Towards DC Energy Efficient Homes

Abstract

The aim of this paper is to shed light on the question regarding whether the integration of an electric battery as a part of a domestic installation may increase its energy efficiency in comparison with a conventional case. When a battery is included in such an installation, two types of electrical conversion must be considered, i.e., AC/DC and DC/AC, and hence the corresponding losses due to these converters must not be forgotten when performing the analysis. The efficiency of the whole system can be increased if one of the mentioned converters is avoided or simply when its dimensioning is reduced. Possible ways to achieve this goal can be: to use electric vehicles as DC suppliers, the use of as many DC home devices as possible, and LED lighting or charging devices based on renewables. With all this in mind, several scenarios are proposed here in order to have a look at all possibilities concerning AC and DC powering. With the aim of checking these scenarios using real data, a case study is analyzed by operating with electricity consumption mean values.

1. Introduction

The targets set by the EU for decarbonization by reducing GHG by 90% compared to 1990 establish a wide need to combat global warming in each sector, being that most emissions are directly or indirectly linked to energy [1]. A household’s need for electricity is varied, and there are many daily actions with effects on electricity consumption, such as turning on lights, charging cells, using desktop PCs, and listening to the radio. All these actions directly affect the needs of the residential and industrial sectors, given that buildings are responsible for 40% of the final energy consumption and 36% of GHG in Europe [2]. In addition, energy trends are growing. Since 2010, consumption has increased by 3% per year and is expected to continue in the coming years [3].

A large amount of household losses is due to the need for rectifiers in home device because of the proliferation of electronics and the development of smart and context sensitive services, i.e., Alexa, Siri, or Aura, [4,5]. The use of DC in the home guarantees simplicity because of the reduced need for components, increasing efficiency and life cycles, and reducing the failure of these devices [6,7]. The optimization of home consumption, in combination with the elimination of or reduction in losses in the current domestic electrical system, allows the achievement of NZEH or even ZEH [8,9]. In this way, control and reduction of home electricity is one of the main pillars to achieve the sustainable development set by the United Nations, which are present in almost every sustainable development goal [10].

GHG reduction entails a transition to renewable sources, with PV being a widely used solution to reduce consumption and emissions in homes [11,12,13]. Continuous improvements in the cost and efficiency of solar panels promote energy flexibility, supplied by PV self-production, and presents one of the highest growth rates, in installed power, across the world over the last few decades [14]. PV installations on building rooftops play a key role in achieving climate goals, and a production of 25% of the actual consumption and a seven-million-ton reduction in CO₂ per year has be estimated [15].

The implementation of EVs represents one of the main opportunities to ensure a strong reduction in local pollution. In fact, one of the EU’s plans is to achieve a vehicle-emission-free logistic in large urban centers by 2030 [16]. The current restrictions on mobility in the urban centers of European cities, i.e., Barcelona, Madrid, or London, along with increasingly restrictive regulations in the production phase of a vehicle, force carmakers the need to increase EV production [17]. Based on this, presently, there are already carmakers without conventional motorization (diesel or gasoline) that aim to achieve EU objectives [18].

The growing trend of EVs in the automotive sector, along with a dependence on portable devices, i.e., phones or tablets, has been of great importance in the evolution of batteries; requiring more capacity in smaller spaces and needing to be more reliable, durable, less polluting, and cheaper [19]. HESS allows better energy consumption in homes, alleviating the variations presented in the demand curve and producing savings in electricity bills [20,21,22,23]. In fact, the European Commission emphasizes HESS as one of the key elements to achieving a neutral and prosperous economy from the point of view of climate change [24].

Electricity consumption is not constant, with most of it being in the central hours of the day, the so-called peak hours. In turn, due to the high demand for energy, the price of electricity at these hours is higher than in other periods, when the demand is lower, i.e., in the valley hours. Therefore, governments seek to encourage consumption in valley hours with attractive prices, even lower than in early morning hours, i.e., the supervalley hours, in which EV charging is sought [25].

On the other hand, the price of electricity varies according to the availability of renewable resources, and lower prices are established when generation from a renewable origin is higher [26]. Due to the need to combat global warming, the world is currently in a period of energy transition towards renewable energy sources, in which a high percentage in the final generation mix will modify the different periods mentioned above, depending on the availability of renewable energy [27].

