# Tag Info

136

Why is mains frequency 50Hz and not 500 or 5? Engine efficiency, rotational stress, flicker, the skin effect, and the limitations of 19th century material engineering. 50Hz corresponds to 3000 RPM. That range is a convenient, efficient speed for the steam turbine engines which power most generators and thus avoids a lot of extra gearing. 3000 RPM is also ...

133

You can use dead weights, but you need a huge amount of weight. For example the biggest pumped hydroelectric system in the world (the Gianelli Hydroelectric Plant in California, USA) uses water stored in a reservoir about 9 miles long by 5 miles wide, lifted through a height of about 300 feet. Even then, it can only supply about 5% of California's ...

91

Let's spin some numbers to further illustrate the poor energy density of gravity-based storage systems. Assume that you have a 100 kilogram lead weight that you can lower into a 10 meter deep hole in your yard. Now, how much energy can it store? This is given by potential energy formula $E = mgh$, thus $E = 100\,\mathrm{kg} \cdot 9.8\,\mathrm{m}/\mathrm{s}^... 63 The maximum continuous power that can be generated for an hour by a fairly fit person on an efficient machine like an exercise bike or rowing machine is$\sim 200$W (olympic-standard track cyclists might manage 400 W). Let's say that a gym is occupied at any time by 10 people who are doing this kind of intense exercise. Then you might just be producing ... 59 We do use mass in a gravitational field to store energy and have done for hundreds of years! Grandfather Clocks Grandfather Clocks have used powering weights since the 1660s. This was when they first gained their tall thin shape. The weights seen in the picture slowly descend as their stored energy is released. In order to add energy back into the system ... 48 The efficiency of a thermoelectric generator is around 5 - 8%. The efficiency of a large steam turbine power plant aproaches 40%. In fact the thermodynamic efficiency of a large steam turbine power plant is over 90%, so it's about as efficient as anything could be. The maximum possible efficiency of a steam driven engine is given by the idealised model ... 45 Another way to store energy in mass is the use of flywheels. You simply take a massive wheel and spin it up to store energy, use an electrical generator as a break to take energy out. Their main limitation for use outside of the realm of professional settings where they can be properly monitored and maintained by experts is the potential to fail ... 40 Power to a water-wheel depends both on the current (amount of water delivered) and the head (vertical drop of water as it turns the wheel). So, the water analogy does have TWO variables that multiply together to make power: current, measuring (for instance) the water flow at Niagara, and vertical drop (like the height of Niagara Falls). Current is NOT the ... 32 Yes, there is a fundamental limit. It comes down to two factors: How many watts of light energy can the source produce for each watt of electrical energy? How many lumens does each watt of light energy correspond to? The first question is straightforward - by conservation of energy, 1W of electrical energy can yield at most 1W of light energy. The second ... 28 I was going to comment on other people's answers, but this was going to become too long. Almost everyone fails to separate Thorium (which is a fuel type) and reactor type. Safety is a function of the reactor type, and molten salt in particular for this question. Does the fuel choice impact ultimate reactor safety? Yes, but to a limited extent. So how ... 27 I'm not sure what all you've read on them, but I'll try to clarify at least a few things. I would certainly disagree with several of your assertions. For starters, you say "...they don't produce anything you could feasibly use as a source of material for nuclear weapons." Thorium reactors use Thorium as a fertile fuel that transmutes into fissile U233. ... 26 In the end, the choice of a single specific number comes from the necessity to standardize. However, we can make some physical observations to understand why that final choice had to fall in a certain range. Frequency Why a standard? First of all, why do we even need a standard? Can't individual appliances convert the incoming electricity to whatever ... 23 Suppose you are using a waterwheel to do some form of work (e.g. grind corn). You need a head of water to make the wheel move, and you could use either 1kg of water at a height of a million metres or you could use a million kg of water at a height of one metre. In both cases the water would do the same amount of work as it flowed through your wheel. The ... 23 Let's work this out from the Stephan Boltzmann law. What color is a fire? If you look at color charts for black body radiators at various temperatures, I estimate it to be about 1000K. (Be careful: some flames are colored by strong emission spectra, making their light very different from a blackbody radiator's color). Glancing around the web from various ... 