Mário João Murta Pina

Ventim Neves
email: ventim@uninova.pt

Alfredo Álvarez
email: aalvarez@unex.es

Amadeu Leão Rodrigues

Mário Ventim Neves

High Temperature Superconducting Fault Current Limiters as Enabling Technology in Electrical Grids with Increased Distributed Generation Penetration

Keywords:

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In the past years, several reasons justified the increasing penetration of dispersed, embedded or distributed generation (DG) in electrical grids. Amongst these highlights common demand on greenhouse emissions reduction by the use of renewable energy sources; energy efficiency and rational energy use; diversification of energy sources [START_REF]Impact of increasing contributions of dispersed generation on the power systems[END_REF]; availability, short construction times and cheap small generator plants; and the advantages of placing generators near consumers decreasing transmission costs [START_REF]CIRED Working Group 04: Dispersed generation. Preliminary report, CIRED Conference[END_REF]. Nevertheless these favorable aspects, DG still raises several relevant technical questions [START_REF] Conti | Analysis of distribution network protection issues in presence of dispersed generation[END_REF], [START_REF] Jenkins | Embedded Generation[END_REF], as network voltage changes (e.g. in partially loaded grids with high renewable generation); power quality issues (e.g. harmonics due to static converters); and conflicts with network protections. The latter relates with increase in short-circuit current levels and grid switchgears' selectivity issues, amongst others.

Restrictions arising from protections conflicts usually prevent additional DG penetration [START_REF] Boutsika | Short-circuit calculations in networks with distributed generation[END_REF]. Adding new rotating generators (e.g. wind turbines) may increase dramatically fault levels, as resulting short-circuit currents consist on phasor sum of original fault currents and new DG generators' fault currents [START_REF] Boutsika | Short-circuit calculations in networks with distributed generation[END_REF]. The latter are not considered when grids protections are originally designed. Simulations found in the literature [START_REF] Conti | Analysis of distribution network protection issues in presence of dispersed generation[END_REF], [START_REF] Tran-Quoc | Technical impacts of small distributed generation units on LV networks[END_REF], demonstrate that adding DG generators increases considerably fault levels which can exceed the current interrupting capacity of the existing switchgears.

An obvious solution to the above problem is to upgrade grid switchgears, adapting their rating to the new levels, but this may turn economically unfeasible. Reducing new generators fault contributions may be achieved by introducing impedances (transformers or reactors) between these and the corresponding grid, which increases losses. Explosive fuses may also be used but these must be replaced after a fault. In recent years, new technologies appeared, as solid state fault current limiters [START_REF] Kunde | Integration of fast acting electronic fault current limiters (EFCL) in medium voltage systems[END_REF], which have high semiconductors losses and are costly, or high temperature superconducting fault current limiters (SFCL). The latter are detailed in the paper.

Next section addresses authors' contribution to technological innovation. High temperature superconducting (HTS) materials are introduced and SFCL classified according to its characteristics and operation principle. Resistive and inductive types are explained in later sections. Future trends and conclusions are drawn in the end.

Contributions to Technological Innovation

The vision of future electric grids envisages coexistence of distributed and central generation, energy storage and bidirectional energy flow, between grid and consumers/producers. SFCL are a promising technology in such environment, addressing issues as safety or power quality. It is thus relevant to develop analysis tools for their dynamic behavior, and authors built such a straightforward algorithm for one type of those limiters (inductive), based on the individual analysis of device's elements (HTS and iron core) [START_REF] Pina | Reverse Engineering of Inductive Fault Current Limiters[END_REF]. It builds FCL's hysteresis cycle, allowing simulating grids' currents and correctly dimensioning switchgears or transformers.

High Temperature Superconducting Fault Current Limiters

Superconducting materials are natural fault current limiters, often recognized as one of its most promising power application. In superconducting state they carry high current densities (up to 10 4 A/mm 2 ) with negligible losses. Under a fault, current density increases. The material loses superconductivity and becomes resistive, limiting current. Once fault suppressed, it returns naturally to the superconducting state. This is the principle of the resistive SFCL, detailed in Section 3. These devices use material non linear resistivity in superconducting/normal state transition.

High Temperature Superconducting Materials

The main macroscopic properties of the first discovered superconducting materials (superconductivity was discovered by the Dutch physicist Kamerlingh Onnes in mercury, in 1911) were the abrupt loss of electrical resistivity under a certain critical temperature and the exclusion of magnetic flux from the material (Meissner effect). For decades, superconductors' highest critical temperature was around 20 K, so they had to be cooled with liquid helium (boiling temperature 4.22 K) in expensive cryogenic systems. The discovery of superconductivity in ceramic copper oxides, as YBa 2 Cu 3 O x , boosted this temperature to above liquid nitrogen boiling temperature, 77 K, reducing dramatically cryogenics' cost. In these high temperature superconductors, magnetic flux is not completely expelled from material's core, rather penetrates it in quantized amounts known as fluxoids in the so called mixed state. The transition from superconducting to normal state, known as quench, depends on the values of current density, temperature and flux density in the material.

Classification of Superconducting Fault Current Limiters

SFCL may be classified according to the way they are introduced in the grid, their use of quench, or employment of iron cores, amongst others.

Single-phase SFCLs' classification is presented in Fig. 1. It depends on how the devices are inserted in the grid: in series (resistive), magnetically linked (inductive) or through a rectifier bridge (rectifier). In this paper resistive and inductive topologies are presented. The rectifier SFCL was abandoned due to high voltage failures and high losses on the semiconductor elements used.

Three-phase SFCLs are the most important devices in power applications. They are built by three single-phase modules or using an exclusive three-phase design [START_REF] Sato | A Study on DC S/N Transition Type Superconducting Fault Current Limiting Interrupter[END_REF]. Although the latter approach uses less HTS material than three single-phase SFCL modules, a fault in one phase affects the remaining ones and this device is restricted to symmetric loads (e.g. motors). Thus, this approach is not presented in the paper.

Resistive Type Fault Current Limiter

The resistive SFCL is represented in Fig. 2. It is built, in its most basic form, by an HTS element in series with the grid, with a shunt resistance, R s , described later.

Operation Principle

In normal grid operation, the SFCL has negligible impedance and associated losses, being electrically invisible to the grid [START_REF] Noe | Investigation of high-Tc bulk material for its use in resistive superconducting fault current limiters[END_REF]. Under a fault, the current exceeds HTS's critical value, leading to quench. HTS's impedance rises abruptly, limiting current.

Reference [START_REF] Paul | Fault current limiter based on high temperature superconductors -different concepts, test results, simulations, applications[END_REF] proposes a simple qualitative explanation for the resistive SFCL behavior under a fault, based on the analysis of the curve that relates electrical field, E, with current density, J, in HTS materials, see Fig. 3 and Fig. 4: -If HTS length is "high", E is "low" so as J and ohmic losses. The heat produced is dissipated by the cryogenic system and material's temperature remains constant. Although this represents the ideal behavior of the SFCL, the amount of bulk HTS used (up to hundreds of meters [START_REF] Shimizu | Proposal of Flux Flow Resistance Type Fault Current Limiter using Bi2223 High Tc Superconducting Bulk[END_REF]) may turn it unfeasible.

-If HTS length is "low", E and J are "high". Thus, the initial current peak may not be sufficiently limited. Losses are "high" and HTS fast heating may not be absorbed by cryogenics, which can lead to material's collapse.

-In a "moderate" length HTS, E, J and losses are also "moderate". In this case there is an HTS slow heating.

The shunt resistance R s protects HTS material against hot spots formation, i.e. regions that enter normal state, increasing locally their resistivity and temperature and degrading the material. R s provides an alternative low resistance path for current.

Resistive

• Pure resistive

Resistive SFCL Demonstrators

Under the framework of CURL 10 project [START_REF] Bock | CURL 10: Development and Field-Test of a 10 kV/10 MVA Resistive Current Limiter Based on Bulk MCP-BSCCO 2212[END_REF] a three-phase 10 kV/10 MVA was build with superconducting bifilar coils as limiting elements. The system operated in field conditions in RWE company's electric grid in Netphen, Germany, coupling two 10 kV buses. Yet, no fault occurred in that period. This concept evolved to magnetic field assisted quench SFCL, referred in the last section.

A limiter using thin film HTS was successfully tested in a 14 kV single-phase grid [START_REF] Sim | 14 kV single-phase superconducting fault current limiter based on YBCO films[END_REF]. 10 kA prospective current is limited to 445 A in the first peak and previously to less than 140 A. Although operation is demonstrated, thin films' high cost, namely for transmission grids with large number of elements, lead to abandoning this approach.

The New Energy and Industrial Technology Development Organization, NEDO, is developing a resistive limiter based on HTS coated conductors. In a 3.8 kV singlephase grid, a 17 kA prospective current is limited to 700 A [START_REF] Yazawa | Superconducting fault current limiter using high-resistive YBCO tapes[END_REF].

