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Putting a loop or more tubing to locally change elevation in the line from the tank out to the collector inlet won't get you much.
Like most aspects of solar energy, it certainly is not rocket science. If you truly know and understand the governing principle driving thermosiphon flow (that is, thermally induced density differences), that's as much explanation as needed to understand why warmer (actually less dense) fluid will tend to stay at higher elevation in any closed fluid loop, and why cooler (actually denser) or additionally cooled fluid will tend to stay at or move to lower elevations. Less dense fluids rise to the top of fluid loops and tend to stay there.
Think of a pipe loop consisting of a solar flat plate collector with a simple line connecting the outlet from the collector back to the inlet of the collector with no positive changes in elevation between collector outlet at the top and collector inlet at the bottom. It's all downhill, or all uphill, and one size line with no intentional storage.
Now, imagine what happens when the collector sits in the sun. The collector and the fluid in the collector portion of the loop heats up. As the fluid in the collector heats up, because of its (now) lower density it rises, just like the air in a hot air balloon, displacing some of the fluid above it and moving that displaced fluid upward and "forward", out of the collector and around the loop. As more heat is added, this circulation continues in one direction ("upward"), and all's well.
Now, thin what happens to fluid temps. in the loop after a sunny day. The sun goes down, the collector and the fluid in it become cooler than the rest of the fluid in the loop, and the fluid density in the collector increases, with the loop volume that was occupied by the formerly cooler (and less dense) fluid now filled by fluid from "above", inducing reverse circulation with nothing to prevent it. As long as the condition exists where the collector and the fluid in it are warmer than the surroundings, and so heat is being removed from the fluid, and the fluid density increases as fluid temp. decreases, the fluid flows "backwards", top to bottom.
Now, imagine a wide spot in the loop and call it a tank, and imagine what happens when the tank is put in two different locations. First, put the tank directly and entirely below the collector. During a sunny day, the collector loop and the much larger quantity of contained fluid will have been operating just like the situation with no wide spot/tank in the line. At the end of the day, the line, and the tank, and the contained fluid will be relatively warm compared to the surroundings. As the collector cools after sundown, the fluid in it will also cool down and so become denser than the rest of the fluid in the loop. Just like the no storage case, the now denser fluid in the collector will then "drop" into the now lower tank, with the specific volume difference induced in the cooler fluid by the temp. drop being replaced by fluid from the "upper" collector connection (the "hot" connection). While all this is going on, the warmer fluid in the tank at a lower elevation really wants to get to a higher elevation (just like the hot air in a balloon) than the cooler fluid in other portions of the line (like the collector) - and it has just the path to do so - "backward" through the line from the tank "inlet", and up to the collector "hot" "outlet". This reverse circulation will continue as long as the temp. of the fluid in the collector remains cooler than the fluid in line "above" it.
Lastly, envision a similar setup, but this time with the wide spot/tank directly above the collector. Same sunny day. Fluid in the collector heats up and flows upward through the collector into the tank just like before, same flow direction and everything. Only difference this time is the location of the tank. Throughout the day, life is good and fluid temps. at day's end are probably close to or a bit more than when the tank was below collector. Now, sun goes down, the collector and the fluid in it cools off same as before and, just like before, because the collector and the fluid in it are getting cooler than the rest of the fluid in the loop (including the tank fluid), the collector fluid gets denser and wants to sink. In the mean time, the warmer (and less dense) fluid in the tank, now at the top of the loop is quite content to stay right where it is, and thus acting as a counter to the cooler fluid wanting to drop and initiate a reverse flow. This push and pull quickly results in a stalemate: Circulation stops, or almost stops. Cooler, more dense fluid goes to the lower portions of the loop and wants to stay there. Warmer fluid, already at the top of the loop doesn't want to go anywhere, and for the most part, it doesn't.
Themosiphon effects or fluid density driving forces increase with relative temperature differences in the working fluid, as well as by greater specific volume (units of volume/mass, as in ft.^3/lbm., or m^3/kg.) changes as f(temp.). For heating applications where fluid density decreases with increasing fluid temp. Storage needs to be above the heating surface to prevent unwanted (reverse) thermosiphoning. For fluids where density increases with increasing temp. (usually not a consideration) tanks need to be below the heating surface. Wwhere fluid density decreases with increased temp. (as in most practical cases, the tank needs to be above the level of the highest heating surface do not necessarily increase with tank elevation changes once the tank is above the level of the highest heating (or cooling) surface. As long as a fluid exhibits a density decrease with temp. increase, a, say, 50 deg. C. temp. diff, in the working fluid temp. in a thermosiphon loop, will produce more flowrate than, say, a 10 deg. C. temp. diff., regardless of the relative diff, in heights between the heating (or cooling) surface and a , i.e. a flat plate solar collector and the fluid height above the collector.
In a twist of nature, and pretty much useless for this discussion unless considering thermosiphon activity below that temp., but perhaps worth noting, water's greatest density occurs at ~ 3.98 C. or so and becomes less dense both above and below that temp.
