Questions tagged [efficient-energy-use]

Use the tag *Efficient Energy Use* in questions having to do with processes or engines or constructions where energy is spent or transferred and the efficiency thereof. Efficiency means both the measurement and the techniques and principles for its optimization.

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26 views

How to find efficiency of a cycle thats not Carnot Cycle?

Here is the Full Question: A monoatomic gas is expanded adiabatically from volume $V_0$ to $2V_0$ and then is brought back to the initial state through an isothermal and isochoric process respectively....
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2answers
30 views

Efficiency of engine with two isotherms one isochor and one isobar

I was creating a hypothetical engine for fun and trying to calculate the thermal efficiency. The engine which I was trying to calculate consisted of: Isothermal compression, Isobaric expansion, ...
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Does incoming light polarization and phase effect the efficiency of photovoltaic cells?

As I understand it, standard photovoltaic cells depend on both the creation of electron-hole pairs and those pairs being created in the depletion zone in a P-N junction. Is seems like the production ...
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Can regular air conditioner be used as heater? [closed]

In winter season, the normal room temperature is generally 6 to 10 degree Celsius, if the air conditioner is working at 25 degree Celsius will it be working as a heater, making the room warm (relative ...
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How is efficiency for MHD generators calculated?

Coefficient of efficiency is defined as useful work divided by the total work done. As I understand MHD generators are working by using the Lorentz force, but how do you get the waste work?
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Maximum possible fuel efficiency at a given altitude

What's the maximum physically or technically possible fuel efficiency (considering common fuels like gasoline or diesel) that can be reached for a 1 ton vehicle at sea level? Can this be calculated? ...
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1answer
45 views

Heater efficiency on maintaining temperature?

I have a central heating system with natural gas (atmospheric) heater warming the water going through the closed loop to radiators. I can set the temperature of the hot water in a fairly broad range, ...
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93 views

What is the maximum theoretical efficiency of conversion of potential energy to kinetic energy and vice-versa?

Am looking at different techniques available for identifying sources of energy. As part of it, have the following question : What is the maximum theoretical efficiency of conversion of potential ...
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43 views

Calculating output of a solar cell

If we are given a particular solar panel with some semiconductor tabs in it, How can I calculate the output of the Solar panel per metre square With the values of ( average hours of sun everyday, ...
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1answer
152 views

How does the Carnot Cycle work?

I am learning the Carnot cycle, which consists of four transformations for a fluid. In the first two transformations the volume of the fluid expands, generating work. This happens in two steps: ...
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36 views

Sound from a rocket

I have been watching programmes commemorating the Apollo 11 mission. One obvious feature of the launch is the sound. It was mentioned that water was sprayed into the pit below the rocket not (...
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1answer
53 views

Why does intercooling improve the efficiency of a Brayton Cycle?

I've read in numerous sources that intercooling improves the efficiency of a Brayton cycle. The explanation given is that it decreases the compression work by minimizing the temperature during the ...
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106 views

Why a reversible engine's efficiency must be equal to that of a Carnot engine

Apparently, in order to prove that an engine must be as efficient as a Carnot engine if reversible is because apparently for its efficiency in either direction: $\eta_E \le (1-T_2/T_1)$ $\eta_R \ge (...
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61 views

Can two quasi-static adiabates intersect? [closed]

I saw a question in my physics book that I've been trying to understand lately. The question is simple: given two quasi-static adiabatic curves, could you find a point of their intersection? Also, ...
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Is it possible to transform energy stored in anti-matter into coherent radiation?

In Travellers, a sci-fi show on netflix, they have 10g of some form of anti-matter floating around. A quick $mc^2$ calculation gives a quantity of $E = 10^{15} J$. For reference, Little Boy released $...
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322 views

How to calculate the power required by a fountain pump?

How can I calculate the power required by a fountain pump, if just the efficiency of the pump, the height of the fountain and the diameter of the water jet is given?
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Should we take the elevator instead of the stairs to save energy?

