind speed fluctuations of some forty percent are the norm over 12 hour periods even at favorable locations. Real fluctuations in power output are much greater, however, since the power generated varies with the cube of the wind speed. And obviously a fluctuating turbine speed will also result in fluctuating AC output from the generator.
Wind turbines can be made to rotate at constant speeds by automatically varying the pitch of the blades but such a ploy sacrifices efficiency. Alternately, rotor slip may be varied in the generator to compensate for variations in rotor speed so as to maintain a constant frequency in the face of wind speed variation, but such a strategy can only accommodate small variances.
The only method in common use today that can assure high conversion efficiency and at the same time rock steady AC frequency control for the output of a wind turbine, or, alternately for an entire wind farm, is to rectify the AC to DC and then reconvert it into frequency controlled AC by electronic means. At present, this generally means a large inverter using IGBT (insulated gate bipolar transistor) power output devices. Since all such devices on the market today are limited to a few kilowatts, large banks of them are required to serve multi-megawatt wind farms. Since such inverters each carry a five figure price, and have operating lives of only a few years, they represent a very large and recurring capital expense. Perhaps for this reason high voltage DC transmission systems for wind have been largely confined to experiments in Scandinavia and have seen almost no commercial employment in the U.S.
One could in theory use a continuously variable transmission on the wind turbine to achieve frequency stability, but the voltage would still fluctuate according to wind speed. Only the electronic inverter can ensure both frequency and voltage stability.
Unfortunately, the inverter cannot maintain line voltage indefinitely in the face of increased electrical draw by subscribers and a simultaneous reduction in wind speed. At best the inverter can only provide ride through in the face of momentary voltage sags. In order to maintain constant voltage into falling electrical impedance caused by increased electrical utilization, the system has to produce more current—basic Ohm’s Law. If the wind speed is dropping as demand is increasing obviously one has a problem.
What the system operator can do to meet local demand beyond his capacity to support it is to draw power from other systems to which he is interconnected—or, in essence, use the same strategy that fossil fuel system operators rely upon today when their generators can’t meet local demand. One simply buys the power from other generator utilities that are not overloaded.
As anyone knows who has experience with electrical utility operation, transmitting power over long distances to meet urgent local demands is an operation fraught with uncertainties even after three quarters of a century of industry experience in modeling large scale electrical systems. Catastrophic regional power outages and cascading system failures characteristically occur when heavy demands for remotely generated power are made by local operators, and no one has yet devised a foolproof method for avoiding them.
Fortunately, urgent short term demands occur but seldom simply because fossil fuel generators are normally sized to meet local demand. But in the case of wind farms the size of the operation is often limited by available land, and regardless of the nominal generating capacity of the farm, temporary cessation of wind can result in no output whatsoever, thus necessitating a heavy draw upon other generating systems.
If interconnected wind resources are distributed over a sufficiently large geographical area, then differences in wind velocity tend to average out, but the problem of managing all of the fluctuating voltages does not go away; instead one merely gains the assurance that the total amount of electrical power within the interconnected system will be adequate to meet the total demand within the same overall area. Making certain that adequate electrical power is everywhere available at all times is quite a different matter, and would appear to require a degree of global network intelligence and automated management capabilities that have not been demonstrated anywhere thus far.
So how does one manage a pure wind powered grid with existing technology? It can be managed by simply by making the power grid somewhat impure.
An obvious brute source solution is the storage of electrical energy or of other forms of energy that may speedily be converted into electricity to meet sudden demands. This is the technique used in most wind based microgrids, and in theory it could be extended to large scale public utilities.
The problem, as we mentioned earlier, is in storing really large values of energy.
Pure electrical storage systems of the sort used in microgrids do not appear to hold much promise, at least in the midterm. Neither lead acid batteries nor the newer redox flow systems scale sufficiently well to anchor the grid. They are at best very local solutions. Reversible or rechargeable fuel cells are rather inefficient and appear inadequate on those grounds quite apart from their current size limitations. Storage of electrical power in super conducting rings represents an intriguing possibility, but at present the technology does not appear highly scalable, and thus far has only provided for ride through power during momentary interruptions. In short, we see no technology on the horizon for storing electricity in values that are sufficient to stabilize a grid made up of intermittent renewable energy resources.
