Figure: 7

Prototype “Solar Tergo” mounted on Boblbee

In the framework of a master thesis project, an existing backpack (Boblbee) was equipped with a click-on unit comprises of a solar panel to generate electrical power and a compartment with electrical energy buffer storage unit including electronic charge- recharge control electronics [Weitjens, 2003]. In this design a flexible PV panel is used to accommodate the shape and a convenient utilisation of the backpack. In addition this curved PV panel enhances the appearance. Mobile products that could be recharged are for example a cellular phone, a PDA, ect. They were placed either in a special compartment outside or just inside the backpack. The recharge process could be done all the time the mobile products were carried around in the backpack both outdoors and indoors.

2.4.2 The Solar Cell White-board

A whiteboard will be used at places where there is sufficient light. A whiteboard usually constitutes of a large area (1 m2 or more) surface. Covering a metal plate with white enamel usually makes a whiteboard. So combining these three facts could result in a practical example of application of the Solar Cell Enamel techniques.

2.4.3 Lawn mower robot

Another example is a solar powered lawn mower [Husqvarna, 2004] see Figure 8. When

there is much sunlight, on sunny days, the grass is growing rapidly. The Solar Cells will convert enough sunlight into electricity to power the solar mower. On cloudy days the grass is growing less rapidly, coinciding with a longer period of recharging the batteries. Intelligence is built into the lawnmower Robot, which controls its mow velocity and path. The synergy by introducing intelligence is apparent.

Figure: 8: Lawn mower robot

2.4.4 Solar car Roof

Solar cells placed on the roof of a car can recharge additionally the batteries [Sunovation, 2004]. The car roofs have usually not flat but curved surfaces.

2.4.5 Solar Sunscreen

Placing solar cells on the box of a sunscreen [Poelman, 2000] is exactly an example in which integration of a function and energy conversion is demonstrated in a practical way. This integration can be pushed even further by making the sunscreen also part of the solar cell. The sunscreen will be used if and only if there is sunlight. On the other side, the solar cell to be functional will also be in need of the sun to shine. Therefore, this example is a clear demonstration of synergy.

M. C Lopez, D. Leinen, F. Martin and J. R. Ramos-Barrado

Laboratorio de Materiales y Superficie. Departamentos de Fisica Aplicada & ingenieria

Quimica. Universidad de Malaga. E29071 Malaga. SPAIN.

Abstract:

ZnO/ZnS bilayers to antireflection coatings for solar cells have been prepared by spray pyrolysis using mixed aqueous solutions of zinc acetate dehydrated and thiourea or zinc chloride and thiourea. The structure, surface morphology, chemical composition and optical properties of the bilayer are investigated as a function of the initial solution. X-Ray photoelectron spectroscopy (XPS) analysis and Ar ion-beam sputter etching was carried out to obtain XPS depth profile of bilayer. Neither carbon nor others by-products which could change the refractive index of the bilayer have been found in the interface. Some differences between the bilayer with the ZnS film obtained from ZnCl2 or Zn acetate can be observed.

1. Introduction:

The efficiency of solar cells dramatically depends on light that can arrive to the active layer. Due to reflection losses, the photogenerated current density (and consequently, the efficiency) reaches a level much lower than he maximum for the standard terrestrial spectrums of light coming from the sun [1-3]. To reduce the reflection of incident light at the surface of the cell, an antireflecting coating (ARC) can be used over the cell [2-6]. This coating can range from a simple layer to a multilayer system of many layers having almost zero reflectance over a wide range of wavelengths [2].

In general, the design of solar cells, follows a step-down interference coating structure: ns>n1>n2>nm, where ns and nm are the substrate and incident medium refractive indices, respectively [4,6]. This choice is based on the spectral stability of the coating and for low reflectance. Stability means that the low-reflectance spectrum changes very slightly with thickness and refractive index variation [4,6].

Zinc oxide and sulphide can be used to form a bilayer because both compounds have appropriate electrical and optical properties.

Zinc sulphide (ZnS) is an n-type semiconducting. ZnS thin film is a promising material for its use in various application devices due to its wide band gap of 3.7eV at

room temperature, its high refractive index (2.35) [7-11], and its low absortion over a broad wavelength range, from 400 to 1400nm [11].

Zinc oxide (ZnO), is and n-type semiconducting [12]. It is a widebandgap (3.24eV) semiconductor material with high optical transparency in the visible and near-infrarred region of electromagnetic spectrum and high refractive index (1.9). Due to these properties, ZnO is a promising material for solar cell applications such as antireflection coating [13].

