I'm trying to figure out how to build a Rube Goldberg inexpensive but functional DIY solar-powered recirculating spa heater for my 500-gallon spa.
My idea is to re-circulate the spa water through a few hundred feet of black rubber 5/8th's diameter garden hose (which I already own) for about six hours a day, using a 12VDC RV water pump (which I already own), a solar panel, and a battery, just to raise the temperature a bit (no, I don't know how much it will raise it yet - that's a huge unknown).
I'm trying to source parts off Amazon to get the base specifications, so that I can at least run some of the easy math (the harder math being the temperature variation).
Starting with the easy math, the volume of each hundred feet of a 5/8ths hose I think is about ~1-1/2 gallons (if I did my geometry right assuming a radius of 0.3125 inches and pi r squared times length for the volume of a cylinder for a volume of 368.14 cubic inches at 231 cubic inches per gallon) so for 200 feet of hose, I'll have about ~3 gallons of water inside the black rubber hose heating up at any given moment with the hose lying on the concrete deck on the perimeter surrounding the pool.
The pump specs say it has a max flow of 6.5 gallons per minute, but I'm thinking, with all the friction and head loss, it'll probably be closer to ~5 GPM. I'm not really sure if that's a good guess though. At 5 GPM, it'd take about 100 minutes, which is an hour and 40 minutes, to circulate all the water. That means I'd get about 3.6 theoretical turnovers in six hours (but that assumes 100% mixing, which won't happen). I'm wondering, do you think 3.6 theoretical turnovers is enough to actually raise the spa's temperature?
While the pump maximum can be as high as 18-1/2 amps, the pump says it is rated at 12VDC and 5 amps, which means it uses 60 watts. Running it for six hours, that's 360 watt-hours, and that's what the solar panel has to replace. I realize a charger has to be greater than the voltage, and I realize 12 Volts is nominal (e.g., a car battery might be 13.4 volts nominally), but for now, I'll keep the math simple at 12VDC, especially since Amazon seems to have plenty of 12VDC solar panels for battery chargers, so I'm looking for a 12VDC solar panel, where I'm thinking a 100W panel might work. I'd have to buy the solar panel as the only panels I have are about 5 watts to 8 watts at 5VDC (for solar outdoor cameras) which I think may be too small.
I know that solar panel won't produce 100 watts all day, but maybe during those peak sun hours around the middle of the day it produces that rated 100 Watts?
Assuming that, if I divide the 360 watt-hours by, say, 5 peak sun hours, that's 72 watts, so I'm thinking a 100W panel should be enough.
But I don't really know.
Then there's the battery, which is the expensive part. I have old car batteries, so maybe I can use them to keep the costs down.
But if I have to buy a battery, I think I'd probably maybe want to use a deep-cycle marine battery, and I know I shouldn't discharge any battery past 50%.
The pump needs 30 amp-hours a day, so I'm thinking I need a 60 amp-hour battery.
But with cloudy days and battery aging, maybe I should add a 25% buffer, which means I'd need around 75 amp-hours?
Given that, would a 75 or 100 amp-hour battery like this one, make sense?
Putting it all together, the hose is about a dollar a foot, the battery is about $210, the pump is around $100, the solar panel is about $40.
Since I already have the hose & pump, the incremental cost would be about $250, which seems like a lot for this kind of project. where the fun is in being successful, not in what it looks like. But right now it's all hypothetical.
If I can power it with an old car battery, that would be a bonus on costs. (I could even charge it at night on a 120VAC battery charger, but that's defeating some of the purpose.)
I'm just trying to figure out from someone who has the experience whether my calculations are reasonable, if 3.6 turnovers will actually heat the water, by how much, and if there's a better or less expensive way to do this.
I'd really appreciate any advice from someone who's done something like this before.
My idea is to re-circulate the spa water through a few hundred feet of black rubber 5/8th's diameter garden hose (which I already own) for about six hours a day, using a 12VDC RV water pump (which I already own), a solar panel, and a battery, just to raise the temperature a bit (no, I don't know how much it will raise it yet - that's a huge unknown).
I'm trying to source parts off Amazon to get the base specifications, so that I can at least run some of the easy math (the harder math being the temperature variation).
Starting with the easy math, the volume of each hundred feet of a 5/8ths hose I think is about ~1-1/2 gallons (if I did my geometry right assuming a radius of 0.3125 inches and pi r squared times length for the volume of a cylinder for a volume of 368.14 cubic inches at 231 cubic inches per gallon) so for 200 feet of hose, I'll have about ~3 gallons of water inside the black rubber hose heating up at any given moment with the hose lying on the concrete deck on the perimeter surrounding the pool.
The pump specs say it has a max flow of 6.5 gallons per minute, but I'm thinking, with all the friction and head loss, it'll probably be closer to ~5 GPM. I'm not really sure if that's a good guess though. At 5 GPM, it'd take about 100 minutes, which is an hour and 40 minutes, to circulate all the water. That means I'd get about 3.6 theoretical turnovers in six hours (but that assumes 100% mixing, which won't happen). I'm wondering, do you think 3.6 theoretical turnovers is enough to actually raise the spa's temperature?
While the pump maximum can be as high as 18-1/2 amps, the pump says it is rated at 12VDC and 5 amps, which means it uses 60 watts. Running it for six hours, that's 360 watt-hours, and that's what the solar panel has to replace. I realize a charger has to be greater than the voltage, and I realize 12 Volts is nominal (e.g., a car battery might be 13.4 volts nominally), but for now, I'll keep the math simple at 12VDC, especially since Amazon seems to have plenty of 12VDC solar panels for battery chargers, so I'm looking for a 12VDC solar panel, where I'm thinking a 100W panel might work. I'd have to buy the solar panel as the only panels I have are about 5 watts to 8 watts at 5VDC (for solar outdoor cameras) which I think may be too small.
I know that solar panel won't produce 100 watts all day, but maybe during those peak sun hours around the middle of the day it produces that rated 100 Watts?
Assuming that, if I divide the 360 watt-hours by, say, 5 peak sun hours, that's 72 watts, so I'm thinking a 100W panel should be enough.
But I don't really know.
Then there's the battery, which is the expensive part. I have old car batteries, so maybe I can use them to keep the costs down.
But if I have to buy a battery, I think I'd probably maybe want to use a deep-cycle marine battery, and I know I shouldn't discharge any battery past 50%.
The pump needs 30 amp-hours a day, so I'm thinking I need a 60 amp-hour battery.
But with cloudy days and battery aging, maybe I should add a 25% buffer, which means I'd need around 75 amp-hours?
Given that, would a 75 or 100 amp-hour battery like this one, make sense?
Putting it all together, the hose is about a dollar a foot, the battery is about $210, the pump is around $100, the solar panel is about $40.
Since I already have the hose & pump, the incremental cost would be about $250, which seems like a lot for this kind of project. where the fun is in being successful, not in what it looks like. But right now it's all hypothetical.
If I can power it with an old car battery, that would be a bonus on costs. (I could even charge it at night on a 120VAC battery charger, but that's defeating some of the purpose.)
I'm just trying to figure out from someone who has the experience whether my calculations are reasonable, if 3.6 turnovers will actually heat the water, by how much, and if there's a better or less expensive way to do this.
I'd really appreciate any advice from someone who's done something like this before.
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