Solar Energy

C.4.NoS Public understanding—harnessing the sun’s energy is a current area of research and challenges still remain. However, consumers and energy companies are being encouraged to make use of solar energy as an alternative energy source. (5.2)

C.4.U1 Light can be absorbed by chlorophyll and other pigments with a conjugated electronic structure.

C.4.U2 Photosynthesis converts light energy into chemical energy: 6CO2 + 6H2OC6H12O6 + 6O2

C.4.U3 Fermentation of glucose produces ethanol which can be used as a biofuel: C6H12O6  2C2H5OH + 2CO2

C.4.U4 Energy content of vegetable oils is similar to that of diesel fuel but they are not used in internal combustion engines as they are too viscous.

C.4.U5 Transesterification between an ester and an alcohol with a strong acid or base catalyst produces a different ester: RCOOR1 + R2OH -> RCOOR2 + R1OH

C.4.U6 In the transesterification process, involving a reaction with an alcohol in the presence of a strong acid or base, the triglyceride vegetable oils are converted to a mixture mainly comprising of alkyl esters and glycerol, but with some fatty acids.

C.4.U7 Transesterification with ethanol or methanol produces oils with lower viscosity that can be used in diesel engines.

C.4.AS1 Identification of features of the molecules that allow them to absorb visible light.

C.4.AS2 Explanation of the reduced viscosity of esters produced with methanol and ethanol.

C.4.AS3 Evaluation of the advantages and disadvantages of the use of biofuels.

C.4.AS4 Deduction of equations for transesterification reactions.

C.4.G1 Only a conjugated system with alternating double bonds needs to be covered.

C.4.ToK1 The claims of “cold fusion” were dismissed as the results are not reproducible. Is it always possible to obtain replicable results in the natural sciences? Are reproducible results possible in other areas of knowledge?

C.4.Aims1 Aim 2: The conversion of solar energy is important in a number of different technologies.

C.4.Aims2 Aim 6: Experiments could include those involving photosynthesis, fermentation and transesterification.

C.4.Aims3 Aim 8: Transesterification reactions, with waste cooking oil, could reduce waste and produce excellent biofuels.

C.8.NoS1 Transdisciplinary—a dye-sensitized solar cell, whose operation mimics photosynthesis and makes use of TiO2 nanoparticles, illustrates the transdisciplinary nature of science and the link between chemistry and biology. (4.1)

C.8.NoS2 Funding—the level of funding and the source of the funding is crucial in decisions regarding the type of research to be conducted. The first voltaic cells were produced by NASA for space probes and were only later used on Earth. (4.7)

C.8.U1 Molecules with longer conjugated systems absorb light of longer wavelength.

C.8.U2 The electrical conductivity of a semiconductor increases with an increase in temperature whereas the conductivity of metals decreases.

C.8.U3 The conductivity of silicon can be increased by doping to produce n-type and p- type semiconductors.

C.8.U4 Solar energy can be converted to electricity in a photovoltaic cell.

C.8.U5 DSSCs imitate the way in which plants harness solar energy. Electrons are "injected" from an excited molecule directly into the TiO2 semiconductor.

C.8.U6 The use of nanoparticles coated with light-absorbing dye increases the effective surface area and allows more light over a wider range of the visible spectrum to be absorbed.

C.8.AS1 Relation between the degree of conjugation in the molecular structure and the wavelength of the light absorbed.

C.8.AS2 Explanation of the operation of the photovoltaic and dye-sensitized solar cell.

C.8.AS3 Explanation of how nanoparticles increase the efficiency of DSSCs.

C.8.AS4 Discussion of the advantages of the DSSC compared to the silicon-based photovoltaic cell.

C.8.G1 The relative conductivity of metals and semiconductors should be related to ionization energies.

C.8.G2 Only a simple treatment of the operation of the cells is needed. In p-type semiconductors, electron holes in the crystal are created by introducing a small percentage of a group 3 element. In n-type semiconductors inclusion of a group 5 element provides extra electrons.

C.8.G3 In a photovoltaic cell the light is absorbed and the charges separated in the silicon semiconductor. The processes of absorption and charge separation are separated in a dye-sensitized solar cell.

C.8.G4 Specific redox and electrode reactions in the newer Grätzel DSSC should be covered. An example is the reduction of I2/I3─ ions to I─.

C.8.IM1 The harnessing of solar energy could change the economic fortunes of countries with good supplies of sunlight and unused land.

C.8.ToK1 A conjugated system has some similarities with a violin string. How useful is this metaphor? What are the underlying reasons for these similarities? What role do models and metaphors play in the acquisition of knowledge?

C.8.Aims1 Aim 6: Students could build an inexpensive dye-sensitized solar cell and investigate their photovoltaic properties.

C.8.Aims2 Aim 7: The properties of DSSCs can be best investigated using data loggers