solar water heating in spain

 Compact vacuum tube solar water heaters ( the type where the tank and collector tubes are joint an one unit  ) are very simple.

I will try and explain how the type that I import and sell works.

the type I have are known as pre heat units the main parts are-

1 vacuum tubes

2 hot water tank

3 copper coil heat exhanger 

4 top up tank

5 frame

The vacuum tubes are the parts that collect the energy, they are similar in construction to a thermos flask interior ie one glass tube inside another with a vacuum between the two, Then inner tube has a special coating that absorbes the solar energy and heats the water inside, the vacuum acts as an insulator , not letting heat escape once inside.

As the water in the tubes gets heated by the sun it becomes lighter and rises into the hot water tank, cooler heavier water sinks to the lowest part of the vacuum tube, all day this circulation continues heating the water up ( in the summer can easily reach boiling point )

This hot water stored in the tank is not used directly ( dose not go to the taps ) the reason for this is that it is at very low pressure, because the tank is open ( has an over flow vent ) If the water were to boil steam and hot water would be dumped, to maintain the water level in the hot water tank there is a small top up tank with a float valve  ( It could be compared with how a cold water storage tank works except the water is hot )

Inside the hot water  tank there is a copper coil heat exchanger , this is where we get the hot water to be used . 

When a hot tap is opened cold water enters the copper coil and is heated then returns to the tap for use.

Very simple !!