Energy Efficiency in Refrigerated Warehouses

The majority of the food supply chain essentially depends on storage methods. Companies require ways to keep food cool and maintain food quality when moving products from one place to another. According to the Food and Agriculture Organization of the United Nations (FAO), the food industry is responsible for approximately 22% of the world's greenhouse gas emissions and nearly 30% of the global energy consumption.

Cold Storage and Energy Consumption

Cold storage facilities, which rely heavily on fossil fuel energy sources, significantly contribute to climate change. Over the last 25 years, various studies of energy performance have concluded that the operation of a low-temperature, industrial-size cold store facility (about 100,000 m3) requires approximately 30kWh per cubic meter per year (30kWh/m3/yr). 

However, the research “‘Energy Performance of Industrial Cold Storage Facilities’”  provides a more accurate best practice benchmark as it incorporates data supplied from analysis of a number of contemporary cold and chill stores using a range of energy-saving techniques. The new curve for best practice performance assumes an energy use level in large facilities which is about 15% to 30% of the generally accepted figure from two decades ago. Furthermore, facilities can use the Specific Energy Consumption (SEC), which is calculated by kWh/m3/yr, as a benchmark to measure system performance and compare the operational energy scenario.

According to the Cold Chain Federation of UK, a modern and well maintained cold store of 100,000 m3 should have a specific energy consumption (SEC) of 10 kWh/m3/yr for the refrigeration system while for a 500,000 m3 store the SEC could be less than 5kWh/m3/yr.

Almost every step of the cold chain — from post-harvest forced-air chillers, refrigerated trucks and shipping containers, to industrial cold storage — requires massive amounts of energy to keep foods from spoiling. And when these energy demands are met with fossil-fuel power, the emissions footprint of cold chains quickly adds up. In 2018, refrigeration accounted for nearly 5% of global energy needs, making these technologies alone responsible for 2.5% of total emissions that year. Additionally, when demand for fossil fuels rises, so do the prices of the products or commodities they store.

Why Does Refrigerated Storage Use So Much Electricity?

For cooling expenditures, the typical refrigerated warehouse uses about 25 kWh of electricity per square foot per year.

High energy usage in cold storage can be explained by thermodynamic principles. Heat naturally travels from hot spots to cold spots due to diffusion. If a refrigerator is shut off, it will eventually reach equilibrium with the outdoor temperature, meaning the internal temperature will rise to match the external temperature.

To remove hot air from inside and maintain low interior temperatures, refrigeration technology requires energy and pressure. This process involves a consistent flow of electrical energy to maintain a temperature disequilibrium, which is essential for keeping the stored products cool. This continuous power consumption is the primary reason for the high electricity costs associated with the food and refrigeration industries.

The primary systems impacting the energy efficiency of refrigerated warehouses are refrigeration units, pumping systems, and lighting. Each of these systems plays a crucial role in maintaining the required temperature and thus significantly contributes to the overall energy consumption.

Refrigerated Systems

Control Systems

Control systems in refrigeration vary widely in complexity, ranging from small programmable logic controllers to full-system controllers. These systems play a crucial role in optimizing different aspects of the refrigeration cycle and managing various parameters to enhance energy efficiency across the entire system. One approach involves utilizing localized programmable logic controllers, which continuously adjust individual components of the refrigeration system to maximize efficiency. 

Alternatively, a centralized control system can be installed to optimize energy usage on a broader scale, improving efficiency throughout the entire system. Supervisory control further enhances oversight by providing comprehensive management of the refrigeration system. In specific cases, implementing floating-head pressure control, which adjusts condensing temperature based on ambient temperature, can significantly reduce compressor load, offering a typical simple payback period of one year and a half.

Similarly, utilizing floating suction pressure control to optimize suction pressure set points based on cooling requirements contributes to ongoing energy efficiency improvements. Additional strategies include optimizing freezer temperature set points to raise temperatures in refrigerated spaces while ensuring product safety and incorporating variable frequency drives (VFDs) for motors operating at variable or constant rpm, enhancing overall system efficiency. These methods collectively contribute to energy savings and promote sustainable refrigeration practices, with potential payback periods as short as two years for certain solutions.

Compressor Efficiency

In cold storage facilities, compressors are notorious energy consumers, surpassing other components in consumption. However, optimizing compressor performance can yield significant improvements in refrigeration energy efficiency. One approach involves reducing the head pressure set point and maximizing the suction pressure set point within feasible conditions. This adjustment effectively lowers the compression ratio or lift, leading to reduced energy consumption. A payback period may be between one to five years.

Furthermore, staging compressor operation to align with the facility's refrigeration load across various operating conditions is crucial. Utilizing one or more compressors at full load and incorporating a compressor with efficient part-load performance as a trim unit can significantly lower the energy consumption. 

Another effective strategy entails installing compressors with varying capacities and employing programmed controls to maximize staging efficiency. In multi-compressor systems, considering a reciprocating compressor with cylinder unloading as the trim unit proves advantageous, as reciprocating compressors can closely match varying loads compared to screw compressors lacking variable frequency drive (VFD) capabilities. Converting existing reciprocating refrigerant compressors to a cylinder unloading strategy further enhances energy efficiency. Additionally, adding VFDs to existing screw compressors that operate regularly at part load, or those functioning as trim units in multi-compressor systems, can improve efficiency. For facilities with separate freezer and cooler spaces, implementing a two-stage refrigeration cycle, although entailing higher upfront costs, proves beneficial. This system provides cooling for two different temperature ranges, significantly enhancing energy efficiency throughout the entire refrigeration process.

