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INNOVATIVE ENERGY STORAGE SYSTEMS FOR PV GRID-CONNECTED APPLICATIONS
H. Colin1, J. Merten1, A.Graillot2, X.Vallvé2, G. Sarre3, A. fedzin4, P. Gaillard5, JP. Smaha6
(1) CEA/INES-RDI, 50 avenue du Lac Léman 73377, BP332, Le Bourget du Lac, France – Tel. +33 4 79 44 45 48- Fax +33 4
79 68 80 49 – [email protected] , [email protected]
(2) Trama TecnoAmbiental S.L. (TTA), c/Ripollès, 46, 08026 Barcelona, Spain - Tel. +34 93 446 32 34 - Fax +34 93 456
69 48 - [email protected]
(3) SAFT, 111 bd Alfred Daney, 33074 Bordeaux, France - Tel. +33 5 57 10 64 09- Fax +33 5 57 10 64 12
79 44 45 48 – [email protected]
(4) ENERSYS, ul. Leszczynska 73, 43-301 Bielsko-Biala, Poland - Tel. +48 33 822 52 23- Fax +48 33 822 52 24
79 44 45 48 – [email protected]
(5) MAXWELL Technologies sa, route de Montena 65, 1728 Rossens, Switzerland - Tel. +41 26 411 8539- Fax +41 26 411
8505 – [email protected]
(6) HAWKER sarl, rue A. Fleming, ZI Est, BP965, 62033 Arras, France - Tel. +33 3 21 60 24 43- Fax +33 3 21 60 25 74–
[email protected]
ABSTRACT: The number of decentralised electricity supply systems based on renewable energies has grown
exponentially during the last years. These systems improve the energy efficiency of the electric system as they are
installed closer to the location of the consumption. However, they are sometimes connected at the end of grid lines where
the grid may be weak. Therefore, these systems may have a significant impact on the electric system or be victim of
malfunctions.
INES-CEA, TTA, SAFT, ENERSYS, MAXWELL and HAWKER have launched a project cofinanced by the European
Commission in order to develop an inverter, dedicated to the injection of photovoltaic energy into low voltage grids, with
special features so that:

The impact on the grid of the PV system is minimised and even more, the system provides grid support on demand,

The performance of this PV system is increased,

The end user is protected against poor power quality and outages of the grid.
The article describes the benefits of the inverter, the sizing of the components, the use of innovative technologies for the
storage system and the field validation of the concept.
Keywords: Small grid-connected PV systems, Battery storage and control, lithium, supercapacitor
1
INTRODUCTION
In order to make PV electricity generation more
attractive from a technical point of view and increase its
acceptability, it is necessary to demonstrate its ability to
supply high-quality service, reliability and profitability.
Power electronic devices that decrease the impact of PV
generation on the grid, that provide additional services
such as power quality to the end-user and the support of
the grid for the utility are enabling technologies for this
increased penetration.
The present paper presents an analysis of the quality
of the grid and the interaction between grid and PV
systems in developed countries. These considerations
have led to the implementation of a project aiming at
designing and developing an innovative inverter fulfilling
the functionalities previously mentioned. Data about the
sizing of the different components, including the inverter
itself and innovative storage systems are presented in this
paper.
2
CONSIDERATIONS ABOUT THE GRID
2.1 Energy supply instability
Disturbances in the voltage supply can cause tripping
or even damage to sensitive equipments. These
disturbances include voltage sags, dips, transients, swells,
harmonics as well as short interruptions. They are
generally caused by weather, accidents or utility
equipment failures.
The interruptions can range from only minor events
lasting few seconds to blackouts such as the ones
experienced in the USA or in Europe on 5th of
November.
The frequency of occurrence of these events depends
on the strength of the electrical system. The situation is
quite heterogeneous in Europe: they are very scarce in
centralised and oversized grids, such as in Germany or
France, whereas they happen relatively often in weak
grids. For instance:
 A study made in 2004 indicates that costumers in
Eastern Europe are facing up to nine interruptions per
year with a total duration over five hours [1],
 A publication of the French Utility EDF reported an
average cumulated duration of interruptions in 2005 of
less than 1 hour,
 A case study in Spain has evaluated the interruption
duration according to the density of connections to the
grid. In urban zones, with high density, the repartition
is illustrated by Figure 1, which shows that a device
able to supply electricity for at least 3 hours would
cover about 90% of the interruptions.
Repartition of interruption by duration in urban zone
40
35
30
25
(% ) 20
15
10
5
0
0-3
3-60
60-120 120-180 180-240
>240
Interruption duration (minutes)
Figure 1 – Repartition of interruption by duration in
urban zone
2.2 Interaction between grid and PV systems
The cumulative installed PV capacity has been
expanding this last decade by about 30% per year. This
increase is related to the share of grid-connected systems,
whose market is driven by feed-in tariffs in industrialized
countries.
Some studies related to PV systems connected to the
grid already show the mutual interaction of PV systems
and the grid.
a) The impact of PV systems on the grid
From a quality point of view, utility specialists’
opinion is that PV private installations do not have any
impact on the grid due to the limited level of power
produced.
But in the case of concentrated PV systems, the
situation may be more critical as it has been shown in
Oota City, Japan [2]: under certain circumstances like
high level of irradiance or low level of consumption -for
instance during week ends- the voltage at end of line, far
from the transformer, increases above the security
threshold and leads to disconnections and energy loss of
the conventional PV system.
From a network planning point of view, a large
coverage of the energy demand by PV systems can have
an influence on the network operation as the production
of green electricity enables to smooth the load curve: the
daytime peak of consumption may disappear, in a case of
pure grid-connection, as it has been illustrated (see
Figure 2) by the study [3]:
no PV penetration
10% PV penetration
20% PV penetration
30% PV penetration
900
800
700
600
phenomena is of great importance for electric systems
managers and producers.
The origin of these disconnections is not yet well
known as very few studies have been conducted on this
topic. Nevertheless some possible criteria are:

