A、The engineering problems with solar power. B、The generation of steam with the latest technology. C、The importance of exploring

[19] We’ ve talked recently about the importance of sustainable energy. We’ ve also talked about the different theories on how that can be done. So far, our discussions have all been theoretical. Now I have a practical question for you all. Can you run a 140,000 kilogram train on just the steam generated by solar power? Well, one engineer, Tim Casselman, believes it’ s possible. And his home city of Sacramento, California should see the technology’ s first test.
    As part of the upgrading of its rail yard, [20 - 1] Casselman, who is an inventor and self-proclaimed steam visionary, is campaigning for a new steam train that runs without any fire and could run on an existing ten-kilometre line, drawing tourists and perhaps offering city commuters a green alternative to their cars. [20 - 2] Casselman wants to build an array of solar magnifying mirrors at one end of the line to collect and focus heat onto water-filled tubes. This would generate steam that could be used to fill tanks on a small steam train without the use of fire. Supplying power to trains in this way would offer the shortest distance from well to wheels, he says, with the least amount of energy lost.
    According to Harry Valentijn, a Canadian engineer who is researching modern steam technology, a special tank measuring 2 by 10 metres could store over 750 kilowatt hours of energy as high pressure steam, enough to pull a two car train for an hour or so. Energy to drive a steam locomotive can be stored in other materials besides water. For example, a team at Tohoku University in Japan has studied materials that can store large amounts of heat. When heated, these materials turn from a solid into a liquid absorbing energy as they change phase. The liquid is maintained above its melting point until steam is required, at which point the liquid is allowed to turn back into a solid, releasing its stored energy.
     [21] Another team at Nagoya University in Japan, has tested calcium compound as an energy storage material. Heating this chemical compound drives off carbon dioxide gas leaving calcium oxide. The gas can be stored under pressure and attained. To recover the energy, the gas is fed back over the calcium oxide. In theory, says Valentijn, this can create a high enough temperature to generate super-heated steam.
Questions 19 to 21 are based on the recording you have just heard.
19. What has the speaker previously talked about?
20. What is Tim Casselman trying to do in Sacramento?
21. What has a Japanese research team tried to do?