Just How Renewable?
What are called renewable energy sources are mostly forms of solar energy. The exceptions are geothermal (radioactive decay heat from the interior of the Earth) and tidal (gravitational forces of the Moon and Sun) energy. Otherwise, in addition to direct solar conversion, there are biomass, wind, wave, and the great-granddaddy of them all — which is not even counted under many ‘renewables portfolio standards’ — water power. These are, respectively, combustible fuels (with all their disadvantages) supplied through photosynthesis ; mechanical energy produced by solar heat, using the whole atmosphere as an engine ; a secondary effect of wind ; and Sun-driven evaporation and atmospheric convection increasing the potential energy of a dense fluid by raising it from a low to a high level.
Of these, only water power is concentrated enough to support large central powerhouses, and then only because geography tends to collect water together on its way downward from high elevations. The remainder are diffuse. This incurs substantial costs, both for the equipment needed to obtain the energy, and in gathering it together and transporting it where it is wanted. To take the example of wind, not only is each wind turbine blade an enormous and difficult piece of engineering, but the largest practicable windmill is quite moderate in output. And one hundred generators rated at one megawatt each cost far more in materials and labour than a single hundred-megawatt unit.
Because of their diffuseness, these sources pose great problems of land use. This is most easily seen in the case of biofuels, which must perforce displace either human food crops or wild vegetation. (It should be remembered that the synthesis of indigo and alizarin from coal tar in the nineteenth century was considered a great advance for human welfare, because land formerly planted in dyestuffs was released for food production.) We see in Indonesia animals such as the rhinoceros and orang-utan being driven to extinction by Germans who fuel their automobiles with biodiesel and think they are being ‘green’. Quantitatively, all the photosynthesis on Earth amounts to about six times the present rate of energy use by humans. Considering that this includes the food crops which present-day agriculture only manages to raise by dint of heroic quantities of artificial fertilizers, produced using fossil fuels, it is to be questioned how much benefit can possibly be expected from this source.
Land turned over to solar power is also, obviously, removed from other uses, although it is more likely to be desert than arable. While the foundations of a wind turbine occupy a moderate area of ground, they require site preparation which can change the character of the land, and the presence of turbines restricts feasible uses for some distance around. Much the same may be said of transmission lines with their towers. Any significant development also requires some kind of access road.
Even hydroelectricity is distinctly imperfect. Reservoirs may drown fertile land or riverside towns, and the barriers interposed by dams often interfere with fisheries and have other adverse effects, economic and ecological, on the rivers. There is also the chance of a dam failure upstream from a large population center. The Aswan High Dam is an extreme example — not only has the cessation of the annual floods with their loads of silt gravely damaged agriculture in the lower Nile valley, it is probably no exaggeration to state that, if it were to be breached, the waters of Lake Nasser would sweep practically the whole country of Egypt into the sea. As a result, not only are feasible sites for new hydro developments limited, the most important being perhaps Inga on the Congo (a situation like Niagara, but much larger), some of those which now exist probably have to be removed.
Renewables demand to be used in their proper role. Like any other technology, there are things they are good for, and things they are not. Solar water heaters, for example, can be extremely effective even in reasonably cold climates. Photovoltaics are great for running low-power devices in odd locations, especially if they may get moved around. Biomass from agricultural waste can be used effectively on farms. In Iceland, a country which is practically one big volcano, it would be absurd not to use geothermal energy. And, because access to even an occasional kilowatt-hour makes a serious difference in the lives of people who previously have had none, solar, wind, or whatever the local conditions will support can play a vital role in isolated districts. Indeed, the economic lift provided by such an installation may be the factor which ultimately makes it practical to extend power lines to a place.
Ultimately, as Dyson, Kardashev, Glaser, O’Neill, and others have pointed out, our civilization must turn to the Sun — that great atomic furnace — for its energy needs. But that energy will be captured, and principally used, not on Earth but in free space. Once off the planetary surface, sunlight is uninterrupted (geostationary satellites, such as those which broadcast television, see a few hours of darkness a year) and unimpeded by atmosphere ; the sunlight can be concentrated to reach any desired intensity, or blocked out to develop intense cold. But, work though we must for that day, it has not yet come. Until then, we have fission to sustain us.
Capacity Factor and You
Civilization demands power
Mathematically, power is energy over time. From the standpoint of pure mathematics, one watt is the instantaneous value of a constant one joule per second, but it is also the average value of 3.6 kilojoules in the course of an hour, whether delivered all in one microsecond at the beginning of the hour, or spread evenly over the whole 3600 seconds. In a practical sense, though, there is a big difference.
When we speak of power, we really mean energy which is there when it is wanted. The electric company charges by the watt-hour ; that is physically 3600 joules, but it represents a watt drawn for an hour, or 120 watts for half a minute, just as you please. If you want to use as much energy in two weeks as you use through the rest of the whole year, though, they will have to install transformers and lines to serve that maximum use, and you will pay for that.
The cost of equipment, in other words, has much more to do with the maximum load it is rated to carry than with the average load in practice. The ratio of the two, which may be framed in terms such as “annual kilowatt-hours divided by peak kilowatts”, is known as capacity factor. The closer the capacity factor of anything is to unity (100%), the more economical it is.
