Solar Thermal

Tiny House Chronicles: More Adventures in Plumbing

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Published on the Doomstead Diner on June 27, 2016

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My preferred system for solar thermal water heating with passive gravity/thermosiphoning circulation was explained in these old diagrams: (for explanatory text see previous tiny house article on plumbing).

diagram1  diagram2

That arrangment offered these advantages:

  1. Hot water cylinder (HWC) is located indoors, hence retains heat better

  2. HWC positioned vertically, hence more effective thermolayering (smaller interface between cold water layer at the base and hot water layer above, hot water being extracted from the top)

  3. The "heat pipe core" type evacuated tube is preferable to the hollow type, because if one tube breaks, the entire system can continue to function well

Unfortunately I was completely unable to obtain my preferred type of indirect small hot water cylinder with large bore internal copper coil capable of thermosiphoning. This forced me to change tack.

diagram3My other alternative is the "hollow core" type evacuated tube system with HWC mounted on top of the tube array, the entire system which sits outdoors. This is often promoted as a stand alone system: cold water is poured into the inlet near the top of the cylinder and after a few hours, hot water is drained from the base diagram 3. The tubes fill with water from the cylinder and this water is heated directly by the sun (in contrast to the heat pipe core type where fluid picks up heat within a manifold which houses the tops of heat pipes which contain acetone).

diagram4Convection currents in the hollow evacuated tubes are set up as shown in diagram 4. Obviously the convection currents cease at night.

The stand alone system does not allow for continuous filling of the cylinder, unless a header tank with ball-valve is attached to the vent at the top of the HWC.

That arrangement was not suitable for my purposes, hence using some lateral thinking I am pursuing the following arrangement where cold water fills from the base of the HWC, hot water is extracted from the top and at night there is reliance on thermolayering to deliver further hot water. The vent connects to a vertical pipe of around 3 metres height, which ensures a constant pressure within the HWC of 3 metres water, above which pressure is expelled out of that vent (option to return it to the top of the header tank within the tiny house is shown in diagram 5).

Diagram5Disadvantages of the hollow core system are the converse of the heat pipe system:

  1. Hot water cylinder (HWC) is located outdoors, hence cools down faster (unless extra insulation is added around it).

  2. HWC sits horizontally, hence less effective for thermolayering

  3. If one tube breaks, water will immediately drain out of the entire system (including the header tank)

One other option would be to mount the array on the roof and use a solar activated electric pump to pump the water from header tank up to the HWC, however that adds electronic complexity, hence I am going with the ground based system at this time.

Having visited a friend at his offgrid homestead who currently uses the "hollow core" type evacuated tubes, I was advised this system can cope with overnight temperatures down to minus 20 degrees C.

In the case of the heat pipe core type, if water is passed directly through the manifold, at night this small volume of stagnant water can easily freeze and break the manifold. Hence in the heat pipe core type, in cold climates, it is necessary to use food grade antifreeze (eg propylene gylcol) as the heat exchanging fluid through the manifold, which then circulates through a copper coil in the HWC. My friend previously had electrolysis problems with the heat pipe / manifold type system, presumably because, in his case, water rather than glycol was directly passed through the manifold. Some systems use magnesium anodes to overcome this problem but another way to minimise that risk could be to use pure (undiluted) propylene glycol as the heat exchange liquid in the manifold, which has an electrical conductivity a thousand times less than that of pure water. He has not however had electrolysis problems with the hollow core type system.

Having received the stamp of approval for my latest arrangement after discussions with my plumber, we will try it out once the plumbing has been fitted in the tiny house.

Current design of the header tank is shown in diagram 6 and the plan is to elevate it above the loft floor on a heavy duty support base as in diagram 7.

Diagram6  Diagram7

The plan to thermosiphon water heated by the wood stove through the header tank remains, however I discovered that the "Hobbit" stove I ordered could not incorporate both external air intake system and the backboiler tank together. Hence because the former option was far more important, I gave up the latter. This has actually worked out favourably because I now plan to harvest heat from a copper coil wrapped around the base of the hot flue and because this will be less efficient than the backboiler tank (which sits inside the combustion chamber) there will be little to no risk of the water in the header tank overheating (the main purpose will be to raise the temperature of the water in the header tank from finger numbing coldness, perhaps 10 degrees C to a tepid temperature, perhaps 20 degrees C. The header tank will then serve as a modest thermal mass heat radiator through the night).

 

G. Chia, June 2016

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