Novel approach for improved control processes
on ageing mine refrigeration systems
PFH Peach
orcid.org / 0000-0001-9445-086X
Thesis accepted in fulfilment of the requirements for the degree
Doctor of Philosophy in Mechanical Engineering at the
North-West University
Promoter: Dr JF van Rensburg
Graduation:
May 2020
Title: Novel approach for improved control processes on ageing mine refrigeration systems
Author: PFH Peach
Supervisor: Dr JF van Rensburg
Degree: Philosophiae Doctor in Mechanical Engineering
Keywords: ageing equipment, control optimisation, cooling, dynamic control, energy savings, gold mines, improved control processes, improved service delivery, ineffective control, refrigeration, temperature reset
Worker safety remains one of the top priorities in South African deep-level gold mines. Due to their operating depths, these mines are heavily dependent on artificial cooling. Large refrigeration systems are commonplace in the South African mining environment, ensuring cold water and air are supplied to underground working areas. The advanced age of the industry in South Africa unfortunately leads to one of its biggest challenges: old equipment using outdated control methods and processes.
There is also a second challenge brought about by the industry’s mature age: that of depleted ore grades as areas are mined out over time. Gold mines in South Africa have seen increased underground expansion in search of new gold-bearing ore bodies to keep operations profitable. The nature of narrow reef mining also means that these expanding and complex underground areas are frequently changing, resulting in an extremely dynamic environment.
Infrastructure past its prime, together with increased distances to active mining areas, presents the gold industry in South Africa with a unique set of cooling challenges. Decades-old refrigeration equipment is operated on original design parameters while mines are deeper and more complex than ever. It is therefore vital that deep-level mine refrigeration systems operate dynamically to account for changes within the environment they service. Current control optimisation methods found in literature mainly focus on operation for cost benefit, and rarely employ strategies for improved service delivery.
Economic stresses on the industry, however, requires a novel approach to improve service delivery of deep-level mine refrigeration systems while still prioritising operational cost. Analysis of available literature focusing on refrigeration control highlighted a clear need to develop an optimisation technique to integrate cost savings and service delivery
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A method was developed to implement improved control processes on ageing refrigeration systems in gold mines for improved performance. The knowledge from the commercial and the mining refrigeration control sector was critically analysed and integrated into a novel solution. This solution took proven methods (e.g. temperature resets) from the commercial field and assimilated them into a mining refrigerating environment. Detailed investigations focusing on the control characterisation of a refrigeration system proved that evaporator temperature resets can be employed to improve cooling performance while simultaneously reducing energy consumption.
The improved control processes were implemented on a case study, referred to as Mine A, at a South African deep-level gold mine. The refrigeration system at Mine A was determined to be over 25 years old and operating ineffectively, as original design parameters were used in the control strategy. An evaporator temperature reset and updated flow control were implemented as part of the improved control processes. After implementation of the improved control, chilled water temperatures sent underground was reduced by 16% during summer months. This was achieved even with the total refrigeration system using 18% less energy.
The improved control meant service water temperatures were reduced by more than 1°C and bulk air cooler air temperatures were reduced by over 2°C during summer months. The control processes also improved the system’s temperature control accuracy by 12% by using 20% more dynamic flow capacity. The dynamic nature of improved control was thus validated, as the refrigeration system could better adapt to external changes in demand and the environment.
All these improvements were present even as Mine A increased production output by 5%. Combined, the improved control processes implemented on Mine A also resulted in daily energy savings of 38 MWh. This energy saving equates to an annual reduction in electricity cost of R 9 million. The results on Mine A validated the effectiveness of improved control processes on ageing mine refrigeration systems.
The main objective of the research presented in this thesis was thus achieved: that is, improving ageing mine refrigeration performance at reduced operating cost.
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Optimising deep-level mine refrigeration control for sustainable cost savings P.F.H. Peach, M. Kleingeld, and J.I.G Bredenkamp
International Conference on the Industrial and Commercial use of Energy, Cape Town, 2017
Common problems faced when considering the implementation of load management initiatives on gold mine refrigeration systems are the implementation costs and time. Expensive infrastructure upgrades are often also the norm in the industry when considering the implementation of load management. In addition to these challenges, the old Eskom Demand Side Management (DSM) model often resulted in rapid deterioration of cost savings after the Energy Services Company’s (ESCo’s) contractual obligation expired. A new DSM model was introduced to rectify this. This study addresses the challenges of implementation constraints and the sustainability of cost savings for load management initiatives on refrigeration systems under the new DSM model. Through optimised control strategy modelling, load management initiatives were implemented on two South African gold mines with limited capital expenditure. This allowed for a quick, low-cost implementation that resulted in sustainable cost savings. The average evening load shift on the two respective mines during summer months resulted in 7.28 MW and 2.00 MW over a period of eight months. This accumulates to an annual cost saving of R 1.76 million and R 215 000 for the two mines, calculated with the 2016/2017 Eskom tariffs. The simulation models predicted the results with an average error of less than 2%. The sustainability of cost savings through optimised control on deep-level gold mine refrigeration systems thus becomes evident.
Control integration of deep-level mine water reticulation sub-systems P.F.H. Peach, J.F. van Rensburg, and J.I.G Bredenkamp
International Conference on the Industrial and Commercial use of Energy, Cape Town, 2018
One common challenge that limits maximised cost saving benefits on mine water reticulation systems is the lack of integrated control strategies between sub-systems. This challenge becomes even more pronounced when energy recovery systems, such as three-chamber pipe feeder systems (3CPFs), are incorporated within the water reticulation system. Investigations showed that the 3CPFs are directly responsible for negatively influencing the performance of conventional dewatering and refrigeration load management initiatives due to a lack of integrated control. This study addresses these challenges through the development of an
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identified as a case study. The water reticulation system of this mine included a 3CPFs which influenced the performance of load management initiatives on both the dewatering and refrigeration systems. By means of control strategy modelling, a solution was developed to optimally use the 3CPFs and improve the performance of the load management initiatives. The control strategy was tested and validated through the implementation of a real-time energy management system. The average evening load shift achieved was 6.3 MW, which can be quantified as a performance improvement of over 400%.