In our university there's a posted sign that encourages students to take the stairs instead of the elevator to save the university electricity during the hot NYC summers. When you climb the stairs ...
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4answers
935 views

Is it more efficient to drive fast uphill?

I know that for a rocket escaping the atmosphere, it's not efficient to travel slowly because even staying stationary consumes a lot of fuel. Does the same apply to a vehicle traveling uphill? In ...
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264 views

Natural gas furnace vs. power plant efficiency

In the USA, residential heaters often use a measurement called AFUE, which is supposed to be a percentage of input energy to output energy. It is typical for a two-stage natural gas furnace to achieve ...
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3answers
234 views

Is the “efficiency” of car brake affected by how much force is applied by the brake?

Someone argues with me that stepping on a car's brakes for long periods of time will wear out the brakes faster than applying the brakes for short periods of time. We were talking about stepping on ...
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Why is it so inefficient to generate electricity by absorbing heat?

When I turn on a heater, it's supposed to be roughly 100% efficient. So it converts electricity to heat with great efficiency, but why can't we do the reverse: generate electricity by absorbing heat? ...
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258 views

Efficiency of any machine

Why cant the efficiency of a machine be 100%? We always see that there is some energy that is lost, mostly in the form of heat. But why is it so? Is it impossible to create a machine with 100% ...
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1answer
75 views

How can I calculate the efficiency of an air-powered car?

An air-powered car is powered using high pressure compressed air stored in a tank. The pressurized tank is connected to an air-powered engine. The volume of the tank is 0.3 m3, and the initial ...
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1answer
39 views

would heating a stove to 400 degrees be more or less energy efficient if done more quickly?

Would heating a stove to say 400 degrees be more or less energy efficient if done over a shorter or longer period of time? say 5 vs 10 minutes. Which would cost more on the electricity bill?
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47 views

Efficiency given two temperatures

Not sure what formula to use for this question? I am new to physics and the regular efficiency formula doesn't seem like the one I should be using. I was thinking the carnot efficiency, since it ...
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1answer
682 views

Stacking thermoelectric generators for a better efficiency

As far as I know, a thermoelectric generator has about 5-8% of efficiency. That means it uses 5% of the heat and converts them to electricity. Rest 95% is dissipated through the other side. So I can ...
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1answer
93 views

How can energy of petrol be compared to electricity?

When driving my car, which is powered by an electrical engine, I'm consuming between 10 and 17 kWh per 100 km (measured at the charging outlet). The car I had before consumed between 5 and 8 litres ...
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1answer
78 views

How much extra fuel is needed to transport a 1kg object over a distance of 10,000 miles? [closed]

I propose the scenario: a 1,300-kilogram, base weight, vehicle is carrying a 1-kilogram textbook with a 35% efficient engine over a distance of 10,000 miles. How much fuel is being burned to carry ...
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70 views

How to prove that an arbitrary cycle in the PV diagram has an efficiency less or equal than Carnot's Cycle? [duplicate]

The statement is: Consider an ideal gas with constant number of particles (N is constant), and an arbitrary thermodynamic cycle (closed curve with no self-interesections) in the PV diagram. Show that ...
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1answer
159 views

Understanding efficiency

This is a repost of a previous question, following the advice of an experienced user. I was told that reversible processes are more efficient than irreversible ones, and that this fact is expressed ...
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2answers
545 views

Understanding entropy and efficiency

A few years have passed since the last time I studied thermodynamics, and I'd like to give me a chance of asking some questions, maybe trivial, I don't know, but I want to clear matters up. Since my ...
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1answer
54 views

Is it correct to find ideal engine's efficiency using celsius scale?

