What is a Pusa Zero Energy Cool Chamber and can I build one cheaply?

A zero energy cool chamber, often called the Pusa cool chamber, is a low cost brick and sand structure that uses water evaporation to keep produce a few degrees cooler and more humid than the outside, with no electricity. It is a double brick wall structure with sand filled between the walls; you keep the sand and the structure wet, and as the water evaporates it cools the inside, which slows down spoilage of fruit and vegetables for a few extra days. It is cheap, simple to build from common materials, and useful for a small farmer who has no cold store and just needs to hold produce briefly before sale or transport. It does not replace cold storage and will not store produce for months, but it buys you time and cuts wilting and quality loss in the short term. Build it in shade, keep it wet regularly, and keep it clean. For the exact dimensions and construction method, get the standard design from your KVK or local horticulture office, since they often have ready plans and may even guide construction.