"Matthew Maddock" wrote:

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.

Tres naturel.

I've seen this system on eBay
http://cgi.ebay.co.uk/ws/eBayISAPI.d...tem=6064859125
can anyone comment on how good these systems are (not necessarily
this one specifically)...

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...

Where we will be, there is plenty of sun, but it also gets pretty cold
in winter too (tho still usually sunny!)

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.

... are the systems any good?

Yes, generally speaking.

will it actually work to a satisfactory degree? or will the electric
backup element always be running?

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

Considering that I can buy a large French "chauffe eau" (pressurised,
highly insulated electric water tank) for around 200€ max,
is it worth it?!!

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 $35 fountain pump (eg Tractor Supply's #3119117) and a $60 1"x300'
pressurized plastic pipe coil in a $170 3'x8'x2' tall metal stock tank (TSC
#2177285) inside an 8'x12'x7' tall A-frame structure with a $100 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:

Y

^
|
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 ALD=180*(PI/2-A)/PI'lower elevation angle (degrees)
110 AUD=180*(PI/2-2*A)/PI'upper elevation angle (degrees)
120 PRINT F;L,ALD,U,AUD,2+L+U

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.

Nick