Energy Transformations: Exploring Alternative Energies

1. Grzegorz Karwasz Anna kamińska University Nicolaus Copernicus Didactical material: Secondary School Part II: Alternative energies

2. „Alternative energies” is not the best title First, we do not say energy „production”, but energy transformations. Physicists believe that the whole amount of energy in the universe is preserved. The total mass, according E=mc2 also enters into this budget. The conversion of a part of mass into energy is the key of energy „production” in the core of Sun, in nuclear plants and in the projected thermo-nuclear plant ITER in Cadarache, France. „Alternative” sources of energy is also not the best world. Sometimes we say „renewable” sources, when we list the wind, the hydropower, photo-voltaic and, sometimes, also burning bio-mass. Every „source” brings own problems. Nuclear power plant, somewhere in Germany, I suppose

3. Mankind, in its history learned to swip from one „source” of energy to another This is an old picture, from 90’ies last century predicting changes in „sources” of energy. Now, we are at the border line of those predictions, so we can compare with reality. The value of this picture is that it shows how specific carriers of energy arrive to their maximum of use (coal at the beginning of XXth century) than they descend. The picture shows also the predictions do not include new inventions: like shale gas The question mark says: solar (now ready), thermonuclear (not ready yet) etc.

4. The most optimistic scenarios assume that the emission of CO2 will stop It is clear that in order to arrest/ slow down the rise of the global warming, emission of CO2 must be stopped. Scientist warn, that the time of permanence of the added CO2 is 150 yrs. Keeping in mind that in the (remote) Earth history the temperatures were by few (1-3ºC) higher, the IPCC panel in its recent report (Dec. 2018) put the goal of not-more than 1.5º rise by 2040. If no measures to stop CO2 emission are undertaken, the temperature will rise by 2ºC. Not more: there is not much coal left to be burnt. IPCC panel, December 2018

5. Alternative, i.e. not emitting CO2, energy sources are needed: eolic, geothermal etc. Geothermal hot water sources were used alreay in antiquity. Some countries, like Iceland, base their economy on geothermal energy. However, as compared with solar-radiation flux (1367 W/m2), the geothermal flux is low, 65 mW/m2. Half of this energy flux comes from still cooling Earth interior, half comes from radioactive decays (238U. 40K, 232Th). The total power capacity is low: 10 GW of world power equals to 20 nuclear power blocks. Geothermal plant, Pozzuoli, Italy Wikipedia

6. Alternative, i.e. not emitting CO2, energy sources are needed: eolic, geothermal etc. More than 40% of electrical power is produced by wind in Denmark (2018). Denmark is particularly windy country. Many other countries invest in wind-based electricity. Even such „solar” countries like Italy, produce some 3% of electricity from wind. The growth, on global scale, is exponential. Ø 10m → 20-30 kW Ø 90m → 3 MW (Vestas) = 1000 housholds IPCC panel, December 2018

7. Photovoltaic sources are the most promising • The efficiency of photovoltaic cells is limited by physical principles: • Photon energy must be higher than the energy gap of the semiconductor. For silicon this gap is 1.1 eV* what means that even IR radiation brings enough energy (red light photons carry 1.8 eV energy) • However, the voltage from a Si photovoltaic cell is not 1.1 V, but lower by 0.6-0.7 V. This is due so-called polarization of the n-p junction. • If photons bring more energy than the gap, the excess of energy transforms into heat. If less than the gap – all energy goes into heat. Therefore, the intrinsic efficiency of a single-stage Si cell is some 10-15% • Bigger efficiencies can be obtained from more expensive semi-conductors, like GaAs, which to some extent is similar to Si: in space flights the cost of PV cell is not important. 16% nei moduli in eterogiunzione; 14% nei moduli in silicio monocristallino; 13% nei moduli in silicio policristallino; 10% nei moduli in silicio microsferico; 6% nei moduli in silicio amorfo.

8. Could we construct more efficient solar cells? The PV cell consists of two types of semiconductors: Si doped with P (n-type, using electrons as charge carriers) and Si doped with B (p-type, with electrons „missing”, i.e. using positive „holes” as charge carriers). n-p junction under illumination: photons (if bringing energy higher than the gap Eg) create pairs of electrons and holes. Electrons „slide-down” along the (inclined) conduction band. Ec is the (lower) edge of conduction band, Ev is the (upper) edge of valence band. EF, is so-called Fermi level that gets adjusted when two semiconductors are connected together. VB is the useful voltage: 0.4 V for Si Practical implementations of PV cell require multiple layers (Source: WikipediaO https://www.sciencedirect.com/topics/engineering/pn-junction

9. Cheap, and flexible PV cells use the white wall paint An essential question in PV cells is „harvesting” of the solar spectrum. The energy of photons should correspond exactly to the band gap: photons with less energy are useless, with higher energies bring a harmful excess of energy. Michael Grätzel overcame this difficulty constructing a cell based on a paint white wall (nanostructured TiO2, a semiconductor with a rather high band gap). Harvesting of the solar spectrum is done by an artificial, brown pigment. Grätzel’s cell can be produced as flexible foils, and are cheap. https://en.wikipedia.org/wiki/Dye-sensitized_solar_cell

10. Photovoltaic sources are the most promising Higher efficiencies can be obtained by a trick consisting in successive „harvesting” of separate parts of solar spectrum (see lesson 2): first the highest-energy photons (blue light); lower energy photons are not absorbed, etc. In three-stage PV cells efficiencies up to 40% has been achieved. The question is, again, about the costs.

