**4. Discussion**

The energy managemen<sup>t</sup> frameworks were mainly researched to adopt energy managemen<sup>t</sup> practices at the technical levels in the industries. However, the reviewed papers emphasized the energy managemen<sup>t</sup> system, ISO 50001, and PDCA cycle, while some studies suggested holistic approaches towards industrial energy e fficiency.

The framework proposed by Christo ffersen et al. was stood out on the Danish industries and emphasized on multiple factors, mostly energy policy, goals and capstone projects aimed at energy savings. Regulation, external relations, company characteristics, and organizational internal condition are the main out-layers of the model to frame the energy management. However, the company size and energy intensity are two factors that can be considered to categorize the industries to apply or analyze the model [44]. The main features proposed by Christo fferen et al. align with ISO 50001: 2011 standard though this model has been replaced by ISO 50001: 2018 [58]. The earlier model encompassed energy managemen<sup>t</sup> system implementation based on PDCA cycle and enlisted few prerequisites that include mainly managemen<sup>t</sup> liability, policy, energy audit, energy performance indexing, energy managemen<sup>t</sup> blueprint, documentation, and so forth. One of the major changes in the recent model is the PDCA cycle modification. "Checking" was the center in the earlier version, whilst "Leadership" became the focus of all cycle components. Figure 3 represents the revised PDCA cycle of ISO 50001:2018. In the minimum requirement segment, the model proposed by Ates et al. comprehended conventional streams towards energy management. One of the significant features is the inclusion of energy manager, whilst ISO 40001 (environmental permit) also act as an enabling feature along with ISO 50001 [40].

**Figure 3.** The "Plan-Do-Check-Act" cycle adopted in ISO 50001: 2018 (Source: [45]).

Looking at the minimum requirement focused model, it is observed that all the energy managemen<sup>t</sup> initiatives are not integrated into the frameworks. Christo ffersen et al. [44] considered energy managemen<sup>t</sup> as a comprehensive managemen<sup>t</sup> system. However, the model does not integrate the energy manager concept. Furthermore, there is no clear guideline about top or mid-level managemen<sup>t</sup> support to achieve energy savings. Though, the involvement of employee to energy-saving related works are suggested. Nonetheless, The ISO 50001 model is a significant protocol [69] along with the proposition by Ates and Durakbasa [40], manifold aspects are still to be explored regards of operational activities in the industrial energy managemen<sup>t</sup> domain. For instance, the principles of sustainability and integral managemen<sup>t</sup> need to be presented at the protocol. In addition, there is very little contribution on the risk managemen<sup>t</sup> and opportunities of energy e fficiency from an integral and strategic point of view, including the planning and control of product lines, process design, projects, and business approaches [69]. Notably, the fruitful operation of the energy managemen<sup>t</sup> system requires the integrated deployment of planned, tactical, and operational levels that align the business culture with sustainable attainment. In this context, the vision that the organization plans to form should be linked to energy e fficiency strategy with organization's survival plan in the market. Additionally, it is necessary to make explicit reference to newly adapted technical features through peer to peer energy managemen<sup>t</sup> platform for optimizing the integration of energy managemen<sup>t</sup> system component with the variable energy demand [70,71]. Moreover, an integrated perspective to control of operational features of each process are required to explore linked to energy e fficiency [69].

In the energy managemen<sup>t</sup> maturity model segment, the model proposed by Ngai et al. based on capability maturity model integration (CMMI), an extension of capability maturity model incorporated five levels according to the behavioral exhibition of the industries [54]. The levels are determined by performance area of key processes [72]. The achievement goals of key process areas must be specified for individual level for further actions. Notably, the propositions of CMM framework has been applied at multiple process enhancement programs in order to achieve the desired quality in the production system [73]. One limitation of this model is inadequate implementation time, having only one factory for consideration. However, the authors have affirmed the acceptability of the model because of prior implementation of managemen<sup>t</sup> practices. On the contrary, Antunes et al. emphasized the PDCA cycle to design the energy managemen<sup>t</sup> framework [64]. Additionally, the authors implied the model with ISO 50001 and incorporated energy managemen<sup>t</sup> practices also. Notable to mention that Finnerty et al. also designed the framework based on the PDCA cycle, keeping on focus on energy managemen<sup>t</sup> practices [66].

