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Old 08-24-2015, 11:57 PM   #11
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I agree.

Small watt electrical heating elements are great for heating water over a long period of time.

Like from solar or a wind turbine.

Nat
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Old 08-25-2015, 12:12 AM   #12
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Originally Posted by nat_ster View Post
Small watt electrical heating elements are great for heating water over a long period of time.

Like from solar or a wind turbine.

Nat
That's what I was thinking... Fill the bucket in the morning, heat the water all day and shower in the evening!
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Old 08-25-2015, 12:28 AM   #13
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Now, imagine all the power from a 100w solar panel going into 5 gallons of water over 8 hours.

The water will be tepid but warmer than the ambient temperature.
5 gallons = 41.5 pounds, 8 hours times 120 watts = 960 watt-hours = 3275 Btu. It can raise the water about 79 degrees F, so if it started at 50 F it could be at 130 F by end of day. But 8 hours is probably on the optimistic side for daily solar production at full power, even in the summer. (and remember we're still ignoring heat lost through the uninsulated walls of the bucket etc)

If instead that water were pumped through a collector designed for solar heating water, or even just left sitting stagnant in a collector big enough to hold the full 5 gallons, it would heat the water much faster. Estimating 1600 kWh/m2 annually from a solar irradiance map for North America off wikipedia, that's 407 watts available on average per square foot. If it's 60% efficient, then it takes just 4 hours to heat the same 5 gallon bucket. A square foot of solar water heater is generally much cheaper than a square foot of solar panel.. this is why you don't often see water heated by PV systems. It can be done, and maybe it's more convenient to store energy in a battery and heat water later instead of storing energy as heated water. But usually hot water is needed at a scale that makes PV-powered heating unattractive.
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Old 08-25-2015, 12:34 AM   #14
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I agree, using a solar water heater would be more efficient.

I will have both.

Also, there are times in the spring and fall when it gets hot in the day time, but freezes at night. This makes the solar water heater harder to use.

However, where I live, when the sun is not out, the wind is often blowing. So I will still use the electricity to heat the water.

Nat
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Old 08-25-2015, 12:37 AM   #15
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5 gallons = 41.5 pounds, 8 hours times 120 watts = 960 watt-hours = 3275 Btu. It can raise the water about 79 degrees F, so if it started at 50 F it could be at 130 F by end of day. But 8 hours is probably on the optimistic side for daily solar production at full power, even in the summer. (and remember we're still ignoring heat lost through the uninsulated walls of the bucket etc)

If instead that water were pumped through a collector designed for solar heating water, or even just left sitting stagnant in a collector big enough to hold the full 5 gallons, it would heat the water much faster. Estimating 1600 kWh/m2 annually from a solar irradiance map for North America off wikipedia, that's 407 watts available on average per square foot. If it's 60% efficient, then it takes just 4 hours to heat the same 5 gallon bucket. A square foot of solar water heater is generally much cheaper than a square foot of solar panel.. this is why you don't often see water heated by PV systems. It can be done, and maybe it's more convenient to store energy in a battery and heat water later instead of storing energy as heated water. But usually hot water is needed at a scale that makes PV-powered heating unattractive.
Yup. I appreciate that but while the plan is eventually to have a hit tank under the bus, the budget says a bucket inside.

Now, if I'd been less concerned with building in absolute privacy then my construction techniques would have been different and I might have put in a pot belly stove.

There are advantages and disadvantages all ways.
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Old 08-25-2015, 11:16 AM   #16
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Will a 120w heater coil ever heat up more than a cup full of water? Do you have a link? I'm curious...



That's only true for the flexible panels or the expensive 12v stuff they sell at RV places (and you know it ). Solar can be found for near $1.00 per watt.
Flexible is the only way to go IMHO.
For completeness I'll add that flexible panels when installed flush to the roof of the bus, though neat and tidy, are the least efficient and most expensive means of powering a bus by solar.

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5 gallons = 41.5 pounds, 8 hours times 120 watts = 960 watt-hours = 3275 Btu. It can raise the water about 79 degrees F, so if it started at 50 F it could be at 130 F by end of day. But 8 hours is probably on the optimistic side for daily solar production at full power, even in the summer. (and remember we're still ignoring heat lost through the uninsulated walls of the bucket etc)

If instead that water were pumped through a collector designed for solar heating water, or even just left sitting stagnant in a collector big enough to hold the full 5 gallons, it would heat the water much faster. Estimating 1600 kWh/m2 annually from a solar irradiance map for North America off wikipedia, that's 407 watts available on average per square foot. If it's 60% efficient, then it takes just 4 hours to heat the same 5 gallon bucket. A square foot of solar water heater is generally much cheaper than a square foot of solar panel.. this is why you don't often see water heated by PV systems. It can be done, and maybe it's more convenient to store energy in a battery and heat water later instead of storing energy as heated water. But usually hot water is needed at a scale that makes PV-powered heating unattractive.
Great calculations! If somebody could chime in with the calculations for heat loss from an uninsulated bucket I'd be much obliged. I would guess that there would be a limit that would be reached well before 130F due to that ambient air temperature.
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Old 08-25-2015, 11:25 AM   #17
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At the moment my internal temperatures are about 97F. The high point was 139F.
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Old 08-25-2015, 01:07 PM   #18
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uploadfromtaptalk1440521822323.pngThis looks interesting - a list of what all the gadgets use. It looks like a raw food diet perhaps supplemented by cooking outside is in order!

Interestingly, I looked at a peltier unit online. Somebody had coupled a peltier cooler unit $9 with a computer fan and a heat sink. They claimed frost would appear almost instantly on the peltier device. The rated power consumption was high at 12v, 5A or 60w.

On the other hand, with one of those units, a simple cooler can be turned into a refrigerator with advantages such as custom size.

If a peltier unit was planned into the power budget, that could be the way to go.

For the moment, I found my breaker box will fit under my kitchen countertop to a rib on the side wall. Now all I have to do is to work out the cabling. I was going to put it at the back and have cabling and conduit. That would have been more expensive though.
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Old 08-25-2015, 01:08 PM   #19
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Great calculations! If somebody could chime in with the calculations for heat loss from an uninsulated bucket I'd be much obliged. I would guess that there would be a limit that would be reached well before 130F due to that ambient air temperature.
This is the sort of question I'd answer experimentally rather than try to calculate or even estimate. Unfortunately, I'm fresh out of 120 watt heaters! Anybody else want to try? If it were me, I'd just set a filled bucket outside in the morning, plug in the heater, and leave it for the day. At some point it'll reach equilibrium where the heat input from electricity equals the heat lost to the ambient and the temperature doesn't rise any higher. I did essentially this same test with a pair of electric 2250 watt baseboard heaters a few winters ago to estimate the heat loss of my bus before I began tearing it apart.
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Old 08-25-2015, 01:16 PM   #20
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It's easy enough to figure wattage and temperature rise. Specific heat of water is around 1 Btu per pound per degree F (it varies with temperature), so if we have a mug holding 8 fluid ounces of water at 50 F and we want to raise it to 120 F, that's 0.52 pounds times 70 degree rise so 36 Btu needed (ignoring heat lost to the mug, ambient, etc). That's 0.0106 kw-hr, 10.6 watt-hours, so at 120 watts input it'll take 5.3 minutes (longer in the real world, where the heater doesn't achieve its rated output and heat is lost to the mug etc).

Man, this was on the tip of my tongue. I was going to write the same thing but went to go make a cup-o-joe and you beat me to it. Now you look like the smart guy...and me...not so much.
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