1 Introduction
The German energy and automotive sector is changing. Since the beginning of the 2000s, more and more stringent regulations, guidelines and limits for the protection of the environment and the climate with regard to the energy and transport sector have been introduced and enforced by the government and regulation authorities.
These include especially EU directives and limits on air pollution, for instance for fine dust and NO
x (frequently exceeded by many cities and municipalities in Germany) and the limitation of the permissible CO
2 emissions from the passenger car mix of an Automobile OEM of 95 g/km from 2020 onwards [
1]. Moreover the commitment to the objectives of the Paris Climate Conference in 2015 shall lead to a reduction of up to 95% of the German CO
2 emissions (based on the emissions of 1990) until 2050, if the 1.5°C objective (permissible limit for the global warming until 2050) is to be met [
2,
3].
As the transport sector is responsible for about 18% on the overall CO
2 emissions in Germany, this has to be converted into a sector that is based on strictly emission-reduced technologies. The share of the energy sector is about 40% of the anthropogenic CO
2 emissions in Germany, because as much of the electrical energy supply is still based and dependent on fossil fuels like lignite, hard coal and natural gas [
4].
Nevertheless in the last few years, especially powered by regulative measures (Renewable Energy Act (EEG), German Energy Act (EnWG) etc.), the share of renewable energies in the gross electricity generation has increased to around 30% in 2015. At the national level and with the view on the development of renewable energy technologies, there is a political commitment to drastically reduce CO2 emissions.
With the “Action Program for Climate Protection 2020” adopted by the German federal government by decision of December 3, 2014, the government reaffirmed that from 2020 onwards 40% and from 2050 onwards 80% to 95% less greenhouse gases are allowed to be emitted by the national economy in comparison to the 1990 reference year [
5].
2 Development of E-mobility in Germany
At the beginning of 2016, the total number of cars in Germany was 45.1 million (the density corresponds to 672 cars per 1000 residents), of which there were 25502 electric vehicles (BEV, PHEV, and REEV), which conformed to a share of 0.057% of the total number of cars. Figure 1 shows the development of the electric vehicle fleet, which has been rising since 2010 [
6].
The declared objective of the German federal government is to rise this stock electric vehicles to 1 million by 2020 and thus to contribute to the reduction of CO
2 emissions in the transport sector. This demand is supported by various measures (discount to retail prices, tax reductions etc.). The changeover of the vehicle fleet is assessed as necessary for the achievement of the set climate and environmental objectives [
7]. However emissions also occur during the charging of the German electricity mix. Unlike in the case of internal combustion engines, these emissions (CO
2, NO
x, fine dust, noise etc.) occur not locally at the vehicle operation, but at the fossil-fueled power plants which supply the required charging energy. These have, for example, specific CO
2, NO
x and fine dust emissions, which are minimal in the use of RES or nuclear power compared to fossil energies. If an electric vehicle is charged with the electricity generated by the German electricity mix, which still contains considerable proportions of fossil energies, the CO
2 emissions will amount to approximately 120–171 g/km for a small electric vehicle. This corresponds roughly to the CO
2 emissions of a conventional SUV, which is significantly more powerful, larger and heavier [
8].
Thus the coupling of E-mobility and RES is necessary in Germany (all nuclear power plants will be shut off in 2022 as a result of a lack of popularity), in view of the achievement of the national climate objectives economically meaningful. Whereby, the possibility of reducing CO2 emissions is, at the same time, ensuring individual mobility.
