# IoT Middleware Platforms for Smart Energy Systems: An Empirical Expert Survey
## 1 Introduction
### Main Contribution  
The digitalization of the energy sector greatly contributes to the transition towards environmentally friendly, low emission energy systems [22]. Several initiatives, such as \"Digitalising the energy sector\" [23] and \"GAIA-X\" [24], aim to provide digital infrastructure and energy services by developing trustworthy intelligent solutions for power generation, energy storage, power transmission and consumption monitoring. The IoT is the backbone of such an intelligent system [25]. Since no standards for IoT middleware or IoT energy platforms exist [26], the requirements of such platforms are unknown, and providers will often start their business based on few available components which might not be sufficient for practitioners' needs. Hence, our study contributes to this digital transformation by gathering the opinions of experts on requirements, promising technologies, limitations, etc., of IoT middleware platforms for smart energy systems. Expert surveys are a powerful research method allowing the systematic study of hard-to-measure phenomena through the aggregation of specialized expert knowledge [27]. Based on expert interviews, we define requirements for IoT middleware platforms; these include topics such as vendor locks, security, data models and communication technologies. In addition, we analyze promising applications, such as control services and demand-side management. To the best of our knowledge, there are no expert surveys on IoT middleware platforms for smart energy systems. To assess the state-of-the-art and the requirements and challenges for IoT middleware in smart energy systems, we devised a two-stage research plan, comprising a literature review and a quantitative expert survey that included (i) city officials who were responsible for smart city projects, (ii) experts in energy and energy technology companies and (iii) experts who were planning and/or operating buildings and districts. The literature review stage included reviewing IoT platform architectures, functional and non-functional requirements and the most common technologies and protocols in order to formulate survey questions that represent the real market needs of the people who operate these services in the energy sector. During the survey process, the experts were asked to list the tools they use/plan to use as IoT platforms, and their current and future applications; and to assess the importance of certain technologies, protocols and features in the IoT middleware frameworks.  
The rest of the paper is organized as follows: In Section 2, we discuss recurring building blocks, concepts, protocols and functionalities found throughout the catalog of IoT middleware solutions for smart energy systems and identify the requirements and challenges in the development and roll-out of middleware platforms. In Section 3, we identify and analyze a set of commercial and non-commercial IoT middleware solutions used in the context of smart energy systems in terms of their functionalities, their non-functional characteristics and their protocols. In the subsequent sections, we present and discuss the results of the survey among building and smart city experts.