Solar Water Heaters
A solar tank is a critical part of
your solar hot water system. The type and size of solar tank you need is
often a poorly understood part of a system, mainly because there are so many
variables to it.
Many systems installed by companies often attach a small box of tricks to the
side of your existing domestic hot water tank. The justification for this is
partly that often you don’t have room for a bigger tank in many mechanical
rooms and it also saves you buying another larger tank. While this is true,
it also means that your solar system will store very little heat during the
good sunny days.
This article will also describe various types and sizes of solar tanks that
can be used for solar water heating. It will describe pressurized and
un-pressurised solar tanks including some places you buy them from.
Most solar water heating systems
installed in people’s homes either re-use the existing water heating tank or
maybe add a second tank of 60 - 80 gallons in size. We also described in this article, why the
tank size you use is VERY important to the solar hot water system. Although
the solar systems still work on smaller tanks, you are also missing out on
much better performance with a larger tank.
What are solar hot water tanks?
The tank is simply like a battery for
electricity, except it stores heat in the form of hot water. Normally a tank
is used to store the heat in hot water.
The solar tanks are mainly split into two categories; Pressurized and
There is another store people can use for solar heat. It is possible to
connect the solar collectors direct to an in-floor heating supply. In this system the concrete becomes the
storage tank. This can work in some
scenarios where temperature control is not as important such as in a garage
or a shop.
These are like the "normal"
tanks you will have in your mechanical room. They are heavy and designed to
hold pressurised water for your house. The "pressure" is normally
supplied by the pressure of your homes incoming main cold water supply from
the city or well.
They hold the water coming into you house from the city water which will be
under pressure from the main city supply. An empty 80 gallon tank can easily
weigh 200 lbs.
You can use a "standard" tank such as a Bradford White tank with no
internal coils and then install your own external side arm heat exchanger
onto it. This will work OK for a small solar water heating system, ie one or
two 30 tube collectors. With this "normal tank" you spend less
money on the actual tank but will spend a bit more in time and money adapting
the side arm heat exchanger to the ‘solar’ tank.
However you can buy tanks specifically designed for solar hot water systems
that have internal heat exchanger coils built into them specifically designed
for solar hot water systems. The lower coil is attached to the solar system
then the upper coil can be used either (a) for connecting to a backup boiler
(ie put extra heat into the tank) OR (b) or connecting to heating such as a
pool or in floor heat (ie to draw heat out of the tank)
This diagram shows the inside of our SolarStor Water Tanks with two heat exchangers.
These are good solar tanks but cost considerably more than a standard tank
without a heat exchanger.
Our double wall heat exchanger tanks come in two sizes, an 80 gallon and a
119 gallon. We also have single coil
solar storage tanks available in 50 gallon size
Weight – Our 119 gallon solar tanks but these will weigh +300 lbs so even
moving them into the house is hard work. When they are full of water they are
very heavy and you need to make sure your floor is strong enough to hold this
The reason for the difference in price between the "expensive"
solar tanks and "cheaper" solar water tanks is often not obvious to
most people. 80 gallons is 80 gallons, but the quality of the internal parts
of the tank can vary enormously.
Some of the things you need to check:
- The diameter and length of the heat
exchanger in the tank, ie the square foot of metal. More square feet =
better heat transfer from the solar collectors to your tank. Our heat exchangers are 7.8 ft2
- More expensive tanks will have bigger
(thicker, longer and better quality) heat exchangers = they should last
longer. Our heat exchangers are
1.5” in OD diameter.
- More metal = better heat transfer to
the tank. The material, if the heat exchanger needs to be stainless
steel or similar. If it’s plain steel tank then it will likely rust and
fail prematurely. Look for a heat
exchanger that is coated to prevent rusting.
multiple collectors on a single tank? Yes you can add as many as you want to a single tank and the tank will
heat up quickly but consider this.....
Let’s say you buy a normal gas fired 40 gallon water heater from Homedepot. Say
the gas burner in this is 40,000 BTUs. Say this will heat the 40 gallons of
cold water to 130 Deg F in 30 minutes.
