38 votes
Accepted

Windmills in empty fields. Why no trees?

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, ...
410 gone's user avatar
  • 3,751
35 votes

How do heat pumps output more heat kW than the electrical kW used to run the heat pump?

In the traditional efficiency equation the power input is the power to run the compressor motor and the power output is how much mechanical power that motor is producing. In that case efficiency is ...
DKNguyen's user avatar
  • 5,496
21 votes

Why does temperature not increase by putting more logs into the fire

Adding more fuel doesn't permit an arbitrarily high combustion temperature because of an unavoidable intrinsic limitation: The reaction has to heat its products. (In the case of wood, this is mostly ...
Chemomechanics's user avatar
15 votes

Can aluminium foil be a thermal insulator?

If furnished in a thin, polished sheet, aluminum will reflect infrared radiation, as for instance in a so-called "space blanket" which consists of a thin sheet of plastic on which a thin and ...
niels nielsen's user avatar
14 votes

Is there a way to make an air conditioner without venting heat outdoors?

The possibilities of transforming heat into any other form of energy are limited by the second law of thermodynamics. Essentially, you cannot simultaneously lower the entropy in a closed system (e.g ...
Dmitry Grigoryev's user avatar
14 votes

How do heat pumps output more heat kW than the electrical kW used to run the heat pump?

A heat pump takes heat energy from one side, compresses it a bit (to get to a usable temperature) and transports it to another side. So you are more or less “stealing” energy from the outside of the ...
Michael's user avatar
  • 318
12 votes

Is there a way to make an air conditioner without venting heat outdoors?

Watercooled , if you don't count sewer as "outdoors".
blacksmith37's user avatar
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11 votes

Can solar panels absorb heat for energy?

Yes, there are indeed combined PV-T (photovoltaic-thermal) hybrid panels that turn some of the incident light into electricity, and have a circulating fluid and heat exchanger to put some of the heat ...
410 gone's user avatar
  • 3,751
11 votes

How do heat pumps output more heat kW than the electrical kW used to run the heat pump?

Heat pumps (commonly) use vapor compression cycles (the following is a short summary of a longer article in tlk-energy.de). To understand this process, you only need the following three key physical ...
NMech's user avatar
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10 votes

Is there a way to make an air conditioner without venting heat outdoors?

Use a (largish) water tank. I've never seen a water tank used as a heat sink for an air conditioner, but they were used for stationary engines some time ago. Typical air conditioners can produce about ...
Pere's user avatar
  • 427
9 votes

Why does temperature not increase by putting more logs into the fire

I agree with Chemodynamics, and I will try to add a different perspective (or maybe two). The fire process is a process where you add a fuel (i.e. energy in the system), and that fuel is gradually ...
NMech's user avatar
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9 votes
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Can aluminium foil be a thermal insulator?

It really depends on what you are trying to do with it. And thermal "conductors" and "insulators" are not a hard and fast distinction in properties like electric conductors or ...
RC_23's user avatar
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8 votes

Is there a way to make an air conditioner without venting heat outdoors?

This question reminds of the concept of a heat bank, a form of heating I experienced in the 1970s. The type of heat bank I am familiar with consisted of a metal box which contained an electric ...
Fred's user avatar
  • 9,667
8 votes

Is there a way to make an air conditioner without venting heat outdoors?

Nobody specifically mentioned geothermal yet but this is exactly the principal used here. Rather than exchanging heat with the outdoor air, heat is exchanged with the ground a meter or two below the ...
jwh20's user avatar
  • 201
8 votes

Is there a way to make an air conditioner without venting heat outdoors?

Surprised they haven’t been mentioned yet because they seem to be all the rage in the spammy ads you see on a lot of sites, but depending (very heavily) on the general atmospheric conditions, an ...
Austin Hemmelgarn's user avatar
7 votes
Accepted

Can an isothermal process also be adiabatic?

