Let’s Talk Tanks


Storing thermal energy is a key component to solar thermal installations


By Bob “Hot Rod” Rohr W


hen talk turns to storing thermal energy, water is often the chosen medium. The goal is to be able to collect energy while the sun is shining


and store it for future use. Many working families con- sume domestic hot water (DHW) in the morning and evening hours. The daytime hours are available for ener- gy collection, with little or no demand. This may apply for many commercial buildings as well. Hotels would be another example of large early and evening DHW loads. Thermal storage has always been a constantly evolv-


ing field. While water seems to be simple and readily available, freeze protection is often required, typically to protect the solar collector. If this is the case, the sys- tem may require a fluid, perhaps glycol, to offer the freeze protection. Other media has been and continues to be researched. Brine or salt solutions are gaining attention again. There are several pilot projects in which solar thermal energy is converted into compressed air for storage, with the hope of storing the air under pres- sure in underground caves, as we do with natural gas in some areas. This compressed air would then be used to


ket. Common to most tanks these days is a good insula- tion layer. Tanks may have a steel, plastic or soft fabric outer jacket. Foam insulated tanks are the most common. An outer jacket is installed around the storage vessel and foam is injected from the top to encase the tank in a tight, infiltration-free cocoon. Other insulation products are being used, including a polyester fleece not unlike the outerwear material, as well as various plastics, such as extruded polypropylene. One tank manufacturer is researching using shredded jeans (cotton) as one of the insulation layers. High tech aerospace products are also being tested. Some tanks have a removable soft foam insulation jacket. The tank ships unjacketed to save wear and tear on the insulation and the finished outer jacket. Many of the residential and smaller commercial tanks


we see advertise 2" of foam insulation. Recent tanks spotted at the European shows indicate 3 – 4" of insula- tion, sometimes in multi-product layers. Tanks are available with internal coils or multiple


coils for heat exchange. A common solar tank contains the DHW and has a coil at the bottom for solar input. Other brands offer a tank within a tank for the heat exchange function. Uber-engineered European tanks may have four or more coils inside, including a stainless steel, copper or alloy coil at the very top. The top coil resides in the highest temperature layer and is often used to heat, or pre-heat, the DHW inside that coil. Another brand has a small stainless steel tank up high


inside the tank for DHW production. Other “bright idea” tanks are oval shaped to fit through narrow doorways, while still offering large capacity. Yet another concept being introduced is tanks with


Figure 1


spin turbines to generate electricity. This of course adds more complexity and phase changes as well as the effi- ciency losses with every change of energy. At the end of the day, many designers and installers


stick with what they know best and choose water for thermal storage. A glass lined, stone lined, polymer or stainless steel tank could contain the DHW as a potable source. If the fluid is used as a heating medium, only steel tanks could be used in a closed system. In some solar drainback systems, the same fluid — plain tap water — circulates through the collector, storage tank and heating system piping. This simple and elegant method eliminates all the heat exchangers, save for the one on the rooftop, aka the solar collector. There are many types and brands of tanks on the mar-


Page 38/Plumbing Engineer


stratification features. Some brands accomplish this with a series of tubes inserted at multiple layers. One brand uses a fabric stratification “lance” inside the tank. Yet another brand has an “active” stratification system that uses circulators attached at multiple levels on the outside of the tank. Lochinvar installs a small LocTemp chamber at the


bottom of some of their tanks to reduce turbulence and encourage stratification (See figure 1). One tank I’ve seen has a rail or track on the outside


that allows sensors to be placed at different levels. Stratifying or layering the tank optimizes the solar col- lector operating temperature for best efficiency. As Jim Huggins likes to say, “A cool collector is a happy col- lector.” The same could apply to a condensing heat source, pulling the coolest water to the boiler to help keep it in condensing mode.


Continued on page 40 June 2011


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