Aalborg Universitet
An Overview of Low Voltage DC Distribution Systems for Residential Applications
Diaz, Enrique Rodriguez; Firoozabadi, Mehdi Savaghebi; Quintero, Juan Carlos Vasquez;
Guerrero, Josep M.
Published in:
Proceedings of the 5th IEEE International Conference on Consumer Electronics (IEEE ICCE-Berlin 2015)
DOI (link to publication from Publisher):
10.1109/ICCE-Berlin.2015.7391268
Publication date:
2015
Document Version
Accepted author manuscript, peer reviewed version
Link to publication from Aalborg University
Citation for published version (APA):
Diaz, E. R., Firoozabadi, M. S., Quintero, J. C. V., & Guerrero, J. M. (2015). An Overview of Low Voltage DC
Distribution Systems for Residential Applications. In Proceedings of the 5th IEEE International Conference on
Consumer Electronics (IEEE ICCE-Berlin 2015) (pp. 318 - 322). IEEE Press. https://doi.org/10.1109/ICCE-
Berlin.2015.7391268
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An Overview of Low Voltage DC Distribution
Systems for Residential Applications
Enrique Rodriguez-Diaz, Mehdi Savaghebi, Juan C. Vasquez, Josep M. Guerrero.
Department of Energy Technology, Aalborg University, Denmark.
erd@et.aau.dk, mes@et.aau.dk, juq@et.aau.dk, joz@et.aau.dk
Abstract—The concept of a microgrid has drawn the interest
of research community in recent years. The most interesting
aspects are the integration of renewable energy sources and
energy storage systems at the consumption level, aiming to
increase power quality, reliability and efficiency. On top of this,
the increasing of DC-based loads has re-open the discussion of
DC vs AC distribution systems. As a consequence a lot of research
has been done on DC distribution systems and its potential
for residential applications. This paper presents an overview of
the LVDC distribution systems used in residential applications.
Several publications that study the potential energy savings and
overall advantages of the LVDC distribution systems are analysed.
Different power architectures and topologies are discussed. The
existing demonstration facilities where LVDC distribution systems
have been implemented are also shown.
Keywords—DC Microgrid, Smart homes, DC homes, LVDC.
I. INTRODUCTION
Nowadays, there is an open discussion on whether to use
AC or DC electrical power systems. This matter can be traced
back to the battle between Edison and Tesla/Westinghouse
more than a century ago [1]. The technology available back
then, made the AC option far more advantageous, consequently
the electrical power systems worldwide are AC-based. Never-
theless, today’s scenario has changed, and DC based power
systems offer interesting advantages regarding simplicity, cost
reduction, and efficiency improvement [2].
So, what has changed that makes DC distribution systems
a stronger candidate?. There are several factors that influence
whether an AC or DC system is advantageous, and the change
of today’s scenario makes those factors yield to a LVDC
distribution system in the future. For instance, DC systems
increase de efficiency of the energy distribution systems, and
easy the integration of decentralise and renewable energy
sources, aiming to reduce the dependency from fossil fuels,
and limit greenhouse gasses emissions.
There are several factors which empower the use of DC
systems instead of AC systems: i) suitable renewable energy
generators, as Photovoltaic Panels (PV) and Fuel Cells (FC),
and energy storage systems, as batteries, are DC-based, ii)
DC loads currently represent 50% of the whole building
consumption, iii) the future integration of the electric vehicle
in the power system, will increase the consumption of DC
devices (batteries) in the buildings, iv) DC distribution systems
are intrinsically more efficient than their AC counterparts, since
in DC there are not reactive power or skin effects, v) intercon-
necting and distributing the energy between mostly DC-based
agents (sources, loads, storage) through a DC power system
avoids unnecessary DC-AC and AC-DC conversions which are
a wasteful of energy. Fig. 1 gives a clearer picture of the above
mentioned aspects showing the reduction of the conversion
stages in the power converters of loads, storage systems, and
sources, when switching from AC to DC distribution systems
in residential applications.
LVDC electrical power systems have been widely used in
applications such as, aerospace, automotive and marine [3].