From an electro technical point of view, the development and rising role of E-mobility for the grid operation is strongly ambivalent. It can help to integrate RES better, relieve grid capacities and complete smart grids as a flexible actor, provided that the electric vehicles are capable of communicating to the grid and the charging processes are smart [
9]. On the other hand, especially in the case of dumb charging (external uncontrolled) of electric vehicles without further restrictions, the charging begins immediately after the charging cable is connected with the maximum permissible power, until the battery is full or the user interrupts the charge. This can be useful in some usage scenarios, when electric vehicles have to be charged as quickly as possible (e.g., on highways) and the electric grid is designed accordingly. In the case of the local simultaneity of several unplanned dumb charging processes in limited grid areas, the single charging powers add to a huge sum power, which can lead to thermal overloads of local grid transformers and lines [
10], voltage drops [
11], phase imbalances [
12] and harmonics [
13]. This clearly shows that the grid integration of electric vehicles can affect the supply quality and safety of the electrical power supply.
3 Development of renewable energies and effects on distribution grids
Since the adoption of the EEG, RES have developed rapidly in Germany and have gradually increased their share on the overall electricity generation. In 2015, around 30% of the gross electricity generation came from RES [
14]. Especially in the east and north of Germany, the construction of the RES installations (good weather-related, space-conditioned and economic conditions) has been particularly advanced over the past few years, focusing on wind turbines and large PV plants. This expansion took place in a completely decoupled way from the actual regional electricity demand, resulting in extreme imbalances between regional supply and regional demand of electricity, leading to renewable surpluses, which have to be transported to regions of demand via the distribution and transmission grids. Since the electric grids were built in the years from the first part of the 20th century to the turn of the millennium, especially in terms of the grid integration of large central conventional power plants and the transmission of the here generated energy (top down of 380 kV level), the grid operation become more and more difficult as the large number of RES installations are connected mainly to the medium voltage level and feed into the high-voltage grid in the fully-supplied grid areas (bottom up to 380 kV level). The north-east of Germany has developed into one of the most challenging regions in Europe regarding the grid integration of RES and the supply-safe grid operation.
The following pattern shows the conditions in the distribution grids of the individual DSOs of this region using an indicator.
The values of the turquoise colored DSO areas of Fig. 2 indicate the relationship between the installed power from RES and the maximum consumption power in 2014 [
15]. In the entire transmission grid of the TSO, taking into consideration (without the grid of Hamburg) the average ratio of 1.8, the installed RE power in this transmission grid area is 80% higher than the maximum consumption power. Thus, enormous overcapacities have already been built up. In the respective DSO distribution grids, the relationship is different. They demonstrate the extreme imbalance between regional installed RE generation and peak load. Especially since the rather sparsely populated areas of the DSOs are still subject to a population decline, regional consumption is at least stagnant. The problem will be exacerbated by the consumer, since more surplus energies from RES are generated. The grid area of the Stromnetz, Berlin (DSO of the capital region) is a special situation, because in this million metropolis, the consumption power is particularly high and the number of installed wind turbines and PV installations is particularly low (e.g., due to lack of space).
Since wind turbines only generate electricity when wind is blowing and PV installations only generate when the sun is shining, the ratios of the incident energy is focused on. Figure 3 shows the enormous increase in the share of RES in the electricity demand demanded by end users in the grid area of the DSO Mitnetz Strom [
16].
By 2015, the share has increased to 85%, so that in 2016, but not later than 2017, it can be expected that the DSO area will be fully supplied with RE. This state is planned for Germany not until 2050 and shows the dimension of the relevance of the problems which have to be solved already today by these DSO. However, it must be emphasized that this is a purely balance-based representation of the energetic states (E/Wh) and it is absolutely doubtful that RE power and consumption power (P/W) are at equilibrium at all times in this grid. On the contrary, regenerative generation fluctuates stochastically and volatile irrespective of the amount of electricity demanded, thus posing a major challenge to grid operation. Figure 4 is an indicator of these negative effects of grid integration of RES [
17].
