Dairy is an indispensable part of the global food system and it plays a crucial role in the sustainability of rural areas in particular. It is a well-known fact that the dairy industry actively contributes to the economies of a number of countries. An increasing demand worldwide is noticeably emerging at present, and the industry is globalising. Milk and dairy products are very essential for human nutrition and development, especially, in children. Although milk is a highly nourishing food, raw fresh milk is highly liable to rot and can be easily spoiled by the growth of microorganisms. Fresh milk is collected from the farm, transported to cooling centres to prevent spoilage, then to processing units to produce other dairy products and finally delivered to the consumers in several ways, as shown in figure 1. The transportation of fresh milk from farms to cooling centres and processing units may take time. Consequently, cooling milk in time becomes major problem associated with raw fresh milk. The milk should be cooled within three to four hours of collecting it from the farm, which otherwise leads to spoilage. Thus, refrigeration plays a vital role in dairy industry.

Refrigeration of Fresh Milk

A variety of products are handled by dairy plants such as butter, ice cream, curd, condensed milk, butter milk, flavoured milk and cheese. The heating and cooling requirements for different dairy products and processes are shown in figure 2. In order to preserve quality and prevent spoilage, the warm and fresh milk should be cooled immediately after milking. The fresh milk should be rapidly cooled to 10°C within two hours of milking and to 4°C within three to four hours. In many temperate and tropical countries, where refrigerated cooling systems may not be available at the producer or milk collection point, the simple small scale methods for cooling milk to 10°C and below can be used. Some of such methods are as follows:

  • Evaporative cooling using a charcoal cooler
    • Cooling with natural water systems – mains, well or groundwater immersion cooling methods include placing the milk cans in a stream, river, lake or tank
    • Surface milk coolers (open cooling systems)
    • Refrigerated immersion cooler or cooling rings

Figure 1: Rural dairy collection and dairy value chain

As soon as the fresh warm milk arrives at the milk cooling centres, it should be cooled to 4°C. This cooling requires considerable use of energy, suitable refrigeration equipment and insulated storage tanks designed specifically for milk. The most commonly used refrigeration system used in milk cooling equipment for milk cooling centres is vapour compression refrigeration system.The basic refrigeration system is made up of a refrigerated bulk tank, a refrigeration compressor unit and an air-cooled condenser unit. A typical bulk milk cooling tank is shown in figure 3. The bulk milk tanks are double-or-triple walled. The compacted polyurethane foam or expanded polystyrene is used for thorough insulation to keep the milk cool for at least 12 hours with a temperature rise of not more than 1°C at an ambient room temperature of 30°C.

The reciprocating type compressor is the most common, which can be open, semi-hermetically sealed or hermetically sealed. Now-a-days scroll compressor is being increasingly used as it is more efficient and uses up to 20 percent less electricity than reciprocating compressors. Condensers use natural or forced air, water or oil to cool the refrigerant. The naturally air-cooled condenser is the most commonly used condenser. The purpose of the condenser is to condense the refrigerant gas by removing the heat. Evaporators are commonly made from copper and located close to the source of the heat to be removed. The compressor and condenser assembly are mounted on the same support frame as the milk tank or on a separate frame adjacent to the tank. The room in which it is located must be well ventilated to ensure that the refrigeration system operates efficiently. In very hot countries, the compressor and condenser assembly can be mounted on an external wall of the milk cooling centre building which improves the efficiency of the system, ensure faster cooling of the milk and reduce energy consumption.

Figure 2: The heating and cooling requirements for different dairy products and processes

The refrigerants used for refrigeration in dairy industry should be in agreement with recent international agreements related to atmospheric pollution, by the gases that contribute to global warming and ozone layer depletion. Natural refrigerants are recognised to be potential permanent solution to phase out the use of synthetic refrigerants which are being restricted progressively. However, there are many challenges associated with the implementation of natural refrigerants. In India, ammonia based vapour compression refrigeration systems are the most preferred mode for cooling in milk processing plants. Ammonia based systems are low pressure systems with very less sophistication. They have good heat transfer properties, low cost and high efficiency. Leakage in ammonia system is easily detected due to pungent odour of ammonia. Ammonia also has zero ODP and low GWP. CO2 is another natural refrigerant which is gaining prominence in the recent days. Compared to other refrigerants, the most remarkable property of CO2 is its low critical temperature (31.1 °C) and high critical pressure (7.1MPa).