Voltage (increase of voltage as mentioned in
clause a, voltage sags),

Frequency,

Impedance.
These disturbances or fluctuations on the grid are
detected by the inverters, which disconnect the PV array
very rapidly.
In given configurations [2], the loss of production
can reach some days more than 50 %.
c) Solution
These two types of interaction between grid and PV
systems are strong arguments in favor of a grid-connected
PV system including a storage function.
The interest is threefold:
Firstly a battery can store the energy in excess

In case of inverter disconnection, the PV
electricity that cannot be fed into the grid is
stored; this avoids losses of energy, and thus
leads to an improved performance ratio of the
system,

Or when the level is consumption is low,
storing energy instead of injecting on the grid
can limit the increase of voltage,
Secondly the stored energy can be supplied to the
loads during the evening peak of consumption so as
to reduce the amount of energy requested from the
grid, and thus smooth the load profile.
Finally in case of grid shortage, which can last from
few seconds up to few hours, the stored energy can
be supplied to the end user during this interruption.
We saw that autonomy of 3 hours would cover 90%
of the interruptions in urban configurations in
developed countries.
3
SOS-PV project
These considerations have led to the project of
developing an multi-functional grid-connected PV
inverter, including a storage function, dedicated to the
injection of photovoltaic energy into low voltage grids,
with special features so that:

The PV system provides grid support on
demand,

The end user is protected against poor power
quality and outages of the grid.
W
500
400
300
200
100
0
00:00
02:00
04:00
06:00
08:00
10:00
12:00
14:00
Day hours
16:00
18:00
20:00
22:00
00:00
Figure 2- Urban load curve in Spain assuming different
rates of PV coverage of the energy demands.
b) The impact of the grid on PV systems
The grid disturbances that PV systems suffer are
mainly their disconnections leading to a loss of
productivity. The understanding of these disconnection
3.1 Market study
To achieve this overall objective, the first step in the
project consisted in an analysis of the potential market
for such a system. This market can be split between short
term and long term applications.

The short term application market is deduced
from the present quality of the networks, the
existing market for PV systems and for
uninterrupted power supply systems,

The long term application market implies a
change in markets and regulations so that the
owner of a SoS-PV system receives a monetary
advantage for supporting the network by
providing energy or by shedding from the grid
on demand.
3.2 Choice of scenario
In the short term scenario, a single feed-in tariff is
applied and the storage is used for UPS purposes and for
storing the PV energy in case of disconnection from the
inverter. In this case, the size of the array is determined
so as to cover the annual consumption and the inverter
should be able to feed the totality of the PV power to the
grid.
In the long term scenario, there are two possibilities:

Either the system is designed for grid support
purpose and for securing the house. In this case,
the inverter does not need to be of the size of the
PV array since injection will happen only when
the grids requires support. The storage system
should have a larger size in order to store the PV
energy for later feeding,