The efficiency of an electrical network in peak hours is lower than in valley hours because of the greater need for electrical generation to meet the consumption in these periods. By means of a home management system, where HESS, PV, and EV are present, supply control is optimized by adapting demand to possible changes produced throughout the day [28,29,30,31]. If losses due to rectifiers are also reduced or eliminated, a high-performance house is the result (NZEH or ZEH). In this way, a combination of these elements makes it possible to keep consumption from an electrical network constant, increasing the level of comfort, and guaranteeing a reduction in electrical consumption and in GHG [32,33,34].

This paper is organized as follows: typical home devices are analyzed in Section 2; DC possibilities and trends are studied in Section 3; different scenarios are described in Section 4; the efficiency of each scenario is discussed in Section 5; and conclusions are outlined in Section 6.

2. Home Devices Analysis

Home devices are often AC powered. However, this means that they may run internally on DC or on AC Generally, electronic devices, such as computers, radios, or DVD players, need DC for their operation, and this makes it necessary to adjust the type of power from the grid. In fact, there are versions of these types of devices that are powered directly through DC, most of them having low energy needs, such as routers, televisions, or LEDs. A battery is an element that operates in DC, and as such portable devices usually also run on DC and sometimes use an external AC/DC converter for operation, such as laptops, vacuum cleaners, or tablets. On the other hand, electrical appliances that use a motor, such as washing machines, clothes dryers, or dishwashers, typically use AC. Furthermore, these devices can be DC compatible through the use of permanent-magnet DC motors with variable driving frequencies, more efficient motors than their AC counterparts, and there are even versions that are used in remote places powered by DC, such as extractor hoods or fridges. Others, such as coffee machines or hair dryers, have commercial DC powered versions [35,36,37].

The most representative devices are categorized in

Table 1. Analysis of different household devices.

Device	Motor	Power (W)	DC Version
Fridge/Freezer	YES 1	100	YES
Illumination	NO	7.5	YES
Dishwasher	YES 1	2400	NO
Washing Machine	YES 1	2300	NO
Clothes Dryer	YES 1	2800	NO
Telephone	NO	0.7	YES 3
Router	NO	12	YES
Printer	NO	210	YES
Laptop	NO	150	YES 3
Monitor	NO	13.3	YES
Desktop	NO	310	NO
Radio	NO	10	YES
Blue Ray	NO	8.5	YES
DVD	NO	11	YES
Game Console	NO	350	YES 3
Set-Top Box	NO	6	YES
Television	NO	183	YES
Coffee Machine	YES 2	1100	YES
Toaster	NO	1090	NO
Extractor Hood	YES 1	146	YES
Oven	NO	3600	NO
Cooktop	NO	6600	NO
Microwaves	YES 2	1270	YES
Air Conditioning	YES 1	5300	YES
Water Heater	NO	2000	YES
Heating	YES 1	5600	NO
Charger	NO	18	YES
Dehumidifier	YES 2	72	YES
Hair Dryer	YES 2	2100	YES
Hair Straightener	NO	100	NO
Iron	NO	2800	NO
Vacuum Cleaner	YES 2	2400	YES 3
Elec. Toothbrush	NO	2000	YES 3

¹ DC motor and variable frequency drives; ² DC motor; ³ it can be portable (uses a battery).

, where it has been determined whether there is a motor, what is the typical power and whether there are currently manufacturers that offer DC versions.

3. Factors for DC Installations

The barriers established in the development and innovation of DC are mainly due to ignorance of the advantages and the great possibilities that this offers. It is generally believed that there is no alternative to AC devices because they constitute the typical applications. However, this is not necessarily the case. This section shows some of the applications of DC, detailing advantages and possibilities.

3.1. Generalization of Electric Vehicles

Over the last ten years, the number of EVs sold across the world has increased considerably. HEVs have been a kind of springboard, and PHEVs are one step closer to pure EVs. Obviously, as long as EVs appear on roads, emissions may be reduced considerably, although this will be true if the origin of the energy is renewable. Nowadays, the number of EVs is very low, but, as the percentage of sales is increasing continuously, recently surpassing 10% in some countries, there are good prospects [18]. The expected trends of the EV market are shown in Figure 1.

