Category Archives: SonSolar

Potentials of Wind and Solar Energy

The potentials of wind power and solar electricity production from PV and concentrating solar power stations are discussed in the following. Except where otherwise indicated, the calculations are based on meteorological data of the European Centre for Medium-Range Weather Forecasts (ECMWF) and, in the case of solar energy, also on data of the National Center for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) [ERA-15] [NCEP 99].

• 3.1 Potentials of Wind Energy

The potentials of national wind energy are dependent not only on prevailing wind conditions but also on factors such as population density or nature preserves and other restrictions. According to [Qua 00], for instance, the realizable wind power capacity on land sites in Germany can correspondingly be estimated at 53.5 GW. A total annual wind power production of about 85 TWh is assumed to be achievable as a result. This figure represents about 17% of total consumption (approx. 490 TWh) and is equivalent to 1600 full-load hours (FLH) of wind power per year. The additional offshore wind potential is taken to be about 79 TWh at nearly 3400 FLH [Qua 00]. In another study, the German offshore wind potential is given as approx. 240 TWh [GMN 95], even though a maximum distance of 30 km has been assumed between the offshore wind turbines and the coastline. This limitation has been rendered superfluous by more recent planning (s. e. g. [BSH 04]), so that a far greater potential may be assumed. If only locations are considered where the water is not deeper than 40 meters, with offshore turbines erected entirely at locations not previously declared nature preserves, military zones, or otherwise unavailable, a "conflict-free” potential of about 67 TWh results according to [IGW 01]. These assumptions may be considered particularly conservative. Permit applications have already been made for water depths of 45 meters [BSH 04], opening the way to a high multiple of "conflict-free” sites compared with the above considerations. Nature preserves and areas used by the military have also come under consideration [BSH 04]. Consequently, the yearly production of several hundred TWh is easily imaginable. In addition, the German portion of potential wind power sites in the North Sea makes up only about one-eighth of the total area with a depth of less than 50 meters (s. also [Czi 00]). Considering the use of the southern North Sea with Denmark’s northern tip as the northernmost point, an area of roughly 200’000 square kilometers with sea floor depths less than 45 meters can be found [Czi 00]. Here theoretically, neglecting all restrictions, an area sufficient for 1600 GW of rated offshore wind power would be available for generating up to 6000 TWh of electricity. This is roughly three times EU consumption, thus demonstrating that even after taking major restrictions into account a huge North Sea potential might still be realisable. Furthermore, shallow areas in other European seas with abundant wind resources would cover more than two times the area of the southern North Sea [Czi 00]. Greenpeace has recently published a scenario in which a capacity of 237 GW offshore wind power would be installed in EU coastal regions by 2020 to produce more than 720 TWh, while covering only 3.4% of the area available after all constraints

had been have been taken into account [Gre 04]. Notwithstanding differing estimates of potential, a significant contribution to electricity production is harnessable. The full use of offshore wind energy necessitates a wide spectrum of cooperative measures among European countries for arriving at the most favourable scheme of implementation.

According to conservative estimates of the Danish company BTM Consult, the technical wind power potential of land sites within the EU and Norway is 630 TWh, corresponding to 315 GW of installable wind capacity [EWEA 99]. The very simplified assumption has been made in this case that all turbines would be delivering 2000 FLH a year, meaning they would operate at an effective average capacity of roughly 23% at each site. In relation to the total electricity consumption within the EU of about 2350 TWh (with Norway, 2450 TWh), this technical potential could thus be harnessed to fulfil about one-fourth of electrical energy demand [DOE 02]. Another particularly detailed analysis of the wind conditions at a relatively narrow strip of land along the Norwegian coastline determined a technical potential of 1165 TWh at an average turbine load of 2900 FLH, not considering any possible restrictions, with the most favourable sites producing 156 TWh from turbines delivering an average of 4100 FLH [Win 03]. According to our own conservative estimates based on meteorological data of the ECMWF [ERA-15], a selection of wind sites within the European Union could generate about 400 TWh of wind energy with an average turbine performance of 2670 FLH using about 150 GW of total installed capacity, taking into account restrictions due to densely populated areas. Under the particularly favourable meteorological conditions prevailing in Ireland and Great Britain, far more electricity from wind power could be produced than estimated here. Due to the conservative assumptions adopted, however, their contribution has been limited to 25% of the total capacity installed in the EU and Norway. The respective electricity generation under these conditions would equal 32% of the electricity consumed in Ireland and Great Britain. In other countries, by contrast, the corresponding figure lies below 10% of domestic consumption. As previously mentioned, an annual average turbine operation of roughly 2700 FLH can thereby be achieved, whereas an even distribution of wind generators within the EU would only allow approx. 2000 FLH to be realized [Gie 00]. If in fact the total achievable potential for Great Britain and Ireland could be exploited, the generated electricity would slightly exceed their current demand. To insure that these possibilities may be realized, the transmission grid to neighbouring countries should be expanded in response to the growing use of wind energy to anticipate and stimulate the multilateral integration of wind power capacities.

