**1. Introduction**

The vehicle-to-grid (V2G) and vehicle-to-home (V2H) technologies refer to the transfer of electricity from vehicles to the grid and the home, respectively. To learn how these technologies have come about, we have studied some of the background literature on the topic. In recent years, because of the newly emerging needs parallel to developing technology and industrialization, the increasing awareness of global energy consumption, global warming, and environmental issues has in many countries forced authorities to make decisions about greenhouse gas emissions. In addition, the constant increase in crude oil prices has led societies to look for alternative fuel types and seek ways to rescue them from oil dependence [1]. Hydrogen fuel cells are a solid option for the future of fuels. Hydrogen is being seen opportunistically by many gas and water companies in Germany for its economic potential in gas-grid networks, facilitating what is known as power-to-gas [2]. Although the hydrogen economy seems propitious, hydrogen itself causes some unresolved problems. Fuel cells are expensive and require a completely new distribution network. Moreover, hydrogen storage also causes certain difficulties, as it is an explosive and a sensitive gas that further limits the large-scale prevention due to its low density [3]. Hydrogen is not found in pure form such as with oil or coal. Thus, hydrogen is bound to another element in nature. For example, water is obtained from the chemical reaction of hydrogen with oxygen, aided by a spark of energy. For this reason, pure hydrogen must be produced in a way that requires energy, such as gasoline. Because of the high energy losses within a hydrogen economy, the synthetic energy carrier cannot compete with electric energy [4]. Thus, electric energy may be considered

as an alternative to conventional fuels. There are both advantages and disadvantages of electricity when it is stored. However, electricity is the most exceptional technology for the future because its infrastructure is ready and its technology is considered to be safe and reliable technology. Moreover, constant increasing fuel prices and emerging environmental awareness have forced societies to look for alternative transport solutions [5]. In the past, electric vehicles (EV) were not a strong alternative for internal-combustion engines (ICE) because of their weak batteries. However, recently batteries and EVs have been developed that have become important alternatives for conventional vehicles. Although battery prices are high and driving ranges are lower compared to conventional vehicles, EVs offer many direct and indirect benefits for society. There are many electricity transportation projects running all over the world, and EV technology and its trade are developing rapidly [6]. For this reason, it is extremely important to act in line with this technology and to obtain further innovations and concept developments [7].

Currently, transportation infrastructure consumes 26% of the annually produced energy; almost all of these energies are from fossil fuels. A total of 11% of the CO2 emission of the world is caused by individual use on roads. The current CO2 concentration that exists in the atmosphere is 286 ppm, and it is increasing at a rate of 2 ppm each year. Under normal conditions, it is predicted that it might be 700 ppm in some regions by 2100 [8]. It is thus of crucial importance that radical changes be made in the usage of fossil fuels to lower CO2 emissions in the atmosphere, otherwise the earth will soon perish. The existing manufacturers must enforce new solutions to ensure efficient fuel consumption with legal restrictions on CO2 emissions. The most important factor that will support these efforts is the widespread use of EVs for individual use [9]. Mass shifting to EVs, although a time-staking initiative, will significantly reduce CO2 emissions, also having numerous added benefits.

EVs store large amounts of energy in a reliable form. Classic electric vehicles may be considered a form of load that charges and uses the batteries from the electricity grid, and charging is a one-way process from the grid to the vehicle [10]. The rate of using electric vehicles increases with each passing day and constitutes an additional burden on the existing grid. In this case, there is a need for extra investment in the infrastructure, and costs are quite high. At this point, the idea of using EVs as an energy source has come to the agenda. In this way, additional investment will be avoided in the existing system [11]. Today, electric vehicles transfer energy to the grid when the vehicle will not be used for a long time or when the electricity sales price is at its highest. This technology is called V2G. Users can charge the battery when energy prices are low and provide both the current energy demand of the grid and gain financial benefits. In the literature, this process is named grid-to-vehicle (G2V) technology. If the energy of the electric vehicle battery is supplied to individual houses instead of the grid, it is called as the V2H technology [12]. A general operation mode of the G2V, V2G, and V2H technologies are given in Figure 1.

**Figure 1.** Operating modes of the bi-directional electric vehicle (EV) charger. (**a**) grid-to-vehicle (G2V) mode; (**b**) vehicle-to-grid (V2G) mode; (**c**) vehicle-to-home (V2H) mode [12].

Because V2G and V2H technologies are the fastest methods of meeting the increasing energy demand and can make an investment in the existing infrastructure, internationally recognized standards have been established. Thanks to these standards, a common language has been created among countries [13].

With the use of EVs together with renewable energy sources, the micro-grid structure is diversified in terms of relevant energy sources [14]. In this respect, as illustrated in Figure 2, the vertical bold line represents the grid, to which several power converters are connected, which converts the AC power of the grid into AC or DC of suitable magnitude for a device or machine connected to it, such as solar array, windmill, vehicles, uninterrupted power supplies (UPS), motor, lighting, appliances, generator, or FACTS devices. The figure shows that EVs are used as an energy source as well as a load when the user desires. This has also established a dynamic grid structure that ensures energy continuity [15].

**Figure 2.** A general system structure for microgrid.

In this study, research in the literature and application types and standards have been investigated in order to operate EVs interactively with the grid and with houses. The grid-interactive bidirectional charging topologies are examined, and the advantages and disadvantages of these topologies are determined. Moreover, the system components are examined, and information on these components are given in detail. The rest of the sections are arranged as follows: Section 2 provides an overview of and introduction to the vehicle-to-X (V2X) technology in general, along with specific description of the V2G and V2H technologies. Section 3 outlines the many different charging topologies of the batteries of the EVs used in V2X technology. Section 4 contains an introduction to the communication standards used in V2X technologies, along with specifications of several such standards. Section 5 is composed of an elaborate discussion about the different types of AC/DC and DC/DC power converters, along with a comparative view of the types. This section is also embellished with a SWOT analysis of the V2G and V2H technologies. Next, the outcomes of this paper is discussed in Section 6. Finally, the conclusions are drawn in Section 7.