Like most aspects of solar energy, it certainly is not rocket science. If you truly know and understand the governing principle driving thermosiphon flow (that is, thermally induced density differences), that's as much explanation as needed to understand why warmer (actually less dense) fluid will tend to stay at higher elevation in any closed fluid loop, and why cooler (actually denser) or additionally cooled fluid will tend to stay at or move to lower elevations. Less dense fluids rise to the top of fluid loops and tend to stay there.
Think of a pipe loop consisting of a solar flat plate collector with a simple line connecting the outlet from the collector back to the inlet of the collector with no positive changes in elevation between collector outlet at the top and collector inlet at the bottom. It's all downhill, or all uphill, and one size line with no intentional storage.
Now, imagine what happens when the collector sits in the sun. The collector and the fluid in the collector portion of the loop heats up. As the fluid in the collector heats up, because of its (now) lower density it rises, just like the air in a hot air balloon, displacing some of the fluid above it and moving that displaced fluid upward and "forward", out of the collector and around the loop. As more heat is added, this circulation continues in one direction ("upward"), and all's well.
Now, thin what happens to fluid temps. in the loop after a sunny day. The sun goes down, the collector and the fluid in it become cooler than the rest of the fluid in the loop, and the fluid density in the collector increases, with the loop volume that was occupied by the formerly cooler (and less dense) fluid now filled by fluid from "above", inducing reverse circulation with nothing to prevent it. As long as the condition exists where the collector and the fluid in it are warmer than the surroundings, and so heat is being removed from the fluid, and the fluid density increases as fluid temp. decreases, the fluid flows "backwards", top to bottom.
Now, imagine a wide spot in the loop and call it a tank, and imagine what happens when the tank is put in two different locations. First, put the tank directly and entirely below the collector. During a sunny day, the collector loop and the much larger quantity of contained fluid will have been operating just like the situation with no wide spot/tank in the line. At the end of the day, the line, and the tank, and the contained fluid will be relatively warm compared to the surroundings. As the collector cools after sundown, the fluid in it will also cool down and so become denser than the rest of the fluid in the loop. Just like the no storage case, the now denser fluid in the collector will then "drop" into the now lower tank, with the specific volume difference induced in the cooler fluid by the temp. drop being replaced by fluid from the "upper" collector connection (the "hot" connection). While all this is going on, the warmer fluid in the tank at a lower elevation really wants to get to a higher elevation (just like the hot air in a balloon) than the cooler fluid in other portions of the line (like the collector) - and it has just the path to do so - "backward" through the line from the tank "inlet", and up to the collector "hot" "outlet". This reverse circulation will continue as long as the temp. of the fluid in the collector remains cooler than the fluid in line "above" it.
Lastly, envision a similar setup, but this time with the wide spot/tank directly above the collector. Same sunny day. Fluid in the collector heats up and flows upward through the collector into the tank just like before, same flow direction and everything. Only difference this time is the location of the tank. Throughout the day, life is good and fluid temps. at day's end are probably close to or a bit more than when the tank was below collector. Now, sun goes down, the collector and the fluid in it cools off same as before and, just like before, because the collector and the fluid in it are getting cooler than the rest of the fluid in the loop (including the tank fluid), the collector fluid gets denser and wants to sink. In the mean time, the warmer (and less dense) fluid in the tank, now at the top of the loop is quite content to stay right where it is, and thus acting as a counter to the cooler fluid wanting to drop and initiate a reverse flow. This push and pull quickly results in a stalemate: Circulation stops, or almost stops. Cooler, more dense fluid goes to the lower portions of the loop and wants to stay there. Warmer fluid, already at the top of the loop doesn't want to go anywhere, and for the most part, it doesn't.
Themosiphon effects or fluid density driving forces increase with relative temperature differences in the working fluid, as well as by greater specific volume (units of volume/mass, as in ft.^3/lbm., or m^3/kg.) changes as f(temp.). For heating applications where fluid density decreases with increasing fluid temp. Storage needs to be above the heating surface to prevent unwanted (reverse) thermosiphoning. For fluids where density increases with increasing temp. (usually not a consideration) tanks need to be below the heating surface. Wwhere fluid density decreases with increased temp. (as in most practical cases, the tank needs to be above the level of the highest heating surface do not necessarily increase with tank elevation changes once the tank is above the level of the highest heating (or cooling) surface. As long as a fluid exhibits a density decrease with temp. increase, a, say, 50 deg. C. temp. diff, in the working fluid temp. in a thermosiphon loop, will produce more flowrate than, say, a 10 deg. C. temp. diff., regardless of the relative diff, in heights between the heating (or cooling) surface and a , i.e. a flat plate solar collector and the fluid height above the collector.
In a twist of nature, and pretty much useless for this discussion unless considering thermosiphon activity below that temp., but perhaps worth noting, water's greatest density occurs at ~ 3.98 C. or so and becomes less dense both above and below that temp.
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