[q]I’m considering design requirements for a cistern system that will utilize two sets of storage tanks. One would be for primary storage, and would be down hill from catchment sources. The other would be the secondary storage, and would be up-hill from the primary storage tanks and dwellings. My thinking is that I’d utilize the secondary tanks for potable water and for generation of electricity; the electrical generation would be by locating a generator along a return line to the primary storage tanks. During the nighttime I’d let as much water from the secondary tank run back into the primary tank (for electrical generation). During the daytime I’d have a solar- powered DC pump refill the secondary tank from the primary tank. Somewhere along my design path I’d have to identify just how much water to retain in reserve in the secondary tanks (to cover for cloudy days [which there shouldn’t be too many where I’m looking to locate to]).
OK, that’s what I’m up to. Now my questions:
1) Is it possible/practical to (re)claim snow? Rainfall is looking to be around 20 inches per year, with about 6 in the form of snow. Does anyone do this?
2) When discussing water pressure generated via gravity, is this calculated from the bottom (pipe/plumbing output location) of the tank? Is the height of the tank not a factor? I realize that because of the variance of water levels one cannot base height on the top of the tank, but could one do so based on an average water level? This point might be a bit moot if the height of the tank isn’t very great (but 20 feet or so could make a difference).[/q]
[a]
A quick and dirty way to retain a necessary minimum of water in the upper tanks might be to place the return/power generation outflow at a level higher than the necessary reserve. That way it could not fall below the reserve level. Snow pack is the source for most of the spring runoff across the US, so of course, it can be reclaimed, but it creates new problems in keeping the water clean. I don’t know if it is economical, since 6 inches of snow is just over a half inch of water, under local conditions here. Using the potable water for generation would cause you to have to upgrade the standards throughout the system to potable water. Perhaps a smaller potable water tank and a larger storage/generation tank might be in order. I think upgrading the entire system would cost a lot in terms of upkeep and continuing costs, although a separate potable water supply might cost more in capital investment. One of the things that drives me crazy (because of the extra expense) is that throughout the US, communities are chlorinating and fluoridating the water used to flush toilets and water gardens! [/a]
[a]
It would be a lot simpler and cheaper to just store your energy in batteries, and use it directly from them. I was once planning a similar system, for my very first Solar house, way back in the mid 1970s. However, I had a whole lake, officially the cleanest in my state, at the bottom of my fifty foot hill, and I was going to dig a pond, in front of the house, at the top. It was a windy site, and I was going to use two water pumping (mechanically) windmills to get the water up the hill.
I see some potential problems with your proposal:
1) You may not be aware of how little power you will store per gallon-foot, and that a tank will likely not be nearly big enough.
2) You have probably not properly researched Solar electric, to realize the low efficiency and high cost.
3) Your efficiency will be extremely low compared to other storage systems, and other alternative energy systems. There are many better ways to spend your effort and money for saving energy, which will actually be a good investment, and will actually save you money, over using fossil fuels.
Here is a simple formula, which will help you calculate the storage potential of water and gravity:
GPM x ft drop /8 = watts
watt minutes/60,000 = kWh
1 kWh = 480,000 gal ft
1 kWh = 9600 gal dropping 50ft
Into that you must factor pump efficiency (maybe 40%) pipe friction losses (will vary greatly depending on size, volume, length, horizontal run, etc (say 20%), generator/ /turbine efficiency (maybe 70%), electrical line losses, plus an additional 10% misc. losses. In the end you will be using up almost all (80+%) of your collected energy, which is low with PVs to start with, just to store it that way, compared to losing less than 25% with batteries and an inverter.
In contrast, the similar system I had considered, used windmills that would cost much less, and would be much more efficient than photovoltaics, so that it could afford some inefficiencies in other places. It would be pumping water directly, so would have no electrical line losses, or battery losses, or their initial costs. A pond will also economically store the minimum 100,000+ gal, of water needed, at a relatively low cost, and in many areas would also qualify for tax credits. I figured that, if we were fairly energy conscious, that we would still have needed as much water storage as 100,000 gal/day, moving up and down our hill to get 4 or 5 kWh. Your system would require much more, unless you use extremely little electricity, in which case you have even more low cost, cost effective options.
Bottom line: You would be much better off with 2 L-16 batteries, to store 2+ kWh, than a 100’x45’x3′ (average) pond, running down 50′, through such an inefficient system.[/a]
[a]
there is one way to make your pumped storage proposal a little cheaper. The solar motor/pump will function as a generator when run in reverse (there is a lot of info around on how to do this). Ergo, you do not need to invest in a separate turbine/generator set, just arrange some by-pass valves to direct generation flow around the pump check valve and your dream can come true. Of course some electrical protection circuits and maybe an inverter will also be needed, but you were going to build the rest of the system anyway, right? [/a]
[a]
OK, yes, I think I see the reasoning. Being yet a novice in all of this, I was thinking that I’d be able to set potable water filtering on the output of the secondary tanks, between them and the dwellings. But perhaps a third set of tanks would be necessary. So, to revise: Primary tanks: repository for initial catchment; direct source for agricultural needs (non-dwelling use), and (circulatory) feed to secondary storage; located below dwellings (catchment surfaces); input filtering for large debris (size?); total capacity not yet determined Secondary tanks: source for “evening” energy and for feeding potable water tanks; located above dwellings and tertiary tanks; capacity not yet determined (volume to refill teritary tank usage and generate required energy) Tertiary tanks: potable water; located between dwellings and secondary tanks, and at a hight above dwellings sufficient to provide for gravity feed pressure (no pumps); input filtering sand(?); output filtering TBD; capacity TBD (3 days times max daily usage- to allow for low sunlight days and maintenance issues).
NOTE: The use of multiple tanks per stage provides for uptime/redundancy and maintenance activities. Filtering is still something that I’ve got to get a handle on. As I’ll need to pump water up to the secondary tank I’ve got to consider the filtering requirements for the pump. I haven’t determined whether I’ll have a composting toilet or not. Heck, I’m struggling to convince my wife that we can get by with a 1,000 square foot home (down from 1,300), discussing a composting toilet at this stage of the game wouldn’t be wise :-) The water numbers might cure the toilet issue: which would you prefer, a shower or a flush toilet? :-) And then there’s the issue with having a wood cook stove… Does anyone know of any in-line (pipe) hydro generators? These would be used to generate “evening” power. Once I’ve determined evening energy requirements I’ll need to map this back to the pump specification, which would then be used to determine the secondary tank storage size (how much water will I expect to need to return to the primary tank [on average] each evening). Also, does anyone have any best practices for ensuring that feed and delivery systems don’t freeze up? My location of interest experiences wintertime freezing (not severe though).[/a]
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