Post by cye on Jul 12, 2014 2:32:36 GMT -5
There are numerous ways of getting solar energy into a hot water tank. One can pump the hot water into the tank directly which does not required a heat exchanger at all, but whilst very simple, this has the disadvantage that anti-freeze cannot be used, and furthermore, in hard water areas, may clog up the internals of your solar panel. To be able to use antifreeze you can either use a tank with a heat exchanger inside the tank, or you can fit an external heat exchanger to the side of the tank.
This thread provides a comparison of the external heat exchanger versus the standard solar tank.
If you use a tank designed for solar, it will have an additional coil in the base of the tank to which the 'solar loop' (the plumbing to and from the solar panel) is connected. The solar coil resides in the base because this is where the coldest water in the hot water tank sits. The solar panel will always work more efficiently when heating cold water as opposed to heating already warmed water, plus the coil being lower in the tank has a larger body of water overall to heat than would be the case if the solar coil were higher up in the tank. Solar tanks are typically larger too, and so the 'thermal energy battery' is effectively bigger with a solar tank.
Most solar tanks are twin coil, the top coil for the boiler (central heating or stove) and the lower coil for the solar. You will also see triple coil tanks; one for oil/gas heating, one coil perhaps for the wood stove, and again the lowest usually for solar.
Now an external heat exchanger is outside of the tank and will gets its feed of cold water from the base of the hot tank and will feed hot water into the upper region of the tank. An external heat exchanger is also commonly known (in american parlance anyway) as a side arm heat exchanger. external heat exchangers are ideal for retrofitting to an existing standard single-coil tank. (one can retrofit an additional internal coil heat exchanger to a standard single coil tank, but we'll not go into this at present . )
A solar water heating application using an external heat exchanger will heat the water in the main tank from the top down, whereas a standard solar tank will heat the water from the bottom up.
The attached video from willis explains this perfectly
www.willis-renewables.com/video.htm
(well almost perfectly, as the vid suggests that a standard solar tank will only start heating when the collector is at 70c, which is very different to how a setup with a standard solar tank would be controlled. Standard solar tanks generally run the solar collector panel at lower temperatures which is more efficient compared with the solasyphon principle - That matter is a bit of clever marketing by willis, i.e., implying that the solasyphon captures as much energy in the same time as a standard solar tank)
How does the eternal heat exchanger heat from the top down? Well it is just a thermo-syphon. For those who do not know how a thermosyphon works using a heat exchanger external to the tank here's how I managed to explain it to myself:
(understanding this also explains why the tank sensor for differential temperature controller goes in a different place compared with a standard solar tank)
[#1] imagine a 1cm wide vertical column of water in the sidearm heat exchanger and the vertical pipework connected to it above and below.
[#2] now imagine a 1cm wide vertical column of water inside the hot water tank.
Imaginary water columns #1 & #2 are the same height, i.e, approx the height of the tank itself, and the two vertical columns are connected both at the bottom and at the top (as the side arm HEX is fed with cold from the base of the hot water tank and is teed into the how water tank not far above the top of the tank)
now how does the thermosyphoning work here? well, when the solar panel heats the water on the HEX side (column #1), this expands and gets lighter overall than the imaginary column of water in the hot water tank (column #2). as the water in the HEX side rises higher (because hotter water is less dense and therefore will float above colder water) it sucks in colder water from the base of column#2, effectively pushing hot water into the top of the hot water tank (column #2) and sucking in colder water from the bottom of the hot water tank. i'm sure there is a better explanation of thermosyphoning available than the above, but that's how i visualise it.
Now for the above to work as described, the solar panel needs to heat the water on the hex side (column #1) to a an average temperature greater than the average temperature of the water in the hot water tank (imaginary column #2). So if the differential temperature controller only sees the tank temp at the base of the hot water tank this will clearly not work, as the panel will only be heating column #2 to a temp above the base of the tank. So somewhere above the middle of the hot water tank will be required for the tank sensor, but where exactly? Well, as hot water stratifies in a tank we can't really be sure where, as the level of the hot water layer in the tank will vary with the amount of hot water in the tank (stating the obvious). So, to be on the safe side all the time, the tank sensor for any sidearm HEX solar setup needs to be at the top of the tank.