The model proposed by Introna et al. is comprised of five dimensions and enables the feature of self-evaluation for the industries towards energy managemen<sup>t</sup> practices. The dimensions are characterized by identifying the necessary elements in energy managemen<sup>t</sup> consumption segmen<sup>t</sup> of the industries [8]. On the contrary, Jovanovi´c et al. focused on ISO 50001 processes as well as PDCA phases, keeping the knowledge base in the model EMMM50001 [53]. The EMMM50001 establishes the relation to EUMMM maturity levels, maintaining ISO 50001 specifications and PDCA phases. Notably to mention that EMMM50001 links the CMMI criteria, also maintaining the ISO 50001.

It can be observed that the majority of the maturity models emphasized on similar type of characteristics and areas to evaluate the energy managemen<sup>t</sup> in an organization by a systematic set of commendations. However, the narrated models demand more time and resources to perform as per their characterization. In addition, looking at the scientific literature, all of the frameworks studied to see the requirements for providing a continuous development path following the PDCA approach and ISO 50001. Notably, few of the maturity models incorporate the implication of dedicated energy manager and top managemen<sup>t</sup> support. In contrast, Antunes et al. [64] a ffirm on top managemen<sup>t</sup> support whilst not integrating the energy manager in the framework. The framework by Introna et al. [8] also did not complied with the energy manager. Nonetheless, Jovanovi´c and Filipovi´c [53] and Finnerty et al. [66] considered top managemen<sup>t</sup> support along with the energy manager in their framework.

Gordic' et al. applied the energy managemen<sup>t</sup> matrixes model in the Serbian car manufacturer industries and critically analyzed the existing energy managemen<sup>t</sup> system with the model [55]. Notably to mention that the energy managemen<sup>t</sup> matrixes models proposed by Gordic' et al., Carbon Trust and Energy Star encompass all key areas to assess the energy managemen<sup>t</sup> practices in the model, with having few modifications at the individual version.

On the contrary, Fleiter et al. [58] and Trianni et al. [52] emphasized on a characterization based model where both of the models are incorporated with specific attributes. The characterization scheme has some implications on policy design and assessment. However, formalization of the groups with categorized attributes enables the option towards relevant aspects identifying the energy e fficiency measures. In addition, Trianni et al. contend a comprehensive scenario on EEMs focusing on the relevant aspects of industrial energy managemen<sup>t</sup> [52]. One of the critical factors, "corporate involvement" for industrial decision-makers is also implied, hence allowing additional feature and an increase in the applicability of the model. In another proposed framework, Trianni et al. incorporated energy managemen<sup>t</sup> practice-based approach. However, the authors acknowledge more compatible space for the SMEs within the model, as SMEs are sought to be entitled to more attention, considering their cumulative energy consumption percentile [74]. In a recent study, Tina et al. persuade the significance of SMEs and the policy implications in the peripheral of the industrial energy sector [74]. Referring to the SMEs, Prashar [67] proposes an energy e fficiency maturity assessment framework that emphasizes SMEs. Notably, the author argues that the common energy e fficiency framework approach does not facilitate fully to the SMEs; hence, a customized maturity framework is significant. The author considered steel re-rolling mill sector of India as the contextual sphere for the proposed framework.

Few of the studies on characterization the energy efficiency measure focuses on financial features. Notably to mention that these models do not frame the energy efficiency measures comprehensively, rather offer some framework without characterizing the energy efficiency measures in-depth. In one of the studies by Pye and McKane, they state that quantification of the accumulated benefits of energy efficiency scheme supports the enterprises perceive the monetary opportunities of EEMs financing [5]. The energy savings features act not as the singular primary driver for the industrial decision process; hence, the authors persuade that energy savings be viewed as a factor of the holistic approach towards energy efficiency programs. Skumatz studied the methods to find out the attributes of EEMs and established the scheme to measure both of the positive and negative secondary benefits stemming from industrial energy efficiency schemes [75,76]. On the contrary, Mills and Rosenfeld provided a framework to understand multiple benefits of energy efficiency initiatives and grouped the attributes into the better interior environment, noise lessening, savings of labor and time, improved supervision of procedure, convenience, water savings and waste reduction, and benefits due to downsizing of equipment [77].

