**4. Distributed Generations in Smart Grid**

A distributed energy resource (DER) is an aggregation of distributed generators, as shown in Figure 4, or controllable loads (conventional or smart) connected to the network in a smart grid. A DER unit, or distributed generation (DG), often blends a variety of energy sources. They are classified into, essentially, two sorts of sources based on their dispatch capacity and source of generation type:

**Figure 4.** Various types of energy sources in the smart grid.

### *4.1. Renewable Energy Sources (RES)*

Solar Photovoltaics: Converting solar energy to electrical energy via mounted semiconductor panels is the primary renewable generating source across the globe. It can produce energy in any mode, stand-alone or grid-integrated, and small-scale (such as rooftop PV in residential areas) or large-scale (centralized power plants). Scheduling, as per weather condition forecasting, boosts production capabilities.

Solar Thermal: Solar thermal power plants use solar energy to heat a fluid to a high temperature to generate electricity. The heat from this fluid is transferred to water, which subsequently forms superheated steam. In a power plant, steam is utilized to run turbines, and mechanical energy is transformed into electricity by a generator. This sort of generating

is similar to electricity generation that uses fossil fuels, except that instead of burning fossil fuels, sunlight is used to heat steam.

Hydropower Plants: The flowing capacity of water is capable of rotating a turbine to generate electricity, which can be the centralized or decentralized mode of operations according to the availability of the water and the water head. Generally, small-scale hydropower stations are used for DSM operations, which are described in Section 5.

Wind Turbines: Wind energy conversion systems (WECs) are also a significant component of DGs where the appropriate wind reach is available. This generation unit is limited to smaller, low-capacity generation units. This allows for small-scale WT unit deployment on the customer side to be possible without affecting the operation of the entire power system as a whole.

Geothermal Energy: Geothermal energy captures the energy from the core of the earth. DGs can be localized around nearby natural geothermal energy sources, such as lava flows, hot springs, geysers, or places that experience direct contact between water and high thermal capacity surfaces. As part of the natural cycle of evaporation and replenishment, geothermal sources can be considered a viable source of renewable energy generation.

### *4.2. Traditional Energy Sources*

Combined Heat and Power: Fossil fuels are the primary sources of CHP that are set to run as centralized power stations, mainly to fulfill the baseload requirement. Fossil fuels are burnt to produce steam, which rotates the turbine to produce electricity. Due to substantial carbon emissions, limited availability of sources, and environmental concerns, the focus has shifted toward renewables.

Fuel-based DERs: To supply supplemental power to the grid, diesel generators and fuel cell (FC) generators often employ readily available fossil fuels, waste-derived fuel, and hydrogen-based production, and they are typically run on-demand rather than always-on. Due to their simple dispatch mechanism and controllability, they are suitable DER units to link to a smart grid design. For the purposes of providing power to emergency loads, they are suitable as a DSM option.

### *4.3. Energy Storage Systems*

ESS is now viewed as a novel technique for adjusting generating capacity to load demand changes, particularly as energy buffers in the situation of the high availability of non-dispatchable generation sources. These ESS capture and store surplus energy generated during off-peak hours, then dispatch it during peak periods when the extra load is needed. They also allow for the optimal redistribution of PV array and WT unit output power throughout the daily scheduling period. In terms of ESS concerned with energy supply, they are categorized as compressed air energy storage (CAES) and hydraulic pumped energy storage (HPES), depending on the method of application. Similarly, ESS focused on power supply include supercapacitor energy storage (SCES), superconductor magnetic energy storage (SMES), pumped storage, and flywheel energy storage (FWES) [21,22].

### *4.4. Waste-to-Energy (Bio-Energy)*

An increase in urbanization is the cause of the generation of a large amount of waste, particularly MSW (municipal solid waste). The thermal treatment of municipal or industrial waste and sludge, as well as medical or industrial hazardous waste, decreases trash disposal in landfill sites dramatically. The produced energy yields a new revenue stream that helps both the local population and the environment through cleaner air, water, and soil. The process starts with collection, followed by segregation, then processing through various stages, such as pyrolysis, and then incineration to produce electricity. Taking into consideration the United Nations' Sustainability Goal of cleaner energy, waste is counted as one of many effective sources of conversion to electricity [23,24]. The different processes, which can be followed to convert the waste, are presented in Figure 5.

**Figure 5.** Different processes of waste-to-energy.