Figure 4 shows implemented enforcement measures in the distribution and transmission grid by the responsible TSO 50Hertz Transmission GmbH, which results from the application of § 13 EnWG. Measures pursuant to § 13 sub. 1 EnWG are grid- or market-related, e.g., optimization of the grid capacity or redispatch. Measures pursuant to § 13 sub. 2 EnWG are forced intervention by feed-in management, whereby conventional power plants or RES can be throttled or shut off locally in the entire transmission grid. Because the interventions are contrary to the willingness of the operator, in the case of a loss of revenue, the operator of the power plant or RES is monetary compensated under certain conditions. The accessibility according to subparagraphs 1 and 2 are cascaded in dependence on the severity of the grid disturbance. Figure 4 shows that the number of measures mentioned has increased for several years with the increase of RES in the grid, and that the TSO are forced to take action on almost every day of the year in order to ensure safe grid operation.
To illustrate the problem, Fig. 5 shows the typical power performance of a 20 kV/110 kV transformer in one of the relevant distribution grids.
The distribution grid considered is located in a region with low load (some small municipalities) and a high number of installed RES. The performance over half a year shows almost exclusively negative values, which means that the power flow is directed toward the 110 kV distribution grid. Therefore, the grid area acts as a generator. Only in very few time intervals does the 20 kV grid draw energy from the superimposed 110 kV grid and acts as a consumer (positive values). The load in the underlying 20 kV grid is by no means sufficient to consume the generation power on-site. Thus, although the grid section is quasi regeneratively fully supplied, the generation power fluctuates strongly, and the excess energy is transported over the superimposed 110 kV or even higher over the 380 kV grids (Fig. 5(a)). The fluctuation of the power transfer even in the process of one day is illustrated in Fig. 5(b). In addition to a highly volatile power generation and high power gradients, a large amount of energy is fed into the superimposed distribution grids throughout this day. To maintain grid stability, the imbalance can only be countered with the transport, storage and throttling of the surplus RE power. Nevertheless, the problem with each new installed RES will exacerbate. The transport is limited by the available line and grid capacities, which are only slowly expanded by the grid expansion and, in sum, do not meet the faster growing requirements due to the construction of additional RES installations. Large-scale electrochemical storage of surplus energy is not possible at the present time, as large-scale economic technologies (sufficient power and energy capacity) are still missing. Due to the long planning horizon, the expected economic risks and social resistance, the construction of a sufficient number of pumped-storage power plants is not very realistic. Despite these current challenges, solutions have to be found in order to reach the postulated objectives of the German Energiewende and the Paris climate protection targets.
To cope better with the increasingly difficult grid operation, which are resulting due to the fluctuating generation of RES, electric vehicles can be integrated as flexible loads or mobile storages within smart grids (electricity grids with high penetration of ICT). They can help to react quickly to changes in grid conditions or supply conditions, to compensate power fluctuations in the grid very quickly, and thus to provide security for supply. Due to the fast-available and fast-reacting lithium-ion battery technology, electric vehicles offer an excellent possibility to support the change in the electrical power supply with a very high share of RES.
4 Charging infrastructure and electric vehicles as a functional load and storage network
In Germany, the penetration of the public space with charging infrastructure can be characterized as restrained in the year 2016. By mid-2016 about 6517 publicly available charging points were available [
18]. The reason for the currently relentless expansion of the charging infrastructure is mainly the lack of commercial business and investment models for charging infrastructures as well as a lack of technological planning security regarding plug-in types, charging types (AC, DC) and charging power capacities. Although many projects have been and still are being done on billing and payment systems, the diversity of alternatives such as radio frequency identification (RFID), smart phone apps, cable-based plug and charge, debit/credit card payment, near field communication (NFC), parking machines, cash payment and the lack of an over-arching standardization with regard to an E-mobility payment platform (roaming) amplifies the investors’ planning uncertainty. In addition, the figures of the stocktaking record should not veil the fact that it is not an area-covering uniformly distributed equipment with charging points. Rather, the charging points are concentrated in conurbations, subsidized project sites and limited regions (Fig. 6) [
18].