The majority of studies on energy e fficiency measures, benefits, and initial characterization frameworks propose few interesting reflections. However, a lack of consistency on the attributes grouping within existing categories from the methodological perspective is observed. It is found that the same attributes are grouped in di fferent categories by di fferent researchers. Moreover, the attributes are categorized and then aggregated again within other segments by di fferent researcher. For instance, "reduced noise" and "improved indoor environment" are framed in two di fferent categories in [77], whereas "reduced noise level" as categorized in "working environment" segment. On the other note, the decision-making process is a grey area keeping mind about the stakeholders. Nonetheless, the earlier characterization framework did not incorporate the energy e fficiency measure implications in a comprehensive way. To be precise, the inclusion of non-energy benefits is not incorporated into the characterization framework. Notably, the inclusion of non-energy features in the modeling factors would double the cost-e ffective potential for energy-e fficiency enhancement, likened to an analysis eliminating those benefit [60]. However, few attributes (e.g., improved air quality, better worker safety, reduction of noise level, and improved working situation) are there in the characterization framework, which are di fficult to quantify [76]. Therefore, speculation is required to articulate the benefits into a comparable cost figure, and hence the assessment turns out to be rather subjective [60].

The study by Ngai et al. [54] features energy managemen<sup>t</sup> with particular process areas in the manufacturing industries. In this study, few guidelines are o ffered to conduct analysis for organizational maturity improvement in terms of energy along with the managemen<sup>t</sup> of utility resources. However, the integration of energy managemen<sup>t</sup> into production process has not been complied comprehensively. This is a significant avenue that needs be to address with utmost attention in future studies considering the technical implications o ffered by Industry 4.0. Notable to mention here, is that increasing the efficiency at the production processes is one of the salient features of Industry 4.0 [78]. The deployment of smart machinery o ffers diverse benefits which primarily includes manufacturing productivity and waste reduction [79]. Therefore, it is worth observing the energy managemen<sup>t</sup> characteristics linked with production process through the lens of Industry 4.0.

Nonetheless, energy managemen<sup>t</sup> towards industrial energy e fficiency has been widely discussed in academia, and several barriers are still persistent in the energy managemen<sup>t</sup> domain. The identification of barriers is important because it hampers or slows down the adoption of energy efficiency measures [80]. Several studies have addressed the barriers which cover energy-intensive industries to SMEs and include regional, national, and transnational perspectives [15,26,27,81–84]. However, most comprehensive studies focusing on energy managemen<sup>t</sup> have been discussed without really looking at the integration of energy managemen<sup>t</sup> into production and operation management. An imperative avenue, therefore, lies to be further explored in future within this research domain.

#### **5. Concluding Remarks and Future Research Avenues**

The paper attributes a review of research works on the energy managemen<sup>t</sup> model for energy managemen<sup>t</sup> practices in the industries. Multiple models have been compiled and structured, maintaining the narrations. Moreover, the energy managemen<sup>t</sup> frameworks were synthesized emanate from the findings in order to facilitate energy managemen<sup>t</sup> in the industries by o ffering necessary benchmarks to the industrial experts. The review findings show that the narrated models can support an organization to assess energy managemen<sup>t</sup> and incorporate insightful contribution to energy e fficiency initiatives. Nonetheless, some of the studies did not comply with a full methodological description and exhibited shorter model validation. In addition, a gap exists between the theoretical concept and practical implementation of energy managemen<sup>t</sup> and its practices. Precisely, majority approaches remain unsuccessful in replicating and scope of actions distinct in energy managemen<sup>t</sup> due to the certain barriers [27,66,85].

Moreover, most of the models have looked the energy managemen<sup>t</sup> as a single unit function, whilst it is a combination of multi-dimensional approaches with the involvement of several functional units in the industries. Let us not forget about the multi-dimensional operational activities in the industries which are conjugal part with energy management. Notably, multi-dimensional approaches are critical to support the process and operational oriented program [86]. Therefore, a comprehensive operational approach should be considered by integrating all the relevant energy flows. It infers to all forms of energy, including externally supplied energy sources as well as internal energy flows. Interestingly, relating the energy managemen<sup>t</sup> into the operational framework integrates the resource efficiency also at the manufacturing level. The raw or auxiliary material consumption might be of interest, considering the direct and indirect impact on energy and resource efficiency in the manufacturing process. Moreover, keeping mind about the non-static nature of energy consumption, the dynamic consumption feature might unveil manifold resource optimization aspects [87].