Firstly, it has to be said that EVs convert over 77% of electrical energy received into power at the wheels, while conventional vehicles operate in the range of 12–30% [38]. This fact makes EVs more efficient than conventional vehicles, at least in terms of the process of energy conversion. Another issue to be taken into account regarding EVs is the battery cost. It is estimated that 30–40% of EV costs corresponds to the battery. Carmakers are working on solutions to this issue by sharing investments in battery plants or investing in their own plants. The result is that the trend of the noted percentages will reach 10% in the coming years [39]. The technological advances in batteries design are reducing weights and prices [40] and provide products that make EVs more accessible each year [41]. On the other hand, some cities and/or countries are establishing deadlines for conventional vehicles [42]. This situation will cause the general use of EVs to be even sooner.

Although the driving range of EVs is shorter than the conventional vehicles, this is improving year after year. There are two recharging types that are available in most vehicles: a full recharge in 3–12 h and fast charge of up to 80% in half an hour. The ways to charge EVs depends on the connector used (presented in Figure 2) and are strongly related to the type of power used (DC or AC) [43,44,45]:

• Unplugged: consists of wireless charging with three possible scenarios: at home, at a bus or taxi station (short time), or on-route.
• AC: for regular charging of EVs. The power is converted into DC inside the EV, so long charging periods are required. The typical connector used for this type of charging is a Type 2.
• AC and DC: an enhanced version of the previous one. The connector has additional power contacts (DC) for faster charging. The connector used is the CCS combo 2.
• DC: fast charging due to exclusive operation of DC. The conversion from AC to DC is performed outside the vehicle. A Type 4 connector, also known as CHAdeMO, is the standard connector.

The charging levels include two AC 1-phase levels (up to 2.3 kW and 7.4 kW), one AC 3-phase level (normally up to 22 kW), and one DC level (from 50 to 175 kW). Nevertheless, if there are several EVs connected to the same charging station, there is a point bias with a consequent reduction in charging speeds [46]. This fact is something that has to be taken into account when assessing energy efficiency in houses with two or more EVs. Fast charging is used for a power transfer higher than 22 kW, and can be carried out in either AC or DC. However, with DC, the charging point is in direct contact with the car battery, and the conversion from AC to DC is not performed inside the EV. The voltage levels of the EV batteries range from 300 to 400 V and the average seems to be around 350 V [47]. The charging efficiency depends on the state of charge and weather, and it may be reduced from 12 to 36% due to transmission and heat losses [48].

Smart charging addresses external control of the charging process of the EV [49,50,51]. It has to facilitate the reliability of the supply while meeting mobility requirements. It should be noted that G2V takes energy availability into account in order to establish periods of charging as well as their speed or whether or not to store energy when there is overgeneration. The opposite process exists as well, V2G, when the battery is used to provide power back to the grid during periods of low energy production.

3.2. Batteries at Home

An increasingly typical solution around the world is the application of HESS since it is an easy way to reduce electrical consumption and, as can be seen in Figure 3, prices have fallen in recent years by more than 50% [19]. In fact, HESS application in German homes has grown by around 50% in each of the last three years, according to data from the BSW–Solar association [52].

The different HESS solutions sold on the market allow it to be adapted to any house, depending on the specific characteristics, i.e., discharging and charging power, voltage levels, DoD, capacity, etc. The round trip efficiency is 95%. Additionally, due to the varied possibilities existing in the market, there is a wide range of prices, from $0.19 kWh/cycle to $1.07 kWh/cycle [54].

Regarding the types of HESSs, there are mainly three different ones that, due to their characteristics, can be applied in houses, i.e., monoblock, stationary and lithium [55]. Nevertheless, the direction of manufacturers, such as LG Chem, Tesla, ABB, Huawei, or ByD, seem to be towards lithium [56]. Lithium offers a greater versatility, faster charge and discharge, longer life cycles, higher DoD, higher power density and higher round-trip efficiency than the other HESS types [57].

The placement of a HESS allows to store energy during periods of lower demand and to power the home when necessary, by improving security and by controlling the supply. HESS makes it possible to improve household electricity consumption, alleviating the variations presented in the load demand and ensuring supply in the case of a grid outage [58,59].