Or the system is operating with real time pricing
and it is of interest either to feed as much energy
as possible during the peak times or to shed
from the network.
For the study it was decided to focus on the long term
scenario.
3.3 Sizing of components
The sizing depends basically on the general
requirements for the system, the energy demand, the solar
radiation and the feed-in tariffs in each country.
a) PV array
We made the assumption that feed-in tariffs may
decrease and disappear in the long term. In this situation
there is no interest to have a large PV array: the right size
is the one enabling to cover the energy consumption.
According to the evaluation of the yearly
consumption of southern and northern countries in
Europe, and the optimum yearly productivity of 1 kWp of
PV modules, it was possible to determine the range of
power to cover the needs of a household: between 4 and
6 kWp.
b) Inverter
In a grid-connected system, the inverter only runs at
100% in few occasions (high irradiance), but has a low
efficiency the rest of the time.
Having a battery in the system permits to undersize it
as it is possible to store the PV energy during the peak of
generation and use it afterwards.
An analysis of the consumption profiles in Spain of
residential users and mixed type (i.e. residential and
SME) users in winter and summer times showed that an
inverter of 1,5 kW would be sufficient to secure the
critical loads (size for grid support and house secure
mode).
In the real time pricing mode, the inverter should
have the same size as the PV array to supply not only
critical loads. In this case connecting in parallel two 2,4
kVA inverters can be an optimum (as it falls below the
single phase limit of the German regulation) and gives
high modularity.
c) Battery
The battery has to be sized in order to fulfill two
ways of operation:

Normal daily operation where the battery is used
to delay the injection of energy into the grid,

Operation in case of grid shortage where the
battery should provide energy to critical loads.
The previous consumption profiles were also used to
evaluate the size of the battery bank in the first mode.
The most constraining scenario led to a daily need of
11000 Wh. As said before 90% of interruptions would be
covered by a storage system with 3 hours of autonomy;
the need for critical loads is therefore 4500 Wh (3 hours
at 1,5 kW).
This makes a total capacity need of 15,5 kWh.
4
INNOVATIVE STORAGE SYSTEMS
The energy for the power quality and UPS functions as
well as for the grid support will be provided by 2 types of
storage systems that proved to be most adapted to this
application after evaluation in the INVESTIRE thematic
network.

A lithium-ion based system,

A hybrid system combining a lead-acid battery
and supercapacitors.
Both types of storage systems have never been used yet
in this application. They have the following advantages:

Maintenance free operations,

Long life duration,

Positive impact on environment.
4.1 Innovative aspects
The first innovation in this project is to associate in
parallel a valve-regulated lead-acid (VRLA) battery with
supercapacitors as a hybrid system in order to provide the
following advantages for the lead-acid battery:

Decrease the effect of the “coup de fouet”,

Limit the depth of discharge,

Absorb the peak power pulses in discharge,

Allow to use a lower capacity thus decrease the
cost,

Decrease the weight and volume.
The second innovation in terms of storage is to use a
lithium battery of large size in an application where it
was never used yet. This type of battery provides :