Conclusions

Resistive SFCL allows fast limiting action, in less than half cycle, since HTS element is in series with the grid. It is also a compact device. Yet, several elements must be series connected in order to achieve desired limiting performance, degrading quench homogeneity which can lead to material's destruction. Current leads are also needed, to make current transition from room to cryogenic environment. Recovery time is usually in the order of a few seconds [START_REF] Paul | Fault current limiter based on high temperature superconductors -different concepts, test results, simulations, applications[END_REF]. Devices demonstrated their technical viability in medium voltage distribution networks, usually associated with DG.

J (A/cm 2 ) J C 1 Flux Creep Flux Flow Normal state

Current Time

Fast heating Slow heating Constant temperature Fault Fig. 3. HTS E-J qualitative dependence. The 1 µV/cm criterion defines critical current density, J c , below which the material is in the superconducting state. Above J c and below normal state, the material is in the mixed state.

Fig. 4. Different types of response under fault in resistive SFCL with distinct lengths (adapted from [START_REF] Paul | Fault current limiter based on high temperature superconductors -different concepts, test results, simulations, applications[END_REF]).

Inductive Type Fault Current Limiter

The inductive type SFCL, introduced in [START_REF] Bashkirov | Current-limiting reactor based on high-T c superconductors[END_REF], is magnetically linked with the power line, behaving as a current transformer with short-circuited secondary, see Fig. 5. The primary is fed by the power line, while the secondary is built by an HTS element, usually a cylinder or a stack of cylinders. The primary and secondary concentrically embrace an iron core forming a closed or open path for magnetic flux.

Operation Principle

In normal operation, the primary's magnetic flux induces currents in the HTS that shield magnetically the iron core, leading to a negligible primary's linked flux (no flux lines penetrate the core) and consequently to negligible impedance (only related to leakage), see Fig. 6. Under a fault, line current increases abruptly and the HTS is no longer able to shield flux. It finds a preferential path through iron where it is amplified, see Fig. 7. Primary's linked flux increases by some orders of magnitude, increasing inductance and limiting current. Current evolution is represented in Fig. 8. Limitation is visible in the first half cycle, although it gets more effective after that.

Inductive SFCL Demonstrators

The most relevant demonstrator found in the literature was implemented by ABB Company, namely a three-phase 10.5 kV/1.2 MVA built in a three leg core [START_REF] Paul | Test of 1.2 MVA high-Tc superconducting fault current limiter[END_REF]. In laboratory, 60 kA prospective currents were limited to 700 A in the first half cycle and to less than 250 A after 50 ms. After 6 months of operation in Kraftwerk am Löentsch hydroelectric plant, Swiss, the device "…did not face any major problems". The re-cooling time for the material was in the order of two to ten seconds.

Conclusions

The inductive SFCL has some important advantages compared with the resistive one. Current leads, source of ohmic losses, are dispensable, limiting impedance is easily adjusted through primary turns, and hot spots are easily suppressed by cryogenics. Yet, it has considerable weight and volume due to iron cores. Large bulk superconductors are also hard to produce in good homogeneity conditions. Nevertheless, technical viability in distribution networks is demonstrated.

Future Trends and Conclusions

Resistive SFCL are object of great research due to their compactness. Yet, hot spot formation is a major issue, as it might irreversibly damage HTS elements. A novel design called magnetic field assisted quench is being developed in the framework of CULT 110 project [START_REF] Elschner | HTS Components for High Voltage Resistive Current Limiters Based on a Magnetic Field Triggered Concept[END_REF], using applied field's quench dependence. HTS element is placed inside a conventional coil and these are shunted. In normal operation, current flows through the "zero" impedance element. Under a fault, a voltage is developed at its terminals. Part of the current deviates to the coil, originating magnetic flux that forces homogeneous and fast quench, avoiding hot spots. Although less compact, robustness of inductive SFCL makes them attractive in distribution networks, where DG is applied. The algorithm developed by the authors [START_REF] Pina | Reverse Engineering of Inductive Fault Current Limiters[END_REF] is thus of utmost relevance.

SFCL limit DG short-circuit currents, allowing its increasing penetration. Although cryogenics is still the most relevant technical challenge in superconductivity, it can be diluted if other HTS technologies are present, as lossless superconducting transformers, flywheels (energy is stored in a rotating mass, using HTS bearings to avoid contact) and SMES (superconducting magnetic energy storage, energy is stored in the magnetic field of an HTS coil). The last ones are related with energy storage and power quality, key issue in the advent of future electric grids, relying on DG, energy storage and bidirectional energy flow and complying with strict standards.

Fig. 1 .

 1 Fig. 1. Classification of single-phase superconducting fault current limiters.

Fig. 2 .

 2 Fig. 2. Resistive superconducting fault current limiter electrical diagram.

Fig. 5 .

 5 Fig. 5. Inductive superconducting fault current limiter electrical diagram.

Fig. 6 .

 6 Fig. 6. Axisymmetric representation of an inductive SFCL in normal operation. Primary's flux is shielded from iron core by HTS induced currents.

Fig. 7 .Fig. 8 .

 78 Fig. 7. SFCL under a fault. Flux penetrates iron core, increasing device's impedance.


Carmen Lungoci
email: carmenlungoci@unitbv.ro

Elena Helerea
email: elenahelerealungoci@unitbv.ro

Evaluation of Supercapacitors Effects on Hybrid Energy Systems for Automotive

Keywords: Supercapacitors, energy systems, energy management, automotive, simulations

This work aims at evaluating the effects of the supercapacitors presence in hybrid energy systems used in automotive. The design and the electrical schema of a hybrid energy system that contains batteries and supercapacitors and propel a synchronous motor are purposed. The motor operating regime is described, detailing the drive evolution of the cycle speed imposed. In these conditions, to model the systems behavior, simulations developed in Matlab/Simulink environment are carried out. Two energies management strategies for the ensemble energy system-motor are implemented. Simulations are done and the energy management is discussed, making the comparative analyses. Applying a current control strategy on the supercapacitors, under two working conditions, functional diagrams are showed and compared. The results obtained highlight the advantages of the supercapacitors.

Introduction

Nowadays, in transport the use of hybrid energy systems represents an important current topic [START_REF] Chan | Modern Electric Vehicle Technology[END_REF] [START_REF] Yap | Hybrid energy/power sources for electric vehicle traction systems: The Institution of Electrical of Electrical Engineers[END_REF]. In a hybrid vehicle, the most utilized batteries are the Ni-Mh and Li-Ion, presenting advantages due to their technology. But energy recovery using a traditional system composed by batteries (SB) is a constraint. The weight, volume and the price of the batteries reduce the efficiency of the system. From this reason, there are many researches to optimize the architecture and the structure of an energy system [START_REF] Steiner | Energy storage on board of railway vehicles[END_REF] [START_REF] Livint | Vehicuel electrice hibride[END_REF].

The useful strategy adopted is an energy system containing many energy sources. Among the existing solutions, the hybrid energy system using batteries and supercapacitors (SBSC) is proposed and analyzed. This work is aims to develop a SBSC system for electrical traction. The architecture of this system is proposed and the studies regarding the structure, functionality and modeling on it are presented. Using the models for the elements components of the system, developed by the authors in the previous papers, the modeling and simulations are realized, in order to obtain the electrical parameters: current, power, energy, in the test conditions of functioning, imposed by a proposed drive cycle. The necessary power resulted is ensured by the hybrid energy system, applying two energy management strategies. In this way, the effects brings by the supercapacitors presence in the energy system are finally underlined.

Contribution to Technological Innovation

The automotive field requires new technological innovations regarding energy demands inside of subsystems. This trend is putting increasing pressure on energy systems from a vehicle to be efficient and run for long periods. From technical point of vu, they must be able to store and deliver enough energy and power.

Batteries are preferred for most applications because of their superior capability to store energy. The disadvantage consist is the poor ability to deliver power.

Supercapacitors offer a combination of high power and good energy performance parameters with commercial relevance. They are very high surface area activated carbon capacitors that use a molecule-thin layer of electrolyte [START_REF] Lungoci | Eco-design of a hybrid energy system for electric vehicles[END_REF]. This new technology gives them the property to storage energy proportional to the charge surface area. So, they are able to hold a very high charge which can be released in a controlled manner.

Taking in account the batteries and the supercapacitors properties, there is a strong trend in automotive towards hybridizing both energy sources, making the energy system more highly and efficient functional from the point of vu of the energy management. The last technological advances in the applications to automobiles are based on this ability of supercapacitors to complete with the batteries.

State of Art

In hybrid vehicles, the load profile is characterized by a low value of a main power with big picks of power requested in transient regimes. In these conditions, a hybrid energy system that contains supercapacitors near batteries (SBSC) ensures performing answers of power, higher like in SB systems.

There are researchers which evaluated the effects of the supercapacitors in the hybrid energy systems and they developed and realized practically the SBSC systems [START_REF] Livint | Vehicuel electrice hibride[END_REF] [START_REF] Lungoci | Eco-design of a hybrid energy system for electric vehicles[END_REF].

The studies from the literature proved the advantages of SBSC systems use, in which batteries ensure a constant voltage and the role of supercapacitors is to deliver/storage the transient picks of power.

