While I was in Africa this summer I worked out how I could aim several mirrors at my old water heater. I had already stripped off half the casing and foam insulation on the water and heater and painted it black knowing that I wanted to acheive some sort of solar water heating but had never really worked out how to position the mirrors to make it really hot!
So, being a mathematician I set about working out the requisite formulas which would help me to space out the mirrors and determine at which angle I should keep them.
It turns out that instead of tracking the sun it’s actually quite simple to place the water heater perpendicular to the sun, and array the mirrors facing north. Aligning the mirrors in the center with the long edge running perpendicular to the sun means that at midday there will be an additional 8 sq. ft of solar power to heat the water that will eventually be contained in the heater.
In the first picture we can also see that there are 2 additional mirrors placed at 45 degrees in either direction to the middle mirrors. These mirrors are testing my hypothesis that at about 11 am (a little past quarter day) and 3pm there will be some additional sunlight that could be caught.
Once I get a little more funding for this project I’ll add a few more mirrors (4-10) more and also start work on a smaller concentrator for my home office.
Later I’ll connect up some actual water pipes, and circulate the hot water into the house for our new radiant heating system that I’m hoping to install this fall.
If you’d like to code for your own project please contact me.
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Theo, I can’t help but laugh out of amazement at your experiments. You have a contagious enthusiasm about being green.
I really like the idea of using a solar heater to pre-heat water for the house, but I’m curious about the practicality of solar water heating. For Lesley and me, our primary need for hot water occurs daily at 6AM for our morning showers, which is prior to there being any amount of usable solar heating. Is there enough insulation/heat capacity to store the heat overnight? Our secondary need for hot water occurs 2-4 times per week in the evening for a much smaller amount of hot water to wash dishes, hands, and once a month or so – cloths.
Have you performed any calculations to determine how much, on average (including weather phenomenon that will effect solar heating capability, i.e. clouds, ambient temperature, convective cooling) you will save by implementing the use of solar heating for your water system?
Also, do you have plans for a more permanent location for the solar heater? I know Lesley would insist that it not be located on our lawn… in fact, unless I could somehow make it very aesthetically pleasing, I don’t think I’d be allowed to put something like this ANYwhere around our house
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greentheo Reply:
September 4th, 2008 at 1:54 pm
Thanks Derrick! I like the green thing… but even more.. I like a challenge. Building a non-conventional solar water heater like this is more fun than anything.
However practically speaking there are two points you bring up:
1.) aesthetics and
2.) practical usage
For 1.) instead of a raggedy old rusted water heater you could use something like this: http://www.solar-components.com/actvdhw.htm … these flat panel heaters are both economical and look really nice.
Eventually I plan to add two of these to the round tank and add a horde of mirrors around those as well. The bonus is you can take those flat panels and stick em up on your roof.
in Regards to 2.) For any water heating system to be practical it needs to have a few things.
1.) a storage tank that is well insulated.
2.) Heat exchangers that take incomeing hot water and store it in the tank… with the exchangers switched off at night to minimize convective heat loss.
3.) If used for hot water for showers etc. a secondary heating unit will need to be in place for the reasons you mention… no solar heat in the morning. Though if your hot water storage tank is large enough even this may possibly be omitted.
I plan to use this heater not for the heating of hot water for showers etc. but rather for a radiant heat system in the house. Ideally it will go in our bottom floor and heat up the slab starting in late summer. As we go into winter the slab will act as a nice thermal flywheel and ideally lose all of its heat by the end of spring. Ideally it will be a nice smooth cycle.
As for real calculations? Simple.
solar BTU per meter square per year: 484.5 million BTU
heat required to raise 1 gallon of water 1 degree farenheit: 10
In an ideal system which exhanges all the heat in say 300 gallons of water incoming at 60 F we might expect to heat up the water to 484.5 m (BTU)/ 365 days /10 (BTU/gallon/degree / 300 (gallons) + 60 (DF) = 502 (DF).
Because the system may not operate at anything more than about 20% efficiency it’s safe to assume that out of a 1 sq. meter system like mine we’d probably acheive a maximum of about 100-120 DF water.
I have currently about 2 sq. meter of light on the water heater so I might be able to acheive 150-200 DF.
Anyhow, we shall see what I acheive.
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wildderrick Reply:
September 4th, 2008 at 3:35 pm
Those solar heaters are quite nice indeed, but there is quite a lot of capital to invest initially. Granted, a couple of the systems on that website qualify for certain tax incentives that could make the system more economical (obviously more research will need to be done to determine the cost effectiveness of going with a system like this). Furthermore, I don’t very much like the idea of having to go shovel off the panels on my roof when it snows. Thankfully, the snow doesn’t stick around for very long in Colorado, so that may not be a serious issue.
In regards to heating your slab, do you have an idea of how you would install such a system? I’d guess that you’d have to lay the piping on top of the slab and then install some sort of subfloor over that, but I don’t think it would provide much heating to the slab. Ideally, the system would be installed as an integral part of the slab when it is poured, but retrofitting a system like that could be very costly.
I think one of the primary factors in your efficiency will not be the amount of heat that you can direct onto the tank, but minimizing the heat loss through the pipe running from your house to the tank. I see that the tank is currently located in the North East section of your back yard. Picturing your yard in my head, I believe this is the ideal location to maximize sun exposure, however, it also places the tank at the farthest location from your house. Unfortunately, this will require more pipe and more insulation ($$$) and will reduce the efficiency of the overall system.
I don’t mean to be a total kill joy! I DO think this system will make a difference, I’m just blithering my thoughts about how to optimize the system. During the winter, for instance, it may be optimal to swap the location of the tank and mirrors to account for the lower arc of the sun, so that may be something to consider as well for the final design. Either way, I think your roof will be the optimal location. I’ll help you get it up there the next time I come over
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Time for the Way-Back machine… Nate was maybe 4 or so ??87??
We lived in a 1200 ft2 home in Aurora, CO with an open basement. The government declared a $7000 tax credit for solar improvements… by magic several companies came up with $7000 systems right then… go figure…
We got one. They put 4 “drain down” units on the roof. There was no water in them until the panels rose above a certain temp, which was every day even in Aurora CO even in the winter. The units were about 4×8 black metal with a grid of copper tubing and a glass front and insulated back and sides. The pumps cycled on automatically sending water from and back to a 300 gal closed system in a heavily insulated tank. The heat built up as long as the water in the panels was hotter than the water in the tank, or if it was too hot in the tank.
There were 2 sets of heat exchanger tubes inside the tank. One set pre-heated water into the regular water heater. most of the time the gas never came on. There was a mixer that mixed in cold water as needed to keep the water in the household system at non-scalding levels.
The second set of exchanger tubes fed a “solar hearth” in the kitchen. It had it’s own thermostat and hot air distribution fan. It could draw down the temp in the big tank until it reached a certain minimum, then automatically turned off. We got heat in the kitchen even in the winter and usually all evening long.
The team even installed an automatic set-back thermostat. All in all a very cool and wonderfully effective system… then we moved… no more deals… no more $7000… oh well. Nice while it lasted!
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