Articles » DIY - Open Storage - Part 2 (Wood Not Oil)
Construction: The tank is constructed in the corner of my basement closest to the boilers and distribution manifold. For the highest level of efficiency heat storage tanks should be as close to the heat load as possible. Two existing basement walls were utilized with two new walls being constructed of cement blocks reinforced with rebar and tied into the floor and existing walls.
Dimensions & Volume: The tank is 4.5'x12'x4'. Once the thickness of the 8” blocks and the 3” of insulation all around are subtracted the interior dimensions should be roughly 42”x130”x45”. This has a cubic foot volume of 142 cubic feet. Since there are 7.481 gallons per cubic foot, 140x7.481=1062 gallons.
Liner: I have chosen to use an EPDM pond liner which is rated at temperatures up to approximately 175°. There are liner choices that can handle much higher temps, but they cost significantly more. I will be using a 15'x20' piece in the tank and making a cover out of a 5'x10' piece along with scrap from the tank piece.
Insulation: I will use 3 inches of polyiso insulation inside the tank on all the walls and on the bottom. Polyiso insulation is a good choice becuase of it's high temperature rating. Be aware that some other types of insulation are not rated for high temperatures. (Here is a link that describes the high temperature limits of various types of insulation www.engineeringtoolbox.com/insulation. . . Here is a link to a chart on the r-values of various types of insulation http://www.coloradoenergy.org/procorner/stuff/r-values.htm)
Update: I have decided to replace my 30 plate heat exchanger with a larger one as my system experiences a lot of idling while charging. I would suggest using 50-70 plates and having larger than 3/4" ports. They are more expensive, but will work better and are still less than copper coils.
Type: I have chosen to use a flat plate heat exchanger to charge and draw from the open tank. A heat exchanger is needed with open storage because the boiler piping is pressurized and the open tank is not pressurized. Other methods of heat exchange include long copper coils or PEX coils immersed in the water. In those instances the pressurized water is pumped through the coils and heat is exchanged into the open tanks water. I chose not to use copper coils because of the current cost of copper and I chose not to use PEX coils because you need roughly 3x as much of them due to lower heat transfer than copper and that would occupy a large amount of space in the tank, as well as taking time to construct them. (Both of the coil options have been utilized successfully in tank construction. I could not find anyone who had successful utilized a flate plate for both charging and drawing from the tank, so my design is somewhat experimental.)
Stratification: Hot water in a tank will naturally want to stratify into layers where the hottest water in near the top. If that stratification can be maintained, there will be hotter usable water near the top of the tank. As a result, when charging the tank water must be drawn from the bottom and returned to the top. When drawing from the tank water is taken from the top and returned to the bottom. In my instance I have chosen to use two pumps on the tank side of the flat plate heat exchange to accomplish good stratification. These pumps will not have integrated check valves and one will pump through the other allowing water to travel in either direction through the pipes. (for a more detailed explanation of stratification see: Thermal Energy Storage: Systems and Applications pg.261)
Size: After speaking to a technician at a heating supply company I have chosen to use a 30 plate heat exchanger. The Tarm Solo Plus 40 produces 140,000 BTUs and with a sending temperature of 180° and a returning tempurature of 160° water will need to travel at 15gpm through the 30 plate heat exchanger to charge the tank.
Pumps: In order to accomplish the 15gpm I will be using Grundfos 15-58 variable speed pumps probably set on the highest setting. There will be a total of four pumps needed for charging and drawing (1-2 are already in place on the boiler side of many systems). One on the return side of the wood boiler, one on the return loop to the oil boiler, and two on the tank side of the flat plate heat exchanger. See System Layout for more details on placement.
Rough Cost of Components
Cement Blocks and mortar - $250
EPDM Liners - $230
Polyiso Insulation - $300?
Gundfos 15-58 Pumps (2)- $160 (4 are needed, but I already have 2 in my system)
Pump Flanges (2) - $30
Honeywell Aquastat $60
DPDT Relay & Socket $12
30 Flat Plate HX - $200 (see update above about sizing)
Rough Total - $1242 (see links page for suppliers)