Design of Solar Milk Cooler | Solar Energy | Energy Management

In this article we will discuss about:- 1. Introduction to Solar Milk Cooler 2. Basic Operating Cycle 3. Solar Milk Cooler Design. 

Introduction to Solar Milk Cooler:

India is essentially an agricultural country with 78 per cent of its population living in rural villages.

Population-wise the distribution of villages is as follows:

About 95 per cent of the totals are small villages with population less than 2000. On an average, the cattle population is 50 per cent of human population in Indian villages. The total milk production per village per day on an average may be taken as 2000 liters; out of which the milk producers consume 25 per cent for self-use and convert about 45 per cent into milk products like ghee, paneer and khoa. Therefore, 500-600 litres of milk per village are available for sale as liquid milk per day.

Normally, at present this milk is sold as such and its chilling is done by the agencies procuring it. A village level chilling/cooling facility will surely be advantageous as it will make possible collection of evening milk also once in a day which is more economical than twice a day collection requiring use of available collection, transport and chilling facilities twice each time operating much below their rated capacities. There will thus be large savings through once-a-day collection. For example reduction in route kilometerage only can result in savings of about 12 1/2 per cent of the procurement cost.

Rural processing of milk can add value to the product and increase rural income. The fluid milk can be cooled to the prescribed temperatures by the producers or their co-operatives and keep the milk fresh till the time it is boiled by the consumers for use. Pasteurisation of milk is not necessary as consumers even other-wise boil the pasteurised milk and wastage of energy for double heating can be avoided.

At present, some manufactures have developed milk cooling and storing cabinets suitable for rural areas.

Typical specifications of such chillers produced by Larsen & Toubro are given below:

The milk is quickly cooled from ambient temperature to below critical temperature and further cooled and stored at 4°C. The cooling is achieved by mechanical refrigeration and therefore, electricity is required for its operation. Unfortunately electrical power is not available in most of the villages and rural electrification is very expensive and practically difficult in far flung areas in a vast country of India’s size.

On the other hand, India is bestowed with plenty of sunshine all the year round, and development of a suitable solar milk cooler to meet the above requirements of cooling and storage of fresh milk can be a boon to the rural economy and white revolution. Few components, no moving parts, operational simplicity, long repair-free life are essential requisites of a good design of a milk cooler for rural adoption.

Basic Operating Cycle:

Solid Absorption-Zeolite Refrigeration:

Zeolite is a mineral found in many parts of the world and has special adsorption properties. It can adsorb water vapour, ammonia, carbon oxide and freons, all with 30 per cent by weight. The zeolite-water vapour combination is the most efficient system as it requires minimum amount of zeolite due to largest heat of vapurisation of water. The adsorption of water in zeolite is a function of temperature and pressure and nonlinear dependence on pressure makes zeolite very favourable for solar applications.

The zeolite is sealed in an air-tight container irradiated by the sun. During the day cycle, the zeolite is heated to a temperature of about 120°C. At about 40°C, water vapour starts desorbing from the zeolite and its partial pressure rises.

At condensing temperature of 40°C, the partial pressure is about 55 mm Hg and vapour begins to liquify as heat is rejected to the environment. The liquid water is stored in a tank. During the night cycle, the zeolite is cooled by convection to ambient temperature and sky radiation.

Liquid water from the storage tank is introduced into the evaporator, where it absorbs heat from the space to be cooled and is converted into water vapour. If partial pressure is 5 mm Hg, the water in the evaporator boils at 2°C.

The zeolite adsorbs the water vapour produced by the evaporator and a partial pressure is maintained below 5 mm Hg. The heat of adsorption is rejected to the atmosphere. The zeolite is fully loaded corresponding to the partial pressure and is ready for next day cycle.

Na₂ AI₂ SiO₂ + ×. H₂O = Na₂ AI₂ SiO₂. × H₂O

The above cycle can be utilized for the development of zeolite solar cooler.

Zeolite Solar Cooler:

Zeolite solar cooler can be used to produce cooling effect and manufacture ice using sun’s rays without any external power and without moving parts. A solar cooler having an efficiency of 15 per cent can produce about 0.9 kWh of cooling for each square meter of collector area receiving a solar input of 6 kWh.

This amount of cooling corresponds to about 9 kg of ice manufactured per day for each square meter of collector. This principle can be used to produce ice, refrigerators to store vaccine, domestic refrigerators, on-site refrigeration of per­ishable produce and tropical fruits, in-the-field refrigeration of high-cost perish­able products etc.

Its major applications can be in developing countries like India where energy costs are high and electricity unavailable, unreliable and expensive. It can be of great importance to the fishing industry, especially for refrigeration of fresh fish and other sea-food in the remote fishing villages in tropical countries. However, it has a great potential for dairy farmers and rural milk producers.

Solar Milk Cooler Design:

Possible design of solar milk cooler can be as per the following specifica­tions:

i. Solar Panel:

The zeolite solar panel resembles a conventional flat-plate collector. After the zeolite is charged, the whole system is outgassed, evacuated and hermetically sealed. The panels have a zeolite capacity of about 50 kg/m² and require a depth of 50 mm.

The upper surface of the panel is painted black for maximum solar absorption. The water loading is 5 to 7 per cent by weight of zeolite for a temperature range of 40°C to 120°C. The inlet and outlet pipes are connected at the bottom of the panel to permit water vapour to enter and leave. The panel is leak-tested prior to use.

The system is ideally suited to local manufacture, but the zeolite will have to be imported and skill is required to prepare zeolite-filled collector and evacute the system. In order to avoid the import of zeolites, synthetic zeolites can be tried. It may be interesting to try strong Acid Cation Exchange resins in either Na or K form as desicants in place of natural zeolites, which are indigenously manufactured.

ii. Condenser:

The air-cooled condenser fin-pipes can be fixed at the back of sloping pan­els. The condenser pressure will be 55 mm Hg corresponding to ambient air temperature of 40°C. The condensed vapours during the desorption cycle run by gravity into the flooded evaporator.

iii. Cooling-Cum-Storing Tank:

The cooling-cum-storing tank can be made from single-embossed panel coils of stainless steel, the inside being plain sheet and outside made of embossed sheet. This design establishes thermosyphon circulation in the walls during the cooling cycle.

The design acts as a very efficient heat exchanger thus reducing the temperature difference between milk and water vapour to less than 0.5°C. As water storage is in the bottom, it makes possible to cool even small quantities of milk. At a pressure of 5 mm Hg, water will boiled at 2°C in the evaporator coils and this low pressure is maintained due to adsorption of water vapour by zeolites.

The walls and roof of the tank are covered with 150mm thick polyurethane insulation having a loss coefficient of 0.2 W/m². °C.

Unlike conventional milk coolers using freons or ammonia as refrigerant and brine as secondary fluid, solar milk cooler works with only one fluid. As there are no moving parts or corrosive chemicals and there is no reduction in the adsorption ability of zeolite, it is estimated, the system would work for more than ten years.

iv. Auxiliary Heat:

The cooler uses only solar energy and can be used in rural areas where electricity is unavailable, unreliable or expensive. The collector panel can be provided with pipes that run through the zeolite panel to provide auxiliary heat whenever the sun is not shining. The farmer could then start a small fire from straw, cow-dung or other local fuels and let the hot gases from the fire pass through the pipes in the zeolite panel, thereby heating the zeolite and causing desorption.