The increasing concern about climate change has incited the European Commission to set up a European Green Deal. The Green Deal provides an action plan to make the economy more sustainable—e.g., by the efficient usage of resources and moving to a clean and circular economy. The key target for 2030 is to achieve a share of renewable energy sources (RES) of at least 32% and an improvement in the energy efficiency by at least 32.5% [1
]. It is obvious that this will lead to an energy landscape which will be highly dominated by wind and photovoltaic (PV) energy. Today, many residential and industrial distribution grid users have already installed PV on their roof. However, at the moment more than 90% of usable rooftops all over Europe are still unused [2
The increasing penetration of PV into the distribution grid can lead to increasing congestion. This congestion could be translated towards higher energy losses and impact on power quality aspects, such as voltage rise and voltage unbalances, which constrain the PV hosting capacity. The many single-phase connected PV systems are causing unbalanced voltage variations at each phase, resulting in power quality issues in the connected appliances [3
]. Concrete cases are already established in certain regions in Germany, where, due to the increase in RES, voltage and capacity issues are occurring [4
]. Those problems can be avoided by implementing flexible solutions, such as demand response, battery storage, or a combination of them. A more radical solution is to reinforce the grid. It is the task of the Distribution Network Operator (DNO) to face this challenge and weigh up the cost for flexibility and grid reinforcement [5
]. The study [6
] investigated the impact of renewable energy sources in the European distribution grid by 2050. It was found that in general, for distribution grids with a share of RES higher than 60%, grid reinforcement is more cost-effective. Additionally, the expected massive shift from internal combustion engine-based vehicles to electrical vehicles (EV) exposes the distribution grid as well to many power quality-related issues. Similar to the issues related to the increasing PV penetration, this causes the necessity to primarily fully exploit the flexibility potential and, if absolutely necessary, reinforce the grid [7
Low-voltage DC (LVDC) distribution systems are gaining more and more attention due to their many benefits. In [8
], the power transfer capability was analysed and it was found that, with the same cable, a 1.28 times greater power could be transported by DC compared to the same system using AC. It is important to mention that for AC a traditional three-phase 400 V AC system was considered, while for DC a unipolar 800 V DC or a bipolar ±400 V DC system was used. Given that, for AC, a four-wire cable was considered, the power transfer capability could even be 2.56 when using two wires in DC in parallel for the positive and zero pole. A higher DC voltage would further increase this benefit. Consequently, for a unipolar DC system with two wires the maximal power can be transferred over a cable length that is 1.80 times longer without violating the maximal voltage drop.
As many household appliances such as EV chargers as well as distributed energy resources (DER) operate on DC, the converter stage could be eliminated. Converters contain semiconductors and capacitors, which are prone to failure. Consequently, the reliability and lifetime of these systems will increase. Secondly, due to the elimination of the conversion stage the system will be more efficient [9
]. In [10
], a loss analysis was conducted on a case with 20 residential houses with a community PV system and a gas engine co-generation unit. The comparison of the losses was carried out with these assets connected to a distribution system operating on 200 V AC and a system operating on ±200 V DC. The whole losses of the DC system are around 15% lower than those of the LVAC system. The same analysis [11
] was conducted but only with a community PV system and an apartment with 20 residential units, operating on the same AC and DC voltage. The most interesting result of this analysis is the fact that an improvement in the efficiency is only achievable for PV installations larger than 11 kW, which produce around 12% of the consumption on a yearly basis.
Notwithstanding all these advantages, LVDC systems still have to face many challenges. Firstly, there is a lack of commercially available products such as converters and protection devices. Protection devices designed for AC rely on the natural zero-crossing of the AC current. For LVDC systems, effective protection could be achieved by using electronic-based protection devices, which are more expensive [12
Today, almost no DC-compatible household appliances exist. It is obvious that a complete shift to LVDC will need investments and will take considerable time. Secondly, for a smoother development of those products, standardisation is needed on, for instance, the voltage level. The maximum voltage applicable for LVDC systems is 1500 V, as specified in the IEC 60038 [13
]. It should be mentioned that there is no single voltage that might fit all devices, as it often depends on the application. The optimal voltage level could be optimised by a trade-off of the following system parameters: efficiency, cost, and safety [14
]. In [15
], a voltage of ±350 V is proposed for high loads, such as electric vehicles or PV installations. At the same time, this voltage could be used to connect household appliances, as the conversion losses will be lower due to the low voltage difference compared to the AC amplitude voltage of 325 V.
