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  #41  
Old 01-05-2010, 04:17 PM
popcorn_karate popcorn_karate is offline
 
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Default Re: The Next Rare Earth Crisis

Quote:
Originally Posted by AemJeff View Post
I haven't totally given up the idea that smaller scales might be workable. The 75% efficiency achieved at Dinorwig surprised me. I have no idea how to quantify the amount of power that could be generated by a water tower size pair of tanks - but, if you powered the pumps with combination of solar and wind, e.g., the cost of storage seems pretty reasonable. If it's possible to generate useful power on that scale, then there might be the germ of a workable idea here. That said, I hear what you're saying.
i loved this idea for a while (water battery) but i do think you would need some good topography to make it work.

15 feet of head
10,000 gallon tank of water
5 cfs
= 3.5 kw for 5 minutes

head(ft) * cfs * efficiency (0.55 for a good system) * 0.085 = kw

1cfs = 448 gallons/minute

the only way to make it really work is to have something closer to 200 feet of head.
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  #42  
Old 01-06-2010, 02:39 PM
cragger cragger is offline
 
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Default Re: The Next Rare Earth Crisis

The energy is dependant on the head, but that is not the only variable. There are many dams, including one near me, that don't have anywhere near 200 feet of head. More is good, but not necessary. Note that in your example, a 10k gallon tank is only about a 10 foot cube. It's not a lot of water for this application, though your figures do support your overall point, with which I agree, that the technique doesn't scale well down to individual home use.

On the other hand, playing with the numbers a bit you can see the merits of the approach. Even on a home level,

3.5 kW x (1/12 hr) x 30 (days/month) ~ 8.75 kWh.

so if one had for example a slightly bigger tank, 15' cube ~25k gallons, you would get nearly 22 kWh/month. Still obviously not a lot of energy, but for certain isolated applications that might be enough to say, keep the refrigerator and freezer running overnight and allow for some nightime lighting if one used solar/wind to fill the tank and did energy demanding activities during the daytime (assuming solar generation). Scaling up a bit more by considering a water store the size of a backyard swimming pool, and you now may have something pretty decent.

There is of course an advantage if you have even a modest topographic aid here. If you have a 20 or 25 foot slope to work with, one could use something like swimming pools for the store - i.e. the dig a hole and drop in a liner type pool, for cost reasons. This would save you having to build a big strong tower and buy steel tanks. It's not hard to picture a system that would store 100 kWh or more per month, probably enough for most home users without worrying too much about being careful about usage.

Without costing out systems, I suspect cost efficiency is one limiting factor in doing this at the individual home scale, though again of course not necessarily the only problem for those on flat, 1/4 acre lots in the local subdivision. I also wonder about the efficiency factor of the energy recovery turbine. At a home use level, one would generally want to draw power at a pretty low rate, well below the 3.5 kw figure you used, and I suspect that there is a minimum flow rate that is required for any given turbine to reach its theoretical efficiency rating. Batteries being expensive and of limited service life, perhaps some hybrid system involving a smaller battery store augmented by a water-based potential energy store would be best, reducing both the dynamic range of efficient output required for the hydro system and the number, size, and attendant cost of the batteries.

All a somewhat long winded way that while one doesn't want to dismiss this out of hand as an option for some users, the method seems better suited in general for larger scale applications at the community level and above.
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  #43  
Old 01-06-2010, 05:11 PM
popcorn_karate popcorn_karate is offline
 
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Default Re: The Next Rare Earth Crisis

Quote:
Originally Posted by cragger View Post
The energy is dependant on the head, but that is not the only variable. There are many dams, including one near me, that don't have anywhere near 200 feet of head. More is good, but not necessary. Note that in your example, a 10k gallon tank is only about a 10 foot cube. It's not a lot of water for this application,
here's a 10k tank you can buy - check out the dimensions and price.

Part Number: T10000IW
Capacity: 10000 Gallons
Size: 141"D x 160"H
Weight: 1803 lbs.
Ships From: CA, AL

(oops - $2400.00 is the price)

so, you would be elevating 82,000 pounds. by the time you build something to support that amount of weight and buy the tank - you are already deep in a financial hole that will probably never be filled.


Quote:
Originally Posted by cragger View Post

3.5 kW x (1/12 hr) x 30 (days/month) ~ 8.75 kWh.
given the assumptions in the original calculation, you would need a 180,000 gallon tank - the cost of the tank would price you out before you tried to elevate it.


