Tuesday, December 18, 2007
Renewable Energy: Wasted Days and Empty Nights
One of the I had to learn when I began to read discussions of renewable sources of electricity is the vocabulary of power. Generators of electricity can be divided into three different types. There are base power generators, these are the units that can be counted on to generate electricity, all the time, except during periods of maintance. A base power unit can be counted on to generate electricity 60% to 95% of the time. Examples of base electrical generators include coal and natural gase fired electrical generating plants, nuclear power plants, geothermal power plants, hydroelectric power generators.
Peak power generators represent a second class of generators. Peak power generators can provide electricity when demand is high. Usually peak demand is during the day time, when stores, offices and factories are using electrical energy for their operation. In the North peak demand may occure on winter nights, when cold waves drive demand for electrical heating. The popularity of lighted Christmas displays is another source of winter night time peak demand. In the South peak demand is associated with summer use of air conditing. Although the absolute peaks of air conditioning driven summer demand are in day time, the air may remain hot at night, bringing about elevated demand for electricity after dark. Sources of peak demand electricity may include pump storage facilities, as well as reserve conventional electrical generators.
Supplemental power is a third category of electrical generator. Supplemental power cannot be counted on to be avaliable on demand. Some renewable sources of energy like wind powered generators cannot be counted on on to provide electricity on demand. Wind power is avaliable when the wind is blowing. Other supplemental generators, for example solar voltaic panels, are only avaliable at spacific times of the day. Since the "fuel" for wind generation is free, and they do not generate greenhouse gases, the use of supplemental renewable power sources, may be desirable to replace base power when the supplementals electricity is avaliable.
Electricity from variable renewable energy sources has to be ballanced, Electrical demand stays constant whether the sun is shining or behind a cloud. For a simple solar or wind generating system a battery might do. Feed the electricity into the battery and then draw it out when you need powet. pump storage facilities are sort of super batteries. You can pump water up a mouintain when the sun is shining, or the wind is blowing, and then let it flow down the mountain through a generating turbine, when consumers want electricity. A more common way to ballance variable renewable power is by using power from a generating source that can quickly be drawn on as the wind drops, or clouds cover the sun. Denmark draws on electricity generated at Norwegian dams to ballance its wind generated electricity.
Electrical generation has to be able to come on line quickly in order to ballance the variations of wind generated electricity. If you have access to hydroelectric power and plenty of water, that will do fine. but California does not have enough hydroelectrical power to ballance its wind generators. Balancing power usually come then from convintional fossil fuel powered electrical generators. These are called back up generators. In order to be able to supply power quickly, the back up fossil fuel generators must be kept running all the time. Power companies call running back up generators "spinning" them. Of course when you run a back up generator, when you spin it, you are burning fossel fuel, and you are producing geenhouse gas.
Opperating an electrical system that draws on wind generated electricity or solar pannel generation, requires generating back ups. This is called redundancy. How much redundancy do you need? That depends on how much electricity you are trying to get from your vaiable renewables. If you are just operating a few wind generators, you may need quite a lot. If you have a lot of wind, wind generators will produce a lot of electricity. Wind generators are rated by the mazimim amount of power they can generate on a very windy day. Unfortunately in most places the wind rarely blows that hard. In a light brieze, your wind generator may only produce 20% of its rated power. If you are commited to produce a certain percentage of your electricity by wind generation that may mean that you want to buy more wind generators to back up your generating capasity. Five wind generators operating in a light breeze will supply the same amout of power as one generator operating in a heavy wind. If you have light breezes 25% of the time, then you may need to buy five wind generators in order to provide the amount of wind generated electricity you want.
Now suppose no wind at all blows 25% of the time. How many wind generators do you need? Well no number of wind generators will get you what you need. So you will need 100% of your electricity from back up generators. How often do you need back up generators? How often have you been out in the wind, and saw a hard wind turn into a breeze, or even simply die away to nothing? You basically have to keep 100% of your back up capacity spinning all the time with wind. In desert environments the sun shines almost all the time, clouds are fairly predictable.
There is usually some warning if the sky is going to cloud over. So spinning back up is not nearly as critical with Solar power as it is with wind. You still need back up capacity with solar, and you need variable online power for ballancing, even if there are only a few clouds in the sky.
There sunlight is much more tricky than you might imagine. The problem of SV intermittency is discussed in "The Character of Power Output from Utility-Scale Photovoltaic Systems," by Aimee E. Curtright and Jay Apt, (Carnegie Mellon Electricity Industry Center Working Paper CEIC-07-05: http://wpweb2.tepper.cmu.edu/ceic/papers/ceic-07-05.asp). The paper is unfortunately password protected.
A quote from Curtright and Apt,
"The intermittency of large-scale PV power for four sites in the American southwest desert is significant, even during daylight hours. These data also imply that site diversity over a ~280 km range does not dampen PV intermittency sufficiently to eliminate the need for substantial firm power or dispatchable demand response. The high correlation between geographically dispersed arrays may indicate that high, widespread clouds are responsible for a portion of the intermittency. Observed rapid and deep fluctuations at time scales of 10 seconds to several minutes may indicate that a component of the intermittency is due to low, scattered clouds with significant opacity. We observe a number of examples of output power rising above nameplate capacity before and after deep drops in power. This may be due to focusing of sunlight around the edges of low clouds."
If the problem exists in the relatively cloudless southwest, how much worse is it going to be in the South East, when there can be a lot of clouds in the sky on "sunny" summer days?
The use of solar power for peak power is further complicated by demands for peak power at night and under adverse weather conditions. You also have to factor in the demand for peak electricity on cold, snowy winter days, and after dark in much of the country. Solar cells don't generate electricity when covered with snow. On hot summer days in Texas, AC demand continues all night. It is often in the upper 90's at midnight after hot Texas Summer days. We need peak power at night.
One other factor that can effect the reliability of solar power is air born dust. Dust can effect solar power both by blocking sunlight on dusty days, and by dust deposits both on solar panels, and on mirrors in Solar thermal mirror arrays. NASA has predicted a return to dust bowl conditions in the Southwest.
You need back up generators for the 12 hours a day the sun does not shine. Although advocates of solar power talk about night time backup systems, they require redundant generating capacity. Thus with stored solar back up, capital costs will be more than doubled. If you use none solar backup those generators will either burn fossil fuels, and thus they will emit CO2, or they will be nuclear powered. But why, if you built nuclear power plants, would you only want to run them at night? It would be cheaper to run them all the time and not worry about solar power.
Either a ST or an SV system ends up being expensive, vulnerable to clouds and dust storms, and is going to occupy a lot of territory, and damage a whole lot of vulnerable desert habitat. But hay, damaging the environment is no big deal to greens.
Building so much redundancy into the electrical generating system costs money. Taking land from other uses and committing it to renewable generated electrical power also has economic costs. Electrical grid systems that gather and transmits electrical energy from far flung renewable energy "farms" and transmits it to cities also costs money. And there is of course the cost of fuel to keep back up generators spinning. Is there a cheaper way to do thing?
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