Due to the relatively low charging or discharging power of single electric vehicles in the charging modes 1 to 3, which are limited to a maximum of a few 10 kVA, load management in the context of the distribution networks only makes sense when several charging processes are aggregated. As a result of the summation, the available maximum power of such a network increases correspondingly and can reach very quickly several 100 kVA to several MVA already in locally limited grid areas. This also applies to the usable energy capacity (kWh, MWh) of the formed mobile storage. Besides, according to Ref. [
19], it is because of the aggregation and load control capability of several connected electric vehicles that their participation in the electricity market is significant.
The so-called aggregators (SCADA systems for charging infrastructure networks) summarize the load management resources of the single electric vehicles and could be optimally used by the operator via ICT-supported management functions in addition to other generation, storage or consumption units, e.g., in pooled networks.
A serious problem has to be addressed in the field of load management—the risk of imbalance, which occurs in relation to E-mobility, when the charging process of an electric vehicle significantly occupies single conductors of the three-phase system more than the others. This compromises the symmetric load on the grid. In Germany, the maximum permissible difference of load of a single conductor to the other two conductors in the low-voltage network is only 4.6 kVA (20 A), which have already been achieved by the charging process of one electric vehicle. The automobile OEMs would, therefore, be obliged to ensure that the vehicles are equipped with three-phase charging devices which are symmetric to the power supply grid. This has just recently been taken into account by upgrading of current models (e.g., BMW i3, Renault Zoe). What also relates to the fact that the requirements for the absolute level of the necessary charging power increase in order to keep the charging time of bigger battery systems as short as possible.
5 Coupling of E-mobility and RES in medium and high voltage grids
Whenever the sum power of several AC and DC charging points is increasing with high availability of connected and charging electric vehicles, their effects can be detected and measured in the medium and high voltage grids. The situation in the charging station pool (CSP) on the central campus of the Brandenburg Technical University Cottbus-Senftenberg (BTU) is exemplified.
The CSP has a maximum total capacity of 110 kVA (limited by a switchgear), but a connection power of about 330 kVA, which results from the 15 AC charging points with a respective maximum permissible supply power of 22 kVA each. The existing bottleneck of 110 kVA in these grids as well as all power flows are monitored by a self-developed intelligent energy management system. According to these and other conditions, the charging processes are fully automatically controlled. Figure 7(a) clearly shows the effect of charging processes of electric vehicles on the grids. The fact that the vehicles are used by BTU staff tempts to load them mainly in the CSP on the campus. This is evidenced by a sharp increase in the total consumption power, since most of the electric vehicles arrive and charge from 07:00 am to 09:30 am onwards. Given a hypothetical view of 15 DC charging points with a connection capacity of 50 kVA each and electric vehicles with battery sizes of 85 kWh (and 50 kVA permissible charging power), a similar profile with a peak power of up to 375 kVA would be the result (with only 50% availability). The data shows that even in small fleet networks of about 10 vehicles, the expected total charge load can be very fast in high kW and low MW ranges.
To use the vehicles rationally in the concepts P2V and V2G, a further variable value is of even greater importance to the specific user, vehicle, operator and grid data. Figure 7(b) depicts the average monthly availability of the vehicles in the CSP. The availability, i.e., the energetic and communicative connection of the electric vehicles with the existing charging infrastructure or the electrical grid, is the basic essential requirement for the use of electric vehicles as a flexible load or mobile storage in a grid area. For this reason, the availability must be recorded and evaluated as precisely as possible in order to be able to make forecasts about future availability and thus load behavior. The data shows that in that case, up to almost 50% of the existing 15 charging stations are occupied by connected electric vehicles, and a relatively stable and high availability (important characteristic of availability for safe usage of electric vehicles as flexible load and mobile storage) can only be expected at certain times (e.g., at high noon until afternoon). At night and in the morning, the availability is close to zero, since the users, of course, using the vehicles for their home journey.
Future smart grids, which are required and necessary in the course of the German energy transition, have to use these and other information in order to selectively manage surpluses respectively shortages from RES or (grid) bottlenecks in certain time segments of a day. Figure 8 shows how such a coupling of E-mobility and RES can look like.
