Let’s Talk Tanks

By Bob “Hot Rod” Rohr

When 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 consume domestic hot water (DHW) in the morning and evening hours. The daytime hours are available for energy 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 evolving 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 system 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 pressure in underground caves, as we do with natural gas in some areas. This compressed air would then be used to spin turbines to generate electricity. This of course adds more complexity and phase changes as well as the efficiency 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 market. Common to most tanks these days is a good insulation 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 insulation, 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 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.

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 collector operating temperature for best efficiency. As Jim Huggins likes to say, “A cool collector is a happy collector.” The same could apply to a condensing heat source, pulling the coolest water to the boiler to help keep it in condensing mode.

Blank tanks, without internal HX coils, are also available. Placing the HX function outside the tank has several advantages. First, the tank is less expensive to build and replace some day. Next, external HX coils can be sized exactly for the job. Another benefit is the ability to service, clean or replace the HX without replacing the tank. This seems to be a more common approach with tanks in excess of 119 gallons.

There are more and more multi-port insulated tanks on the market. These are designed to be installed as buffer tanks and can be useful for heat pumps, micro loaded hydronic systems or solid fueled boilers. Many of the new buffer tanks offer large ports at multiple levels for high flow rates and good air elimination. Think of this type of tank as a super-sized hydro separator device. With the correct application, the tank serves as a buffer, hydraulic separator, air and dirt eliminator and a central location for the control sensor or sensors.

Another unique feature I noticed on Jenni tanks, built in Switzerland, is that the connections were placed on the side of the tank with an angled connector pipe. This helps eliminate any thermo-siphoning that might be possible with a single outlet port on the very top of the tank. It eliminates what is known as single pipe thermo-siphoning. Cold water connections at the bottom were configured the same way.
The following are some exotic tanks and collection methods:

• How about an aluminum tank built in Sweden, developed by Salzburger Aluminum of Austria. This product, named the ECOScience, is manufactured from recycled products, including an EPP insulation shell, and provides a weight reduction of approximately 60% compared to a steel vessel.

• A copper tank with a glass fiber lamination for additional strength comes from SolarDome of South Africa.

• Stockbro, from Sweden, has a dual chamber tank that allows loading in the bottom and heat removal from the top. DHW is produced inside a stainless steel inner coil. This provides a simple four connection tank. View the cool YouTube animated video of their Siljan tank in operation.

• Ouraset of Turkey builds a tank with all piping connections located on one flange. This greatly reduces the number of ports welded onto the tank.

• Ezinc, also located in Turkey, builds a very attractive solar tank with the pump station and controller attached to the tank in a streamlined package. They also offer a corrugated stainless steel upper coil for DHW production.

• Thermo siphon solar is very popular in mild climates. Most of these systems have a tank mounted integral to the collector, on the rooftop. Transsen of Brazil builds a tank with additional outer anti corrosion protection for use in harsh outdoor conditions. These tanks were featured in a recent issue of Sun & Wind Energy magazine.

So, there is a quick update on some of the tank technology on the market produced domestically and abroad. “Tanks” for taking a look.

Bob “Hot Rod” Rohr has been a plumbing, radiant heat and solar contractor and installer for 30 years. Rohr is a longtime RPA member and Plumbing Engineer and Phc News columnist. Bob joined Caleffi North America as manager of training and education.