In order to optimize the structure, to improve the functionality and to grow the energy efficiency of the SBSC systems used in automotive, new analyses are necessary, the improvement of the existing models is required and the development of new models is expected.

Research Contribution and Innovation

In the present research improvements on the energy systems used in automotive are proposed. The main objectives of this work are: -To present the ensemble hybrid (batteries-supercapacitors)-synchronous motor; -To simulate the functioning regimes under different running conditions; -To implement two energies management in order to highlight the contribution of the supercapacitors in automotive presented system.

System Description

The architecture of the proposed supply/propulsion system consists of a batteries pack and a supercapacitors pack, being utilized in a hybrid vehicle (Fig. 1). The battery pack is directly connected to the DC bus, providing a constant voltage for the system. Mechanical load of the system is represented by the inertia and brakes of the vehicle. Propulsion is ensured by MSA traction motor. The DC/AC inverter, coupled to MSA, transforms the electrical DC power provided on the bus into AC power and controls its parameters. The power supplying the inverter is delivered by the both: main and the secondary power source. Supercapacitors pack represents the secondary power source, ensuring optimal performance for the motor in punctual operation regimes. The bidirectional DC/DC converter connects the supercapacitors pack to DC bus, controlling current flow and allowing energy recovery while breaking. The functioning regimes of the motor are modeled according to the proposed running drive cycle -3ECE. The entire power demand by the motor is ensured by the batteries and supercapacitors pack, designed according to the speed cycle.

The aim of simulations is to obtain a quick and consistent response to transient power requests from the motor, in order to obtain a high energy efficiency of the entire system. The model used for simulations includes a batteries pack consisting of 23 elements, providing a constant voltage in amount of 280V. Supercapacitors pack introduced in the simulation model has the following parameters: total capacity in amount of 5.8 F, voltage across terminals 150 V and internal resistance 0.19 Ω. Electrical propulsion motor is a permanent magnet synchronous motor, with the following parameters: U n = 230 V; I n = 42,9 A; M n = 35 Nm; n n = 3300 rpm; P n = 12,1 kW. Energy efficiency is to be determined in simulations, by analyzing obtained responses.

Two energy management strategies are presented:

1. The necessary of the electrical power requested by the motor is entirely ensured by the batteries (SB system), during the drive cycle period; 2. The same necessary of power is ensured by the batteries and the supercapacitors. The energy management is treated, observing the temporal responses obtained by simulations under the under drive cycle conditions for each case above. Running cycles set (3ECE) is composed from several European urban running cycles, containing different acceleration, constant speed and breaking phases. There are presented three levels of rotational speed for the motor: 1100 rot/min, 2200 rot/min, 3300 rot/min, the motor dynamics having easy accelerations and breakings.

Energy Management Using SB

The following work assumptions have been considered:

• The unique energy source used is the pack of batteries; • The entire electrical power requested by the motor is ensured by the batteries and the braking energy of the motor is completely taken by the batteries; • The model of batteries is an internal resistance in series with an electromotive voltage source; • For the synchronous motor was used the experimental model developed by the authors and detailed in [START_REF] Lungoci | Modular test bench for a hybrid electric vehicle with multiples energy sources[END_REF]. In Fig. 2 the necessary of current (I bus ) variation during the 3ECE cycle is presented. It can be observed the many peaks of current corresponding when crossing to another level of speed. In Fig. 3 the current variation curve on the batteries during the same cycle are obtained. Fig. 4 shows the electrical power required by the traction motor during the cycle and Fig. 5 presents the power electrical variation at the pack of batteries in the same period. Graphics shows that during the running cycle, batteries must ensure all the power picks required by the motor, which implies their overload. On the other hand, the batteries must take the entire breaking power reinserted by the motor in the DC bus, which is practically impossible. The SB system is not sufficient to ensure the motor functionality, being required to use a hybrid energy system, like SBSC, that can answer efficiently of the dynamics of the motor.

Energy Management Using SBSC

The electrical model of the supercapacitors used in simulations is composed by a variable capacity in series with a resistance. The pack of batteries is simple modeled by an ideal voltage source. The synchronous motor has the experimental model detailed by the authors in the previous works [START_REF] Lungoci | Modular test bench for a hybrid electric vehicle with multiples energy sources[END_REF]. The functioning regimes of the motor are given by the running drive cycle 3ECE. The power control principle utilized for this SBSC, which establishes the contribution of each source in supplying the motor, is based on the separation of medium power demand from the transitory power demand and consists of a first order low pass filtering function. The filter is applied to the current supplying the motor obtaining a reference output current for controlling the supercapacitors pack. In order to implement control strategy for the supercapacitors pack, the transfer function of a first order filter has been utilized. To highlight the filter action on the system functionality, the time constant will take two values: first, 3 s, than simulations are repeated for value of 6 s. Simulations has been done using the Matlab Simulink software.

The motor current is in Fig. 6. In Fig. 7 is layout the current variation that must be ensured by the pack of batteries, when using the constant time filter τ = 3 s. In Fig. 8 is shown the variation of the current when τ = 6 s. Fig. 10 shows the reference current applied to the pack of supercapacitors for τ =3s, and in Fig. 11 for time constant τ = 6 s. The reference current is resulted after the filter at the converter entry, when batteries and supercapacitors are used together. The graphics prove an important filter of the current delivered by the batteries. Supercapacitors take efficiently the power picks and batteries are smoothed. Graphics prove a better management of current sharing on both energy sources, which ensures together the necessary of electrical energy of the motor.

Discussions of Results

In this section, a comparative analyze of graphics in both energy management strategies is done. The power variation curves for the ensemble SBSC-synchronous motor are detailed. In Fig. 11 is represented the requested power by the motor. In Fig. 12, Fig. 13 and Fig. 14 are analyzed the results obtained after the utilization of both energy management strategies (SB and SBSC) and after utilization of the power control strategy on the supercapacitors. Therefore, is presented the power variation on the batteries terminals in the three cases:

1. using SB like energy management strategy;

2. using SBSC like energy management strategy and a power management strategy with a filter having τ = 3 s; 3. using SBSC like energy management strategy and a power management strategy with a filter having τ = 6 s. Comparing the graphics obtained, it can be deduced: 1. When using SB systems, power and energy on batteries have high values and high picks, that crave for batteries; 2. Using SBSC systems, the transient picks of power and energy on batteries are smoothed; 3. Growing-up the time constant value of the filter used for power control strategy on supercapacitors, the power/energy on batteries becomes smaller and smoother. Therefore the contribution of the supercapacitors in the energy deliver/storage processes became more important and the system energy efficiency is improved.

Simulations effected highlight the effects of supercapacitors in the hybrid energy system used in automotive. With this research there are certified:

• the validity of the SB and SBSC system models;

• the validity of the energy management strategy with SBSC;

• the validity of the control strategy of the supercapacitors pack based on power sources separation; • the contribution bring by the supercapacitors in hybrid energy systems.

Conclusions and Further Works

The use of SBSC as power control strategy leads to an optimized model of the propulsion system, in accordance to requirements. The presence of supercapacitors in power supply/recovery systems used for electric traction increases battery lifetime and allows a fair design in terms of weight and volume. Therefore, the entire system can be design at lower power requirements, power dimensioning being separate from energy dimensioning of components, ensuring therefore increase of efficiency, reduction of dimensions, costs and weight. The advantages offered by power systems with batteries and supercapacitors recommend them for applications in transportation, especially where high power peaks are required.

The paper presented an analysis regarding power control and energy management when utilizing SB and than a SBSC-synchronous motor system, in functioning conditions imposed by standard 3ECE running cycles.

A command strategy has been implemented for the supercapacitors pack in order to optimize system performance and energy efficiency.

Results of the analysis prove that supercapacitors particularly reduce current peaks applied to/from the battery and improve acceleration characteristics, range and efficiency of the vehicle, especially when the running cycle determines a lot of current peaks.

For the future, experimental results are expected and new models for the components of system will be implemented.

Fig. 1 .

 1 Fig. 1. Architecture of the power system used in hybrid electrical vehicle.

Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .

 2345 Fig. 2. Motor current I bus during 3ECE cycle. Fig. 3. Current I bat of batteries during cycle.

Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .

 6789 Fig. 6. Current I bus at the traction motor.

Fig. 11 .

 11 Fig. 11. Power P bus at the motor terminals.

Fig. 12 .Fig. 14 .

 1214 Fig. 12. Batteries power P bat , SB-motor. Fig. 13. Batteries energy W bat , SB-motor.

Fig. 15 .

 15 Fig. 15. Batteries power P bat , SBSC-motor, with filter constant τ = 6s. The energy management is underlined from the energy variation curves resulted from simulations in the Fig. 13, Fig. 16 and Fig. 17.

  .

Fig. 16 .

 16 Fig. 16. Batteries energy W bat, SBSC motor, τ = 3s.

Fig. 17 .