Lately these systems have made their way into electrical power
systems for industrial applications, especially in the telecom-
munication industry. In data centres, LVDC architectures have
been widely studied [4], [5], and several facilities are currently
using LVDC distribution systems. Data centres demand high
reliable systems, where the integration of UPS systems is
a priority, hence the installation of DC distribution systems
reduce the conversion stages significantly, making the system
more efficient. For instance, the Lawrence Berkeley National
Laboratory has shown that a 28% efficiency improvement can
be achieved by switching from AC to a DC distribution system
[6].
Introducing the LVDC distribution systems also for com-
mercial and residential applications seem like the next rea-
sonable step. Brian T. Patterson, founder of Emerge Alliance,
has shown the importance of the DC technology in a future
electrical grid ”enernet”, and the Zero-Net-Energy buildings
(ZEBs) [7].
AC distribution systems have been recently loosing ground
against DC, however, regarding residential applications, DC
systems still have a long run ahead. The lack of regulation and
standardization, and development of protections, are probably
the main challenges that DC power systems need to overcome,
before being considered a suitable option to replace AC power
systems.
II. ADVANTAGES, CHALLENGES AND BARRIERS OF
LVDC DISTRIBUTION SYSTEMS FOR RESIDENTIAL
APPLICATIONS
The advantages of LVDC distribution systems have been
already pointed out, however a deeper discussion and analysis
is required, in order to see the true potential of this technology.
Several studies have addressed the efficiency improvement
and energy savings of switching from AC to DC systems in
residential applications.
In [8], [9], the energy savings obtained by using a DC
distribution system in residences in United States were studied.
The study was carried out for several different locations across
the country, and for different system’s topologies. Distribution
topologies with and without energy storage systems were
Wind
Turbine
M
FC
M
FC
PV
Fuel
Cell
Electronics
LED
Lights
AppliancesHeat Pump
EV Storage
Grid
Grid
Wind
Turbine
PV
Fuel
Cell
Electronics
LED
Lights
AppliancesHeat Pump
EV Storage
DC Distribution SystemAC Distribution System
AC
DC
Fig. 1. Conversion stages reduction when switching from AC to DC distribution systems for residential applications.
considered. The results showed that the use of DC could yield
to great efficiency improvement, especially when an energy
storage system is installed. The energy savings estimation
are 5% for the case of a non-storing system, and 14% for
the storing system. The difference of energy savings is a
consequence of the consumption profile of the residential
loads, which peak in the afternoon and evening, while the
PV production peaks at noon. Therefore, with an energy
storage system, the excess power generated by the PV panels
can be stored and used afterwards, avoiding the DC-AC-DC
conversion losses of sending the excess power into the grid.
There are more optimistic studies that aim to achieve 25-
30% energy savings [10], [11]. However, the environment
conditioning loads (cooling and heating) need to be taken into
account. Also, in order to obtain a fair comparison between
the different AC/DC distribution systems, comparable AC and
DC loads need to be use for both systems. The energy savings
achieved by using extremely efficient DC loads, instead of
regular AC loads, should be not taken into account.
LVDC distribution system still need to face important
challenges and barriers before been implemented in residential
systems. the main challenges and barriers can be summarised
as follows:
• The lack of standards and code is probably the main
issue that needs to be solved. Several organizations
as Emerge Alliance (EA), the European Telecommu-
nications Standards Institute (ETSI), the International
Electrotechnical Commission (IEC), IEEE and others,
are already actively developing the necessary regula-
tion and standards.
• Safety and protection issues derived from the use of
DC. New DC protection devices and schemes are
required, in order to ensure people’s safety [12].
• The lack of industry and products for DC distribution
systems. When analysing DC systems in residen-
tial applications, it is easy to notice that there are
barely commercial products ready to be used with
DC voltage. For instance, in DC appliances/devices,
small modifications are required to make them ”DC-
ready”, since most of them already have a DC/CC
conversion stage connected to a rectifier stage [13],
[14]. However, there are no DC products in the market,
aside from recreational vehicle appliances running on
12 VDC.
III. VOLTAGE LEVEL IN LVDC DISTRIBUTION SYSTEM
FOR RESIDENTIAL APPLICATIONS
The lack of standardization is evident when observing
the voltage levels used for LVDC distribution systems. As
mentioned before, most of the configurations use the data
centres voltage levels (i.e., 380-400 VDC), however is it really
necessary?. Power consumption of a regular home is much
lower than the consumption of the data centre, therefore, lower
DC voltages could be used, without significantly increasing the
distribution losses, while increasing safety in the system. For
instance, power distribution up to several hundred watts, can be
efficiently performed using 48 VDC [15], which would cover
all the IT, electronics and entertainment equipment.
most of the configurations use the data centres voltage level
(i.e., 380-400 VDC), however is it really necessary?
