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Home » Refrigeration technology and system designs: the continual quest for efficiency

Refrigeration technology and system designs: the continual quest for efficiency

  • marimac 

By Benjamin Brits

With the growing global demand for refrigeration expected to maintain an upward trajectory over the next ten years, continual developments in products, equipment, services and engineering design to meet efficiency requirements will continue – as well as the drive to reduce food waste.

A CO2 installation showing the gas coolers. Image credit: © Cold Link Africa | Benjamin Brits

A CO2 installation showing the gas coolers. Image credit: © Cold Link Africa | Benjamin Brits

Taking a closer look at all the elements involved, one realises the vastness of the cold chain. The taxonomy here would possibly run into many thousands of items. From the simplest sensors to the most extravagant plants the world has seen, or the technology involved in ‘plug ‘n play’ equipment to the doors, seals and insulation of display cabinets or freezers. The advancement on all fronts to achieve better control, accuracy and ultimately efficiency is ongoing.

Of late, world bodies and authorities have become increasingly vocal around two particular factors that persist to come to the fore – which also relate directly to the cold chain: energy consumption and food waste. Now, as the ‘custodian’ in the preservation of perishable products the cold chain will continue to remain essential in food supply chains (and food safety), and further as one of the largest users of energy, continual innovation is again essential as globally the struggle to manage and forecast energy demand becomes a greater and greater task.

“The improvement of refrigeration technologies overall has made it possible to extend the shelf life of food, which has a relevant impact on reducing food waste and increasing food safety. The type and mix of equipment used in a refrigeration system plays a definite role in the stability of temperature and humidity where food is stored in a cold store or supermarket outlet, and thus affects the impact in the objective of preservation”, says Carel Industries.

The whole point of maintaining the cold chain is to avoid the deterioration of the properties of any perishable product to the point of affecting its marketability or ‘value’. This factor depends on the combined action of specific enzymes typical of tissue cells and the microbes that contaminate food.

The overall effect is the formation of organic substances with a low molecular weight (containing nitrogen and sulphur) that modify the smell, taste, and colour, when considering foodstuff as an example, in a negative or unpleasant way. That hydrolytic action also breaks the product’s structure, frequently causing irreversible modification to some rheological properties (softening of the food, for example).

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The cold chain remains the best way to limit or slow down microbial multiplication and thus produce a reasonable preservation period for food perishables. This period can vary, even considerably, depending on the type of food, its physical state, its history before being placed on the market (good hygiene practices during processing immediately prior to being placed on the market), the time of year, the specific batch of product, botanical variety, and so on.

Among the different steps in the cold chain, the choice of reliable technologies cannot be more emphasised. Produce may constantly be exposed to factors such as temperature variability due to the influx of ambient air, proximity to lights, defrost cycles, air flow, and differences in set point temperature from the producer to a retail cabinet. In South Africa, there is a general perception of clients wanting only the ‘cheapest solution for now’, without any forethought to the future and how these decisions will impact their businesses down the line.

“In recent years, most of these variables have been improved notably thanks to more accurate design and improvements in technologies which include DC inverter technologies, that together with the use of electronic expansion valves and more advanced control and supervisory systems, have led not only to system energy savings but also greater stability of the parameters in various refrigeration systems from complete plants to stand-alone cabinets”, Carel Industries states further.

The changing landscape of refrigeration systems

Another major factor in the equation here that does not necessarily affect all components, but is a major one, is the phasing out of older global warming potential and ozone depleting refrigerants (Chlorofluorocarbons or CFC and Hydrochlorofluorocarbons or HFCFC) that has resulted in a worldwide drive towards natural refrigerants. Ultimately narrowing down the list – three ‘main contenders’ remain: ammonia (R717), carbon dioxide CO₂ (R744) and propane (R290). Each of these have their own best suited application, pros and cons (or risks to be more technically correct).

