PV System Components

Advanced ElectronicsLandstown High School STEM &

Technology Academy

PV was developed for the space program in the 1960’s

What is a solar cell?

• Solid state device that converts solar energy directly into electrical energy

• Efficiencies from 10%- 80%%• No moving parts• No noise• Lifetimes of 20-30 years or more

Cross Section of Solar Cell

How Does It Work?

• The junction of dissimilar materials (n (+) and p (-) type silicon) creates a voltage,

• Energy from sunlight knocks out electrons, creating a electron,

• Connecting both sides to an external circuit causes current to flow,

• In essence, sunlight on a solar cell creates a small battery with voltages typically 0.5 volt DC,

Combining Solar Cells

• Solar cells can be electrically connected in series (voltages add) or in parallel (currents add) to give any desired voltage and current,

• Power (Watts) output is calculated P = I x V• Photovoltaic cells are typically sold in modules (or

panels) of 12 volts with power outputs of 50 to 100+ watts.

• These are then combined into arrays to give the total desired power or watts.

Cells, Modules, Arrays

Photovoltaic Array for Lighting

Telecommunications Tower

Remote Water Pumping

Solar Lanterns for Landscaping

The PV Market

• As prices dropped, PV began to be used for stand-alone home power.

• If you didn’t have an existing electrical line close to your property, it was cheaper to have a PV system (including batteries and a backup generator) than to connect to the grid.

• As technology advanced, grid-connected PV with net metering became possible.

Other System Components

While a major component and cost of a PV system is the array, several other components are typically needed. These include:

• The inverter – DC to AC electricity• DC and AC safety switches• Batteries (optional depending on design)• Monitor – (optional but a good idea)• Ordinary electrical meters work as net meters

PV On Homes

• PV can be added to existing roofs. • While south tilted exposure is best, flat roofs do very

well. • Even east or west facing roofs that do not have steep

slopes can work fairly well if you are doing net metering since the summer sun is so much higher and more intense than the winter sun.

• The exact performance of any PV system in any orientation is easily predictable.

Photovoltaic Array on Roof and as an Overhang

Other Mounting Systems?

• If it is impossible or you don’t want to put a PV system on your existing roof, it is possible to pole mount the arrays somewhere near the house as long as the solar exposure is good.

• Pole mounted solar arrays also have the potential to rotate to follow the sun over the day by installing a sun tracking system,

• Sun tracking systems can provides a 30% or more boost to the PV system performance.

Pole Mounted PV

Roof Integrated PV

• If you are doing new construction or a reroofing job, it is possible to make the roof itself a solar PV collector.

• This saves the cost of the roof itself, and offers a more aesthetic design.

• The new roof can be shingled or look like metal roofing. A few examples follow.

Solar Roofing Shingles

PV System Battery Sizing

Advanced ElectronicsLandstown High School STEM &

Technology Academy

Series & Parallel Circuits

Battery– A combination of two or more cells.– Negative terminal is also called the cathode,

Primary cells– Cells that cannot be recharged.

• A dry cell; also referred to as a carbon-zinc cell.• Alkaline cell.• Lithium cell.

Secondary cells– Cells that can be recharged.• Lead-acid battery or wet cell.• Nickel-Cadmium cell or Ni-Cad.

Connecting Cells and Batteries– Series• Series-aiding:

– IT = I1 = I2 (current stays the same),

– ET= E1 + E2 (voltage is added together)

– Parallel• Current expressed as IT = I1 + I2 , Current is added

together,• Voltage expressed as ET = E1 = E2, Voltage stays the

same,

Connecting batteries

• When cells and batteries are wired together in parallel then the amount of current increases,

• When cells and batteries are wired together in series then total voltage increases,

Series C

ircuit

Series CircuitWhen cells and batteries are wired together in series then total voltage increases, but the current stays the same.

Parallel CircuitAll the positive terminals are connected together, and all the negative terminals are connected together. The total current (IT) is the sum of the individual current of each cell or battery.

3A 3A 6A

Sizing a PV SystemSolar Panels

• Solar modules/panels are typically sold by the peak watt. – That means that when the sun is at its peak intensity (clear day

around midday) of 1000 watts per m2,

• a solar module/panel rating at say 100 Wp (peak watts) would put out 100 watts of power.

