Water Treatment Challenges in Thermal Storage Systems

Thermal energy storage (TES) systems are cooling systems that can use ice banks, brine systems, or chilled water storage tanks to capture BTUs for the purpose of removing a heat load at another point in time. In practice, the chillers for the TES operate outside peak electrical load hours and store the BTUs in the preferred form for use during peak electrical load hours. This practice reduces strain on the electrical grid and provides both cost and energy savings for the owner.

Energy storage requires space. When treating systems that use chilled water storage tanks, the sheer size of the systems poses economic challenges. System piping can total several miles of linear pipe footage. Tanks can vary greatly in size, with tank volumes of over a million gallons being common. Overall system volumes can be in the multi-millions of gallon. Due to their design, TES systems have areas of low flow velocity. Biofilm can become established and particulates can accumulate in low flow areas, especially in the storage tanks. Low flow velocity can contribute to corrosion, biofilm formation, and fouling that lead to shortened system life and loss of heat transfer efficiency. 

Corrosion, fouling, and microbiological control are concerns with any closed loop. These concerns are addressed with the application of suitable corrosion inhibitors and biocides, as well as with mechanical solutions, such as filtration. Routine monitoring with field test methods for inhibitor levels, corrosion byproducts, and microbiological growth coupled with corrosion coupon or corrator results helps ensure program success. Periodic validation with laboratory methods provides additional insurance that the prescribed treatment program is working as designed.

The economics of treating such a large volume of water favors the use of concentrated corrosion inhibitors. Soluble silicates and concentrated azoles can provide very economical corrosion control as compared to other approaches. Phosphates, molybdate, and nitrite treatments are alternatives but phosphates and nitrite are nutrients for microbiological growth. Well established microbiological communities can be difficult to eradicate or control, particularly in such large systems that receive variable flow. Molybdate, while an effective corrosion inhibitor, is restricted in its use in some locales and is expensive by comparison. 

Microbiological control in systems with millions of gallons of water is expensive with traditional non-oxidizing biocides. Furthermore, the degradation residues of traditional biocide can eventually become nutrients for microbiological growth. For these reasons, chlorine dioxide is a preferred treatment approach.  As compared to 45% glutaraldehyde and 1.5% isothiazolin, the cost of a chlorine dioxide generator can be recovered in the first year if quarterly additions of biocide are required. A typical non-oxidizer dose to a one million gallon system costs $6,000 to $10,000 per dose while the cost for a typical dose of chlorine dioxide in the same system is about $300.00, assuming no oxidant demand.

Chlorine dioxide has very good efficacy at low dosages. The degradation products do not contribute to bacterial growth. Being a dissolved gas, chlorine dioxide diffuses to low flow areas of the system and also penetrates biofilm. It is less reactive than traditional oxidizing biocides, and its selectivity for target contaminants is higher while its corrosivity at target dosages is lower.

Chem-Aqua can bring a wide variety of water treatment solutions to your door. We offer chemical and mechanical solutions to meet most needs, and our products are backed by years of application experience.  If you have a water treatment problem needing a solution, please contact your local Chem-Aqua representative.

Written by: Michael McDonald

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