Wednesday, December 13, 2017
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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 attached a small box of tricks to the side of your existing domestic hot water tank.  The justification for this is partly that often you dont 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 pressurised and un-pressurised solar tanks including some places you buy them from.  It is also part of a longer article on heating homes with solar hot water using a real life example of a home using a large 1700 gallon tank.

 

The article wll be in two parts.  This part (part 1) will describe large solar tanks, and part 2 (to follow shortly) will describe the actual house set up.
 
Although the SHW systems are great at heating your domestic hot water, the question is always asked "can I heat my home with it".  The answer to this is both "Yes" and "No".  The full answer to this was covered in this article.  
 
Most solar water heating systems installed in peoples 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.
 
Why we dont sell solar tanks for solar systems?
We often get asked why we dont have tanks for sale.  The answer is simple.  There is no point you paying to ship a tank all over the country when you can a buy them locally.  

The country is so big that it is not really worth while us buying tanks, shipping them to us, just to ship it back out again to you.  This adds costs at every step.The whole idea about our solar company is to try and make it more affordable and to educate people to the best ways of installing a solar system.

We hear from people all over the country with sources of cheap cost effective tanks they find and use for their solar hot water systems.

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 Un-pressurised.  
 
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.  It can work in some scenarios.  See page 26 of the article here - Solar collectors direct into in-floor heating system.

Pressurized solar tanks

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. 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 heat exchanger onto it.  This could 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 adding the external heat exchanger to it.
 
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 infloor heating (ie to draw heat out of the tank)
 
This diagram shows the inside of a Buderus Solar Tank.  These are good solar tanks but very expensive, +$3000 each last time we checked just for the tank.
Buderus Solar Water Heating tankYou can buy other "solar" tanks similar to the above Buderus tank for less money, but you will still be looking at paying between $1500 and $2000 for each 80 gallon tank, before you pay for installation.
 
You can also buy 120 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 weight.
 
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 very 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 expensive tanks will have bigger (thicker, longer and better quality) heat exchangers.  More metal = better heat transfer to the tank.
  • The material, if the heat exchanger needs to be stainless steel or similar.  if its a plain steel tank then it will likely rust and fail prematurely.
  
Ive been told I can add multiple collectors on a single 80 gallon tank?
Yes you can add as many as you want to a single tank and the tank will heat up quickly but consider this.....
 
Lets 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 isnt shining the gas will heat more water.  With a solar collector this cant happen, you can not heat the water with solar when the sun isnt 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.  Its really not worth it.
 
OK... so now we know you need more storage, what next...
Lets add some more pressurised tanks. This sounds easier than it is.  In the photo below, for example, 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.  The tanks alone will cost you approx $1800 each just to buy, then you have to plumb them together making sure the flow is balanced across all the tanks.  You will need to buy more piping, insulation, balancing valves etc.
 
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. 
As you can imagine, trying to do this for solar system on larger commercial solar water heating projects is simply uneconomical.
 
 
multiple pressurized solar tanks
 

Remember to install a pressurized tank in Canada it has to have CSA approval which not only limits the availability of suitable tanks but adds significantly to the cost of the tanks.  The majority of a tanks volume is just air or space so it costs a lot to ship them.  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 you end dead, but your house insurance might not pay out!

OK.... so far you have told us pressurised tanks are OK but not practical for more than one or 2 x 30 tube  collectors at a time, now what? 
 
Un-pressurised  solar tanks - This is why many companies started making un-pressurised 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 holds heat.
 
You then "pre-heat" the water you use in your home or building by running it through a heat exchanger or copper coil immersed in the large solar tank.
 
The photos below are from a company called Trendsetter Industries who make custom unpressurised solar tanks.  These 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 way below 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 much less than multiple pressurized tanks.
 
You can see several coils hanging in the tank.  These are sized so you can add multiple bank of collectors to put heat into the tank and also so that you flow your infloor 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 Hasse Tanks.  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 and were designed in Germany for the european market where the use of solar is much more advanced than it is here in Canada.  However this advanced technology comes at a price, the largest tanks will cost $20,000 to $30,000.

The image below is taken from the Hasse Tank website as it shows these tanks being used as total home heating solutions with solar, wood burners and other sources of heat in the same tank.

Hasse Tank - Solar hot water storage tanks

The 350 gallon tanks cost in the range of  $7000 - $10,000.  When you compare the cost of buying 4 or 5 separate 80 gallon tanks and plumbing them together the cost is still much better.  When you get to several thousand gallon tanks the $$ per gallon is much lower.
 
 Other custom solar tank Manufactures are  STSS Co Inc
 

The DIY Approach - Large unpressurised solar storage tanks - 

All the above professional 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 dont mind getting your hands dirty, and are feeling adventurous then the DIY tank approach will save you a lot money.  
 
