User:Macintoshiba/sandbox

Solar drain back system
A solar drain back system is a type of solar water heating system where the system is protected against freezing or overheating by draining the water from the collectors and above roof piping into a drain back vessel. A big advantage of these types of systems is that they are low maintenance since the collector loop fluid usually is tap water which doesn't degrade. (Although sometimes a little bit of Propylene Glycol is added for extra safety).

The main disadvantage of a drain back system is that the system must drain completely. If due to a mistake the system cannot drain completely than either the collector and/or above roof piping will (eventually) start leaking due to frost damage. So typically a drain back system either operates with minimal maintenance for many years or it break's in the first winter it experiences.

Moreover the collector and collector loop piping needs a slight slope so as to ensure all the water can be drained. Usually this slope needs to be somewhere between 1 to 3 cm/m. Although a slope of 0.5 cm/m may be possible, this however makes the installation very difficult. For example great care most be taken to ensure that above roof piping doesn't sag. Also the collector inlet(s) need to be high enough above the roof so as to ensure that the cold (and hot) collector piping has enough height to slope down and enter the roof.

Operation
Overheating and frost protection is provided by simply shutting off the pomp. The water in the collector and piping than simply drains back to the drain back vessel. The collectors can reach a maximum stagnation temperature of typically around 200°C-250°C depending on the collector type and location. The pump at startup first runs at full power since it needs to push the water up to the highest point. If the collectors are at a high temperature than collector will be quenched, and it's temperature drops rapidly from stagnation temperature to boiling water temperature of around 100°C and some high temperature steam will be produced. When the highest point is reached the water falls down and acts like a pulley. The pumping power can therefore be reduce to a lower power setting. The pump continues to run as long as as sufficient heat can be gained. When to pump shuts down the water drains from the collectors and piping and the system is in rest.

Pressurized drain back system
Pressurized drain back systems usually employ harp flat plate collectors or evacuated heat pipe tubes. However any type of collector can be used as long as the collector can be fully drained, is rated for the working pressure and the collector doesn't break when it's quenched from the stagnation temperature to the cold water inlet temperature. Sometimes a small restriction is placed before each collector so as to ensure that the flow over all the collectors is equally distributed. Since the system is a closed system, corrosion is usually not a big problem. Besides a pressure relief valve, sometimes also the installation of a vacuum breaker is required.

Non pressurized drain back system
The majority of installed solar systems in China however employ all glass evacuated tube collectors. These types of collector are both very cheap and efficient. Obviously these type of collectors cannot resist a lot of pressure (typically about maximum 0,2 to 0,5 bar over pressure) so the system has to be a non pressure system and is open to the atmosphere. Most systems in China are therefore simple non pressure thermosyphon systems. Recently pressure less drain back systems have been installed using these type of collectors. Combining the advantages of a drain back system and the all glass evacuated tube collectors. The system use the vertical type all glass evacuated tube. The water in the piping will drain, but the water in the evacuated tubes remains. The water in the evacuated tubes however will not freeze due to thermal inertia. Although the temperature may drop during the night. During the day even a little bit of sun will raise the temperature above freezing and melt the ice. Only for the North of China this is no longer the case and Glycol based systems are installed instead.

It's important to note that all glass evacuated tube collectors (usually) will break when they are quenched from it's stagnation temperature to the cold inlet water temperature. Therefore the tubes need to be filled with water all the time.