With hot water solar heating panels that heat a copper cylinder with no pump, to an internal heating coil, does the "flow" from the solar heating panel run to the top of the coil in the copper cylinder or the bottom, bearing in mind that with a gravity system hot water rises?
Z.
The solar coil is usually the lower coil in a cylinder, with the flow going to the top tapping.
Presumably for a gravity system the cylinder would be sited above the solar panels. difficult if they are on the roof. However i'm pretty sure that you can't have a gravity fed solar system, most panels and cylinders require pumps. The pipework diameters would be too small to get good flow.
With no pump in the solar panel circuit, the solar panels have to be BELOW the hot water cylinder, just as a solid fuel boiler has to be below the hot water cylinder. This can be achieved by mounting the panels at low level, for example in the garden or on a garage or other lower roof.
The pump is generally mains powered, but some systems use a small PV module that drives a DC pump. A mains pump needs a differential thermostat that only runs the pump when the solar collector is hotter than the water tank. A direct PV driven pump needs no control.
It is possible to have an entirely gravity driven solar system - some of the early systems worked like that. As OM says it's necessary to have the panels lower than the cylinder and have low resistance pipework, The panels need to have a "vertical" flow - so that rules out modern vacuum tube (heat pipe) types as the water flow is entirely horizontal in them along the top. Generally the performance is better with a pump and thermostatic controls. Agree too that the flow from the panels goes to the upper tapping on the cylinder - in effect the solar water cools as it gives up heat to the cylinder so wants to fall.
- Andy.
Does anyone actually have a pumpless solar installation ? you need a hot fire to get much sense out of a gravity fired heating system even of the back boiler kind, and as already noted, the heat source has to be the lowest point, as water expands when heated. The pump power can always be photo-voltaic if mains is not on-site.
Mike
Thanks Olympus, that is all I needed to know. I am aware that the heating panelwill have to be below the cylinder. The system wil be very basic and will just produce mainly limited tepid water. I have discovered that I can have a pretty good wash and shave with just over a mugful of hot water.
Z.
I grew up in a house with a back boiler. The gas fire/boiler was on the ground floor, and the tank was on the first floor. So hot water from the boiler naturally rose into the tank by convection.
A pump was only needed for the central heating, which took its heat from the hot water tank.
That's not going to work if the panels are on the roof, but could if they are at ground level.
Nice setup. Love the blue and white modesty screen which would double as a wind break.
I will not be heating the whole house, just producing limited hot water for washing and shaving.
Doesn't look ideal for gravity flow - you'd have to orientate it on a steep slope with the connections high & low (not both at the same level) - and then those small contorted channels look like they'll provide quite a resistance to the flow (and being horizontal not ideal for gravity circulation).
The old school way was to take a cheap ordinary radiator, paint it matt black and stick a sheet of glass in front of it (the posh ones were put in a wooden box with some rockwool insulation behind to reduce losses). They can be quite effective - I managed to scald myself with a homemade one when I was about 11.
you need a hot fire to get much sense out of a gravity fired heating system
Temperature often isn't a problem with solar - or rather one of the major challenges is controlling high temperatures. With the system I've got at home (vacuum tubes) not only are plastic pipes on the solar circuit a complete no-no, but soldered copper isn't acceptable either - the stagnation temperature of the panel can exceed the melting point of plumbers solder! So it's copper and compression fittings only (alternatively flexible stainless steel pipe). If it's not open vented then you need some way of dealing with excess pressure if the pump fails - some systems rely on the water in the panel itself flashing to steam and pushing the rest of the water content away from the heat source - so oversized (as well as heat resisting) expansion vessels are the normal (as well as pressure relief values).
- Andy.
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