Dynamic optimisation of deep-level mine refrigeration control P.F.H. Peach, J.I.G Bredenkamp, and J.F. van Rensburg
South African Journal of Industrial Engineering, November 2018 Vol 29(3) Special Edition
Artificial cooling remains vital to the deep-level mining industry. This is mainly to ensure a safe underground working environment. Unfortunately, the refrigeration systems of South African gold mines are outdated. Ageing infrastructure and increasing distances to active mining areas present this industry with a unique set of cooling challenges. Mines are deeper and more complex than ever, yet their refrigeration systems operate according to original design specifications. The importance of mine refrigeration systems adapting to a dynamic environment becomes evident. One solution to this challenge is to re-evaluate the control of refrigeration systems based on dynamic cooling needs. Control optimisation strategies were implemented on a case study to improve the overall performance of a refrigeration system. The implementation of these strategies resulted in multiple benefits for the mine. The operational performance of the refrigeration system was optimised, resulting in both service delivery improvements and energy efficiency. During summer months the chill dam temperature decreased by 1°C at a lower energy consumption of 38 MWh per day. This accumulates to a financial cost saving of R 9 million per annum. This study proves that operational performance increases are possible through dynamic control optimisation of deep-level mine refrigeration systems.
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Firstly, I want to thank the Lord for blessing me with the talents to pursue my dreams. Without Him this venture would not have been possible.
To my parents, Carel and Marietjie, thank you for all the support, love and words of encouragement throughout the years. Without you I would not be the man I am today.
To my sister, Retha, who never hesitated to help or encourage me when I needed motivation. Thank you for all the love, support, and phone calls to distract me from all the hard work.
To my supervisor, Dr Johann van Rensburg, and mentor, Dr Jean van Laar, for their guidance and assistance with writing up this thesis.
To my friend and mentor, Dr Johan Bredenkamp. Your inputs and guidance during the writing of articles were of great value to me.
To ETA Operations (Pty) Ltd and Enermanage (Pty) Ltd for the financial support to complete this research. Thank you to Prof. Eddie Mathews and Prof. Marius Kleingeld for making it possible for me to complete this study.
Finally, I would like to thank my colleagues and friends who supported me during the course of this research. I am truly blessed to have such wonderful people in my life.
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Abstract ... i
Publications from research ... iii
Acknowledgements ... v
Table of Contents ... vi
List of Figures ... viii
List of Tables ... x
List of Equations ... xi
List of Abbreviations ... xii
List of Units ... xiv
1 Introduction & Background ... 2
1.1 Refrigeration concept in the deep-level mine environment ... 2
1.2 Refrigeration and control optimisation – A critical analysis ... 8
1.3 Refrigeration and control optimisation – Research comparison ... 35
1.4 Problem statement ... 37
1.5 Research objectives ... 38
1.6 Novel contributions ... 38
1.7 Document overview ... 42
2 Improving refrigeration performance through novel control optimisation ... 44
2.1 Background on mine refrigeration control ... 44
2.2 Introduction to methodology ... 50
2.3 Identification ... 51
2.4 Detailed investigation ... 53
2.5 Control characterisation ... 56
2.6 Solution development ... 62
2.7 Solution verification and validation ... 66
2.8 Conclusion ... 69
3 Improved refrigeration performance through novel control optimisation ... 71
3.1 Introduction ... 71 3.2 Identification ... 71 3.3 Detailed investigation ... 73 3.4 Control characterisation ... 75 3.5 Development of solution ... 81 3.6 Validation ... 84 3.7 Conclusion ... 96
4 Conclusion and recommendations ... 99
4.1 Summary ... 99
4.2 Novel contributions ... 101
4.3 Recommendations for future studies ... 102
Reference List ... 104
Appendix A: SAJIE Article ... 115
Appendix B: Detailed investigation – Mine A ... 130
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Figure 1: Cooling systems required at lower depths [4], [9], [22] ... 3
Figure 2: SA gold mining depth vs. production [1], [25], [27]–[33] ... 5
Figure 3: Centrifugal chiller commissioned in 2013/2014 ... 6
Figure 4: Centrifugal chiller commissioned in 1991/1992 ... 6
Figure 5: Refrigeration concept on deep-level mines [34] ... 45
Figure 6: Methodology overview ... 50
Figure 7: Simplified mine refrigeration layout – Example ... 53
Figure 8: Control characterisation process flow chart ... 57
Figure 9: Control characterisation – Example 1 ... 60
Figure 10: Control characterisation – Example 2 ... 61
Figure 11: Refrigeration system basic layout – Mine A ... 72
Figure 12: Detailed refrigeration circuit – Mine A ... 74
Figure 13: Evaporator exit water temperature – Mine A ... 78
Figure 14: Normal distribution of average hourly exit temperature – Mine A ... 79
Figure 15: Evaporator water flow – Mine A ... 80
Figure 16: Normal distribution of average hourly flow – Mine A ... 81
Figure 17: Historical monthly average ambient temperatures – Mine A ... 82
Figure 18: Average ambient temperature comparison – Mine A ... 84
Figure 19: Average seasonal BAC air outlet temperature comparison – Mine A... 88
Figure 20: Average summer refrigeration power consumption – Mine A... 89
Figure 21: Average winter refrigeration power consumption – Mine A ... 90
Figure 22: Summer evaporator flow comparison – Mine A ... 92
Figure 23: Normal distribution of average hourly flow – Summer ... 92
Figure 24: Winter evaporator flow comparison – Mine A ... 93
Figure 25: Normal distribution of average hourly flow – Winter ... 93
Figure 26: Normal distribution of average hourly exit temperature – Summer ... 94
Figure 27: Normal distribution of average hourly exit temperature – Winter ... 95
Figure 28: Pre-cooling sub-system layout – Mine A ... 132
Figure 29: Chiller evaporator side sub-system – Mine A ... 132
Figure 30: Chiller condenser side sub-system – Mine A ... 133
Figure 31: Surface storage and BAC sub-system – Mine A ... 134
Figure 32: Dam capacities – Mine A ... 135
Figure 33: Chiller 1-3 compressor data – Mine A ... 136
Figure 34: Chiller 1-3 heat exchanger data – Mine A ... 137
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Figure 37: Detailed schematic layout of refrigeration system – Mine A ... 139
Figure 38: EnMS evaporator interface layout – Mine A ... 141
Figure 39: EnMS pre-cooling/recycling interface layout – Mine A ... 142
Figure 40: Variable flow EnMS coding implementation– Mine A ... 144
Figure 41: Evaporator outlet water temperature – Summer ... 145
Figure 42: Chill dam temperature – Summer ... 146
Figure 43: BAC outlet air temperature – Summer ... 147
Figure 44: BAC water flow - Summer ... 147
Figure 45: Evaporator water flow – Summer ... 148