High energetic efficiency (> 95 %),

Operation whatever the state of charge of the
battery,

Low weight and volume,

Low life cycle cost.
4.2 Lithium-ion based system
The battery bank is based on Saft’s Li-ion VL45E
elements, which can supply a maximum of energy within
a compact and light packaging. It has a low self-discharge
and gives an excellent reliability during its whole
lifespan. It is highly suited to any charge/discharge
cycling application that demands a battery with
drastically reduced weight and volume.
Elements are gathered by group of 14 cells in a
module; each module includes an electronic board for
safety management, cells balancing and data acquisition
(thermal and electrical state of the storage system).
According to the energy needs, 8 modules are assembled
in a cabinet according to a rack configuration (see Figure
3).
Figure 4 – Lead-acid battery
Figure 3 – Li-ion modules in cabinet
During normal operation the battery voltage will stay
between 380 V and 448 V; thus, there is no need of a
DC/DC converter to connect it to the 400 V DC bus
linked to the inverter. In case of emergency, during an
interruption of supply from the grid, this voltage may
drop down to 336 V.
The 112 Li-ion cells are able to provide the requested
15,5 kWh especially at the end of life of the battery (20
years).
4.3 Hybrid storage system
The hybrid storage system is composed of a lead-acid
battery and a supercapacitor, which will cover the power
peaks. The goal of this parallel construction is to extend
the lifetime of the battery by suppressing the high
currents.
a) Lead-acid battery
The battery bank is based on Enersys’ VRLA
batteries
with
AGM
construction.
Internal
electrochemical design has been adapted to the cycling
requirement in order to achieve a long cycle life (7-10
years).
To ensure this long life, daily cycles are made
between 50 and 90 % of the state of charge, which
statement leads to a global energy capacity of near 28
kWh. The reserve autonomy, in case of grid shortage, is
higher than 5 hours (between 50 and 20% SOC).
A prototype of the VRLA battery is designed to
achieve a low cost battery with longer life and better
reliability. This prototype is based on a 12 V module in
an existing box and lid with revised electrochemistry:
adapted grid design, adapted paste formulation, specific
AGM separator (see Figure 4 ).
The battery bank is made of 32 cells of 2 V put in
series, giving a nominal voltage of 64 V. A DC/DC
converter steps up the voltage up to the 400 V of the DC
bus. The battery management system is included in the
inverter.
b) Supercapacitor
The supercapacitor is based on Maxwell’s
BOOSTCAP® cells. It is sized to level off the peak
current from the battery and thus ensures the function of
a low pass filter.
The role of the supercapacitor is to supply current to
the loads if the demand exceeds the maximum tolerable
for the battery (i.e. maximum battery current allowed).
During the charge and discharge phases the voltage
across the BOOSTCAP® terminals changes from a
maximum value to a minimum value. In order to use the
available stored energy at high efficiency a voltage
variation of 50% should be set. The unit is coupled to the
VRLA battery DC-bus through a bidirectional DC/DC
converter.
One aim concerning this component was the increase
of the cell voltage of the capacitor, which results in an
increased power as well as energy density via appropriate
selection of the electrode/electrolyte combination and
improved electrode design, and the increase of the
electrochemical stability regarding temperature domain
and cycle life via the selection of electrode materials with
tailored pore structure, novel electrode design, and a
matching electrolyte.
The supercapacitor is made of 2 modules of 18 cells
of 2,7 V put in series, giving a operating voltage of 97,2
V. The capacity of each module is 165 F (see Figure 5).
Figure 5 – Supercapacitor
The module includes cell voltage management
electronics where each cell is monitored; this leads to
improved efficiency and operation.
c) Operation
The principle is that the battery delivers the energy needs
and the supercapacitor supplies the power needs.
In the case the storage system has to provide the
loads with energy, if the demanded peak current exceeds
the maximum current allowed for the lead-acid battery,
the supercapacitor is requested to supply this current;
otherwise the battery is discharged accordingly.
The supercapacitor is recharged by the battery itself
or the PV generator according to the load demand. The
battery is recharged by the PV generator (normal daily
charge and equalization charge regularly or when the low
state of charge threshold is reached).
5
VALIDATION
The individual components are tested individually to
check their characteristics and performance before
integration, and then the whole system is tested in the
field.
5.1 Battery testing
Tests are performed to check the initial characteristics
of the storage systems (capacity, efficiency, internal
resistance, self-discharge) and their cycling ability
according to the load profiles defined previously.
The test of the storage system (whole unit that is
connected to the 400V DC bus, i.e. storage device +
converter + BMS) are done independently of the
technology and all procedures are applied to one system
of each technology.
5.2 Inverter testing
Tests are performed to check the characteristics and
performance of the inverter: static power efficiency, total
harmonic distortion, power factor, start-up sequence,
losses, reaction to disconnections, and behavior of
MPPT.
Some tests related to the inverter with the storage
function will also be performed according to the standard
concerning UPS installations IEC 62040.
Electromagnetic compatibility tests are also foreseen.
5.3 System testing
Four PV systems (two with the Lithium based storage
and two with the hybrid storage) are going to be tested in
the field to validate the new functions: 3 sites (2 in Spain
and 1 in France) have been selected which exhibit
different classes of grid weaknesses and different ratios
between PV generation, storage, and consumption.
Typical single household system with a PV power of 2 to
3 kWp will be considered. The installations and site
management will be executed including monitoring
equipments. The systems will be tested (reaction to grid
interruptions and voltage perturbations) and monitored
during at least 5 months of operation for what concerns
the energy flows and the grid stability parameters. The
data resulting from the field tests and the field operation
will then be analysed for quantifying grid stabilisation
services provided and the efficiency of the systems
components as well as PV production comparing to
conventional PV inverters.
6
CONCLUSION
The present paper deals with the integration of
innovative storage systems in the PV grid-connected
installations in order to address the different problems
encountered in the low voltage distribution grids.
The interest of a storage function in the PV gridconnected systems is to improve the mutual impact
between grid and PV systems, and increase the
performance ratio of the PV installations.
The modular architecture of the system enables to
select two different storage technologies using the same
system components: either on lithium-ion or a
combination of lead-acid batteries and supercapacitors.
Based on a long term scenario, in which the system is
designed for grid support purposes and the security of
supply for the house needs, sizing has led to a large
storage system in order to store PV energy for a deferred
feeding (11 kWh for daily cycling and 4,5 kWh as
reserve for emergency use) and an inverter power (2,4
kVA) much lower than the PV generator power (4 to 6
kWp).
Test procedures have been defined on component and
system level in order to validate the concept in the field
and assess the different storage solutions selected.
7
REFERENCES
[1] ERRA, EU accession Countries Working Group,
Quality of Electricity Supply – Comparative Survey,
April 2004
[2] Y. Ueda, K. Kurosawa, Performance Analyses of
battery integrated grid-connected residential PV
systems, 21st European Photovoltaic Solar Energy
Conference, Dresden, 2006
[3] A. Graillot, X. Vallvé, M. Perrin, E. Bosch, Interest
of a storage system in PV grid-connected
installations, 21st European Photovoltaic Solar
Energy Conference, Dresden, 2006
Acknowledgments:
This program has been conducting with the support of the
European Commission.