Spray pyrolysis is a useful alternative to the traditional methods to obtain antireflection coatings for solar cells. It is particular interesting because of its simplicity, low cost and minimum waste production. The spray pyrolysis allows coat big surfaces and it is easy to insert in an industrial line of production. This method of deposition allows obtain thin films with different chemical, physical and morphological properties; the characteristics of the films depend on the spray rate, temperature of the substrate and high of the nozzle values. ZnO/ZnS bilayer obtained by spray pyrolysis technique shows a smooth and homogeneous surface without by-products.

The measurements of the angular dependence were performed under the solar simulator, on a cooled rotating measurement block. The change in current was determined for the incidence angle varying between -75° and +75°. The main bus bars were perpendicular to the rotation axes during the measurement. The values for positive and negative angles were always averaged. In Fig. 3a, the angular dependency of the normalised short-circuit current /sc(e)/[/sc(0) cos(e)] is plotted as a function of the incidence angle в. The curves are obtained by averaging the results obtained on 6 modules with AR layer and 6 without and show the departure from the ideal cosine law at angles higher than 45°. A significant improvement is given at high incidence angle by the AR layer, where the angular losses remain minimum. In Fig. 3b, the total current gain in % given by the AR layer is shown.

The average gain can be fitted with a single exponential growth curve. Starting from 2.65% at 0° it reaches 3% at 45°, 6% at 60° and 12% at 75°.

Fig. 3. a) Normalised current as a function of the light incidence angle for the glass without and with AR layer (The data points at 80° and 90° have been extrapolated). b) Gain in current given by the AR layer as a function of the incidence angle.

3.4 Outdoor modules

The nominal operating cell temperature (NOCT) of the modules has been determined by analysing 200 values of the module temperatures, for illuminations between 700-900 W/m2 and air temperatures between 20°-25°C. In average, the module with the AR layer was found to be 1.2°C warmer than the module with the normal glass, with an average backskin temperature of 43.4° and 42.2°C respectively (temperature at the backside of the module measured on the Tedlar foil).

Simon Furbo, Louise Jivan Shah and Louise Overvad Jensen
Department of Civil Engineering
Technical University of Denmark
Building 118, DK-2800 Kgs. Lyngby
Denmark

Email: sf@bvg. dtu. dk
Fax: +45 45 93 17 55

Esben Larsen
0rsted. DTU

Technical University of Denmark
Building 348, DK-2800 Kgs. Lyngby
Denmark

Email: ela@.oersted. dtu. dk

Goran Olsson
Sunarc A/S

Gronlandsvej 14, DK-4681 Herfolge
Denmark

Email: olsson@sunarc. net

1 INTRODUCTION

Experiments have shown that an antireflection surface of a glass cover can increase the transmittance by reducing the normal 4 % reflection at the air-glass interface [1]. Different techniques of the antireflection treatments are sol-gel deposition [2], direct plasma enhanced chemical vapor deposition [3] and acid etching of the glass [4].

The company SunArc A/S applies an antireflection surface to the glass by a special etching process.

Investigations have shown that for incidence angles between 0° and 70° the solar transmittance of a glass cover is increased by 5-9 %-points and the efficiency of a flat plate solar collector is increased by 4-6 %-points by using a glass cover equipped with antireflection surfaces by the company SunArc A/S, [5].

In this paper measurements of the efficiency for a marketed PV module with the normally used glass cover and with the same glass cover after an antireflection treatment of the outer surface of the glass by SunArc A/S are presented.

The efficiency of the PV module was measured outdoors under different weather
conditions: For sunny periods with a small part of the radiation being diffuse
radiation and with incidence angles for the direct radiation of 0°, 15°, 30°, 45°, 60°

and 75°, and for cloudy periods with a small part of the radiation being direct radiation.

A. Khairy Aboul-Soud, Moataz M. Soliman, Alaa S. Hafez
Faculty of Engineering, Alexandria University, Egypt.

* Institute of Graduate Studies and Research, Alexandria University, Egypt.

ABSTRACT:

The aim of this paper is to determine the optimum parameters associated with the use of the quantum well for high efficiency Alx Ga1-x As/GaAs solar cells.

Two types of cells are considered, single quantum well and multi quantum wells solar cells. For SQW the optimized parameters are: well location in the intrinsic region, well width and aluminum mole fraction x. It is found that the optimum location of the quantum well should be away from the high recombination zone around the center of the space charge region and closer to the p region. The optimum well width is found to be 200A due to the enhancement of quantum confined stark effect for the thick well. Mole fraction of 0.2 is an optimum value taking into consideration the trade off between the open circuit voltage and the short circuit current.

