A Dam failure in Missouri
Posted by Heading Out on December 14, 2005 - 10:35am
The Lake is part of a pumped storage system put in for balancing electric power supplies. Essentially, when demand is light, the turbines pump water from a lower reservoir up into an upper lake. Then at times of high demand the water flows back through the turbines generating 350 Megawatts of power.
The system is useful since the use of coal power for electricity generation is best applied to systems that run under steady load. Load however fluctuates during the day. Thus the storage and then use of power using the pumped storage concept is, to a degree, similar to a large battery in the system. The power station can run under steady load, and the variations are provided for by running the turbines in either direction depending on the level of demand.
Oil and gas fired power plants are more flexible and thus do not require this type of control, since the fuel flows can be more easily controlled. (Think of the difference between a coal fire and a gas stove).
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Thanks, John
It's hard to find a sandstone section that can hold pressure in the tilted mess that composes a mountain range. Using a secondary reservoir is probably the only thing that makes sense in that location. Since the jury is still out on the failure, my 2 cents is that it is ice wedging in local strata that caused weakening over time. Only way to stop this is to grout all the cracks in the reservoir, and with minor earth shakes weekly, that may not be realistic.
Combine them with a substantial amount of load from (partially or completely) electric vehicles, you could make the load curve pretty much flat all day, every weekday. This would let hydro systems operate in "run of the river" mode. New systems without reservoirs would vary their output strongly with the seasons, but if they produced enough energy to justify the capital cost, you couldn't beat the price of the fuel.
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Since fuel costs decline from oil to gas to coal to nuclear while capital costs increase from oil to gas to coal to nuclear (typically), the high capital cost/low fuel cost plants should run as much as possible while the low capital cost/high fuel cost plants should run only when the price of electricity exceeds the cost of fuel.
What that means in practice is just as described - nuclear is must-run, coal is intermediate and oil/gas supplies the peaks. For a nuke to load follow is like having a big mortgage on your home and spending 6 months away in your camper, leaving your expensive home unoccupied.
A run-of-thumb for pumped storage is you get 3 MW out for the 4 MW you put it.
BTW, the Helms Pumped Storage plant in the Sierra Nevadas was built in tandem with the Diablo Canyon Nuclear plant on the California coast. Nukes and pumped storage make a great pair. Unfortunately dams are much riskier than nukes.
Dispatchability, as you seem to be indicating is true about different kinds of implementations of nuclear, is a continuum rather than two end-members. I'm curious where coal really fits into the picture. Thoughts? Thanks!
Old coal is from old, dirty plants that are fully amortized or close to it - the mortgage is paid off. Their cost is mostly variable operation costs, mostly fuel. These plants may have annual emissions limits so often can't run full out all year. Plus, they require a lot of maintenance as coal plants get a lot of wear-and-tear.
New coal is bigger, has more pollution control equipment, and has a big mortgage. These need to run more hours to cover their fixed costs to carry their capital.
The economics of dispatch would then run new coal as often as possible and old coal just when the market price is above the variable costs.
They both can be dispatched but old coal will get turned down before new coal. Nuclear will get turned down rarely.
As to wind, it is non-dispatchable since its variable cost of operation is zero plus most governments require it be treated as "must-run". However, it has little or no capacity credit since the winds can die down without warning. The burden for the grid then is that wind must be backed up with "spinning reserve" - a power plant operating at less than 100% load that can pickup whatever wind drops off - without disrupting the frequency, voltage, VARs, or critical transmission line(s) on the grid.
Wind is a pain! It's only advantage is its zero fuel cost so it can be credited only with the fossil fuels that don't get burned minus the cost of spinning reserve. Plus, you can't site it anywhere you want so the transmission lines have to operated to allow for its ups and downs - that lowers the transmission system capacity and flexibility.
Having more than a few percent wind on your grid is asking for trouble and higher costs since the tail soon wags the dog.
What I have heard about single or small groups of MW power wind powerplants is that they increase the quality of power for the neighbours connected to the same distribution line buy giving a source of reactive power and voltage stability when there is wind. When there is no wind it gets as bad as it use to be anyway.
