Dr. Michael Riese, PH.D. Cold Logic P/L

Newton’s first law in part states that an object at rest will remain at rest unless acted upon by an external force. And this is where we are at the moment. Our industry, specifically in the mixed-use refrigerated warehouse sector is at rest. Or differently expressed, at a standstill when it comes to ongoing improvements, new ideas, and implementation of new technology that has the potential to make systems and plants more energy-efficient and creates ongoing savings for our clients.

The current approach of liquid overfeed systems has been well established over the last 30 plus years and is well understood concerning design requirements and methodologies. However, with very few exemptions, design changes are relegated to the periphery and the fundamental underlying principles are not challenged. Essentially, the prevailing dogma appears to be the question of why change a good thing that our industry has going right now?

And the answer will be, that change will be coming. It will either be on our terms or external terms. Moving back to the analogy from the beginning, it is clear that it will be much easier for external forces, such as financial, political, or environmental change to pressure us into a new direction in which we have little control and little input in the means of implementing these changes.

This leaves us, as designers, consultants, contractors, maintainers, and operators with the challenge to create change from within; to create an internal force that overcomes the resistance to change. While the possibilities are endless, the argument can be made that using the concept of energy benchmarking to foster change and advocate for new first cost funding models that consider ongoing savings during the lifetime of the plant is the low-hanging fruit right in front of our eyes.

In the November 2020 and February 2021 editions of Condenser, Stefan Jensen of Scantec Refrigeration in Australia introduced the argument for the use of specific energy consumption (SEC) benchmarking to compare the performance of mixed-use refrigerated warehouse facilities. His writings center around the comparison of best practice values for specific energy consumptions and currently published and established SEC values from different industrial installations and reference literature. His arguments detail the benefits of reducing the SEC and flow on effects of operational expenses for a given facility. He shows a reduction in energy consumption up to 74 percent in a like for like comparison between two nearly identical sized and used facilities but with different refrigeration plant design principles.

This improvement in energy efficiency is clearly an indication of the low hanging fruit that can be realized by our industry. And while the use of best practice SEC values is a lofty aim to strive for (Mr. Jensen offers some suggestions on how this can be achieved), the reality of establishing benchmarks based on current industry practice and then advocating and challenging contractors and designers to improve on those values can be considered a relevant approach.

To examine the reality of creating a moving benchmark and considering the impact of individual improved SEC values on the installed plant SEC average, the energy performance data presented in Figure 2 of the Feb 2021 edition of Condenser is reproduced in Figure 1 with permission from the author and will be further examined as part of this article.

The data provided by Mr. Jensen has been re-examined for underlying patterns and as shown in Figure 1 have been grouped into 10 different bins using logarithmic sorting and separation. Using this approach and in direct comparison with Figure 2 of Mr. Jensen’s article in February 2021, several different results are immediately visible. Firstly, when using a logarithmic X-axis display it is evident that there is a significant void of data in the facility size between 10,500 m3 and 28,000 m3 , which hence has led to a first bin size that is different from the remaining 9 bins. Those remaining 9 bins are all split into equal sizes when considering the exponential range of the overall available data. Hence secondly, when averaging the SEC for each bin (2 through 10), it can be seen that the average SEC for bins 2 and 3 are significantly influenced by the data provided from low-charge, centralized DX plants and specifically in bin 3 leading to a very low standard deviation of the data.

Lastly, when considering bins 4 through 10 alone, it appears that there is a linearized relationship between the individual bin average SEC values, but sometimes with a significant spread of the data’s standard deviation and also significantly higher values when compared to Mr. Jensen’s proposed best practice SEC values. Figure 1 Reproduction and averaging of SEC data provided by S Jensen in Condenser Feb 2021. Bin 2 through 10 are split into equal logarithmic intervals. SEC averages for each bin are plotted in the center of each bi and are not indicative of the average facility size in the respective bin.

