The Farm SunFridge is an innovative, off-grid, batteryless, cold storage facility for perishables that uses a combination of evaporative cooling and solar refrigeration to store harvested farm products on farmers’ fields or in farm communities. This facility offers smallholder farmers inexpensive access to cold storage even without electrical connection and improves their control over the marketing of their crops.
The Farm SunFridge can be built by farmers with locally available materials. It incorporates several innovative features into its design: it uses wetted fabric over iron mesh walls for passive, evaporative cooling, autoclaved aerated concrete blocks (AAC) and styrofoam panels for insulation, a solar-powered, mini split inverter air-conditioning unit for refrigeration, water-based thermal storage in place of batteries for nighttime cooling, and a novel solar sensor that better balances refrigeration demand with available solar energy.
Unique among clean energy storage concepts, the Farm SunFridge can be conveniently constructed all at once or developed in stages, as time and finances allow. In the first stage, farmers build the evaporative cooling (EC) chamber with locally available materials, which provides modest cooling. In stage two, solar refrigeration is installed with minimal additional investment and no major structural modifications.
The Farm SunFridge has been constructed in farmers’ fields in villages in Haryana, Delhi, and Rajasthan states in India. A fourth Pusa FarmSunFridge has been built at Pusa Fair grounds.
Farm SunFridge Specifications
~Rs 5 Lakhs (~ $7000)
3 x 3 x 3 meters.
2000 kg produce
Internal Daytime Temperature:
8 – 10°C
Internal Nighttime Temperature:
12 – 14 °C
* when the daily ambient maximum temperature is approximately 45 °C
Research, design, and construction of the Farm SunFridge.
Design and Construction HighlightsThe Farm SunFridge is a one-of-a-kind, innovative design that utilizes the combined effects of evaporative cooling and solar refrigeration.
Design Features And MaterialsThe outside walls of the Farm SunFridge are composed of a special nylon felt fabric that absorbs and disperses water and permits the exterior of the structure to be cooled by evaporation. The cooled outer walls remove approximately 20 to 30% of the heat load, permitting the use of a smaller solar panel array and refrigeration system.
The fabric is stretched over an iron mesh, which provides extra strength and security to the walls. The fabric is wetted by a drip irrigation pipe that is sewn into a pocket in the fabric at the top of the wall.
The roof and floor are made up of reinforced cement concrete and are covered by a layer of lightweight autoclaved aerated concrete (AAC) blocks. The roof is supported on 4 columns made of reinforced concrete both of which are also covered by AAC blocks.
Insulation of the structure interior is essential to protect from heat gain from the surroundings and the soil. Inexpensive, 5-cm thick, high density (22-32 kg/m3) styrofoam panels on inner walls, floor and ceiling (two layers) are used to insulate the structure. The floor is covered in iron sheeting to protect the styrofoam.
Refrigeration is supplied by a modified mini split inverter air-conditioning (IAC) unit. It is powered by an inverter (model, MPP Solar) solar-powered refrigeration. 14 solar panels (350 W each, 39.2 V operating voltage, 8.94 A operating current) are used to power the inverter. The inverter is of a special design that can be run without batteries and without a grid tie. This permits us to draw the energy for refrigeration and water pumps directly from the solar panels.
There is no solar power at night, so a form of energy storage is needed. The Farm SunFridge uses a simple cold-water reservoir made of 4-inch diameter PVC pipes holding approximately 650 L (170 gallons) of water and fitted with a specially-build flow control system. This allows the Farm SunFridge to use small solar panel arrays and a small capacity refrigeration system, reducing costs. A stainless steel or plastic drum (120 L) is used as a heat exchanger for cooling water during the day. This water is circulated through the overhead thermal reservoir throughout the day.
Because light levels fluctuate throughout the day and with changing weather, a pyranometer is used to sense the light levels and a microcontroller is used to regulate refrigeration demand to prevent overloading the inverter.