Solar Power
The total solar power arriving on the Earth at this distance from the Sun is approximately 1400 Wm-2 (estimate total power Sun is emitting?). This covers the whole electromagnetic spectrum, including the visible region, ultraviolet, and infrared - though because of the high temperature of the Sun, the spectrum is biassed towards higher frequencies.
(i) Water panel on roof of house. Efficiency about 50 %. Usual design with glass covering, blackened copper pipes, insulation behind. (Estimate the length of time it might take to warm up a bathful of water 10oC, using panels on the roof?)
(ii) Solar cells. Uses silicon. Only use a part of the solar spectrum, E = hf (rather like the photoelectric effect), giving a theoretical maximum efficiency of only 21 %. Reflection of the light off the surface reduces efficiency to 18 % maximum.
Heat Engines
e.g. Steam engine (steam pushes piston), car engine, steam to turn turbine in power station.
Ordinary efficiency defined as usual, which is equivalent to (Q1 - Q2 )/Q1 x 100 %. This
is obvious, if you consider that, by Conservation of Energy, the difference Q1 - Q2, the difference between the heat into the engine, and the heat out of it, must equal the energy converted to other forms egg. external work. This looks fine; you can imagine all the heat in being converted to external work, with Q2 was zero, giving 100 % efficiency. Unfortunately this is not possible, because - for example - to get the steam to do work, you have to have a pressure difference between the steam on one side of a piston, and, say, condensed steam - virtually a vacuum - on the other side. Just having steam at high pressure pushing on one side of a turbine blade, is no use if the same steam, at the same high pressure, is pushing on the other side too. How will you get the steam to condense? By removing the heat that is in it, using cooling water, for example. Which is fine, except that this heat you are removing is some of the energy you hopefully put in at the beginning, intending it to be converted to useful work. OK, then, perhaps you could re-use the heat you have removed. But this heat is at lower temperature, and is much harder to get mechanical work from.
Maximum efficiency =( T1 - T2 )/ T1 x 100 %. (Proof follows from reasoning using the Carnot Cycle and Entropy, and is not in the syllabus)
Thermal Power Station
The middle stage of this is a heat-mechanical energy transducer i.e.. a heat engine. Coal, say, enters at one end, is pulverised, and then burned in a boiler. Water enters the boiler in coiled pipes, lower down, at about 20oC, and is boiled to steam at about 540oC, which leaves at the top. It hits one side of the blades of a turbine, rotating the connected dynamo to generate electricity; on the other side of the turbine, the steam is condensed back to water at 20oC, using water from outside, usually from a nearby river, which is heated up to 150oC, in the process. This hot water is partly released as steam in the cooling towers, and partly, perhaps, used to heat nearby houses or greenhouses.
The efficiency of this process, in total, from the chemical energy in the coal to the electrical energy from the dynamo is only about 34%.
(The alternating voltage from the dynamo is stepped up, using a transformer, to hundreds of kilovolts before transmission, since the same amount of energy can be transferred to the users by supplying large voltage and small current, or small voltage and large current (P supplied = VI). Since the power loss to heat in the supply cables I2R (where R is the resistance of the cable) increases with current, it is more efficient to decrease the supply current, and therefore increase the supply voltage)
Thermal Fission Reactor
This produces heat, so the only differences are in the way the heat is produced. When a Uranium 235? nucleus is hit by a slow neutron, the unstable combined nucleus splits, giving two smaller nuclei, three neutrons, and heat energy. If enough further Uranium nuclei are nearby (a critical mass) a chain-reaction ensures that the process continues.
23592U + 10n goes to 14857La + 8535Br + 3 10n + Q
A graphite (soft carbon) (or heavy water - deuterium oxide) moderator is needed, combined with the Uranium, to slow the neutrons down, because otherwise they tend to go straight through the nucleus without forming the unstable combination.
Control rods, made of Boron (coated onto steel) which absorbs neutrons, are held above the reactor core, not so far down as to stop the chain-reaction, not so far up that you get a nuclear explosion.
The heat of the fissions is carried away from the core by a suitable fluid, with, for example, a high specific heat capacity. This fluid, which will become radioactive, circulates in a sealed loop. Then there is a heat exchanger (intertwined pipes), which transfers the heat to water, converting it to steam for turning a turbine, as above.
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