1. Photovoltaic Systems Engineering Review of Energy and Power
SEC598 (SEC501)
Photovoltaic Systems Engineering
(SEC501 Solar Engineering and Commercialization, I)
Session 01b
Review of Energy and Power
August 22, 2018

2. A Quick Review of Energy and Power
Energy is a measurement of ability to do work. It is expressed in units of joules (J)
Power is the rate at which energy is employed. It is expressed in units of watts (W). One watt equals one joule per second

3. A Quick Review of Energy and Power
Energy is a measurement of ability to do work
Work is defined as an amount of force applied to an object it to move it a certain distance:
The work done to move an object of mass m against the force of gravity to a height h is:

4. A Quick Review of Energy and Power
Energy is a measurement of ability to do work. It is expressed in units of Joules (J)
Energy is expressed in two different forms:
Kinetic Energy – energy in motion
Potential Energy – energy in position
Consider the
pendulum

5. A Quick Review of Energy and Power
PE = mgh h
PE = 0

6. A Quick Review of Energy and Power
Lets suppose a ball is moved to the top of the ramp, and it gains potential energy of
If the ball is moved to the top of the ramp in 100 seconds, the power required to do this is
If the ball is moved to the top of the ramp in 1 second, the power required to do this is

7. A Quick Review of Energy and Power
The original units for energy and power came from thermal science
One calorie (1 cal) is the energy required to raise one gram of water by one degree Celsius (1oC)
One kilocalorie (1kcal) is the energy required to raise one kilogram of water by one degree Celsius (1oC) – this is 1 food calorie
One British Thermal Unit (1 BTU) is the energy required to raise one pound of water by one degree Fahrenheit (1oF)
The proper unit for energy (work) in the scientific community is the Joule (J)
1 cal = J
1 BTU = J

8. A Quick Review of Energy and Power
The most common unit for energy in energy policy and engineering circles is the kilowatt-hour (kWh)
1 kWh = 1000W . 1hour
= 1000W sec
= 3,600,000 W.sec = 3,600,000 J
= 3.6MJ
But the energy content of fuels (like coal, oil) is usually quoted in BTUs
1 kWh = 3.6MJ J/BTU = BTU
The BIG unit of energy measurement is the Quad:
1 Q = BTU = x 1011 kWh = 293 TWh

9. A Quick Review of Energy and Power
So what is the size of 1 kWh, in common, everyday values?
Suppose Prof. Kozicki decides to read a comic book before retiring for the night, so he switches on a reading light with a 100W bulb. He reads for 10 hours, then switches off the light.
Prof. Kozicki’s light bulb has consumed an electrical energy value of:
100W * 10hours = 1000Wh = 1kWh

10. A Quick Review of Energy and Power
Another example of 1 kWh
Now suppose Prof. Kozicki decides to go to Argentina and climb Mt. Aconcagua. How much potential energy does he gain during the climb?

11. A Quick Review of Energy and Power
Energy is “conserved” – it is neither created nor destroyed. But it can be converted from one form to another
Energy is also expressed in several different formats:
Optical Energy
Chemical Energy
Mechanical Energy
Thermal Energy
Electrical Energy

12. A Quick Review of Energy and Power
Optical Energy is the energy contained in light; Optical Power is the rate at which optical energy is delivered.
Chemical Energy is energy stored in atomic bonds, and in the motion or separation of ionic charges. The lead acid battery, or the AA dry cell, stores chemical energy.
Mechanical Energy is the energy in moving or displaced objects at both microscopic and macroscopic scales. A spinning flywheel has kinetic mechanical energy; water stored behind a dam has potential mechanical energy.
Thermal Energy refers to the energy associated with heat and its transfer

13. A Quick Review of Energy and Power
Electrical Energy is the energy contained in electrons and other charged particles; Electrical Power is the rate at which electrical energy is delivered to an “electrical load”
Electrical Power is calculated by multiplying the Electrical Current and the Electrical Voltage in the electrical device
Current is a measure of the flow of electrons (amperes)
Voltage is a measure of the potential energy (volts)

14. A Quick Review of Energy and Power
Electrical power can be “delivered” from an electrical source (a battery, a generator, a solar cell) and is consumed or dissipated by an electrical load (a motor, an electrical stove, a computer)
An examination of the power equation shows that the power is zero when the current is zero (open circuit) or the voltage is zero (short circuit)

15. A Quick Review of Energy and Power
Energy is “conserved”
It is neither created nor destroyed.
The expression, “The solar module created 100 joules of energy” is scientifically incorrect
But it can be converted from one form to another
The expression, “The solar module converted 100 joules of optical energy into 20 joules of electrical energy and 80 joules of thermal energy (heat)” is technically correct.

16. A Quick Review of Energy and Power
In the Energy Enterprise these terms are often used
The “volume” of electricity reflects the notion that electricity represents a number of charges moved through a system – electrical energy
The monthly utility bill lists the number of kWh utilized by the house – it is called a “volumetric” bill
The electricity “demand” is the rate at which electricity is delivered to the system – electrical power
The electrical “capacity” of an electrical generating station (or storage system) is the maximum value of the rate the energy can be delivered – electrical power

17. A Quick Review of Energy and Power
A few words about units, cont.
The SI format is Number Modifier Unit
750,000 W is expressed as 750kW
7,500,000 W should not be expressed as 7500kW, but rather 7.5MW
k, M, etc

18. A Quick Review of Energy and Power
A few words about units
The organization, SI, has established these “attachments” to simplify Scientific Notation
Order of magnitude
SI name
109
Giga (G)
1012
Tera (T)
1015
Peta (P)
1018
Exa (E)
1021
Zetta (Z)
1024
Yotta (Y)