Category Solar Electric Power Generation – Photovoltaic Energy Systems

Improvement of Cell Irradiance by Reduction of Optical Reflection

At a fixed (non-tracking) PV installation the incidence angle of solar irradiance is rarely perpendicular so refection losses increase in comparison to the possible minimum (at plane surfaces). At unfavorable conditions, for example a facade-integrated PV system in low latitudes, the reflection losses of direct irradiance may amount up to 42% (Krauter 1993c).

Reflection losses can be reduced by changing the surface properties (structuring), a better matching of the refractive indices of the incorporated optical layers above the solar cell or by additional anti-reflective coatings.

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Improvement of Irradiance on a Solar Cell

10.1.1 Improvement of Irradiance by Tracking the Sun

Подпись: Fig. 10.1. Fixed mounted PV modules and 2-axis tracking of a 6.6 kWp, PV generator (Hans Grohe, Offenburg). Irradiance on a solar generator can be improved by tracking the sun’s path from sunrise to sunset. In the case of Berlin,

Germany the yield increases by 25-30% in summer and by 0 -10% in winter. Most single-axis tracking devices follow the sun’s path from east to west (azimuthal tracking) at a fixed inclination. Two axis tracking devices also follow the seasonal change in the elevation of the sun’s path. An example of a two-axis tracker in comparison to a fixed PV-generator is shown in Fig. 10.1. The costs for a tracking device are considerable. In many cases the additional yield is not able to compensate for the extra costs (Erturk 1997). In some parts of the Third World (e. g...

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Optimization

The objective is the maximization of the yearly electrical energy yield. This task is carried out by examining the effect of the actual environment in terms of irradiance, optical reflections, ambient temperature, wind as well as the performance of the different components and their interactions.

Optimization of cell-reaching irradiance:

Taking into account direct and diffuse irradiance (considering also angles of incidence spectrum and polarization) and the albedo.

Solar-electrical converter / solar cell:

Technology, feasibility, availability, cost-effectiveness, and environmental soundness. Effects of the operation environment (spectrum, incidence angles, temperature, shadowing, albedo, contacts, wiring) on the power output.

Solar modules, PV generator:

Implementation of the PV generator...

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Variation of Location

While the specific electrical energy requirements do not vary notably for most of modern manufacturing facilities of PV components all over the world, the specific CO2 emissions depend very much on the power plants (nuclear, hydro, fossil etc.) producing the electricity to operate production facilities of PV and system components. The CO2 intensity of electrical power plants and of national electrical grids may vary considerably (between 17 and 1,140 g of CO2/kWhel), as can be seen in Table 9.4.

Remarks:

Due to a considerable increase of wind and solar energy contribution in some countries (e. g., Germany, Spain) during the last years, the participation of “other renewable energies ’ ’ in Table 9.4 has changed (e. g., in Germany from 1.9% to 6...

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Effect of PV on Reduction of CO2 Emissions in Germany

While the calculations given above indicate the possible reduction CO2via PV, the calculations below give an evaluation of the total possible reductive effect of PV applied in Germany.

The reduction effect of emissions is even higher if not just the actual composition of the electrical grid is taken into account, which emits on average 0.53 kg/kWh, but also the priority of policies and energy planning; e. g., energy consumption in Germany is stagnating at a high level, and additional power generating capacity will be used for substitution of existing power plants...

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Specific Emission Balance

Table 9.2 shows the commutated energy expense (divided in electricity, fuels, and non-energy consumption) and the CO2-emission of a PV-power plant in Germany. The CO2-balance in Table 9.2 was based on the sectors of energy expenses; for electricity the actual power plant composition was applied.

T able 9.2. Energy expense and CO2-emission for the production of PV – power plants

Electricity

Fuels

NEC 4

Cell

type

Energy CO2 in D CO2 in BR (kWh/kWp) (kg/kWp) (kg/kWp)

Energy CO2 (kWh/kWp) (kg/kWp)

Energy CO2 (kWh/kWp) (kg/kWp)

sc-Si

5,144 2,726 360

1,152

346

228

52.4

mc-Si

2,530 1,341 177

1,630

489

450

1,035

1) Non-energetic-consumption: use of energy carriers as construction material. characteristics: electricity: 0...

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CO2 Reducing Effects by the Use of PV

To examine this effect, a process-chain-analysis for a complete cycle of production, use, and recycling is carried out. In carrying out that, the results from the examinations above will be taken into account:

• Energy expense for production

• Transport and installation

• Yield of the PV generators during their lifetime

• Recycling

Due to a lack of industrial experience in recycling of PV modules, a recycling rate of 25% was taken for granted. This value represents a lower limit, other examinations such as Bruton et al. 1994 claim that values of up to 70% are feasible. Figure 9.3 shows the principal difference in terms of CO2-emission resp. CO2-reduction between conventional and PV power plants...

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Process Chain Analysis

The process chain analysis examines the different stages of the manufacturing process of the energy system in form of a micro-analysis. The system is described as a process chain, if final energy or an energy service is achieved from an energy resource passing through different process steps (Spreng 1995). For each single process of this chain, not just inputs and outputs have to be defined, but also all materials and equipments which are causing an indirect energy expense. Also by-products and other outputs leading to additional energy expenses have to be determined and have to be considered.

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Fig. 9.2. Scheme of the energy-, CO2- and material flows in a single process chain.

At first, the consumption of energy for operation has to be determined...

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