Sizing Solar Hot Water Tanks

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 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, and will never perform like a proper solar tank with internal heat exchanger

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-pressurized 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 will 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 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.

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-pressurized.  

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. Here is a good link for space heating solutions direct into concrete as the storage mass – Solar Home and Space Heating.

These are like the "normal" tanks you will have in your mechanical room. They are heavy and designed to hold pressurized 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 in floor heating (i.e to draw heat out of the tank)

This diagram shows the inside of a SolarStor residential solar water tank. These are great solar tanks but are more costly than retrofitting a regular tank.  However the performance will certainly be superior as it has extras like better insulation and thermos-well ports for accurate water temperature readings.

You can buy SolarStor tanks in three sizes; 50gallon, 80 gallon and120 gallon solar tanks.

The reason for the difference in price between the "modified" regular solar tanks and "proper" 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 compare:

• The diameter and length of the heat exchanger in the tank, i.e. 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.
• Better Insulated – Solar tanks will be better insulated
• More ports for sensor to accurately read the water temperatures.

I’ve 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.....

Let us say you buy a normal gas fired 40 gallon water heater from Home Depot. 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 is not shining the gas will heat more water. With a solar collector this cannot happen, you cannot heat the water with solar when the sun is not 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.  It’s really not worth it.

OK... so now we know you need more storage, what next...

Let us add some more pressurized solar tanks (proper solar 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. $2000 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.

Remember to install a pressurized tank in Canada it is a must to have CSA approval which not only limits the availability of suitable solar 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 pressurized "solar tanks" with heat exchangers for sale on the internet but a lot of them are not CSA approved and it is technically illegal to install them. In the case of a pressurized 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 pressurized tanks are OK but not practical for more than one or 2 x 30 tube  collectors at a time, now what? 

Un-pressurized solar tanks - Therefore many companies started making un-pressurized tanks. All an un-pressurized 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 STSS who make custom unpressurised solar tanks. These tanks come in almost any size you want but the standard range from 120 gallons up to 5000 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 banks of collectors to supply heat to the tank and separate coils can be added to remove the heat to distribute to heating sources such as radiant floor heating, domestic hot water or pools.

STTS tanks are professional un-pressurized tanks that work very well, they are very well designed, have a warranty and a track record with certifications and engineering. Best is they will cost you 1/3-1/4 the price of a pressurized solar tank!

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 low (large volume of water), the collectors can still heat the tank as the collectors are cooling. In other words you have produced solar energy to your water tank all day long maximizing the output available.

With a smaller 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 midmorning. 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. In other words you have only use 1/3 of your solar system capacity to generate heat energy.

You need also to consider at what angle you have placed the collectors.  If you are trying to heat a solar 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 solar 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 don’t 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!

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 vapor and you get a huge expansion of the fluid. Therefore in a closed loop solar system you will need large expansion tanks and pressure relief valves to control this over pressuring. If you don’t then something in the system will burst. With smaller storage tanks you might also consider 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. The best way to control a solar system is by using large amount of solar storage.

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.  This section below will explain this:

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. 

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 temperatures (Tm) and the (Ta) Ambient outside temperature the less efficient the collector is (less power it will generate).

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.

Graph source  https://www.phcppros.com/ December 2008

1. 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. i.e. 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, they work great when you live in a hot place, in the summer, when the air temperature is hotter than your pool, but as the temperature difference gets bigger and you have less sunlight then they don’t work well at all.

The middle line, the flat plate collectors, are not quite as efficient 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.  

Therefore 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 solar 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.