A lot of reports now exist where you would read statements such as ‘what will win the race – ammonia or CO₂ as the dominant global refrigerant’ or reports highlighting the risks over rewards of each. Speaking to several industry players, the debate may go on for a very long time into the future as engineers and companies alike will have their natural preference, but many are also confident that systems of the future will incorporate more than one refrigerant (operating as required per temperature range of course).

“Historically, refrigeration systems always took a standardised approach when it came to their design and construction. Over time, cooling trended toward no longer being engineered as much as a replication-model exercise using decades-old, but proven designs. This was the case for any new facilities. Ultimately, the result was that specifically industrial cooling systems started falling behind when it came to inclusion of the latest available technology. Times have however quickly changed due to several factors. Facilities around the world are facing higher average ambient temperatures while at the same time need to reduce energy-use and minimise climate-impact. Innovative technologies and improved refrigeration systems that employ natural refrigerants have quickly gained traction.

As a result, there has been a dramatic shift in the approach to refrigeration, but more so improved designs that professionals in this space are responsible for, and the facility design and operations as a whole will have to continue to evolve”, notes Permacold Engineering.

Natural refrigerants as the major drive in engineering design changes have increasingly proven their value and safety over the last decade. The move to higher-efficiency, leaner systems that rely on natural refrigerants like CO₂ and ammonia, are essentially already the norm on a global scale, particularly as more international investment is seen in South Africa, and as we compete on the international level, such clients are become increasingly aware and more demanding in better sustainability throughout the supply chain.

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“As the shift towards natural refrigerants progresses, looking back, CO₂ technology has shown rapid adoption locally. What was first introduced into South Africa started out with equipment for retail clients where subcritical CO₂ designs were used. We then started to move to first world transcritical technology and systems based on Beijer Ref Italian company, SCM Frigo designs. Our first transcritical unit was supplied into South Africa in 2018. From then onwards the technology and range of CO₂ equipment has just grown. The range in CO₂ equipment today includes everything from small applications right through to multi-megawatt facility applications. We can offer a full solution on CO₂ units while this as a refrigerant allows a green solution for our clients and to meet the demands, we are moving in that direction. We all know that we need to be moving to natural refrigerants, and the reasons behind this. CO₂ is obviously not the only green or natural solution out there. On smaller applications in future, we will definitely see R290 coming up the ranks into the selection pool as well. From the commercial side we believe CO₂ will be prominent while on the higher end of the industrial scale, above three megawatts, ammonia will remain the most competitive solution for now. We will continue to support all of our clients in whichever natural solution they choose,” says Metraclark, part of the Beijer Ref Group.

Many plug ‘n play cabinets and island fridges/freezers already make use or R290 (propane) refrigerant in retail applications. Image credit: AHT
Many plug ‘n play cabinets and island fridges/freezers already make use or R290 (propane) refrigerant in retail applications. Image credit: AHT
An ammonia refrigeration system installation. Iamge credit: © Cold Link Africa | Benjamin Brits
An ammonia refrigeration system installation. Iamge credit: © Cold Link Africa | Benjamin Brits

Permacold Engineering adds, “In the future, the refrigeration systems operated and maintained in many facilities will no doubt have to meet certain criteria and may be characterised by having a smaller footprint, be limited in their power consumption, and rely on the latest technologies developed, catering to natural refrigerants only. The increasing demand for high efficiency, low maintenance systems will also inevitably force a change in the typical approach to systems designs as clients themselves will seek out far lower energy bills, the reduction in use and costs of resources such as water, sewage and chemicals, and improved sustainability of system and component materials.”

The South African sector has seen the largest growth in CO₂ systems over the last two years as well as the availability and supply of CO₂ system components from international suppliers.

