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Author Topic: Anyone used solar water heating on their bus conversion?  (Read 3935 times)
Kevin Warnock
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« on: November 10, 2009, 12:02:49 PM »

Hello,

I was wondering if anyone has used solar water heaters on the roof of their bus conversion? I have been reading Home Power Magazine, and they really urge readers to use solar for water heating, even in the winter.

I was thinking that I could make a homemade system where I put pipes directly touching the inside roof of the bus, since I still haven't spray foamed my bus yet. I could paint the solar water heater section of my roof black to soak up more heat. I don't know how many square feet I would need, but I bet it's a small area since I would only be heating a small amount of water, maybe no more than five or ten gallons.

Alternatively, I could just bolt one of the commercial collectors to the roof alongside my solar panels for making electricity. That would certainly be easiest.

Anyone out there with a lot of solar water collectors on the roof plumbed to radiators or under floor piping to heat the air in the bus? I am just wondering how 'green' one can go with a bus conversion if money were no object.

Thanks
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Iceni John
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« Reply #1 on: November 10, 2009, 01:00:34 PM »

My plan is to make a simple solar water heater to either preheat the water to the water heater during cooler months, or to completely heat the water during hotter months.   I will probably make it from black rubber hose zig-zagged into 4' square shallow boxes that are lined with aluminum foil and covered with Lexan  -  many years ago in England (long before most folk were aware of solar power) I saw an outdoor swimming pool that had an array of such home-made panels;  the owner could swim comfortably almost year round, and in the summer he said his biggest problem was preventing the water from boiling by the time it reached the last panel!   He was using the existing filtration pump to move the water through the panels, so it was essentially free heating.   Even overcast days have a surprising amount of solar energy available for PV cells and solar water heaters.   Any reason this idea wouldn't also work on a bus roof?

I will put a walkway on the roof, accessible through one of my exit hatches, to allow easy access to cleaning the PV and water panels;  I will also put a water spigot up there so I can easily wash down the panels to keep them clean.   The panels will be attached at their inner edges with hinges to the walkway, so they can be pivoted up to 45 degrees to catch the sun efficiently, and they will lay down flat to the roof when under way.   A simple cranking mechanism, similar to those used for awnings, will raise and lower them.

The only expensive part of a solar water heater may be the high temperature-resistant pump to circulate the water through the panels.   The PV panels should be generating enough electricity during sunnier months to easily power the pump, so again it should be essentially free heating.   A 10 or 12 gallon LPG water heater will be the storage tank for the hot water, and a simple thermostat will control water temperatures by running the system only when sufficient solar power is available.

I am intrigued by the idea of putting the solar collector inside the roof space.   It will look clean (and stay clean!), but may lose some efficiency compared to external panels.   Definitely worth some thought . . .

You say "If money were no object".   Money, or lack thereof, is a Big Object with me  -  I am furloughed three days a month, so I now have time to work on my bus but no money to buy what's needed!.   I think a simple home-made solar water heater won't take long to pay for itself, and shouldn't cost much if it's DIY.

John, in sunny SoCal
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Sean
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« Reply #2 on: November 10, 2009, 01:09:48 PM »

Hmm.

Incident solar radiation at 40 degrees lattitude is about 446 watt-hours per square foot on an eight-hour day when the sun is shining.  That's about 1,520 BTU.  Also, that's the total -- it does not account for any losses in the collector system.

One BTU is the amount of energy required to raise one pound of water one degree Fahrenheit, so 8.34 BTU is required to raise one gallon of water that amount.  So for example, to raise ten gallons of water from 65 to 120 would require 4,587 BTU.

In a system with 100% efficiency, in the desert, on a cloudless day, you would need three square feet of collector for your ten gallons of water.

In the real world, efficiency of a solar heating system seldom approaches even 50%, so you would need to double that.  Assuming you have any heat loss in the storage system, figure on starting with hotter water, and adjust accordingly.

When all is said and done, I would guess you would need close to twenty square feet of collector to get ten gallons of water usably hot, and then only on sunny days.

I would not recommend installing this inside the roof and painting the roof black.  That will have the undesirable side effect of heating up the interior of the coach, and any energy savings from the "free" hot water will be instantly negated by having to cool the interior.

A more practical way to get free hot water is to use waste heat from the engine(s).

-Sean
http://OurOdyssey.BlogSpot.com
« Last Edit: November 15, 2009, 08:08:35 AM by Sean » Logged

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Jeremy
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« Reply #3 on: November 10, 2009, 02:03:47 PM »

My Dad, who is a retired engineer with nothing better to do with his time, did a lot of research last year into the practicalities of building a home-made solar water heating system. In the end he decided that to do it properly required a lot more design and manufacturing sophistication (evacuated thin-wall glass tubes and such) than could be achieved on a DIY basis, so he spent over 6,000 ($11,000 or so) on a high-end professional system. I thought he was mad, but he has shown me the results and I have to agree that the performance is very impressive. The financial payback is still measured in decades though, so I still think he was mad. (His argument, and he is probably right, is that the cost of natural gas will rise significantly in the next ten years or so, which will make buying his system a smart investment).

Given the limitations of a bus I suspect you would need the efficiencies of a 'high-tech' system to see any worthwhile results, but the cost and fragility (evacuated thin-wall glass tubes!) of such as system is likely to make it totally impractical. But I may be wrong.

Jeremy
« Last Edit: November 10, 2009, 02:17:48 PM by Jeremy » Logged

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Kevin Warnock
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« Reply #4 on: November 10, 2009, 03:19:02 PM »

Thanks for the great comments.

