Design of a Solar-Powered Milk Cooling System for Smallholder Dairy Farmers in Plateau State

Milk spoilage at the farm level due to lack of cooling infrastructure is one of the most significant constraints on dairy value chain development in Plateau State, North Central Nigeria, where electricity supply is unreliable. This study designs and evaluates a solar-powered milk cooling system for smallholder dairy farmers producing between 50 and 200 litres per day. The system design integrates a photovoltaic array, battery storage, and a modified direct-expansion refrigeration unit. Engineering calculations for heat load, solar panel sizing, and battery capacity were conducted based on ambient temperature data from Jos and the thermal properties of fresh whole milk. A prototype was fabricated and tested at a dairy cooperative in Barkin Ladi, Plateau State, using 120-litre capacity insulated stainless steel tanks. Performance was evaluated over 30 days, measuring cooling rate from initial milk temperature to the target of 4 degrees Celsius, energy consumption, temperature maintenance over 10 hours, and effect on milk quality as assessed by acidity and total plate count. Results showed cooling to 4 degrees Celsius was achieved within 90 minutes, and temperature was maintained within plus or minus 1 degree Celsius for 10 hours on a full battery charge. Milk quality indicators remained within acceptable limits throughout the cooling period. The study draws on solar refrigeration literature for agricultural applications. Findings provide a technical design reference for rural dairy infrastructure investment. Keywords: solar cooling, milk preservation, dairy, Plateau State, solar energy