PV System Design and Installation LO3 Electricity Basics

Electricity Basics (15% of test questions) Task/Skill 3.1. Explain difference between energy and power 3.2. Define basic electrical terms 3.3. Describe the use of digital multi-meter 3.4. Calculate simple circuit values

Flow of electrons (negative charge) through a circuit Measured in terms of electron flow per time Units = 1 Amp =Amps 6 x electrons per second Symbol = ‘I’ Current

Voltage Pressure pushing electrons through circuit Electric Potential Units = Volts Symbol = “V”

Voltage = The “height” tank that pushes water through a circuit Current = The “gallons per minute” of water flowing through a circuit Water Analogy

Power = Rate of delivery of energy Power = Current (I) x Voltage (V) Units = Watts or kilowatts Power

Energy Energy = Work done by electricity Energy = Power (watts) x Time (Hours) Units = watt-hours or kWh

For all electrical devices, the power that they supply or consume is the product of the potential across the device and the current that flows through the device. PV module Current-Voltage Curve Reference 3 IV Curves

Conductors are those substances in which the outer electrons (typically only one or two in each atom) can move freely from one atom to the next Insulators are substances in which all the electrons in the atoms of the substance are tightly bound. The electrons do not easily move from one atom to the next. Semi-conductors is a general term for a class of substances which allow current to flow, but in a limited or controlled fashion. Types of materials

Measure of opposition to flow of electrons through a conductor Resistance lowers the voltage in a conductor but not the current Resistance = Voltage / Current Units = Ohms Resistance

Ohm’s Law

Direct Current (DC) Voltage and Current are Constant over Time Electrons flow in one direction only DC Power Sources Batteries DC Generators Photovoltaic (PV) modules Etc. Types of Electricity

Historic in nature. Used by electrical engineers and engineering text books Reflect actual electron flow. Used in introductory texts and by scientists Current Flow Notation / Different Conventions

Series Circuits Voltage is additive, current remains the same Parallel Circuits Current is additive, voltage remains the same DC Circuits

Series Circuits V total = V1 + V2 + V3+ V4 Current is constant Parallel Circuits I total= I1 + I2 + I3+ I4 Voltage is constant Reference 3 DC Circuit Calculations

Current and voltage are constantly changing from positive to negative in value of time. Change in values represented as a sinusoidal curve. Cycle – One complete change from positive to negative values Frequency (Hertz) = Number of Cycles per Seconds Household AC power typical is 120/240 V AC at 60 Hz Alternating Current (AC)

“Effective” AC current and voltage Can apply Ohm’s Law V (RMS) = I (RMS) x R RMS = x Peak Peak = 1.41 x RMS Peak to Peak = 2 x Peak Root Mean Square (RMS) 170Volts 120 Volts

Types of AC Loads Resistive Loads Incandescent Lights Resistance Heaters Power factor = 1 Reactive Loads Motors Compressors Capacitors Power factor < 1

Power Factor AC loads such as motors, compressors, etc. cause current and voltage wave forms to be out- of-sync with each other. The degree that the two wave forms are out-of-sync is measured by a phase angle, phi. The cosine of phi is called the Power Factor (PF). In a typical residence, the power factor is from 0.8 to 0.9

Apparent and Real AC Power Power (apparent) = I (RMS) x V (RMS) However, current and wave forms, may be peaking at different times, therefore the “instantaneous” power (I x V) will not be the same as the “time average” power. We use the Power Factor (PF) to calculate the “real” power used by an AC motor load, Power (real) = I (RMS) x V (RMS) x PF of device

Current and Voltage Curves Resistive Load PF=1Reactive Load PF<1

Real and Apparent Power Reactive Loads Real Power = Power used by in circuit. Expressed in “Watts”. Apparent Power = Power supplied to circuit. Expressed in “kVA” Power = Real Power Factor Apparent Power

Reference 5 Main Load Center

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