In many remote places like farms, households which are far away to the grid there is great need for electricity. Photovoltaic system offers possibilities to satisfying this need and they provide a free and clean energy source from the sun and the other hand resolve a social problem in this remote area. The investment cost to connecting this region where the households are dispersed to the log distance from each other, in especially in the mountain region is very high for example 10.000-12.500 Euro/km. Starting point in this study was the distance from the grid connection is 4-5 km. It would mean over 50.000 Euro investment in case of grid development. In this context for the electrification of the area one way is the stand alone PV system what can be extended with another energy sources and the second is the electrification by micro grid that is indicated for villages with 100-300 households. The present paper designs and simulated a stand-alone PV system developed for this remote area. The figure 4 shows the principal design of a photovoltaic system with alternating current output.

The PV system presented in figure 4 consists of 12 photovoltaic module (1) with a peak power 40Wp each and the type is Kyocera KC 40. The charge regulator (2) is C 40 Trace and the used inverter (3) is Piccolo 21. The produced energy is stored in a battery bank (4) and the type Rex Homas. The monitoring and the data logging are realised by the ENERPAC-data logging unit. At our latitude an energy supply based exclusively on photovoltaic requires large photovolatic generators due to the fluctuation is solar radiation. The same is true of photovolatic system that has to have great availability. Hence, a mixture of generator types is generally combined to from hybrid system. Combining PV generator and motor other wind generators ensures the same power security as in a public grid. In this way was design the system the component structure especially the battery bank and the monitoring equipment permit this enlargement of the system using another classical and RES sources, like wind hydro and fuel cells. In the future if the user want completely independent of fuel supplies an electrolyser and a hydrogen storage system can be integrated.

Till now we assumed active materials for our simulations which are already in use for the design of organic solar cells. Now, how would it affect the behavior of an organic

solar cell’s light absorption, if we expanded the range of optical parameters. Respectively, what kind of materials would be desirable for optimal power conversion and what optical parameter would provide the theoretical optimum of light absorption efficiency. Considering this we always kept in mind the practical relevance of used optical parameters concerning the producibility of the material. For each layer we have to determine the thickness di, the refractive index ni and the index of extinction Ki. That

leads to a parameter space with the dimension 3i for our optimization algorithms in the general case. For the four-layer-solar-cells we start with a 12-dimensional parameter space. Due to constant parameters of the ITO and the PEDOT layer the processing times for the optimization of the remaining six parameters could be kept in operable

limits. We figured out the optimum of the 6-dimensional hyper surface which provided the maximum of Aeff with the optimized parameters di, ni and к for the active layers. In order to gain a better understanding of the usefulness of the optimized parameters we varied both of them at a time in certain ranges while the others were fixed according to the optimized Aeff.

The figures 7-9 show Aett in dependence on the optical parameters of the active layers.

These graphs are intended to give a concept where to go searching for new solar cells’ materials. Appropriate optical parameters are not the only criterion for the improvement of solar cells materials but it will help us to estimate the energy absorption potential of new materials in a very straight way.

Table 1 shows the results for different installation scenarios. It can be seen that for all scenario, the energy production from the PV system is depended on the installation conditions. The scenario 3 shows the higher energy out put but when compared with other scenario it is a very small percentage. Scenario 3 is 0.28% (1,429 kWh/year) and 2.05% (10,349 kWh/year) higher than scenario 2 and 1 respectively.

This value only shows the pre-feasibility technical study of the project. It needs more detailed study such as financial study in further works. Also, for the details of engineering work from the actual site is a very important issue as well.

Acknowledgements

We thank Tesco Lotus, Ek-Chai Distribution System Co., Ltd., Thailand for their financial support. We are also grateful to several people for contributing invaluable input and advice during the preparation of this paper.

References

Bakos, G. C., Soursos, M., Tsagas, N. F., 2003. Technoeconomic Assessment of a Building-Integrated PV System for Electrical Energy Saving in Residential Sector. Energy and Building 35. 757-762.

IEA, 2001. PV System Installation and Grid-Interconnection Guidelines in Selected IEA countries. Report IEA PVPS T5-04:2001

IEA, 2002. Reliability Study of Grid Connected PV Systems: Field Experience and Recommended Design Practice. Report IEA-PVPS T7-08: 2002 PVS 2000 User Manual. PVS for Windows: Simulation and Sizing of Photovoltaic Systems. Econzept Energieplanung GmbH Freiburg Germany.

The ideal rim seal should be 100 % gas and moisture tight and at the same time have a thermal conductivity equal to that of the evacuated aerogel to avoid thermal bridge effects along the glazing perimeter.

However, such solution does not exist and the main task has been to develop a solution as close to the ideal solution as possible.

Several ways exist for minimizing the thermal bridge effect: 1) use of materials with a low thermal conductivity, 2) minimizing the material thickness, 3) increasing the heat flow path length or a combination of the three [3].