Quote:
Originally Posted by cragger View Post
All a somewhat long winded way that while one doesn't want to dismiss this out of hand as an option for some users, the method seems better suited in general for larger scale applications at the community level and above.
or someone with significant topography on their property. you could build a holding pond, skipping the expense of a tank, and use the topography for the head, avoiding the need for a tower.

i was totally stoked on this idea for about 6 months before doing the calculations and realizing i really needed to get onto another idea, at least in my location.
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  #44  
Old 01-06-2010, 06:21 PM
cragger cragger is offline
 
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Default Re: The Next Rare Earth Crisis

You miss the point of the "x 30 days" in the calculation. Using the example of solar generation, you would re-fill the upper water store every day using solar power. You need to store enough energy to get you through to the next generation period, not build up a month's-long store at once. Looking at things on a month long scale just seemed to be useful since we typically get electric bills on a monthly basis, so there is a little "feel" to that sort of timescale.

This neglects extended periods of bad weather/no wind or whatever of course, just back-of-the-envelope stuff that shows theoretical potential, though not necessarily at a $/kWh cost that is attractive. On the other hand, a large energy store using current deep discharge batteries isn't what you would call cheap either.

This leads to the point Osmium was alluding to in looking at this as a grid-level question. Like many things, a cooperative approach is likely the most efficient. I favor a hybrid system, with millions of small points of power generation going down to the home level, combined with large generation sources on the grid as well - dams, wind farms, wave power, etc. once the inevitable transition to renawables happens. I would envision the majority of storage for load leveling would be at scales above the home.

Afterthought: one readily sees that combining the functions of conventional dams with the addition of recycling of water as energy stores provides obvious synergy as a very efficient partial solution.
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  #45  
Old 01-07-2010, 05:04 PM
popcorn_karate popcorn_karate is offline
 
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Default Re: The Next Rare Earth Crisis

Quote:
Originally Posted by cragger View Post
You miss the point of the "x 30 days" in the calculation.
or perhaps you missed that you would need a 180,000 gallon tank to run it for 1.5 hours each day of the month as you suggested?

Quote:
Originally Posted by cragger View Post
Using the example of solar generation, you would re-fill the upper water store every day using solar power. You need to store enough energy to get you through to the next generation period, not build up a month's-long store at once. Looking at things on a month long scale just seemed to be useful since we typically get electric bills on a monthly basis, so there is a little "feel" to that sort of timescale.

This neglects extended periods of bad weather/no wind or whatever of course, just back-of-the-envelope stuff that shows theoretical potential, though not necessarily at a $/kWh cost that is attractive. On the other hand, a large energy store using current deep discharge batteries isn't what you would call cheap either.

This leads to the point Osmium was alluding to in looking at this as a grid-level question. Like many things, a cooperative approach is likely the most efficient. I favor a hybrid system, with millions of small points of power generation going down to the home level, combined with large generation sources on the grid as well - dams, wind farms, wave power, etc. once the inevitable transition to renawables happens. I would envision the majority of storage for load leveling would be at scales above the home.

Afterthought: one readily sees that combining the functions of conventional dams with the addition of recycling of water as energy stores provides obvious synergy as a very efficient partial solution.
agree with this stuff. distributed energy generation and storage is where we need to be going.
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  #46  
Old 01-07-2010, 08:27 PM
cragger cragger is offline
 
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Default Re: The Next Rare Earth Crisis

Quote:
Originally Posted by popcorn_karate View Post
or perhaps you missed that you would need a 180,000 gallon tank to run it for 1.5 hours each day of the month as you suggested?
Perhaps. From your BOE calculations, the 10k gallon tank delivers:

3.5 kW for 5 min - close enough for BOE for sure

3.5 kW x 5 min /60 min/hr = .2916667 kWh/day reflecting the energy recovered each time you run the upper tank through the generator.

.291666. kWh/day x 30 d/mo = 8.75 kWh/mo

for the 10k gallon tank you suggested. Perhaps you are referring to my suggestion that with a slightly greater head, and a larger water store one could easily envision a home system with a storage capacity in the neighborhood of 100 kWh/mo?

100/8.75 ~ 11.4

suggesting you need a bit more than an order of magnitude increase in head, gallons of storage, or some combination thereof. i.e gallons x N and head x P with N x P ~ 11.4 < the factor of 18 as you suggest. (i.e. 1.5 hours/day = 90 min at 5 cfs = 18 x 5 min) , presumably how you got to the 180k gallons figure). As I recall I suggested that a combination of a modest increase in head (20' vs. 15' gives a factor of 4/3, and a larger water store such as swimming pool size: 11.4 x 3/4 x 10k gallons ~ 85.5k gallons for example). Or bump the head up to 25 feet. So on.

Now my algebra and ability to click the right buttons on Microsoft's calculator are both always suspect so please feel free to expound in greater detail on what error I am making here. Lacking such, we could pretend to be physicists who, being within an octave let alone an order of magnitude, could consider ourselves to be in agreement that the technique seems applicable, but of questionable economic efficiency at the household scale outside of specific users with strong economic motivations and/or situations likely including topographic advantage in reducing implementation cost. On the other hand, a technique of such efficiency at larger scales that we are a rather sad and stupid bunch if we fail to use it collectively at the grid-level scale.
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