06-16-11, 02:28 PM | #1 |
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Designing & building a solar hot water tank
With the solar panel racks designed to my liking, I am moving on to designing the tank for the solar hot water setup. I think I'm actually going to build the water tank first as I can use it as a temperting tank for my domestic hot water until I get the solar panels installed and working.
The tank will be made of wood, and hold around 400 (updated to 650 now, see later posts) gallons of water. It will be built similar to Gary's DIY solar hot water tank. The main differences are that mine is going to be smaller, it lined with a different material, and insulated mostly with cellulose instead of foam board. These changes are mostly to decrease cost. The smaller tank size matches my 200 sqft of collector with a rule of thumb that you need 2 gallons per sqft of collector. I'll go through the heat gain calculations in another post to see exactly why more than 400 gallons of water is not needed and might actually be bad for the system depending on how you intend to use it. Anyway, I got it somewhat modeled up over lunch today. As you can see, its very similar to Gary's tank. I will be doing some FEA to see if the plywood thickness can be reduced. This could save a decent amount of money.
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06-20-11, 12:56 PM | #2 |
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I ran a few simulations over lunch today. The weight of just over 400 gallons of water is roughly 3,600 lbs. That is more weight than most cars, so its nothing to play around with.
The one thing I struggled with was getting the water weight forces to come out exactly how they would in the real world. The bottom of the tank is obviously going to see the full 3,600 lb force of the water, but what about the sides? At the bottom they'll see the most pressure, but at the top it will be less. So, I'm still searching for a formula that will help me figure that out. For now, I used 1 psi as my baseline. The base of the tank is 3,400 sq/in, so roughly 1 psi. I used this for the sides as well. Here is what I got: This is with 15/32" thick plywood. Try to ignore the colors as they don't truely tell you the whole story. The graph on the right shows the peak stress and the scaling. This is with 19/32" thick plywood. Again make sure to reference the graph as the scale varies. Now with 23/32". And once again with 3/4" thick plywood. As you can see, the 3/4" plywood showed quite a bit more strength than the other two thicknesses. I'm not quite sure why the huge reduction in stress. But, its not really consequential. Plywood has a tensile strength of 4500 psi. So, even the 15/32" can hold up to the stress of the water (assuming my simulated pressure isn't way off). I'll guess I'll run a few more simulations with even thinner plywood. However, there is a point of diminishing returns I'm sure. The 19/32 4x8 sheet only costs $3 more than the 15/32 sheet, so there isn't huge cost savings to be had.
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06-20-11, 01:19 PM | #3 |
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So, I talked with an engineer at work and he cleared up the numbers I should be using and the forces involved. I was using 1psi where I should have been using 1.3 psi. So, the stresses should go up about 30%. I'll rerun the simulations with the new numbers to make sure this is the case. I'll also incorperate the correct loading on the sides of the tank.
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06-20-11, 09:15 PM | #4 |
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Interesting. I just figured tanks were something you had to buy, but it looks like Gary has pulled it off.
I would caution that wood, ceramics, and other materials with brittle failure modes don't always behave the way the FEA models suggest. Minor cracks and porosities that would have little effect on a ductile material will make the strength of brittle materials more unpredictable. Stress concentrations, such as at fasteners, will be significant. I wonder... could you use steel hoops (or fiberglass) for their tensile strength, to allow you to reduce the amount of plywood required? My intuition tells me a square tank would be better than rectangular, in terms of both cost and heat retention. Btw, what software are you using? |
06-21-11, 07:11 AM | #5 |
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Yep, Gary has a few different ideas on DIY tank designs on his site thankfully. However, I believe his is the cheapest and easiest to build and insulate. A 120 gallon tank retail for $1000+ and I'd need a few of them! My current cost estimate for the tank and liner (no plumbing) is about $250 in materials.
I agree, a square tank would be more thermally efficient. The currently modeled tank can hold about 650 gallons of water. Since I only need 400 I will be shortening it up a bit length wise. However, I still have a rectangular footprint to put the tank in, so that will also be determining the shape of it to some extent. The tank (and the rack) is modeled in SolidWorks and analysed with CosmosWorks.
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06-21-11, 12:08 PM | #6 | |
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I redid my calculations on the tank sizing and decided that going with a larger tank will actually be worth it. The end result is the lower temperature increases collector output enough to be worth the reduced tank temperature.
Here is a quote from another post where I explained the calculation: Quote:
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06-21-11, 12:28 PM | #7 |
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So, I just finished up running the next simulation with the 650 gallon tank and 15/32 plywood and things look pretty good. A max stress of just under 3000 psi puts us in pretty good shape. I didn't bother correcting the load distribution on the sidewalls. If its good now, it'll only be better with less loading on it.
Here is the stress distribution. I'll probably add some corner supports like Gary did with his, especially since I'll be using thinner plywood. Without something solid to screw into, it'll be hard to make a strong joint.
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06-21-11, 01:38 PM | #8 |
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I'm curious what are the dimensions of the tank you're thinking about?
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06-21-11, 02:32 PM | #9 |
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The base of the tank is a 4x8 sheet of plywood. There is 2x4 framing around it and the side sheets are 48" tall. The dimensions of the inside of the tank are 40"x88"x48". If filled to the brim, it'll hold about 730 gallons. With the fill level 6" below the top, you get 640 gallons.
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06-21-11, 02:58 PM | #10 |
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The stress distribution looks the way I would expect it to look if the box had a lid and was filled with a compressed gas... I would expect that an open box filled with water would show some indication of greater stress toward the bottom of the sides and end, and a reduction in stress to zero toward the top of the box. -AC_Hacker
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