HOW IT WORKS

Solar water heaters work by the phenomenon of "Thermosyphon" natural convection circulating the hot water in the collector panels into the insulated hot water tank above, (hot water is lighter, it rises while cold water is heavier, it sinks). No pumps, or motors are involved. The cold water descends from the bottom of the storage tank to the bottom of the collector panel where it is heated up by solar energy. The water rises up the panel as it heats and returns to the tank where it rises to the top of the tank ready for use.

EFFICIENT THERMOSYPHON FLOW MEANS MORE HOT WATER FOR YOU

Obviously in this weak thermosyphon convection flow system (there are no pumps), other factors being equal, the highest water temperatures are achieved by solar water heaters whose thermosyphon circulation paths are the most efficient and offer the least drag, i.e.:
1. shortest thermosyphon loop, directly from panel to tank, no twists and turns,
2. no bottlenecks, freer flowing through greater number of valves or feeder pipes into tank from panel, no sharing of a common manifold,
3. minimum or zero horizontal run (hot water wants to go up,
and cold water down, not horizontally), etc. An efficient free flowing system with minimal drag will start to operate even under cloudy sky conditions, while a system with a high threshold of drag resistance (such as single feeder pipe systems) will operate only under sunny conditions or shut down under cloudy skies (you will then have to switch on the electric backup heater). The efficient system will react faster, deliver more hot water per day, working from sunrise to sunset, while the inefficient system is more lethargic; it starts work later and shuts down earlier.

DRAWBACKS OF SOME CONVENTIONAL SOLAR WATEH-HEATERS BOTTLENECK - SINGLE FEEDER PIPE

If you look carefully at a conventional solar water heater on your neighbour's roof, you will notice the single feeder pipe running from top corner of the panel to the tank and the single return flow pipe running from the tank down the side of panel. Where it enters the tank, this single feeder pipe system causes a

Bottleneck or Constriction m flow diameter.

This is because it collects hot water from numerous (10 to 20) inclined heating pipes into one small single horizontal feeder pipe which feeds a large 60 gal tank. This bottleneck can be compared to 16 traffic lanes squeezed into one single lane -a traffic jam results. Thermosyphon flow slows down, a hot spot develops in the panel, more heat is lost through the glass, and less hot water is collected in the tank.

CONVENTIONAL SOLAR HEATERS HAVE HORIZONTAL FLOW IN THEIR THERMOSYPHON CIRCUIT
(very inefficient)

The bottleneck problem is further aggravated by hot water stagnation due to the horizontal angle of the upper manifold. The hot water flow stagnates as it is forced to travel horizontally for up to 2 metres across the top of the panels before it can enter the tank. The hot water wants to rise, does not want to move horizontally. It just sits there in the horizontal manifold. This leads to hot water stagnation, especially at the far end of the manifold furthest away from the inlet into the tank.

WHY IS HOT WATER STAGNATION IN THE HORIZONTAL MANIFOLD INEFFICIENT?

Because a hot spot develops in the panel the stagnant hot water is trapped in the manifold, getting hotter and hotter but the heat is reradiated back out through the glass. This heat lost through the glass should have been collected safely in the insulated hot water tank.
 
 

Detail of flow in horizontal manifold far from feeder  
pipe to tank
Detail of flow horizontal manifold close to feeder  
pipe to tank
CONVENTIONAL SINGLE VALVE
Hot water is trapped in horizontal manifold, cannot rise any more (too far from single feeder pipe into tank) Smooth flowing hot water in only 3 or 4 heating pipes closest to the single feeder pipe. This is because the hot water can slowly make its way to the single feeder pipe nearby and continue rising into the tank
 
WHAT HAPPENS ON CLOUDY DAYS?

On cloudy days the thermosyphon force is reduced because the temperatures are lower. (Thermosyphon/convection force depends on how hot the water in the panels are, the hotter the water, the stronger the thermosyphon force). Solar Water Heaters with a high resistance to thermosyphon flow, such as conventional single feeder pipe systems with horizontal manifold may not work as well on cloudy days as the reduced thermosyphon force may not be sufficient to push the hot water into the tank.

HOW DOES IT AFFECT YOU?

You may run out of hot water on cloudy days with an inefficient conventional solar water heater and have to switch on the electric backup heater (very expensive, goes against why you purchased a solar system in the first place, i.e. to save money and not use electricity.)