Improving Energy Efficiency
Moisture loss from fresh produce causes it to shrivel or wrinkle and lose quality. Therefore, proper measures must be taken during cold storage to minimize moisture loss which also represents a direct loss of saleable amount...
Refrigeration of perishable food products is an important and fascinating application area of heat transfer and thermodynamics. Refrigeration slows down the chemical and biological processes in food and accompanying deterioration and loss of quality. The optimum storage temperature of most fruits and vegetables is about 1°C to 4°C. But this is not the case with some fruits and vegetables like bananas, cucumbers (summer season produce) that experience undesirable physiological changes when exposed to low temperature usually between 0 - 10°C. The resulting tissue damage is called the chilling injury and is characterized by internal discoloration, soft scald, skin blemishes, soggy breakdown and failure to ripen. The severeness of chilling injury depends upon both the temperature and length of storage at that temperature. Moisture loss from fresh produce causes it to shrivel or wrinkle and lose quality. Therefore, proper measures must be taken during cold storage to minimize moisture loss which also represent a direct loss of salable amount. Moisture loss can be minimized by (a) keeping the storage temperature as low as possible. (b) Keeping relative humidity of storage room as high as possible and (c) avoiding high air velocities. However, air must be circulated continuously throughout the refrigerated storage room to keep it at uniform temperature. To maintain high quality and product consistency, temperature swings of more than 1°C above or below the desired temperature in storage room must be avoided. Waxing reduces moisture loss and thus slows down shriveling and maintains crispiness in some products such as cucumbers, mature green tomatoes, peppers and turnips. But a wax coating that is too thick may actually increase decay especially when no fungicide is used. Sprouting of onions, potatoes and carrots becomes a problem in a storage that are not adequately refrigerated. The problem can be controlled by using sprout inhibitors. Heat treatment or radiations can be used to control decay and kill insects and micro-organism on or near the fruit surfaces.
Fresh and shrivelled Kinnow fruit
Chilling injury in Guava & Tomato
Properly waxed and over waxed Kinnow
Refrigerated spaces are maintained below the temperature of their surroundings thus there is always a driving force for heat flow towards refrigerated space from surroundings. As a result of this heat flow, temperature of the refrigerated space will rise to the surroundings unless heat gained is promptly removed. Fresh fruits and vegetables are live products and they continue giving off heat that adds to refrigeration load of cold storage room. The storage life of fresh perishables like fruits and vegetables can be extended by several weeks by cooling. We cannot do much about the physiological characteristics of perishables but we can certainly alter the environmental conditions to more desirable levels through heating, cooling, ventilation, humidification, dehumidification and control of oxygen levels.
Cold storage projects, while performing the important task of preservation of foods, consume a lot of energy for their operation. Energy efficiency is, therefore, very important for cold stores. Normally, energy efficiency of any product or system is defined as ratio of (Work out put) / (Energy input). The ratio is always less than one or in percentage less than 100%. So, cold stores are not very energy efficient and with increasing cost of electricity, their margins are shrinking. Under prevailing business model of cold stores, the farmers are not willing to pay more tariffs and the large number of cold store owners feels that there is no incentive to modernize. Currently, the store owners do not take any responsibility of the weight loss and quality deterioration during storage and generally service levels are poor.
Ever since potato processing industry has realized the importance of quality of potatoes (high dry matter, low sugar, firm and free from other defects) there is increase in cold stores with better storage conditions, the processers particularly large companies like Pepsi, McCain and ITC are insisting on the following storage conditions-
• Use of sprout suppressant – CIPC which also enables storage of potato between 10°C-12°C
• High humidity- more than 95% to avoid shrinkage and maintaining firmness of the potato which helps in peeling while processing.
• Maintaining CO2 level- less than 3000 ppm which avoids black hearts and stress to potato.
Considering theses requirements of major companies, substantial improvement needs to be made in the existing stores. The retrofitting budget for a typical chamber of a traditional cold store for 2000-2500 tonnes is quite high. However, the modernization definitely improves the quantity and quality of stored potatoes. But before arriving at this decision, the cost benefit ratio, the use of energy and its potential leakage points be investigated thoroughly. Reducing energy use makes perfect business sense. It saves money, enhances the reputation of your business and promotes the fight against climate change. Energy saving doesn’t need to be expensive. Upto 20% can be cut in many refrigeration plants through actions that require little or no investment. In addition, improving the efficiency and reducing the load on a refrigeration plant can improve its reliability and reduce the likelihood of a breakdown.
Any energy efficiency initiative dealing with refrigeration should start by reviewing the heat gains on your system. If you understand the nature of these gains, you’ll be able to manage the amount of cooling that needs to be done and make energy savings. Heat gains include warm air entering cold room and heat produced by electrical equipment within the cooled space. The single largest load on cold rooms is usually caused by warm air getting through open doors. This typically accounts for 30% of the total heat gain by a cold room. Gaps between insulated panels or at points where pipes penetrate the walls can also allow a small but constant stream of warm moist air into the store.