The rim seal solutions used in sealed glazing units should both act as gas and vapour barrier as well as a structural element for keeping the desired glass distance.

In aerogel glazing the glass distance is kept by the aerogel layer, which has the sufficient strength to serve as spacer when evacuated. Therefore rim seal solutions for aerogel glazing do not need any structural strength, which makes foil solutions possible.

Metal foils with a thickness larger than 0.1 mm and glass are the only materials that are 100% tight against gas and moisture diffusion. Metal foils with a thickness < 0.1 mm are not airtight due to pinholes. Different laminated plastic foil solutions developed for vacuum insulation panels have a very low permeability that may be sufficient if a limited lifetime of the glazing is allowed.

Glass is considered too fragile leaving metal and laminated plastic foils as the most suitable solutions. The thermal bridge effect has been calculated for different metal foils and for a laminated plastic foil developed for vacuum insulation — the Mylar® 250 RSBL300 from DuPont [6]. The foil is made of several different plastic layers and a 13 nm thick aluminium layer. The total foil thickness is less than 0.1 mm. The thermal advantage of laminated plastic foil relative to stainless steel foils is shown in Figure 2.

The barrier properties of the Mylar® foil are according to specifications given by the manu­facturer (ASTM tests F1249 and D3985) sufficient to keep the required vacuum for at least 30 years if protected against water and UV-radiation.

Solar energy Total glazing U-value as function of window size

transmittance and foil rim seal solution

The advantage of aerogel glazing compared to other highly insulating glazing units are the high solar energy transmittance, which in cold climates has a large influence on the annual energy consumption for space heating.

The basic aerogel made as part of the European projects has a solar energy transmittance of approximately 70% for an aerogel thickness of 15 mm. A subsequent heat treatment of the aerogel to a temperature of 425 °C has shown to improve the optical quality

considerably and increasing the solar energy transmittance with approximately 6 %-points. Placing the aerogel between two layers of glass would reduce the solar energy transmittance due to absorption and reflection in the glass panes. A common 4 mm float glass absorbs approximately 10% of the solar energy and the iron content in the glass furthermore changes the colour of the transmitted daylight. Therefore float glass with a very low iron content makes progress, which reduces the solar energy absorption to less than 1% almost independent of glass thickness.

The reflection losses of the glass panes amount to approximately 8% for a single layer of glass. This value can be changed by surface treatment of the glass panes. A commercial durable treatment has been developed by the Danish company SUNARC A/S [7] and is mainly applied for solar collector covers. The surface treatment reduces the loss due to reflection to approximately 3%.

Table 1 shows the estimated benefit of using anti reflective treated low iron glass for aerogel glazing and common low energy glazing units. An improvement of the solar energy transmittance of approximately 13 %-point for both type of glazing is found, but even if the triple glazing is fully optimised the solar energy transmittance will still be lower than for the non-optimised aerogel glazing.

Jochen Benz, Ursula Wittstadt, Beatrice Hacker

Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany

Fernando Isorna, INTA, Madrid, Spain;

Antonio M. Chaparro, Ciemat, Madrid, Spain Loreto Daza, Ciemat & CSIC Madrid, Spain

Photovoltatic systems are widely used in autonomous power supply systems. One application that has become more important during the last years is the power supply of telecommunication equipment. An availability of 100% over the whole year makes hybrid systems necessary, especially in regions with distinct seasonal variations of insolation. A PV-hybrid system with electrolyser and fuel cell has been developed within a joint European consortium in the project FIRST (Fuel Cell Innovative Remote System for Telecom). Surplus energy in summer is stored in form of hydrogen to be used up by a fuel cell during periods of low insolation in winter. The system supplies the power for a telecommunication equipment. Operating experience from a test site will be reported.

Set-up of the test system

The autonomous power system has been designed to supply a consumer with 100-300 W (power depending on channel usage, average: 160 W) power over the whole year. The consumer is a trunked radio base station which gives wirebound telephony services to locally attached users, e. g. in remote areas or for developing countries where no grid infrastructure is available.

Thin film CuInSe2 (CIS) type PV modules (Wurth Solar, Germany) with a power of 1.4 kWpeak are connected to a lead acid battery of around 19 kWh. It allows for the load to be supplied autonomously for 5 days. .The battery serves therefore as a short term energy storage. A charge controller with maximum power point tracking (Wurth, Germany) puts the energy from the solar panels into the battery A PEM electrolyser (Fraunhofer ISE, Germany) with a maximum power of 1 kWei produces hydrogen at a pressure of 30 bar using the surplus energy during high insolation. The produced gas is fed into a gas purification unit and subsequently stored in metal hydride tanks (CSIC, Spain) with a volume of 70 Nm3 of hydrogen. During longer periods of low insolation (e. g winter), hydrogen is fed to a PEM fuel cell (Nuvera, Italy) which supplies the necessary power if the state-of-charge of the battery is low. An energy

management system (EMS) (Fraunhofer ISE, Germany) which will be described in more detail below has been developed on basis of a microcontroller. The system (see Figure 1) has been set up inside a container at the site of Ciemat (Madrid, Spain). A flow diagram of the system is shown in Figure 2. Permanent data acquisition is done by a data logger. Some parameters are acquired from the EMS’ internal quantities.