 17 Fig. 17. Batteries energy W bat , SBSC motor, τ = 6s.


Radu Musat
email: r_musat@yahoo.com

Elena Helerea
email: helerea@unitbv.ro

Characteristics of the PTC Heater Used in Automotive HVAC Systems

Keywords: PTC heater, HVAC, thermal comfort, vehicle, characteristics

The present heating method which uses the cooling system of the internal combustion engine of the vehicle takes a lot of time to heat the interior air. In order to improve the heating process, auxiliary devices are required. The new innovative techniques propose as auxiliary heating device positive temperature coefficient heaters. For a better control of the temperature, the parameters of these devices should be known. This paper deals with a detailed description of the new types of positive temperature coefficient heaters used in automotive field. The test bench system developed for determining the parameters and characteristics of the positive temperature coefficient heater is described. The obtained data of the thermal resistance and voltage-current characteristics are used for controlling the temperature in order to design and control the positive temperature coefficient heaters properly.

Introduction

The passenger's thermal comfort inside a vehicle is ensured by using heating, ventilating and air conditioning (HVAC) systems.

Modern internal combustion engines (ICE) with fossil fuels offer improved efficiency and dissipate less heat to the cooling system of the engine. On cold days, these engines produce little heat excess and are unable to provide the amount of heat for warming the vehicle interior up to a comfortable level [START_REF] Daly | Automotive air conditioning and climate control systems[END_REF].

In the automotive field, different technologies are available to provide the heat and to quickly ensure the passenger's thermal comfort. The conventional heating system uses the heat loss produced by ICE, which is transferred to the cooling system and then used for heating the vehicle interior air. This conventional heating method has disadvantages: it loses about 60 % of the heat and the interior air temperature increases slowly. To improve the conventional heating method, auxiliary heating devices are required [START_REF] Kassakian | The Future of Electronics in Automobiles[END_REF].

This paper describes and analyses an auxiliary heating device proposed to be used in the automotive field. Also, a test bench system is developed for determining the parameters and characteristics of the positive temperature coefficient (PTC) heater.

The obtained data of the thermal-resistance R = f (T) and the voltage-current U = f (I) characteristics will be used for controlling the temperature in order to design the PTC heater for vehicle thermal comfort system.

Contribution to Technological Innovation

In the paper an innovative technique is proposed: a PTC heater is used as auxiliary heating device because of the specific thermal and electrical properties: thermal selfregulation, high responding time, quickly removing the condensed drops from the windscreen and improving the visibility through it.

Based on the experimental results described in the paper, a model and simulation of the PTC heater are proposed. An integration of the PTC model in the vehicle heating diagram for overall heating process simulation will be made in order to design and control optimally a PTC heater.

State of the Art and Existing Problems

Since the 1990's, research has been developed regarding the auxiliary heaters that warm up the interior air as soon as the vehicle is started [18-21]. There have been difficulties as regards the fact that the auxiliary heaters demand a large quantity of energy, and consequently they operate only when the engine is running, which increases the fuel consumption and the pollution.

After the significant increase in the fuel prices in 2006, much automotive research has shown renewed interest in fuel-efficient vehicles. In gasoline-fuelled vehicles, about 40% of fuel energy is wasted in exhaust heat, while 30% of energy is transferred through the engine coolant. Because of that, these engines produce little heat excess, especially in winter time, in city traffic and traffic jams, and they are unable to warm quickly the passenger compartment to a comfortable level [START_REF] Daly | Automotive air conditioning and climate control systems[END_REF].

Various vehicle models from leading manufacturers around the world are now equipped with PTC auxiliary heaters. In 2007, 65% of all diesel vehicles in Europe were equipped with an auxiliary heater and this is expected to rise to 90% by 2010. It is obvious that PTC heaters will not be limited to diesel vehicles but will be also used in gasoline-powered ones as well [START_REF] Ehsani | Modern Electric, Hybrid Electric, and Fuel Cell Vehicles -Fundamentals, Theory, and Design[END_REF].

Commonly used PTC materials include high density polyethylene (filled with graphite), polymeric and titanate ceramic materials (which work by grain boundary effects). PTC heaters based on barium titanate ceramic (BaTiO 3 ), having high resistance and power-dissipation characteristics, are mostly used in automotive industry [START_REF] Amin | Piezoresistivity in semiconducting positive temperature coefficient ceramics[END_REF], [START_REF] Heywang | Bariumtitanat als Sperrschichthalbleiter[END_REF], [START_REF] Huybrechts | The positive temperature coefficient of resistivity in barium titanate[END_REF], [START_REF] Wang | Electrical properties of PTC barium titanate[END_REF], [START_REF] Mamunya | PTC effect and structure of polymer composite[END_REF].

Several authors have studied the resistance -temperature dependency of PTC heaters [START_REF] Heywang | Resistivity anomaly in doped barium titanate[END_REF], [START_REF] Huybrechts | The positive temperature coefficient of resistivity in barium titanate[END_REF], [START_REF] Jonker | Some aspects of semiconducting barium titanate[END_REF], [START_REF] Schedel | New PTC Heaters Control High Voltages in Hybrid and Electric Vehicles[END_REF], [START_REF] Apostol | Obtaining the High Performance PTC Thermistors based on Barium Titanate[END_REF]. Over the years, various theoretical and experimental models have been developed to describe the PTC heater effect, e.g. in semiconducting BaTiO 3 , such as Heywang (the most accepted model), Jonker and Wang [START_REF] Heywang | Semiconducting barium titanate[END_REF], [START_REF] Jonker | Some aspects of semiconducting barium titanate[END_REF], [START_REF] Wang | Electrical properties of PTC barium titanate[END_REF].

A comparative analysis between PTC thermistors and heaters sustains the development of new heating solutions.

PTC Thermistors and Heaters

PTC thermistors have specific features: thermal self-regulating and efficient energy consumption, no danger of fire since there are no glowing parts, no excess temperature protection required, no smell, no radiation and oxidation, fast thermal response time, temperature setting from +50 °C to + 320 °C, compact design and long life service [START_REF] Jones | PTC Thermistor Introduction[END_REF].

PTC thermistors have a wide variety of applications, which can be divided into two distinct categories [START_REF]Advanced thermal Products Inc[END_REF]. The first category utilizes the resistance-temperature or voltage-current characteristics of the PTC thermistor. These are known as self heated applications and include: self-regulating heaters, PTC over-current protectors, time delay, motor starting etc. The second category comprises zero power or sensing applications (e.g. temperature sensors and control) [START_REF]Advanced Thermal Products Division[END_REF].

The PTC thermistor has a low value of electrical resistance at low temperatures. When a voltage is applied to the cold elements, a high current is generated and the value of resistance rises with the temperature. As soon as the current flows through the device, the elements warm up by electrical dissipation until a steady state is reached and the resistive elements have reached their working temperature. This triggers the reduction of the electric current because the electric charge is unable to cross the boundaries of the tiny crystals at high temperatures. For this reason, the manufacturers of auxiliary heaters propose the use of the PTC thermistor principle [18], [START_REF] Hella | [END_REF], [START_REF] Termico | [END_REF], [21].

The PTC heater is mounted inside the HVAC system, after the heat exchanger. In Fig. 1, there is illustrated the position of the PTC heater and the air flow inside the HVAC system.

The PTC heater is connected to the vehicle electrical system and it is controlled by the Engine Control Module (ECM), HVAC control panel and relays [START_REF] Amsel | Use of CAE-Methods to analyse the influence of new electrical systems behaviour on tomorrow's 42V[END_REF]. In Fig. 2 there is illustrated the PTC heater control diagram. The PTC heater, illustrated in Fig. 3, consists of a moulded plastic plate (1), an electrical connector (2) and heating resistive elements with fins (3). The PTC heating resistive elements (Fig. 4) consist of small metallised ceramic plates [START_REF] Amin | Piezoresistivity in semiconducting positive temperature coefficient ceramics[END_REF], which are layered alternately along the unit core with aluminium radiator elements (2). These layers are held together by spring elements in a frame. The aluminium elements ensure the electrical contacts and transfer the heat to the passing heater air flow [START_REF] Stone | Introduction to Internal Combustion Engines. 3rd Edition[END_REF]. The heating resistive elements are separated in different heating circuits in order to adapt the heating power to different requirements and they act as positive temperature coefficient resistors. The maximum value of the temperature of the elements is around 165 °C, if no air flows through the heating system [START_REF] Daly | Automotive air conditioning and climate control systems[END_REF].

The heating elements are switched progressively in order to reduce the demand factor of the generator, to prevent load dumps and ECM problems caused by high currents.

The PTC heater operates only: (i) at low external temperatures; (ii) when insufficient heat is supplied by ICE cooling system; (iii) at reduced loads of the generator.

The total electrical power consumption for a PTC heater ranges between 900 W and 2000 W, depending on the air volume of the vehicle interior. Vehicles equipped with PTC heater are fitted with a more powerful generator.

Experiments and Results

In order to model and simulate the processes which take place in the HVAC system, data related to the characteristics of the auxiliary heating devices are required. For a PTC heater, it is important to analyze and measure the temperature -resistance characteristics and the voltage-current characteristic in order to design the PTC heater and to determine the optimal thermal comfort.