For the MQW solar cell, the optimized parameters are the barrier to well width ratio and the spacer thickness. It is found that barrier to well width ratio should be less than 0.25 to enhance the resonant tunnelling between adjacent wells in order to maximize carrier collection through electron tunnelling. The optimum spacer thickness is about 300 A to prevent the impurity scattering effect into the space charge region.

1- INTRODUCTION:

Quantum well solar cells were proposed [1], as a method to increase the solar cell efficiency. Several models were introduced to reduce the dark current in QWSC [2-3] in order to increase the efficiency of the solar cell. It is found out that the current depends on quantum well position contrary to spectral response. Other models [4-5] studied the effect of quantum well width and aluminum mole fraction x on the output voltage of the QWSC. The study of barrier to well ratio [6-7] and the spacer thickness also discussed [8]. These models study different parameters for SQW and MQW solar cells but didn’t optimize these parameters in order to achieve maximum photovoltaic conversion efficiency. Figure (1) shows the energy band diagram of the quantum well solar cell under illumination. An intrinsic region is inserted into a conventional p-n solar cell to extend the field-bearing region. The quantum wells (QWs) extend the absorption below the bulk band-gap Eg to threshold Ea. If the field is maintained across the i-region, the carriers produced by the extra photons absorbed in the well escape to the bulk cell and contribute extra current with high quantum efficiency at room temperature.

The PV system (Fig.1) which is the subject of this study is located in a site characterized by these coordinated geographical. This site has a Northern latitude equal to 22°, a longitude 6° and an altitude 1400m. PV generator of this system is a power of 750-Watts peak and it has a surface of 6 m2. It is directed towards the south is tilted towards north with an angle equal to 10°. This system east constitutes by the following elements:

1. A PV field, which is consisted of 16 sell panels, each module with a power of 45 Watts peak. The maximum tension is 40 V and its maximum current is 20 A.

2. A battery which consists of 12 lead accumulators of 2 V each one connected in series to ensure a nominal voltage of 24 V

3. An inverter for converts the tension continues with an alternating voltage

4. A regulator which makes it possible to manage the circulation of current between the battery and the load

The monitoring of a PV system consists in following its operation through the fine measurements taken using an automatic measurement chain. This

operation allows to control the operation of the system on the one hand and to detect the possible anomalies of the failures which can this present on the one hand and to detect the information on the reliability of the elements constitute the system and on its capacity to satisfy the load. Thus, thanks to the database which it provides, monitoring makes it possible to determine the performance of PV system considered, to evaluate the quality of its sizing and to optimize the various elements.

Vladimir N. Baboshin, RNTs «Applied Chemistry»

Aleksander B. Pavlov, RNTs «Applied Chemistry»

Valery P. Fokanov RNTs «Applied Chemistry»

Vyacheslav M. Andreev Ioffe Physical Technical Institute

Maxim Z. Shvartz Ioffe Physical Technical Institute

Leonid M. Vinogradskiy, RFNC-VNIIEF

Viktor A. Eroshenko, RFNC-VNIIEF

Victor A. Kargin, RFNC-VNIIEF

Sergey K. Sobolev RFNC-VNIIEF

Introduction. Lasers directly pumped by the solar light attract the uprising attention of the scientific community due to their possible implementation in those fields where energy supplies become extremely expensive. Solar pumped lasers can be implemented in the following endeavors:

• energy transmission between satellites, which can be specially useful for solution of energy problems associated with low-orbit space crafts, as well as energy transmission to the Earth;

• technological applications (cutting, welding) associated with production of big — size space-borne constructions;

• multiple tasks evolving monitoring of the Earth surface and atmosphere, specifically for weather forecasting and environmental studies;

• laser jets for inter-orbital travel of space-borne crafts featuring low consumption of operational substance and the admissible time of maneuvering (10 to 30 days);

• space communications.

The advantages of photo-dissociation iodine solar-pumped laser (PDISPL) can be referred from the following:

a. PDISPL is a gas laser, that is why its active medium does not feature such the indications of inhomogeneity as thermal lens and heterogeneous refraction, which are specific to the active medium of solid-state lasers as a result of pumping energy absorption.

b. The active medium of the gas laser does not feature such the restrictions as, for instance, the rods. That is why PDISPL can be provided with the big active medium volume with homogeneous gain.

c. Solar-pumped iodine laser can reach the diffraction limit of radiation divergence less than 5*10-6 rad (~1”).

d. The laser can operate at the low pumping level of -100 — 200 solar constants. In the previous laboratory studies the pumping flux approximated to ~ 1000 solar constants and higher, which is hardly applicable for the real space-borne laser.