The accumulation of energy in HESSs is carried out in DC, so connection to household devices requires a DC/AC conversion. On the other hand, as seen in Section 2, a large number of household devices operate in DC, even though they are AC powered. As such, it is necessary to convert the current twice to run most of home devices, and this results in an inefficient system. The layout of a fully DC scenario facilitates the ability to more easily and directly connect HESSs, improving the energy efficiency of a house. This, along with the flexibility of HESSs, enables to achieve emission reduction goals by creating a NZEH.

3.3. Availability of DC Household Devices

As explained in Section 2, home devices are generally DC compatible because most internal components are available in DC versions, i.e., electronics, motors, or lighting. The market for these appliances is focused on off-grid, as well as specific applications, such as recreational vehicles and boats, but there is a lack of available products for grid-connected systems [60,61]. Manufacturers, such as Dometic, Alphatronics, Unique, or NIWA Solar, develop DC products in order to solve the needs of isolated homes, rural areas, ships, caravans, etc.

DC appliances can meet quotidian needs with lower voltages than their AC counterparts, usually 12 V DC or 24 V DC rather than 110 V AC or 230 V AC [62,63]. Therefore, the security levels of DC appliances are much higher than those of their AC counterparts. In addition, these can be connected to batteries or renewables, such as PV systems, in an efficient way because they do not need intermediate conversions.

3.4. LEDs

Typically, LEDs are AC powered; however, they have a great potential to be DC powered as their internal operations are in DC. The needs of efficient household devices make LED an easy way to reduce electrical consumption because of their high lumen/consumption ratio and their long life cycle [64,65]. Figure 4 shows a comparison of the efficiencies between different types of home lighting. As can be seen in Figure 5, the LED industry is growing rapidly, with well-known manufactures, such as Philips or Panasonic, offering different DC solutions for residential applications, i.e., 12 V DC, 24 V DC, or PoE.

AC/DC conversion failures are among the most common failures with LEDs, having a probability of 64% [56]. In this way, DC-powered LEDs are more reliable than their AC versions because of the elimination of the AC/DC conversion, reducing the probability of failure because of the lower need for components [67]. However, DC LEDs might require DC/DC conversion, although this is between 5 and 10% more efficient than the AC/DC one. In addition, when the overall efficiency is taken into account, DC LEDs needs 15% less energy than AC LEDs to produce the same lumens [57].

3.5. Distributed Generation and Renewables Available at Home

Distributed generation is an option that guarantees sustainable and secure energy in homes. One of the main sources of distributed energy is solar PV, produced on the rooftops of houses. The potential for solar PVs is wide, and it can be estimated that it is possible to generate 25% of all current energy consumption [15]. Nowadays, the percentage of PV rooftop panels covering a surface is lower than a 10% of the available space, but, as can be seen in Figure 6, sales are increasing and will continue to do so in the long term. Thus, in 2019, the installed power in Europe has doubled that of the previous year and the forecast for 2021 is to become the highest ever in terms of solar installations in a single year [68].

The prices of PV panels range from 0.05 €/kWp to 0.6 €/kWp depending on the efficiency, the material and the structure. These prices are expected to be lower in the coming years because the increase in demand, since it has been shown that a growth in production reduces costs [70]. Typically, PV systems in the residential sector generate between 1 kWp and 10 kWp with the grouping of modules in series or parallel arrays. The power of these modules is between 160 W and 300 W at 36 V [71]. Despite the great potential and the adjustment that it presents to homes, the production of PV energy is in DC, as such, nowadays, converting it to power household devices is necessary [72,73].

As the objective is the reduction of domestic electricity consumption, the efficiency of a PV system depends on the amount of power transformed rectifications using this type of production. Therefore, the more DC appliances there are, the greater the PV system efficiency.

3.6. Installation of DC at Home

Most devices in homes today operate in DC, and, with the current trends in technologies, more and more appliances will need to run in DC [74,75]. A DC power supply for these devices directly increases the efficiency at home because it needs fewer conversions, which is on average 14% more efficient [76]. In addition, the heat losses produced by these conversions are reduced because they are usually carried out internally in the devices. Thus, DC-powered appliances favor a greater simplicity and a smaller number of elements. Therefore, DC has the ability to improve the resiliency, reliability, sustainability, energy efficiency, and safety of homes.