23 Here is a simple way to keep this stuff straight. Power is always the product of an effort variable and a flow variable. In hydraulic systems, the effort variable is pressure and the flow variable is the flow rate. For flow in open channels, the effort variable is typically very small (but not zero) and the flow variable is very large. BTW power exchange ... 21 We do. Just the weights we use are made not from lead, but from water. Many water reservoirs are also used to store energy by pumping water up when you have energy surplus, and letting it come down through the generators when you need energy. All you need for this purpose is two or more reservoirs at different altitudes. 21 Now lets imagine two hoses... one has twice the resistance (meaning it has smaller physical width than the other). But we also make sure that both hoses have the same current (meaning that the smaller hose has twice the voltage (water pressure)). If we were to hit some toy windmills with the water coming out of each of these hoses (from the same distance of ... 19 my question is about whether it's possible in principle The answer is yes. and whether anyone tried it. The answer is by all chances, no. So, how come? The effect The thermoelectric effect for electricity generation (called the Seebeck effect) is the phenomenon that a voltage is generated at a temperature different across the ends of a conductor:$$... 16 The conductor material (copper, aluminum, whatever) expands when heated. When the temperature increases, the length of the power line between two towers increases due to thermal expansion, and the line sags because of the increased slack. 16 There is no contradiction here. In fact, the power equation is often represented in different ways: \begin{eqnarray}P & = & IV\\ P & = & I^2R \\ P & = & V^2/R \end{eqnarray} The equations can be manipulated depending on which variables you are controlling. Most commonly, your circuit has a particular resistance. Your power source ... 15 What is the size/scale of a wood fire that is producing 1kW? Based on what my understandings re energy content of fuels and combustion processes, about 0.5 to 1 cubic inch per minute of typically dry wood in an open fire. Based on an utterly superb 80 page Wood Fuels handbook which I discovered along the way - about the same, rather to my surprise. See at ... 15 You have the general idea right, but the following statement is subtly wrong The pressurized fuel/air mixture is ignited and this increases the pressure inside the combustion chamber even more Unlike in a piston engine, the ignition of the fuel air mixture in a turbine engine increases the mixture's volume while pressure stays relatively constant. ... 15 Power sources can work in two modes : control voltage (CV) or control current (CC). In CV mode, the voltage is imposed, and the output current is adjusted depending on the load. This is for instance the case at home, where the electrical plugs deliver 100V (or 220V depending of country), regardless of what is plugged in it. In this case,$P=V^2/R$is the ... 15 I understand that a watt is a unit of power (change in energy per unit time) that describes the rate at which physical work can be done Right. The key thing to observe is that energy can move in both directions. It can move from the "supply" to the "load", but it can also move from the "load" to the "supply". The power at any given instant is found by ... 14 When you are going from an equation to a proportionality statement you need to be mindful of what is being kept constant.$V=IR$means that$I$varies directly with$V$if$R$is constant.$P=IV$means that$I$varies inversely with$V$if$P$is constant. The only time you could get a contradiction is if you are comparing situations where the power is ... 14 Yes, these exist. There's one here in Portland, OR, USA called The Green Microgym. But it doesn't generate much energy. People wildly overestimate how much energy a human can produce. They claim to "have generated 20% of our own electricity" but it is done by "combining human and solar power". They claim to use "Energy-producing cardio equipment (... 13 If you express power loss in a power line as$V^2/R$, the$V$in that expression is the voltage difference between the two ends of the power line, not the voltage difference between the power line and ground. To supply a fixed amount of power$P_L$to a load, if the voltage at the load$V_L$is larger, the current$I=P_L/V_L$can be smaller. If the power ... 13 The two other answers address the frequency issue. The voltage issue is much simpler. If the voltage is too high, you run the risk of arcs between conductors. The minimum distance between conductors before an arc appears is proportional to voltage. At 240V, you arc at a distance of a few millimeters in air, depending on humidity. More voltage gets clearly ... 12 Within power systems such as regional or national electricity grids,$\frac{\mathrm{d}^2E}{\mathrm{d}t^2}\$ is called the slew rate: it's used to denote the rate of change of power demanded from, or supplied to, electricity grids. It's typically either expressed as MW/s or GW/h, being two time periods of interest in balancing electricity grids. ...

Only top voted, non community-wiki answers of a minimum length are eligible