Figure 3: The bulk milk cooling tank

Energy Requirement for Refrigeration

The dairy industry ranks fifth among the most energy-intensive industries after oil, chemical, pulp and paper mill, and iron and steel making industries. The industrialised countries have modern large-scale milk processing plants with about 100 tonne of milk intake. Even smaller plants with a daily intake of about 30 tonne of milk are generally equipped with modern machinery. The energy requirement in modern milk processing plants is shown in table 1. Also, the monthly electrical energy consumption (kWh) for 22 farms over 12 months (mo) for all major energy-consuming processes in the dairy industry is represented graphically in figure 4.The energy requirement in therefrigeration sector plays a very significant role in the overall energy requirements of a modern milk plant, often constituting above 40 percent of the total electric power consumption. This makes research on the reduction of energy required for refrigerating the dairy industries interesting and challenging.

Methods to Reduce Energy Consumption

There are several methods identified that can be added to the milk cooling systems to reduce the refrigeration requirement and to capture waste heat:

  • The condenser temperature should be as low as possible. For this, the correct size of the condenser should be ensured. A small condenser for the refrigeration indicates a small initial outlay, but running costs increase greatly. A condenser that is oversized, however, can sub-cool the refrigerant and affect the function of the expansion value.
    • The evaporator temperature should be as high as possible. To achieve this, the evaporator should be of correct size. The evaporator should be kept clean and defrosted when necessary, especially, when cooling air to below 0°C, as ice can build up on the coil. Hot gas from the outlet of the compressor can be used to defrost freezers, but control must be accurate. The defrost water may then be used elsewhere in the plant.
    • The compressor should match with the load. If a compressor is oversized, it will operate at only partial load, and the energy efficiency may be reduced. A sequencing or capacity control system to match the compressor with the load could help to improve efficiency.
    • The precooling of warm fresh milk using mains or well water will reduce the energy requirements of the refrigeration system to certain extent, thereby, reducing the cost of cooling. Precooling can be done by using both mains/well and chilled water in one operation. The chilled water alone increases the cooling rate and helps to maintain the milk quality.
    • Refrigeration heat recovery (RHR) units are used to make a refrigeration system more efficient by collecting heat which is normally released into the air and using it for heating water which is used for various other purposes in the plant. The pre-coolers and RHR units are competing technologies. It is usually more cost effective to use a RHR than pre-cooler.
    • The cleanliness of a farm’s refrigeration system plays an important role in its maintenance. The coils that are dirty will reduce efficiency and increase operating costs. So, most of the systems are provided with a watch glass that can be used to determine if the refrigerant needs to be recharged.
    • Leakages of refrigerant can reduce a system’s efficiency by 40% and should be kept to less than 2% of the annual charge.

Figure 4: Monthly electrical energy consumption (kWh) for 22 farms over 12 months (mo) for all major energy-consuming processes.