If instead of this, say you buy the same sized tank but this one has a 160,000
BTU gas burner ( 4 x the size). All this means is the water will be heated 4 x
as quick. You still only have 40 gallons of hot water.
With a gas burner this is OK, when the sun isn’t shining the gas will heat more
water. With a solar collector this can’t happen, you cannot heat the water with
solar when the sun isn’t shining! 40 gallons of hot solar water will not last
long when the sun goes down. If you are trying to heat a home or garage with it
then the heat in the tank will be gone very quickly.
If you put 3, 4 or 5 solar collectors on a 40, or even an 80 gallon tank, you
will have the same problem this. It will heat the tank quickly, often by midday
the tank will be at nearly boiling point and you then have to install systems
to "dump" the heat. Unless you
have a pool to use the solar heat, oversizing collectors on a tank is not a
Adding more tanks for more storage
One easy way to gain more storage is to add more pressurised tanks. If you
have 3 x 80 gallon tanks you can plumb them together so the water flows
through them all together giving you an effective 240 gallons. When adding tanks together to from more
storage, it is important to make sure the flow rates are balanced. If you get the "balancing" wrong
then all the flow from the solar collectors goes the path of least resistance
and only one tank heats up which is a waste of money and stored energy.
multiple pressurized solar tanks
Remember to install a
pressurized tank in Canada and the USA it has to have CSA/UL approval which
not only limits the availability of suitable tanks but adds to the cost of
the tanks. As you go to larger sizes
above 120 gallons, the costs skyrocket as the certification cost versus the
demand requires manufactures to recoup their cost by raising prices. There are pressurised "solar
tanks" with heat exchangers for sale everywhere but a lot of them are
not CSA approved and it is technically illegal to install them. In the case
of a pressurised tank, the CSA approval is important because if it blows up
and there is no approval, not only might someone get hurt, but your house
insurance might not pay out!
Un-pressurised solar tanks - All an un-pressurised solar tank consists of is
a huge tank which holds water. The solar heats the water but this water does
not get replaced or used in your home or building. It simply gets hot and
You then move the heat energy to your home or building by running it through
a heat exchanger or copper coil immersed in the large solar tank or and
external heat exchanger such as a plate heat exchanger outside the tank
Un-Pressurized tanks come in almost any size you want but the standard range
from 100 gallons up to 2500 gallons. These tanks are very well insulated and
provide a cost per gallon below what you could hope to buy with the
pressurized tanks. These are excellent tanks suitable for residential and
commercial applications but still cost a considerable amount of money, but
cost less than multiple pressurized tanks.
You can see several coils hanging in the tank. These are sized so you can add
multiple banks of collectors to put heat into the tank and also so that you
flow you’re in floor heating or the domestic hot water line through the tank
where they are pre-heated.
Trendsetter - large un pressurized solar tanks
There are other manufactures of such un-pressurised tanks such as Cocoon
These tanks range in size from 300 gallons up to 10,000 gallons in size. Even
the 10,000 gallon tanks are designed to fit through a normal doorway then are
built on-site in the room. This way you can get tanks of 10,000 gallons into
an existing building. They are super well insulated with wall thickness up to
8”. The advantage of the Cocoon Tank
is that it is made mostly of foam and as such is affordable to purchases and
The DIY Large unpressurised solar storage tanks
Professionally Manufactured un-pressurized tanks will work very well, they
are very well designed, have a warranties and a track record with
certifications and engineering. However they do cost a lot of money.
If you don’t mind getting your hands dirty, and are feeling adventurous then
the DIY tank approach will save you lot money. Almost any large vessel can be modified to
build a large un-pressurized storage tank.
You can also use almost any other tank you can find.
- Scrap yards - find old milk tank
trucks, old oil filed tanks etc. We found a 1000 gallon insulated
stainless steel milk tank at a Calgary scrap yard for under $1000.
- Old oil field tanks - modern oil tanks
need to be double walled to prevent spillage should one wall get a leak.
Hence there will be single walled tanks for sale. They will be messy and
need cleaning but could work.