Fadia M. A. Ghali

Electronics Research Institute
Cairo, Egypt
Email: fadia@eri. sci. eg

Abstract

Conventional power plants have formed the skeleton of the electrical energy sources. Together with the A. C. and D. C. transmission and distribution networks, enterprises have been well established. Combining the concepts of New and Renewable Energy Sources (NRES), Distributed Generation (DG) and Hybrid Energy Systems (HES) create new perspectives and possibilities to design an optimal generation and distribution system under consideration of economy, energy efficiency and environmental aspects. However, it will also result in more complex problems in the hybrid distributed generation power systems. Therefore, this paper discusses one of the common problems in power systems; voltage collapse problem, and the effect that can be detected by introducing DG to an existing power system. The problem simulation has been carried out using the specialized software package MODES. Results and discussion are introduced.

1. Introduction

Conventional power plants, utilizing fossil fuels with different capacities, have formed the skeleton of the electrical energy sources. Together with the A. C. and D. C. transmission and distribution networks, enterprises have been well established. With the tremendous industrial breakthrough, huge amounts of electrical energy were demanded. However, the expected shortage of fossil fuels and the increasing awareness for the environmental impacts of such systems result in introducing new alternatives. One of these alternatives is the concept of New and Renewable Energy Sources (NRES), such as solar, wind, biomass and geothermal energy sources. Another alternative, which can be applied for the conventional as well as the renewable energy sources, is the Distributed Generation (DG). A third alternative is the concept of Hybrid Energy Systems (HES).[1,2,3] Combining all these alternatives create new perspectives and possibilities to design an optimal generation and distribution system under consideration of economy, energy efficiency and environmental aspects. However, it will also result in more complex problems in the hybrid distributed generation power systems.

One problem with the perforations in PVs solution is that the freestream velocity, if set to model natural buoyancy, depending on the height of the fagade and the size of the duct behind the panel, can be quite small. The best flow regime for heat transfer is turbulent flow, and so it may be necessary to induce turbulence with other methods. Heat-transfer-enhancing ribs are now routinely employed in blade cooling applications. They are usually employed along two opposite surfaces of internal passages within rotating gas turbine blades [10].

The geometric variables in this case include: Rib height to duct hydraulic diameter (e / Dh ); Rib pitch to height ( P / e ); and Duct aspect ratio ( W/ H ), over a range of Reynolds numbers that encompass those reasonably achieved through flow due to natural buoyancy effects or with the addition of a fan extractor.

Work conducted by Hong and Hseih [11] in 1993 showed that staggered, as opposed to in-line, ribs increase local Nusselt number levels in the developing flow region and that these levels can be maintained for fully developed flow further downstream for certain combinations of duct aspect ratio and Reynolds number.

Fig 2. Square duct with aligned rib turbulators perpendicular to the flow.

Rib roughened duct walls can however, cause localised hot spots to develop along the surface wall in the base region behind the ribs. Hwang and Liou [12] in 1995, published research papers covering their work on the use of perforations in the ribs. They found that having an array of staggered holes in the axial flow direction, in the ribs, has a beneficial effect on heat transfer rates. For their optimal rib configurations, peak heat transfer was observed at a rib open area-to-face area ratio of 0.44. Flow visualisation analysis also showed that incorporating the perforation arrays in the ribs significantly reduced the hot spots in the base region behind the ribs. Changing rib shape to semi-circular or triangular-shaped ribs of the same height was also found to reduce the hot spots, while maintaining Nusselt number distributions. A proposal to include v-notch grooves positioned midway between adjacent square ribs was introduced by Zhang et al [13] in 1994, and proved to improve heat transfers by a factor that can be as high as 3.4 above that for a smooth walled duct, while ribbed walls alone yielded a factor of 2.4, for the same pressure drops.

Recent trends in this area have been to align the ribs at an angle to the main flow direction. The use of inclined ribs induces a secondary motion parallel to the ribs, which is found to improve the thermal performance of the cooling passages. Iacovides et al

[10] published results on their experimental investigations on both local thermal and hydrodynamic data for these internal cooling flows. The secondary flow motion leads to a uniform distribution of the turbulence intensities by causing increased mixing and thus more uniform heat transfers across the duct. Inducing turbulence is relevant in the PV problem, particularly at the lower heights up a fagade, as the air in the duct is relatively cool and the large temperature difference can be fully exploited by the turbulent flow.

