132 votes
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Why bother buying efficient lights if you are already heating your house?

Yes, it does make sense to buy efficient light bulbs in this case. Here is why: Modern state-of-the-art heating systems use heat pumps to do the heating work. Rather than just "burning" the ...
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71 votes

Why is it so inefficient to generate electricity by absorbing heat?

"So, why can't we do the reverse?" Because of the Second Law of Thermodynamics! Very, very roughly, heat is 'thinly spread' energy and won't spontaneously organize itself into the 'concentrated' ...
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69 votes

Why bother buying efficient lights if you are already heating your house?

Hot air rises You'll have noticed the radiators around your house are close to the floor - under it if you're lucky enough. Your lighting, on the other hand, is close to the ceiling. The hot air they ...
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  • 1,089
54 votes
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Why is it so inefficient to generate electricity by absorbing heat?

tl;dr- Current technology absorbs temperature gradients, not heat. As temperature gradients become arbitrarily large, their information content nearly approaches the heat's information content, such ...
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49 votes
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Why is the efficiency of human cells less than the efficiency of an Otto engine?

Can we compare alive cells with heat engines at all? No, not really, because the living being isn't only a heat engine. There are three main points I want to make here. 1. Homeostasis Requires ...
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29 votes
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How does lowering your thermostat save energy?

The rate at which your home loses heat is proportional to the difference of temperature between the inside and the outside. This is Newton's law of cooling. Hence, a higher temperature home will lose ...
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  • 1,100
26 votes

Why bother buying efficient lights if you are already heating your house?

The cost of replacing incandescent bulbs is greater than the cost of longer lasting LED lights The other answers are correct in that: In many places fossil fuels like natural gas provide cheaper ...
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24 votes
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Are neurons more energy efficient than transistors?

Not surprisingly, it isn't so easy to get the power consumption of a cell. What is the power consumption of a cell? makes various estimates. One estimate for a human cell is $$P_{cell} = 3 \cdot 10^{-...
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23 votes

How does lowering your thermostat save energy?

Firstly, the specific heat (more precisely the specific enthalpy) of humid air varies a bit with temperature, so going from 72 to 74 degrees F will take a bit more energy than going from 66 to 68 F. ...
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  • 2,241
18 votes

Why bother buying efficient lights if you are already heating your house?

There's also the generation efficiency that comes into play. In general, about 60% of energy used to produce electricity is lost in conversion, so for every Watt-hour your light bulb consumes more ...
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17 votes

Why is it so inefficient to generate electricity by absorbing heat?

When you use heat to produce electricity (or any other form of energy), you are limited by Carnot efficiency: $$\eta = 1 - \frac{T_C}{T_H} $$ You cannot produce any useful work using a heat source ...
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15 votes
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Why is the Carnot engine the most efficient?

However, isn't any closed loop on a PV diagram reversible? The arrows can simply be drawn in the reverse way to create a refrigerator. If any closed loop is reversible then why does the specific ...
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  • 359
14 votes

Why is the efficiency of human cells less than the efficiency of an Otto engine?

Why is the efficiency of human cells less than efficiency of an Otto engine? It's not. You are comparing two very different things. The low value of 18 to 26% efficiency you found for the human body ...
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11 votes

Why is the Carnot engine the most efficient?

All the answers here are wrong to an extent - or at least very misleading. The Carnot cycle is not the highest efficiency cycle of all possible cycles, it is only one of an infinity of cycles all of ...
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  • 423
11 votes

Why bother buying efficient lights if you are already heating your house?

It's highly dependent on where you live and what room you are lighting (Leaving out stuff like gas heating, just looking at electric heating) According to the 1st law of thermodynamics, energy cannot ...
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  • 219
8 votes

Does the Carnot efficiency apply to photoelectric conversion?

Prism or diffraction grating does not separate the frequencies into parallel beams of rays falling on the same area as before. It increases beam divergence, which increases the area needed to capture ...
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7 votes
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Natural gas furnace vs. power plant efficiency

A power plant efficiency is electricity out / energy in. All the heat generated is by definition wasted energy. In your house's heater all heat generated is work out - so the only losses are heat ...
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7 votes

Are neurons more energy efficient than transistors?

This doesn't directly answer your question, but I think this article gives a good feel for just how much more complex a single biological neuron is than an individual node in an artificial neural ...
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7 votes
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Does the Carnot efficiency apply to photoelectric conversion?

100% quantum efficiency does not translate to 100% efficiency. It simply means 'one electron out per photon in'. A standard single-junction cell has a maximum thermodynamic efficiency of ~33% at 100% ...
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  • 194
6 votes
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How does the Carnot Cycle work?

The typical depiction of the Carnot Cycle is with the use of a closed cylinder containing an ideal gas fitted with a piston with a shaft that extends outside the cylinder to interact with the ...
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6 votes

Why is it easier to ship goods by water than by land?

There is a nonlinear dependence of drag on speed through the water for a ship's hull- which asymptotically approaches zero at zero speed. This means that you can use a single mule to (slowly!) pull a ...
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5 votes

How does lowering your thermostat save energy?

Forget about the furnace kicking in at 2 degrees below the preset or whatever. This is immaterial. The heat is not needed to produce this temperature rise, but rather to compensate heat loss with the ...
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  • 151
5 votes

Which is more mechanically efficient, a bicycle or a unicycle?

According to Wikipedia: A bicycle chain can be very energy efficient: one study reported efficiencies as high as 98.6% Given that on a unicycle, there is no chain, it ought to heave fewer losses ...
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5 votes

Which is more mechanically efficient, a bicycle or a unicycle?

A bicycle is mechanically more efficient as a vehicle. In the unicycle we can't harness the kinetic energy of the vehicle. The unicycle is direct drive, no gears, no "coasting". The system (...
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5 votes

Why does heat engine need to reject energy to environment?

1)You must have clear on your mind that engines do not work on any thermodynamic cycle.Per example, a diesel engine "consumes" fuel and air and expels an equal amount of exhaust gas. Nothing returns ...
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  • 3,526
5 votes
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Are all heaters exactly as energy efficient as each other?

Further to Samuel's answer, beyond pure energy efficiency, another measure of efficiency is how efficiently the heater converts the heat energy it produces into the heat that makes you comfortable. ...
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  • 216
5 votes

Why bother buying efficient lights if you are already heating your house?

Heating house with electricity is one of most expensive ways (if not THE most expensive). Normally houses are heated with coal/oil/firewood/natural gas/heat pumps/RTGs, and only rarely with electric ...
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  • 67
5 votes

Does the Carnot efficiency apply to photoelectric conversion?

The carnot limit only applies to heat engines which use thermal energy flows to produce mechanical work. So it does not apply to electrical power generation using moving fields to drive the flow of ...
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4 votes
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Shouldn't solar cells be nonreflective?

Good question! Solar cells made of silicon are naturally quite reflective (30%), and there is quite an art to making them less so - because any sunlight reflected is not available for power ...
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