Inside the Mechanical Room: Commercial Solar Thermal Storage, Exchangers, Pumps and Controls
Inside this Article
All the components of residential solar thermal systems are present in larger installations. In the latter, however, the parts can increase in scale to the degree that they may be hard to recognize at first glance. Although the tanks, piping and pumps are larger in commercial systems, residential scale controls are often used. Today, while most residential SHW systems are based on preengineered designs and component packages, larger projects require custom engineering. In the October/November 2008 issue of SolarPro, we covered the roof work involved in large commercial thermal installations. Here we go inside to the mechanical room to examine the design considerations necessary for large scale systems.
Large solar heating systems incorporated into new buildings are usually included in the overall mechanical plans. The plans drawn up by the mechanical engineer will be detailed enough to allow plumbing contractors with relatively little solar experience to accomplish the installation. However, having an experienced solar technician on-site will keep common mistakes to a minimum and streamline installation.
Retrofit jobs, however, often require solar contractors to design the system in addition to installing it. The system design determines the size of the components and project feasibility. Making room for the storage tank is probably the first consideration in any large retrofit job, because adequate space for the collectors and storage tank(s) is a make or break proposition for any solar heating system.
Most professionally engineered projects will specify a pressurized storage tank for large solar water heating systems. This is particularly true with installations requiring tanks with capacities greater than about 2,000 gallons. Large pressurized vessels can cost three to 10 times more than unpressurized tanks. They are typically custom built and can have long lead times in procurement. Tanks are often manufactured in a cylindrical shape with a domed top and bottom (or sides in a horizontal tank). Pressure tanks are built from stainless steel or steel with a glass liner to assist in corrosion protection, similar to a typical electric or gas water heater.
The term atmospheric is often used to describe open or unpressurized tanks. A tank need not be completely open to be unpressurized, and it might appear to be a pressure tank on quick inspection. Atmospheric tanks can be constructed of any material with a suitable temperature rating. Lined steel, stainless steel, lined fiberglass and polypropylene are all used in atmospheric tank construction. Unpressurized fiberglass tanks resemble a giant egg and are made for vertical and horizontal installations. Steel and stainless tanks are usually cylindrical, and polypropylene tanks can be either cylindrical or rectangular.
Bladder tanks are sometimes used for large SHW system storage because they can be constructed on site. Ethylene Propylene Diene Monomer (EPDM), a rubber roofing material, is used for the bladder. This material has a service temperature of about 300oF (149oC), which is sufficient for an atmospheric tank. A bladder tank starts with a cylindrical or rectangular frame that can withstand the hydrostatic pressure of the water to be contained. At least 2 inches of high-temperature foam insulation line the tank sides, top and bottom, and the bladder is then folded into the cavity. No bottom penetrations are made in a bladder tank; instead, an inverse "U" pipe circulates water from the tank bottom and keeps the pump primed if the tank is used for drainback. The heat exchangers are simple rolls of copper tubing and used with all types of unpressurized tanks. EPDM tanks have a life span of at least 15 to 25 years. Some bladder tanks do start leaking, possibly due to inadequate material thickness or because the EPDM was not folded properly during tank construction.
Storage tanks are usually sized to the building water heating load, with their size proportional to the collector surface area. Others factors that influence sizing include the temperature required, the total load, the local climate and available solar irradiance. Rules of approximation will get you into trouble with large SHW systems, but many throughout the US are designed with ratios of 1 square foot of collector to 1.5 to 2 gallons of storage volume—the same ratio as most residential systems.
In many large systems, the storage tank is designed to be used as the drainback tank. The only difference related to storage in small and large drainback systems is tank size. The same goes for expansion tanks in antifreeze systems. Very large systems with hundreds of collectors can have expansion tanks as large as typical household water heaters.