Figure 46: Refrigeration power consumption – Summer ... 149
Figure 47: Average chiller status - Summer ... 149
Figure 48: Chiller inlet/PCD-2 water temperature - Summer ... 150
Figure 49: Evaporator outlet water temperature – Winter ... 151
Figure 50: Chill dam temperature – Winter ... 152
Figure 51: BAC outlet air temperature – Winter ... 153
Figure 52: BAC water flow – Winter ... 154
Figure 53: Evaporator water flow - Winter ... 154
Figure 54: Refrigeration power consumption – Winter ... 155
Figure 55: Average chiller status – Winter ... 155
x
Table 1: Previous research evaluation matrix – Mining sector ... 21
Table 2: Previous research evaluation matrix – Commercial sector ... 33
Table 3: Mining vs. commercial sector research – Comparison ... 35
Table 4: Common chilled water consumers on deep-level mines [34] ... 46
Table 5: Identification – basic information required ... 52
Table 6: Investigation – detailed information required ... 54
Table 7: Validation KPIs ... 67
Table 8: Identification – Mine A ... 71
Table 9: Control elements – Mine A ... 75
Table 10: Proposed evaporator temperature reset schedule ... 84
Table 11: Baseline vs. optimised period ambient conditions – Mine A ... 85
Table 12: Performance improvement validation – Mine A ... 86
Table 13: Reduced operating cost validation – Mine A ... 89
Table 14: Dynamic flow control validation – Mine A ... 91
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Equation 1: Vapour-compression cycle energy balance ... 47
Equation 2: Specific heat equation for water ... 48
Equation 3: Modified specific heat equation for water... 49
Equation 4: Specific heat equation example– Low volume flow and exit temperature ... 64
Equation 5: Specific heat equation example– High volume flow and exit temperature... 64
Equation 6: Current chiller cooling ... 83
xii
3CPFs - Three-chamber pipe feeder system
ACP - Air Cooling Power
AHU - Air Handling Unit
BAC - Bulk Air Cooler
COP - Coefficient of Performance
ChWLT - Chilled Water Leaving Temperature
ChWST - Chilled Water Supply Temperature
CWLT - Condenser Water Leaving Temperature
CWST - Condenser Water Supply Temperature
DSM - Demand Side Management
EMUT - Equal Margin Utility Theory
EnMS - Energy Management System
Eskom - Electricity Supply Commission of South Africa
HVAC - Heating, Ventilation and Air Conditioning
IDM - Integrated Demand Management
IGV - Inlet Guide Vanes
KPI - Key Performance Indicator
NPSHA - Available Net Positive Suction Head
NPSHR - Required Net Positive Suction Head
PID - Proportional-integral-derivative
xiii
SCADA - Supervisory Control and Data Acquisition
TMY - Typical Meteorological Year
VRT - Virgin Rock Temperature
xiv
LIST OF UNITS
Symbol - Unit - Description
°C - Degrees centigrade - Temperature
K - Kelvin - Temperature
kg/s - Kilogram per second - Flow rate
kJ/kg - Kilojoule per kilogram - Specific energy
kJ/kg·K - Kilojoule per kilogram Kelvin - Specific heat capacity
kW - Kilowatt - Power
kWh - Kilowatt-hour - Energy
m - metre - Distance
1
Chapter 1 – Introduction & Background
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This chapter introduces important background information to aid in formulating the outcomes of this thesis. Various challenges affecting the deep-level mining and refrigeration environments are discussed. A comprehensive literature survey is summarised within a critical analysis of available research. The problem statement and research objectives for the study are formulated from the background information and literature provided. Novel contributions to the field are presented before the chapter is concluded with a brief overview of the entire thesis.
2
1 Introduction & Background
This chapter explores the importance of artificial cooling in the South African deep-level mining environment. The challenges faced by deep-level mining and refrigeration systems in particular, will be discussed. A critical analysis of cooling optimisation techniques for improved service delivery is conducted to identify and highlight shortcomings in this field. The problem statement, research objectives, as well as novel contributions for the study are formulated from the background information and literature provided.
1.1 Refrigeration concept in the deep-level mine environment
With South African gold mines reaching beyond 4 km below the surface with virgin rock temperatures (VRT) exceeding 60°C, the underground environment poses one of the industry’s biggest risks [1]–[4]. It is these harsh environmental conditions that workers are exposed to on a daily basis which highlight the importance of artificial cooling within the industry.
1.1.1 Importance of artificial cooling in deep-level mines
Gold mining has been an important part of the South African economy since gold was discovered on the Witwatersrand basin during the 1880s [5], [6]. Continuous mining taking place for over a century has forced mining companies to go ever deeper in search of gold-bearing ore bodies [7]. Underground mining in South Africa started surpassing depths where conventional air ventilation systems became insufficient for cooling during the 1960s [8], [9].
The Mine Health and Safety Act of 1996 limits the exposure to 27.5°C wet-bulb at the station and 32.5°C wet-bulb in the stopes [10]. Large refrigeration and Bulk Air Cooler (BAC) installations have since become commonplace to manage underground environmental temperatures within these legal limits. These machines not only mitigate the effects of auto compression and the environment, but also those of increased mechanisation [8].
The primary objective of these large cooling and ventilation systems remains that of maintaining worker safety [11]. The effects of high wet-bulb temperatures on human productivity has also been well documented in research [12]–[16]. There is thus a secondary benefit to regulating the underground environmental temperatures, i.e. improving worker productivity [17], [18]. As better temperature regulation equates to improved production, it can also improve profit margins for deep-level gold mines. Additional research is, however, required to confirm to which extent improvements in cooling service delivery will impact production [17].
3
Mining deeper has become inevitable as ore bodies located closer to surface have long been depleted [7]. Cooling systems used to maintain safe and productive conditions underground become larger and more complex as mines start encountering higher VRTs at lower depths [9]. These cooling systems are designed to function under specific conditions such as ambient temperatures, production targets, depth and VRT, to name a few [4], [9].
Any changes to the environment to be cooled or method used to mine will therefore require appropriate changes to the cooling system in question. Typical cooling infrastructure required at varying depth is depicted in Figure 1. Studying Figure 1 in more detail, it can be seen that cooling starts off with conventional air ventilation systems, moving on to water and ice as higher specific heat capacity is required to remove more heat from the environment [19]–[21].
