HEAT STORAGE

Without a place to store heat, solar collectors won’t be much use for that Saturday night bath. Eighty gallon solar hot water tanks cost about $1000 each and I wouldn’t blame you if you broke down and bought a few to speed up this SHW project.  Building a solar collector from scratch would be enough to satisfy most, but if you’re a hard core do-it-yourselfer like me you’ll want to build your own heat storage system. Commercial SHW tanks contain a large copper coil on the inside for exchanging heat into the water. You’d need to modify an inexpensive holding tank with welding equipment to build this kind of commercial tank but there are simpler less expensive ways to build a heat storage unit.  

My first solar hot water tank was made from an 80 gallon holding tank that I salvaged from a recycling yard for $10. The one in the picture was purchased from a plumbing supply house brand new for $100. To transfer collector heat into the tank my friends and I wrapped the tank with two 60 foot coils of ½ inch copper tubing hooked in parallel. After pulling the copper tight against the tank with bailing wire we wrapped some chicken wire around the copper and plastered it to the tank with cement.

It’s not a very pretty picture but this external heat exchange system worked fine and my first solar hot water customer, Luke Dyers was pleased with the results. Cold well water normally fed directly into the gas hot water tank had to go through this tank first.

If heat transfer coils were bonded to his gas hot water tank, solar heat transfer would not be practical. Isolating the solar heat storage area from the solar heat collection area is a good idea. It’s also a good idea to keep the fossil fuel system separate from the solar heating system. Solar hot water systems are normally used to preheat water for fossil fuel systems.

Multi Tank Heat Storage

This three tank system demonstrates the value of a multi tank heat storage vault.

HEAT INPUT
Tank 1 will always be the warmest because it is the first to exchange heat coming from the collector. The second tank is heated in a like manner though it will never get as hot as the first. Tank 2 is the warmer tank. Tank 3 is designed to suck the last bit of heat from the already cooled collector fluid. This warm tank will always be the coldest of the three tanks. Fluid returned from this tank will be nice and cold and ready for efficient heat transfer when it’s returned to the hot collector.

Cold well water flows through tubes near the tops of the tanks where storage tank water is hottest. Well water is heated in three successive stages. Tank 3 preheats well water for tank 2 and tank 2 preheats the water for tank 1, the warmest tank. 
Multi tank heat storage systems maximize heat transfer efficiency. 
Notice that the collector fluid heat exchange coils are located near the bottom of the tank. Do you think this is a good idea? Why or why not?
Domestic heat exchange tubes are located near the top of the tank. Do you think this is a good arrangement? Why or why not?

If sufficient storage tanks and sufficient collectors are used fossil fuel heating would be unnecessary. For a slide show demonstrating the construction details of a multi tank heat storage system for home heating click here: Multi Drum Heat Storage Vault

You may need some plumbing assistance to build this multi tank heat storage vault. 

MATERIALS FOR A 1' X 3' X 1' MULTI TANK HEAT STORAGE VAULT

1. Three 10' lengths of 1/2 inch copper tubing.
2. Sixteen 1/2 inch elbows, four T's solder etc.
3. Two 1x12x8's, one 2x2x8
4. 60lb Mortar mix
5. Three small (13 cu ft) garbage bags.

HEAT INPUT PLUMBING

First cut a three foot length from a 1x12. This will be the bottom of the heat input exchange system. Then attach the 2x2 frame to this as shown in the picture.
Cut four pieces of copper tubing 40" long and then drill four 3/4 inch holes 2" apart into the ends of the 2x2's. 
Insert these 40" long copper tubes into the 2x2's.
Next sweat the copper tubes together with elbows and T's as shown in the picture.

After the plumbing is done pour cement into the 2x2 frame and be sure make it level with the top of this frame. Congratulations! You have just constructed the heat sink for the heat input system.

HEAT STORAGE VAULT

Now let's build the heat storage vault.
First cut two 11" pieces of 1x12 and round off the edges as shown in this picture. These will be the tank barriers and they will be exposed to water so they should be protected with oil paint. 

Cut two end pieces 1 x 12 x 1'
Cut two side pieces 46.5 inches. These side pieces will have 1/2" deep notches  3/4" wide centered one foot from both ends of the board. Examine the following picture to see the position of the notches.  

After cutting and shaping and sanding these notches you can assemble the heat storage vault with screws. Make sure that the tank barriers fit loosely and are well sanded before continuing.

Cut notches 3/4" wide and 2" deep into the tops of the tank barriers. These notches should be centered and spaced about 2" apart. Sand all rough edges, remove the tank barriers and paint the assembly with oil base paint.

After the assembly is dry line the inside of the vault with a few sheets of plastic to make it waterproof.

HEAT OUTPUT PLUMBING

Insert the tank barriers with notches cut into the top. 

Now assemble the heat output plumbing as shown in this picture.
Cut four 1/2 inch copper tubes 30" long. 

After assembling  the elbows and T's so that the 30" lengths of copper tubing are spaced about 2" apart remove the assembly and sweat the joints away from the heat storage vault. Remember the heat storage vault is now lined with plastic. 

Cross supports hold the heat storage vault together, and also hold the tank barriers down.

For large tanks solid Styrofoam insulation is used. Can you imagine the buoyancy force of ultra light insulation under six feet of water?

Without cross supports solid insulation tank barrier walls would pop out of the tank as soon as water is added.

Best of luck with this project. it's a good one.

Give the dimensions of your heat storage vault and and run some experiments to determine the rate of heat transfer into the heat storage vault. Don't forget to upload your results to the Bulletin Board