To measure and obtain the temperature-resistance curves, R = f (T), a test bench was developed. The test bench, illustrated in Fig. 5, consists of a heating camera (type Carbolite PF/200), a cooling camera (type Derby DK 9620), temperature sensors (placed inside the heating/cooling camera) and a data acquisition system.

To measure and obtain the voltage-current characteristics U = f (I), a test bench was also developed. The test bench, illustrated in Fig. 6, consists of a battery (type Rombat Pilot Diesel Hybrid, 12V / 77Ah), an ammeter, a voltmeter, temperature sensors (placed on PTC heating elements) and a data acquisition system.

All the acquired data are processed by means of MATLAB tool. To make the experiments, a 1000 W / 13.5 V PTC heater sample was used. This heater consists of four power stages (two stages of 333 W and two stages of 166 W). In Fig. 7, there is illustrated the electrical diagram for the PTC heater sample. As it can be seen in Fig. 8, the obtained data show that the resistance initially decreases with the increase in temperature. In the low temperature region, the resistance curve has a negative temperature coefficient. The minimal values of the resistance obtained at specific threshold temperatures, T t and the calculated values for the temperature coefficient of the resistance α R (for the negative and positive slope), for each stage are given in Table 1. For temperatures greater than T t , the negative temperature coefficient is changed to a positive one.

Table 1. The values of the resistance R, the threshold temperatures T t . and the temperature coefficient of the resistance α R for each stage.

Stage The values of the temperature coefficient of resistance require specific thermal regulating process characteristics of the PTC heater. Based on these coefficients, the PTC heaters can be properly designed. The thermal resistance characteristic is appropriate for preventing the PTC heater overheating.

R [Ω] T t [°C] α R-[K -1 ] α R+ [K -1 ] 1
As it is shown in Fig. 9, the voltage-current characteristics are linear until the PTC heating elements have reached their working temperature and a steady state is reached. After these values, the electrical current drastically decreases.

Warming up of the PTC heater depends on the electrical power inside the elements and can be described with the following relationship:

dt dT C T T t T R t U t I I U PTC A ⋅ + - ⋅ ⋅ = ⋅ ) ( ) , ( ) ( ) ( ) ( 2 δ . ( 1
)
where: U(t) -voltage supply; I(t) -electrical current through the PTC heater; δ -heat dissipation of PTC; T -present temperature of PTC; T A -ambient temperature; C PTC -heat capacity of PTC; dT/dt -time temperature variation.

Based on the voltage and current data values, the dissipation power -temperature characteristics for each stage are obtained and illustrated in Fig. 10.

As it can be seen in Fig. 10, there is a maximum dissipation power corresponding to the threshold temperatures for each PTC heater power stage. The obtained values of the parameters and the characteristics can be compared with those from specialty literature. The PTC heater tested above proved to have the same characteristics with those from a typical PTC thermistor model. The developed test bench allows testing these characteristics rapidly, efficiently and accurately.

The high responding time is the typical feature of the PTC heater. This is useful after the vehicle is started: till the engine reaches its operating temperature, the conventional heating system does not supply the necessary heat for the interior air. This additional heat can practically be accounted to the PTC heater.

Conclusion and Future Work

This paper deals with an analysis on auxiliary heating devices used in vehicle thermal comfort systems. A test bench system was developed for determining the parameters and characteristics of PTC heater and measurement data were processed.

Based on the experimental results described in the paper, a model and simulation of the PTC heater was developed. An integration of the PTC model in the vehicle heating diagram for overall heating process simulation is to be made.

The PTC heaters are necessary in some conditions (e.g. winter time) to increase the passenger's thermal comfort. The PTC heaters are ideal for thermal comfort control due to their capacity of thermal self-regulating and high responding time.

The challenge is that the PTC heater demands a large quantity of electrical current, so it operates only when the engine is running and thus increases the fuel consumption and the pollution.

New higher capacities of PTC heater would be possible with a 42 V vehicle wiring system. The transition to 42 V voltages, the decentralisation of power distribution and the development of more complex energy sources will provide new innovative systems to supply PTC heaters properly.

Since vehicles are getting more efficient, they free less heat for the heating system; thus, by including auxiliary devices consuming electric energy, the heating time will be improved and the engine efficiency increased. The electric energy is taken from the engine through the generator/battery system, which reduces the overall efficiency of the vehicle. Consequently, an adequate energy balance and management should be accomplished.

Increasing the overall efficiency is the main objective in any PTC heater design and can be achieved by different techniques, such as: improving the technology of materials, manufacture and control processes.

Fig. 1 .Fig. 2 .

 12 Fig. 1. HVAC system. Fig. 2. PTC heater control diagram.

Fig. 3 . 4 .

 34 Fig. 3. PTC heater components. Fig. 4. PTC heating resistive elements.

Fig. 5 .

 5 Fig. 5. Test bench scheme implementation for thermal resistance characteristics.

Fig. 6 .

 6 Fig. 6. Measurement scheme for determining U = f (I) characteristics.

Fig. 7 .

 7 Fig. 7. Electrical diagram of the PTC heater sample. In Fig. 8 and Fig. 9, there are illustrated the experimental results representing the characteristics R = f(T) and U = f(I), for each stage.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Thermal resistance characteristics. Fig. 9. Voltage-current characteristics.

Fig. 10 .

 10 Fig. 10. Dissipation power -temperature characteristics for PTC heater sample.


Aitor Mata

Bruno Baruque
email: bbaruque@ubu.es

Belén Pérez-Lancho

Emilio Corchado
email: escorchado@usal.es

Juan M Corchado
email: corchado@usal.es

Forest Fire Evolution Prediction Using a Hybrid Intelligent System

Forest fires represent a quite complex environment and an accurate prediction of the fires generated is crucial when trying to react quickly and effectively in such a critical situation. In this study, an hybrid system is applied to predict the evolution of forest fires. The Case-Based Reasoning methodology combined with a summarization of SOM ensembles algorithm has been used to face this problem. The CBR methodology is used as the solution generator in the system, reusing past solutions given to past problems to generate new solutions to new problems by adapting those past solutions to the new situations to face. On the other hand, a new summarization algorithm (WeVoS-SOM) is used to organize the stored information to make it easier to retrieve the most useful information from the case base. The developed system has been checked with forest fires historical and experimental data. The WeVoS-CBR system presented here has successfully predicted the evolution of the forest fires in terms of probability of finding fires in a certain area.

Explaining the WeVoS-SOM Algorithm

Case-Based Reasoning systems are highly dependent on stored information. The new algorithm presented here, Weighted Voting Summarization of SOM ensembles

Introduction

A hybrid artificial intelligence system [START_REF] Abraham | Hybrid learning machines[END_REF], [START_REF] Herrero | MOVIH-IDS: A mobilevisualization hybrid intrusion detection system[END_REF] is presented in this study. It is applied to generate predictions about the evolution of forest fires. That kind of systems combines symbolic and sub-symbolic techniques to construct more robust and reliable problem solving models. In this study a system based on Case-Based Reasoning (CBR) [START_REF] Watson | Case-based reasoning is a methodology not a technology[END_REF] and Topology Preserving Models [START_REF] Kohonen | The self-organizing map[END_REF] is presented and successfully applied.

Case-Based Reasoning systems have the potential to use past information in order to generate useful knowledge that may be used to solve new problems. Those systems should organise the information handled in order to improve the way that information is used. When the amount of information stored in a CBR system grows, the results obtained are normally better. But the growth of the case base (internal structure where the data is accumulated) also implies a more difficult retrieval process, where more information has to be considered in order to obtain the best possible collection of data.

The summarization algorithm presented here, WeVoS-SOM (Weighted Voting Summarization of SOM ensembles) [START_REF] Baruque | A weighted voting summarization of SOM ensembles[END_REF] represents the organizing system of the internal structure of the data in the CBR system. With such an inner organization, it is easier to locate the new data that is introduced in the system and to retrieve the needed information to solve new problems.

The combination of both, the generalization power of the CBR methodology and the organizational capabilities of the WeVoS-SOM algorithm generates a system that has been used to generate predictions in a natural environment such as forest fires. Historical data have been used to check the correction of the system, where the predictions generated were compared with the actual past data.

This paper is organized as follows. In the second section the Case-Based Reasoning methodology is introduced and briefly explained. In the third section, the novel summarization algorithm presented in this paper is explained and analyzed. Next, WeVOS-CBR, the developed system is described, and its main phases are detailed. Finally, some results of the application of the system to the forest fire case study and some conclusions are shown.

Introducing the CBR Methodology

Case-Based Reasoning [START_REF] Watson | Case-based reasoning is a methodology not a technology[END_REF] origins are in knowledge based systems. CBR systems solve new problems acquiring the needed knowledge from previous situations [START_REF] Aamodt | A Knowledge-Intensive, Integrated Approach to Problem Solving and Sustained Learning[END_REF]. The main element of a CBR system is the case base, a structure that stores the information used to generate new solutions.

