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Keeping thermocline in a solar water heater
I seem to be inventing, in these conceptual notes, a combination
thermosiphon/active solar water heater. Reviewing after 12 years, I'm
not sure if this idea has a chance of working or not; certainly most
systems I've seen just assume and accept turbulent mixing and
extra-power-burning pumps as the design foundation and build from
there, rather than more patiently explore how even subtle tweaks of
elevations and flow-shaping devices might optimize whole-system
design. If impractical for now, it was fun to think about; and maybe
it'll be practical later, for someone, which is the point.
- Background: Solar water heaters produce automatic flows as the sun heats the water in them, the water expands as it heats, and then it automatically rises, producing flow. In some designs this thermosiphon effect can enable pump-less operation, though with slow flow rates. Alternatively, actively-pumped piping through solar collectors and tanks achieves better flow rates, and you can put the inputs and outputs wherever you want, as a designer, at the cost of more and potentially unnecessary pumping energy.
- Consider a thermosiphon tank 2 feet above the solar collectors,
perhaps in addition to a storage tank elsewhere. 2 feet is
enough distance for the solar heating to produce enough
thermosiphon effect for the passive subsystem to flow. Then the
heater will send heated water up to the tank automatically, and
the coldest contents of the tank will descend, to fill the heater
at its low end with the coldest fraction of the water to be
heated: ideal.
- Precis: A combined thermosiphon/active system can minimize
pumping wattage.
- On stratification's benefits see "Thermal
Stratification in Storage Tanks." by Mohammad Ali
Abdoly, U Texas PhD Thesis (Abstract here:
http://adsabs.harvard.edu/abs/1981PhDT........75A,
viz.:
"Thermal stratification of a fluid in storage tank is a natural process that takes place due to the decreased density of the fluid at higher temperature. This natural process creates a transition zone temperature gradient between cold and hot fluid zones, called the thermocline. Thermocline storage is attractive for the following reasons: (1) The working fluid can be withdrawn from the top of the storage tank for use at higher temperatures than would be possible with a non-thermocline system. (2) At the same time the working fluid can be sent to the solar collector at lower temperatures than possible with a non thermocline system, thereby increasing the operating efficiency of the collector."
See https://www.osti.gov/biblio/5663663 - Use flow softening geometry to optimize temperature
stratification within the thermosiphon tank (old idea). The
coldest water should be what is sent to the solar collectors,
which are most efficient on cold input water. And the hottest
water should be what is taken off for actual end use. Gentler
inflows might preserve and maximize stratification, to best use
the different temperatures in the tank.
- My idea: provide a conical, reverse-funnel structure at the inlet
to a storage tank to slow the flow of liquid entering the tank.
The angle and length of the cone/funnel should be derived based
on the Reynolds number (turbulence characteristics) and
thermosiphon flow rates of the liquid, etc.
- 40-80 liters per hour is a sunny daytime flow rate range seen in
Figure 6 of this paper (http://www.nrel.gov/docs/fy06osti/39734.pdf,
in systems with 1 to 2 inch diameter thermosiphon pipes.
- Consistent with my concept, others have found that a large
(diffusion target) plate produces better system performance than
a smaller curving plate which directs hot water upward by
reducing backup heating requirements by 3 to 7% (See "Thermal
stratification in small solar domestic storage tanks caused by
draw-offs" by Ulrike Jordana, and Simon Furbo).
- On the other hand, a "settling mesh" target produces better
performance than some other type of diffuser.
- I propose to design for laminar-current or blob movement (think
of the Lava Lamp), rather than turbulence-based or continuous
diffusion or mixing, so that natural channels created by the
movement of different-density columns in the liquid, may be
enhanced by the inlet and outlet geometry.
- In particular, an entry horn or slowdown funnel (opening wider in
the direction of flow) can reduce the flow rate from the inlet
piping flow rate to the thermocline non-disrupting flow rate. If
the emitted stream or blobs move slowly enough, then they will
self-stratify into the thermocline just as fast as they emerge,
rather than turbulently or diffusely mixing and deteriorating the
thermocline. If the funnel axis is horizontal and its open end
is the full height of the tank, then through correct
selection of the cone angle of the funnel, the emerging stream
should be able to slow to the desired fraction of its entry
velocity before boundary layer separation occurs, separating the
stream from the walls of the funnel and directing itself into the
thermocline due to the differential buoyancy of hotter and cooler
liquid layers. The contrary design desiderata come from the
desire to control turbulence or mixing, namely, that the
separation of the entering liquid from the funnel walls not
happen before the flow rate is reduced enough produce blob-flow
behavior, that is, the rate at which the water will seek its own
stratification level rather than mix through turbulence.
- Alternatively, if the funnel be a vertical, downward-oriented
sub-duct within the tank, it should end at the design slowdown
point, thereby releasing blobs or currents to flow down or
sideways or sideways-then-up, hopefully to settle at their target
point on the thermocline. This design makes the
self-stratification a matter of the random settling of blobs
within a thermocline, rather than the possibly more-efficient
self-stratification of a stream directly aimed, from the side and
thus more horizontally, at its target layer.
- A diffuser within the inlet fitting or piping may aid in
laminarizing the flow.
- Use a reduced-capacity circulator pump to exchange cold and
heated water between thermosiphon tank and the storage tank.
Instead of a few degrees temperature difference as in a normal
active system, here the return water has already been through the
collectors many times using thermosiphon pumping and is quite
hot, and cycles far less frequently through the storage tank.
The storage-to-collector inlet connection is located on the
thermosiphon feed line to feed the coldest water directly to the
collectors. The collector-to-storage outlet is located on the
top of the thermosiphon tank to remove the hottest,
temperature-stratified water.
- Note that thermal conductivity through tank walls destroys the
thermocline (aluminum tanks faster than steel; steel faster than
glass; glass has thermal conductivity similar to water which
would make it not an accelerant to thermocline breakdown). The
mechanism is that some heat goes from a layer into the wall, down
through the wall, and heats wall-adjacent water at a lower layer,
which then becomes a buoyant blob that rises and mixes all kinds
of water in the area. So a little conductivity can go a long
way. Prefer glass walls.