Because of the advantages of DC and the growing trends of its application, not only in the different household devices, but also in different fields (presented in Section 2 (see EV, PV or LED)), it is necessary to consider a change in the home distribution system [77,78,79]. Figure 7 shows a design of a possible DC distribution that could occur in homes in the next few years.

It is not surprising that, due to these virtues (among others), certain clusters and trends emerge in order to standardize and promote the development of DC, such as Emerge Alliance or PoE. The EMerge Alliance is an association with different affiliated organizations, such as ABB, IEEE, IEC, Cisco, and Bosch, that develops standards for the implementation of DC. Among these standards, the standard that establishes the use of 380 V DC for distribution in buildings and 24 V DC for occupied spaces stands out [74]. PoE is a technology that has been developed with current trends in technology that incorporates power supply into a LAN infrastructure. Current voltages are established according to the IEEE 802.3 standard with values between 42.5 and 59 V DC [80].

There have also been countless projects and studies on DC that extol their advantages, such as the Bosch DC microgrid or the Pulse projects. The Bosch DC microgrid is a project created by Bosch, in North Carolina in 2017, to reduce energy consumption at a local fitness center and an emergency shelter through a DC energy management system. As a result, the system increased its efficiency by 13% and reduced its consumption by more than 55,000 kWh/year [81]. Pulse is a project that was completed in 2018, carried out at the Technical University of Delft, in order to achieve an energy neutral building using an intelligent system with DC, solar and storage [82].

3.7. Use of Exercise Devices to Charge Batteries

More and more people have HEDs at home. When exercising using with these devices, there is energy involved in the movement, and most of that energy is lost. Therefore, it is possible to take advantage of that work, regardless of whether it is treadmill running, pedaling on a stationary bicycle, or lifting weights [83,84]. The energy produced by HEDs is in DC. HEDs can be connected to a battery with low losses, converting 94% of the energy produced through the HED into electrical energy; thus, reducing household demand [83]. In fact, depending on the duration and the intensity of the exercise performed, it is possible to meet the consumption needs of certain appliances [85].

Through the use of HEDs, activity is promoted from the achievement of a goal, energy and economic savings. With the encourage of movement at home, new applications can also appear, which enhance the achievement of goals, either by competing with other people or with certain challenges for a period of time.

3.8. Standarization of DC Connectors

Due to the wide range of applications, DC connectors can be of different types. From the well-known USB, and its power delivery extension of up to 100 W, to Ethernet, also up to 100 W, there is a wide array of options; for example, different types of USB (type A, type B, type C, type D, and also the mini and micro versions, mini A, micro A, mini B, and micro B), lightning, thunderbolt, etc. Currently, USB connectors are used to charge the batteries of electronic devices (smartphones, tablets, laptops, wearables, etc.), to supply power to some peripherals (scanners, webcams, headsets, speakers, etc.), and to supply power to small size devices, such as fans, handheld vacuum cleaners, juicer machines, mixers, etc. [75,86] However, the limit of power does not presently allow to connect every device, although research and development in the mobile industry will soon allow phones to be charged at 125 W with an efficiency of 98% [87], and so it is possible that load powers will be extended in the coming years. In fact, in the market, combined AC and USB sockets are available, due to their generalized use, and also USB sockets can be found in public access places, such as parks, squares, bus stops, shopping centers, etc.

The different DC voltage levels can be easily obtained or maintained with a buck-boost converter. These types of converters can increase or reduce voltage levels when necessary through two main parts, the buck converter, which reduces the voltage, and the boost converter, which increases the voltage. The efficiency of buck-boost converters is 98% and they are widely used in consumer electronics, control applications, or power amplifiers [88].

4. Possible Scenarios for Electrical Installations at Home

Homes entail a large percentage of final energy consumption. Therefore, reducing electrical demand is necessary in order to achieve decarbonization milestones. As seen in previous sections, the presence of DC in homes is varied, needing to carry out transformations for practically all devices. In addition, with the growing trend of EVs and self-consumption with batteries, a reinforced framework for DC is presented. On the other hand, DC/AC converters are more efficient than AC/DC ones, with typical values of 90% and 80%, respectively [89]. This section describes the pros and cons of four scenarios for increasing home efficiency based on electricity use.