The DIY tank market can be started with American Solar Technics - These tanks are not as "polished" as some of the other tanks but are affordable and work well.  They also make a "200 gallon soft tank" which involves some DIY work on your own, but means it can be shipped easily.  This tank was installed in Vancouver and works well.  A photo compilation of a solar hot water system built with a 200 gallon soft tank can be found on this webpage.
 
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.
  • Plastic Water Tanks - Tote tanks and potable water tanks.  The 1700 gallon tank we describe in the story below was bought from Barr Plastics in Abbotsford BC.
  • Call into you 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.
  • 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 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 over heat.  

 

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 of 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, as the tank temperature is still fairly low (large volume of water), the collectors can still heat the tank as the collectors are cooling.  

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 will be cooling down through the afternoon and evening but the tank will still be very hot so no solar heating of the tank can occur.

What angle should you install the collectors at?

You need also to consider at what angle you have placed the collectors.  If you are trying to heat a plastic tank for winter space heating then your collectors should be at a steeper angle to pick up the winter sun which is lower in the sky.  If they are angled steeply they will not heat the tank as much in summer.  
 
In a closed loop system with a plastic tank,  80 galls per 30 tubes is an absolute minimum volume as 30 tubes will heat 80 galls to nearly boiling point on a good summer day.  Hence if you dont want the tank overheating in summer, 160 galls per 30 tubes should limit the temperature to approx 55 Deg C (130 Deg F).  Remember there are a lot of variables to this!
 

Overheating of solar systems - Closed loop or drain back solar system

Whenever a 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 pipe will boil and turn to vapour and you get a huge expansion of the fluid.  This is why in a closed loop solar system you will need expansion tanks and pressure releif valves to control this over pressuring.  If you dont then something in the system will burst.  You might also need a heat dump to remove excess heat from the system.
 
As the solar system increases in size, controlling this heat gets more complicated and expensive. 
 
To avoid any overheating, installing the solar system as a drain down solar system will prevent overheating, especially for larger solar water heating systems.  We will try to write another article on the benefits of drain down solar systems soon!
 

 

All solar collectors (evacuated tubes, flat plate collectors and un-glazed pool heaters are more efficient when heat cold water than they are heating hot water.  This section below will explin this

Advantages of the larger solar water tanks

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.

Solar collectors work more efficiently when heating low temperatures water in the tank.  As the solar collectors heat the tank up this means the efficiency of the collectors decreases the hotter the tank gets.  Hence with larger tanks the water is not as hot, but there is more of it so your collectors work more efficiently for longer.  This can be seen in the graph below. 
 
Efficiency of solar collectors graph
 
Calculating collector efficiency is complicated and not easy to do without a computer.  Not all collectors reports will show a graph like the one above.
 
What this shows is that the greater the temperature difference between the collector temperature (Tm) and the Ambiant outside temperature the less effecient the collector is.
 
The higher the temperature different between the outside temperature (Ta) and the Collector temperature (Tm) the less efficient the heating works.  The maximum efficiency is at the X axis = 0 (ie the collectors are just warmer than outside temperature, the maximum Power (W) is 1762.  On the right side, at X = 100, the power output drops to about 1180.
 
However note that even at high temperature differences the collectors still work well.  In the above case, if it is - 40 deg outside and the water is +60 then the collectors still work well.
 
Compere the above graph to the one below.
 
Solar Collector Efficiency graph
Graph source  http://www.plumbingengineer.com -  December 2008
 
Two things to note about this graph.
1.  On the left, the higher the line intercepts the Y axis the better the collector is at capturing sunlight under ideal conditions.
This shows that unglazed pool heating collectors are very good at capturing the sunlight.  ie they are black and no glass to get in the way of heating the rubber pool collectors.  However look how steep the black line is.  Yes they are very efficient but as soon as the temperature differential increases (moving right along the graph) the efficiency of then drops like a stone.

This means, work great when you live in a hot place, in the summer, they work great at heating your pool water. but as the temeperture difference gets bigger and you have less sunlight then they dont work well at all.

Th middle line, the flat plate collectors, are not quite as effecient as the pool collectors but the line is not as steep.  They have some insulation in them (the glazing) that helps prevent some wind and cold climate stealing heat from the collector.

Notice that both the pool and flat plate glazed collectors line both hit the x axis.  What this means in practice is that this shows that they both have a maximum temperature differential, above at which they will NOT heat water any more.  

This is why in the cold and cloudy conditions of Canada in the winter months, you cannot heat a pool with a unglazed rubber solar collector and why the performance of flat plate glazed pool collectors is so poor in the winter months.

Yes, if you want to heat your house and still collect heat from your solar collectors when it is sunny outside but -40 Degrees C or cloudy then evacuated tubes will still produce heat.  The SRCC test reports show that in cold and cloudy conditions, evacuated tube collectors can produce 1100% (yes 1100%) more heat than the flat plate collectors will.  See here for full details.

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