MULTIFUNCTIONAL MODULE LITHIUM-IONSTORAGE ANDPHOTOVOLTAlC CONVERSION OF SOLAR ENERGY

J.F. Reynaud', c. Alonso", P. AI0"isi 1, c. Cabal", B. Estibals 1

, G. Rigobert 2, G. Sarre 2

, H. Rouault', D. Mourzaqh",F. Mattera", s. Genies 2J

1CNRS , Laboratory for Analysis and Architecture of Systems (LAAS), France2SAFT , France

3CEA-LITEN, Laboratory for Energy Compounds (LCE), France4INES-RDI/CEA-LITEN, Laboratory for Solar Systems (L2S), France

Fig. 1. Descriptive of the full photovoltaic power system.

Fig. 1 shows the multifunctional module integrated ina full photovoltaic power system with its differentconfigurations.

2. Stand-alone configuration, alternative current:saving of space (no technical room), easier toinstall, easier to transport on the site, and flexibilityto adapt the system to the expectation of users.

AdaptationStage 1

AdaptationStage 2

AdaptationStage3

Module LiPV +(MLiPV)

3. Grid connected configuration: easier to integratethanks to the compactness of the module, easier toinstall, and flexibility to adapt the system to theexpectation of users.

1. Stand-alone configuration, direct current: saving ofspace inside a mobile home, easy to use (even fora non expert user), no need for a specific place forthe batteries, easier to install and to replace, andno need to modify the existing systems.

Registered in a fast-growing context, photovoltaic powersystems could be designed for different applications.This paper presents a multifunctional module, notedMLiPv, where all the components, panel, battery,converters, electronic control unit and cablings arecombined in a compact single unit. This module offersdifferent possibility of configurations with theiradvantages:

The high degree of integration of this moduleinvolves new designs and technologies for all insertedcomponents. Today, lead-acid batteries are the mostcommonly used technology for photovoltaic energy

INTRODUCTION

ABSTRACT

A large part of photovoltaic research is dedicated todeveloping innovative integration technology for themodule. Now the use of a storage function in suchintermittent power supply systems is crucial in order toget better adequacy between the peaks of energeticconsumption and production. The integration degree ofphotovoltaic systems will be improved if all thecomponents: panel, battery, converters, electroniccontrol unit, cablings can be combined in a compactsingle unit. Different battery technologies are currentlyused with photovoltaic systems along with some of thecharging and discharging techniques that are available.Lithium-ion technology appears to be the best candidatebecause of its high energetic efficiency, high specificenergy density, deep discharge ability, its long cyclinglife and its possible different shapes (cylindrical orprismatic styles) to obtain an integrated design. Tooptimize the battery performance, different conditionsare required and achieved by control sub-systems,integrated into battery and module construction. Theelectronic management system presented in this paperaims to optimize the use of the battery making theoverall system more reliable and cost effective. On theone hand, a research algorithm of the optimal power tocharge and discharge an advanced lithium-ion batteryfor photovoltaic application is necessary to obtain anoptimum electronic management system. On the otherhand, a Maximum Power Point Tracking is alsoincorporated into the electronic control unit, to optimizethe solar array production under varying atmosphericconditions. This paper will describe a new concept of anintegrated multifunctional module using lithium-ionbatteries and new control algorithms.

The context of our research is a new national projectnamed Module LiPV constituted by a consortium of twolaboratories specializing in solar energy, material ofenergy storage and architecture systems, and oneindustry experts in batteries. This project depends onthe French National Research Agency (ANR). Theobjective of our work is to demonstrate the feasibility ofa new concept of a multifunctional photovoltaic system.

Today, photovoltaic (PV) energy has a highthreshold of technical viability and constitutes a serioussolution to respond to the total energy consumption.

978-1-4244-1641-7/08/$25.00 ©2008IEEE

storage due to their low cost and their great availability[1]. However, the battery is the component which moststrongly influences the photovoltaic system cost andreliability. There are three possible reasons which couldexplain this situation:

• Photovoltaic systems impose specific operatingconditions.

• The battery and cell characteristics (life time,efficiency) are different from one to another.

• The battery management is often notappropriate to the battery technology.

To square up these difficulties, conventionalphotovoltaic power and storage systems must bereviewed and improved to be efficient during charge anddischarge without degrading the life time of the battery.A new concept of energy management and storagesystem is presented in this paper to smooth theintermittent energy production from the PV generator tothe load.

