The Vocabulary of Electrical Power

Solar Under The SunSolar School

May 2010

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• The purpose of this presentation is to review electrical terms such as volts, amps, and watts

• And to relate these terms to components in a solar power system

Picture of Picture of

Four 6-volt batteries, Part-au-Prince 50 amp and 20 amp circuit breakers

Picture of Picture of

Four 155 watt solar panelsArchaie, Haiti

Four 125 amp-hour batteriesArchaie, Haiti

DC vs. AC electricity

• Photovoltaic (PV) panels and batteries are sources of DC power– In a DC system, current only flows in one direction

• Electric utilities, generators, and inverters are sources of AC power– In an AC system, current changes direction tens of

times a second.

Water analogy for DC electricity

• Current is the flow of electrons in a circuit. – Units of current are amperes or amps– Think of current as the water flowing through a

hose

• Voltage is the electrical potential– Units of voltage are volts– Think of voltage as the water pressure in a hose

Current

• Current can only flow when there is a complete loop– V = I*R,

where R is resistance– If the load goes

to 0 ohms resistance, thisis called ashort circuit

+

-

LOAD

current, I

current, I

Voltage

• Voltage exists even if there is no current• When the resistance

is infinite, is an open-circuitcondition

+

-

current, I = 0

current, I =0

12 volts12 volts

Power

• Power is the rate at which work is done• Units of power

are watts• Power is the product

of current and voltage

• P = V * I

+

-

LOAD

current, I

current, I

V V

Power Examples

• A solar panel is outputting 4 amps at 25 volts. How much power is the solar panel producing?

• P = 4 amps * 25 volts = 100 watts• A 150 watt solar panel operating at rated

output provides 5 amps of current. What is the full-output voltage?

• P = V*I, so V = P/I = 150 watts/5 amps = 30 volts

Energy

• A photovoltaic panel converts solar energy (light) into electrical energy

• A battery stores energy and delivers energy• Units of energy are watt-hours• E = P * t, where t is time

Energy examples• How much energy will a 155-watt PV panel

produce when it operates at rated power for 5 hours?

• E = 155 watts * 5 hours = 775 watt-hours• We will often refer to Energy in a particular time

period– How many watt-hours will a solar array generate each

day if it operates at rated output for 5 hours/day? E/day = 155 watts * 5 hours/ day = 775 watt-hours/day

How many solar panels?

• In a SUTS solar power system, the PV panels must provide enough watt-hours of energy each week to meet the watt-hour needs of the equipment and to keep the batteries at full charge.

• This calculation will normally be performed by a graduate of Solar 2

How many batteries?

• The batteries must be able to provide enough watt-hours of energy to meet the watt-hour requirements of the equipment for at least 3 days of no sun, while maintaining at least 50% of their total charge.

• This calculation will normally be performed by a graduate of Solar 2

Deep-cycle batteries

• Batteries for solar power systems should be “deep-cycle”.

• This means they are designed to provide a steady flow of power for an extended period of time

• Common sources of deep-cycle batteries are golf-cart batteries and wheelchair batteries

Batteries

• Batteries are characterized by two numbers– Their voltage (usually 6 or 12 volts)– Their amp-hour capacity

• Amp-hours are an indicator of how much charge a battery can store, and

• Amp-hours are an indicator of how many hours a battery can supply a given number of amps

How does this relate to Solar 1?

• Solar 1 graduates and Solar 2 graduates will complete the Solar Suitability Survey (SSS) during the partnership development visits with the operating partners

Solar Suitability Survey

• One component of the survey is determining what local or regional sources of solar equipment exist

• Where a network exists (like Haiti) a solar supplier will have already been identified

Camp Hopewell, CWU circa 2008

The initial system still used to test equipment and ideasDC Slo-pump replaced with Sun Pumps circulation pumpAC pump tested and DC pumps adopted

Archaie, Haiti July, 2009

4 x 155-watt panels

Water treatment system with UVdisinfection

Submersible pump

Water fountains at school yard across the street

Security lighting for water building

Institute Racine, Port-au-Prince, Haiti March 2010

2 155-watt solar panels

Water treatment system withozone disinfection

Lighting for water building

Outlets for charging computers and cell phones