Thermo Storage Options?

[Yianie]

Hello, I have a 90 tube (evacuated tube) hot water solar system.  It has an 80gal storage tank.  I want to increase the heat capacity for storing large amount of hot water.  I use the hot water to heat my floor (hydronic heating).  I see that a lot of people built wooden square tanks with a rubber lining, others collect old water tanks.  I was wondering if anyone has seen or built thermo storage,tanks of water? or sand? Wooden tanks? any ideas will be greatlhy appreicated.

[Crofter]

The conductivity of sand is much less than water. A great amount of heat can be stored in a sand bed but only if you apply or extract it slowly otherwise the fluid goes through with no transfer. To transfer at the same rate as into a water storage you need much greater tubing footage on close spacing and will consume more energy to circulate the transfer fluid.

Sand storage has some good points but they are better at flattening seasonal peaks rather than daily peaks.

[Terry]

Hot dry sand will flow like water. Any small holes let out huge amounts of sand. Built a wood fired heater in late 70's and used sand for heat storage. Run heat transfer fluid through copper pipes to take heat into house. Worked, but not the best. Have plans to build wood fired heater this year using water as storage medium and incorporate solar and microco gen.
Terry

[Ronmar]

The type of tank will really depend on if your system is pressurized or non pressurized.  Obviously a pressurized system needs a pressure vessel such as a domestic hot water tank to store hot water.  A non pressurized system could use about any insulated tank, as long as system head pressure does not force water out a vent or filler port when the system shuts off.  I have never personally played with sand storage, but I have heard the same thing, they need larger internal heat transfer surface area and have a slower heat transfer rate. 

So is your current tank maxed out in storage on a regular basis( in addition to what you are consuming, the tank reaches peak temp and stops collecting while there is still sun available)?  If your tank was sized correctly initially, this should be a rare occurence.  Larger storage usually means an increase in avalable heat(more collectors) or an alternate source(CHP installation, wood or waste oil boiler:)).

[mbryner]

Hi Yianie,

Welcome.   

I have a homemade thermal tank.   I used a 1000 gallon concrete septic tank, capped at the ends, and lined with rigid foam.   It's sealed w/ silicone.   Heated with copper tube heat exchangers from a wood stove.   I've only had it working for a few weeks but it preheats water well.

Remember, water has a relatively high specific heat, 5 times as high as concrete or brick.   Even though brick or concrete weights more per volume than water, water still will hold more heat per volume.   

If you want to see homemade wooden tanks, here's one:

http://www.builditsolar.com/Experimental/PEXColDHW/TankConstruction.htm

and another:

http://www.builditsolar.com/Projects/WaterHeating/KnickSystem/Tank.htm


Marcus

[hal]

I have also have a septic tank insulated with pink foam.   I strongly recommend using a EPDM liner as thick as you can afford.    While I have seen installs in basements and garages.   I installed my tank outside of my home.   

[farmer0_1]

i am headed in the same direction for the floor heat on my replacement house,  i already have a wood outside boiler waiting to install and will be looking for some massive storage.   i have been watching gsa auctions for gov surplus but haven't seen anything useful yet.