Figure 1: Cooling systems required at lower depths [4], [9], [22]
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 Depth [m] V e n til at io n O n ly C o n ve n tio n al Su rfa ce B A C U ltr ac o ld Su rfa ce B A C D e d ic at e d R e fr ig e ra tio n P la n ts Su rfa ce P la n ts + U n d e rg ro u n d A ir C o o lin g U n d e rg ro u n d R e fr ig e ra tio n P la n ts Average VRT 20°C 40°C 60°C Su rfa ce Ic e P la n ts
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The operation of these large and complex refrigeration systems is, however, not without fault. Their operation, and subsequently the safety of underground workers, is subject to many factors, some outside of the mine’s control. The main contributing factors to the ineffective operation of refrigeration systems will be elaborated on in the following section.
1.1.2 Industry challenges impacting on refrigeration performance
There are multiple challenges that threaten the effective operation of these large cooling systems discussed in the previous section. Although these challenges seem to worsen over time, throughout the past few decades it has expressed the same concern: The South African gold sector needs to explore new avenues to combat the age and complexity of both their mining operations and the infrastructure used to support it.
“Rising costs, falling ore grades and a stagnant gold price are steadily eroding the economic viability of gold mining in South Africa” – 1994 [23]
“Dealing with orebodies past their prime, aging infrastructure and labour issues, new gold projects in South Africa that will mine even deeper reserves will have to mine differently for the industry to remain profitable” – 2010 [7]
“Its mines are old, deep, with falling grades and productivity, and rising costs.” - 20182
These extracts taken from the past few decades show that if gold mining in South Africa is to survive, the industry must adapt. The first step to this adaptation was expansion of the mines in search of more gold-bearing ore to improve production outputs [17], [24], [25]. This expansion came at a cost, as mines had to go deeper beneath the earth’s surface, and farther away from the shaft areas in search of the precious metal [7], [24]. As expansion progresses and temperatures underground rise, worker productivity starts to decline [17], [18].
This decline in production has been evident ever since the South African gold mining industry hit peak production in 1970 [26], [27]. Projections based on the Hubbert Peak Theory estimate annual production figures below 50 tonnes as soon as 2023 [5], [28]. Figure 2 shows the annual production along with the maximum mining depth of South African gold mines since the 1950s [1], [25], [27]–[33]. The information displayed in Figure 2 illustrates that, despite mining deeper than ever, production continues to decline annually.
2 A. Seccombe, “Minerals Council says 75% of SA’s gold mines unprofitable,” Business day live, 2018. [Online]. Available:
https://www.businesslive.co.za/bd/companies/mining/2018-07-11-minerals-council-says-75-of-sas-gold-mines-unprofitable/. [Accessed: 04-Mar-2019].
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Figure 2: SA gold mining depth vs. production3 [1], [25], [27]–[33]
Figure 2 thus highlights that the installation of new equipment to combat the environmental conditions is not feasible from a life-of-mine perspective [34]. More effective use of current cooling and ventilation infrastructure is consequently required to enable mining at these extreme depths [34]. The age and maintenance procedures of the infrastructure in question, however, present another challenge to the industry entirely as it leads to ineffective operation and reduced efficiency [2], [22].
The existing refrigeration infrastructure used on South African gold mines is, for the most part, outdated [34]. One study showed that the average age of refrigeration systems used in the Free State province of South Africa is over 25 years [22]. Some gold mines have been continuously operating these machines for more than 35 years [22]. A mine’s cooling needs can drastically change during the course of a few decades, while the effectiveness of the machinery used to support its operation deteriorates over this same period. Figure 3 and 4 illustrate the difference between a relatively new refrigeration chiller and one that has been in operation for 27 years.
3 “The top ten deepest mines in the world,” Mining Technology, 2019. [Online]. Available:
https://www.mining-technology.com/features/feature-top-ten-deepest-mines-world-south-africa/. [Accessed: 09-Jul-2019].
0 200 400 600 800 1000 1200 0 500 1000 1500 2000 2500 3000 3500 4000 4500 1950 1970 1975 1985 1990 2005 2015 2023 An n u al G o ld Pro d u ctio n (to n n es ) De p th Bel o w Su rf ace (m ) Year
Production Production (predicted)
6 Figure 3: Centrifugal chiller commissioned in 2013/2014
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This outdated refrigeration infrastructure is still expected to provide sufficient cooling to larger, more complex mines. Using outdated control methods mostly based on original design parameters (static control) is also commonplace within the industry [34]. Static control severely limits the system’s ability to react to changes within the mine or environment [34]. This leads to major operational inefficiencies in deep-level mine refrigeration systems [22].
Upgrading existing equipment or, as previously mentioned, installing new infrastructure to address these operational efficiencies, is not feasible based on many life-of-mine estimates. When taking the economic and financial stresses on the industry into account, this becomes even less feasible [2], [35]. Abundant research on the weak financial performance of South African gold mines is available, mainly identifying the following issues as the main contributors [24], [35]–[38]:
• Declining ore grades, • weak gold price,
• industrial action (strikes),
• reduced worker productivity, and • increased operational costs.
Regardless of the causes, the weak financial performance of gold mines during recent years means one thing for capital expenditure: there simply isn’t money [34], [39]. This means large refrigeration complexes cannot be installed or commissioned as capital is used to keep operations running [39]. Even if the installation of new equipment were possible, the additional concern of operational expenditure remains.
Large refrigeration systems do not only come at a significant upfront capital cost, they are also costly to run due to their operational expenditure. These systems require intensive maintenance to keep them running optimally, something mines are already struggling to keep up with [2]. They also consume copious amounts of electrical energy to operate [40], [41]. The impact of intensive energy usage coupled with the exorbitant hikes in electricity rates is inevitable: South African gold mines cannot afford to run and maintain new machinery, let alone spare the capital required to obtain such items [42], [43].
Existing machinery must therefore be exploited to ensure maximum potential benefit from their operation during these uncertain times. This should also be conducted at minimum cost, as every cent spared improves the forecast and position for the gold mine industry within South Africa. As refrigeration systems are critical to the continued operation of deep-level mines,
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optimising their operation should alleviate some of the pressure the challenges mentioned in this section exert on the industry.
From the information showcased in this section, it becomes clear that solutions to many of the industry’s challenges have yet to be established. Refrigeration systems’ operational performance has been shown to be one of the key areas of concern within these broader challenges. Before this specific problem can be addressed, however, critical analysis of research within the field of refrigeration optimisation is required to ensure a complete understanding of what has been done. This understanding of previous research is vital in developing a novel solution to the challenges presented.
1.2 Refrigeration and control optimisation – A critical analysis
A detailed summary of available research within both the mining and commercial sectors is presented in this section. The commercial HVAC sector will be investigated as it is a mature industry which can lend valuable insights into methods not considered within the mining environment. This information will then be used to compile critical analyses on the available research. The critical analyses of the research will be simplified with the aid of evaluation matrices.
