Cold storage is a remarkable tool for preserving perishable food. In addition to preventing deterioration, cold storage has real economic importance, since it promotes international trade and maintains the quality of agricultural and fishery products. The applications of cold storage to a wide variety of perishable foodstuffs have become firmly established in little over a century since its introduction. One of the most important factors affecting the quality, safety and storage life of agricultural and food products is temperature. In order to maintain the highest quality of food, the immediate refrigeration processing either cooling or freezing after the harvest and storage and transportation in a low temperature environment are recommended for perishables. The low temperature environment can be achieved and maintained through a cold chain in which the temperature is consistently at required levels. Today, the majority of foodstuffs being treated and maintained by refrigeration in cold storages, for instance, in Indian supermarket sales, account for 70 per cent of all food sales.
Currently, in the light of energy crisis, energy efficiency is crucial for sustainable development particularly in developing countries like India. The energy efficiency is also a key mitigation strategy for addressing climate change. With the urgency of climate change requiring both immediate and long-term actions and the volatility of energy markets, the importance of efficiently using energy throughout the world, including India, is clear. Increased prices of electricity and other fuels force refrigeration based cold storages and commercial buildings to reduce their energy consumptions and demands or to implement the means for conserving energy. Therefore, the need of the hour is to minimise the energy consumption and energy demand, which consequently reduces emissions at the thermal power generating stations.
In view of the energy conservation opportunities in industrial, commercial and residential sectors; one of important areas is cold storage industry, which needs to be addressed with immediate attention. Preservation of perishable commodities, like potato in the cold storage, is an energy intensive process and consumes significant amount of electrical energy. The proper and timely preservation of the perishable commodities is crucial for retaining their quality, natural flavour and for minimising quantity degradations. A sizeable amount (approximately 10-15 per cent) of the total potato production is gone wastage every year due to shortage of the storage facility in the country like India. Currently, available storage facility is sufficient only for 50 per cent of the total potato production in the country as shown in Figure 1 and 2. Moreover, the poor operation of refrigeration units in the existing storage facilities not only increase the energy consumption but also responsible for poor quality of refrigerated product. The Indian cold storages are mainly run on the grid electricity and their energy expenses account approximately 28 per cent of the total costs. However, continuous efforts are being made by researchers and engineers to improve the energy performance and indoor storage environment in the cold storages.
Figure 2: Cold storage requirement per population in important countries.
Fresh foods like fruits, vegetables, meat, poultry and dairy, require an uninterrupted cold chain due to their perishable nature. By controlling parameters of temperature, humidity and atmospheric composition, along with utilizing proper handling procedures, cold chain service providers can increase the product life of fresh foods for days, weeks or even months. These services allow fresh products to hold their value longer, increasing their transportability and providing opportunities that expand their market reach. The exact structure of each cold chain varies significantly depending on product and customer requirements; however, the goal of a properly designed cold chain system is to safely move temperature-sensitive products in a way that reduces waste, maintains the quality and integrity of the product and limits opportunities for bacterial contamination. A complete cold chain system may include post-harvest precooling or freezing, processing, temperature controlled warehouse or storage, retail or distribution and refrigerated transport between locations.
Cooling technology for cold storages
The performance of cold storage only depends on the temperature. Lower the temperature inside the cold chamber, greater will be the performance. To achieve the low temperature to storage, a refrigeration system is employed. This refrigeration system works on vapor compression. The refrigerant R-22 is used as working fluid for vapor compression refrigeration.
Components of Cold Storage
The cold storage consists of following components:
Compressor: Reciprocating or screw type compressors are generally used in the cold storage. The reciprocating and screw compressors are the best suited for use with refrigerants which require a relatively small displacement and condense at relatively high pressure, such as R-12, R-22, Ammonia, etc.
Condenser: It is an essential part of the cold storage plant. It is applied to work at higher side at the constant pressure. Three types of condenser are frequently used in the cold storage – atmospheric or air type, water cooled and evaporative condenser. The condenser fan can also be used to force air. For enhancing the heat transfer, Aluminum fins are employed in condenser with a regular gap of 2 mm.
Receiver: It is basically storage vessel designed to hold excessive amount of refrigerant not in circulation. Refrigeration systems used to vary heat loads, or systems using a condenser flooding valve to maintain a minimum head pressure during low ambient temperatures, will require a receiver to store excess refrigerant. Liquid receivers are employed at liquid line as near as to the outlet of condenser. The piping between condenser and receiver should be so arranged that it can enable for free drainage. A receiver’s storage capacity is based on 80 per cent of its internal volume at a refrigerant temperature of 90°F as per ARI Standard 495. Generally, a receiver is selected to receive 90 per cent of the total system charge to provide adequate reservoir during high loads and to allow the refrigerant to be isolated between the condenser and the receiver during repairs.
Filter-Drier: A filter drier in refrigeration system has two essential functions: first, to absorb contaminations present in the system, such as water, which can create acid and second function, to provide filtration. The functions of filter dried are accomplished by use of desiccants within filter drier. The three most frequently used desiccants are molecular sieve, silica gel and activated alumina.