The hybrid power supply needs a central and reliable control to run the system closely to the predefined rules. It also calculates system states like the battery’s SOC and State-of-Health, the current End-of-Charge-Voltage etc.. All system states, setup values and system parameters reside in a small scale database, with capabilities of automated backup to nonvolatile memories.

Component control has been implemented by the mean of state machines. A control kernel frequently checks if certain conditions (e. g. limits exceeded, inputs set, times elapsed etc.) are fulfilled to bring a component from one state to another. On entering a new state a list of tasks is executed (e. g. switching outputs, set values etc.). This way changes to the components can easily applied without programming and future reuse of code is possible.

The EMS has interfaces of different kinds (digital, analogue, serial) to all components it is attached to, shows important information on a display and provides a serial-line interface for setup, maintenance and continuous monitoring or logging of data. Via this connection it is possible to remotely control the power supply system from an operation and maintenance centre (e. g. by the telecom service provider).

The features described are all implemented within a low cost microcontroller platform, using a robust embedded operating system, robust to power losses.

Operating experience

The system has been set into operation during spring 2003. Overall system behaviour has been reported by Ciemat, analysing the energy flow within the system [1]. Operation data of the period from May to September 2003 will be presented in the following section. In the summer period, the fuel cell stack has been running only for frequent self-tests. Thus, we will concentrate on the data of the hydrogen production by electrolysis.

The electrolyser is started by the EMS system if the calculated state-of-charge of the battery reaches a certain value. This value is a linear function of daytime. Operation is possible from 9 am till 4 pm and only during summer months (March — October). In order to prevent the electrolyser to be damaged by a high operating voltage, the battery voltage is reduced with the help of the charge controller to 56.4 V which equals a voltage of 1.88 V per cell. Additionally, premature aging of the battery is avoided by the following rule: Electrolyser operation is only possible if the relative PV power is above 65% of the nominal PV generator power. These limitations and their value are a result of the opimisation of the life-time cost (TALCO).

Figure 3 shows the typical operation behaviour of the of the electrolyser in the test field for one day. The electrolyser starts at around noon after the battery has been loaded during the morning from the PV panels. The voltage of the electrolyser corresponds to the voltage of the battery as the electrolyser is directly coupled to the battery. The constant rise in electrolyser current shows that the electroctrolyser has not yet reached its stationary operation point. At 4 pm the electrolyser is shut down by the EMS following the rules as mentioned above.

The electrolyser did show less —o— I Hectrolyser —■— SOC ♦ U Battery —O— U Electrolyser

current flow as expected. In laboratory tests, the current at operating conditions (70°C,

30 bar) was 20 Ampere, whereas the current reached only a maximum of 11 Ampere in the

test field. An analysis of the data ^

shows for this effect to be mainly due to the temperature behaviour of the electrolyser stack.

The characteristics of electrolyser cells for different temperatures are shown in Figure 4. As can be seen, the current is very sensitive to the operation temperature.

Compared to an operation temperature of 70°C, only 70% of the current density can be reached at 40°C. As the current is

directly coupled to the hydrogen production rate (see (1)), this leads to a reduction in hydrogen production of the same amount.

VH = 0.418^ xI (1)

where U0 is the open cell voltage (1.23 V), N the number of cells in the stack, and U the operating voltage. Thus, the operating voltage has a direct influence on the temperature behaviour of the stack.

As the electrolyser is directly coupled to the battery, low battery voltages lead to a slow temperature increase of the electrolyser stack. Figure 5 shows the temperature behaviour of the electrolyser during operation in the test field. When no hydrogen is produced, the temperature of the electrolyser stack follows the ambient temperature. As soon as the electrolyser is set into operation, the temperature increases. For the day depicted in Figure 5, the temperature rises from 24°C to 46°C during 4.33 hours of operation. As discribed above, this results in an increase in current, and thus an increase in hydrogen production rate over time. Still, the design temperature of 70°C is not reached and hence, a the maximum current does not exceed 11 Ampere.

The time of operation during one day of the electrolyer is varying following the weather conditions (see rules described above). The behaviour of the stack temperature and the current with different hours of operation is compared in Figure 6.

It can be seen, that the general temperature behaviour is the same for all days. Longer operation leads to higher operating temperature and thus, to an increase in electrolyser current. A comparison of Figure 6a) and 6b) shows that an interruption of hydrogen production leads to lower operation temperature (4 K) for the same overall time of operation.