The learning capabilities of CBR systems are due to its own structure, composed of four main stages [START_REF] Aamodt | Case-Based Reasoning: Foundational Issues, Methodological Variations, and System Approaches[END_REF]: retrieve, reuse, revision and retain. The first stage is called retrieve, and consists in finding the cases (from the case base) that are most similar to the new problem. Once a set of cases is extracted from the case base, they are reused by the system. In this second stage (reuse), the selected cases are adapted to fit in the new problem. After applying the new solution to the problem, that solution is revised to check its performance. If it is an acceptable solution, then it is retained by the system and could eventually serve as a solution to future problems.

As a methodology [START_REF] Watson | Case-based reasoning is a methodology not a technology[END_REF], CBR has been used to solve a great variety of problems [START_REF] Baruque | A forecasting solution to the oil spill problem based on a hybrid intelligent system[END_REF], [START_REF] Mata | Forecasting the probability of finding oil slicks using a CBR system[END_REF]. It is a cognitive structure that can be easily applied to solve problems such as those related with soft computing, since the procedures used by CBR are quite easy to assimilate by soft computing approaches. CBR has also helped to create applications related to quite different environments. Different kinds of neural networks such as ART-Kohonen [START_REF] Yang | Integration of ART-Kohonen neural network and case-based reasoning for intelligent fault diagnosis[END_REF] or Growing Cell Structures [START_REF] Diaz | Gene-CBR: A case-based reasoning tool for cancer diagnosis using microarray data sets[END_REF] have been combined with CBR to automatically create the inner structure of the case base. Some effort has also been devoted to the case-based maintenance issue [START_REF] Liu | An Association-based Case Reduction Technique for Case-based Reasoning[END_REF].

It is easy to understand that the case base is one of the key elements of a CBR system. It is crucial to dispose of a great amount of data, but properly organized. The quantity is important, but if there is no order within the stored cases, it could become impossible to obtain all the knowledge that such an accumulation of data may offer. This is why an algorithm like the WeVoS-SOM that is going to be explained next, is so useful to CBR systems.

(WeVoS-SOM) (Baruque et al.) is used to organize the data that is accumulated in the case base. It is also used to recover the most similar cases to the proposed problem.

The main objective of the new fusion of an ensemble of topology preserving maps [START_REF] Kohonen | The self-organizing map[END_REF] algorithm presented here, WeVoS-SOM, is to generate a final map processed unit by unit. Instead of trying to obtain the best position for the units of a single map trained over a single dataset, it aims to generate several maps over different parts of the dataset. Then, it obtains a final summarized map by calculating by consensus which is the best set of characteristics vector for each unit position in the map. To do this calculation, first this meta-algorithm must obtain the "quality" [START_REF] Pölzlbauer | Survey and Comparison of Quality Measures for Self-Organizing Maps[END_REF] of every unit that composes each map, so that it can relay in some kind of informed resolution for the fusion of neurons.

The final map obtained is generated unit by unit. The units of the final map are first initialized by determining their centroids in the same position of the map grid in each of the trained maps. Afterwards, the final position of that unit is recalculated using data related with the unit in that same position in every of the maps of the ensemble. For each unit, a sort of voting process is carried out as shown in Eq. 1:

M p m p M p m p q q x x = m) V(p 1 , 1 , , (1)
where, V p,m is the weight of the vote for the unit included in map m of the ensemble, in its position p; M is the total number of maps in the ensemble; x p,m is the binary vector used for marking the dataset entries recognized by the unit in position p of map m; and, q p,m is the value of the desired quality measure for the unit in position p of map m.

The final map is fed with the weights of the units as it is done with data inputs during the training phase of a SOM, considering the "homologous" unit in the final map as the BMU. The weights of the final unit will be updated towards the weights of the composing unit. The difference of the updating performed for each "homologous" unit in the composing maps depends on the quality measure calculated for each unit. The higher quality (or the lowest error) of the unit of the composing map, the stronger the unit of the summary map will be updated towards the weights of that neuron. The summarization algorithm will consider the weights of a composing unit "more suitable" to be the weights of the unit in the final map according to both the number of inputs recognized and the quality of adaptation of the unit (Eq. 1). With this new approach it is expected to obtain more faithful maps to the inner structure of the dataset.

This algorithm generates a structure where similar data is placed close together, with a clear relationship between the distribution of the initial data and the structure obtained by the algorithm. This relation between reality and inner structure is quite useful to a CBR system, where the stored information must be used in future situations to obtain future solutions.

Next, the system developed combining the CBR methodology and the WeVoS-SOM algorithm is explained, focusing on the main phases of the CBR cycle.

An Hybrid Forecasting System

CBR have already been used to generate predictions in complicated environments where different parameters were involved [START_REF] Corchado | FSfRT: Forecasting System for Red Tides[END_REF]. In this occasion, the CBR methodology is used in combination with a summarization of SOM ensembles algorithm, in order to improve its results. The WeVos-CBR system presented here is able to generate predictions using past information as a source of knowledge to solve new problems. The available information is divided into cases that are stored in the case base. Those cases are structured, using the WeVoS-SOM algorithm.

When a new problem should be solved by the system, then the most similar data to the problem is retrieved from the case base. Then, the inner organization generated by the WeVoS-SOM algorithm is useful to recover those elements more similar to the one that is introduced in the system as a problem. The retrieved cases are used to generate the new solution, by feeding a neural network, trained to generate solutions. If the solution generated is good enough to be proposed to the user, then it is also stored in the case base, to serve as new data to solve new problems. All this process, covering the four main phases of the CBR cycle explained before, is covered by the next sub-sections.

Creating the Case Base and Retrieving the Most Similar Cases

When the case base is created the WeVoS-SOM algorithm is used to structure it. The graphical capabilities of this novel algorithm are used in this occasion to create a model that represents the actual variability of the parameters stored in the cases. At the same time, the inner structure of the case base will make it easier to recover the most similar cases to the problem cases introduced in the system.

The WeVoS-SOM algorithm is also used to recover the most similar cases to the problem introduced in the system. The case base is organized by using the WeVoS-SOM algorithm. This organization keeps similar values stored close one to another. When a new problem comes into the system, then its virtual position into the inner structure of the case base is calculated. The system tries to store the problem into the case base like if it was a solution. That virtual allocation serves to calculate the position of the problem into the case base, and to recover those elements that are located close to that virtual position. Those retrieved cases are used in the next stage to generate the solution.

Adapting the Retrieved Cases to Generate a New Solution

After recovering the most similar cases to the problem from the case base, those cases are used to obtain a solution. Growing RBF networks [START_REF] Karayiannis | Growing radial basis neural networks: merging supervised andunsupervised learning with network growth techniques[END_REF] are used to generate the predicted solution corresponding to the proposed problem. The selected cases are used to train the GRBF network. This adaptation of the RBF network lets the system grow during the training phase in a gradual way increasing the number of elements (prototypes) which work as the centres of the radial basis functions. The error definition for every pattern is shown below:

p k ik ik y t n l e 1 ( 2
)
where t ik is the desired value of the k th output unit of the i th training pattern, y ik the actual values of the k th output unit of the i th training pattern. After the creation of the GRBF network, it is used to generate the solution to the introduced problem. The solution will be the output of the network using as input data the retrieved cases.

Revising and Retaining the Proposed Solution

In order to verify the precision of the proposed solution, Explanations are used [START_REF] Sørmo | Explanation in Case-Based Reasoning-Perspectives and Goals[END_REF]. To justify and validate the given solution, the retrieved cases are used once again. The selected cases have their own future associated situation. Considering the case and its solution as two vectors, a distance between them can be measured by calculating the evolution of the situation in the considered conditions. If the distance between the proposed problem and the solution given is smaller than the distances obtained from the selected cases, then the proposed solution is considered as a good one.

Once the proposed prediction is accepted, it can be stored in the case base in order to serve to solve new problems. It will be used equally than the historical data previously stored in the case base. The WeVoS-SOM algorithm is used again to introduce new elements in the case base.

Case Study and Results

Forest fires are a very serious hazard that, every year, cause significant damage around the world from an ecological, social, economical and human point of view [START_REF] Long | Mapping fire regimes across time and space: Understanding coarse and fine-scale fire patterns[END_REF]. These hazards are particularly dangerous when meteorological conditions are extreme with dry and hot seasons or strong wind. For example, fire is a recurrent factor in Mediterranean areas.

Table 1. Variables that define a case in the WeVoS-CBR system.

Fires represent a complex environment, where multiple parameters are involved. In this sub-section, a series of applications and possible solutions are explained. There are different approaches to the forest fire problems [START_REF] Mazzeo | A Multitemporal Robust Satellite Technique (RST) for Forest Fire Detection[END_REF], [START_REF] Arrue | An intelligent system for false alarm reduction in infrared forest-fire detection[END_REF], [START_REF] Muñoz | Forest fire detection using NOAA/16-LAC satellite images in the Araucanía Region[END_REF] and [START_REF] Rodríguez | An Adaptive System for Forest Fire Behavior Prediction[END_REF], including all the main phases existing in the evolution of this kind of problem. The WeVoS-CBR system presented here has been applied in this study to generate predictions in a forest fire scenario. Forest fires represent a great environmental risk. The main approaches that have been used to solve this problem come first from the detection of the fires [START_REF] Mazzeo | A Multitemporal Robust Satellite Technique (RST) for Forest Fire Detection[END_REF], where different techniques have been applied. Once the fire is detected, it is important to generate predictions that should help to take decision in those contingency response situations [START_REF] Iliadis | A decision support system applying an integrated fuzzy model for longterm forest fire risk estimation[END_REF].