Figure 8(a) shows the possible principle of the energetic coupling of E-mobility and RES on the 110 kV high voltage level, with separated networks of spatial wide distributed CSPs and RES on the low voltage and medium voltage level. The black arrows indicate the directions of the power flow, so RES are, basically, just generating units. CSPs and RES are connected separately via their own transformers to the 20 kV grid. Relevant data from the E-mobility provider (E-mobility SCADA: demand forecast, predicted availability profiles, load management and storage capacities, actual data etc.) and RES operator (RES SCADA: supply forecast, actual data etc.) will be transferred. And these data allows the grid operator (DSO SCADA: transfer capacities on the transformers, line and equipment utilization etc.) to send control or demand signals, which will be formulated according to the state information from its medium and high voltage grids. This would enable the DSO to optimize its medium and high voltage grid operation by the grid integration of the CSP.
Similar effects can be demonstrated by the principle in Fig. 8(b). In this case, however, CSPs, RES and also stationary storages as a third flexible actor (e.g. electrochemical short-term storages) are already energetically connected to the low voltage level, because they are spatially near distributed. Under consideration that the communication is cross-linked, which, in this case, combines E-mobility, RES, and stationary storages (RES SCADA: supply forecast, predicted availability profiles, load management and storage capacities, actual data etc.), the power flow or grid operation can already be optimized in the low voltage level. The optimization also depends on the specifications of the DSO (DSO SCADA) and, thus, directly affects the distribution grid operation. Principally, locally generated energy from RES has to be used in this way by locally available CSPs or stationary storages in order to improve the grid operation on all depending voltage levels.
6 Conclusions
As shown, the electrical power supply is part of a transformation process, which is articulated in Germany by the slogan “Energiewende” in the course of the development of the decentralized RES and the replacing of central conventional power plant capacities. For years, this process has been clearly felt by the grid operators. In addition to the opportunities of this development for climate and environmental protection, there are also considerable challenges regarding the system integration of RES. Distribution grids in the north and east of Germany are fully-supplied or already over-supplied by the annual generated renewable energy. The DSO grids act as generators in the high voltage grids, for which they have not been designed in the last decades. This is, on the one hand, due to the partly poorly populated and less industrialized counties, and, on the other hand, by the extreme expansion of RES installations. These two effects intensify each other and the DSO is in this situation in the duty to distribute the strongly increased RE shares.
Parallel to the German energy transition, since a few years have also seen the recognition that transport sector has to be changed. A “mobility transition” could call this transformation process, which should lead to a politically driven replacing of the internal combustion engine in the automotive industry. The changeover to battery electric vehicles should help to avoid successive emissions in the transport sector in order to meet the objectives of German, European, and international climate and environmental protection agreements. However, the grid integration of necessary charging infrastructures and electric vehicles has to be made smart. Dumb charging processes (extern uncontrolled) can threaten the safe grid operation and thus endanger the security of supply. In addition, emissions from the operation of the vehicles are only avoided if the electricity comes from RES, which themselves do not cause any emissions such as CO2, NOx and fine dust on site.
The sectoral coupling of electrical power supply (especially RES) and mobility offers the opportunity to make grid operation more flexible on the part of consumers. This makes it possible to react more flexibly to unplanned fluctuations in the RE supply as well as to optimize the charging of the traction batteries from RES. As a result, the intelligent grid integration of P2V and V2G concepts of E-mobility creates a win-win situation. On the one hand, the further expansion of RES is supported because its effects on the grids are reduced; on the other hand, the coupling reduces very clearly resulting emissions of the transport sector. In total, this helps to achieve the objectives of climate and environmental protection.
In addition to P2V and V2G, there are other technology concepts such as P2H, P2C, and P2G. These approaches are also expected to drive the system integration of RES by sector coupling (heat, cold, gas). Further research and development is also desirable in order to achieve a holistic transition of the energy supply, which is primarily based on RE.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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