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Any other 'bakers remember Abhi's water tower? |
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You mean the desert-sited anno-generator? |
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thumb: No... are you sure it was Abhi? |
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islander: I've wondered about this myself. There are three questions you have to ask yourself when storing energy:
1) How efficiently can I convert the energy when storing it?
2) How much is lost over time when storing it?
3) How efficiently can I convert the energy back when I need it? |
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Not having the answers to any of these questions, I'll leave it there... |
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half: The problem I have with going solar only where I live is we tend to have extended periods of rainy and overcast wheather...particularly during the monsoon months. |
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st3f: I agree, obviously I'd have to do alot more research to find out how much energy can be generated with an artificial "waterfall" and how much energy it would take to recirculate it (I think this will be my biggest obstacle...I think I'll need several solar water pumps depending on how high the tank is and the volume of water that needs to be recirculated). My premise for this idea came as I was searching for a standby generator for my house. I got to thinking about how I could use natural resources instead of a motor to turn the generator. I'm open to any suggestions on how I could possibly achieve this. |
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Potential energy stored in your water tank:
mass * gravity * height
or
weight * height |
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Thus, 100 gallons of water 50 ft high is 56,549 Joules of potential energy, or about 15.7 watt-hours. |
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Note: the following method uses back of envelope calcs and web research - take with grain of salt. |
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1. Search for household electricity usage: Washington residents use 18kWh/day.
2. Search for pump efficiency (Ep): around 60%
3. Search for motor efficiency (Em): around 80%
4. Search for solar panel energy per square meter: around 0.16 kWh per day in Los Angeles
5. Search for turbine efficiency (Et): around 65%
6. Search for alternator efficiency (Ea): around 45%
7. Create equations:
For ease and because this is an international audience, let's use metric units. To get 18kWh/day (PE), you'll need that amount of energy from stored water, reduced by the listed efficiencies. Let's assume your storage tank is 10m above your turbine (head). The potential energy in water at this height is mass * head * g. Reducing this by the appropriate losses, we get PE = m * h * g * Ep * Ea. Solving for the mass of water we need: m = PE / (h * g * Et * Ea) = 18000 * 60 * 60 / (10 * 9.8 * .65 * .45) = 2,260,600 kg of water. Divide that by the density of water (1000 kg/m^3), and you get 2260 m^3 volume (596,640 gallons). Tripple this to make sure you have enough for a cloudy day or two.
<note>the final volume of water seems a bit off to me - I'll check my math when I have a minute</note> |
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Now, how large does your solar panel have to be? To reverse this process and pump the water back up, you'll need the same amount of energy (18 kWh/day) reduced by the pump and motor efficiency. E = PE / (Ep * Em) This leaves us with 37.5 kWh/day. Since we get about 0.16 kWh per day per square meter in Los Angeles and I used Washington for the daily energy use, let's cut this number in half for 0.08 kWh per day. This leaves us with around 470 square meters required. At about $200/square meter, that's $94,000. |
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End result: possible, but expensive. |
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worldgineer, lurch: Thanks for the info. |
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worldgineer: I was with you up until the end...what is the significance of Kwh per day per square meter?...is this in reference to the amount of surface area of solar cells required? If so, then how about I supplement the solar charge with a automobile alternator? Would you know if it would be possible to help draw the water back up if I sealed the system (and thus no new air would enter) and used the weight of the falling water to draw the water back up? |
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FarmerJohn: I considered the steam concept but I don't think I'd ever be able to evaporate and condensate enough water to keep up with the volume of water that will be needed to turn the turbine. |
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//what is the significance of Kwh per day per square meter?// Yes - this is the area of solar panels you'll need. Sure you can use a gas-powered generator to supplement, though you might want to just switch to a gas powered pump (or get rid of the whole system and switch to a generator) if fuel costs/environmental issues aren't important to you. |
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//draw the water back up if I sealed the system// No, you're trying to get something for nothing here. Plus, if you seal the system you get rid of the storage value of this system - your pump will immediately turn your turbine, and your turbine will only turn when the pump is going. |
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Another possiblity: collect rain to help power this, since you'll need a large tank anyway. |
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the basic problem is that you will lose 50% of the energy that
you produce from the solar panels
converting it to hydro electric. |
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generally speaking that's too much a loss to size an economical
system however using water storage tanks at a higher head is a very ecological solution for power storage (as opposed to batteries) and clever. |
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two possibilities exist
though... |
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1) a house that ran on 12V or 24V DC instead of 120V AC with small loads probable could be sized for a system with 50ft of head but sizing your tanks and your pump(s) will depend on the the typical loads you run in your house and how much "reserve" energy you want and how much capital you have for the "real" source of the energy (PV solar panels). |
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2) A house that was wired for 120V and was tied to the "grid"
could also be sized for such a system. the basic concept though is completely different
to make it efficient... |
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the basics for a grid tie net meter system is that you produce energy and sell it to the power company at the rate you produce it. When you use power you buy it back from the power company. The "big" economic advantage is that during peak hours the price per kWh is 4X higher than off peak. |
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that means if you run you water pumps at off peak rates and run your turbine and solar panels at peak times you can effectively produce 4X the electricity as expected...or means the over all size of your system can be 4X smaller and more cost effective...it would seem a 500W hydro turbine could then produce 2kW of off peak power which is pretty decent for a small home but since peak hours are 6 and you need 100 gal per min to run turbine at 50 ft head then you need tanks
that are 36000 gal in capacity and 8 in pipe... |
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so it would work and if your in ca you can get incentives to deal with about half of the costs... |
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You are just speeding the process of rain. Aren't you? |
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Checking Worldgineer's math....It would be much more efficient to directly use the solar power rather than pumping water up to re-release that water again. (Energy efficiency of solar panel = 0.15?) Now, how large does your solar panel have to be? |
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18kWh/day = (0.16 kWh/m^2*day) * (0.15) * (0.80) * (0.60) * (0.65) * (0.45) * (square meters needed) |
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5341.9 square meters needed |
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P = (5341.9 square meters) * ($200/square meter) |
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More like $1,068,376.07 for the solar panels alone. Not to mention the highing contractors to build a damned reservoir, installing the pumps, valves, and panels, the cost of operation and maintenance, etc. Nuclear is far cheaper and more efficient by comparison. |
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