No solar system can run without an expansion tank. But many water boiler installers and engineers have no idea how to size an expansion tank for solar.
It is often believed by many engineers and contractors new to solar that an expansion tank is sized (in btu or Kw) solely to the output of the solar system. This is only one component of the equation to look at.
In a boiler system we look at the difference in temperature that the boiler runs at, typically 20-70C (70-158F) and the volume of liquid in the system. As this is a very controlled system (we can easily turn of or on the boiler), the rules of thumb are well known. A 30kw (100mbtu) boiler in most houses with a liquid volume of 80L (20gal) using floor heating or old cast iron rads, will need a #30 tank (about 8 gal) and will likely never see much change in pressure.
If we took a solar system with the same output, 30kw which is about the same as about 15 flat collectors (depending on collector size and efficiency) and used the same #30 expansion, the system would go over pressure on the first fine day and probably blow the relief valve. But the pressure relief valve should be the last line of defense for the system. (BTW… I would put a #120 tank on that one)
The first line of defense in any solar system is the expansion tank. It is there to take up whatever pressure there is in the system and keep it at an equilibrium. As the storage tank heats up, the temperatures in all parts of the system increase and the expanding liquid must go somewhere and the only place it can go is the expansion tank. When the storage tank can hold no more heat and the pump shuts of, the collectors will continue to increase in temperature until it equals the heat loss of the enclosure. In a good flat collector this will be at 200C or higher (250C+ for vacuum collectors)
So, instead of a deltaT of perhaps 60C max (20 deg nominal) for a boiler, we can have a deltaT of 160C or more if you remember that some components of the solar system can be at -30C at 2AM on January 1st and 200C+ on a hot summers day.
On the hottest days of the year and when the storage tank is full of heat and pump is off, some of the water/glycol (propylene glycol should be used……never ethylene glycol or car antifreeze) will turn to steam. No matter at what pressure this happens, the water in the mixture will expand hundreds of times its volume and push the rest of the liquid out of the collector and down towards the pump station and expansion tank. Therefore, the rule of thumb is that the acceptance volume of the expansion tank must be greater than 10% of the system liquid volume (typical systems will expand this much on a regular basis) plus the entire volume of the collectors and the near collector piping.
Here is an example. A Viessmann system with 300L Viessmann tank and two flat collectors contains about 30L of liquid. Collector volume is 4L so the expansion tank must accept the 4L plus 3L of normal liquid expansion plus the volume of the liquid in the first 20 feet of piping (lets assume 3L for now). Expansion tanks for solar run at a higher static pressure than heating expansion tanks (typically 2-3 Bar or 30-45psi…1 bar is = to 1 atmosphere or 14.7psi) to be able to keep enough pressure to get the liquid to the top of some tall roofs. A tank will probably have 50% of its true volume filled (membrane will be flat across) when started up for the first time.
The total volume needed will be in the neighbourhood of 10L X 2 = 20L (5 gal) tank with a 10L acceptance volume. The equivalent tank in the heating style would be a #30 by Amtrol, for example. But….I am not through yet. These tanks are not designed either for the pressure or for the temperature they can be subjected to. In North America, where true solar expansion tanks are not sold in the average plumbing wholesaler, we have opted for the equivalent sized domestic water expansion tank which can take a higher pressure and larger changes in pressure and temperature.
I remember a number of years ago I was called in to look at a 10 collector system consisting of 30 Thermomax tubes each (30m2 system). Not only was there no pressure relief valve on the system, there was a #60 expansion tank. There was there no pressure in the system and when I went to the roof to see the piping I found 1″ copper elbows half blown apart. This system was a mess and the collectors were garbage. I found out after that the system was designed by a reputable mechanical engineering firm which refused any help from collector manufacturer (pride goeth before a fall).
Lesson………never undersize your expansion tank. A bit more glycol and a bit more expensive tank can create a system where the glycol does not have to be changed for 10 years.