The hydraulic diameter for the test rig is initially set to 0.125m, by the constraints of the optimal duct depth to length ratio as described by Brinkworth, 2004. A rib height to hydraulic diameter ratio (e/Dh) of 1.6 will be used, matching those of Hwang, 1998, and Jubran et al., 1996, giving a rib height of 20mm. A rib thickness of 12.5mm will be used giving a height to thickness ratio (e/t) of 1.6, matching that of Cavallero et al., 2002 and again Hwang, 1998 [19]. A pitch of 125mm will be used, giving a pitch to rib height (P/e) ratio of 10 will be used, which a common mid-range parameter used frequently by many researchers, including all those previously mentioned in this field. The use of small P/e ratios is found to decrease overall heat transfer coefficients and promote hot spot development. These parameters will be set to compare the effects of straight ribs, staggered ribs, angled and v-shaped at 60° ribs, and perforated ribs against the Nusselt numbers seen for a smooth duct.

Dimples

In general, internal cooling has been enhanced using rib-turbulators. However, the separated flowfield over discreetly mounted ribs can induce cooling non-uniformity and thermal stresses, along the relatively high pressure drop induced, and the difficulty in manufacture further takes form the ribs attractiveness. In recent years, the concept of using indented (dimpled) surface has attracted attention due to the high heat transfer enhancement and lower pressure loss penalty associated with dimples, with up to 2.5 times greater heat transfer over smooth plates and only 1.6 times the penalty loss or half that of rib turbulators, Acharya et al [20]. Serving as a vortex generator, a concavity promotes turbulent mixing in the flow bulk and enhances the heat transfer. According to Mahmood et al., 2001, these regions of high local heat transfer are a result of vortex

Fig 3 The flow structure showing induced vortices exiting a dimple (Zhou et al., 2000)

pairs and vortical fluid that is shed periodically from each dimple. The outward shedding or ejection of fluid produces heat transfer augmentation from the periodicity and unsteadiness of the vortical fluid, and the strong secondary fluid motions of the vortical fluid and vortex pairs near the surface.

As both Mahmood and Chyu both found that the in dimple heat transfer was not as great as that seen after the dimple, the geometric details used by Chyu et al., 1997, will be adapted as this incorporated a larger pitch. They did however analyse the heat transfers differently, with Chyu et al. using area averaged Nusselt numbers, and Mahmood et al., using the total surface area for this calculation, resulting in lower values. The pitch set to be approximately 2.3 times the diameter of the actual circular opening of the concavity was so chosen as that in heat exchangers arrays, e. g. pin-fins and tube bundles, where this ratio generally optimizes their heat transfer enhancement, if arranged in such a fashion. The dimple profile was manufactured using a 19.1mm ball nose cutter, cutting out an opening of diameter 8.24mm and to a depth of 4.8mm. Given the availability of ball nose cutters of diameter up to 25mm, the Chyu profile will be replicated with both longitudinal and traverse pitches set to be 2.3 times the open cut diameter [20].

Having included the effect of increased area (17% for hemispheric dimple), the level of heat transfer enhancement over a smooth wall of 2.2 to 2.7 was seen, which is comparable to most of rib turbulators but lower than some of the more complex ones, which are ruled out for our purpose due to manufacturing constraints, Han et al.,1995. Chyu et al., noted that the trends observed appeared to be insensitive to channel height, or W/H ratio (W/H was 4 in their case), or the Reynolds numbers they checked for, Chyu et al., which were as low as Re=1250 in the case of Mahmood et al [20].

The following criteria can be formulated for a suitable hole contact realized by the metal — grid supported organic p-conductor. The distance between the grooves of the microstruc­ture where the microgrid is located has to be sufficiently small to minimize the series re­sistance. In contrast, the ratio between the lattice distance of the grid and the grid width of the conducting lines should be as large as possible in order to minimize the shadowing effect. The metal grid lines have to be sufficiently conductive to minimize the series re­sistance. The optimum lattice distance for a given type of structure is calculated on the basis of the following assumptions: A width of 10^m for the gold grid conducting lines was taken which could be realised in experiments (figure 5). The contribution of the alu­minum top electrode and the gold grid to the series resistance is neglected. A conser­vative value of 105Qa was taken for the PEDOT CPP105d-sheet resistance. The current voltage characteristics of a infinitesimal small elementary cell with the following charac­teristics: Voc = 600mV, Isc = 15mA/cm2 and FF=0.55 was taken as input parameters.

As a consequence, the series resistance of the PEDOT-layer is excludet. The following non-linear differential equation (1) was solved under consideration of the listed boundary conditions (2) and (3) [8].

2 = Poj(U(x))

w

i( 2 ) = 0