Energy Forms and Energy Conversion

Energy

• The capacity of vigorous activity• The ability to act• The capacity of a body or a system to do work

(and heat).

Mechanical EnergyKinetic + Potential

• E = U+K

h

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U +K = mgh +mv 2

2

Chemical Energy:Microscopic Version of the Potential Energy

Potential energy of a bond between atoms forming a molecule

Thermal Energy:Microscopic Version of the Mechanical Energy

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ΔEthermal =CΔT

C – heat capacity

For a gas the thermal energy is just the kinetic energy

In general thermal energy has kinetic and potential components (potential energy stored in the vibrating bonds)

Electrical Energy: Potential Energy of Electrostatic Interactions

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Eelectrostatic =q1q2r

q1 q1r

Electromagnetic Energy

Energy in oscillating electromagnetic waves

These waves can be considered as massless particles called photons traveling with the speed of light

Photon energy hv (h –Planck constant, v frequency)

Nuclear Energy

Potential energy of bonded protons and neutrons forming nucleus

The energy amount is so high that during nuclear energy release it can be observed that mass is reduced according to

E=mc2

Energy Conservation

Energy can not be created or destroyed – it can be only converted

Energy of the system can be changed by the flow of energy form or to the outside in the form of work, W, and heat, Q

ΔE = W + Q

Work

Work = force times displacement

W = Fd

The unit of work is the same as the unit of energy

The unit of force

thus the unit of energy

This unit is called Joule

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kgm2

s2

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kgm

s2

Heat

Heat, as work is also a process – it is essentially work done by molecules thus it can change the energy of the system

Energy and Power

Power is the energy change rate

W = ΔE/Δt

The unit of power is Watt (W)

The World uses about 15 Terra Watts of power, with about 2.5 TW of electricity

Energy Units - I

Joule = Watt x second

Kilowatt-hour = 3.6 Megajoules– on your electric bill

Therm = 105 Megajoules = 100,000 British thermal units (BTU) – about 100 cubic feet natural gas burning equivalent

BTU – energy needed to heat one pound of water by one °F

Energy Units - II

Calorie - heating 1 g of water by 1 °C = 4.2 Joules

Electronovolt – energy gained by moving one electron across potential difference of one Volt = 1.6 x 10-19 Joules (one Volt = 1J/Coulomb)

Energy = Volt x Coulomb = Volt x Ampere x time

Power = Volt x Ampere

Energy Conversion

Useful output is generally one that can produce work (mechanical, electrical)

Sometimes heat can be useful e.g., to heat a house

Chemical Energy ConversionMost important is combustion – potential chemical energy is transformed to kinetic energy of the gas (high temp and pressure). For example,

Yields about 240 kJ/mole energy leading to a high pressure gas. This energy shows as the kinetic energy

This energy can be used directly as mechanical energy (car) or converted to other useful energy, such as electrical energy

Alternator

Converts mechanical energy to AC electricity

Rotating magnet

Stationary wire

Transformer

Can easily change the voltage of an alternating current

Why AC won over DC

AC transformers can easily increase and decrease voltage

The input power is P=VI (voltage times current)

The voltage on a transmission wire is V = IR

The power loss on the transmission wire is Ploss = I2R, and since I=P/V, Ploss =R P2/V2

Higher voltage means much lower transmission loss

This was a rare time that Thomas Edison lost big time to Tesla and Westinghouse (War of Currents).

AC vs. DC in the future

Distributed power generation, e.g., by solar, allows to avoid transmission power losses due to short lines.

Transformation losses are eliminated

Many devices work on DC only

So Edison might be right in the future

Solar to Electricity

Photon excites negatively charged electrons and separates them from positively charged holes – this generates electrostatic energy and associated voltage (DC)

Limits of efficiency is mostly due to two factors

Not all photons lead to excitation

Electrons and holes recombine and just generate heat

Efficiency – Carnot Cycle

Efficiency = Work/Input heatHot heat is useful

Efficiency and Thermal Energy Storage

Carnot conversion efficiency

(Thot-Tcold)/Thot*100%

Increases with increasing Thot

But stored heat radiates at the rate ~ (Thot)4

thus the loss increases with increasing Thot

Efficiency in General

Efficiency = Useful Energy/Energy Input

Typical fossil fuel power plant up to 40%

Hydro – about 90%

Solar 6-40%, 15% typical

Combustion engine ~ 30%

Electric engine 70-90%

Photosynthesis few percent

Energy Returned on Energy Invested

EROEI = Usable Energy/Energy Expended

Sankey Diagram – Electric Plant

Sankey Diagram – USA