Electrical energy may, however, be stored by converting it into mechanical energy. Pumped hydro is one such method, and here large amounts of water are pumped into a reservoir and then allowed to cascade out through a hydroelectric turbine. The process is reasonably efficient, but building such facilities, which of necessity must be fairly enormous, is inordinately expensive.
Another technique for storing mechanical energy is compressing air in cylinders and subsequently using it to operate pneumatic engines. Again efficiency is fair, but the cost of high pressure storage would be extravagant in a large public utility.
And finally there are high velocity flywheels which may achieve conversion efficiencies in excess of 90% but which require costly precision machinery and expensive, ultra-heavy duty construction materials. Flywheels are already in use in some generation facilities but nobody is assuming that they represent any comprehensive solution to the energy storage problem.
Hydrogen itself, as we have indicated, is often advanced as the solution to the energy storage problem, and we have seen many proposals for using wind energy to create DC which would then be used to operate large electrolysers for producing hydrogen. The hydrogen would subsequently be stored on the spot or piped to remote locations and ultimately consumed in utility grade fuel cells or hydrogen powered turbines. Local storage, as we have discussed, seems by far the better option.
Unfortunately, the use of vast numbers of electrolysers for massive production of hydrogen would introduce a new cost element of staggering magnitude that we have not even considered thus far. The U.S. currently generates quadrillions of watts of electricity to meet current needs, and, as we have seen, the requirement for greatly augmented generation to supply hydrogen for transportation as well as stabilizing the grid would result in many more quadrillions being generated. Much of this total would be consumed by electrolysers, which are currently priced at $1,000 per kilowatt. At such pricing that could mean hundreds of trillions of dollars for electrolysers alone, more than the entire industrial output of the nation. Of course one would expect electrolyser prices to drop over time, but they would have to drop by orders of magnitude to make a hydrogen economy remotely feasible.
Quite apart from the issue of electrolysers, we must also consider the current art in hydrogen energy conversion devices, principally fuel cells and Hydrogen internal combustion engines.
Attempting to stabilize a grid with utility scale hydrogen fuel cells alone would be quite infeasible today for a number of reasons. Available units carry extremely high prices and produce relatively small outputs, and neither limitation seems amenable to any quick solution. Indeed, all of the larger fuel cells constructed to date have been designed to operate on fossil fuels not hydrogen, and, in any case, they take hours to reach their requisite operating temperatures and thus would be used for primary rather than fill-in power.
Furthermore, fuel cells are low voltage DC devices and have no ability to augment their outputs to follow a load. They do offer steady, predictable power outputs but otherwise they enjoy no advantages over wind turbines. They’re simply not a good solution for stabilizing a wind based grid.
Hydrogen turbines are a different story and probably could be built cost effectively with today’s technology, though considerable research would be required to optimize the designs because no hydrogen gas turbine is currently in production, and, to our knowledge, only one is in use. Hydrogen fueled internal combustion turbines certainly have been built and operated on an experimental basis, and they clearly have the potential to scale to enormous size. The real issue then becomes how much of the total electrical capacity would have to be allocated to hydrogen turbine generators to ensure grid stability, and how much infrastructure for storing and transmitting hydrogen would have to be built.
The current thinking in Scandinavian countries, where wind accounts for a greater percentage of total electrical generation than elsewhere, is that the grid becomes difficult to stabilize when wind becomes more than 20% of the total. If that figure cannot be improved upon then one is condemned to massive redundancy and inefficiency. Most of the available wind power in a fully renewable grid could not be used directly to serve subscribers but instead would go to operate electrolysers which would produce vast amounts of hydrogen for turbine installations—at about a 25% energy efficiency level over the complete cycle from electrolysis through combustion. The latter, presumably, would function analogously to the natural gas turbines used today, following electrical loads quite accurately and serving extended areas by means of enormous generation facilities putting out thousands of megawatts of total power.
Of course the possibility exists that the grid can be made much smarter than it is now, and that the 80% requirement for stable power can be much reduced, but that internal combustion turbines can be entirely eliminated cannot be assumed at this time. Unless one proposed to anchor the grid with nuclear plants, a prospect most wind advocates view with sheer, unmitigated horror.