“Using CO₂ as a refrigerant is definitely not anything new though. This refrigerant has in fact been used for over 15 years already, and in different forms. At the beginning stages of its use, CO₂ was primarily used for low temperature applications (freezer applications). In the medium temperature applications (chillers), and the high temperature (drop-down areas). CO₂ wasn’t used as a refrigerant because it operates at high pressures and the technology at that stage did not exist to use it. Typically, at that time, designs made use of a cascade method, while today the use of CO₂ has been very well refined and can be used for all applications. Transcritical CO₂ plants are common as they enable the handling of low to medium temperatures and high temperatures through the inclusion of intermediate or parallel compressors. This method has now been in use for about six years and locally various larger supermarket groups have been shifting over to transcritical CO₂ over recent years”, according to Matador Refrigeration.

Although adoption of natural refrigerant plants has seen a lot of activity for bigger players, initial capital costs are higher, and so at this point in time not all companies have the necessary funds to make a shift and must rely on maintaining their existing plants.

“We have to all understand that natural refrigerants are the future and are going to take over the market at some point whether companies like it or not. This is a particular drive for internationally recognised chains who are on the fast track to go green but further than that these new systems are far more energy efficient and provide significant savings in operational costs quite quickly. CO₂ plants (especially transcritical) also work differently because there is for example no condensing (when the refrigerant reaches transcritical state), and state change occurs through gas coolers. Liquid tanks are also different to traditional Freon systems, as well as the compressors and piping. Due to the high pressures involved copper cannot contain the pressures reached during transcritical state so stainless-steel tubing need to be used,” notes Matador Refrigeration.

Not all geographic areas however can accommodate CO₂ systems currently. In Northern Africa, for example, the technology is not there yet in terms of using transcritical in high ambient conditions. In time it is expected that this challenge will be resolved and a successful solution in all ambient conditions will be developed. The main concern in this (under current technology circumstances) is the standing pressure (the plant not being in-use for whatever reason). Plant equipment or any vessel currently on the market is just not able to withstand the overall pressure as this may exceed 110 Bar. Equipment may well be able to be manufactured of course, but the costs would be excessive.

“We have chosen to offer CO₂ solutions to our clients because in various trials we have discovered that this refrigerant could produce equal results in large scale industrial plants where ammonia has been a better solution in the past. Also, when considering the other natural refrigerant – R290, volumes required in commercial and industrial applications cannot be met, or don’t make sense,” Matador refrigeration adds.

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The latest in equipment and components

Each category in the refrigeration sector is continually developing to enhance efficiency through not only improved design but new techniques, materials, and electronic controls.

“As the sector evolved a lot with Freon plants over the year, so too will the next stage be in terms of natural refrigerants. Each small component in a system makes a difference in the overall goal of energy savings. What we have been trying with our CO₂ plants is de-superheating in the system which is quite new method and cools the gas in a quicker way. Other equipment performance worth mentioning is the improvements in variable frequency drives and fan motor technology,” says Matador Refrigeration.

Facility insulation, door and loading area seals, internal facility heat loads, and general monitoring and control methodology are all additional factors that continue to gain improvement in facility lifespan and plant performance.

“Variable speed drives, which are really a complex system of control hardware, power supply and software, are designed to adjust the power supplied to a motor from the main power input to modulate operating speed. They can manage the performance of compressors, pumps, or fans. This means that these components can operate at variable capacity, bringing significant energy savings to refrigeration systems. Compressors are the most energy consuming components of refrigeration circuits, thus the use of inverters to increase their performance is being rapidly extended.

In practice, inverters provide the best way to avoid inefficient on/off cycles that reduce the compressor’s seasonal efficiency. This means that, at part load or in low load conditions, an inverter-driven compressor adapts cooling capacity to system requirements without stopping it completely. Currently, the most efficient technology for inverter-driven compressors is called BLDC (BrushLess Direct Current) or simply DC. Permanent magnet brushless motors differ from more traditional asynchronous motor technology in that the compressor rotor consists of a permanent magnet instead of an electric coil. This allows higher motor efficiency (no energy is consumed to magnetise the rotor, as in the case of asynchronous motors) and a wider range of speeds, from 600 to 8000 rpm, while asynchronous motors are limited to 1500-6000 rpm. These features of DC compressors, together with the use of inverters, highlight their efficiency at part loads, giving higher seasonal efficiency and performance in terms of cooling or heating capacity control, with precise load management and constant control of the compressor envelope. It should be noted that inverter technology cannot be used without adopting electronic control systems that instantly calculate the optimum compressor speed, and electronic expansion valves, the only expansion technology that can adapt to the variations generated by the compressor,” notes Carel Industries.