- Call into your local plumbing supplier
/ wholesaler. Ask it they have any gas or electric tanks that have been
returned for a warranty reason. Very often the tank is still OK but the
electric element might have failed. All you are after is the tank.
- Try Kijiji or EBay for old tanks.
There are some limitations to
using a plastic tank.
- You will need to use some ingenuity to
build and modify the tank to your use.
- You will need to insulate it yourself
or come up with some novel insulation methods as described below. Read
this story on heating homes with solar collectors.
- The plastic tanks are not usually
designed to take very hot water. A custom solar tank will be capable of
storing hot water 80+ deg C (170 Deg F), but a cheap plastic tank tank
is likely to start to get "floppy" about 55 Deg C (130 deg F).
However remember if you use a very large tank then it is unlikely to overheat.
Larger solar tanks mean your system works more efficiently
A simplistic explanation of what this
means in practice is that with a very large solar tank when the tank starts
at 50 degrees F in the morning. The solar pump stations are generally set to
turn on the pump when the collectors are 10 degrees hotter than the tank. As
soon as the sun comes up and the collectors heat up to about 60 degrees F
they will be heating the tank.
The temperature differential in this case is only 10 degrees so they are
working very efficiently.
However as the sun goes down in the afternoon and evening, the tank temperature
is still fairly low but holding a lot of energy (large volume of water), the
collectors can still heat the tank even as the collectors start to cool as
they are still much hotter than the tank right down to after sunset. In this way we are maximizing the amount of
heat energy being stored throughout a day.
If you only have a small tank, as the tank heats up quickly the collectors
can only heat it when they are hotter than the tank. In summer a 40 or 60
gallon tank can be very hot 60 to 80 deg C (140 Deg F or 176 Deg F) by
midday. This means all afternoon the solar collectors can do nothing as they slowly
start to cooling down through the late
afternoon and evening but the tank will still be very hot so no solar heating
of the tank can occur.
Remember in September, the collectors can still be well over 100 Deg C at 5
pm. A 60-80 gallon tank in a normal home will have reached 80 + Deg C by
midday. This means all afternoon the tank will be at its maximum temperature
(set for safety) and not only are you wasting solar heat (no more room to
store it) but you will spend money running a pump to dump the heat to a heat
dump unless you have a pool to dump to.
Temperature is not energy – Most customers
often assume a hotter system means more energy. This is not true. In fact in solar water heating theory, the
colder the system operates at, relative to the outside temperature, the more
efficient it will be. Put in another
way, a collector is most efficient when the fluid it is heating is the same
temperature as the outside temperature.
This happens when the collector first starts up as the heating fluid
in the collector is the same temperature as the outside temperature. As the heating fluid gets hotter than the outside
temperature the power of the solar collectors decline.
All solar collectors (evacuated
tubes, flat plate collectors and un-glazed pool heaters are more efficient
when heating cold water than they are heating hot water. However vacuum tubes have the best
efficiency curve for high temperature differential heating. When you buy a solar collector, you want it
to be heating water for as many hours as possible. There is no point buying
it if it is only going to heat water for a couple of hours in the middle of
the day. A larger tank will ensure this
Overheating of solar systems – Expansion Tank
Whenever a solar storage tank reaches
the maximum temperature it can hold (set by you) the solar pump turns off to
protect the tank. At this point in a closed loop solar system the fluid in
the collector header will boil and turn to vapour and you get a expansion of
the fluid in the entire system as a result of the steam gas. This is why in a
closed loop solar system you will need a properly sized expansion tanks to
control this over pressuring. If you don’t
then something in the system will burst.
Sizing the expansion tank is very
important as is the type of tank you purchase. Most expansion tanks such as Amtrol are not
meant for the higher operating temperatures in a solar thermal system. All our pre-packaged solar heating system come
with the appropriately sized solar expansion tank made by Italian solar
manufacture Zilmet. These tanks have a
high temperature EPDM rubber bladder able to withstand higher temperatures.
As the solar system increases in size, controlling this heat becomes more
important and only a properly engineered solar water heating system should be