Evaporator Efficiency

Evaporators in industrial refrigeration systems are known for their energy-intensive nature, but there are several proven energy-efficiency measures available for optimizing their performance. One effective method involves fine-tuning the floating suction pressure set point to optimize compressor efficiency, ensuring energy savings. Regularly cleaning the evaporator coil enhances heat-transfer efficiency, and offers a payback period of several weeks. Installing controls to switch constant-speed evaporator fans to an on/off cycle reduces fan run time, contributing to energy conservation. 

Another valuable approach is the installation of Variable Frequency Drives (VFDs) on evaporator fans in refrigerated spaces with variable loads, allowing continuous optimization of fan speed based on dynamic refrigeration requirements and resulting in a payback period of one to four years. Additionally, optimizing the minimum-speed setting for evaporator fans using VFD control further enhances efficiency. Reducing the frequency and duration of timed defrost cycles to the minimum necessary ensures proper defrosting while minimizing energy consumption, especially during periods of lower ambient temperatures. Replacing timed defrost systems with sensor controls for evaporators reduces the heat added to the cold storage space, with a payback period of two to four years. Retrofitting with high-efficiency evaporators that extract heat from refrigerated spaces using minimal fan energy improves the overall energy efficiency of the refrigeration system. Opting for evaporators that defrost with water or hot gas instead of electric resistance defrost conserves energy. 

When evaporator fans have a fractional horsepower rating, installing evaporators with electronically commutated (EC) motors proves beneficial. Additionally, replacing existing shaded-pole evaporator fan motors with EC motors significantly reduces energy consumption by up to 65%, with a typical payback period of one to three years. These measures collectively enhance energy efficiency and contribute to sustainable industrial refrigeration practices.

Condenser systems

Condenser systems play a significant role in cold storage energy consumption, and there are several strategies available to enhance their efficiency. Programming control systems to optimize the floating head pressure set point is a crucial step in minimizing energy usage. Additionally, adjusting the minimum speed setting for condenser fans and lowering the fixed condensing pressure set point to the safest possible level contribute to energy conservation efforts. Regular cleaning of condenser surfaces improves heat transfer efficiency, typically offering a payback period of one month to one year.

Descaling water-cooled condenser tubes enhances water flow and heat transfer efficiency, further optimizing performance. Retrofitting condenser fan Variable Frequency Drives (VFDs) and implementing associated controls can effectively optimize condenser fan speed, with a payback period of one to two years. Installing on/off controls for single-speed condenser fans or high/low/off controls for two-speed condenser fans aids in efficient operation. Upgrading from an air-cooled condenser to an evaporative condenser proves advantageous for improved efficiency. Another option is considering an oversized condenser installation, which decreases head pressure and enhances compressor efficiency. Additionally, recovering heat from the condenser for use in the glycol slab heating system further maximizes energy utilization, creating a comprehensive approach to energy efficiency in cold storage facilities.

Pumping System

In refrigeration systems, inefficient pumping and heat or friction losses in piping can result in unnecessary energy consumption. To mitigate this, incorporating Variable Frequency Drives (VFDs) in refrigerant pumps for applications with varying fluid flow requirements proves highly beneficial. VFDs enable pumps to adjust their speed according to the system's needs, optimizing energy consumption and ensuring efficient operation. Additionally, enhancing insulation on refrigerant piping helps minimize refrigerant heat gain, further reducing energy wastage. These measures collectively contribute to energy conservation and promote the overall efficiency of refrigeration systems.

Lighting Systems

Implementing high-efficiency lighting systems not only reduces energy consumption but also lessens the heat generated, decreasing the burden on refrigeration systems. One effective step is transitioning to LED lighting technology, which has gained popularity in cold storage facilities due to its unmatched energy efficiency, excellent performance in cold temperatures, and minimal heat production during operation. 

LEDs are particularly advantageous in freezer environments and extremely cold settings where lighting frequently toggles on and off, thanks to their superior cold-temperature restrike capability. Another energy-efficient option is replacing High Intensity Discharge lighting with high-bay linear fluorescents like T5 and T8, offering up to 50% more energy efficiency and significantly lower heat emission in warehouses. To further conserve energy, occupancy sensors can be installed in intermittently used areas, reducing lighting energy usage by up to 75%. 

Aisle-specific lighting control, guided by occupancy sensors, allows for efficient grouping of goods by storage and retrieval timeline, minimizing activity and lamp run time. Ensuring optimal coverage areas for existing occupancy sensors and photo sensors enhances their effectiveness. Utilizing photo sensors to turn off interior lighting in areas with ample natural light and adjusting timer-controlled lighting to minimize operating hours further contribute to energy savings and promote sustainable lighting practices in cold storage facilities.

Conclusion

Optimizing energy efficiency in refrigerated warehouses not only reduces costs but also minimizes environmental impact. Implementing strategies like advanced control systems, efficient compressor and evaporator management, and high-efficiency lighting can lead to significant energy savings and quick payback periods. By adopting these measures, companies can enhance sustainability in their operations. 

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