The criteria against which previous research needs to be evaluated can be reduced to four main points. Each of the four criteria will be discussed in more detail with sub-criteria expanded upon as necessary. The evaluation matrices will also be developed based on these criteria. How the main criteria link up with the challenges mentioned in section 1.1 will also be discussed. The four main criteria are:
• Cost savings, • capital expenditure, • control optimisation, and • service delivery.
Abundant research on deep-level mine refrigeration control optimisation for cost savings is available, partly due to the ongoing weak financial performance of gold mines. It is important to evaluate to what extent these energy and cost savings were prioritised. The main question here is: were savings achieved at the expense of service delivery? The sustainability of these initiatives needs to be assessed to see how practical it is to implement long-term initiatives on such systems.
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Capital expenditure will also form part of the focus criteria as numerous initiatives involving refrigeration control optimisation were funded through third parties (Eskom’s Demand Side Management). This method of funding infrastructure upgrades and the installation of new equipment have ceased after changes to Eskom’s Integrated Demand Management (IDM) model in 2015 [44]. It will also be noted whether unplanned maintenance, large scale reconfiguration or installation of new equipment was funded by the mines themselves. This might have been possible in the past, but consecutive years of weak financial performance have forced gold mines to limit capital expenditure.
Control optimisation makes up a crucial part of the critical analysis as it can indicate how effectively a machine is used. How frequently optimisation techniques are implemented will show whether the mine is exploiting their current systems to their maximum potential. It will also need to be confirmed whether optimisation happens at the machine, or at a systems level. The dynamic operation of these systems is also important, as they need to adapt to the changing environment and demand.
Finally, the level of service delivery of these systems as described in literature will be evaluated. As mines are continually becoming deeper, providing adequate cooling becomes more important. How savings initiatives affected the cooling service delivery of mines will be assessed. The techniques used to enable improvements in cooling service delivery will form a key part of the analysis.
1.2.1 Refrigeration control in the deep-level mining sector
Abridged summaries of research available on mining refrigeration control optimisation are given below. The summary of each study was compiled based on the criteria as set out in the introduction of this section. Each of the studies presented hereafter is summarised in Table 1 in the form of an evaluation matrix. The critical analysis of summarised research within the mining sector continues in section 1.2.2 (page 20).
Study M-1 (2003) [45]
This thesis focused on optimising the operation of underground refrigeration plants and ventilation fans for minimum electricity cost. Research showed that some mines oversupplied working areas with cooling, indicating that there is scope to reduce electrical load on refrigeration and ventilation equipment. Simulations and optimisation models were developed to determine the most cost-effective method to achieve a specific minimum required Air Cooling Power (ACP) underground. Using a mine as a thermal energy storage device to shift load on refrigeration equipment was also discussed.
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The simulation model was tested on an actual case study mine. The model was verified by comparing the calibrated simulation with actual underground conditions. Results showed that energy savings were possible in the case study without reducing service delivery. Furthermore, by reducing the cooling supplied to the minimum required levels to maintain productive work, the case study mine could almost double energy savings. The study showed that significant energy and cost savings are achievable through optimising the control of the refrigeration equipment.
Study M-2 (2006) [46]
Research focused on implementing load reduction methods on a mine’s refrigeration system. A simulation model was used to test an optimised load shift control philosophy on a gold mine’s refrigeration system. The simulation assisted the study in proving safety regulations would be adhered to even when load management is implemented. New controllers were installed to execute the developed load management strategies.
A successful evening load shift was achieved after installation of controllers and implementation of optimised control philosophy. In addition to load shifting, the study also noted an overall energy efficiency due to improved control. The study proved that Demand Side Management (DSM) can be implemented on a mine’s refrigeration system without adversely affecting cooling service delivery.
Study M-3 (2007) [47]
The study investigated implementing load-shifting measures on mine refrigeration systems through improved control strategies. A cascade mine surface refrigeration system was automated as part of a DSM initiative to enable improved control strategies. Improved control strategies were developed and verified by aid of simulations. Operational parameters such as cooling supplied, temperature and dam levels were all monitored and determined to stay within acceptable limits.
The developed strategy was implemented on a mine using a Real-time Energy Management System (EnMS) that was integrated into the mine’s Supervisory control and data acquisition (SCADA). The study emphasised the importance of a mine’s thermal storage capacity to enable load shifting of refrigeration systems. Post-implementation assessment of the case study showed the project achieved an over-performance with regards to load shifted from evening peak.
11 Study M-4 (2007) [48]
This research focused on the sustainable implementation of DSM load shifting initiatives on gold mine refrigeration systems. Mathematical modelling was integrated into simulation models to improve the accuracy of results obtained. A unique protocol was also developed to easily determine whether a refrigeration system has potential scope for load shifting. The study highlighted optimised control as the only sustainable method for achieving load shifting on refrigeration systems.
Four refrigeration systems across South Africa were identified and submitted to Eskom DSM for approval. The study focused on two of these projects as Real-time EnMS were implemented on the refrigeration systems of two deep-level gold mines. Results show significant load shifting was achieved without compromising the operation or safety of the mines in question.
Study M-5 (2008) [49]
The study focused on implementing DSM initiatives on underground refrigeration systems after previous studies’ success in implementing such measures on surface machinery. The study showed load shifting initiatives can successfully be implemented on underground refrigerating systems by means of a revised control strategy. Load shifting was also shown to have minimal effect on service delivery, as machine operation and safety parameters were not compromised during the course of this research.
Study M-6 (2011) [50]
A case study is presented for energy savings on a deep-level mine’s water reticulation system. A simulation of the integrated water reticulation system (refrigeration and dewatering) is presented and used to develop a suitable optimisation strategy. The optimisation strategy was implemented on the case study by means of a real-time EnMS. This resulted in each component being optimally controlled without negatively affecting any other component within the integrated system.
Results showed an overall reduction in energy consumption as well as significant cost savings due to time-of-use optimisation of water reticulation equipment. The study does not investigate any effects on the cooling service delivery as a result of the integrated control strategy.
12 Study M-7 (2012) [51]
This study focused on improving mine cooling system performance through the implementation of energy saving measures on cooling auxiliaries. Large capital investment was funded through the Eskom Integrated Demand Management (IDM) model to refurbish auxiliary equipment and install new control equipment. The results showed significant energy savings were realised when improving the control management of auxiliary equipment.
The study improved the cooling performance of refrigeration equipment purely through reducing the energy input into the system. No improvements on cooling service delivery were present as this was not the focus of the study. As with most other studies on mine refrigeration control, the aim was to reduce the supply to match the demand more accurately.