The land-based wind power potentials in the EU are limited to the estimated levels identified above, due less to technical and meteorological restrictions than to the population densities of particular regions. If it were possible to use land areas freely, electrical energy requirements could be fulfilled many times over with wind power alone (s. Fig. 1). Restrictions due to the high population density are of secondary importance in many distant windy regions surrounding Europe. The population densities of northern Russia and western Siberia, northwestern Africa, and Kazakhstan lie between 0 — 2 inhabitants/km2 and are thus at least two orders of magnitude below those of Germany with its 230 inhabitants/km2 (s. e. g. [Enc 97]). In addition, these areas are steppes, deserts, semi-arid regions, or tundra of practically no inherent economic value, so that wind electricity generation may be instituted as a beneficial means of "farming in the desert”. The potential electricity production from wind power is shown in Fig. 1. Even considering only land sites at which more than 1500 FLH can be achieved (within the rectangle roughly 40% of the land area), and without further restrictions, the area shown within the rectangle comprising Europe and its neighbours could deliver 120’000 — 240’000 TWh of electricity from wind power at a installation density of 4 — 8 MW/km2. This result constitutes a maximum of about one hundred times the current electricity demand of the

EU or fifty times the electricity consumption of all countries within the selected area. If only the best wind sites with the highest production at an installation density of 8 MW/km2 were employed, just 4.3%o of the land area would be required to provide the equivalent of the annual electricity consumption of the entire area within the rectangle shown on the map. About 2.5% of the area would be adequate for covering the equivalent of the electricity demands of the EU. Furthermore, the area covered by the turbines and accompanying infrastructure themselves is typically only about 2% of any land dedicated to wind farming. (The figure of 2% applies generally to wind farms consisting of individual turbines of 600 kw rated capacity. The area is reduced if larger single units are employed.) Therefore, the land space required for generating the equivalent of total EU electricity consumption is actually less than 0.05% of the entire marked geographical area. By comparison, the roughly 6% of total land area in Germany currently sealed by streets, buildings, and other infrastructure covers a thousand times bigger fraction of space.

Fig. 1 Potential of average annual electricity production from wind energy of the years 1979 — 1992; meteorological data: ECMW. The theoretical generation potential of wind energy, shown in the red quadrangle when land areas are used with over 1500 FLH, is between 120,000 and 240,000 TWh (turbine placement 4 — 8 MW/km2).

The three regions previously mentioned — northern Russia with northwestern Siberia, northwestern Africa, and Kazakhstan — each offer a greater wind energy potential alone than required for meeting EU consumption requirements in their entirely. In the following treatment, therefore, only the areas within these regions with the highest yields have been considered. In Tab. 1, the size of the areas selected for the analysis, the installable turbine capacity for a conservative assumption at a moderate installation density of 2.4 MW/km2, the expectable average production of the turbines assuming wide-range turbine placement over the selected area, and the expectable yearly output are given. Because of the data used, the estimates tend to be conservative. In the case of southern Morocco, for instance, measurements have shown that load factors of far more than 4500 FLH may be assumed directly on the coast at favourable locations [ER 99]. In Kazakhstan, measurements and other investigations likewise indicate that yields significantly over 4000 FLH may be expected [BMW 87] [Nik 99]. The higher the topographical complexity of the terrain, the more significant the underestimation tends to be. Wind potentials in [CGM 03] calculated from Rise for the region at the Gulf of Suez in Egypt represent the most extreme underestimation of wind conditions in any complex terrain known thus far to the author. A comparison of this map and the date with the data depicted in Fig. 1 indicates a maximal average production of roughly 2200 FLH at low spatial resolution (like the data derived from ECMWF data, which build the basis of the scenarios), while the high-resolution Rise

