31 votes

Why don't hydroelectric plants use cascades of turbines instead of single turbines?

The exhaust gasses are compressible fluids, whereas liquid water is not. Here's an animation of how a gas turbine works: https://www.youtube.com/watch?v=gqNtoy2x5bU At the combustion stage, the gas ...
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  • 5,930
30 votes
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Why is hydro-electric power still scarce in some places?

Because most of the water is at the lowest point is can be, the ocean. Second issue is that you need quite a bit of vertical drop for water power to be a viable power source. There are DIY hydro ...
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16 votes
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Why don't we just build a giant wind turbine?

Not everything scales linearly. In particular, the cross-sectional area of supports required scales faster than height of a structure, all else held constant. This explains why ants have tiny thin ...
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  • 11.2k
15 votes

What are the disadvantages of concentrated solar thermal power?

There are no silver bullets when it comes to decarbonising the grid. That's good news: it means you can stop looking for one, and accept that every form of generation has its pros and cons. ...
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  • 3,701
11 votes

Why is hydro-electric power still scarce in some places?

There are two issues, harnessing and market. You need both to be viable. A market, makes most of the 70% of the land mass covered by water unusable, because even if you could harness the power, it ...
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10 votes

What are the disadvantages of concentrated solar thermal power?

Everything we do as humans has consequences of some sort. Although not a huge downside (in my opinion), critics have often cited the plant's supposed adverse effects on wildlife: Wikipedia Article: ...
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  • 101
9 votes

What are the disadvantages of concentrated solar thermal power?

In the UK, the sun does not shine every day. We also need the most power in winter when the sun shines the least. Therefore every solar power station must be backed up by anther power station that ...
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9 votes

Relationship between wind velocity, turbine rotor diameter, and power output

It's not a simple relationship. First let's deal with the kinetic energy of the wind passing through the rotor. The mass of air passing through it in one second, $m$, is equal to the the density of ...
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  • 3,701
9 votes
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How to correctly size the blades for a small educational wind turbine?

As it happens, I just recently went through that calculation myself for a different site. Given the following facts from a quick web search, it isn't difficult to work out the numbers. The maximum ...
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  • 6,749
8 votes

Why is hydro-electric power still scarce in some places?

Rivers in a flat landscape are often unsuitable because you need a height difference to make the turbine work. Let's say a 1 meter height difference is required. There are many areas where the river's ...
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  • 736
7 votes

Are abandoned nuclear smoke stacks strong enough to be filled with water (for pumped storage)?

Those aren't "smoke" stacks but cooling towers designed to let hot air rise and expand optimally. These things are typically 200 m high. Static pressure of 200 m of water is pretty high. They were ...
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6 votes

Why don't hydroelectric plants use cascades of turbines instead of single turbines?

What's missing so far is an explanation why you can't expand from high pressure to atmospheric in a single stage gas turbine. There are two types of gas turbines - impulse and reaction turbines. Both ...
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  • 1,580
6 votes

Why aren't gravity batteries everywhere?

So, this battery costs US$70, weighs 406 grams, and holds around 200,000 Joules of energy. To get the same energy from that weight as a "gravity battery" you'd have to pick it up over 49 kilometers ...
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  • 2,218
5 votes
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What is the theoretical maximum energy efficiency of hydro turbines?

Something can only be 100% efficient when a lot of generous assumptions are taken into account. If you assume: No friction (turbine surface, turbulence, nozzle, or bearings) Perfect cup design (100%...
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  • 7,469
5 votes

Generating power from fruit

You seem to be asking about using fruit juice as the electrolyte of a battery. Yes, that's possible, but you don't get a lot of power that way. The power also doesn't just come from the fruit juice. ...
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  • 11.2k
5 votes

Why don't we just build a giant wind turbine?

The cost of a wind turbine is not proportional to its size - like everything that needs to be manufactured, the cost exponentially increases as manufacturing becomes more complex, beyond what has been ...
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5 votes

To what degree are wind and water turbines exchangeable?

Given that water is roughly 1000× as dense as air, and incompressible besides, it should be fairly obvious: A wind turbine would be destroyed by the forces that water would put on it, and a ...
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5 votes
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Could future fusion energy exist without the input of any petroleum or natural gas products?

The energy cost of obtaining Deuterium and Lithium is minimal compared to the energy released by the fusion process. There are many references that provide relevant information but a sensible one to ...
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5 votes

Wind turbines with down tower power units. Why aren't they more common?

The math doesn't make any engineering sense. A 5MW turbine typically rotates at about 10 RPM, near enough 1 radian per second. Assuming you had reasonably big sprockets on the chain drive, say 2 ...
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  • 12.5k
5 votes

Why aren't gravity batteries everywhere?

Consider 1 kWh = 1000 W * 3600 seconds = 3,600,000 J. 1 kg lifted 1 m = ~10J. Let's use 1 metric tonne. 1000Kg. So we get 10,000 J per meter of elevation. 3,600,000 / 10,000 = 360 meters. Around ...
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4 votes

What is the power and max flow rate required to rotate a 300 W wind turbine?

You need to push air through the turbine at 11 m/s, or just above. That's what the rated windspeed means: it's the lowest windspeed at which the turbine hits rated power, at standard atmospheric ...
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  • 3,701
4 votes

Clutch role in a micro hydropower

The clutch provides an instant disconnect either for scheduled maintenance or in an emergency. If the water flow is stopped instantly, then the energy of that moving pipeful of water has to be ...
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  • 13.9k
4 votes

Why is hydro-electric power still scarce in some places?

In years past I would have been a fan of hydroelectric power but I have read1 about some of the downsides over the years. These include: Major ecological problems including blocking fish migration. ...
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  • 8,249
4 votes

Can lithium ion batteries solve the world's energy storage problem?

I would like to answer this question slightly differently. The challenge with rechargeable batteries (Lithium Ion), each charge discharge cycle causes internal resistance of the battery to increase. ...
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4 votes

How large would a solar panel array need to be in order to supply all the world's energy?

The U.S. National Renewable Energy Lab did a study of solar installations in the U.S. in 2013 and determined that for large-scale solar PV systems, 7.9 acres (31,970 $m^2$) of land are required on ...
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  • 402
3 votes
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What theoretical efficiencies can wind / hydro turbines achieve?

The theoretical maximum efficiency of a wind turbine is the Betz Limit, which is $\frac{16}{27}$ ( ~ 59.3%). The result dates back to 1919. The Betz limit is, as you note, speed invariant. To get an ...
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  • 3,701
3 votes

What are the disadvantages of concentrated solar thermal power?

Concentrated solar power plants are best located in isolated areas that receive a lot of sunshine all year round, which basically means arid or semi-arid regions. Most of these regions don't have ...
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  • 9,462
3 votes
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What is the exact definition of renewable energy

overview: Renewable energy is one for which the input energy is renewed on human timescales. Remember, energy is always conserved. It doesn't come from nowhere. Pretty much all the energy we use on ...
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  • 3,701
3 votes
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Relationship between wind velocity, turbine rotor diameter, and power output

The Betz limit will give you a good theoretical maximum from your wind velocity and swept area. Your real world turbine will fall somewhere below this number based on its efficiency. For measuring ...
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  • 7,469
3 votes

Why don't hydroelectric plants use cascades of turbines instead of single turbines?

The reason why a hydroelectric generator is fundamentally different to a gas turbine is because water under pressure is not a gas, and does not change size significantly as energy is extracted from it....
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