Study M-8 (2013) [52]
This thesis summarises the work of five academic articles focusing on improving mine cooling system electrical performance for energy savings through the implementation of a variable water flow strategy. All research focused on energy savings with mention of mine cooling service delivery never being adversely affected during the implementation of research. Improvements in cooling service delivery were not part of the scope. The work compiled for each article is summarised below:
Article M-8.1 (2013) [53]
The first article focused on estimating the potential saving of implementing variable speed drives (VSDs) on mine cooling systems on a large scale. Areas where VSDs would result in the most energy savings were also investigated. Findings showed that VSDs can lead to significant energy savings when implemented on a cooling system’s pumps and fans. Results of a VSD pilot case study corroborated the findings of the article.
Article M-8.2 (2015) [54]
The second article focused on developing and simulating a variable flow control strategy based on the findings from M-8.1. The control strategy’s aim was to match the cooling supplied with the cooling demand of the mine. A simulation model was developed to predict energy savings for the developed control strategy. The simulation model was verified and found an electrical savings potential of up to 33% was possible if implemented on Kusasalethu gold mine.
13 Article M-8.3 (2013) [55]
The third article centred on the development of a real-time EnMS for the implementation of automatic control, optimisation and monitoring of variable flow strategies (M-8.2 research). The developed EnMS was implemented on four different cooling systems for verification. The main results of the article were the integration the EnMS directly to the mines’ SCADAs and energy savings due to the implementation of the real-time control and optimisation.
Article M-8.4 (2013) [56]
The fourth article described the results achieved from implementing the methods described in
M-8.2 and M-8.3 on a practical case study. The results showed that a total energy reduction
of 32% was achieved on the cooling system on a deep-level mine case study. Cooling service delivery was not adversely affected during the implementation of improved strategies through the real-time EnMS. The study thus concluded: variable water flow strategies can realise large improvements in energy efficiency without compromising mine cooling requirements.
Article M-8.5 (2013) [57]
Conference proceedings summarising the main results from articles M-8.1 to M-8.4.
Study M-9 (2013) [58]
This research aimed to integrate saving measures between the cooling and ventilation systems of a deep-level mine. The study found that projects are implemented in isolation of each other, leading to no clear cost saving measures being implemented for the benefit of an integrated system. A surface BAC peak clip initiative was combined with existing load management and energy efficiency strategies to ensure cost savings throughout the cooling and ventilation systems.
The results after implementation indicated a 38% decrease in electricity cost on the mine’s cooling and ventilation system. This translated to a 16% saving on the mine’s total electricity bill. The research noted the mine’s cooling service delivery was not negatively affected by the combined strategies. Improving cooling service delivery was not part of the scope of research.
Study M-10 (2014) [59]
This research focused on implementing variable flow control on a cascade mine cooling system. VSDs were installed on the transfer pumps of a case study refrigeration system as part of an Eskom DSM initiative. The energy efficiency of the cooling plant increased after
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optimised control was implemented through the use of the installed VSDs. Payback periods based on VSD installation cost were calculated with the energy cost savings results. The study did not investigate improving the cooling service delivery through control optimisation.
Study M-11 (2014) [60]
This research focused on monitoring the performance of mine cooling systems and how it is affected by maintenance interventions. A maintenance schedule was developed to ensure optimal performance of refrigeration equipment. Results clearly proved that improved maintenance resulted in better cooling service delivery. The secondary focus of the research looked at operational interventions and their effect on the system’s energy consumption.
The effect of changes in operation between the chillers and surface BACs were identified and optimised for maximum cost benefit. An evening peak clip of the surface BACs was incorporated into optimised control to realise energy savings on the chillers. The saving was a result of chillers being switched off along with the surface BACs due to reduced chilled water demand.
Although the research showed improved cooling performance on refrigeration systems, it did so through improved maintenance scheduling and not control adjustments. The developed control strategy was also aimed at reducing energy input, keeping cooling service delivery the same after implementation. The study did, however, recommend that the effect of control optimisation on cooling service delivery (positive and negative) be investigated in more detail.
Study M-12 (2014) [61]
The integration of multiple DSM initiatives on a deep-level mine’s water reticulation system was investigated to improve overall cost savings. The study found that, by integrating the dewatering and refrigeration DSM load shifting initiatives, cost savings were maximised. Operational performance was not negatively affected by implemented initiatives. The study did not include investigating energy efficiency or cooling service delivery.
Study M-13 (2014) [62]
Implementing improved strategies to sustain mine cooling system energy saving measures were investigated. Deteriorating DSM initiates were identified and targeted for re-commissioning through the improved strategies. The performance of the energy savings measures on mine cooling systems were improved via reduced electrical input. Sustained
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energy savings were shown to be achieved. Improving cooling service delivery was not part of the scope of the study.
Study M-14 (2014) [63]
The study looked at optimising the refrigeration system of a platinum mine through implementation of an energy efficiency DSM initiative. Various flow control strategies were implemented through the use of VSDs on surface refrigeration equipment. Underground demand was also investigated by reducing wastages of secondary cooling equipment. The study noted that cooling service delivery remained unaffected during the course of the initiative’s performance assessment period. The study concluded that generic variable flow strategies on platinum mine cooling systems can result in significant energy savings.
Study M-15 (2015) [64]
The research focused on combining different DSM strategies to improve cost savings on platinum mine cooling systems. A control strategy was developed to combine energy efficiency and load shifting initiatives. Simulation software was used to determine the effect of the combined control strategy before implementation. The combined strategy proved successful with significant cost savings being achieved along with over 44 MWh energy reduction per day. The combined strategy did not negatively influence nor improve the cooling service delivery.
Study M-16 (2015) [65]
Cost-effective management strategies were investigated on platinum mine cooling systems. The study focused on implementing a manual load shift on refrigeration equipment as no funding was available under a revised Eskom IDM model. Using simulation techniques, the implementation of load management without the need to install additional infrastructure was investigated. Results proved that manual implementation of such strategies were cost-effective, although not as reliable as a fully integrated EnMS.
Study M-17 (2016) [22]
This study focused on reconfiguring the cooling auxiliaries on refrigeration systems for optimal performance. Special emphasis was placed on the age of typical deep-level mine refrigeration equipment and its effect on machine performance. Four of the cooling system’s sub-systems were identified for reconfiguration. Due to financial constraints, only one sub-system (transfer
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pump VSD installations) was reconfigured. This reconfiguration was part of an Eskom DSM initiative, requiring the mine to realise energy savings in exchange for funding.