As the planet heats up too, according to International Energy Agency (IEA), it’s estimated that globally 3.5 times more cooling will be required by 2050 than today. At present, refrigeration and air conditioning systems already consume around 15% of global electricity production.

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“In addition to the transition to natural refrigerants, another hot topic these days is the use of heat pumps in refrigeration applications. GEA recognised the application potential and benefits of heat pumps in production early on and became a pioneer in the field of modern heat pump technology. This provides customers with significant, measurable operational benefits as well as a future-proof solution that helps them to achieve their sustainability-related goals”, GEA Refrigeration Technology noted in a product update.

Power supply constraints in some parts of the world are putting further emphasis on energy efficiency in the expanding infrastructure of the refrigeration sector and having to use that same power supply.

A refrigeration and heat pump plant installed at Wipasz - Poland’s leading poultry producer. Image credit: GEA Refrigeration Technology
A refrigeration and heat pump plant installed at Wipasz - Poland’s leading poultry producer. Image credit: GEA Refrigeration Technology
A typical transcritical CO₂ plant. Image credit: Matador Refrigeration
A typical transcritical CO₂ plant. Image credit: Matador Refrigeration

“In a study conducted recently in South Africa we aimed to test the energy-saving potential that could be realised by using variable speed technology, specifically focusing on scroll compressors. By quantifying the saving, one can then easily calculate the savings over the full lifecycle cost considering the initial capital outlay and we further aimed to prove that lifecycle cost outweighs initial capital outlay when using energy-saving technology. Based on the results of this study, it is clear that when using even basic technology, a saving could be realised by merely changing one element of the cycle. Further savings could hence be reached by potentially changing, for example, thermostatic expansion to electronically controlled expansion, implementing smart defrost cycles, including defrost on demand and pulsing of fans during the off cycles.

“Putting the electricity saving aside, further savings and optimisation features to be gained by using this technology include increased food safety due to accurate suction control, which directly leads to an upsurge in food shelf life, reduced sound levels due to compressor technology and EC fans, a smaller footprint – one unit to run several cold rooms – as it can adjust to load, lower maintenance cost due to fewer components, reduced complexity and capital cost on installation. It can safely be said that a minimum of 30% in electricity savings could be realised in equally sized condensing units, when changing over from fixed speed scroll technology to inverter scroll technology. When looking at the expanded criteria, it is completely plausible to assume that the saving will most likely lead to even greater saving in electricity when sizing the units correctly, and combining the inverter scroll technology with the appropriate energy-saving evaporator controls,” says Danfoss South Africa.

Sources:

  1. ASHRAE
  2. Carel Industries
  3. Danfoss South Africa
  4. GEA Refrigeration Technology
  5. Matador Refrigeration
  6. Metraclark
  7. Permacold Engineering