Variable

The data used to check the WeVoS-CBR system was a subset of the available data that has not been previously used in the training phase. The predicted situation was contrasted with the actual future situation as it was known (historical data was used to train the system and also to test its correction). The parameters taken into account to create the cases stores in the system are listed in table 1. The proposed prediction was, in most of the variables, close to 90% of accuracy. To create the cases, the geographical area analyzed was divided into small squares, each of which were considered a case, with all its associated parameters (longitude, latitude, wind, pressure, temperature, etc.). The squares determine the area to be considered in every case. The problem is represented by the current situation of the area (all its parameters and the presence or not of fire). The solution is represented by the situation in that area in a future moment (same location but parameters changed to next day, or next step -if less than a day is considered in every step-).

In table 2 a summary of the results obtained is shown. In this table different techniques are compared. The table shows the evolution of the results along with the increase of the number of cases stored in the case base. All the techniques analyzed improve its results when increasing the number of cases stored. Having more cases in the case base, makes easier to find similar cases to the proposed problem and then, the solution can be more accurate. The "RBF" column represents a simple Radial Basis Function Network that is trained with all the data available. The network gives an output that is considered a solution to the problem. The "CBR" column represents a pure CBR system, with no other techniques included, the cases are stored in the case bases and recovered considering the Euclidean distance. The most similar cases are selected and after applying a weighted mean depending on the similarity, a solution is proposed. The "RBF + CBR" column corresponds to the possibility of using a RBF system combined with CBR. The recovery from the CBR is done by the Manhattan distance and the RBF network works in the reuse phase, adapting the selected cases to obtain the new solution. The results of the "RBF + CBR" column are, normally, better than those of the "CBR", mainly because of the elimination of useless data to generate the solution. Finally, the "WeVoS-CBR" column shows the results obtained by the proposed system, obtaining better results than the three previous analyzed solutions.

Conclusions

The comparison of the WeVoS-CBR system with other techniques and systems has shown its advantage in terms of efficiency and quality of the provided results. The performance of the WeVoS-CBR system was also compared with a previous local version of the system, with the same artificial intelligence techniques implemented, but without the use of agents and services. The tests performed consisted of the execution of the same series of predictions in both systems. There were 50 different prediction requests to be generated by the system. It was shown that, the current system was faster than previous versions (a 70% faster in answering 50 successive requests)and the number of crashes produced when the load of work was increased, was quite lower (80% less crashes when using the current system).

With the optimistic results obtained after applying the presented system to the forest fires scenario, a future line of investigation will be the application of this system to other complex environments where its predicting and generalizing capabilities could be successfully applied.

Table 2 .

 2 Percentage of good predictions obtained with different techniques.

	Number of cases	RBF	CBR	RBF + CBR WeVoS-CBR
	100	36 %	38 %	41 %	44 %
	500	40 %	44 %	47 %	51 %
	1000	48 %	51 %	59 %	65 %
	2000	52 %	58 %	66 %	73 %
	3000	56 %	62 %	70 %	78 %
	4000	61 %	66 %	73 %	85 %
	5000	66 %	70 %	78 %	90 %

Acknowledgments

This research has been partially supported through projects of the Junta of Castilla and León BU006A08; project of the Spanish Ministry of Education and Innovation CIT-020000-2008-2 and CIT-020000-2009-12. The authors would also like to thank the vehicle interior manufacturer, Grupo Antolin Ingenieria S.A., within the framework of the project MAGNO2008 -1028.-CENIT Project funded by the Spanish Ministry.


Figueiredo Ramos
email: framos@ipg.pt

António Cardoso
email: ajmcardoso@ieee.org

Adérito Alcaso
email: aderitona@ipg.pt

Hybrid Photovoltaic-Thermal Collectors: A Review

Keywords: Hybrid photovoltaic/thermal (PV/T) collector, photovoltaic (PV), thermal (TH), heat transfer, efficiency

Solar energy can be converted directly into electric and thermal energy through photovoltaic cells and thermal collectors, respectively. However this conversion, in particular the photovoltaic, has a reduced efficiency. A solution proposed to increase this efficiency is with the hybrid solar structure, which consists in the junction of the photovoltaic panel and the thermal collector in a single module. The interest on these solar systems led the International Energy Agency to create a "Task" on this subject. This paper presents a review of the research in this area, presenting the definitions of the related collectors and results of their characteristics, as well as some ideas for future studies.

Introduction

At the beginning of the XXI century the energy issues have been occupying a central place in the international concerns. These concerns are related to ecological aspects, namely climatic concerns with the global heating and climate changes, economic aspects, tied to the scarcity of resources of fossil origin and geopolitical aspects, regarding the difficulty of energy transportation mainly through politically unstable zones.

Due to the vital importance of the energy sector in the countries economies and from all the issues mentioned above, it is necessary to establish new energy and environmental politics in order to find a balance between the environmental, technical and economical viability of energy production and consumption, promoting thus a sustainable development.

The European Union (EU), geographic area that is strongly dependent on external primary energy, considers that, relatively to the energy sources, a strong increase of renewable energies in global energy production is desirable, in order to reach a 15% contribution until 2010. The concerns of the EU on energy and environmental politics integration are reported in the new "Energy Politics for the Europe". This politic proposal must be the basic instrument for the reduction of the Greenhouse Gases and the increment of the energy production with renewable sources, among others. Also in Portugal, the Government has defined the "National Strategy for the Energy" to promote energy production on the basis of renewable sources, with the capacity of exploration in the country, being the solar energy one of them.

The technology associated with the exploitation of solar energy, namely the independent photovoltaic and thermal systems, for conversion of the solar radiation into electric and thermal energy, respectively, is already a mature technology. However, one of the inconveniences associated to this traditional technology, particularly the photovoltaic one, is its low efficiency, leading necessarily to great areas of implantation for these systems, which is a major limitation. That is particularly important in applications where the availability of free lands is more limited. Moreover, the solar radiation increases the temperature of photovoltaic cells, decreasing their efficiency, and the natural convection cooling of these modules, due to climatic conditions, is not enough to reduce the temperature in order obtain a desired maximal efficiency. To surpass these inconveniences, investigators had considered the integration of the thermal and photovoltaic systems in a single device for conversion of solar energy with simultaneous production and consumption of electric and thermal energy. The goal of this integration is to generate more energy per unit area than with the photovoltaic panel and the thermal collector, installed independently. These devices of solar co-generation are called "Photovoltaic-Thermal" collectors/panels or "PV/T". In contrast to the independent photovoltaic and thermal solar systems, the hybrid devices have not reached yet a technological maturity and are still being studied. By recognizing their potentialities, the International Energy Agency has created a task, "Task 35 -Solar PV/Thermal Systems", whose aim is to increase the common knowledge of the PV/T solar systems, the promotion of their development, the market introduction of these solar systems with quality and commercial competitiveness and also contributing to the standardization of functional tests and surveillance of commercial characteristics of these systems in the construction sector. Under supervision of Task 35 some publications were developed [START_REF]PV/T Collectors: Technologies Combine to Increase Output[END_REF][START_REF] Hansen | IEA SHC Task 35 PV/Thermal Solar Systems. World Renewable Energy Congress, Document Number DE[END_REF][START_REF] Zondag | Commercially Available PV/T Products[END_REF] being foreseen for publication in 2009 a final report, with the goal of presenting all the information about the PV/T solar systems.

Contribution to Technological Innovation

In this paper, a review of the research on PV/T collectors and their application in heat and electricity production will be presented, illustrating their potentialities.

The performance of PV/T collectors depends on many variables, being the temperature one of the most important, and, although promising, there is not yet a consensus about the viability of this technology and its application, since it can differs from region to region. For example, in the northern countries, with lesser solar radiation, the PV/T collectors can be used, to a large extent, integrated in buildings to lower the temperature of the PV modules and to supply hot air for preheating of interior spaces. In countries with warmer climate, as it is the case of the Mediterranean countries, the PV/T collectors can be used, for the most part, to lower the temperature of PV modules and heating domestic waters.

More research is needed, namely regarding modelling, structure dimensioning and topology aspects. A setup is being developed in order to experimentally validate this research, which will also comprises a modelling and computational simulation based on techniques of distributed parameters and finite elements, in order to obtain a better temperature distribution in the PV/T.

PV/T Collector Types

Different possibilities are described in the literature for the construction of a PV/T. The basic structure uses a thermal fluid for cooling photovoltaic cells, allowing the increase of their conversion efficiency and recovering some of the various heat losses that in another way would be simply transmitted to the environment. This allows the production of thermal energy and its direct application in domestic water heating, swimming pools or drying substances, among others.

