Wind powered pump-storage plant

System

hi there,
i'm a student working on one of my small assignments where i have to propose a renewable electricity generation system to a small town. The demand to fulfill is 3 MW. I'm interested in proposing a wind powered pump storage plant.
The concept is something like when there is a high wind supply then the excess amount of power shall be diverted to the hydro station to store the water in reservoir. When there is a low supply of wind then the demand shall be fulfilled by the hydrostation.
I want some help in correcting the way I'm doing calculation on plant installation. I think that here the main source of generation is the wind turbine and that the hydrostation is serving to fulfill the spare capacity because of intermittent nature of wind. Is this thought correct?
You can also call it as the concept of capacity credit.

Secondly, if hydrostation is the capacity credit for wind power then as per an empirical formula provided by our course teacher:
capacity credit (for wind) = (installed wind capacity)^0.5
can i use the above formula to find the total MW of capacity for the hydrostation?

please help me with the above two questions, ASAP.

Thanks a lot!!

PS.
This assignment is very small scaled. It is just to examine student's thinking ability. All loses are considered as negligible and the students are free to install waht they want but the capacity values should be CORRECT.

Cariboocoot

Re: Wind powered pump-storage plant

Yes the hydro storage would serve as a kind of "battery" to store excess energy produced by the wind turbines thus mediating the output.

I have to say I don't like the parameters set up for the exercise. The losses are not "negligible" by any means, and to relegate them to the scrap heap is, in my opinion, teaching poor practices.

In practical terms there are at least the following problems to such a design:

1). Site wind. You'd be amazed at the number of wind turbines (especially small scale) that get erected on sites which really don't have enough wind to produce, much less produce surplus. Especially considering the next factor.
2). Elevation for hydro reservoir. Simply filling a tank will have limited power capacity, as the available stored potential kinetic energy will drop off quickly as the tank drains. Pumping the water uphill will give an added advantage. Hydro storage has been discussed on this forum before, and largely deemed impractical on small scale.
3). Energy transfer losses. Far from being negligible, these are significant to the design of a system such as this. Wind turbines aren't very efficient to begin with. Turning that mechanical power into electrical power and then back into mechanical power to pump water will use up much more. The friction losses of the water going through the pipe into the reservoir will not be small either. And then you have to lose more when the stored potential kinetic energy gets converted back in to electricity.
4). Demand variations. Having to "ramp up" power production on demand is a major problem for all utility systems. To be able to quickly turn the stored power back into electricity at a second's notice isn't easy. Chemical batteries can manage this, but are impractical for large-scale storage. In the real world, the system you propose would require multiple turbines and a quite complex computerized monitoring system to anticipate and "spin up" additional turbines as demand increases.

You could perhaps cut out some of the losses by using the turbines to pump directly, continually refilling the reservoir as wind permits, and supplying the electric from turbines kept spinning continuously.

As for the ability of the hydro to produce, it is a function of being able to spin the turbines at the correct speed with enough pressure behind them to overcome the resistance of the (varying) magnetic field interface. In simple terms, you either need a very large volume of water flowing very fast (geared down to provide the necessary torque), or a lesser amount with significant pressure (head) behind it.

The formula supplied by your teacher is, in my opinion, too simplistic to be of any real value. Why not ask him/her if it should apply to the hydro side as well, since this is a totally theoretical exercise?

BB.

Re: Wind powered pump-storage plant

More or less, the capacity of the pump should match the capacity of the wind turbine if you assume the wind turbine can reach 100% of its output in high winds...

The capacity figure would normally be the assumed average power output of the wind turbine--Over time which would be around 30-35% in a good wind installation... I.e., The average energy (power*Hours) would be something like:

• 3MW = 3,000 kWatts (kilowatts--units a home power meter bills in)
• 3,000 kWH * 24 hours per day * 365 days a year * 0.33 capacity = 26,280,000 kWH per year

The average western home uses around 1,000 kWH per month (with conservation, no air conditioning, and using natural gas/propane/etc. for hot water, heating, cooking:

• 1,000 kWH per month per home * 12 months per year = 12,000 kWH per year per home
• 26,280,000 kWH per year per wind turbine / 12,000 kWH per year per home = 2,190 homes worth of power

From a power system design calculation, wind is highly variable and cannot be controlled (less power needed at night, more power needed during day)... So, the typical installation (my assumptions, you can make your own) is that 100% of the turbine energy goes into pumping water up to the reservoir. This would be "opportunity pumping". You collect 100% of the wind power to pump into the reservoir (based on whatever energy is available from the wind turbine).

The hydro-generators have a stored energy source (lake) that can easily be turned on and off (throttled) to meet the demand.

Since you are told to assume no losses (normally, pumped hydro is, at best 80% efficient--which is actually very good as engineering losses go), there is no reason for you to draw power directly from the wind turbine to power the town--just power the lift pump. If you chose to draw power from the turbine to power the town (save an energy conversion loss), it could possibly be done by sharing energy with the hydro-generator--but you would have to decide if that makes sense in terms of your report.

-Bill

icarus

Re: Wind powered pump-storage plant

Question?

Which is a more efficient conversion of energy? Using wind to pump water directly via mechanical pump or using a wind turbine to produce power, convert that to mechanical energy to pump water? My intuition is the former.

In my recollection effective water pumping can be done with lower wind speeds than electric power generating. The torque curves of wind mills for power as opposed to water pumping are quite different, which is why you don't see man water pumping mills converted to power mills. (as I was lead to believe decades ago!)

Tony

Cariboocoot

Re: Wind powered pump-storage plant

Tony is right, of course.

In fact with the right location this could become practical. Massive capital outlay, but ... For a seaside town with a nearby hill (west coast NA). Put an open reservoir up the hill; could be used for recreation. Wind turbines down on the coast pumping water up as able and keeping the reservoir filled. Water flowing back down from reservoir to hydro station which would generate the electricity. Puts the major control point at hydro, which would need regulating anyway. You could add direct solar powered pumps to supplement the wind turbines too.

SolarLurker

Re: Wind powered pump-storage plant

Is the proposal theoretical or real?

Is this a real town some place or just a school exercise.

Can you add solar, PV negatively correlates with wind well.

Could you use solar thermal collectors to heat and store water, then use the heated water to run stirling engines when needed?

MisterB

Re: Wind powered pump-storage plant

SolarLurker wrote: »

Can you add solar, PV negatively correlates with wind well.

Well said. As conditions become more sunny--high atmospheric pressure--the winds tend to die down. As atmospheric pressure lowers and clouds come in, the wind comes with them. There are exceptional places where it's always windy and deserts where it's always sunny but most places both come into play at different times and rythms.