Basic refrigeration system types

  • Single-stage compression system: This configuration comprises the components of a traditional refrigeration system, as well as a pump and a liquid separator. The high-pressure liquid refrigerant flows from the condenser to the expansion valve, which regulates the pressure and delivers liquid refrigerant to the liquid separator. From there, the refrigerant in the liquid state, is pumped to the evaporator and then back to the separator This ensures that the compressor does not receive any liquid. The refrigerant in the form of vapour at low pressure rises and is drawn back into the compressor before repeating the entire cycle again.
  • Two-stage compression system: This is the next evolution of the industrial refrigeration system, suitable for low temperature refrigeration applications, providing high efficiency and low compressor discharge temperatures. In this type of system, there are two compression stages, as the name implies. There is also a tank, called an intermediate cooler, between the receiver and the expansion valve. There is a coil inside the tank, where the main refrigerant flow passes through before entering the main expansion valve the refrigerant continues its flow via the separator, the evaporator and back to the separator. Another refrigerant flow comes out of the main line and is sprayed into the tank via an expansion valve to produce a cooling effect: as it is sprayed and evaporates in the tank, it cools the submerged coil. This sub-cools the main refrigerant flow inside the coil before this reaches the main expansion valve. The vapour refrigerant drawn out of the separator flows to the low-stage compressor to increase its pressure. From there, it flows into the intermediate cooler, which helps condense the refrigerant. The vapour refrigerant is drawn out of the intermediate cooler and flows to the high-stage compressor, before flowing into the condenser and repeating the entire cycle.
  • Cascade systems: In this configuration there are two set of compressors, one in a high temperature circuit and another in a low temperature circuit. A heat exchanger between the two circuits, called the cascade condenser, acts as an evaporator for the high temperature circuit and a condenser for the low temperature circuit. The two refrigerants can be the same or different for each circuit One common practice is to use R717 for the high temperature side and R744 for the low temperature side. This means that less ammonia is used, and the system is more efficient compared to a two-stage ammonia-only system.
    These systems primarily utilise large reciprocating compressors – although screws are used as well – for the CO₂. These compressors, however, are only capable of subcritical CO₂ compression. As a result, the compressors may look a bit odd to someone familiar with ammonia systems. The compressors themselves are relatively small, and the motors are relatively large – the exact opposite of an ammonia system.
  • Transcritical CO₂ System: Transcritical CO₂ systems are continuing to grow in capability and size. They offer a particularly great solution for cold-storage freezers. Transcritical CO₂ systems utilise racks of small compressors; however, as compressor technology allows them to become larger, the racks will in turn get smaller. Additionally, these systems commonly use hybrid heat-rejection systems called adiabatic gas coolers that utilise less water, send less water to the sewer and often require no chemical treatment of the water. Along with such systems, it is reasonable to expect to see improvements on the controls side.
  • Low Charge Ammonia System: Optimisation trends of current technologies for low charge ammonia refrigeration systems are moving toward reduced ammonia charges for every application. The key is the application of correctly engineered and sized equipment.Overall, stick-built low charge ammonia refrigeration systems will operate like a typical ammonia system in use today with key exceptions: All components will be smaller, and a large high-pressure receiver typically will not be used. More precise engineering will mean the ammonia charge is no more than necessary to operate the system. Expect these systems to continually be optimised, utilising less refrigerant while becoming more efficient and getting smaller at the same time.Packaged low charge ammonia refrigeration systems offer the benefit of faster installation and are excellent for cold storage in high ambient temperatures and coolers. They also eliminate the need for an engine room; instead, they use localised systems.As the name indicates, the charge for these systems typically is lower, and the systems are even smaller than stick-built ammonia systems. Some employ special feed and control techniques to keep the charge to an absolute minimum. In practice, these systems will often appear as a sort of miniaturised engine room located on a facility roof or on a concrete pad outside.A properly designed low-charge optimised system uses less than 2-7 kg of ammonia (from 0 06 kg/kW to 1 3 kg/kW⁹), and therefore fewer vessels, fewer pipes, smaller pipe diameters and no pumps. Nevertheless, it still needs an equipment room.A packaged ammonia system eliminates the huge quantities of ammonia inventory and piping by moving to smaller self-contained systems that are usually placed on the roof/ground outside, avoiding any dangers due to leaks These self-contained systems have an ammonia charge of about 0 6 kg/kW and usually combine the compressor, evaporator valve system and control systems into one easily installed and movable packaged system.

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