Solar Milk Cooling: Study case Sidi Bouzid, Tunisia
- Solar milk cooling system
Solar milk cooling system
The solar milk cooling system, an innovative technology to cool milk on the farm, is entirely based on renewable energy. The system is composed of conventional 40-litre milk cans with an ice compartment and removable insulation. The ice is produced by a solar powered freezer that can harness solar radiation with help of an adaptive control unit. As shown in Figure B2.8, the system is composed of photovoltaic panels, small batteries, an adaptive control unit, a charge controller and a commercially available direct current refrigerator with an integrated fan and 25 two-litre plastic cans for the ice blocks.
The system can cool down 30 litres of milk by using ice as cooling medium. This facilitates a flexible cooling of the milk on the farm or during transport. Milk cooling is an important stabilization treatment, but it demands a high amount of energy within the dairy chain.
- Two systems installed on farm
Study Case: Sidi-Bouzid, Tunisia
To test this milk cooling system in the field, 10 milk cooling systems were installed in 7 small farms in two regions: Zitouna and Hania, Sidi bouzid Tunisia.
Some farms had two units (Figure B2.9), and others had one, depending on the farm’s productivity.
The dairy sector in Tunisia is one of the country's most strategically important sectors, accounting for 11 percent of total agricultural production. Milk is usually collected twice a day. Typically, the collector brings the milk at a temperature of 30°C to the collecting centre where it is directly pumped to the refrigeration facilities to be cooled to 4°C. Under warm conditions, the collector faces the risk of spoilage. In many cases, the collector rejects the milk from some farms because it has spoiled in the time between the milking and the arrival of the collector.
Moreover, milk collection is usually done using tanks with an average volume of 500 litres. The milk of the different farms is mixed in this larger tank, which can also create a high risk for the collector. Depending on the volume of the spoiled milk, the whole tank could be affected. In some cases, one farm could provide more than 90 litres of spoiled milk.
On the different tested farms, the uncooled milk that was delivered to the collecting centre was of 'bad' quality. This can be caused by inadequate hygiene on the farm, but the primary cause is the long waiting time between milking and the collection. In addition, due to a variety of reasons (e.g. bad weather and the lack of lighting); the collectors may not pick up the evening milk at some farms in isolated areas. In this situation, the farmers are unable to sell their milk. They either use it for their own consumption or put it in cold water overnight.
The introduction of the solar milk cooling system has shown considerable results in improving thermal conditions and milk quality during transport and overnight storage.
Thermal conditions and milk quality assessment using the solar milk cooling system
In the different farms the solar milk cooling system was successful in keeping the milk at a temperature lower than 15 °C during the first two hours. This temperature is favourable for keeping the milk safe up to 6 hours. Transport took around 2 hours in the studied farms. For the overnight storage, the milk temperature remained around 8 °C after 12 hours.
The milk quality of the cooled and uncooled milk was assessed with different tests (acidity test, alcohol test, total bacterial count) after transport and overnight storage. Based on these tests, the milk cooled with the solar system was of higher quality than the uncooled milk and was always accepted by the collecting centre.
Potential advantages of the use of the solar milk cooling system:
Increase of quantity of milk sold in isolated farms
With the solar milk cooling system, the collector can collect the evening milk with the next day's morning milk and be assured that the milk will be acceptable.
If a farm has a minimum of 15 litres of uncollected evening milk, as was the case for one farm in this study, the farmer can earn an extra income of minimum EUR 3.75 per day, which comes to EUR 112 per month and around EUR 1 350 per year.
Preservation of milk quality
As of yet, there is no quality-based pricing in Tunisia, but it will be introduced in 2017 and offer a price premium for cooled milk. This premium, which provides a bonus per litre of cooled milk, will be a great incentive for farmers and collectors to use new technologies to cool down milk on farms. With the use of the solar milk cooling system, the farmers can avoid having their milk rejected and at the same time earn extra profit for their production.
Precooling service for the milk-collecting center and avoidance of CO2 emission
With the solar milk cooling system, the milk arrives at the collecting centre after two hours with a temperature of 15 C° instead of 30 C°. Precooling is considered as a key step toward achieving an efficient cooling system. The electricity needed to cool 1 litre of milk from 30 C° to 4 C° (assuming a mean coefficient of performance of one for the heat pump) is around 0.03 kilowatt-hour (Kwh) per litre. With the solar cooling system, the electricity needed would be reduced to 0.013 Kwh per litre, a decrease of 0.017 Kwh per litre. As electricity is relatively inexpensive in Tunisia (EUR 0.06 per kwh), it may not be worthwhile from an economic standpoint to use the solar system for energy savings. However, if carbon emissions are taken into account, given Tunisia’s electricity emission factor (0.572 kg of CO2 equivalent per Kwh), there will be a decrease of 9.7 grams of CO2 eq. per litre with the solar milk cooling system based on carbon accounting.
An average milk collecting centre cools around 40 000 litres of milk per day. With this volume, daily CO2 emissions would be around 686 kg. If all farmers delivering milk to the collecting centre used a solar milk cooling system, daily CO2 emissions would be reduced by more than half, to around 297 kg.
Payback period of the system
The total price of the system is around EUR 2 700 Euros and the price of a litre of milk in Tunisia is EUR 0.25. The payback period of the innovation would vary depending on the volume of milk cooled and sold per day. The period would be around two years, if the farmer successfully sells 14.5 liters per day. However, the solar cooler could be shared by a cluster of farmers, as the system is designed for a maximum capacity of 60 litres per day. Each farmer could have his or her own milk cans. In this way, the costs of the system will be divided on the different farmers and the payback time will be shorter.
Source: Mrabet, F., Torres Toledo, V., Salvatierra Rojas, A. & MüllerMrabet, J. Institute of Agricultural Engineering,Tropical and Subtropical Group University of Hohenheim, Germany.
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