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Friday, July 27, 2012

Peak Shaving

Peak Shaving:  The production of power to take care of the highest electrical load that comes on the system.  It can involve having a stand by plant ready to fire up during peak demand.  It can involve having extra generating capacity in a hydro electric dam so that for short periods water can be used faster than it is entering the dam to take care of this short term load or it can involve pumped storage in which water is pumped into a reservoir during periods of low demand to be used for peak shaving.  There are also ways of avoiding having peak demand and that is what this blog is about.

The blog came about because of an announcement by our New Zealand Commissioner for the environment that solar water heaters are not the answer to our energy problems.  She argued that since solar water heating is at a nadir just when we have our winter peak demand, it doesn't eliminate the need for a dormant stand by generator to peak-shave. Stand by power generators are very expensive since you have to bear the capital costs and the return is poor because of the limited time that the equipment operates.   In this the commissioner is completely correct.  An electrical grid must be able to supply the greatest load that is demanded.  A few cloudy days in New Zealand in the middle of a cold snap and our electrical need surges and we are heating water with electricity rather than sun.  With our present electrical system, she definitely has a point.  However it is somewhat blinkered.

First, and simplest, whenever we heat water with sunshine, we eliminate the use of coal to do the same thing.  This reduced our carbon foot print and since we were generous enough to be one of the first to sign up to Koyto, it is rather expensive for a small country to have a carbon footprint.  Not only does coal produce the most carbon pollution per kWh produced but it is a far too valuable resource to be burnt.  Coal can be used as the feed stock for a huge range of products and if used for this purpose instead of being burnt, would last for thousands of years.

Of more importance, though, is the ways we can eliminate having peaks of power demand.  This involves a number of measures, all of which shift electrical demand away from times of peak energy need.  Smart grids are a large part of the answer.

The best incentive to get people to use power when it is available rather than exactly when they would like to use it is price.  So first we set up a system of varying power price depending on demand.  We have, say A)Power on Demand.  This would be for your lights and your computer which you want to be able to switch on when you need them.  Power on demand would be at the full 22c per kWh no matter when you used it.  B) Option 1.  Option one would be somewhat cheaper.  A Signal would be sent down the line or over a dedicated phone chip that Option 1 power is available.  The power company would send this signal when they had some extra generating capacity they wanted to sell.  Perhaps their dams are full and they would rather sell more power than have the water run over the spill way.

The home owner would have smart appliances.  For instance, he might set his hot water cylinder to option 1 and would only heat water when this option is available.  However to fine tune the system there should probably be a few more levels.  Lets say Option 2, 3 and 4.  Each one is a little less expensive than the previous one.  Each one is signaled by the power company when they have extra capacity and the consumer can dial which option they want on each electrical device they have in the house.

The home owner will also have a read out somewhere in the house to show which option is on offer at the moment.  The smart retired couple, for instance, would watch for less expensive power and do their bread making, hedge trimming and so forth when power is less expensive.    Our equipment must also have the options Take When Available and   Once Started Don't Stop.  Your water heater can turn off and on as often as the power priority changes.  Your dish or clothes washer is a different matter.  When you start the cycle, you don't want it to stop until the cycle is finished.  You might end up doing part of the cycle at a price greater than you dialed  and hence the Once Started Don't Stop option.

The benefit of all this is that power use is shifted away from what formerly were peak hours and the generating capacity of the grid is used more effectively.  The more sophisticated the smart grid, the greater value of all forms of renewable energy, including solar hot water heaters (and electric cars).

Of course a further option is pumped storage.  It is typically between 70 and 87% efficient and New Zealand is probably very suitable for this option due to its high mountains and hills.  Renewable energy becomes particularly effective when you have this form of "battery".

1 comment:

Matthew Whittaker said...

Far easier, just use hydrogen as an energy store in a similar way as backup plants are used at the moment, your suggestion does help with the peak but if reducing CO2 output is your goal then its a misnomer due to the energy required to completely update the grid. Decentralizing the grid could help too. Rather than redesigning the grid (which has massive energy costs woven in) just store excess energy in hydrogen, much more versatile than other stares and a significantly reduced impact than hydro electricity of fossil fuels. This way you can implement decentralised production (solar, wind etc) and use the excess to store hydrogen for using during peaks. I like the suggestion of people being smarter about energy use such as how tokyo coped with fukiashima but we have to recognise the energy required to redo the grid would negate its impact and where we are using energy at the moment it is invariably fossil fuel. This way we can mitigate the volatility in production of clean energy via storing excess in hydrogen for use during peaks (cold winters night). No matter what you do you will always have peaks in winter and lulls in summer and inversely the most energy (certainly via solar, the starting subject) is available in summer. Using hydrogen as a store we can take the excesses in summer and save them for winter kind of like squirrels :)