The PV/T collectors can be classified primarily with regard to the existence, or not, of auxiliary mechanisms for concentration of the solar radiation. The collectors with concentrators use reflective devices in order to increase the amount of solar radiation in a small area (in the PV face). The collectors without concentrators are of flat type, like usual PV panels, and the solar radiation directly reaches the area of the collector. The collectors are still classified in accordance with the existence of a solar tracking system and the type of thermal fluid that can either be liquid (water) or gas (air).

Regarding the concentrating type, some authors [START_REF] Coventry | Performance of a Concentrating Photovoltaic/Thermal Solar Collector[END_REF], [START_REF] Smeltink | 40 kW PV Thermal Roof Mounted Concentrator System. 4 th World Conference on Photovoltaic Energy Conversion[END_REF] had developed PV/T collectors that consist of a parabolic reflective surface that concentrates more solar energy in the photovoltaic cells, whose material must support the high working temperatures of these collectors in the receiving area. In the backs of the photovoltaic arrangement, the thermal fluid (normally water with antifreezing characteristics) cools the PV cell taking out the heat for the production of thermal energy. Other authors [START_REF] Yang | A 3kW PV-Thermal System for Home Use[END_REF] developed a structure with a different version of concentrating PV/T, as shown in Fig. 1, with a system composed by two parabolic mirrors. In this system, that must include a solar tracker, the first mirror concentrates the solar radiation of small wavelength in photovoltaic cells for production of electric energy, whereas the second parabolic receives the radiation with higher wavelength that however crossed the first mirror and that later will be reflected in a pipe that has a fluid in its interior, that serves as a way of thermal transference. This system, according to its authors, proved to be efficient and economically competitive as compared to the traditional power plants.

With respect to flat-type PV/T collectors, more often used than the concentrating ones, these are similar to common thermal collectors and photovoltaic panels, which are cheaper although less efficient. The flat PV/T collector is usually constituted by the PV component, whose material is the conventional doped crystalline silicon, and the TH component, joined each other, being the later behind the first one. This PV/T module can be glassed or not, i.e. it can include or not, in its structure, a glass placed above the photovoltaic cells.

Fig. 2 shows the structure of a typical flat plate PV/T collector showing the PV module, the heat extraction equipment -metallic plate constituted by pipes that absorb heat and functions as a cooling system of the PV module attached to it -the glass cover and the thermal isolation that has a function of preventing great heat losses through the equipment walls. These collectors are suitable for roof installation or other areas of easy access, not only because they allow an easy and adjusted integration in constructions, but also because they occupy less space than the photovoltaic and thermal modules used separately. The flat plate PV/T collectors can be divided in two types: water and air, depending on what fluid passes through the pipes. When using water it is usually associated to another liquid with antifreezing characteristics, so that it does not freeze so easily when the environment reaches negative temperatures, preventing damages in the system. According to the typical PV/T collector shown on Fig. 2, there are authors [START_REF] Bhargava | Study of a Hybrid Solar System -Solar Air Heater Combined with Solar Cells[END_REF], [START_REF] Aste | Experimental Evaluation of the Performance of a Prototype Hybrid Solar Photovoltaic-Thermal (PV/T) Air Collector for the Integration in Sloped Roof[END_REF] that propose the air as a thermal fluid, showing that for the same amount of solar radiation, the efficiency of the photovoltaic module is always lower than the thermal module and also that the increase of solar radiation results in an increase of the efficiency of the two modules, PV and TH.

Other authors [START_REF] Tripanagnostopoulos | Application Aspects of Hybrid PV/T Solar Systems[END_REF][START_REF] Zondag | PV-Thermal collector development -an overview of the lessons learnt[END_REF][START_REF] Zondag | Development and Applications for PV Thermal. 17 th European Photovoltaic Solar Energy Conference[END_REF][START_REF] Zondag | PV-Thermal Domestic Systems. 3 rd World Conference on Photovoltaic Energy Conversion[END_REF] analysed a PV/T collector with water as thermal fluid and multicrystaline silicon PV panel, with and without a glass cover. They had stated that using this cover in a collector increases the thermal efficiency, but reduces the electric efficiency due to the additional losses by reflection. These authors also consider the choice of the thermal fluid as one of the most important selections to make, having the liquid ones (water) a better performance than the air collectors.

The evaluation of the PV/T systems and their application areas are analyzed in [START_REF] Elia | Dimensioning and Efficiency Evaluation of Hybrid Solar Systems for Energy Production[END_REF]. The authors consider that thanks to the use of PV/T collectors, which they call as solar co-generation technology, there are some practical and technical improvements regarding the installation of PV and TH systems separately. These improvements are: a lesser installation area, reduced installation costs and the fact that PV and TH modules use different parts of the solar spectrum (PV -wavelength of the visible one; TH -wavelength of the infrared one), making the flat PV/T hybrid technology well adjusted to explore both radiations, due to the fact that the silicon, main component used in the PV cells, is partially transparent to the infrared radiation. It is important that the PV/T collector absorbs the maximum radiation that it receives, using for this a good PV and a thermal plate with a great absorption of heat.

PV/T Collectors Performance

A realistic system for the simultaneous conversion of the solar radiation in electric and thermal energy through the PV/T is schematized in Fig. 3. Apart from the PV/T collector, that converts the solar radiation, the system includes a possible pump and hot water storage for the thermal system, and a charge controller, a battery and a DC/AC converter, for the electric system. There is also a supervision and control system of the conversion process. The performance of PV/T system depends on factors like the temperature of the thermal fluid, the amount of the mass flow of this fluid, the number of glass covers, the configuration of the thermal flat plate module and the supervisory and control system.

In the analysis of a PV/T collector [START_REF] Vandaele | Combined heat and power from hybrid photovoltaic building integrated components: results from overall performance assessment[END_REF] it was reported that the reduction of the temperature verified in PV module, due to the thermal fluid, was superior to 10 ºC which implied an increase of about 5% in the photovoltaic efficiency.

As stated before, in a flat PV/T collector without additional glass cover the electric efficiency is bigger than with a covering glass. In this case the PV module can even reach a bigger temperature than a conventional PV panel and there is an increase of reflected radiation on the top of the collector. To reduce the verified reflection the glass cover must be highly transparent to the solar radiation [START_REF] Furbo | [END_REF].

When the thermal resistance between PV and TH modules increases, this implies the existence of a great difference in temperatures -high temperature gradientbetween them, leading to the increase of thermal losses and a reduction of PV performance. In PV/T collectors the thermal efficiency increases with increased heat transfer coefficients, that is, with the reduction of thermal resistance. With glass covers the thermal efficiency of the collector is bigger.

Modelling and Simulation

The modelling of a PV/T system can be based on the known base models of the PV and TH modules, with an additional path between them. The model can then be implemented in a software package. The majority of studies used the TRNSYS (The Transient Energy System Simulation Tool) simulation program or the MATLAB/SIMULINK. TRNSYS was initially developed only for thermal systems but has been also applied to PV/T collectors with electrical systems. In [START_REF] Kalogirou | Use of TRNSYS for modelling and simulation of a hybrid PV-thermal solar system for Cyprus[END_REF] a simulation was presented considering meteorological data of a typical year of the city of Nicosia, Cyprus. The goal of this study was the optimization of the water flow in the system. In [START_REF] Jong | System Studies on Combined PV/Thermal Panels[END_REF] different PV/T systems had been analysed with TNRSYS, namely for the cases of water heating in a house and preheat of the ventilation air in a school. The simulation was executed by a period of one year, with typical climatic data from Holland. For the areas of research and development TRNSYS is used significantly when the simulation is focused in TH part, even though the simulation environment also allows the inclusion of PV models, [START_REF]Energia Solar Térmica -Manual Sobre Tecnologias, Projecto e Instalação. Comissão Europeia, programa ALTENER[END_REF], [START_REF]Energia Fotovoltaica -Manual Sobre Tecnologias, Projecto e Instalação. Comissão Europeia, programa ALTENER[END_REF]. Fig. 4 shows a TRNSYS model used to simulate a PV/T solar window [START_REF] Davidsson | Performance of a multifunctional PV/T Hybrid Solar Window[END_REF]. In [START_REF] Járdán | Control of a Combined Photovoltaic/Thermal Energy System[END_REF] a PV/T system was analysed, in this case, with MATLAB/SIMULINK and the obtained results were compared with the experimental ones, concluding that the considered system was possible, although needing more research.

Conclusions

This work has presented a review of the available literature on PV/T collectors, mainly of flat plate type. The results show that the PV/T efficiency is sensitive to many variables and a more detailed study seems to be necessary in order to obtain an optimal PV/T collector with improved efficiency and reduced costs, in order to be economically competitive.

Fig. 1 .

 1 Fig. 1. A schematic structure of a concentrating PV/T collector [6].

Fig. 2 .

 2 Fig. 2. Cross section of a flat plate PV/T collector.

Fig. 3 .

 3 Fig. 3. Block diagram of PV/T system.

Fig. 4 .

 4 Fig. 4. Block diagram of a PV/T TRNSYS model [21].

Acknowledgement. This work was supported in part by the Portuguese Foundation for Science and Technology (FCT) under Project Nº SFRH/BD/50276/2009 and Project Nº SFRH/BSAB/950/2009.