DESCRIPTION OF THE MULTIFUNCTIONALMODULE

lithium-ion electrochemistry used (Le. nickel oxide oriron phosphate).

The module "Panel/Converter / Li-ion Battery",noted MLiPV, could work in two cases as follows:

• MLiPV working alone,• MLiPV working in parallel with different

equivalent modules.

Parallel association of n MLiPV modules allow higherpower on the order of n*MLiPV depending on theapplication. The number of n modules MLiPV is equal tothe number of PV panels necessary to supply one load.

The system is sized to ensure the continuity of thesupply energy for four days with random meteorologicalconditions. The system is not, at this stage, bi­directional and does not accept energy injection fromthe grid.

The developed operating system takes intoconsideration the various characteristics of theindividual battery cells and manages depending on theload profile, how the energy flow has to be controlled.

Panel

Battery

Fig. 2. Design of the multifunctional lithium-ion module,(PVSEC Milan, September 2007).

+

Panel

ElectronicControl Unit

Charge/DischargeRegulator

Switchedmodepower

converter

PVarray

module

Battery

Fig. 3. Functional architecture of the multifunctionalmodule MLiPV.

The elementary architecture studied in this paperincludes a PV generator, a storage system and anoptimal conversion chain including MPPT and electronicmanagement system with different electrical functions tobe connected to a load with a maximum security. Fig. 2shows the integrated design of the multifunctionalmodule which has been presented in the 22ndEuropean Photovoltaic Solar Energy Conference inMilan (September 2007).

Fig. 3 describes the functional architecture of themultifunctional module MLiPV with all sub-systemsincluded.

The principal function of this architecture is totransfer the maximum power from a PV panel and storeor restore the energy to the load. For that, the electronicmanagement system will be designed and implementedon a microprocessor to control all sub-systemsdescribed below.

The PV generator of the module delivers themaximum energy of one panel. To satisfy the powervalue defined by the application, several modules MLiPVcould be associated in parallel. The power of the PVpanel in this project can be between 75W and 200Wdepending on PV cell technology.

The de-to-de converter connected to the PVgenerator is associated with a Maximum Power PointTracking (MPPT) optimizing the research of theMaximum Power delivered by the PV generator. Thenon-reversibility of this static converter will protect thepanel from a possible battery discharge of electricalcurrent.

The battery, elemental component of themultifunctional module, will be a lithium-ion battery.Electronic management system and charge/dischargeregulator will ensure the management of the battery,protection and control system included in the de-to-deconverter. Control algorithms would be adapted to the

978-1-4244-1641-7/08/$25.00 ©2008IEEE

A. Maximum Power Point Tracking

To maximize the photovoltaic module efficiency, it isnecessary to have the operating point between the PVgenerator and the load very close to the MPP of the PVsource. However, as the MPP changes with radiationand seasons, it is difficult to track it at all radiations. Toovercome this problem, a solution consists inintroducing an adaptation stage (a static converter)between the PV source and the load associated with theMaximum Power Point Tracking (MPPT) working as animpedance adaptation.

Different types of MPPT exist. Their differencesdepend on the type of implementation, the theoreticalcontrol principle and the acquisition parameters. Indeed,the existing tracking methods reach the MPP more orless successfully.

In our case, the multifunctional module uses an MPPTcontroller created by the Laboratory for Analysis andArchitecture of Systems (LAAS-CNRS). Indeed, in thislaboratory, different MPPT algorithms have beendeveloped for photovoltaic applications since 1997. Theconversion chain of Mupv will use the recent optimalMPPT, which has demonstrated excellent behaviors[2,3]. For example, in steady state the MPPT efficiencyis about 99.6 0/0, and during brutal radiation variations,the recovery of the new maximum power point is inferiorto 5 ms. Fig. 5 shows an experimental result of a boostconverter associated with a MPPT controller. All theseperformances will be integrated in the multifunctionalmodule.

B. Energy Storage System

Electricity production systems based on renewableenergy and, in particular, photovoltaic systems need anenergy storage system because of the intermittentenergy source. Nowadays, photovoltaic systems oftenuse energy storage systems based on lead-acidbatteries due to economic and industrial availabilityreasons. But these conventional technologies of batteryhave environmental drawbacks and limits that canreduce the development of systems by random life timeleading to a high possession cost. Today, storagesystem represents sixty percent of the cost of the fullsystem. That would be reduced to thirty percent withnew technologies used in this project [4]. Due to theircharacteristics and their high potential of evolution,lithium-ion batteries are showing possibilities to run overthe actual limitations in photovoltaic systems. Lithium­ion technology has remarkable energy efficiency (970/0)[4] and higher than conventional technologies (anaverage of 800/0 for lead-acid batteries) given the factthat there are not parasitic reactions in cycle.

