solar tank water heater
Dave
questions now that I have started. I have removed the tank from an old
gas water and am going to paint it black. I have inspected the anode and
it still looks ok. In this setup, does it even need the anode? I am also
curious as to how far away from ones house this will work as in my
current situation, to get the most sun (without putting it on the roof),
I would have to place it farther away from the in house water heater or
lose all the west sun. Any and all info appreciated.
Thanks in advance,
Dave
Gary
You didn't say what kind of solar hot water heater it
is, but it sounds like a batch heater -- i.e. the tank
is the collector?
Anode Rod:
Since your collector tank sees pretty much the
same environment as a hot water tank,I would
think it would be good to retain the anode rod.
Heat loss from pipe:
For 3/4 inch copper pipe with the 3/8 inch thick hardware store
type foam insulation on it, you will lose about half the heat you added
to the water in the pipe in an hour(1), and about 3/4's of the heat in
two hours.
If (for example) you moved the collector enough to add 30 ft of
extra pipe between the collector and your hot water tank, and you
have (say) 5 hot water draws a day that are spaced a couple hours apart,
you will lose the heat you put into
(30/43) (5) (3/4)= 2.6 gallons/day
(43ft of 3/4 pipe holds 1 gal)
of the water. If your system is working pretty well, this might
be around 5% of what your daily production. Maybe not too big a deal?
You could reduce this by:
1) Using more insulation. If you doubled the thickness of the
insulation to 3/4inch, you would cut the heat loss by about
25%.
2) If you lump your heavy hot water draws together, you will
lessen the effect of pipe heat losses. In addition, if you time
your major water draws for late afternoon to early evening, you
will use the water before it starts the night time cool down --
this is probably more important than the pipe losses.
Losing Western exposure Problem:
If you are trying to locate your collector such that it
has enough sun to work well, and you have obstacles to
contend with, I would suggest this approach:
Get the CAM 417 "Sun Chart: Orientation" writeup from this site in Seattle:
www.cityofseattle.net/dclu/camlist/camlist.asp
Go through the procedure it recommends for each of your candidate
collector locations.
This procedure is for locating windows for good gain, but the
process is the same for your collector.
To use this method, you will need a sun chart for your location,
you can get one here: http://solardat.uoregon.edu/SunChartProgram.html
Look for a location you can place the collector
such that it gets at least 6 hrs of continuous sun
(ideally centered around noon). Aim the collector such
that the sun is perpendicular to the collector glazing in
the middle of the daily collection period.
Some additional rules of thumb for solar batch hot water heaters:
Glazing should point roughly South (+- 20 deg)
Glazing tilt should be roughly equal to latitude (+- 20 deg)
Use about 1 ft^2 of glazing per 2.5 gal of collector water tank capacity.
Ideally, provide 20 to 30 gals of collector tank capacity per person.
The inside surfaces of the box that encloses the tank
should be designed to reflect sunlight onto tank.
Cold water should enter at bottom of tank, and hot water
should be taken off at top of tank. This takes advantage
of stratification.
(vertical tanks encourage stratification, which is good)
Use copper pipe for plumbing.
Water weighs 8lb/gal -- tanks full of water need good support.
If you live in a climate where it gets below freezing:
Freezing of the supply lines is a common problem.
If you can integrate the solar water heater with the south
wall of the house, this eliminates freeze problems.
If you can work out some form of insulated cover that
is manually or automatically placed over the tank or the
glazing at night, this is a major gain, since it greatly
reduces night time heat loss.
Go Build It!! :-) Gary
(1) Explaining Pipe Loss Used Above:
This is a measured value, for these conditions Thot water = 122F,
Tambient = 59F, still air, insulation is the foam stuff that hardware
stores sell that is slit to allow you to apply it over the pipe, 3/8
inch thick. Temperature was down 33F in first hour, 15 more in 2nd
hour. The temperature was measured with thermocouple tucked between
insulation and pipe, and taped over. Also tried replacing about 5 ft of
insulation with some much thicker (1.5 inch)insulation, and did not get
as much gain as the heat loss equation predicts -- not sure if this is
different k values, or end effects.
Dave
just one large tank although I may make it larger by adding more tanks
inline. My problem here (in NW Florida) is that my house has not been
ideally located on the planet :) and the front of my house faces west,
the back east, and the south side is the at the opposite end of the
house from where my existing hot water heater is. And to top it off, the
south side has only a little yard space and then the woods, which blocks
most off the good location for solar. The north side as equally small
yard space and has woods on that side too. Putting it on the roof is
also out of the question. So, I will try to draw in text.
W
xxxxxxxxxxxxxxxxxxxxxxxxxxx
x x
x x
S x x N
x x
x O x
xxxxxxxxxxxxxxxxxxxxxxxxxxx
E
The house is about 30' wide and 60' long and the hot water heater is
represented by the letter "O". What would work in this situation?