The potential for photovoltaic electricity generation has been estimated for Germany to lie

Fig. 2 Potentials for average electricity production from photovoltaic generation derived for the years 1983-1992 Module = 14%, System = 11.5%, Orientation East-West with Slope = Latitude; met. data: ECMWF and NCEP.

data corresponds to 6000 FLH (for better comparison, see also [Czi 01]). Even if this example is particularly extreme, such underestimation is rather typical for complex terrains, making clear that the scenarios represent a very conservative approximation of actual possibilities and thus provide compelling reasons for further argumentation, since — as the example shows — there must be substantially better wind potentials worldwide at many places than can be inferred from the data bases used for the scenarios. It appears certain that high-yield locations would be exploited first if they were known, whereby high potentials could be expected at high quality sites. [31]

Country

Annual Production

Total area selected

Potential rated Power

Potential

production

[FLH/a]

[km2]

[GW]

[TWh/a]

Min

0

Max

Northern Russia and

North-western

Siberia

3000

3100

3400

140.000

350

1100

North-western

Africa

Southern

Morrocco

3200

3400

3700

50.000

120

400

Mauritinia

2650

coast

3000

3250

inland

44.000

105

320

Kazakhstan

2500

2600

2800

90.000

210

550

Table 1. Expectable turbine output for wide-area wind energy deployment in distant regions of high wind yield, total area of the selected regions, assumed installable turbine capacity at 2.4 MW/kW2, and expectable yearly output. The output varies within partial areas within the regions, as reflected in the specifications Min, 0 and Max (expanse of each partial area roughly 1.125° in NS and EW direction).

at about 190 GW (150 TWh), some 120 GW (95 TWh) of which would be on rooftops [Qua 00]. This figure corresponds to an average yearly full-load capacity of 770 FLH or 780 FLH on roofs. Our own calculations have shown that good modules employed on rooftops with optimum angular position and unaffected by shadows could produce about 950 FLH. The difference to the values given in [Qua 00] is due primarily to the inclusion of shadow and disorientation factors. Fig. 2 shows the potential yearly electricity production from PV.

Table 2 contains the potentials and FLH for a number of countries.

Rooftop PV Potentials

Load

Country or Area

P

EG

L0

Lopt

[GW]

[TWh]

[FLH]

[FLH]

Germany

120

95

780

950

Portugal

10

14

1100

1350

Finland

5

4

660

800

Algeria &Morocco

81

96

1200

1450

Mauritania & Senegal

32

42

1300

1700

Total EU 15

550

470

850

1050

A second variety of solar electricity generation makes use of linear concentrating of solar radiation in parabolic mirror arrays (s. e.g. [Gre 03]) (Similar configurations, not yet constructed in operational size for power plants, have been realized with linear Fresnel reflector arrays [Sol 03].). With this technology, the desert regions of northern

Fig. 3 Potentials of average annual heat production from parabolic linear concentrating mirror fields for solar power plants for the years 1983-1992; met. data: ECMWF and NCEP.

Tab. 2 Potential power (P) and electricity generation (EG) from PV (Module = 14%) on roofs as well as simplified assumptions on the expectable average equipment duty factor (L0) under consideration of the losses due to shadows and roof disorientation, or under optimum conditions (Lopt). It has been assumed that the same roof area per inhabitant is available in all countries as in Germany an that it is distributed in th countries according to the population. [32]

Africa could satisfy 500 times the electricity demand of all EU countries. Since domestic consumption is comparatively low, however, this high solar energy potential could only be realized to a significant extent if solar power were exported outside the northern African region. The output of this solar thermal power plants with parabolic arrays depends crucially on their design. Therefore, the performance characteristics can be stated only with reference to the design parameters. The use of thermal storage units is of major importance in this respect. The quality of the site can be determined by the heat production of the mirror array, independent of the specific parameters of the power plants, as shown in Fig. 3.