11. Is burning biomass „ecological”? Wood was a traditional source of heating before the industrial era. Today, wood „pellets” become quite popular. Are they ecological? Read what wikipedia says: „Burning biomass releases carbon emissions, around a quarter higher than burning coal (per energy unit), but has been classed as a "renewable" energy source in the EU and UN legal frameworks, because plants can be regrown. Using biomass as a fuel produces air pollution in the form of carbon monoxide, carbon dioxide, N2O, NO, NO2 (nitrogen oxides), VOCs (volatile organic compounds), particulates and other pollutants at levels above those from traditional fuel sources such as coal or natural gas.” Source: https://en.wikipedia.org/wiki/Biomass A cogeneration plant in Metz, France. The station uses waste wood biomassas an energy source, and provides electricity and heat for 30,000 dwellings.

12. The choice of energy sources depends on their costs As it is clear from this extensive study by US Energy Dpt. the range of costs is wide: the most expensive is off-shore wind. Coal is cheap, unless capturing of CO2 is applied. Petrol is still the cheapest. And the cost of photovoltaics strongly depends on oil: production of silicon consumes huge amounts of energy. Energy policy requires experts. US Department of Energy

13. What about nuclear plants? The cost of electricity in France is below the EU-28 average. This is partially thanks to a big share of the nuclear (i.e. uranium-based) plants. Nuclear plant (1 GW) consumes about 1 kg of uranium/ day, compared to 10,000 tons of coal in traditional plant. Another problem are deposits of used fuel (i.e. highly radioactive „ashes”). But under Eastern Europe the magmatic, stable rocks are as much as 70 km thick. What is expensive is the nuclear plant: 500 MW block costs some 4-5 bln $. Korea builds 4 blocks in Arab Emirates.

14. Thorium is one of alternatives for uranium Usually tacit fact about uranium-based plants is that reach ores get exhausted. Some fuel has been produced cheaply as „diluted” uranium from nuclear bombs. But with low contents of uranium, the ores risk to become a „yellowcoal”. There is another chemical element, neighbouring with U in Mendelev periodic system: thorium. It is almost non radioactive, and abundant, say, in Baltic sands. Thorium-based reactor has been proposed by Italian Nobel-prize winner (for his discoveries on elementary particles) Carlo Rubbia. The advantage of such a reactor would be that it is intrinsically stable: thorium needs an external source of neutrons. The point is that no country wants to build it. GK is expert of EU, evaluating research projects

15. When will be the thermonuclear plant ready? • - Can we control the thermonuclear reaction (i.e. the synthesis of He nuclei from H, like in Sun)? • Scientists learned to control it some 50 years ago, using great magnets, in machines called tokomak. • But the reaction does not produce energy? • Already from 20 years we get net energy from the reaction: 3H + 2H → 4He + n, for example in JET in England. • So why we do not use it? • Because we need to add the costs of keeping currents in magnets, cooling etc. • The first machine, producing net 500 MW energy should be ready in Cadarache, France at about 2025. The first industrial plant – at about 2050 (in Korea?) GK is expert of IAEA, working on atomic processes in ITER

16. Future is various, when it comes It is very risky make predictions on the future technologies. We usually extrapolate what we already know. In 2100 the human demand for energy will not be lower than now. Known „sources” like water, wind, PV will be used. Solar radiation can be concentrated by huge mirror, heating water for a turbine (solar thermal electricity). Other combinations are also possible. Storage of enegy will be a problem: hydrogen is one of solutions. With no doubt, new specialists will be needed. Courtesy: Dr Johannes Töpler, Deutscher Wasserstoff-und Brennstoffzellenverband

17. The real problem will be not production, but storage Almost all energy „soruces” in the previous picture are based on wind and solar, i.e. based on the weather:producing energy when they can not when it is needed. As seen from the picture here, already in next year in Germany a problem will arise wih the lack of power when there is no wind neither Sun. Hydrogen production from PV and its storage in huge tanks is one of possible solutions to fill-down energy gaps. Courtesy: Dr Johannes Töpler, Deutscher Wasserstoff-und Brennstoffzellenverband

18. Experiments with photovoltaics • Infinity of table toys run from photovoltaic sources (toy cars, animals, calculators). Please, identify as much as possible and bring them to school. • The teacher prepares different light sources: LED, luminescent, old with W wires inside. Please, prepare several, even of the same type. Sign the nominal power consumed by all of them. • Students are divided into groups 3-4 persons and move, with their „toys” from one light source to another: check, what is the maximum distance from the light source that you toys still works. Make notes. • Leave 15 minutes to discuss results. Conclusions will not be easy: each photovoltaic cell can have different efficiency and spectral response, and each light source either.

19. Debate for students The teacher divides the class into groups, that for the next lesson will prepare arguments in favour of selected „sources” of energy: photovoltaic, eolic, hydric, nuclear (?), thermonuclear, alternative nuclear (thorium), biomass • Every group prepares analysis of benefits and costs (investment, current running, dismantling, social costs, health risks) • Please, defend your „source” of energy, as much as possible: do no present disadvantages to other groups, until they ask: this is a political debate

20. We finish again with cover page from „Focus”: Hydrogen era arrives In which way the new fuel will make the world more just, more pacific, cleaner. Hydrogen does not add carbon to atmosphere