Energy Efficiency

Energy Efficiency

Reducing the amount lost—also known as increasing efficiency—is as important to our energy future as finding new sources because gigantic amounts of energy are

lost every minute of every day in conversions.

Conservation of Energy vs. Energy Conservation

• Energy Conservation is about reducing consumption by lifestyle changes and technological fixes.

• Conservation of Energy is a law of nature that states that the amount of energy in the universe is constant and that energy is neither created nor destroyed, only transformed from one form to another.

Energy is Conserved• When we use energy, it doesn’t disappear.

We change it from one form of energy into another.

• Don’t we create energy at a power plant?– No, we simply transform energy at our power

plants• Doesn’t the sun create energy?

– No—it exchanges mass for energy• The net energy of the entire Universe is

constant. The best we can do is scrape up some useful crumbs

Energy Conversions• Though the total

energy of a system is constant, the form of the energy can change.

• A simple example is that of a simple pendulum, in which a continual exchange is converted between kinetic and potential energy

Energy Conversions• There are three primary sources of energy

readily available for human consumption1.Chemical Energy

• Fossil Fuels and Biomass

2.Nuclear Energy • radioactive minerals and geothermal

3.Radiant Energy• solar and wind

• We convert these primary sources into more useful energy through energy conversion devices.

Energy Conversion Devices

• An energy conversion device converts one form of energy into another.



Review of Work and Energy

• Work is a force over a distance.• Energy is the ability to do work.• Too put it another way……

Energy is money in the bank

Work is when you use that money by cash, credit card, check, etc

Review of Work and Energy

Energy can do work as1. Work against inertia2. Work against gravity3. Work against friction4. Work against shape• Work against

combinations of above

Law of Energy Conservation• The Law of Energy Conservation states

that when work is done the energy can be converted from one form to another but the total energy remains constant.

The energy input should be equal to the energy output as the woman overcomes inertia and friction to do work by pushing the box.

Work and Energy• Work is a way to transfer energy to an

object.• The box obtains energy by the work

perform by the woman to overcome inertia and friction.

The total work is to the change in potential and kinetic energy added to the box from the woman.

W = Δ (KE + PE)Δ means “change in”

Thermal Energy

• All matter is made of atoms/molecules, which move around randomly

• Thermal Energy is the energy available in the internal motion of atoms or molecules

• Thermal Energy increases when atomic motion increases

• Friction and other contact forces generate thermal energy by increasing the atomic motion along the surface of contact.

Work and Thermal Energy• As the woman pushes the box, the

molecules of the box in contact with the floor increase in motion due to friction

• Thus the box obtains additional energy by the thermal energy generated due to friction.

The total work can now be represented .

W = Δ (KE + PE +TE)Δ means “change in”

Work and Energy Change

• The result of work is that one or all of the following energy changes has taken place:– Increased kinetic energy (Work against

Inertia)– Increased potential energy (Work

against Gravity and Shape)– Increased thermal energy (Work against

Friction)

Perpetual Motion??

• A simple example is that of a simple pendulum, in which a continual exchange goes on between kinetic and potential energy

• Why won’t the pendulum swing back and forever?

“Waste Heat”

• As the pendulum swings back and forth in encounters air resistance and friction

• This converts some of the energy in this system to thermal energy which is transferred to the surrounding environment as heat.

Total Energy = Δ (KE + PE + TE)

• The TE is wasted because is not useful in the original system (pendulum)

High Vs Low Quality Energy

• When energy is converted in a system, two types of energy occur:

• High Quality (USEFUL) Energy– Organized and Concentrated– Great ability to perform USEFUL work.

• Low Quality Energy– Disorganized and Dilute– Little ability to do work.– Usually in the form of “Waste Heat”

Energy Flow in a System

1) The total energy in the system is constantEnergy Input = Energy Output

2) Some of the energy input will be converted into less useful thermal energy and transferred into the surrounding environment as “waste heat.”Useful Energy Output < Total Energy Input

System

Energy Input Energy Output

Energy Output = Energy Input (1st Law)

Useful Energy Output < Energy Input (2nd Law)

Energy Conversion Devices and Efficiency

• The efficiency of an energy conversion device is a quantitative expression of the balance between energy input and useful energy output.

• The meaning of the word ‘useful’ depends on the purpose of the device.

100%convertedenergyorinputenergytotal

outputworkorenergyuseful(%)Efficiency

Energy Conversion Devices and Useful Energy

Incandescent Light Bulb

• Very thin tungsten filament that is housed inside a glass sphere.