The heat may be employed in a conventional thermal power plant to generate electricity at an efficiency of about 35%. If heat storage is included in the overall design, a larger linear mirror array is employed to charge the storage medium during the day. In this way, electricity may be produced throughout the night while supplanting the fossil fuels otherwise necessary for continuous operation of the plant. The storage facility therefore provides greater flexibility and reduces the cost of the solar electricity produced, since the conventional part of the power plant utilizes more solar heat, which during the night is delivered from storage. Therefore, the specific costs of the conventional part are lower, while not entirely compensating for the investment in storage capacities. In order to estimate the achievable electricity generation at certain locations, it is assumed that the storage has been generously dimensioned for 14 FLH, so that the solar heat produced in the mirror field will never be partially wasted due to the limited influx capacity of the conventional section. Such a parabolic trough power plant could attain nearly 5600 FLH in southern Morocco (western Sahara). Farther south in Mauritania, more than 5800 FLH would be possible, while 3000 FLH could be expected at a good location on the Iberian Peninsula.

Solar Energy in the Canton of Geneva (Switzerland)ssessment of the Solar Energy Resources and. Setting-up of a Public Multi-Stakeholder Strategy. for the Promotion of Solar Energy

Marcel Gutschner and Stefan Nowak
NET Nowak Energy & Technology Ltd
Waldweg 8, CH-1717 St. Ursen, Switzerland
Tel. +41 264940030, Fax: +41 264940034
marcel. gutschner@netenergy. ch, stefan. nowak@netenergy. ch

Oliver Ouzilou and Jacobus van der Maas
ScanE, DIAE, Department for Environment and Energy
Canton of Geneva, Switzerland
PO 3918, 1211 Geneva 3
Tel +41 223272092, Fax +41 223272094
olivier. ouzilou@etat. ge. ch, jacobus. vandermaas@etat. ge. ch

The Canton of Geneva (Switzerland) is renowned for a strong energy policy promoting both renewable and local energy and a sustainable development. Besides tangible quantitative targets, the policy also aims at optimising the interfaces and interactions between the relevant stakeholders (authorities in energy issues, building / urban design and land planning, multi-utility, customers, etc.) in order to facilitate and promote solar energy. This paper focusses on two issues in the wider context of the energy policy and activities supporting the implementation of renewable energies: 1) Assessment of the solar energy resources and 2) Setting­up of a public multi-stakeholder strategy for the promotion of solar energy.

Purpose

The objective of the "Assessment of the solar energy resources and Setting-up of a Public Multi-Stakeholder Strategy for the Promotion of Solar Energy” covers two interrelated issues.

Within the energy policy, the Canton of Geneva (Switzerland) aims at optimising the interfaces and interactions between the relevant stakeholders (multi-stakeholder strategy with authorities in energy issues, building / urban design and land planning, multi-utility, customers, etc.) in order to facilitate and promote solar energy.

The assessment of the solar energy resources is part of the process and provides information to strengthen strategies in order to exploit the solar energy resources and potential available on its own territory, particularly in its building stock.

Key questions for the built environment

A number of barriers must clearly exist be overcome before the construction sector can become deeply involved in carbon trading markets. These include

the availability of suitable technologies and awareness of them on the part of building and property professionals, owners and occupants. There is also an important need for access to financial expertise, means of coping with uncertainty in the process and level of transaction costs relative to benefits. As one construction professional expressed it, “The key to green office buildings lies not so much in developing the technical side but in adjusting the ecoomic arguments in favour of more sustainable solutions” (McKee 2003).

The following three case studies, taken from US examples in Northern California indicate some of the technologies and design approaches available to implement energy efficiency in practice, and how these might relate to carbon trading.

Case study 1: Hewlett Foundation, Menlo Park

Fig 4: Hewlett Foundation, Menlo Park, California.

Architects (shell): B. H. Bocook; (interiors): HPS Architects

The William and Flora Hewlett Foundation is a philanthropic organization created by one of the founders of Hewlett Packard. Its purpose built accommodation, completed in 2002, is located next to the Stanford campus. The 48000 sq. ft. building is designed to provide office and meeting space for the Foundation.

It also provides facilities for staff, including a gym and refreshment area.

The plan is a shallow U-shape around a central courtyard: due to the shallow plan, most office areas have windows. The interior zones on the upper floor have either rooflights or clerestory lanterns. The side windows are openable.