I think I may have an interesting space for the collector that I hadn't considered before... the destination sign location. I removed the sign over the windshield and it's a good sized area, 8 feet by 1 feet in size. It's sloped and about 10 inches deep. The reason this area might be good is that the driver compartment is not conditioned air in my conversion when parked. There is a wall and door right behind the driver, blocking off the entire driver compartment completely, with weather stripping around the normal house door. I figured it's so hard to insulate that huge front glass area that I wouldn't even try. So I can paint the area for the solar collector black without worrying about extra heat gain, as that area was to be covered in fully black window tint anyway.

I have no idea if I'll really do this, as Sean presents some compelling math above that suggests this idea won't work very well. But it's interesting to think about.

Thanks
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Jeremy
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« Reply #5 on: November 10, 2009, 03:40:08 PM »

Assuming the area above the windscreen is approximately vertical, placing the collector there will probably work nicely so long as you always park with the bus facing South and on a 55 - 60 degree upward slope.

The actual direction and angle depends upon where you are on the planet of course, and to a lesser extent on the time of year. But it's more important than you might think -  with PV panels at least it is far more cost effective to buy a cheap panel and a good sun-tracking mount than it is to buy an expensive panel and just stick it on a flat roof.

Jeremy
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Iceni John
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« Reply #6 on: November 10, 2009, 04:10:06 PM »

I don't dispute the theoretical aspects of this at all.   From practical experience I know that the water in my small 2.5 gallon solar shower left all day in the sun will be too hot for a comfortable shower after about 2.00 or 3.00 PM, on a typical 90 F late-spring day in the Anza-Borrego desert (about 33 degrees North).   Any reduction in the amount of fuel needed to heat water is worthwhile, especially if it has cost very little to make the system.   Using waste heat from the engine is not a viable option if one is parked for weeks at a time.   Also, the solar panels (PV and water) provide some shade to the roof, similar to the "tropical roof" sometimes used on older Land Rovers, and that should slightly reduce interior temperatures.

If one can make a system that works reasonably well for very little money, what's not to like?

John  
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NewbeeMC9
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« Reply #7 on: November 10, 2009, 08:47:53 PM »

Assuming that you are not somewhere that it will freeze.  You can make one out of black pipe, ABS if you can use it for potable would be cheapest, or CPVC and paint it black.    Use a large diameter pipe for the for the reservoir on top.  Each end would have two taps, one high and one lo.  One end would be cold supply(lo) and heater return (hi), other end  cold heater supply(lo) and hot to use (hi)  don't for get to put a vent in it.

Now for the heater, use some Pipe, Tee's, and elbows and make an array with vertical pipes and the cold on the bottom side and hot return on top other side.  Have the heater lower than the reservoir at least a few feet, more is better, hang off sunny side of bus and tilt out or however you choose to do it.  This will cause "natural circulation" and you won't need a circulation pump. Makes it much simpler and cheaper.


Kinda like this
http://www.motherearthnews.com/uploadedImages/articles/issues/1979-09-01/059-070-01pic.jpg

another cool thing about the Natural circulation is when the sun is off of it, it does not circulate and cause cooling of the stored heated water.


 You could make it so it can be on any side and take down when you leave.  many ways to do it. just have cold come off bottom go lower and keep it out of sun then heat returns to top.  Don't forget safety releif

Let us know what you do.

 Interesting project
« Last Edit: November 14, 2009, 05:02:26 AM by NewbeeMC9 » Logged

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mc5a Jon
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« Reply #8 on: November 13, 2009, 11:38:02 PM »

If you don't plan to use destination sign, and shipping to Kansas isn't too much I'd like to buy it. I am only interested in the rollers not the hand crank or any thing visible from the inside. I too am interested in green power for my bus. I am going about it a little differently, I plan to build a wind generator on a pole that will lay flat on top of my 68' mc5a then be raised and secured with cables. It should keep the battery voltage up above 20(inverter cut out). It would be a nice dream if I could run the A/Cs on wind but I'd need as many wind generators as would fit on my roof and one hell of an inverter. Running lights, the frig, tv, and water heater may not be totally out of the question as it takes over 24 hours to run my house batteries down to 20 volts just running some of those little things. Anyway don't throw out the destination sign if you still have it.
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steve5B
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« Reply #9 on: November 14, 2009, 05:37:24 AM »



   Hey  1967 MCI 5A,


   Have you ever thought about using a large "FRENSAL" lens?  They always have some on EBay.  It's a solar furnace that will

   heat water like you won't believe.  You might want to look into it!

   Steve 5B.....
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Kevin Warnock
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« Reply #10 on: November 14, 2009, 09:50:53 PM »

Sorry, the destination sign was a flip cell type, and I junked it as Luminator would not help me reprogram it to display new text.
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Jerry W Campbell
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« Reply #11 on: November 15, 2009, 07:01:37 AM »

My plan for next spring is to is to build a 2 x 3 ft. box and use 3/8 copper tubing with a small solar pump with it's own solar panel. when the sun shines on it the pump pumps. I have a 10 gal tank.
Jerry
« Last Edit: November 15, 2009, 07:03:30 AM by Jerry W Campbell » Logged
Kevin Warnock
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« Reply #12 on: November 16, 2009, 11:59:30 AM »

Jerry,

Please post a message to this board once you have some information about how well your plan works.

I love your idea.

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Old Scool Bus
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« Reply #13 on: November 21, 2009, 07:40:38 PM »

a
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Old Scool Bus
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« Reply #14 on: November 21, 2009, 07:49:38 PM »

b
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