Many Indian regions suffer from bad water quality: More than 75% of the country has moderately hard to very hard water. Chloride levels are also high in specific areas when water is obtained from boreholes. The “desired” legal chloride limit for drinking water is 0.25 g/litre, but is allowed to increase to 1 g /litre. As a result, solar thermal storage tanks gradually corrode and tend to develop leaks. Statistics obtained from the national Helpline show that over 70% of product-related complaints are about leakages in storage tanks. The photo shows a corroded extremely scaled hot water tank.
Photo: Jaideep Malaviya
In response to the tanks’ corrosion problems, the May 2013 standard of the Ministry of New and Renewable Energy (MNRE) requires a guarantee period of at least five years on storage tanks (see the attached document). Under the title “Storage Water Tank for All Glass (Glass in Glass) Evacuated Tubes Solar Collector”, the ministry summarises on 10 pages the requirements and tests which are mandatory to receive grants from the government. In order to ensure that the storage tanks used in solar thermal systems perform without incidents for at least 5 years, the MNRE specifies the following eligible technical solutions for the inner tank:
- Stainless steel conforming to certain grades and having a sheet thickness of at least 0.5 mm for capacities of up to 300 litres
- Mild steel with anti-corrosive coating and a sheet thickness of at least 1.5 mm for capacities of up to 300 litres
- Galvanised iron with suitable anti-corrosive coating and a sheet thickness of at least 1.5 mm for capacities of up to 300 litres.
Due to the growing market penetration of solar collectors in India and the poor water quality in many states, manufacturers are now focusing their R&D on designing storage tanks which will last at least an uninterrupted 5 years even in regions with a high chloride content and hard water. The industry is offering several solutions here:
- Sacrificial anode made of, for example, magnesium or aluminium
- Closed-loop system with heat exchanger
- Anti-corrosive coating, often made of epoxy polymers
- Enamelled coating
A couple of companies produce coated and enamelled tanks to prevent chemicals in the water from coming in direct contact with the tank’s metal wall. Collector and tank producer Emmvee Solar was the first to set up an enamelling line for inner tanks.
Several manufacturers use stainless steel tanks to avoid corrosion, but the capacity of these tanks is usually no more than 500 litres. Economically, mild steel is the most feasible material for greater capacities than 500 litres.
Tank manufacturer Inter Solar Systems has favoured the anti-corrosive coating for a longer lifespan of its tanks after expanding its production capacity as reported by solarthermalworld.org. Pragnesh Raiyani, Managing Director of system supplier Redren Energy agrees: “Bad water quality can call for frequent replacements of sacrificial anodes maybe every 5 months. Anti-corrosive coatings are the ideal solution.”
Find attached the following documents:
Storage Water Tank for All Glass (Glass in Glass) Evacuated Tubes Solar Collector (May 2013)
Selection of suitable solar water heating systems
Indian Standard for drinking water IS: 10500
This text was written by Jaideep Malaviya, an expert in solar thermal based in India (firstname.lastname@example.org)