Some Case Studies

A few researchers studied the dairy refrigerating systems and implemented different methods in order to reduce the energy requirements and increase the efficiency of the system. The low grade heat which is available at the condenser side can be recovered and used for heating purposes. Various prominent options to recover the waste heat from the system are using heat pump system, regeneration system and tri-generation system. Heat pump option using a natural working fluid is considered as most viable, which can improve the overall COP of the plant. Singh et al. (2017) put forward an idea to integrate a trans-critical CO2 heat pump system with internal heat exchanger (IHX) to utilize the rejected heat from the ammonia based refrigeration plant to preheat the boiler feed water and thereby, reduce the overall coal consumption. The boiler feed water used is ground water that is available round the year at almost constant temperature 27°C. The condenser of ammonia based refrigeration system was coupled with evaporator of the CO2 based heat pump system in a way that the evaporative cooling system was eliminated which resulted in saving of some ground water. Figure 5 shows the CO2 heat pump system with IHX working in a trans-critical mode, coupled with existing ammonia based refrigeration and boiler system.The CO2 heat pump with IHX meets simultaneous heating and cooling demands. The evaporator of the heat pump system takes up heat from the ammonia cycle and boiler feed water is preheated utilizing the heat rejected by the gas cooler. They carried out the analysis using the actual heating and cooling load and demand data from an ammonia based milk refrigeration plant situated at warm climatic condition in northern India where the annual temperature ranged varies between −2°C and 48°C. They observed resulting changes in primary operating cost and energy saving. Total payback period (PBP) of the proposed tans-critical CO2 heat pump system was also computed using the total life cycle cost (LCC) approach. They found out that approximately 37 kW heat was recoverable by employing trans-critical heat pump system and about 5000 litres of ground water per week, consumed for evaporative cooling, was saved. They concluded that total reduction rate in CO2 emission and cost of energy was approximately 45.7% and 33.8% respectively. Also, total PBP of the trans-critical CO2 heat pump system with IHX was calculated to be approximately 40 months only.

Figure 5: Schematic of the proposed trans-critical CO2 heat pump system with IHX

Now, we know that existing refrigeration systems in the productive process employed to conserve the dairy products are largely responsible for high levels of energy consumption, and in some companies they consume about 60% of total energy used. In a study by Alves et al. (2014) the main causes that have been detected which contribute to such high spending were found to be deteriorated door junctions, poor insulation along the edges of cold chambers or lack of awareness of human resources for good behavioural practices. Many of these causes can be easily corrected with immediate results in terms of savings, estimating that these can be in the order of 20%.Thus, they intended to apply a mathematical methodology to estimate energy savings resulting from the application of energy efficiency measures in cold chambers from the dairy industry. The methodology developed was applied in real cold chambers from the dairy industry, more specifically in the cheese area of Nisa, Portugal. There was a moveable tight door to enter the interior which was usually opened for a long period of time to execute work tasks by 4 members of the staff.The total heat to be removed from the chamber was equivalent to the sum of all penetrating thermal loads with several origins. Figure 6 discriminates all those loads and the sources of origin. The identification of the most influent parameters in the global consumption was done by a sensitivity analysis, which consisted in a study of the variation ofsimulated consumption as a function of inputs defined in table 2.

Figure 6: (a) Classification of penetrating thermal loads;         (b) Sources of thermal loads

For the sensitivity analysis, the variation in power consumption for varying internal temperature and door opening frequency are shown in figures 7(a) and 7(b) respectively. The consumption of power reduces with increase in internal temperature and increases with increase in door opening frequency. The power consumption for halogenated light, fluorescent light and LED light has been shown in figure 7(c). The LED light and fluorescent light show similar consumption of power while the halogenated light consumes more power compared to the others. The number of staff inside the chamber generates the lowest weighted thermal load in final consumption (+0.11 kWh/person), among all inputs evaluatedas shown in figure 7(d).

It is observed that reducing the time of door opening and installing an air curtain device is really interesting due to the energy saving it could provide which is more than half of the original consumption. So, it is possible to reduce consumption by 67 % through the application of the proposed measures and a payback time of up to four years.

Figure 7: Sensitivity analysis for energy consumption by varying input parameters (a) internal temperature,( b) door openingfrequency, (c) lighting type and (d) number of people inside the chamber.

Energy Saving Potential of Future Technologies

A number of technologies are under development for use in the near future. Some of the most promising include:

  • Absorption chillers allow cooling to be produced from heat sources such as fossil fuels, incinerated garbage, biofuels, low-grade steam, hot water, exhaust gas or even solar energy, generally, using a lithium bromide and water refrigerant. While the cost of absorption refrigeration is relatively low compared with compression refrigeration (1 kW of refrigeration for 1 kW of energy), absorption chillers can utilise a waste heat source, thus emitting less greenhouse gases than conventional vapour compression refrigeration systems
    • Greater use of renewable energy sources such as solar electricity (PV), solar thermal, wind energy, biomass, geothermal heating and cooling.
    • Greater system integration by use of heat pumps, Combined Heat and Power and Trigeneration.

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