As has already been mentioned, a radical shift to LVDC in the distribution system and the residential load will take much time and investment. A hybrid structure is today more feasible and could pave the way for a gradual transition [16
]. In this article, a grid architecture is presented where the load is connected on a traditional low-voltage AC (LVAC) system while the PV installations and battery energy storage system (BESS) are connected to an LVDC backbone. This architecture is especially applicable in urban areas with clusters of town houses or apartments. Instead of having a PV installation for every individual grid user with a DC/AC inverter each, one larger PV installation is applied on the roofs for every building cluster and connected on the LVDC backbone. The same applies for the BESS, as a larger BESS is connected to the LVDC backbone instead of multiple small BESSs. The LVDC backbone is connected to the point of common coupling (PCC) of the LVAC system via one large DC/AC inverter. Hence, the PV and BESS connected to the LVDC backbone could be considered as a shared asset to provide the aggregated load demand on the AC side.
By aggregating residential loads, a larger share of the total load could be covered by the produced PV energy—i.e., the self-sufficiency index (SSI) increases. At the same time, this leads to a minimal storage need for the same SSI [17
]. In [18
] the benefit of shared PV and BESS compared to individual PV and BESS is evaluated based on the historical load data of 21 houses. This was achieved by analysing the share of the produced energy that is directly consumed by the individual or aggregated load—i.e., the self-consumption index (SCI). The SCI increases by 15 percent points when the PV energy is shared compared to in individual PV installations. When BESS and PV are shared, the SCI increases by 11 percent points compared to individual BESS and PV. The study [19
] demonstrated by a very in-depth analysis, based on a set of distribution networks containing different realistic communities, the benefit of community BESS. It was found that a community BESS could be sized 35% smaller than an individual BESS for the same SSI. A shared PV installation can be very interesting for apartment residents. The large roof areas can offer economies of scale and the aggregation of the different residential loads leads to an increase in the SCI. An Australian study [20
] showed that the SCI could increase by up to 30% when aggregating 25 apartment residents. The higher the aggregation level, the higher the increase in SCI compared to the SCI for an individual grid user. However, the study also also showed that, from a certain aggregation level onwards, the SCI starts to saturate, which means that the benefit of aggregation decreases.
A higher aggregation level leads to a higher load factor, which could be explained by the fact that the baseload increases proportionally but the peak demand increases non-proportionally due to the non-synchronity of the peaks. Moreover, the six parameters describing the load duration curve of an aggregated load demand show a denser distribution as the aggregation level increases. This would make the demand more predictable and so improve the flexibility potential of the community [21
]. The aggregation of demand and sharing of generation and storage are activities that are stimulated by the European Union (EU), aiming at incentivising citizens in order to get them involved in the energy market. The EU presented in 2019 in their Clean Energy Package two new actors: Renewable Energy Communities (RECs) and Citizen Energy Communities (CECs) [24
]. It is not within the scope of this article to discuss the legal aspects of these new actors more in detail. However, it is important to notice the evolution to a more collective approach in the energy system, both in the legal framework and in the rising number of pilot projects in that context [25
]. The aim of this study is to investigate the benefits of an LVDC backbone in a community compared to a traditional AC grid. The possible benefits could be divided into the following aspects:
Conversion and cable losses: As has already been discussed, in an LVDC backbone grid architecture the number of converters is highly reduced. However, the number of conversion stages is the same compared to a traditional AC grid. The multiple small DC/AC inverters are replaced by one common inverter. The comparison of both will be performed based on converter loss models. Secondly, the cable losses will be analysed for both architectures. In a traditional AC grid, the produced and stored energy can be directly consumed by the grid user, while for an architecture with an LVDC backbone additional cable losses occur on the DC side. Nevertheless, the losses can be reduced due to the higher operating DC voltage and the reduced unbalance.
SCI and SSI: It has been shown in various other works that the aggregation of demand and sharing of PV and BESS increases the SCI and SSI. In this work, this will be analysed taking into account the cable and converter losses that occur. This will be carried out for three situations: a traditional AC architecture with individual PV and BESS (i) without energy sharing and (ii) with energy sharing, as well as (iii) an LVDC backbone architecture with energy sharing.
Savings in assets: The savings in PV and BESS are here analysed for an LVDC backbone compared to a traditional AC grid with individual PV and BESS and the possibility to share energy. The savings will be calculated assuming, as a starting point, that the same benefits in terms of SSI should be achieved as with a traditional AC architecture with energy sharing. The savings in assets could be translated into a saving of materials and therefore an environmental benefit.