• The climate data at a given site summarizes the solar intensity data in terms of peak sun hours, – the effective number of hours that the sun is at that peak intensity on

an average day.

• If the average peak sun hours is 4.1, it also means that a kw of PV panels will provide 4.1 kw-hr a day.

Sizing and Calculating

• To determine the number and size of the batteries we will need, there are some thing we need to determine,– Load (number of kw being used),– Battery capacity,– Location of the panels,– Type of mounting system,

Battery Sizing I

• If your load is 10 kw-hr per day, and you want to battery to provide 2.5 days of storage, then it needs to store 25 kw-hr of extractable electrical energy,

• Since deep cycle batteries can be discharged up to 80% of capacity without harm, you need a battery with a storage of 25/0.8 = 31.25 kw-hr.

• A typical battery at 12 volts and 200 amp-hour capacity stores 2.4 kw-hr of electrical energy.

• So how many batteries would you need?

Battery Sizing II

To calculate how many batteries:• We use the relationship between battery energy (E) in kw-hr and battery

capacity (amp-hr), • E(kw-hr) =capacity(amp-hr) x voltage/1000

– E = 200 amp-hr x 12 volts/1000= 2.4 kw-hr– So for 31.25 kw-hr (2 ½ days) of storage we need

31.25 kw-hr/2.4 kw-hr/battery = 13 batteries• How many batteries would you need for only one day of storage? 13/2.5 =

– 5.2 batteries

• If we are happy with one half day, – we need only 2 or 3 batteries,

Example

• Typically, Landscape lights are rated at 20w,• If we wanted to design a PV system to run these

lights for 30 days per charge how many batteries would we need?

• 12 volt battery = • E = 200 amp-hr x 12 volts/1000= 2.4 kw-hr

• Load = 20w x 30 days = 600w/1000 = .6 kw-hr• .6 kw-hr/2.4 kw-hr = .25 batteries• So how many batteries do we need?

Thinking About Solar Energy

• When the sky is clear and it is around midday, the solar intensity is about 1000 watts per m2 or 1 kw/m2, or– In one hour, 1 square meter of the earth’s surface

facing the sun will intercept about 1 kw-hr of solar energy,

• What you collect depends upon surface orientation and collector efficiency,

Sizing a PV System to Consumption

• A PV system can be sized to provide part or all of your electrical consumption.

• If you wanted to produce 3600 kw-hr a year at a site that had an average of 4.1 peak sun hours per day,

PV Size in KWp = 3600 kw-hr 4.1 kw-hr/day x 365 days/yr x 0.9 x0.98

= 2.7 KWpNote: the 0.9 is the inverter efficiency and the

0.98 represents the loss in the wiring.

Photovoltaic Systems

Charge Controllers

Charge controllers manage interactions and energy flows between a PV array, battery bank, and electrical load.

Single-stage battery charging is simpler, but multistage battery charging brings batteries to a higher state of charge.

•Charge controllers protect batteries from overcharge by terminating or limiting charging current.

•Charge controllers protect batteries from overdischarge by disconnecting loads at low battery voltage.

Most charge controllers include displays or LEDs to indicate battery voltage, state of charge, and/or present operating mode.

Shunt charge controllers regulate charging current by short-circuiting the array.

Series charge controllers regulate charging current by opening the circuit from the array.

•Maximum power point tracking manipulates the load or output voltage of an array in order to maintain operation at or near the maximum power point under changing temperature and irradiance conditions.

•Diversionary charge controllers regulate charging current by diverting excess power to an auxiliary load when batteries are fully charged.

•Controllers designed for hybrid PV systems must manage multiple current sources simultaneously.

Photovoltaic Systems

Inverters

Inverters are available in many different configurations and ratings. Usually converting 12 volt DC power to AC.

Stand-alone inverters are connected to the battery bank and supply AC power to a distribution panel that is independent of the utility grid.

Interactive inverters are connected to the PV array and supply AC power that is synchronized with the utility grid.

BATTERIES

• Batteries can be used to provide long-term or short-term electrical supply in case of grid failure.

• Many grid-connected houses choose to have a small electrical battery system to provide loads with power for half a day in case of outage.

• Larger number of batteries are typically used for remote grid-independent systems.