4.1. Scenario 1: Current Scenario

Keeping the configuration used nowadays, in which a house is wired in AC. Each device is connected to sockets and performs the rectification internally. The advantage of this scenario is that no change in the supply system is needed. On the contrary, the energy losses produced in the home are large because of the need for AC/DC conversion for the devices, resulting in a higher probability of failure of the appliances and a reduction in life cycle. The configuration of this scenario is presented in Figure 8.

4.2. Scenario 2: AC/DC Converters Close to Sockets

In the case of keeping the traditional AC configuration with a high generalization of AC/DC converters close to sockets, the efficiency will be low due to the losses from the converters, even though it can be higher than in the current scenario because there is a converter by the socket and not in the device. However, the small improvement obtained in this scenario can be neglected. The advantages are that it can be a good option for a transition period, maintaining both possibilities (AC and DC) for all devices and that it can be a simple and cheap option that allows old houses to be adapted. The possible transformation of the current scenario is shown in Figure 9.

4.3. Scenario 3: AC and DC Distribution along the House

As in the previous case, this can be a good solution for a transition period; however, the costs of adapting homes will be higher. It consists of keeping the classic AC configuration and, besides, a DC distribution along the house. The advantages are that carrying AC and DC wiring in the house, instead rectifying in the sockets, allows a great reduction in losses in the home due to there being less need for AC/DC conversion in the system. If there are HESSs or renewables, the system will be much more efficient than those previously presented. Due to the distribution of DC, the consumption of the devices that use this type of power is reduced, according to Equation (1).

$$E_{DC}=E_{AC} · {\rho }_r$$

(1)

where $E_{DC}$ is the energy consumed in the DC distribution by any specific device, $E_{AC}$ is the energy consumed in an AC distribution by the same device, and ${\rho }_r$ is the efficiency of the conversion AC/DC.

As an EV can be powered directly using DC, its consumption is reduced twice: in the DC/AC inversion and in the AC/DC conversion inside the vehicle, in comparison with an EV powered using AC in this scenario. The equation to obtain the consumption is the one expressed in Equation (2).

$$E_{DC}^{EV}=E_{AC}^{EV}-E_{AC}^{EV} · {\rho }_r-E_{AC}^{EV} · {\rho }_i$$

(2)

where $E_{DC}^{EV}$ is the energy consumed by the EV in the DC distribution, $E_{AC}^{EV}$ is the energy consumed by the EV using AC in this scenario, and ${\rho }_i$ is the efficiency of the inversion DC/AC. The double distribution that occurs in this scenario is shown in Figure 10.

4.4. Scenario 4: DC Distribution and DC/AC Inversion in Essential Cases

In this case, the power is converted into DC at the entrance point of the house. Inside the house, DC is the only type of power used, unless it is for some essential purposes (e.g., washing machine). The house may have a HESS, based on renewables or not, but most of the household devices, such as EV chargers and electronic devices, are powered directly in DC. The advantage here is that there is just one distribution system of power around the house (instead of two as in the previous case) and that there are no losses due to AC to DC conversion though the disadvantages may be the voltage drop with the distances, as well as the prices of a completely new installation. In this scenario, DC devices and EV are going to have the same consumption as in the third one. However, AC devices would have a higher consumption than in the previous scenarios, due to the specific inversion of DC/AC needed. The consumption of AC devices is according to Equation (3) in this scenario.

$$E_{AC}{\rfloor }_4=E_{AC}{\rfloor }_1-E_{AC}{\rfloor }_1·\left(1-{\rho }_r\right)+E_{AC}{\rfloor }_1·\left(1-{\rho }_i\right)$$

(3)

where $E_{AC}{\rfloor }_X$ is the energy consumed by an AC device in the scenario X. This DC home configuration can be seen in Figure 11.

