Category Nanophysics of Solar and Renewable Energy

The single junction solar cell was described in Chapter 3, before Figure 3.17. The limiting efficiency of a single junction cell is 31% at one sun illumination, but as high as 40.8% with concentrated sunlight. The Si single-crystal solar cell shown in Figure 3.17 is the most common first-generation solar cell. Elaborations on this cell reaching 24% efficiency will be described in Chapter 6, but the bulk of the Si cells are characterized, as in Figure 5.5, in a range 15-20%. The crystalline Si solar cell is built on a wafer of Si, which is typically 200-300 p thick, cut from a large crystal using a wire saw that wastes also one wire-width of the Si single-crystal boule (http://www. omron-semi-pv. eu/en/wafer-based-pv/wafer-preparation/slicing-the-ingot. html.)...

9.1

Intermittency of Renewable Energy

Hydrogen is available, but tightly bound, in water and many other compounds. To produce hydrogen from water requires an energy input. Hydrogen is not a source of energy, but rather a means of storing energy. A hydrogen infrastructure for storing and transporting energy is a possible alternative to batteries to store energy from solar cells. In a large-scale scenario, liquid hydrogen might be transported by railroad tank car, analogous to liquid natural gas (LNG), rather than by electric transmission over power lines, which are costly and require connected plots of land. One, but not the only, clear route to produce hydrogen from solar energy is via photovoltaic (PV) cells followed by electrolysis of water...

Another potentially inexpensive form of solar cell is the dye-sensitized solar cell. This approach is based on an absorber that is partly titanium oxide, TiO2 in its anatase tetragonal form, and partly dye molecules. Anatase can be prepared in a nanoporous form (sometimes, described as mesoporous, although microscopy shows particles of 10-80 nm diameter) by various procedures, one described as hydrothermal proces­sing of a TiO2 colloid.

This oxide has a bandgap of 3.2 eV, which means that the maximum light wavelength absorbed is (1240eVnm)/(3.2eV) = 388 nm. Thus, only a small part at the UV end of the solar spectrum is absorbed. Spectacular extension of the light absorption range to at least 700 nm has been demonstrated by coating (sensitizing) the nanoporous anatase with dyes...

One scenario for future distribution of energy is in the form of hydrogen, which might be compressed or liquefied. The “hydrogen economy” scenario has been promoted along with the idea that automobiles may switch to electric motors for which the electricity comes from fuel cells run on hydrogen gas stored in the car’s fuel tank. A scenario of this sort is being tested in Iceland, which has renewable energy resources and no oil, coal, or gas.

Hydrogen as a fuel is attractive because it burns to produce water, with no adverse effect on the atmosphere. Hydrogen is not abundant in its free form; for example, the availability of hydrogen as a byproduct of commercial liquefaction of air to produce nitrogen and oxygen is limited...

Here (Fig. 7.14) solar cells are mounted vertically on the edges of the light collecting plates, which have dissolved dyes. The upper plate collects blue photons, the dye reemits (fluoresces) and that light is sent to the cells on the edge. The fluorescence quantum efficiency of isolated dye molecules approaches unity. (Total internal reflection helps keep the reemitted light inside the plate, until absorbed in a solar cell).

The redder light passes through and is caught in the second plate that has a red dye. The effective collecting area is multiplied by this scheme, and the scheme also effectively involves two separate bandgaps, leading to higher efficiency in principle.

In the collection of the reemitted light from the dye molecules, two methods, fluorescence (prompt response) and phos...

One of the questions on many aspects of solar and renewable energy is the optimum scale of the device or system. We have seen in Figures 5.4 and 5.5 concentrating solar thermal installations on the scale of 11 MW and 25 kW, respectively, with a good indication that the present efficiency of the smaller installation is nearly twice that of the large installation. (It is not clear what the cost in $/W comparison is, but it probably favors the large system.) It has been argued above that much higher efficiency than the present 15% should be available in the solar tower systems with advances in engineering and in materials. We will see later that arrays of small concentrating solar cells built into panels are commercially available using small plastic Fresnel lenses (as suggested in Figure 5...

Inert electrodes such as stainless steel or platinum, immersed in water (containing a small addition of ions to promote conductivity), will evolve hydrogen gas at the cathode and oxygen gas at the anode if ~1.9 volts is applied. The chemical potential energy associated with a molecule ofhydrogen is 1.23 eV, so the efficiency ofthe electrolyzer is stated as 65%. According to Turner [109], the efficiency of commercial electrolyzers is in the range 60-73%. (Actually, about 4% of commercial hydrogen production is by electrolysis, and about half that is by electrolysis of brine (NaCl plus water) with chlorine gas, the primary desired product, hydrogen sometimes being abandoned.)

If the electrolyzer is connected with a photovoltaic array (PV) of efficiency 12%, connecting enough PV cells in seri...

The operation of this device follows the steps indicated in Figure 6.17: step 1 is absorption of a photon by a dye molecule. Typically, the dye is a metal-organic ruthenium complex with response in the wavelength range 700-900 nm, light that is not absorbed by the titania. This is the idea of dye sensitization, to extend the absorption range of the photocell to utilize more of the solar spectrum. In step 2, the excited state electron jumps from the dye molecule into the conduction band of the TiO2. The transfer is rapid from the dye only if the dye level lies ~0.2 eV above the conduction band, and reverse transfers from conduction band to dye are found to be slow...