• Electricity runs through the filament, since the filament is so thin it offers a good bit of resistance to the electricity.

• This resistance then turns electrical energy into heat.

• The heat is enough to make the filament white hot and the “white” part is light.

• The filament literally incandesces because of the heat.

Electrical Energy In Radiant Energy Out


Energy Conservation in an Incandescent Light Bulb

• When a convince device such as an incandescent light bulb converts electrical energy to radiant energy, the energy input is equal to the energy output.

However…….• If we were to measure the energy in and the energy

out of the incandescent bulb for one second, we might get energy numbers that look something below.

• This would suggest that energy is not conserved!!! Where did the other 95 J of energy go?

Electrical Energy In100 J (joules)

Radiant Energy Out5 J (joules)


“Missing Energy”• Energy can change into more than one form

simultaneously. • If you feel a light bulb it is very hot. The "missing"

energy was converted into low quality thermal energy.

Electrical Energy In100 J (joules)

Radiant Energy Out5 J (joules)

Thermal Energy Out95 J (joules)


Efficiency of an Incandescent Light Bulb

• Suppose you paid $100 a year to light your home with incandescent light bulbs.

• Since these light bulbs have an efficiency of 5%, only five dollars of this payment went toward paying for the light!

• The other $95 of energy was wasted as unused thermal energy!

100%convertedenergyorinputenergytotal

outputworkorenergyuseful(%)Efficiency

5% 100Joules 100

Joules 5BulbLight of Efficiency

Efficiencies of common energy conversion devices

• The table on the next slide shows the energy efficiency of common energy conversion devices

• The numbers shown are typical but they can be different for different models of the same type of device details or for the same device, depending on whether it is used and maintained properly.– For example, your car engine will be more efficient if

you change the oil regularly.• Note that the most efficient devices are those

that produce heat as the use energy output (electric drier and electric heater)

Efficiencies of common energy conversion devices

• In a multistep system, the overall efficiency is equal to the product of the individual efficiencies.

• For example, in a electrical power plant there are three main energy conversion devices: the boiler, the turbine, and the generator.

Overall Efficiency

The Boiler• The boiler

converts chemical energy from the energy source (coal, natural gas, uranium) to thermal energy in order to generate steam.

• A boiler has an efficiency of 88%.

The Turbine• The turbine

converts the thermal energy from the boiler to mechanical energy to spin the generator.

• A turbine has an efficiency of 40%.

The Generator• The generator

converts the mechanical energy from the turbine to electrical energy.

• A generator has an efficiency of 98%.

Efficiency of a Power Plant

• The efficiency of the power plant can be calculated by multiply the individual efficiencies of the boiler, turbine and the generator.

Conservation of Energy

Chemical Energy in Fuel

Electricity(Useful Energy)

Waste Heat

Total Energy In = Total Energy Out

100% Use Energy Input = 35% Use Energy Output

Thermal Pollution• 65% of the

potential use energy of the power plant is released into the environment as “waste heat”

• This unwanted heat alters the surrounding environment and is referred to as thermal pollution

Thermal Pollution

Thermal Pollution

Overall Efficiency of a Light Bulb• Once the energy is generated at the power

plant, it has to be transmitted to building to be used by energy conversion devices.

• The efficiency of most power lines is 90%• What is the overall efficiency of an

incandescent light?

Overall Efficiency of a Light Bulb

Compact Fluorescent Light Bulb• A compact fluorescent light bulb

(CFL) produces less waste heat than an incandescent bulb, so the overall efficiency rises to around 5%— better, but still a small fraction of the original.

• Although CFLs cost more upfront, they last longer an a household could save up to $40.00 per light bulb for a 5 year period by using CFL instead of incandescent bulbs.

Light Emitting Diode• Light Emitting Diodes

(LEDs) generate relatively little heat, last 100 times longer than an incandescent lightbulb, and convert about 25% to 35% of electrical energy to light.

• Additionally, they do not require bulky sockets or fixtures and could be embedded directly into ceilings or walls.

Cost Comparison between LEDs, CFLs and Incandescent light bulbs

LED CFL Incandescent

Light bulb projected lifespan 50,000 hours 10,000 hours 1,200 hours

Watts per bulb (equiv. 60 watts) 6 14 60Cost per bulb $35.95 $3.95 $1.25KWh of electricity used over

50,000 hours 300 700 3000

Cost of electricity (@ 0.20per KWh) $60 $140 $600

Bulbs needed for 50k hours use 1 5 42

Equivalent 50k hours bulb expense $35.95 $19.75 $52.50

Total cost for 50k hours $95.95 $159.75 $652.50