The simulated control strategy that was implemented was chosen based on energy efficiency, as savings were a part of a contractual obligation. The study showed that cooling service delivery can be improved through reconfiguration of auxiliary equipment. Unfortunately, the lack of actual results to verify all the simulated strategies shows that there is a need to test more of these methods on a practical system. Improving cooling service delivery without capital expenditure by only using control strategy improvements should also be investigated, as reconfiguring entire sub-systems remains an expensive endeavour.
Study M-18 (2016) [66]
This study looked at improving the energy efficiency of a deep-level mine’s water reticulation system by integrating underground refrigeration and dewatering systems. Existing variable water flow control strategies were implemented on the underground cooling system through the installation of VSDs. Water control valves were implemented on the mining levels to reduce flow during low demand periods (according to mine schedule). The integrated control of these two methods was implemented by using a real-time EnMS.
Chiller energy consumption decreased as demand decreased because of the mine water control valves. Reduced water volumes in the system made it possible to implement load shifting on the refrigeration system and large dewatering pumps of the mine. The results showed that significant energy and cost savings are possible through the implemented strategies. These strategies are, however, expensive as they require large amounts of capital for new equipment.
The research did not investigate improving cooling service delivery as it was not required on the case study mine. The newly installed equipment was only used to lower the demand, after which the supply was matched for maximum energy savings.
Study M-19 (2017) [67]
Load management control strategies were developed and implemented on closed and semi-closed loop surface cooling systems. Emphasis was placed on underground ambient temperatures and how load shifting affected them. Underground temperature sensors were installed and incorporated into a real-time EnMS. Successful surface cooling system load shifts were implemented on two case studies with underground working conditions staying within safe limits.
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Results of ambient underground conditions, however, showed cooling service delivery was negatively affected during periods when load management was implemented. The study noted that these changes were within regarded safety limits, with temperatures recovering soon after the system was started up each evening. The study, therefore, did not aim to improve cooling service delivery through control strategy adjustments.
Study M-20 (2017) [44]
Research focused on optimising mine refrigeration systems for sustainable cost savings. Control strategies’ sustainability needed to be improved to ensure consistent energy savings due to changes in Eskom’s IDM model. Suitable load shifting control strategies were developed and implemented on two case studies. Emphasis was placed on sustainability of cost savings, i.e., minimal effect on cooling service delivery. Simulations of improved control strategies indicated that they could be implemented sustainably.
Results showed that using the thermal storage capacity of cooling systems minimised negative effects during times when chillers are switched off. Control strategy implementation and thus cost savings were also sustained over long periods on both case studies. As with most load management initiatives, the research only aimed to save costs by exploiting different electricity tariffs for different times of day. Improving cooling service delivery was not part of the scope of this research.
Study M-21 (2017) [41]
This research focused on improving the performance of existing energy efficiency initiatives on mine cooling systems. An energy performance strategy was developed and implemented on two case studies that had existing, under-performing DSM initiatives being implemented. Adjustments to the variable water flow control were made as part of the energy performance strategy. Results proved that under-performing energy efficiency initiatives can benefit from optimised control strategies for sustainable performance.
Although the study improved the performance of the refrigeration systems in question, it did so by reducing energy input. Some of the KPIs even improved under the implementation of the performance strategy. Global cooling service delivery improvements were, however, not present in either of the two case studies.
18 Study M-22 (2017) [2]
This study aimed at optimising load management on deep-level mine refrigeration systems through maintenance and operational improvements. An existing DSM initiative was evaluated and showed substantial negative impact on cooling service delivery during periods where chillers were switched off. A combination of maintenance, reconfiguration and an optimised control strategy was investigated to improve the service delivery during periods where load management was implemented.
Certain maintenance and reconfiguration work was completed (mainly on the pre-cooling circuit), after which the control strategy was updated to account for the new system parameters. Clear improvements in cooling service delivery were present after implementation, with chill dam temperature reductions of over 2°C. The cost impact of load management was also improved after implementation. Results clearly show improved cooling service delivery is possible by combining maintenance, reconfiguration and control strategy optimisations.
Unscheduled maintenance and reconfiguration, however, is expensive to implement. Large capital expenditure can be required based on the extent of equipment to be reconfigured. Using only control optimisation to improve service delivery was not the objective of this research. This shows that scope still exists to improve cooling service delivery of mine refrigeration systems through control optimisation alone, reducing cost by eliminating capital expenditure.
Study M-23 (2018) [68]
The study focused on developing an identification model for cost-effective electricity savings on mine cooling systems. The developed methodology assisted with the identification of cost savings on integrated refrigeration systems on deep-level mines. The methodology was implemented on a case study and identified additional scope for cost saving on a surface BAC. Savings increased after the implementation of a BAC peak clipping while underground temperature remained within acceptable limits.
The study focused purely on cost savings on mine cooling systems, with cooling service delivery not forming part of the scope. The integrated nature of mine cooling systems was discussed, indicating that small changes can have large effects elsewhere in the system. Keeping the integrated nature of these systems in mind, new control strategies can be developed for improved cooling service delivery.
19 Study M-24 (2018) [40]
Research focused on the integration of a deep-level mine’s water reticulation sub-systems to improve the performance of load shift projects. Existing strategies such as variable water flow control on refrigeration and underground dam preparation on pumps were used to integrate the control of three sub-systems: the surface refrigeration system, conventional dewatering pumps, and an energy recovery system. The energy recovery system in question was a three-chamber pipe feeder system (3CPFs).
The control strategy aimed to adapt the control of the 3CPFs (high volume flow rates) to maximise the load shift benefit on the remaining two systems. This resulted in significant increases in existing load management performance on both the refrigeration and dewatering systems (additional savings were also present on the 3CPFs). Results show that load shifts could be conducted on consecutive days, indicating that cooling service delivery was not adversely affected. No mention, however, is made of the exact effect the control strategy had on cooling service delivery performance.
Study M-25 (2019) [69]
The study focused on dynamically controlling chiller compressors for improved performance on mine cooling systems. A dynamic temperature set-point algorithm was developed in conjunction with an ambient dry-bulb temperature prediction model. The dynamic control was implemented on an actual case study on a South African gold mine cooling system. The control algorithm adjusted chiller compressor guide vane angles in real time based on ambient conditions.
The system proved to decrease power consumption by up to 46% during the evening peak period. In addition to the energy saving, cooling service delivery was improved by 15%. This research shows the importance of dynamic control of mine refrigeration equipment for optimal operation. The research proves that service delivery improvements are possible by only utilising dynamic control adjustments.