5. Case Study: Scenarios Analysis and Comparison

Even though a full comparison in terms of energy consumption and price would be the proper option, a different point of view is going to be presented here. In order to compare the scenarios on an equal basis, two hypotheses have to be stated before analyzing them:

• The cost of the installation is the same for all scenarios. The installation may include a HESS, a classic AC distribution of power and/or a new DC one, it may contain DC/DC converters, classic sockets, DC types, etc. The tendency of the price of the full installation in all scenarios will converge due to the competitiveness, the reduction of the prices considering economies of scale and the evolution of prices of HESSs.
• Energy consumption due to the different processes of conversion related to the HESS, AC/DC and DC/AC, is not considered here. In this case, the justification is that those losses are not part of the energy distribution. Instead, they are part of energy storage. This process would be critical in the future and the price of the related losses would be covered by charging the HESS during the low-price period and by discharging it during the high-price one. In fact, the low-price period corresponds to a high availability of renewables in the electrical network. Therefore, those losses are part of the energy that is going to be discarded. Note that if the scenarios also contain HESS and PV, the efficiency will be higher. Thus, there are fewer losses because no rectification is needed. For example, for a typical value of 90% in the DC/AC conversion and 80% in the AC/DC one, a house will be 16% more efficient.

The values presented in Figure 12 have been taken for household devices, according to the study made by the IDAE [90]. In that study, average values of consumption by household devices, taken from Spanish homes, were assessed. The methodology used in that study was telephone and face-to-face surveys.

For EVs, a consumption of 0.15 kWh/km can be considered and 12,000 km/year can be taken as a common value. Therefore, a consumption of 1800 kWh/year can be used in this case study.

The results are shown for two different cases: with and without EVs, and can be seen in

Table 2. Efficiency comparison of the different scenarios respect to the current one, evaluating the impact of EVs on them, all units in kWh.

Device	AC/DC	Scenario 1	Scenario 2	Scenario 3	Scenario 4
EV	DC	1800	1800	1260	1260
Illumination	DC	410	410	328	328
Fridge	AC	655	655	655	721
Freezer	AC	131	131	131	144
Washing Machine	AC	254	254	254	279
Dishwasher	AC	245	245	245	270
Clothes Dryer	AC	71	71	71	78
Oven	AC	178	178	178	195
Television	DC	119	119	95	95
Computer	DC	145	145	116	116
Standby	DC	229	229	183	183
Other Appliances	DC	75	75	60	60
Total without EV		2512	2512	2316	2469
Savings without EV		-	0	196	43
Total with EV		4312	4312	3576	3729
Savings with EV		-	0	736	583

and Figure 13.

The devices identified with DC are those that internally consume that type of power and those identified with AC do not consume DC internally and therefore should not be supplied with DC.

For AC/DC conversion an efficiency of 80% was considered and 90% was considered for DC/AC according to a mean value of the current devices.

In Scenario 2, no differences were considered with respect to the first one, due to the low values in the reduction of losses in the AC/DC transformations close to sockets instead of in each DC device. This means that a small difference can be found, but it is not very meaningful.

In Scenario 3, some important differences can be found due to avoid AC/DC conversion in DC devices (EVs, illumination, electronic devices, etc.). The savings in this scenario are 7.80% (without EV) and 17.07% (with EV).

Scenario 4 also avoids the AC/DC conversion in DC devices but also considers the DC/AC one in AC devices. Therefore, the final consumption is higher than in the third one. The savings in this scenario are 1.00% (without EV) and 13.10% (with EV).

Whether EVs are taken into consideration or not only affects Scenario 4 because of the proportion of DC and AC devices, from the point of view of the total energy consumption, which are different, and the DC/AC conversion is meaningfully reduced in terms of percentage.

6. Conclusions

Electric batteries are improving considerably year-by-year, mainly due to the evolution of smartphones, but also for other reasons (EVs, PVs, etc.). The trends in prices, capacities, weights, and pollution is clearly going in the right direction. One of the main beneficiaries might be the energy efficiency of homes, which can use batteries in order to, among other things, adapt consumption to the best tariffs, directly power EVs in DC, store power from renewables, and increase the number of DC-powered home devices.

Most of these factors are related to DC powering and, therefore, alternative scenarios for electrical installations may be better than classic/current ones, where AC power is distributed. Three new scenarios are proposed here, where the distribution of DC power is considered from different points of view: on its own, in parallel to AC, or even not considered for most of the electrical installations in homes. These scenarios and the classic one are analyzed and compared with mean values of a Spanish home and under two considerations: with or without EV. The obtained results show that the scenario where AC and DC power are distributed simultaneously is the most efficient one in both cases, and, taking EVs into account, it increases the efficiency dramatically.