Solenoid valve: A solenoid valve is an electromechanical valve often used to control the flow of liquid or gas. Solenoid valves are widely used in many applications and are commonly used in refrigeration and air conditioning systems. Its function is simply to turn refrigerant flow on and off. Solenoid valves provide fast and safe switching, reliability and compact design.
Evaporators: It is also called cooling coil of the cold storage units. It plays a vital role in cooling the cold chamber and applied at low pressure side and pressure should be uniform throughout the cooling process in the refrigeration units. The refrigerant of liquid type from solenoid valve is passed down to a low-pressure liquid and passed on to an evaporator mounted on cold store wall from solenoid valve is admitted to the evaporator as shown in figure 3 and then there, the liquid refrigerant starts absorbing the heat from ambient medium thereafter it boils and then converted into the vapour phase. The temperature of boiling refrigerant in evaporator must always be less than that of ambient medium so that heat absorption can be there by the refrigerant. The dry expansion type of evaporator is being used to cooling the chamber of the cold storage unit and mount to wall of cold store. The fan used in evaporator to force the air.
Fan: In order to save power in the part load operation periods, fan works on cycled operations. Variable frequency drives are used on the fans to save power consumptions.
For uniform and rapid cooling within cold store, the crates of product should be so stacked that cold air from evaporator can be enabled to move throughout each individual crate. It is essential that crates are so arranged that air channel can be made in cold store for direct air movement and also there should be some gap between the crates and walls to enable refrigerated air to absorb the heat of conduct through the walls. The arrangements of stack are accomplished by providing the certain gap among the columns of stack as shown in Figure 4. In cold storages, an innovative design has been performed to reclaim the cold energy for a warm weather. Conventionally, this was done by installing vapour compression refrigeration systems, necessitating tremendous electrical power to drive the refrigerant compressor working in low temperature maintenance of cold storage.
From average temperature distribution of different arrangement, it has been observed that on increasing the gap between the columns of stack, the tendency of achieving low temperature within the cold store is more. This is happened due to on increasing the gap between the columns of stack, the width of air channels in different arrangements increases accordingly. The increase in width of air channels give the way to air to be circulated to each individual crate within the cold storage. The increase in gap is also influenced the running time of cold storage. Lager gap provides more air circulation, resulting to achieve the optimal temperature of product rapidly.
Use of Renewable Solar Energy in Cold Storage
When solar radiations are interpreted on solar collector then it converts solar energy into the electrical energy which is being stored into the batteries i.e. the batteries are charged by solar energy. The inverter is used to convert electrical energy from 24V DC to 220V AC. This energy is supplied to AC electric motor, which is used to run the refrigerator compressor as shown in Figure 5.
The followings are the benefits of use of renewable solar energy in cold storages:
The solar panel can be effectively used in summer to produce refrigeration effect using solar hybrid refrigeration cycle.
The operating cost of the solar hybrid refrigeration cycle is low.
The coefficient of performance of the hybrid system is high.
It does not need to be connected continuous to power grid nor using any consumable fuel as power source.
Solar hybrid refrigeration systems are more applicable in remote areas where conventional cooling is difficult and solar energy is always available.
This system is more suitable because pollution free working fluid use as a refrigerant.
Energy Conservation Options in Cold Storages
The energy conservation in the cold storage is now getting more attention throughout the world in regards to protect earth’s environment. The energy efficiency of cold storage is highly dependent on various parameters such as arrangement of stack, maintenance of cooling temperature range, type of stored items etc. Various energy saving options have been observed in the analysis of surveyed data available in literature. A few options can be implemented immediately and other can only be incorporated effectively at the time of planning and development stage of cold storage. The exterior surfaces are painted by different colours without taking the advantage of high reflection properties of the colors. The energy consumption can be reduced significantly by selecting the colour of exterior surface appropriately. Nearby plantation and other shading of building might be effective options for immediate improvement in energy efficiency as well as for minimising the worst environmental effect. The overall energy consumption can be minimised significantly by choosing the optimum aspect ratios and the respective orientation of the cold storage building. The construction materials used in the cold storages are conventional and have not been arranged in any specific manner. The thickness different materials need to be optimised.
To save energy the following practices must be implemented:
1. Reducing Heat Loads
2. Uses of latest energy saving equipments
3. Proper Insulation
4. Efficient maintenance practices
5. Automation & Integration.
Moreover, the cold storage should be designed to be mounted on a concrete platform which greatly reduces the heat gain from the soil. The exclusion of floor insulation was justified in this design in order to minimise both material cost and hence, cost of construction.
Challenges and Barriers to Cold Storages
While it can be challenging to operate a business in any international environment, one of the greatest challenges to cold chain expansion in a developing market is the lack of infrastructure necessary to sustain cold chains. Transportation systems in many of these economies can make reliably transporting refrigerated products in a timely manner difficult to near impossible. In addition, lack of reliable power for cold storages, power hookups for reefer trailers at ports and transportation hubs, and adequate facilities at the final customer locations add further costs and complications.