But the grid feeding the distribution line must be solid enough to not care much about sudden additions or withdrawals of a few MW.
I do not know how different grids are run. The spinning reserves and grid in the nordic countries are built to handle a sudden loss of the largest generator, busbar or powerline, about 1200 MW. The 1600 MW EPR being built in Finland means that this has to be uprated. I have not heard anything about costs for adding more spinning reserve only new high tension lines. I guess we have unused margins in the hydro powerplants. There is a non spinning reserve in gas turbines and hydro powerplants that can be started within 15 minutes to absorb another worst case fault after 15 minutes. ( These margins do however no loger exist if it gets realy cold due to too little investments, "greens" closing two reactors and two few spare thermal powerplants since the deregulation of power production. )
These kind of margins seems to be the prudent ones to have. They should enable the use of a fair ammount of wind power before needing enlargement.
If sudden loss of all wind production occur seldom you could have as much wind power as your largets single producer or rather spinning reserve wich is your point. If if is possible to forecast for 15 minutes or longer it gets even better.
The grid bottlenecks for more wind power is over here usually in the 20-130 kV network. Its not built to handle large loads/inputs in the areas where the wind is good and few people are disturbed by the turbines. The cost for strenghetening the grid can be a large part of building a group of wind turbines and there is a debate on who should pay for it.
The economical bottleneck is roughly that they need to get half as expensive to build or electricity at least twise as expensive to buy.
EVERY well-designed grid ensures that its largest unit is less than 10% of total capacity - that's an old rule of thumb.
Systems also have to have "blackstart" units. Most power plants require major external power supplies (usually from the grid) to start or restart after a trip.
A lack of adequate numbers of blackstart plants made recovery from the last Northeast North American grid collapse much more difficult and lengthy. I hope someone is fixing that.
Here in California, the utilities maintain hydro units with some resevoir capacity that will automatically start spinning and come up to voltage when the connected grid goes dead. Then the system dispatcher can use it to bootstrap the other tripped plants. In our case, the nukes get first call to backup the diesels.
In addition, one can design the plant for what's called "net load rejection" where the output breakers to the grid can open but the plant keeps running supplying its own "hotel loads" so that it can immediately be reconnected to the grid. However, this is a difficult trick and doesn't always work. It's really difficult for a nuke!
But when the percentage of wind power becomes so large that you need more spinning reserve you need to build a MW of reserve for each MW of wind power and keep is spinning wich is very bad for the economy. You can work around this if you have large loads that can be shed quickly like hydrogen production or heating of houses with a large thermal mass.
Nuclear net load rejection: (I have have gotten it right, I have not worked with it. )
When the load disappears the turbine runs like a vacuum cleaner with a clogged pipe, it overspeeds due to lack of resistance to its movement. If you quickly close the steam valve to the turbine you get a preassure spike in the steam lines due to the living energy in the steams movement, as in the "thud" noice in a water pipe when you quickly close av faucet. You have to close the valve and get rid of the steam so you dump it directly to the condenser thru another valve. But the condenser is built to get rid of the left over heat and condence the steam to water after the turbine has taken all that it can so this is marginal for the condensers capacity. The preassure in the condencer rises and this is not good for the turbine. You quickly have to get the reactor to deliver less steam. Closing the valves helps partly since it gives a higher preassure and with a higher preassure you get less boiling and more of the energy stays in the hot water. Then you need to get the nuclear reacton from adding so much energy to the water. In a BWR you slow down the recirculation pumps that forcing water into the reactor core. This gives a smaller percentage of liquid water in the core, less moderation, neutrons dissipate out of the core and the power output goes down. Phew!
I do not know how it is done in a PWR. In a BWR the control rods are mostly used to form the active region in the core and I think it is the same with a PWR and the power output is regulated by adding a neutron absorbent to the water, boron. Inserting the control rods untill power output falls and then withdrawing them about as much must be the quickest to do. Whitehall, is this correct?