In [16], an analysis of the influence of the voltage level
on the efficiency has been performed. The study shows that,
using 380 VDC as voltage levels for supplying energy to the
high power loads (kitchen appliances and air conditioner) only
brings a efficiency improvement of 0.3 %, when compared
with 120 VDC. 120VDC is still considered extra-low voltage,
2Vdc
2Vdc
+Vdc
-Vdc
PV
Array
Load 1
Load 3
Energy
Storage
Load 2
Grid
M
Fig. 2. Bipolar Type DC Microgrid Concept.
hence, the damage cause by a electrical shock is reduced.
Different studies also conclude that, for residential applica-
tions, 48-120 VDC systems distribute the energy efficiently
[17]. In addition, the Emerge Alliance 24 VDC Occupied
Space Standard is intended to be used to supply energy to IT
and electronic equipment, therefore, the energy of low-power
loads/appliances, typically in bedrooms and living rooms, can
be also distributed safely and efficiently at lower voltages.
IV. LVDC DISTRIBUTION SYSTEM TOPOLOGIES
It has been pointed out before that there is a lack of
regulation and standardization on this technology. Therefore,
there are several different configurations and voltage levels
that can be employed. Since DC distribution systems have
been widely implemented in the telecommunication industry,
the voltage levels used for residential application seems to
converge to the standards used in data centres (380-400 VDC).
However it is still far from being standardised, and several
topologies of LVDC distribution system are being studied.
A. Bipolar Type Distribution Systems
The concept of using a bipolar type distribution system
brings some advantages over the unipolar type counterpart. The
distribution system concept is shown in Fig. 2. The distribution
in the system is made by a 3-wire line, with positive, negative
and neutral line. It can be easily appreciated that this concept
reduces the voltage level respect to ground, which makes
the distribution system safer for the users. Also, this concept
allows the converter on the load side to choose from three
different voltage levels, +V
dc
, −V
dc
and 2V
dc
, furthermore, the
system increases the reliability of the power supply, because,
in case of a fault in one the lines, the energy can still be supply
using the other two lines [18].
B. Unipolar Low Voltage DC Distribution Systems
This configuration has been designed for low power sys-
tems. In India has aimed to installed 20 GW of solar power
Conventional AC Microgrid DC Microgrid Concept
Fig. 3. Variable DC Bus Voltage Microgrid developed by Robert Bosch LLC.
installations by 2022, by means of the Jawaharlal Nehru
National Solar Mission (JNNSM). The JNNSM intends to
bring electricity to rural areas, where there was not electricity
available before. This program has motivated the research and
development on LVDC systems, as they easy the integration
of renewable energy sources and storage systems, achieving a
simpler, cheaper and more efficient systems.
An analysis of the deployment of a 48 VDC system for
integration of PV panels and high-efficient DC loads in multi-
storied building in India has been performed [19]. The studied
showed that the DC system is more efficient and also brings
cost savings for the users, by reducing the electricity bills, and
the cost of the system. In [20], the conceptual implementation
of low power solar system is shown. The system is designed to
cover the minimum needs of a low-income household in India.
The system is formed by a 125 W PV Panel, 48 V battery, 18
W LED tube, 5 W LED bulb light, 32 W BLDC fan, and one
cell phone charger.
Even though, this system can not cover the power require-
ment of a household in the well-developed countries, it shows
that when aiming for minimizing the cost of the system, LVDC
distributions systems have no competitors.