Assuming that there is agreement amongst the readership that the chosen method of data segregation is valid, it is now worth examining the potential impact of achieving and accounting for better than average facility SEC values and their impact on changing the benchmark values for each bin. For this purpose and as shown in Figure 2, the worst SEC in each bin has been replaced by a single data point in each bin that is more energy-efficient than the respective average SEC. Mr. Jensen has already demonstrated that SEC improvements over 50% can easily be achieved and hence a moderately low improvement target of 20% is used here. For the data bins 2 and 3 very little change in the average SEC is evident as a result of the data set modification because the plants captured in those data bins are already exhibiting a very low SEC when compared with the other data provided. The largest changes are evident in bins that show a large range distribution and hence standard deviation of the source data.

Figure 2 shows a demonstration of the influence of the removal of the single worst data point from each bin and replacing it with a new SEC value 20% lower than the original bin average.

Figure 3 shows the before and after averages for each data bin. Visual guidelines have been added for easier comprehension and they are solely used as visual aids, not as fitted trendlines. Overall though it is evident that a small change in data can have a significant impact on the overall average SEC for each bin. And while it appears that bin 10 is relatively unaffected by the improvement of SEC, it shows that larger improvements are possible in bins 1 through 9.

Focusing on the bottom guideline that essentially connects the improved average SEC for bins 2 and 3 with bin 10, it shows that it should be possible to achieve a performance average and SEC that is significantly better than what is currently the case. While it is not even close to the proposed best practice SEC, it is a more readily achievable target for our industry. Utilizing a moving average approach, this benchmarking system provides the opportunity for designers and contractors to gradually improve the plant designs, control systems, and energy efficiency. Figure 3 Application of indicative linear trendlines to the old and new bin averages for clarity. Bin averages in Bins 2 & 3 are not taken into account for the red and green dotted lines.

If the data described above is now considered holistically, it has been clearly shown that based on existing, published data, SEC trends and benchmarks can be established. While there are good arguments that ultimately a best practice benchmark is the final goal to aim for, it may not be the best approach at this time for the refrigerated warehousing industry as a whole. Mr. Jensen has shown that significant improvements to the SEC can be made by moving away from traditional liquid overfeed systems and further development of centralized, low-charge DX plants will lead to significant improvements in energy consumption and operational expenditure for the owner. However, this requires research, education, and significant changes to established thinking in the ammonia refrigeration industry.

It is expected that there will be significant resistance to such approaches and hence at this point, the course of creating moving average benchmarks will lead to small step improvements and a gradual drive in the industry to become more energy-efficient. The published data show that plants that are more efficient than the respective bin average SEC benchmark have been constructed using established design approaches and only require specific changes in operation and design to achieve above-par performance.

In conclusion, it must be recognized that change to our industry will be coming. It is up to us to decide how to approach this change and how active a role we want to play. Only if we create change from within us, strive to overcome the resistance that keeps us at rest and maintains the status quo, are we able to freely decide our way forward. If we do nothing, change will be imposed on us by other interest groups and entities that make decisions based on their interests and are potentially directly contradicting what best practice can look like. Mr. Jensen in his articles has clearly shown where our industry can potentially go when considering the SEC for mixed-use refrigerated warehouses.

These lofty ambitions have to be recognized for what they are: Potentially long-term ambitions that require large-scale change on all levels, from designers and contractors through to operators and maintainers. However, large-scale change also brings with it increased risk and as such will be an unattractive aspiration for many. Alternatively, and as demonstrated in this article, small step change that requires small amounts of efforts with limited risk increase are a more suitable way to harvest the low hanging fruit that is SEC benchmarking. Gradual change and adaptive SEC benchmarks, over time, will foster best practice, and a better understanding of performance by interest groups inside and outside of our industry. It will foster the promotion of competition and reward the best performers. This change will allow our industry at all levels to create a drive that will propel us from the resting position that we have occupied for so long and allow us to determine our future.