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Chapter 3

treatment for low-contaminant waste (Gumy et al., 2006). Multiple- effect humidification (MEH) desalination units indirectly use heat from highly efficient solar thermal collectors to induce evaporation and condensation inside a thermally isolated, steam-tight container. These MEH systems are now beginning to appear in the market. Also see the report on water desalination by CSP (DLR, 2007) and the dis- cussion of SolarPACES Task VI (SolarPACES, 2009b).

In solar drying, solar energy is used either as the sole source of the required heat or as a supplemental source, and the air flow can be generated by either forced or free (natural) convection (Fudholi et al., 2010). Solar cooking is one of the most widely used solar applications in developing countries (Lahkar and Samdarshi, 2010) though might still be considered an early stage commercial product due to limited overall deployment in comparison to other cooking methods. A solar cooker uses sunlight as its energy source, so no fuel is needed and operating costs are zero.Also, a reliable solar cooker can be constructed easily and quickly from common materials.

3.3.3

Photovoltaic electricity generation

Photovoltaic (PV) solar technologies generate electricity by exploiting the photovoltaic effect. Light shining on a semiconductor such as sili- con (Si) generates electron-hole pairs that are separated spatially by an internal electric field created by introducing special impurities into the semiconductor on either side of an interface known as a p-n junction. This creates negative charges on one side of the interface and positive charges are on the other side (Figure 3.5). This resulting charge separa- tion creates a voltage. When the two sides of the illuminated cell are connected to a load, current flows from one side of the device via the load to the other side of the cell. The conversion efficiency of a solar cell is defined as a ratio of output power from the solar cell with unit area (W/cm2) to the incident solar irradiance. The maximum potential effi- ciency of a solar cell depends on the absorber material properties and device design. One technique for increasing solar cell efficiency is with a multijunction approach that stacks specially selected absorber materials that can collect more of the solar spectrum since each different material can collect solar photons of different wavelengths.

PV cells consist of organic or inorganic matter. Inorganic cells are based on silicon or non-silicon materials; they are classified as wafer-based cells or thin-film cells. Wafer-based silicon is divided into two different types: monocrystalline and multicrystalline (sometimes called ‘polycrystalline’).

3.3.3.1

Existing photovoltaic technologies

Existing PV technologies include wafer-based crystalline silicon (c-Si) cells, as well as thin-film cells based on copper indium/gallium disul- fide/diselenide (CuInGaSe2; CIGS), cadmium telluride (CdTe), and thin-film silicon (amorphous and microcrystalline silicon). Mono- and

Direct Solar Energy

Anti-Reflection Coating

n-Type Semiconductor

Front Contact

-

Electron (-)

Hole (+)

Recombination

p-Type Semiconductor Back Contact

+

Figure 3.5 | Generic schematic cross-section illustrating the operation of an illuminated solar cell.

multicrystalline silicon wafer PV (including ribbon technologies) are the dominant technologies on the PV market, with a 2009 market share of about 80%; thin-film PV (primarily CdTe and thin-film Si) has the remaining 20% share. Organic PV (OPV) consists of organic absorber materials and is an emerging class of solar cells.

Wafer-based silicon technology includes solar cells made of monocrys- talline or multicrystalline wafers with a current thickness of around 200

  • m, while the thickness is decreasing down to 150 m. Single-junction

wafer-based c-Si cells have been independently verified to have record energy conversion efficiencies of 25.0% for monocrystalline silicon cells and 20.3% for multicrystalline cells (Green et al., 2010b) under standard test conditions (i.e., irradiance of 1,000 W/m2, air-mass 1.5, 25°C). The theoretical Shockley-Queisser limit of a single-junction cell with an energy bandgap of crystalline silicon is 31% energy conversion efficiency (Shockley and Queisser, 1961).

Several variations of wafer-based c-Si PV for higher efficiency have been developed, for example, heterojunction solar cells and interdigi- tated back-contact (IBC) solar cells. Heterojunction solar cells consist of a crystalline silicon wafer base sandwiched by very thin (~5 nm) amorphous silicon layers for passivation and emitter. The highest-effi- ciency heterojunction solar cell is 23.0% for a 100.4-cm2 cell (Taguchi et al., 2009). Another advantage is a lower temperature coefficient. The efficiency of conventional c-Si solar cells declines with elevating ambi- ent temperature at a rate of -0.45%/°C, while the heterojunction cells show a lower rate of -0.25%/°C (Taguchi et al., 2009). An IBC solar cell, where both the base and emitter are contacted at the back of the cell, has the advantage of no shading of the front of the cell by a top electrode. The highest efficiency of such a back-contact silicon wafer

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