The building is fully air-conditioned: however, the system uses displacement ventilation supplied from the 18” raised floor, with local controls. On the upper floor, the ceilings are open — increasing potential for natural ventilation and well-controlled lighting.

Large roof overhangs shade the upper floor windows; some lower windows are also shaded by roof overhangs, while others are sheltered by the colonnade which adjoins the courtyard. Lighting controls are sensitive to room utilization, and switch off after 10 mins when not required.

Case study 2: Jasper Ridge Biological Preserve, Woodside

Fig.5:Jasper Ridge Biological Preserve, Woodside, California. Architect: Rob Wellington Quigley Architects.

The building has a roof mounted PV system sufficient to power exit signs and emergency lighting in the event of a power cut.

This building provides a field study base for the educational activities of the Jasper Ridge Biological Preserve (JRBP), attached to Stanford University. It is located a few miles from the Stanford campus, about 30 miles south of San Francisco.

The 9 800 sq. ft. building, completed in 2002, is a linear, single story form housing 2 classrooms, a herbarium and administrative offices, a research lab and ancillary spaces including a cold room. Toilets and showers are accessed from outside, and are not within the conditioned envelope of the building. The building’s main axis runs east — west: most eye-level glazing is oriented almost due south. North-facing glazing at high level lights the ancillary spaces; north-facing monitors and rooflights bring daylight to the center of the plan.

The building is naturally ventilated: high ceilings in the main areas provide a plenum and clerestories can be opened manually to give stack ventilation in summer.

Clerestory windows are opened in the evening for night purging: thermal mass is provided by the floor. To minimize heat gain — and heat loss in winter — the building is highly insulated, with walls and roof meeting R-30 standard. High performance glazing is used: double glazed low-e panes with thermally-broken aluminum frames. These are not standard items in California, and were shipped in from out of state.

This building has both PV and solar thermal panels. The PV installation is almost invisible from ground level: the amorphous silicate panels are mounted on the south­facing internal slope of the roof. The solar thermal installation dominates the building’s southern fagade. Six panels sit on the south side of the roof monitors, while the rest are mounted between the eye-level and clerestory glazing of the main spaces. These panels have a secondary function in providing extensive summertime shading to the main south facing windows; the roof overhang gives some shade to clerestories. The primary, glycol-filled loop heats a massive water tank located in an ancillary space: this supplies low-level radiator panels in the lab and offices and ceiling mounted forced convective heaters in the classrooms. A small propane furnace provides a back-up system for cool, dull spells in winter.

Layer preparation and stability

In a common research project of Flabeg GmbH & Co. KG, Merck KGaA and the Fraunhofer Institutes ISC and ISE, a production process for AR solar glass was developed, allowing the coating of panels up to 1.5 x 2.5 square meters. After edge processing and surface cleaning, the porous SiO2 AR layers are obtained by sol-gel dipping of the glass followed by a thermal curing at 630°C taking place simultaneously with the glass toughening, and therefore minimising the production costs. The critical steps in the preparation of the glazing are the glass cleaning and a good control of the SiO2 sol particle size, in the range of 30 nm. A precise control of this size is necessary to achieve films with good adhesion properties and the desirable refractive index. Typical AR layers are about 130-140 nm thick, with a refractive index of 1.27, and are deposited on both sides of the panel. The AR coating process can be applicable to any kind of glass (float glass or rolled glass with texture). Fig. 1 shows the light transmittance through low iron patterned glasses with and without the AR layer. The broad transmission maximum at 99 % around 600-650 nm indicates that the % X condition for reflection minimum is fulfilled in this range. Note that the reflection measured through a glass is roughly the sum of the first air-glass reflection and of the second glass air reflection (which adds 4% to the primary air-glass reflection).

An important issue is the stability of the AR layer, because modules are expected to stay several tens of years in the field. To clarify this point, two main test procedures were performed: accelerated durability tests under severe conditions in accordance to IEC 1215 and real life outdoor exposure tests.

