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Solar water heating system..

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Posted by Matthew Maddock on June 19, 2006, 6:51 am
Hi all,

I am new to all of this, but have had a long term interest.

We are moving to France in a few months to a house that needs
renovating etc. There is no existing hot water system, so that
got me thinking that instead of putting in the usual electric
water heating system that is common in France, a solar system
might be nice.

I've seen this system on eBay
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item `64859125
can anyone comment on how good these systems are (not necessarily
this one specifically).  Where we will be, there is plenty of
sun, but it also gets pretty cold in winter too (tho still usually

I guess I really want to know, are the systems any good? will it
actually work to a satisfactory degree? or will the electric backup
element always be running?  Considering that I can buy a large French
"chauffe eau" (pressurised, highly insulated electric water tank)
for around  200 max, is it worth it?!!

Thanks in advance,


Posted by Christian Kaiser on June 19, 2006, 10:11 am
You still need some backup heating (Oil, Gas, Electric, ...) as you can not
guarantee you will always get a sunny day at least every n-th day. "n" is
dependent on the system layout and size. For me, it's 5-7, depending on the
water usage.

See www.invest-tools.com/pub/solsys for a description of my system.  I'm
still happy with it.

"Will it be worth it" - well that depends a lot on future energy prices. If
you take the current prices, usually "no" is the answer unless you do a lot
yourself and you get cheap and still qualitatively good parts (repair
costs!). You should find out how much electricity is usually used by the
electric heating, then you can calculate an approximate cost curve. I cannot
tell for France.

Here in Germany, I would assume electric heating is too expensive, so it
would be worth it. I never did calculate it, as I did not need it. Assuming
you need 5 kWh per day (which is approx. what we need, 2 adults, 3 kids),
it's 400 EUR/year here. Just compare it to the sum you need to pay for the
solar system, and add 1/4 of the electricity costs (75% solar, 25% backup
heating if the system is well designed). Plot the curves and look at the
crossing point.

Just as a rough calculation, of course.


Posted by nicksanspam on June 19, 2006, 2:10 pm

Tres naturel.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item `64859125

Evacuated tubes with glass-to-metal seals tend to crack over time.
http://www.sunpowersolutions.com  distributes Apricus systems
with no glass-to-metal seals in France...

How many kWh/m^2 of sun falls on the ground and south walls? Grenoble
(where I worked) is similar to Phila, with 620 Btu/ft^2 on the ground
and 1000 walls on an average January day with a 34 F daytime temp.

Yes, generally speaking.

The solar hot water fraction can be close to 100% if you spend enough money.

That's a cheap heat storage tank and cheap backup fuel, if rarely needed.

You might buy one and build a solar preheater (since you'll be renovating)
with a $5 fountain pump (eg Tractor Supply's #3119117) and a $0 1"x300'
pressurized plastic pipe coil in a $70 3'x8'x2' tall metal stock tank (TSC
#2177285) inside an 8'x12'x7' tall A-frame structure with a $00 8'x12'
Dynaglas polycarbonate south wall at a 30 degree tilt, which might collect
0.9x12(4x620+8cos(30)1000) = 101.6K Btu of sun and lose 6h(Tg-34)8x12/R1
= 576Tg-19.6K on an average day, where Tg (F) is the A-frame air temp.
If we collect Q Btu/day of useful heat, Tg = 210.4-Q/576.

If a 4'x12' tank cover collects 90% of the sun that enters the glazing
(91.4K Btu, with the help of 2'x12' of Big Fins on the south tank wall)
in Tw F water and loses 6h(Tw-Tg)1.5x6x12 = 648(Tw-Tg), Q = 305(352-Tw).
Tw = 140 F makes Q = 64.5K Btu (19 kWh) per day, with Tg = 98 F. It might
look like this, viewed in a fixed font:

  7'|                            . <------------------------- sun
    |                         .          
    |                      .     ru  .                        south -->
    |                   .                              
    |               U.           ru down-reflected upper ray              
    |             .                     .      
    |          .                 ru              
    |   (x,y)    45                       .  
    |    . <---------- sun       ru         \          
    |   . rl                                .8'
    | L.        rl               ru           \
    | .                rl down-reflected lower ray   (not to scale)
    |. 67.5 degrees            
  2'|----------------------------F               .
    |  i |  300' pipe coil  |  i B
    |  n |                  |  n i                 .
    |  s |                  |  s g              
    |  u |300 gal stock tank|  u F                   .
    |  l |                  |  l i
    |    |                  |    n      white       60 .
  0' ------------------------------------------------------------------> X
    0'                           4'                    8'

The north wall could be made from 4'x8' 2" double-foil polyiso boards.
... 500 W of standard PV panels under a water duct on the lid might
produce 1000 W of electrical power...

20 PI=4*ATN(1)
30 F=4'max focal distance (ft)
40 A=PI/8'kerf angle (radians)
50 X=F/(1+1/TAN(A)/TAN(2*A))'x breakpoint (ft)
60 Y=X/TAN(A)+2'y breakpoint (ft)
70 L=SQR(X^2+(Y-2)^2)'lower segment length (ft)
80 H=8*COS(PI/6)'height (ft)
90 U=SQR((F-X)^2+(H-Y)^2)'upper segment length (ft)
100 ALD0*(PI/2-A)/PI'lower elevation angle (degrees)
110 AUD0*(PI/2-2*A)/PI'upper elevation angle (degrees)

focus  -- lower segment --       -- upper segment --      total segment
(ft)  length (ft) elev (deg)    length (ft) elev (deg)    length (ft)

4     3.061468    67.5          3.522649    45            8.584117

With lots of insulation, the 300 gallon tank could provide 300x8.33(140-60)
= 200K Btu of water heating over 5 cloudy days.


Posted by Robert Gammon on June 19, 2006, 2:53 pm
 In the deep southeast Texas area that I live, in spite of 42 inches of
rain that we receive annually, A direct gain solar hot water heater
means that we spend very very little on hot water heating.   Scald risk
is avoided because the water in the hot water tank is generally cooler
than the water coming down from the solar collector.

Lots of different designs out there for solar hot water, some good,
some no so good.  Choose carefully and you CAN dramatically cut your
hot water costs.  It would be best to look for products sold and
serviced in Europe.  Cast your eyes a bit further than France as you
may find that both Germany and Italy offer decent solar products.

As interesting as Nick's ideas are, I don't think that you want to
build a home brew solar hot water heater.

Posted by Gary on June 19, 2006, 5:20 pm
 Robert Gammon wrote:


Why not?



"Build It Yourself" Solar Projects

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