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THERMAL STORAGE Cont’d from pg. 34

If you also use the tank to buffer boiler cycling, two formulas can be used to calculate thermal storage/buffer tank size. Formula 2 is for a fixed output boiler, heat pump, etc. Formula 3 is for modulating type equipment. For modulating equipment, the formula considers the lowest turndown rate. Same for heat pumps with two stage capacity. The tank size of course decreases when the equipment can vary its output. So, consider the multi-functionality of a thermal storage tank.

Formula 2

The Flipside

Of course, there are some cons to storing energy in heated or chilled water. It’s heavy, large insulated tanks can get expensive and you must have the space required to house the tanks.

Taken to perhaps an extreme is a company in Switzerland that builds multi-thousand-gallon thermal storage tanks. The tanks are so large they are sometimes partially buried in the ground below the building. One example is a system and tank that stores solar energy which supplies heat and hot water year round to an eight-unit housing project. There was some planning and cost involved in that undertaking. Check out www.jenni.ch for some large tank projects.

Certainly, a tank could accept energy input from multiple sources. Solar, waste heat from refrigeration equipment, biomass boiler, district energy systems, heat pump, and co-gen units, are some examples. So, if a time comes to have multiple energy options, storage can become a part of that.

Where:

V= minimum buffer tank volume (gallons)

T= minimum heat source on time (minutes)

Qhs= rated heat output of heat source (BTUH)

Qloadmin= minimum concurrent heating load when heat source is on (BTUH)

∆T= change in average tank temperature during heat source on time (°F)

Formula 3

Where:

V= minimum buffer tank volume (gallons)

T= minimum heat source on time (minutes)

Qhsmin= minimum stable heat output of heat source (BTUH)

Qloadmin= minimum concurrent heating load when heat source is on (BTUH)

∆T= change in average tank temperature during minimum heat source on time (°F)

Another readily available material for thermal storage is concrete. Concrete is the most consumed product in the world. We all recognize this heavy, rock aggregate product that surrounds us almost anywhere we travel. The obvious location for thermal concrete storage would be a radiant slab.

Maybe you have a basement with a slab, or concrete thin mass radiant on upper floors. All that you need to utilize that as thermal storage is some embedded tubing to transfer the energy in and out of that concrete mass.

Another option, seen in some upper Midwest construction, is a sand bed radiant system. Long-time solar guru Bob Ramlow has spent years tinkering with these installations in Wisconsin. Below a slab would be 12 to 24 inches of sand with PEX tubing loops. Energy is transferred from thermal collectors into the slab anytime the sun shines. The energy comes out passively, just radiating up to the concrete slab above. Heat output control can be a challenge in the fall season, before heat loads kick in. Operable windows are commonly used to regulate excess output.

Another option would be to insulate between the storage bed and the slab, with a thermal battery to pull the energy out via circulation. Take it only when it is needed.

Here are some numbers to know for a typical concrete mix slab. 29.4 BTUs required to raise a cubic foot of concrete 1 degree. Consider a 4-in. thick, 5,000 sq. ft. slab, and about 61 yards of concrete. It would require 48,421 BTU for a 1 degree rise in temperature.

This works in both directions. The slab re-radiates at that same rate for warming a space. This is where over-heating happens. When that energy stored in a slab, which has a mind of its own, wants to it will transfer to any area around it that is at a lower temperature. Sometimes referred to as the flywheel effect, this happens whether you are ready or not for that heat output.