In Section 2
, the analysed grid architectures as well as the case study are presented. The consumption data and the solar irradiance data used in the study and the applied methodology for calculating the power losses and the SCI and SSI are discussed in Section 3
. The LVDC backbone voltage applied in the study is determined based on an efficiency optimisation in Section 4
. In Section 5
, the final results are presented and discussed. Finally, further research aspects and conclusions are formulated in Section 6
and Section 7
4. Optimal Voltage Level
For this study, the determination of the optimal voltage level is confined to the efficiency aspect. A holistic approach taking into account, e.g., the safety aspect or the economic aspect, for the determination of the optimal voltage is certainly a requirement in the light of the development of such grid architectures in the near future as well as the establishment of the according standards. Moreover, as already discussed, the efficiency optimal backbone voltage depends strongly on the system voltages. This implicates that a case-by-case approach is required therefore, or at least a classification in function of the system scale. However, the system voltages selected in this study are surely representative for grid systems of this order of magnitude. For the median aggregated load of 107.9 MWh and the installed PV power amounts 107.9 kWp and the BESS capacity amounts 107.9 kWh.
The voltage is directly related to the power transfer capability; the higher the voltage, the more power can be transferred with the same cable. Inversely, when the power is considered constant, a higher voltage leads to a lower current and a smaller required cable section. The cable section is sized according to the maximum cable ampacity delivered by the manufacturer and shown in Appendix A.4
with the additional condition that the voltage in the network does not exceed the imposed voltage limit. Due to lack of international or national standards, this voltage limit is based on the Finish SFS-6000 standard which limits the DC voltage to the range –25% to +10% [53
]. Secondly, this voltage limit is assumed as a compromise to minimise the conversion losses whilst installing an affordable and, from practical point of view, a manageable cable.
In Figure 9
a, the median of the losses at DC-side is plotted in function of the backbone voltage and the total cable length for the presented case. The length of the cable between the different connections is in proportion with the cable lengths given in Table 1
. The analysis is limited to voltages within the upper limit level of 1500 V imposed by the IEC 60038 standard for LVDC [54
]. A margin of 100 V has been applied for eventual voltage rise. It is obvious that a low voltage leads to considerable higher conduction losses. As the voltage increases the conversion losses are manifesting to a higher extent, which can be explained by the increasing voltage ratio. Although, for longer cables the cable losses are manifesting on a higher degree. This is clearly perceptible in Figure 9
b, while the optimal voltage amounts 700V for cable lengths up to 525 m, for longer cable lengths the optimal voltage amounts 1200 V. For a cable length of 400 m, the optimal voltage is 700 V and the losses are almost 4.5% of the total produced or discharged energy.
6. Further Investigation
It has been proven that the LVDC backbone architecture has many benefits, especially in terms of SSI and conversion efficiency. This grid architecture has the potential to be implemented in existing grids where the distribution cable is highly congested and a reinforcement is needed. A second possible application are new districts where the penetration of PV is expected to be high. Furthermore, the LVDC backbone could also be a way to optimise the electrical vehicle (EV) infrastructure integration in the low voltage distribution grid. Advantages of EV integration on DC are inter alia simplified control and better efficiency [55
]. These advantages could be quantified during later investigations.
An important parameter related to the efficiency is the voltage level. This has been considered as a constant 700 V in order to obtain maximal efficiency. However, a dynamic voltage level could possibly be more efficient given the inherent voltage variations of the PV installations and BESSs. In a future work a voltage optimisation control will be set up and the dynamic voltage solutions will be studied. More generally, the optimal voltage level should also be assessed to following aspects: safety, reliability and cost. After all, the usefulness of a bipolar LVDC backbone will be investigated which could be interesting when having multiple PV installations of different scale.
One of the main differences between the AC grid architecture and the LVDC backbone architecture is the number of converters. For an LVDC backbone the number of converters can be drastically reduced. Moreover, it is found that the nominal power of the converters is not proportional to its weight, which means that a larger system will have a lower specific weight than a small system (Figure 13
a). As a consequence, this grid architecture contributes to the reduction in the material footprint which is promoted by the European Commission, notwithstanding the need of an additional cable [57
]. The latter has to be compared with the material footprint for a reinforced AC grid with the same capabilities in order to obtain a fair comparison. Furthermore, the cable will be exposed to its maximal ampacity only during a limited number of hours a year, i.e., when the PV production is maximal in the summer. This means that the cable could eventually be sized smaller provided that the emergency ampacity would not be exceeded. Additionally, the material needed for grid connection of PV and BESS as well as the protection devices for both architectures should be compared. Based on these aspects a complete analysis could be made assessing the potential material reduction [58
Additionally, the orientation and tilt angle of the PV installation is in this study considered the same for the whole installation. The study [59
] showed that for individual consumers a benefit in terms of SSI is possible as well as lower cable losses due to lower peak power. However, it should be investigated whether a multi-oriented aggregated PV installations in a community could lead to a further increase in the SSI and a decrease in the losses in the LVDC backbone.
