# Internet of Things for Modern Energy Systems: State-of-the-Art, Challenges, and Open Issues
## 2 Key Features in the Energy Internet
### Energy Sources  
In modern energy systems, generation of energy is distributed among the system making many aspects of the system such as stability, reliability, and security more complex. Talking about MGs, energy is generated mostly by Power Electronic-based (PE-based) energy sources, which brings more constraints like controllability to the operation management problem. In [24], new indices are introduced to indicate controllability of DGs in a distributed system, as system variables' thresholds are affected by DGs' capacities and their locations in an energy system. Energy sources, as the main building blocks of every energy system, have to be studied from the different points of views such as their stochastic nature, controllability, and emission awareness. These three main viewpoints are discussed as follows:  
* Stochastic and deterministic nature of the energy source Some energy sources such as wind and solar power have stochastic behaviors meaning that they are directly influenced by the weather and their energy generation would be a nonlinear function of atmospheric variables. Thus, it is unlikely that their power production level stays the same from one hour to the next, or during the day and at night [25]. On the other hand, there are some energy sources such as conventional ones (thermal powers from coal, petroleum, and natural gas or hydroelectric power from high velocity of running water) that have a fixed nature meaning that the system output can be defined based on certain deterministic inputs. By increasing the level of uncertainty in energy systems, it becomes necessary to develop mathematical programming techniques in order to find the optimal production schedule. This leads to uncertain unit commitment models for energy systems in which stochastic nature of the thermal, hydro, and renewable generation units are taken into account through optimization under data uncertainty methods [26].
* Controllability of energy sources Many factors affect controllability of the system. As an example, the control system of a wind turbine has to deal with stochastic nature of the wind. Mostly, a probabilistic energy source is defined as an uncontrollable one while conventional energy sources can be treated as controllable energy sources. The other factor that affects control system design of energy systems is the system size, i.e., bulk energy sources' control systems differ from the ones for distributed energy sources. This is also true for big and small energy consumers control systems. In this regard, the level of controllability varies based on the system size, the nature of the energy source, and the system configuration [27; 28].  
\(\bullet\) Emission-awareness  
As global warming becomes one of the most important issues of this world, supplying energy in an environmentally friendly manner is a matter of importance. Some energy sources such as renewables inherently have an emission-aware nature and can play a great role in clean air/environment policies [29; 30]. As it is reported in [30], it seems that Renewable Energy Sources (RESs) are the only solution for the future global emission challenge, and there is going to be more investments in solar and wind power.