Thanks,
Dave
Gary
Hi Dave,
I think the best thing to do would be to try that procedure on the
Seattle web site with the sun chart for your area. But, as a start, you
could just download the sun chart from
http://solardat.uoregon.edu/SunChartProgram.html – just enter 30 deg
latitude, and hit the “create chart” button. This will give you the
elevation of the sun as a function of time of day and month of year.
You can use this to get an idea at what locations and times the sun will
be above your obstacles. Just stand at the candidate collector location
with sun chart in hand and check South, SE, and SW. One thing you have
going for you is that because of the low latitude of Norther Florida,
the sun spends a lot of time above 40 deg elevation. I would try 1)
along the South wall of house, 2) the North yard – is there a North yard
location that can see over house?, 3) The SE and SW corners of house and
angled so that they see some of the South sky???
If you can't find a good all around location, you could think about a
South facing location in which you might loose a couple months in the
winter when the sun is low, or ESE or WSW pointing location where you
don't quite get 6 hours every day.
Getting a location with good sun is probably more important than
minimizing your pipe run lengths.
The payoff for batch hot water heaters is quite good, so you can afford
to make some compromises, and still get a good return.
Good Luck!! Gary
Dave
The ideal location looks to be the North side of the house as I
have been monitoring it the last few days and it gets sun "all day" over
the house. I am going to study the charts some more and try to figure
the best spot based an the lowest level the sun will be. Any ideas on
how to roughly figure for instance how far the shadow will moved based
on a latitude change of the sun (like for 10 degrees, how many feet)? I
bought a bunch of copper pipe and am still doing some thinking on some
advanced stuff for future reference as my wife says she has to shower in
the early morning as compared to the evening. I was thinking of actually
coming up with a way to "pump" the hot water into maybe a second hot
water heater in the garage that is not electrically connected but is
insulated for the morning batch of saved hot water...might not be
practical, but then again it may be very practical :)
Thoughts?
Gary
> Gary,
> The ideal location looks to be the North side of the house as I
> have been monitoring it the last few days and it gets sun "all day" over
> the house. I am going to study the charts some more and try to figure
> the best spot based an the lowest level the sun will be. Any ideas on
> how to roughly figure for instance how far the shadow will moved based
> on a latitude change of the sun (like for 10 degrees, how many feet)?
The length of the shadow is:
LS = Height/ tan(Elev)
Where LS = length of shadow
Height = height of obstacle
Elev = Elevation angle of sun
For example if obstacle Height = 15 ft, and sun Elev 50 deg, then
the length of the shadow is:
LS = 15/ tan(50) = 15/1.19 = 13 ft
Decreasing the Elev by 10 deg to 40 deg:
LS = 15/ tan(40) = 15/0.84 = 18 ft
Decreasing Elev another 10 deg to 30 deg:
LS = 15/ tan(30) = 15/0.58 = 26 ft
I
> bought a bunch of copper pipe and am still doing some thinking on some
> advanced stuff for future reference as my wife says she has to shower in
> the early morning as compared to the evening. I was thinking of actually
> coming up with a way to "pump" the hot water into maybe a second hot
> water heater in the garage that is not electrically connected but is
> insulated for the morning batch of saved hot water...might not be
> practical, but then again it may be very practical :)
I think you are wise not try to get your wife to change her shower time :-)
If you don't put in the extra storage tank, the water in your collector
tank might have an overnight temperature drop of about:
Qnight loss = (Aglaz)(Tamb - Ttank)(12hr)/ Rglaz
= (30ft^2)((120F+95F)/2 - 60F)(12hr) / 1 = 17000 BTU
Ttank drop = Q/(Wwater)(Cwater) = 17000/(750lb)(1) = 23F
(This assumes 30 sq ft of glazing, a 90 gal tank/collector,
afternoon tank temperature = 120F, and average night temperature
of 60F. Insulated back as sides on collector)
So, your tank temperature might drop from about 120F to about 97F
overnight. Even with this drop, you are still getting quite a bit of
benefit from the solar heater (i.e. your hot water tank is getting 97F
water, rather than 50F(?) water it would get from the "street" supply).
For water draws later in the day, you would get fully heated water.
You could reduce the night time drop with double glazing, or with some
form of manually or automatically deployed insulation over the
tank/collector or the glazing.
Given this, I'm not sure its worth adding the complexity of pumps and
circulation loops etc???
Gary
> Thoughts?
> Thanks,
> Dave
>
Dave
pressure testing now after cleaning and removing the old mineral
deposits. This is the second tank as the first one is maller and from a
gas water heater and has the hole down the center for the flue. I was
thinking that tank may have some kind of advantage (like getting heat
through a 4" tube for winter heat) but I believe there are better ways
for that. I am going to start with one tank and no pumps and go from
there...I appreciate the feedback,
Dave