The focus was, however, on the chiller compressor control, and not the dynamic operation of the entire cooling system and its auxiliaries. Scope exists to dynamically optimise the entire system’s (including auxiliaries) control for improved cooling service delivery. Existing methodologies applied for energy savings should be investigated for re-application to benefit cooling service delivery. Dynamic operation of these re-applied strategies should also be considered, as it will further optimise the performance of mine cooling equipment.
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1.2.2 Mining sector critical analysis
The detailed evaluation matrix of relevant research within the mining sector can be seen in Table 1. The information in Table 1 is colour-coded to show which criteria were included in the scope of each relevant study. Green indicates the study focused or considered the criteria, with red highlighting a shortcoming within the research. A summary of each main criteria’s findings in research is presented after Table 1.
21 Table 1: Previous research evaluation matrix – Mining sector
Study Year Reference Energy Efficiency Load Management Saving Sustainability Evaporator temperature reset Condenser temperature reset 3rd Party funding Maintenance/ Reconfiguration/ New Installations Integrated Cooling
Auxiliary Control Dynamic control
Negative result on service delivery
Chilled water temperature reset for improved service
delivery Service delivery improvement M-1 2003 [45] ✓ ✓ ✓ M-2 2006 [46] ✓ ✓ ✓ ✓ ✓ M-3 2007 [47] ✓ ✓ ✓ M-4 2007 [48] ✓ ✓ ✓ ✓ M-5 2008 [49] ✓ ✓ ✓ ✓ M-6 2011 [50] ✓ ✓ ✓ ✓ ✓ M-7 2012 [51] ✓ ✓ ✓ ✓ M-8 2013 [52] M-8.1 2013 [53] M-8.2 2015 [54] M-8.3 2013 [55] M-8.4 2013 [56] M-8.5 2013 [57] M-9 2013 [58] ✓ ✓ ✓ ✓ ✓ M-10 2014 [59] ✓ ✓ ✓ ✓ M-11 2014 [60] ✓ ✓ ✓ ✓ M-12 2014 [61] ✓ ✓ ✓ ✓ M-13 2014 [62] ✓ ✓ ✓ M-14 2014 [63] ✓ ✓ ✓ ✓ M-15 2015 [64] ✓ ✓ ✓ ✓ ✓ ✓ M-16 2015 [65] ✓ ✓ M-17 2016 [22] ✓ ✓ ✓ ✓ ✓ ✓ M-18 2016 [66] ✓ ✓ ✓ ✓ ✓ ✓ M-19 2017 [67] ✓ ✓ M-20 2017 [44] ✓ ✓ ✓ M-21 2017 [41] ✓ ✓ ✓ M-22 2017 [2] ✓ ✓ ✓ ✓ ✓ ✓ M-23 2018 [68] ✓ ✓ M-24 2018 [40] ✓ ✓ M-25 2019 [69] ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Service delivery Control optimisation
Cost savings Capital expenditure
✓ ✓
22 Cost savings
According to literature, energy efficiency and load management initiatives make up the bulk of the research conducted on mining refrigeration systems. As previously mentioned, this is mainly due to the mines’ weak financial performance over the past few years. Interestingly, sustainability of these initiatives has only become a priority in the past few years, corresponding with an update to Eskom’s IDM model in 2015. Lastly, temperature reset strategies (evaporator and condenser) for energy saving purposes are not widely implemented, with only two studies focusing on these methods.
This is not done despite evidence that evaporator temperature reset strategies result in decreases in energy usage. Further investigation showed that savings initiatives on mine refrigeration heavily focus on not negatively affecting cooling service delivery. Potential decreases in energy usage are sacrificed at the expense of better service delivery. This demonstrates that supplying adequate cooling underground remains the number one priority for deep-level gold mines, despite the financial stress on the industry.
Condenser temperature reset strategies are not present in research literature. This displays a significant shortcoming within the mining industry regarding seasonal optimisation. Condenser temperature reset strategies are usually employed to reduce energy usage when ambient conditions change, e.g. summer to winter. The lack of research in this area confirms that mine refrigeration systems are operated in a static manner. Cooling systems are thus not dynamically optimised based on the ambient operating conditions.
Capital expenditure
An overwhelming majority of the studies made use of third-party funding to enable reconfiguration of equipment or new installations. The initiatives implemented to ensure savings were, in most cases, enabled by costly upgrades to existing refrigeration systems. Very few examples of such capital expenditure are available were no third-party funding was involved. Again, this highlights the weak financial position of the gold mining sector in South Africa.
Control optimisation
There is ample research available showcasing the impact of optimising refrigeration system cooling auxiliaries. This supports the notion that these systems are not operated for maximum benefit at minimum cost. The research is, however, heavily biased towards optimisation for
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energy savings. Very few studies used control optimisation of auxiliary equipment to enable improvements in cooling service delivery.
Optimising the control to enable dynamic operation of these systems was only considered in one study. This dynamic control was only implemented on machine level, not considering the dynamic operation of the entire system (including its auxiliaries). The dynamic operation of deep-level mine refrigeration systems is thus a major shortcoming in the field of current research.
Service delivery
Most of the studies only mention service delivery when evaluating whether savings initiatives influence it negatively. Most of the studies showed that, overall, cooling service delivery was not negatively affected by the implemented initiatives. These conclusions were regularly based on average results on certain key performance indicators (KPIs). Minor instances of rising temperatures that could be recovered through comeback strategies were thus reported as having a neutral affect.
Very few studies considered improving service delivery a main objective. Some studies merely mentioned that developed methods can be considered to optimise service delivery instead of reducing energy consumption. Another shortcoming in literature is that of using a chilled water temperature reset to enable improvements in service delivery. Results from select studies can be interpreted to indicate potential to improve service delivery through these types of control resets. It is, however, neither mentioned nor confirmed in any research discussed within this critical analysis of literature.
1.2.3 Refrigeration control in the commercial sector
Abridged summaries of research available on commercial refrigeration control optimisation are given below. The summary of each study was compiled based on the criteria as set out in the introduction of this section. Each of the studies presented hereafter is summarised in Table 2 in the form of an evaluation matrix. The critical analysis of summarised research within the commercial sector continues in section 1.2.4 (page 32).
Study C-1 (2005) [70]
Two commonly used temperature reset strategies on single-duct constant volume air handling units (AHU) are compared in the study. The aim was to determine whether return temperature or outside air temperature reset is more effective in reducing AHU energy consumption. The results clearly show resetting the temperature based on the outside air temperature to be the