Good operation practices for lowering energy costs
• Introduce good door management and keep the door of your cold store closed whenever possible. This will keep warm air and moisture out, and energy costs down.
• Make sure airflow from the evaporators is not obstructed.
• Run your cold store at the highest possible temperature for the product.
• Ensure the product loaded into your cold room has not warmed up by being left in an ambient temperature area.
• Switch off the lighting in your cold room when it is not in use
Energy Consumption Profile of System Components
1. Compressor Consumption Is Highest : 60% ~90%
Typical savings with low voltage drives – 10-20%
2. Evaporator Fans Consumption is 5~15%
Typical savings with low voltage drives – 20-50%
3. Condenser is 3~5%
Typical Savings with low voltage drives – 15-20%
4. Brine Pump is 2 ~3%
Typical Savings with low voltage drives – 20-40%
Compressors & Condenser
The compressor is the heart of the refrigeration system. These are always the single most intensive energy consumer in the system. The compressor raises the pressure of the refrigerant from the evaporator to a level that will allow the heat to be rejected to ambient air at the condenser. The difference between the refrigerant temperature in the evaporator (evaporating temperature) and the condenser (condensing temperature) often determines how hard the compressor has to work. The larger the difference, the more work will be required by the compressor and the more energy it will consume.
In almost all cases, the single most effective energy-saving action you can take is to reduce the temperature difference. For every degree that this difference is reduced, you will save around
3% of the compressor energy. The main method of reducing temperature difference is to lower the temperature at which heat is discharged in the condenser (condensing temperature). Traditionally, condenser control (head pressure control) systems were programmed to run all year round at a condensing temperature designed for summer conditions. Changing the control to allow the temperature to reduce in cooler weather offers a great potential saving.
Compressor Condensor Technology upgrade
Variable speed control of a compressor offers the ability to match the cooling capacity to the actual need based on measurements in the refrigeration system. Costs are reduced both through energy savings when running at lower speed, but also through lower installation costs caused by optimization of the refrigeration system and the compressor itself. Variable speed controlled also reduce the need for compressor packs because the individual compressor is adjusted to the actual need. The lifetime of the compressor is increased due to few starts and stops which cause mechanical wear and tear
An evaporator is so called because the liquid refrigerant inside evaporates at low pressure. This is what creates the cooling effect. It is mounted on the wall or ceiling of a cold room (figure 5.0). If the evaporator is blocked or is not controlled properly, the cooling will be inadequate – and your energy costs will rise. Like any heat exchanger, evaporators must be the right size for the job. If an evaporator is too small, the compressor will have to work harder and longer. It will also have to defrost more often, increasing your energy costs. Evaporators in most applications need to be defrosted periodically. While this is usually done with timers, intelligent controls can detect when a defrost is required and will ‘defrost on demand.
Many cold rooms are set too low – set the temperature only as low as needed. Turning the thermostat up by just 1°C will reduce energy use by up to 2%. The evaporator controller is usually the room thermostat – make sure this is set as high as possible without compromising food or process quality Keep an eye on your evaporators – if you see a permanent build-up of ice on the coil, something is wrong. Evaporators should be cleaned when they get dirty. They lose performance as dirt builds up. Fans lose performance in the same way. Get your technician to include a thorough deep-clean of the evaporator coils when necessary.
Appropriate regular plant maintenance will save money by ensuring the refrigeration plant
operates efficiently, reducing service costs and making interruptions to your business by breakdowns less likely.
The maintenance schedule will be determined by the size and complexity of the refrigeration plant. As a minimum the schedule should cover the following points:
• Refrigerant levels in the receiver and the liquid line sight glass.
• Refrigerant leak testing and repair as required
• Condenser cleaning, especially air cooled types (the frequency of cleaning will depend on the condenser location and its surrounding environment).
• Condenser fan and pump condition and condition of safety equipment such as fan guard.
• Evaporator cleaning.
• Operation of the defrost system. Condition of fans and safety equipment should be covered as per the condenser
• Compressor oil levels and on systems which have suitable gauges fitted, suction and discharge temperatures and pressures. Accuracy of gauges, guards, operation of all safety controls.
• Checking of control parameters to the optimum set point.Suction superheat to ensure that the expansion valves operate properly.
• Checks for undue noise and vibration.
• Condition of insulation.
• Condition of door seals on cold stores
Cost savings of up to 50% are possible by making sure that your refrigeration plant is well operated and maintained. Also, improved reliability will reduce the chance of unplanned stoppages or business interruption. Appointing a good maintenance contractor is key to achieving these savings. Also, it is possible to reduce running costs by up to 15% by re-commissioning equipment, especially multi-compressor systems, so this should be included in your maintenance contract.
AUTHORS CREDITS & PHOTOGRAPH
Dr Mahesh Kumar
Dept. of Processing
& Food Engineering,
Dr B V C Mahajan
Dr Swati Kapoor