Actual active materials for lithium-ion batteriespresent excellent properties of cyclability and life timeestimated at twenty years. In this project, activematerials work at a range of temperatures higher thanclassical operation conditions. So, researches will bebased on the storage system adaptation to improveefficiency and reduce the cost of the multifunctional

978-1-4244-1641-7/08/$25.00 ©2008IEEE

module. Firstly, SAFT will study a conventional coupleof electrodes, Lithium Nickel Oxide (LiNi02) for thepositive electrode and natural graphite for the negativeelectrode. Secondly, an emergent couple of electrodes,natural graphite (LiC6) for the negative electrode andiron phosphate for the positive electrode (LiFeP04), willbe studied and integrated to the multifunctional moduleby the Atomic Energy Commission (CEA). LiFeP04 hasbeen investigated intensively as potential positivematerial for lithium-ion batteries because of its highstability (even at high state of charge) and moderatecost and toxicity. The couple LiFeP04/FeP04 operatesat 3.45 V vs. Lt/Li, which makes it stable in almost allelectrolytes commonly used in lithium batteries. In otherwords, LiFeP04 becomes today one of the mostattractive material for automotive and stationaryapplications.

So far, LiFeP04/C shows excellent performances interm of specific capacity, power, energy density andsafety towards electrolyte but has to demonstrate longlife capability. High density prototypes are under testingat CEA-LITEN.

c. Electronic Management System (EMS)

The basic task of an electronic management systemis to ensure that optimum use is made of the energyinside the battery and that the risk of damage inflictedupon the battery is minimized [5]. This is achieved bymonitoring and controlling the battery's charging anddischarging. Battery management, power managementand energy management define the full electronicmanagement system. Battery management involvesimplementing functions that ensure optimum use of thebattery. Power management involves theimplementation of functions that ensure a properdistribution of power through the system and minimumpower consumption by each system part. Energymanagement involves implementing functions thatensure that energy conversions in a system are madeas efficient as possible.

Management of cell batteries and conversion chainsrequires the monitoring of the state of the system inmaintaining healthy operational condition. For thisreason an electronic management system will beimplemented to reduce the continuous expense ofenergy storage with improvements in storage lifetimeand reliability.

Due to the influence of internal battery parameters(material, design, dimension, and technology) andexternal battery parameters (charging and dischargerates, temperature) on the lifetime of the battery [5], anequalizing circuit on each battery cell has to bedesigned. Improvements of the lifetime of the battery willbe observed. A protection circuit will be implemented inthe battery management to overcome all securityproblems.

Power management will be included in the design tooptimize the power consumption of the multifunctional

module. The research is aimed at finding an optimummethod of adequately powering all sub-systemsintegrated in the module.

Energy management will be designed to regulate theenergy flow from the PV generator to the battery and tothe load, and from the battery to the load. Acharge/discharge regulator has to be placed in themodule to regulate operations and improve energymanagement by running the battery temporarilyindependent of the load profile and cutting the batteryoff from the remaining stand-alone system.

Ipv*Vpv

70.00

60.00

50.00

40.00

30.00 r----- -------------- ---- ----~------ ---:/-------------- -:- -------------------- -----~---- -----------------------~- ----- ----t- --------- -----I

25.0020.0015.0010.005.00

20.00 L-__ -.L.- --'--__--'- ---'--_L-.----------'

0.0

SPECIFICATIONS OF THE MULTIFUNCTIONALMODULE

Vpv

To respond to the energy consumption in housingand other applications, the module takes into accountdifferent characteristics. The mass of the multifunctionalmodule can not exceed 15kg (storage included) and itsdimensions are equal to those of the panel. Thereliability of the system should be a minimum of 15years. The mechanical resistance of the module has torespect European standard NF EN 61646 and therobustness of the interfaces and the mechanical loadshould be high. Linked to climatic stress, the moduleshould comply with the European standard NF EN61215. Table 1 shows operating temperatures for thecomponents.

°C TMax TAverage TMinPanel + 85 <40 - 40

Battery +60 < 30 -20Electronic + 85 <40 -40

Table 1. Operating temperature of the multifunctionalmodule.