Wind and solar have a hard time delivering power. Sunlight doesn’t come when you need it. It comes when the geometry of the celestial spheres says it must ; ditto the tides, although they, at least, aren’t blocked by cloud cover. Wind is capricious, as are the waves it whips up, and even where they do follow a halfway predictable course, that doesn’t have any relation to human needs. As a result, the capacity factor of renewables other than hydroelectric and perhaps geothermal tends to be quite poor.
No ground-based photovoltaic array will ever deliver its nominal rated power, because that is calculated for “Zero Air Mass” — sunlight not attenuated by passing through the atmosphere. Its peak power may reach fifty or seventy, perhaps even ninety per cent of that, depending on latitude and elevation. But, for any site a good fraction of the time, and for most sites all of the time, the sunlight is coming in obliquely through a deep blanket of air which absorbs and scatters light. As a result, it is not uncommon that the average power (over the course of a year) produced by a photovoltaic array should be a tenth, or less, its nameplate rating. The Massachusetts Museum of Contemporary Art paid twenty times the average American’s annual income for an array which often generates nothing at all. A solar-thermal plant, in a reasonably favourable situation, may attain a capacity factor of 30% — not because of any inherent superiority, but because its rating is based on the actual sunlight at the site.
Wind presents special problems. There is never “too much sun” for sunpower (although light does eventually break down photovoltaic elements), but there can easily be too much wind for windpower. Every wind turbine has a critical speed below which it generates no power at all, and another above which it has to back off or even stop, to avoid mechanical damage or overloading the generator. (Sometimes the safety mechanism fails, and the generator catches fire, or the turbine hub sheds its blades like forty-meter, five-tonne spears. The bare mention of such an event instils in anyone with a working self-preservation instinct the ardent desire to be elsewhere.) As a result, even in areas known for windiness, electricity from wind has a tendency to be highly variable. It may take five or more generators to obtain an average power equal to the rating of one, and this still does not assure continuous power.
These problems can be ameliorated by installing many solar and wind generating elements, amounting to several times the anticipated demand, across geographically distinct areas with different weather patterns, and tying them together with long transmission lines. Even such a costly measure, however, still leaves open the possibility of an extended period of widespread calm and overcast. Furthermore, if those long lines cross international boundaries, power transfers may be restricted to exert political pressure, as has often been done with oil and gas. Such large-scale interruptions would be infrequent, but none the less intolerable, and some other form of generating capacity would have to be built to meet at least the minimum needs.
Coping with intermittency
Power which is not available on demand has hidden costs. Many things can be built to operate on energy which is only available part of the time, but the capacity factor problem applies here too. For instance, if a water-supply system has to work on intermittent power, its pumps must be made larger, in order to move the required volume of water in the available time ; and likewise the water-towers, to hold that volume until it is needed. This means more expensive machinery and construction. Similarly, refrigeration plants can be built to produce ice when power is available, but they will be more complex, costly, and prone to break down than those which operate on a demand basis. Indeed, they will also use more total energy. Those large loads which can tolerate rapid changes in power level, including aluminum smelters and shopping-mall air conditioning systems, are already in use as load-levelers, in return for a hefty discount on their electric rates.
Other applications, such as household lighting, are not amenable to demand-shifting. The power for them has to come from somewhere. Water, unlike air, can be held back by dams, and Manitoba Hydro throttles back its water turbines when its wind turbines take up the load, letting the water in the reservoir back up until it is needed. This is practicable because hydroelectric stations are normally built with a capacity exceeding the average flow of the rivers they sit on, to take advantage of seasonal variations — although Manitoba is now installing additional generators, which will stand idle much more of the time than the original plant, just to balance wind capacity. But not everywhere is blessed with such enormous hydro potential.
In some places, excess power from intermittent sources is actually used to pump water uphill into hydroelectric ponds, allowing the water turbines to run at a higher capacity factor than their natural supply would permit, and thus shoulder the load when it comes onto the system. Compressing air in huge reservoirs, to be let out again through suitable engines, has also been tried, although the problems are considerable. Electrochemical storage, including closed-loop fuel-cell systems, has been the subject of considerable study, but remains quite expensive (among other drawbacks, such as fire risk).
The most common way of handling the power loads on systems supplied with intermittent energy is, in fact, to run combustion plants in parallel. This results in a loss of efficiency, because fuel is being burned much of the time to keep the water hot (or whatever is required to pick up the load at a moment’s notice), and also means higher maintenance costs and worse reliability, because machinery is more apt to break down when it starts up and shuts down frequently than when it runs continuously. And, even with plenty of ‘spinning reserve’, a sudden lull or spike in generation, due to (say) a coincidental variation, lasting only a few seconds, in wind speeds over a large area — which is statistically inevitable — can destabilize the whole distribution network, potentially interrupting power delivery for hours.
Or failing to cope
Almost everywhere that energy policy concentrates on renewables, energy costs and fossil fuel use have both increased. In Germany, for example, where the use of coal had almost been eliminated only a few years ago, both mining and burning are big business again. Denmark’s vaunted investment in wind turbines has led to power costs well above those in heavily-nuclearized Finland, with carbon dioxide emissions close to what they would be if all the power came from coal. This is the strongest possible indictment which could be brought against new energy sources : not only do they not deliver cheap, plentiful power, but they do not displace fossil fuels.