Figure 6: Three days of electrolyser operation a) 2.2 hrs (with interruption), b) 2.1 hrs, c) 4.3 hrs

Conclusions

A PV hybrid system has been designed for autonomous power supply of telecommunication equipment. The sizing of the components has been done by life-time cost optimisation using the simulation tool TALCO. Additionally, the rules for operating the system taking have been deduced from the results of simulations over the life-time.

An Energy Management System has been designed in a way it seamlessly uses those rules for its control decisions. Robustness and the possibility of future series production has emphasised.

Operation experience has been gained in a test system set-up in Madrid, Spain. The summer period has been investigated. An analysis of the electrolyser operation shows that less hydrogen is produced than expected from design parameters. The main cause is the temperature behaviour of the electrolyser stack. This problem might be solved by operating the stack at a voltage (up to 2.3 V per cell) during start-up. This would increase heat production and subsequently lead to a higher hydrogen production rate. Technically, this can be done either by a reduction of cells in the stack or by an additional DC/DC converter. The second possibility is favoured as voltage can be adjusted easily to the needs of the hybrid system at different weather conditions. Further work should also be done in including a dynamic model of the electrolyser in the design tool.

While the component prices (especially those hydrogen components which today are not commercially available) heavily influence the overall system profitability, the work shows the practical feasibility to build complex hybrid systems.

Acknowledgements

The work presented has been financed by the European Commission (FIRST, ERK5-CT-1999- 00018).

The authors want to thank all the partners (Project website: www. inta. es/first) for the co­operation during the last four years.

1.1 Problem definition

In interconnected power systems, the forced outage of one of the main generating units would lead to unstable voltage profile at a number of B. B.s, that definitely does not meet the power quality regulations. This unsatisfactory operating condition may result in the disconnection of a number of loads according to certain priority list, or to reschedule the operation of existing units. Another solution could be the reconfiguration of the network. These solutions are really costly and would be penalized. The introduction of prescheduled controlled (e. g. fuel cell) or non-controlled (PV or wind) DG will offer a better solution.

1.2 Scenario of voltage collapse

Figure (1) shows the schematic diagram of an interconnected power system; consisting of several generating units and voltage levels. This system has been proposed by as test system for studying the problems associated with interconnected networks installing distributed generation[6]. The installed capacity of this system is 12 GW. The voltage collapse is initiated by the following events:

1- A ramp change of loads connected to the nodes N204 and N206 to 80% by rate 20%/min.

2- Unit M4 outage is recorded at t=3min.

3- Double line outage of V451 and V452 (which connect nodes N4 & N5) at t=5 min.

4- Line outage V2_3 (which connect nodes N102 & N103) at t=6 min.

This scenario is a common practice in interconnected power systems combining generation units and distribution networks supplying a variety of load types. Moreover, the ramp change of loads at two non-adjacent nodes reflects a real slice of dynamic loads. On the other hand, it gives the proper way to simulate the added DG at these nodes in order to test the voltage collapse problem.

Rudi Kaiser’, Vojtech Svoboda, Fraunhofer-Institute for Solar Energy Systems, Germany; Heinz Wenzl*, Beratung fur Batterien und Energietechnik, Germany; Ian Baring-Gould, National Renewable Energy Laboratory, USA; Nigel Wilmot, Murdoch University, Australia; Florence Mattera, Commissariat a l’Energie Atomique — Groupement Energetique de Cadarache, France; Stathis Tselepis, Centre for Renewable Energy Systems, Greece; Frans Nieuwenhout, The Netherlands Energy Research Foundation, Netherlands; Carlos Rodrigues, National Institute for Engineering and Industrial Technology, Portugal; Adolfo Perujo, Joint Research Center- Institute for Environment and Sustainability — Renewable Energies Unit (ISPRA), Italy; Alan Ruddell, Energy Research Unit (ERU) at Rutherford Appleton Laboratory, UK; Per Lundsager, Risoe National Laboratory, Denmark; Andreas Jossen, Centre for Solar Energy and Hydrogen Research, Germany.

Renewable energy systems (RES) are unique among energy supply systems because their performance and design depends entirely on the location and climatic conditions. As a result of design options, user requirements and local climatic conditions, there are enormous variations between RES. This paper describes a method how to evaluate the data on monitored renewable energy systems (RES) in such a way that the operating conditions and performance of individual components and the system as a whole can be analysed and compared. It shows how categories of similar use can be defined and recommendations for each category can be made to assist in the selection of the most suitable products.