C. Variable DC Bus Voltage Distribution Systems
The concept of a variable voltage DC bus distribution
system aims to maximize the energy efficiency by eliminating
the converter of the renewable energy generator. Robert Bosch
LLC has implemented this concept in a DC demonstration
microgrid in Charlotte, North Carolina, USA. The system is
shown in Fig. 3. This concept allows to supply the energy with
only one conversion stage between the PV array and the load,
which minimizes the conversion losses. The AC/DC converter
performs the voltage regulation of the DC bus voltage accord-
ing to a maximum power point tracking (MPPT) algorithm for
the PV generator. In contrast with the conventional microgrid
configuration, the MPPT converter, is not in the path from PV
to load, which enables higher efficiency and higer reliability
100 V, 60 Hz
PV
Array
Wind
Turbine
Battery
Appliances
Electric
Vehicle
Smart
Switchboard
Grid
Smart
Energy
Manager
Wifi
ZigBee
In-Home
Display
Smartphone
100 V, 60 Hz
(Grid Blackout)
Data
Storage
Fig. 4. LVDC Energy Distribution System in Fukuoka Smart House.
than systems using a dedicated MPPT converter [21]. The
analysis showed that the system can improved PV energy
utilization up to 8%.
V. EXISTING FACILITIES WITH LVDC DISTRIBUTION
SYSTEMS
Japan is one of the leading countries regarding LVDC dis-
tributions systems implementations. A demonstration facility
has been built in island City in Fukuoka City. The facility
was inaugurated in April 14
th
of 2012. The power architecture
of the house is shown in Fig. 4. It consists of a hybrid AC-
DC distribution system, renewable energy sources, an energy
management system, and loads. The AC system, which is fed
from an inverter connected to a common DC bus, is only used
to supply power to the AC loads in the house. The common DC
bus is running at 380 VDC which interconnects the renewable
generation, the energy storage systems and the DC loads.
In Tohoku Fukusi University in Sendai City, a microgrid
has been implemented and it has been running since 2008 [22].
The microgrid has several generation sources, gas engines,
phosphoric acid fuel cell (PAFC), and PV arrays, and an AC
an DC distribution systems. The DC loads, and buildings are
supplied by the IPS, which is essentially a DC microgrid
formed by a 400 VDC bus which interconnects the loads,
renewable energy sources (PV), and a Valve-Regulated Lead
Acid (VRLA) battery as an energy storage system. The facility
in particular, and the technology in general, gathered the
attention of the Japanese government, since it kept working
autonomously from the grid during the earthquake in the
Tohoku area in 2011, while the main grid was down for three
days.
In Taiwan there is a demonstration facility built by the
Elegant Power Application Research Center (EPARC). The
system is formed by energy generators (PV panels, wind
turbine and a fuel cell), energy storage devices (Li-ion battery
and flywheel), DC loads (appliances and equipment), a monitor
and control center, and a interconnection with the main grid
[23].
In Europe, in comparison with Asia there are barely
demonstration sites of LVDC microgrids. However, Philips
Research has built an office lighting test bed installation in
Eindhoven. The system uses both 380 VDC and 230/400
VAC for distribution of the energy generated by the PV
panel to the LED lighting. The results have shown that the
efficiency of the LVDC distribution system is 2% higher than
the AC counterpart. The energy savings are achieved by mainly
reducing the conversion stages between generator and load,
and the transmission losses in the cables [24].
VI. CONCLUSION
This papers has reviewed the benefits and current topolo-
gies of LVDC distribution systems for residential applications.
Various studies, regarding the potential energy savings and
voltage levels, have been presented, as well as the demon-
stration facilities in which LVDC distribution systems have
been already implemented. The studies have shown that a DC
system will definitely increase the efficiency, power quality
and reliability, however the challenges that DC-based systems
need to overcome compared to the existing AC-based systems
are still lacking, such as the safety and protection coordination
aspects.
Several power topologies were shown, however a deeper
analysis is required about which topology/configuration can
optimize the efficiency, minimize the cost and investment,
and maximize power quality and reliability. An interesting
observation comes from a comparison among ultra-low voltage
levels (i.e., 48-120 VDC) and conventional voltage levels
in telecommunication industry (i.e., 380V) which helps to
point out the improvements of using low voltage DC systems,
such as higher safety without compromising the efficiency
enhancement.
The lack of standardization and regulation is still the main
challenge that this technology needs to face. LVDC distribu-
tions systems show great advantages when the integration of
renewable energy sources, together with storage systems, are
required. The development of commercial available solutions
and devices, is the next step to boost the installation of LVDC
systems, especially when minor modifications are needed in
the devices’ power converters.
In remote areas, where the main electricity grid is not
available or nearly not available, hence, the energy is supplied
by renewable energy sources. In this applications, LVDC
systems system are already the first choice for distributing the
energy, especially for extremely low-cost systems.
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