The following laboratory tests were passed successfully:

• Condensed water climate test at 40°C and 100% relative humidity (acc. DIN 50017): no significant degeneration or changes

• Mechanical resistance by Crockmeter-test (acc. DIN EN 1096-2): after 1000 cycles no significant visible changes, change in solar transmittance < 1%

• Humidity-heat-test of AR-PV laminate modules at 85°C and 85% relative humidity for 1000 hours (acc. IEC 1215): no visual degeneration, change in energy output < 5%

• Temperature-cycle-test of AR-PV laminate modules (-40°C to +85°C for 200 cycles, acc. IEC 1215): no visual degeneration, change in energy output < 5%

Besides, climate test in SO2 atmosphere (cycling: 40°C, 100% rel. humidity, 8 hours to 18- 28°C, 75% rel. humidity, 16 hours, 5 ppm SO2, 23 cycles), freezing test (-20°C, 48 days) and boiling test (100°C, 10 minutes) did not modify the optical properties of the layer. Laboratory tests even under severe conditions can only give an indication of the durability behaviour of new materials or systems. Therefore outdoor weathering tests have also to be performed to check the suitability of the layers in real conditions.

Test samples have been exposed at different locations spread over Germany and monitored for 3 years at the date of writing this paper. The AR glasses have been evaluated by visual inspection and solar reflectance measurement. During this test period no effects and changes could be observed. Fig. 1b shows a summary of the test data.

<D

О

£=

03

-І—* — t—‘

E

w

c

TO

Wavelength [pm] a)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

100 80 60 40 20 0

Fig.1. a): Optical transmission as a function of the wavelength for a low iron glass with and without the porous SiO2 AR layer (AR layer on both sides). b) Solar Reflectance (reflectance weighted over the solar spectrum) of AR-coated and uncoated glass as a function of outdoor exposure time.

In addition, the outdoor results showed no negative effect of contamination or soiling of the glasses due to the AR coating. A possible increased adherence of dust or particles in or on the porous AR structure is not observed. The porosity of the AR layer is in the range of a few nanometers and therefore much lower compared to the typical size of dust particles in the range of several microns. Because of these different dimensions, a particular interaction has indeed not to be expected.

As a conclusion, the mechanical resistivity, as well as the chemical inactivity and the nanostructure of the AR layer seem to combine to confer porous SiO2 AR glass the necessary stability for long term outdoor applications.

System design

1.1. Electrical energy demand

Analysing the solar and wind potential of the regions the PV system is the on way in the electrification of the remote settlements that can be extended with another sources if the energy demand will be growing. For the present case study the energy consumption is presented in Table 1.

Load type

Unit

Power

(W)

Number of loads (W)

Load

Power(W

)

Hours/Day

Daily Energy Consumption (Wh)

Indoor lighting in rooms

13

3

39

5

195

Outdoor lighting

8

1

8

16

128

TV, radio

50

1

50

5

250

PC

80

1

80

1

80

Electronic

equipment

2

1

2

8.5

17

Total

179

670

Table 1. Detailed analysis of PV system

Figure 3. Average daily electrical energy demand for the application

It can be see that the electronic equipment and the TV set appliances is the important portion of energy consumption and in critically meteorological case can be reduced the functioning time. The figure 3 shows the average daily energy demand of the application

Numerical Simulations

2.1 Optimization of layer thicknesses

In dependence on the wave length an organic solar cell absorbs the incident light in different ways. While one part of the light energy is absorbed as heat, another part is

invested in the generation of electron-hole pairs, called excitons. This mechanism occures at any point inside the solar cell and the density of created excitons depends on the light absorption density at a given point. Excitons are separated only by utilization of the „built in field" at the pn-junction of the active layers (Fig. 2). Since the diffusion length of the electron-hole pairs is very short, the chance to reach the pn-interface and the separation as a consequence is very slim. It turned out that the average of the diffusion length is only 10nm in each direction from the interface [13]. For that reason most of the generated electron hole pairs recombinate almost instantaneously while only the excitons created near the pn-interface contribute to the generation of photocurrent at the end of the day. As a consequence one has to place a high amount of absorbed energy to this narrow area around the pn-junction interface what requires a proper distribution of the absorption density in the photovoltaic device. As shown in chapter (2) the absorption density ‘a’ does not follow a trivial function and due to the strong dependence on the conductivity ‘o’ of the optical parameters, ‘a’ does not correlate with the distribution of the electric field |E|2 in dependence on the position vector. While the square of the electric field |E|2 is continuous at the interface between two layers, ‘a’ is not. As a consequence the density of absorbed light energy differs

Fig. 2: Setup of an organic solar cell

within adjacent layers even near their interface. Taking this into account and considering the diffusion length of the solar cell materials we calculated the absorption in a region of only 10nm from the active interface both in the p-layer and in the n-layer and summed it up to a value called „effective absorption“ Aeff. In order to gain a better understanding of the behavior of the absorption density we figured out the optimal layer thicknesses of the active layers copper-phthalocyanin (CuPC) and BBP-perylene of our solar cell models.

