## 2 State of the Art
### Energy as a Service  
Decarbonization, greater decentralization, and increased digitalization of energy systems are driving rapid changes in the energy sector today. Another facet of this effort is the involvement of larger consumers, such as manufacturing firms, with the goal of achieving more sustainable energy production and consumption systems using service-oriented business models [50]. As a result, energy providers are among the key suppliers making an effort towards shifting their business models to provide IPSSs in order to improve their sustainability, thereby increasing the added value of their offerings [51].  
Using cutting-edge digital technologies, namely Cloud systems, IoT, and BDA, energy companies are attempting to combine centralized and distributed energy systems into a more complex system. In addition, Energy Sales Companies (ESCs) are becoming more digitalized to keep the electricity supply competitive and affordable while maintaining the supply of electrical power at adequately high levels. Consequently, stronger synergies between the manufacturing (demand) and energy (supply) sectors are required. In addition to that, innovative solutions are required to make the electricity market fit for distributed energy while also requiring the industrial sector to shift to energy-efficient manufacturing [52]. In the research work presented in [53], the authors analyzed how PSSs have originated. Their work also extends to highlighting the current applications of PSS, and they conclude with providing directions for further research based on the current trends. However, the contribution of [PERSON] et al. [53] involves the discussion on the importance of building and maintaining relationships between suppliers and customers. What is more, key PSS drivers will be the reduction in environmental impact as well as the cooperation with authorities. The Product Service System (PSS) value proposition strategy is suggested as part of the mix of innovations necessary to move society toward more sustainable futures, based on extensive research [44]. Additionally, the authors in [50] present a design and development of an Industrial Product Service System (IPSS) business model, aiming for the collaboration between ESCs and manufacturing companies. Moreover, an adaptive production scheduling approach considering the power usage of manufacturers in response to time-varying energy prices is presented. Similarly, the authors in [35] proposed a methodology for conducting resource planning for the production and installation of IPSS, and according to this, the architecture of a Cloud-based tool is developed. The energy provision can be considered as a Product Service, which involves the contract as a product and a set of services. That leads the energy provider and the customers to mutually benefit through the performed collaboration [54; 55].  
In the literature review performed in the previous paragraphs, it can be highlighted that maintenance is an important aspect to the lifecycle of the products. As a result, the arising challenge is to develop intelligent solutions targeting the reduction in unexpected equipment downtimes, which could affect in a positive manner the reliability of modern manufacturing equipment.