This is obviously a tricky procedure where all parts of the control system has to do the right things within fractions of a second or you get overspeed on the generator or overpreassure somewhere in the system and then an automatic shutdown, it also shuts down if you overcompensate.
There has been two large grid failures in southern Sweden since I started to learn about different kinds of engineering. During the latest the nuclear reactors had about 75% success with the net rejection.
Another problem is that you cant run the reactor on two low power our you will get an accumulation of neutron absorbing substances that hinders a power increase even if you withdraw the control rods. Then you have to shut down and wait for them to deacy naturally, you get the same need for a pause after an emergency shutdown. So a nuclear powerplant leaving the grid can not come back immediately.
You seem to have a pretty good handle on these issues!
Yes, net load rejection is a difficult transient for a big nuke. BWRs with this capacity have to have oversized condensers and integrated and anticipatory control systems on the reactor, the turbine control valves, the generator, and the turbine bypass valves.
On PWRs, they can blow off steam to the atmosphere and dump heat into the condensate system. The first time we tried this transient in a test at Diablo Canyon, when the relief valves opened, the jets of steam were so strong that the siding panels on the turbine building got sucked off and flew a mile through the air! Imagine dumping 3500 megawatts of thermal energy into the air - a million pounds/hr of high pressure steam (850 psig) - talk about loud!
For both designs, xenon buildup is accomodated with initial excess reactivity. It can be a problem if one is near the end of a fuel cycle and is in coast down mode. One may have to start a refueling outage early but this sensitivity is limited to the last month or so of an 18 month fuel cycle.
As to the success rate, it's definitely not 100%.
One can have lots of wind on a grid - you just have to pay for it in spinning reserve, grid remote controls, and transmission line upgrades. Above a few percent, it is definitely has declining marginal utility.
I got what I concider technical general knowledge and since most things are built with the same physical building blocks its only to piece together the puzzle och perhaps put togheter an new one with the pieces on hand. I can bet you a beer on being able to describe the overall function of more then 50% of any random system in a powerplant. But I might need a dictionary to do it in english.
Understanding a process, its components and how everything is interrelated is the easy part. The hard part is to describe it in math and then optimise it.
A PWR can have as large a condenser as a BWR but why build it if you can dump non radioactive steam to the atmosphere? Its not acceptable for a BWR since there is no heat exchanger between the reactor core and the turbine island. (I dident think about that option for PWR:s, this is not a throughly overworked texts. )
Perhaps you can tell med why PWR:s dont have a steam condensing pool?
BWR:s have steam dump pools where steam from an insulation vale closure of the get blown thru overpreassure alves into a pool of cold water inside the containment where it condenses. (I write this to keep it intresting for the general reader. Did you get my email? )
As far as I know PWR:s have a larger containment withouth such a pool and dump the steam inside the containment. The containments preassure can then be lowered by pumping sump water from the bottom of the containment thru sprinklers in the roof. The size of the containmnet is probably due to the need to have the heat exchangers, that is steam generators inside it and to be able to lift components for service.
Do this automatically give enough volume to make blowdown into a pool unneceserry? It seems like it would be a nice passive system to keep the preassure buildup down and give less wear on installations inside the containment.
A BWR has a much larger volume of water within the vessel so the added complexity of a "suppression chamber" is worthwhile and much cheaper than a pressure containment. That allows the flashing steam bubble through an internal pond of water and condense.
Note that these are separate from the steam condenser under the main turbine which is part of the power production cycle. We can make use of it for safety issues like net load rejection.
http://www.whatdoesitmean.com/index862.htm
Skinner said reports of the geysers were first received Friday by hunters who reported the gushing holes of gas, water and mud to the local game warden, who then contacted the Corpora-tion Commission.
http://www.enidnews.com/localnews/local_story_347005144?keyword=topstory
Must be some of that abiotic methane I've been hearing about ;)
This means that there were no outlet for the water when overfilling. Its enourmously stupid to not have a safe overfill outlet with more capacity then the pumps. This is absolutely basic for regular hydro dams.