Finally, an economic analysis will provide insight into the feasibility of such a grid architecture, which is important for, e.g., investors, grid operators, and energy cooperatives. Only when it is proven that the investment in the infrastructure is beneficial and reduces the cost for society compared to the traditional architecture could this system be realised. This means that a total cost of ownership analysis should be performed as well as a payback analysis. These analyses should include the cost for grid connection, the system balance cost (e.g., PV modules, converters, mounting frames, and cables) as well as the labour cost. Figure 13
b illustrates that the specific cost per power unit for inverters decreases as the nominal power increases, which means that a larger system is considerably cheaper than multiple small converters [43
]. The cable investment cost constitutes mainly the cost for earthworks, such as the trenching or digging work, and the purchase cost of the cable. Based on data published by the Flemish DNO [60
], the cost for the placement of a cable is only about 10% of the total investment cost. As a consequence, the placement of an additional cablewill not constitute a substantial share of the total cost.
In this article, the benefit of a hybrid AC/DC grid architecture with an LVDC backbone for the connection of PV and BESS has been analysed. Firstly, the optimal voltage level of the LVDC backbone was investigated. It was found that for cable lengths less than 500 m, the conversion losses take the upper hand. As a consequence, the optimal voltage is equal to the nominal voltage of the PV installation and BESS. For longer cable lengths, the optimal voltage increases in order to reduce the cable losses. Due to the optimal voltage selection, the conversion losses are drastically reduced compared to a traditional AC grid. The reduction amounts to 12 percent points. Regarding the cable losses, they increased by 0.9 percent points due to the additional cable and due to the high PV power injected into the LVDC backbone. The reduction in voltage unbalances as well as the absence of PV injection in the AC grid causes the cable losses on the AC side to reduce by 0.4 percentage points.
The SCI is found to increase by 5 percent points compared to an AC grid with individual assets and 2 percent points compared to an AC grid with shared assets. The SSI increases by 12.5 percent points compared to an AC grid with individual assets and 6 percent points compared to an AC grid with shared assets. In both the SCI and SSI, the saving in losses of the LVDC backbone have an impact. These benefits can be translated to a reduced amount of energy injected to the grid and a more significant reduction in energy purchased from the grid. The increase in SSI is only achievable for grids where the PV penetration level is higher than 64%. As the penetration level increases, the benefit of an LVDC backbone increases compared to an AC grid with shared assets. Furthermore, it has been found that for an LVDC backbone without BESS, the SSI is less favourable. Only when the BESS size is larger than 0.18 kWh per kWp installed PV, does the SSI become higher.
When a constant BESS size of 1 kWh/kWp is considered, a saving of 13.5 percent of PV capacity is realisable by aggregating PV and BESS in an LVDC backbone instead of having an AC grid with shared assets. This means that the BESS size can also be reduced to the same extent. When the PV penetration level is 100%, a saving of BESS capacity of 22% is achieved. In addition to this, the number of converters is highly reduced and only one large DC/AC inverter is needed. Based on manufacturer data, it has been found that the specific weight decreases with the increasing nominal power of the inverter. These material savings as well as the economic viability of the LVDC backbone could be further investigated in future work. However, the economies of scale of energy communities coupled with the reduction in PV and BESS size for an LVDC backbone will likely lead to a shorter payback time. This, in turn, leads to a better return on investment, and the possibly of a lower energy invoice for the members of the community.
The benefit of a dynamic voltage in terms of efficiency will similarly be investigated and a dynamic voltage optimisation control will be developed. Regarding the power quality issues in the low-voltage distribution grid caused by the high PV penetration level, this is highly reduced due to (i) the segregation of load and generation storage and (ii) the aggregation of PV and BESS on DC. After all, this study showed that the LVDC backbone architecture has various benefits and can be a solution for an optimal integration of PV and BESS, aiming at an increased SSI ofor energy communities.