Solar energy
C.4 Solar energy (2 hours)
Pause for thought
(Image from Brightstar Eg)
This sub-topic seems to have the wrong title! If you look at the ‘Understandings’ and Applications and skills’ you will see that they all refer to energy from biomass. It is of course true that the ultimate source of biomass energy is the sun, which provides the energy for photosynthesis to occur, but ‘Solar energy’ usually describes converting the energy directly from the sun into electricity or heat. This may be through the use of photovoltaic cells or dye-sensitized solar cells (covered in the AHL sub-topic D.8) or thermal power plants, which concentrate the sun’s heat and use it to boil water to drive a steam turbine. There is also the passive use of the sun’s energy, e.g. positioning windows on the sunny side of a building and using the sunlight to heat absorbent materials on the walls and floor. This is known as space heating.
It is worth students knowing that the energy that falls from the sun onto the Earth in just one hour is enough to satisfy all the world’s energy needs for one year. However, currently solar technology converts less than 0.1% of the sun’s energy into a useable form. Plants also store less than 0.1% of this energy through photosynthesis. The more we are able to harness the energy from the sun the less reliant we will be on fossil fuels and other polluting forms of energy such as nuclear energy.
Nature of science
Although harnessing the sun’s energy is a current area of research and challenges still remain, consumers and energy companies are being encouraged to make use of solar energy as an alternative energy source
Learning outcomesAfter studying this topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesThe only feature of a molecule that can absorb visible light that needs to be covered is a conjugated system with alternating double bonds. International-mindednessNothing is listed on the syllabus under this heading. |
Teaching tipsThis sub-topic just concentrates on solar energy obtained through photosynthesis and is relatively straightforward. I start by making sure students understand the basics of photosynthesis, including the overall equation, and then explain that chlorophyll contains extended conjugation of alternate double and single bonds. The energy required to excite electrons in these conjugated bonds lies in the red and blue regions of the visible spectrum so chlorophyll absorbs red and blue light and transmits green light, which explains why leaves are green. The absorbed light is converted into chemical energy. The chemical energy is stored in plants and the biomass can be used directly as a fuel, e.g. wood, or it can be converted into biogas, which is mainly methane and carbon dioxide. Alternatively, the carbohydrates can be fermented by enzymes in yeast to produce ethanol, which can be used directly as a fuel or combined with gasoline (petrol) to produce gasohol. Discuss the advantages and disadvantages of using biomass as a fuel to supplement sub-topic C.1: Energy sources. Vegetable oils, which are triglycerides of propane-1,2,3-triol (glycerol) and fatty acids contain much stored chemical energy but they have high molar masses and hence low volatility so cannot be used directly as diesel fuel. Explain the basics of transesterification including the need to use excess alcohol to move the equilibrium to the product side and the need for a catalyst. Then show how it is applied to convert vegetable oils into esters with much lower molar masses, which can be used as diesel fuel as they are much more volatile. Give your students considerable practice at writing esterification and transesterification equations using triglycerides. | Study Guide
Page 146 QuestionsFor ten 'quiz' questions (for quick testing of knowledge and understanding with the answers explained) see MC test: Solar energy. For short-answer questions see Solar energy questions together with the worked answers on a separate page Solar energy answers. Vocabulary listphotosynthesis |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
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Other resources
1. Most good videos on photosynthesis tend to focus on the biology rather than why chlorophyll absorbs visible light but for background information on the whole process this video by Paul Anderson from Bozeman Science is worth watching.
2. This video animation by the Biofuel Research Channel shows how biodiesel is actually produced commercially. It could usefully be supplemented by getting students to write the equations for the different steps as they take place.