If a process is both isothermal and adiabatic, it is implied that the work done on the system is being stored somewhere other than the internal energy of the working fluid. (Or conversely, if the ...
Carlton's user avatar
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7 votes
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Heat pipes vs fluid cooling

Heat pipes are very useful when you have limited space to work with or you want an entirely sealed and self contained unit. An interesting example of this is that heat pipes are used to cool the ...
Chris Johns's user avatar
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7 votes
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Radiators in series or parallel?

The efficiency of any radiator (heat exchanger) is a function of the temperature difference between the two fluids in question. All else being equal, a heat exchanger with a greater temperature ...
Dave Tweed's user avatar
  • 6,874
7 votes

Windmills in empty fields. Why no trees?

There is a non-engineering reason as well: the people most open to placing turbines on their land are farmers who are interested in the extra income. Turbines are not (yet) placed near built-up areas,...
Simon Richter's user avatar
7 votes
Accepted

Why is it more common to use ceiling fans instead of air pumps?

Mainly cost difference. And also, a fan is sufficient for the job. The problem being solved is thermal stratification. That's caused by a lack of vertical mixing; the warm air rises to the top, the ...
410 gone's user avatar
  • 3,751
7 votes
Accepted

Why are home geothermal systems usable anywhere in the U.S. but industrial geothermal plants limited in geographical use?

Because they're two completely different systems that have, in some circles, accidentally been given the same names. The things that are sometimes (particularly in the USA) called home geothermal ...
410 gone's user avatar
  • 3,751
7 votes
Accepted

In a diesel engine, does more heat leave the engine through the tailpipe or the radiator?

They are very nearly equal for typical four-stroke non-turbo diesels under load. A turbo diesel under load should have slightly more radiator loss than exhaust loss. At the bottom is a link to the ...
Phil Sweet's user avatar
  • 4,696
7 votes

Is there a way to make an air conditioner without venting heat outdoors?

There are phase change materials (Similar to the cold pack for injuries) that at some point in time was the all the rage. Basically what they did was storing and releasing heat energy. However, they ...
NMech's user avatar
  • 24.3k
6 votes
Accepted

Do heat pipes work in any orientation?

Orientation does often matter. As Carl's answer mentions, the liquid can get from the condenser to the hot interface via capillary action, but most common heat pipes are designed assuming gravity ...
Olin Lathrop's user avatar
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6 votes
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Difference between temperature and heat

In engineering terms heat (energy) and temperature are two different things. One (of many) real life examples is the kitchen stove. When you turn on the kitchen, (and leave it at a set point), the ...
NMech's user avatar
  • 24.3k
6 votes
Accepted

Why does the author assume the temperature to remain constant, in this case, along the axial direction?

For a steady state solution, (after a loooong time), because the temperature of the air is considered constant the heat lost $\dot{q}$ to the environment will be equal to the heat lost from the fluid $...
NMech's user avatar
  • 24.3k
5 votes
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How can I determine the thermal conductivity of an open container?

If you are interested in knowing why your tea is cooling in different times, you already answered the question yourself. It is due to the different thermal conductivities of the different items you ...
idkfa's user avatar
  • 1,734
5 votes

If I need a very small heat engine, should I consider external combustion?

I have done a lot of work with small sterling heat engines. While decreasing the size generally does have a favorable effect on your heat transfer, your energy density doesn't increase substantially ...
ericnutsch's user avatar
  • 8,136
5 votes

Does venturi effect helps in reducing temperature of a room?

I don't think it will drop the temperature at all. What it might accomplish is increase the velocity of the air in the room. That will in turn affect the convective coefficient The convective heat ...
NMech's user avatar
  • 24.3k
5 votes
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Furnace Combustion Air Properties

the outside air is colder, but the flame temperature is still much much hotter than that so the difference cold inlet air makes on the outlet temperature of the furnace will be small. The same ...
niels nielsen's user avatar

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