Unfortunately, the integration of energy managemen<sup>t</sup> into operational activities have been little explored. It becomes even more imperative while we look to adopt Industry 4.0 keeping in mind about the manifold complex technical features consist of Internet of Things (IoT), big data, cloud computing, and so on in the industrial plants. Many scholars predict that the exponential progress in the promises of manifold technical features o ffered by Industry 4.0 might a ffect the production activities in the industries inclusively. In addition, there are high chances of modification in the traditional industrial actions that cover the processing of elements and material, grinding, and assemble/ dismantle. This is emphasized in Industry 4.0 concept and its implementation, where we pursue to pool the common features with the enormous potential of digital technology [88]. However, it is understood as a necessary incremental approach aimed at further optimizing the energy system without seeking to disrupt it in principle. In the energy e fficiency domain, the energy managemen<sup>t</sup> and its practices have already influenced the production scenario in a broader aspect, and this inclination should remain as long as we allow the nexus between Industry 4.0 and energy e fficiency. On the other hand, energy productivity investments in present as well as the recent technologies must be conveyed through the implementation of energy managemen<sup>t</sup> and its practices [89]. Energy managemen<sup>t</sup> practices and energy services are acknowledged as fundamental solutions; the diminutive e ffort is being paid in characterizing them [24]. Notably, assessment models for supporting the industrial decision-makers

emphasizing detailed activities for better energy managemen<sup>t</sup> is deficient. Therefore, it is imperative to consider the energy managemen<sup>t</sup> in multiple aspects keeping mind about the complex nature of industrial energy system [31].

Interestingly, energy managemen<sup>t</sup> has implications on asset maintenance, e.g., on maintenance procedures. As energy managemen<sup>t</sup> includes the control of energy-consuming devices to optimize energy consumption, manual toggling on and o ff devices depending on requirement is a rudimentary custom of energy management. The initiation of mechanical and electrical equipment (e.g., timers for programmed toggling, bimetallic strip thermostat, pneumatic and electrical transmission system, and so on) provided means for early energy managemen<sup>t</sup> schemes in the form of automatic temperature control. Nowadays, the inclusion of direct digital control in energy managemen<sup>t</sup> has retrofit benefits that allow device monitoring linked to maintenance procedure, thanks to energy managemen<sup>t</sup> and its practices. The comprehensive data recommend that while energy managemen<sup>t</sup> does improve the accuracy and response of a system in the industries, the energy managemen<sup>t</sup> routines facilitate partially asset maintenance [90]. It infers to monitoring or log building equipment performance while consuming the energy resulting increasing magnitude of all benefits covering maintenance and cost avoidance benefit. Unfortunately, much of the energy managemen<sup>t</sup> studies have bypassed this retrofit fact while focusing on the energy managemen<sup>t</sup> framework. So far, the integration of energy managemen<sup>t</sup> with asset managemen<sup>t</sup> has not been widely explored, and several questions remain unanswered at present. Therefore, more research needs to be undertaken to fit the asset maintenance into energy managemen<sup>t</sup> framework in a comprehensive way.

In addition, the narrated models have little explored the sustainability feature integrated with energy e fficiency, pointing to the optimization of resource utilization [91]. We must consider the paradigm that allows industrial energy managemen<sup>t</sup> e ffective for the companies. In this context, it would be certainly interesting to visualize the energy managemen<sup>t</sup> through Industry 4.0 technologies and solutions, may contribute to improved sustainability performances of the companies. If Industry 4.0 is expected to unveil enormous directions not only to energy managemen<sup>t</sup> but also the sustainability field, the challenge definitely lies on the integrational aspects with energy–industry–sustainability nexus. Therefore, the future research avenues should reflect the energy managemen<sup>t</sup> framework complying the diverse directions and encompassing the operational management, Industry 4.0 along with sustainability features.

**Author Contributions:** Conceptualization, A.S.M.M.H. and A.T.; methodology, A.S.M.M.H. and A.T.; formal analysis, A.S.M.M.H. and A.T.; investigation, A.S.M.M.H. and A.T.; writing—original draft preparation, A.S.M.M.H. and A.T.; writing—review and editing, A.S.M.M.H. and A.T.; visualization, A.T.; supervision, A.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
