Water Storage Tank Sizing for Solar Powered Irrigation

Introduction

Water storage tanks are cost-effective solutions for providing round the clock water supply using solar powered irrigation system (SPIS). They are available everywhere, simple to operate, easy to maintain and thus are better design alternative to batteries. They also increase the reliability of the SPIS.

The energy is stored as potential energy in the tanks and is converted into kinetic energy when the water flows. The tank needs to have enough elevation compared to the downstream system for the water to flow.

Advantages of a water storage tanks :

• Instant water supply during any point of the day when the demand exceeds the water supply using the solar powered water pump
• Emergency water storage in case of pump failure or low irradiance (during mornings or evenings as well as on cloudy day)
• Cheap alternative to battery storage

Sizing a water tank

There are different methods for sizing a water tank. The impotant thing to keep in mind is that the size should be big enough to meet the peak water need during the hottest month but not over-sized to reduce investment cost.

The size of the water tank depends on :

• Solar irradiation
• Water demand and use (based on seasonal variation)

Water tanks are generally designed to meet the demands for the month where the difference between the hourly supply and the demand is the largest. There are different guidelines for adequately sizing the water tank and are mentioned below:

1. Quick guideline for sizing water tanks mentioned in the book, “Solar Pumping for Water Supply” .

	Water storage tank sizing
Generator/grid powered	0.5-1 x daily water requirement
Hybrid (solar+ generator/grid)	0.5-3 x daily water requirement
Stand-alone solar	1-3 x daily water requirement

2. Hourly supply vs demand method: In this method, the hourly water supply is compared with the hourly demand for the month with the least irradiance. Then, the water storage tank is designed to meet this difference.