So far we have only talked about one hot water tank : the hot water storage tank that stores the water heated by the sun ready for use by you or your grimy kids.
Why would you possibly want another tank?
Well it's all about trying to reduce the use of the gas/electric booster as much as possible.
Imagine this scenario:
At the end of the day, you have a nice tank full of sun-heated hot water. That evening you all have your baths/showers etc and drain the tank of hot water.
The hot water system will then have to refill your storage tank with cold water and if you have an electric boost in the storage tank, it will have to heat the entire volume of hot water ready for the morning. This will use a lot of electricity as the element works long and hard to heat up all that water.
However if you have a second tank, which has been quietly filling with warm-to-hot water whilst the main tank was full, this will flow into the main tank as the main tank is depleted. Even if the water from this second 'preheater' tank isn't 100% as hot as it needs to be, it wil be a lot hotter than cold, mains water, and your electric boost will need a lot less energy to get it up to temperature.
You may be wondering why not simply use a bigger storage tank? The reason is all about 'maximising the solar gain' of the collector. This bacially means that the colder the water that goes through the collector, the more heat will be transferred from the sun. So it is more efficient to 'pre-heat' a second tank of cold water with the sun, than to keep a large, single tank chock-full of hot water all the time.
So to summarise: if you have an electric boost and use lots of water in the evening, a second, preheater tank will save you a good deal of electricity. The obvious disadvantage of a 2nd tank is that it is going to add to the upfront cost and complexity of your system.
OK, so now you know about the 2nd tank option, let's move on to our penultimate option: Open or Closed Loop?