Hi, I am considering installing solar panels in my home to heat water only. 9 people living here so lots of hot water used. I have been recommended to install 5m2 of tubes and a 500l tank and am wondering if this sounds right?Also has anyone used Kingspan Thermomax HP200 tubes? Thanks
I would prefer to have two cylinders rather than one huge one if you are going down that route. I think 500L is too large as a single cylinder for 5 sq M of panels in most cases. Also, 500L cylinders are often more expensive than two 300L ones. If you use a 300L main cylinder and a second 300L or 200L as a pre-heat for this, then on sunny days you would end up with 500L of hot water, whereas on dull days you might at least get 300L of useable hot water. There are multi-cylinder solar controllers designed for this specific purpose. You feed the solar panel into the main cylinder until it is up to temperature, then use a motorised valve to divert the panel to the second cylinder.
Personally, if I was bathing 9 people, I would go for more than 5 sq m as well. In terms of balancing your expenditure, the configuration you suggest is spending too much on cylinders and not enough on panels to my mind.
There are two very specific types of vacuum system - tubes and flasks. Tubes have a single layer of glass with a small flatplate inside them. The downside is that after 20 years, when the vacuum has failed (and it will) then you have to replace the entire tube, which is very expensive. Also, the vacuum is held in by a seal welding copper to glass which is vulnerable to failure because of differint coefficients of expansion with temperature. Provided you fit a heat dump, this shouldn't be an issue, and I think the brand you mention has resolved this and recommends heat dumps.
The Chinese have been making a flask system in which you have two glass layers with the vacuum between them, just as in a thermos flask. Although their "noon" efficiency is lower, this system is actually every bit as efficient because they passively track the sun as it moves from east to west, and this makes up for the loss in noon efficiency. The plus is that replacement flasks are about €5 each, so if they get broken, or lose their vacuum after 20 years, replacement is both simple and cheap. Although the usual prejudice against Chinese systems is out there (and quality of Chinese panels is definitely variable), I prefer their system for its simplicity. I have had both systems on my house for the last 6 years and both are holding out well.
Lastly, in terms of sizing your cylinders, since you are opting for a vacuum system, you might consider letting the cylinder(s) store more heat by bringing them to a higher temperature. You can do this provided you don't have hard water. 300L at 85 degrees is the same heat storage as 420L at 60 degrees. You will be using a thermostatic mixing valve anyhow (hopefully...) to bring this down to a safe temperature at the taps.