I am solving a problem from my book which asks to calculate the maximum efficiency of an engine whose hot body is at temperature 300°C and cold body is at 25°C. Using 1- Tc/Th, I got efficiency equals ...
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3answers
348 views

Water turbine power generation for large and small turbines

If you have a small water turbine and a larger water turbine in a river, would the larger turbine produce more power or would they produce the same because of the equal speed of the river?
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What is the average range of voltage of lightning in sky?

I wondered how much energy is required to produce , such a loud sound? What is the average range of voltage in sky? Is there any possible way to store it ,and use when needed?
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Why is the efficiency of human cells less than the efficiency of an Otto engine?

I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest ...
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1answer
310 views

How is the energy efficiency in bicycles accounted for? [duplicate]

Bikes are more efficient than walking, but that's only because roads are flat so wheels make sense, right? Does it make sense to say that this fact is simply gained by putting a lot of energy into ...
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1answer
66 views

How much realistically usable energy can be obtained from matter/antimatter interaction?

The high energy density of a matter/antimatter system is well known. However, depending on the nature of the material, most of the energy from the interaction is released in the form of photons (...
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3answers
641 views

Calculating the electrical power generated from a wind turbine [closed]

Heres the question: A wind turbine has blades that sweep an area of 2000 m^2. It convrts the power available in the wind to electrical power with a efficiency of 50%. What is the electrical power ...
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2answers
156 views

Are all heaters exactly as energy efficient as each other?

I many applications, efficiency is measured in terms of how much of the input energy is wasted generating heat instead of the actual type of energy you want. For example, lightbulbs generating heat ...
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1answer
119 views

How can I get 100% fuel efficiency from an IC Engine in a car?

I understand that the current technology of internal combustion (IC) engines gives about 20% for a normal car. I want to understand what I can do to achieve 100% or near 100% efficiency in the same ...
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4answers
3k views

Why does heat engine need to reject energy to environment?

The most efficient thermodynamics cycle is the Carnot cycle, it involves 1/ Receiving heat from hot source, expanding and doing work on the surround 2/ Receiving work from environment, being ...
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177 views

How much money can I save on gas by losing weight?

I read somewhere that if you carried around a penny in your car, over the lifetime of the car, you would end up paying an extra penny in gas than if you weren't carrying that penny. I was wondering ...
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Do you know the principle which says that connecting two sources of similar kind produces a waste and destruction? [closed]

There is a great article, called commutation cells, which states that you cannot transfer kinetic energy from one container to another immediately, bypassing the potential energy storage. Otherwise, ...
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5answers
2k views

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

Human physiology aside, I wonder which device is more mechanically efficient, a bicycle or a unicycle?
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1answer
103 views

Heating 2 sets of 1L water vs 1 set of 2L water [closed]

Can heating 2 sets of 1L water or 1 set of 2L water have a variation in terms of fuel efficiency? Can one be more fuel-efficient than the other? (migrated from chemistry site)
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Otto and Carnot efficiency comparison

Lets say both engines operate between same temperature limits. Carnot eff: $$ e_\text{Carnot} = 1-\frac{T_L}{T_H} $$ Otto eff: $$ e_\text{Otto} = 1-\frac{T_4-T_1}{T_3-T_2} $$ assuming ...
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seperate engines working vs single engine

What is more fuel efficient: Two identical cars running at 50mph or one being tied to the other and only the first dragging both at 50mph? Or what requires less fuel: Two cars to start from stop and ...
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5answers
3k views

Engine fuel consumption vs power

Something that has bothered me for a long time is why a 600 hp engine uses more fuel per kilometer than a 80 hp engine. Let pretend I have two equal cars (same shape same weight etc) except for the ...
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energy efficiency - dryer vs hang+dehumidifer

When I have wet clothes coming out of the washing machine, I can either tumble-dry them in an electric clothes-dryer, or I can hang them to dry. But I can only hang them in a basement. The basement ...
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Is “eco” shower mode actually using less power? [closed]

At work we have electronic showers in the gym, they have three modes: * (A snowflake, Which I guess means no heating at all) ...