In our simulations we assumed a thickness of 140nm for the ITO layer as it was purchased on 1mm thick glass. We found an effective thickness of arround 140nm for the PEDOT layer of most of our solar cell models as well. For that reason we stayed with dITO=140nm and dPEDOT=140nm through all our simulations unless noted otherwise. Due to its almost constant refractive index throughout the visible sun light spectrum, the covering glass only decreases the amount of light absorption, but does not effect the location of the maximum and minimum of the absorption density inside the photovoltaic device. For that reason we left the glass out of consideration. Furthermore we assumed perpendicular incident light with a wave length of 550nm. In order to gain a tool for a quick determination of proper layer thicknesses we calculated Aeff for a given range of layer thicknesses and for given active materials. The figures 3-4 show the effective absorption Aeff in dependence on the thickness of the active layers dCuPC and dBBP-

perylene. As one can see there is a distinct maximum of Aeff, which corresponds to special thicknesses of the layers. The top-view (Fig. 4) provides the optimized layer thicknesses like a “map”. For this special model we found 55nm for dCuPC and 70nm for dBBP-perylene. In addition to that we found a big difference between the maximum and the minimum of effective absorption. A factor of almost twenty between the worst and the optimal case of efficient light absorption clarifies the importance of the optimal parameter setup for the solar cell layers.

Fig. 3-4: Effective absorption (Aeff) in dependence on the thickness of the active layers. The maximum indicates the optimal setup for given solar cell materials

Upon this thickness maps (Fig.4) which show the usability of given layer thicknesses we calculated the spatial distribution of absorption density in the whole solar cell. Figure 5 shows the absorption density in dependence on the position vector ‘z’ for a non — optimized organic solar cell with dCuPC=200nm and dBBP-perylene=150nm. As one can see ‘a(z)’ is almost at a minimum both in the CuPC layer and in the BBP-perylene layer. A solar cell with this configuration will never be a performer. No mater how good the transport properties of the materials are. Since ‘a(z)’ is a measure for the amount of generated excitons, one can see that most of the generated excitons will be lost by

recombination.

Fig. 5: Absorption density in dependence on the position vector z’ for an non-optimized organic solar cell

Fig. 6: Absorption density in dependence on the position vector z’ for an optimized organic solar cell

In contrast to that the following solar cell (Fig. 6) with the layer thicknesses dcupc= 55nm and dBBP-perylene=70nm is an optimal performer in terms of energy absorption efficiency. The absoption density in the BBP-perylene layer, which is the main absorber in this special case, reaches its maximum near the active interface.

The presented examples show the best and worst case for a given wave length of the incident light. According to given light spectra and taking into account geographical and geological aspects of the place of destination one is able to gain an optimal setup for any situation.

Computer software tool

In this study computer software PVS2000 will be used for energy analysis. With the simulation program PVS it has a powerful instrument to plan and dimension grid — connected. With the program, the user is able to compute the PV energy output, characterize and evaluate the operation of the system, compare different variants of the system or optimize system parameters such as the tilt angle or the battery capacity. In developing the program special emphasis was put on ease of use, a clear and comprehensive display of the results, as well as short computation time. A time step procedure that represents the performance of a PV system correctly has been chosen for the simulation program PVS. It offers sufficient flexibility to treat special systems. The system behavior is simulated by balancing the calculated hourly energy flows. By this study only the energy produced from the PV system is needed to be known. The main initial inputs are:

— PV generator capacity 350 kWp and other PV module characteristics

— Inverter capacity 350 kW and other inverter characteristics

— Weather data (solar radiation, temperature)

— Install condition as follow the previous scenarios

The aerogel glazing

Monolithic silica aerogel (aerogel) is a highly porous material with pore diameters in the range of 10 — 100 nm. The porosity is above 90%, which combined with the nanometre pore size makes the aerogel a highly insulating material with a thermal conductivity lower than of still air [1]. Further decrease in thermal conductivity can be achieved if evacuated to a rough vacuum, i. e. below approximately 50 hPa in which case the thermal conductivity in the pore gas is almost eliminated [2].