This thread provides a comparison of the external heat exchanger versus the standard solar tank.
If you use a tank designed for solar, it will have an additional coil in the base of the tank to which the 'solar loop' (the plumbing to and from the solar panel) is connected. The solar coil resides in the base because this is where the coldest water in the hot water tank sits. The solar panel will always work more efficiently when heating cold water as opposed to heating already warmed water, plus the coil being lower in the tank has a larger body of water overall to heat than would be the case if the solar coil were higher up in the tank. Solar tanks are typically larger too, and so the 'thermal energy battery' is effectively bigger with a solar tank.
Most solar tanks are twin coil, the top coil for the boiler (central heating or stove) and the lower coil for the solar. You will also see triple coil tanks; one for oil/gas heating, one coil perhaps for the wood stove, and again the lowest usually for solar.
Now an external heat exchanger is outside of the tank and will gets its feed of cold water from the base of the hot tank and will feed hot water into the upper region of the tank. An external heat exchanger is also commonly known (in american parlance anyway) as a side arm heat exchanger. external heat exchangers are ideal for retrofitting to an existing standard single-coil tank. (one can retrofit an additional internal coil heat exchanger to a standard single coil tank, but we'll not go into this at present . )
A solar water heating application using an external heat exchanger will heat the water in the main tank from the top down, whereas a standard solar tank will heat the water from the bottom up.
The attached video from willis explains this perfectly
www.willis-renewables.com/video.htm
(well almost perfectly, as the vid suggests that a standard solar tank will only start heating when the collector is at 70c, which is very different to how a setup with a standard solar tank would be controlled. Standard solar tanks generally run the solar collector panel at lower temperatures which is more efficient compared with the solasyphon principle - That matter is a bit of clever marketing by willis, i.e., implying that the solasyphon captures as much energy in the same time as a standard solar tank)
How does the eternal heat exchanger heat from the top down? Well it is just a thermo-syphon. For those who do not know how a thermosyphon works using a heat exchanger external to the tank here's how I managed to explain it to myself:
(understanding this also explains why the tank sensor for differential temperature controller goes in a different place compared with a standard solar tank)
[#1] imagine a 1cm wide vertical column of water in the sidearm heat exchanger and the vertical pipework connected to it above and below.
[#2] now imagine a 1cm wide vertical column of water inside the hot water tank.
Imaginary water columns #1 & #2 are the same height, i.e, approx the height of the tank itself, and the two vertical columns are connected both at the bottom and at the top (as the side arm HEX is fed with cold from the base of the hot water tank and is teed into the how water tank not far above the top of the tank)
now how does the thermosyphoning work here? well, when the solar panel heats the water on the HEX side (column #1), this expands and gets lighter overall than the imaginary column of water in the hot water tank (column #2). as the water in the HEX side rises higher (because hotter water is less dense and therefore will float above colder water) it sucks in colder water from the base of column#2, effectively pushing hot water into the top of the hot water tank (column #2) and sucking in colder water from the bottom of the hot water tank. i'm sure there is a better explanation of thermosyphoning available than the above, but that's how i visualise it.
Now for the above to work as described, the solar panel needs to heat the water on the hex side (column #1) to a an average temperature greater than the average temperature of the water in the hot water tank (imaginary column #2). So if the differential temperature controller only sees the tank temp at the base of the hot water tank this will clearly not work, as the panel will only be heating column #2 to a temp above the base of the tank. So somewhere above the middle of the hot water tank will be required for the tank sensor, but where exactly? Well, as hot water stratifies in a tank we can't really be sure where, as the level of the hot water layer in the tank will vary with the amount of hot water in the tank (stating the obvious). So, to be on the safe side all the time, the tank sensor for any sidearm HEX solar setup needs to be at the top of the tank.