The objective is to obtain results complying withquality standards and predefined performances andleast cost. All these specifications will be respected andwill have an impact on the electronic and storagedesign.

SIMULATION RESULTS

Fig. 4. Panel Characteristic Ppv(Vpv) and deliveredpower by the panel Ppv.

EXPERIMENTAL RESULTS

This section is focused on different examples ofexperimental results. Tests were realised with aprototype of a boost converter associated with a MPPTcontroller.

Fig. 5 presents an experimental result of theresearch of the maximum power point, in steady-state,when the system is submitted at a constant irradiationlevel, high power in this case, for a MPPT designed byLAAS.

Ch1 5.0'11 Ch2 SOOmYCh4 1.0A Q

The operation of the complete system is quitecomplex and the analysis, regarding the number of sub­systems, is a real challenge. The development anddesign of the system can be done by the use ofmodeling and simulations.

Fig. 4 gives the Ppv(Vpv) simulated characteristics ofthe PV generator model function of solar irradiations.Oscillations around the maximum power point validatethe behavior of the simulated model.

Malhl 2.0W 10.0ms

Fig. 5. Steady state behavior of PV array variables anddc-dc converter.

Fig. 6 shows the behaviour of the PV electricalvariables when occurs a brutal increasing of the sunradiation. In this type of change, the PV currentincreases immediately. The PV voltage remainspractically unchanged and then the duty cycle remainsidentical allowing instantaneously a new recover of theMPP. The phenomenon is the same when the PVcurrent decreases (low sun radiation). In this type ofMPPT control, the losses of efficiency are minimisedduring transitory.

978-1-4244-1641-7/08/$25.00 ©2008IEEE

increase the system reliability; (d) the possibility ofadapting the power easily by putting the modules inparallel.

ACKNOWLEDGMENT

This project titled "Module LiPV" is funded in part byANR - Photovoltaic (French National ResearchAgency).

Max(M1) 22.14W

PPV REFERENCES

[1] R. Kaiser, "Optimized battery-management systemto improve storage lifetime in renewable energysystems", Journal of Power Sources 168 (2007) 58­65.

Cell capacity and voltage according to current at +20°C

Fig. 6. Response of the system submitted to brutalincreasing of solar radiation.

Finally, Fig. 7 shows a discharge characteristic of ahigh energy type lithium-ion battery, designed by SAFT,with different rates of discharge values. Characterizationof the multifunctional module can be made usingsimulation and experimental results of each sub-system.

[5] Johnson. A. Asumadu, M. Haque, H. Vogel, C.Willards, "Precision Battery Management System",Instrumentation and Measurement TechnologyConference (lMTC05), May 2005.

[4] F. Mattera, "Confidential report of theMultifunctional Module Lithium-Ion project", 2007.

[2] R. Leyva, C. Alonso, and A. Cid-Pastor, "MPPT ofphotovoltaic System using Extremum-SeekingControl," IEEE Aerospace and Electronic Systems,vol, 42, Jan. 2006, pp. 249-258.

[6] M. Glavin, W.G.Hurley, "Battery ManagementSystem for Solar Energy Applications", UniversitiesPower Engineering Conference, 2006. UPEC '06.Proceedings of the 41st International, Vol. 1,pp.79-83.

[3] Cabal C., Alonso C., Cid-Pastor A., Estibals B.,Seguier L., Leyva R., Schweitz G., Alzieu J.,"Adaptive digital MPPT control for photovoltaicapplications", International Symposium on IndustrialElectronics (lSIEO?), June 2007 pp. 2414 - 2419.

-3C-2C-CI2 -cCJ3-CP.i

~~---------~----------+=~---~---------+------~,-------~----="'~------='",,",,-

;

Fig. 7. Discharge voltages at different constant currentand +20°C.

CONCLUSION

The overall cost of a stand-alone PV system can bereduced with new battery technologies and new controlalgorithms which achieve high battery lifetime andreliability, under continuously varying atmosphericconditions, which give rise to intermittent PV energyproduction. In this paper, a new concept of amultifunctional module has been presented, consistingof the integration of all components in one structure.Advantages of the proposed module are: (a) the MPPTcontroller employed in the conversion chain ensuresmaximum of the energy is transferred to the battery, andthus better exploitation of the PV generator is achieved;(b) the battery lifetime is increased because of theutilization of lithium-ion technology; (c) the operatingmanagement strategy is a significant approach to

978-1-4244-1641-7/08/$25.00 ©2008IEEE