Beside the low thermal conductivity a high solar energy and daylight transmittance is achieved, which makes aerogel a very interesting material for use in highly energy efficient windows [3]. The compression strength of aerogel is sufficient to take up the atmospheric pressure if evacuated but the tensile strength is very low, which makes the material fragile, i. e. if in contact with liquid water the surface tension in the pores would demolish the aerogel structure. So, the application of aerogel for window glazing requires the aerogel to be protected against water and tensile stress. This can be achieved by placing the aerogel between two layers of glass and apply a gas and vapour tight rim seal. When evacuated to a rough vacuum only compression stresses will be present in the aerogel due to the external atmospheric pressure.

Figure 1 shows the advantage of aerogel windows relative to commercial available low energy glazing for which the reduction in U-value is achieved by multiple layers of glass and low emissive coatings — measures that all reduces the solar energy and daylight transmittance. But aerogel glazing has a solar energy transmittance equal to plain double glazing and at the same time has a heat loss coefficient equal to the best triple layered gas filled glazing units. Monolithic silica aerogel is the only known material that has this excellent combination of high solar and light transmittance and low thermal conductivity — material parameters that makes it possible to achieve a net energy gain during the heating season for north facing windows in a northern European climate as the Danish climate.

The utilization of the passive solar energy passing through the windows is an important factor in reducing the annual energy consumption for space heating in northern European countries and has been the background for the research and development projects HILIT

[4] and HILIT+ [5] financed in part by the European Commission. The objectives for both projects were to improve the aerogel elaboration process with respect to material properties (both thermal and optical) and process parameters (drying, duration, safety, etc.) and to develop final aerogel glazing prototypes with a total U-value lower than 0.6 W/m2K and a total solar energy transmittance above 75%.

Summary and Conclusions

For the optimization of silicon cell manufacturing processes with respect to a reduction of breakage losses an i ntegral concept was developed and applied in industrial manufactu­ring lines. The concept consists on a combination of different approaches.

Measurements of the cell strength after specific manufacturing processes reveal, whether these processes induce or reduce damaging. The damage and crack formation and evolution history is investigated by fractographic and microscopic inspections of original cells broken during manufacturing. Fracture relevant processes are analysed by experi­ments and by numerical modelling in order to determine the acting mechanical or thermo­mechanical loadings. Numerical simulations by means of finite element analyses help to optimise the processes, because the influencing process parameters can be varied in the computer. Testing methods such as acoustic techniques or proof-tests are applied for the early detection and elimination of cells which are not able to survive further processing.

By a combination of the results of these approaches the causes of breakage losses can be localised, analysed and deleted.

[1] Beinert, J., Kordisch, H., Kleer G.: Studies on Fracture and Strength of Photovoltaic Silicon Wafers, World Renewable Energy Congress VII (WREC 2002), 2002, Cologne, Germany, Pergamon, 2002

Safety of the System

The described system differs from existing installations using hydrogen because it employs components that are not yet widely applied. Additionally, it should be sufficiently safe and fault forgiving in order to be operated by "normal people”.

Due to its physical properties hydrogen differs from other gases. The main relevant points are the low density, that it is tasteless, odourless and invisible, its wide explosion range and that is tends to detonate.

The safety analysis for hydrogen systems [6] suggests that it can be handled without an unacceptable risk. However, the design of the installation has to take into account the peculiarities of hydrogen. Additional requirements stem from the fault forgiving properties mentioned above.

1 Results

As a result of this research project, a procedure for identifying configurations of autonomous energy systems which

• are safe to handle,

• sufficiently reliable,

• optimized as to the number and type of equipments required,

• energetically efficient.

will be available. Additionally, the relationship between the degree of redundancy, component lifetimes and their repair times can be analyzed.

Since the research is ongoing, calculation results will be communicated in the oral presentation at the conference.