A simple Solar Water Heater
The total amount of solar radiation falling on each square
meter in the UK is around 1000kWh per year - of this, it should be possible to
collect between 25% and 33% . For a typical domestic water heating installation,
a total of about 4 sq. metres is considered appropriate.
The amount of water which can be directly heated to a 'usable' temperature
is fairly small so it is usually better to use the panel to preheat cold water
in a separate tank before it is fed to the main hot water tank. The solar storage
tank should be about 50 litres per 1 square meter of panel, however this is not
very critical. The tank and all interconnection pipework should be well insulated
to avoid loss of collected heat. The best position for the panel (UK) is in an
unshaded position angled to face just west of south at about 35 degrees to the
horizontal. Other orientations between SE and SW and various tilts from 10 to
50 degrees cause only small reductions in the overall energy collected.
Flat Solar Collector Design
The simple, single panel design shown here uses a standard pressed steel central
heating radiator as the solar collector. These are relatively cheap and readily
available either as new or second-hand (if using a second-hand radiator panel,
remove any decorative paint from the front surface and re-prime as necessary).
The thermal efficiency of a collector using a central heating radiator should
be comparable with many commercially available designs. However the relatively
large water content will make it slow to respond especially at low levels of solar
radiation. The panel should be a single panel without fins and with tapped connecting
holes at all four corners so that the necessary 'diagonal' flow of water is easily
achieved. Other types with only two connections can be used providing that the
connections are at diagonally opposite corners. The panel needs to be painted
with matt black oil paint to give a high absorbent surface. All pipework within
the case should be insulated to prevent collected heat escaping back into the
case.
The size of collector used in this design is not defined other than by 'h'
and 'w', this allows you to build the case to suit your particular size of radiator
panel. Try to keep the size of panel about 1 sq. metre or less, larger panel are
heavy and can be difficult to handle especially on a roof. If you can build the
case in it's final position, the job will generally be easier.
The case for the panel is a simple wooden box made from pressure treated timber
(alternatively a good quality timber preservative could be used). Single front
cover glazing is shown, using 3mm window glass - for cases over about 1 metre
in any direction, use separate pieces of glass, you'll need to add addition glazing
support bars across the front of the panel to fix them. Always measure the completed
case before you buy the glass and buy it cut to size - leave a 2mm gap around
the glass to allow it to expand. The front glazing cover should overhang the lower
edge of the case by about 12 mm to allow rain to drain off without running onto
the case. Glazing clips fixed to the inside of the lower edge of the box are used
to hold the cover in position.
The insulation fitted behind the collector needs to be of a high temperature
type as the temperature can reach up to 140 degrees Celsius if the water is not
circulated through the panel. Other, cheaper alternative materials (such as polystyrene)
are unsuitable as they can shrink or even melt. Air movement between the back
of the panel and the insulation should be avoided so make sure that all gaps are
filled.
It is virtually impossible to make the collector totally water tight over a
long period, even if the rain is kept out, some internal condensation will occur.
To prevent this becoming a problem, insert three or four 5 mm 'breathe' holes
along the lower side of the case just in front of the insulation.
Cut away of solar collector
Material list for the solor collector
Note: most dimensions are shown h+x and w+y - where h and w are the height
and width of the particular panel you are using. Measure these before you start
and just add x or y as appropriate.
- Timber - Softwood, planed all round, preferably tanalised or, alternatively,
treated with a good quality timber preservative. Timber sizes shown are standard
nominal size - it will be smaller having been planned.part identity size
(par)length quantity A 125x25 w+150mm 1 B 125x25 h+125mm 2 C 100x25 w+100 1 D 25x12 w+100 1 E 25x12 h+75 2 F 50x25 h+100 2 (or 3 where w is greater
than 1metre)G 45x12 W+150 1 (mitre corner (both
ends))H 45x12 H+150 2 (mitre corner (one
end))J 50X50 cut
to suit4 (or 6 where w is greater than 1metre) - Plywood Exterior quality 9mm, h+150 x w+150
- Absorber Press steel, single panel radiator without
fins - with termination at all four corners to enable a diagonal flow (or at 2
diagonally opposite corners) - Glass w+95 x h +135mm, 3 mm, (measure the collector
box to check the size before you purchase - fit separate pieces of glass to keep
all dimensions less than 1 m, this will involve additional support bars across
the front of the panel as required) - Glass retaining clips - 2 per maximum 1 m edge
- Corner plates (250mm x 100mm, mild steel - bent 90
degrees) 4 off - Insulation 50 mm thick, high temperature mineral fibre
- Connecting pipework and connectors - copper, size
and quantity to suit - Metal foil (such as kitchen foil) as required
- Glue - PVA wood glue as required
- Putty or glazing tape as required
- Screws etc. as required
Basic Solar Collector Case
Assembly of the solar
collector
- Cut the timber, treat all sawn ends with a good quality
timber preservative. - Paint the collector panel using as thin a coating as
possible of high temperature, black, oil base paint (black car exhaust paint is
a good suggestion). - Glue and screw the sides together (A, B, and C).
- Glue and screw the backing ply onto the frame.
- Screw the corner plates into position.
- Glue and screw (from the back) the cross braces (F)
into position. - Drill a number of 5 mm drain holes in the lower side,
just in front of the insulation. - Lay the panel in position within the case, mark the
position of the pipe entry points on the frame. Remove the absorber and drill
the holes for the pipe entries. - Glue and screw strips D and E to the inner sides of
the case so that they offer a flat surface for the glass on the same line as the
top of the lower side (C). - Cut and fit the insulation between the cross braces
and the case. - Cover the insulation with metal foil.
- If not being built in position, this is a good time
to position and secure the case. - Lay the panel in the case and secure using the holding
down blocks on the cross braces. - Fit the pipework between the panel and the rest of
the system, fill the gaps around the pipes where they enter the case using a suitable
flexible sealant. - It's probably best to leave the front cover off until
the system has been filled with water and the system checked for leaks. - Fit the glazing retaining clips to the lower side of
the case. - Using putty or glazing tape, fit the front cover and
secure by screwing strips G and H to the sides of the case.