III: Photovoltaic Fundamentals:

Distributed Generation:

  Solar Energy is a form of 'Distributed Generation.'  Distributed generation refers to the small-scale production of electricity on a local level, rather than large-scale industrial power plants.  This is factor of vital importance because centralized energy production creates situations where power companies are able to control the price of electricity and where the multitude of the masses are dependent on one source controlled by one company for electricity.  Just as small-scale food production on a local level reduces our collective dependency on large-scale agriculture and the multinational conglomerations, so does solar reduce our dependency on coal and nuclear energy.  Distributed generation also implies a higher level of energy security and local resiliency, as well as a lower impact on the environment.  In addition, "efficiency gains no longer come from increasing generating capacity, but from smaller units located closer to sites of demand." (Takahashi, et al; Policy Options to Support Distributed Resources; U. of Del., Ctr. for Energy & Env. Policy; 2005) In other words, it is also more efficient to have a distributed generation grid rather than large centralized plants.  

Basics of Electricity:

 Watts = Volts x Amps

   This is the most fundamental equation of electricity.  Voltage is considered to be electromagnetic force.  Amperage is the current.  Watts are the usable energy and the product of electromagnetic force and current.  The equation may be manipulated easily by dividing watts by either volts or amps in order to find the missing variable. 

   The hydraulic metaphor is often used to understand electricity.  For example, a wide massive river with a slow current is similar to a high voltage, low amperage flow of electricity.  A waterfall with small amounts of water falling very fast down a cliff is said to be high amperage yet low voltage.  A strong river with white water rapids would be both high voltage and high amperage. 

   A concrete example is that a 60-watt light bulb running on a 120v system will draw a current of 0.5 amps.  However, a 60-watt bulb running on a 12v system will draw a current of 5 amps. 

   What is necessary to understand is that all photovoltaics put out Direct Current, or DC, electricity.  An inverter is what changes the waveform of the electricity from DC to AC (Alternating Current), so that it may be used for residences and businesses. Most modern electrical grids run on AC electricity and the DC must be changed to match the grid waveform before it can be interconnected. 

   Key Photovoltaic Terminology to Understand:

 PV Module:  A module is a solar panel.  Each module is made up of solar cells of crystalline-silicon.  The back is glass and the sides are aluminum.  The term 'solar panel' was originally used to describe a collection of modules placed on a racking system and pre-wired, which was then shipped to the job site.  The term has stuck and the words 'module' and 'panel' are now used interchangeably, however, it is more correct to say 'module' when talking about a single solar panel and an ‘array’ as a collection of modules.  

 Standard Test Conditions (STC): STC is defined as 1,000W/m2 @ 25C (77F) @ 1.5 Am.  This is the laboratory testing of each module, which gives an output rating of Voltage Open Current (Voc) and Current Short Circuit (Isc).  This rating system does not include any derate values, therefore, it does not reflect the actual output of the PV array.  Other rating systems, such as the CEC (California Energy Commission) and PTC (PV USA Test Conditions) provide more accurate output ratings based on more realistic conditions during the test. 

 Balance of System (BOS) Components:  BOS components refer to the hardware used to complete the installation of a solar energy system.  BOS does not refer to the modules or the inverter, but rather the smaller pieces used for construction, such as waterproof flashings for attaching the base of the rack to the roof rafters and specialized bolts.

 PV String & PV Array:  A string is a group of modules wired together.  An array is one or more strings wired together that create an entire system.  Most residential solar systems are comprised of 2 strings of solar modules wired in series, which are then connected in parallel, which creates a high voltage and low amperage PV array.

 Temperature Coefficient:  Solar modules put out more power when the temperature is lower.  PV systems must be designed to account for the maximum possible voltage, thus, we find the temperature coefficient on a chart that associates to the coldest temperature ever recorded at the site location (i.e. the maximum possible voltage that an array could produce while receiving a peak sun hour on a very cold day).  This number is used in equations to size PV arrays. 

 An Astronomical Unit (AU) is 93 million miles, or the distance between the Earth and the Sun.  The speed of light is 186,000 miles/second making solar radiation take around 8.3 minutes to reach Earth.

 I-V Curve: The Max Power Point Tracker (MPPT) continuously tracks the amperage (I) and voltage (V) intake from the system.  The MPPT balances the volts and amps to create the highest output Wattage in real time.  The I-V curve shows the volts and amps on an XY axis graph. 

 Solar Irradiance:

 Solar radiation power per unit area, or watts per meter squared (W/m2). 

 Solar Irradiation: 

Solar radiation power per unit area per amount of time, or watt-hours per meter squared (Wh/m2).

          For example:  600 W/m2 for 8 hours equals 4,800Wh/m2 or 4.8kWh/m2. 

 Solar Constant:

Irradiance value for Earth @ 1,366W/m2.  This could also be understood as how powerful photons from our Sun are at a distance of 1 Astronomical Unit. 

 Air Mass:

Distance of atmosphere that sunlight travels through.  Air Mass is equal to 1 when the sun is directly overhead at sea level.  Air mass is effected by the time and day of year, as well as by the latitude, longitude and elevation of a specific location. 

 Peak Sun:

The solar irradiance that reaches the Earth's surface.  This is equal to 2/3 the solar constant @ 1,000W/m2. 

 Peak Sun Hours:

Amount of hours of peak sun per day that a specific location receives.  Santa Cruz CA is at a latitude of 36 degrees North and receives around 5 peak sun hours per day.  This is a key element to the design of PV systems. 

 Insolation:

Killowatt hours per meter squared per day.  (kWh/m2/day)

 Solar Declination:

Plus or minus 23.5 degrees.  This is also the tilt of the Earth and is constituted by the seasonal shift in tilt as we orbit the sun annually.  Summer solstice has a +23.5 solar declination, the equinoxes have a declination of 0, and the Winter solstice has a solar declination of -23.5 degrees.  In addition, 23.5 degrees North creates the Tropic of Cancer, whereas 23.5 degrees South creates the Tropic of Capricorn. 

 Solar Time:

A time scale based upon the apparent motion of the sun through the sky.  15 degrees longitude is one solar hour.  Thus, plus or minus 15 degrees of longitude constitutes 60 minutes of solar time, or, changes a 'time zone.'  Greenwhich, England is the prime meridian.

 Solar Altitude Angle:

The angle between the observer, the horizon and the sun.  This is between 0 and 90 degrees.  This is basic trigonometry. 

 Solar Azimuth Angle:

The angle of the sun traveling horizontally from East to West.  S=0; W= -90; E=90; N= +/- 180 

 Solar Window:

Area of sky between Summer and Winter solstices.  This is critical for photovoltaic array orientation and optimization. 

 Efficiency:

Efficiency, in terms of solar energy, is the conversion of photons to electrons.

 Crystalline Silicon:  (CSi) Mined from quartz and silica sands.  This is the raw material for making solar modules and was used to make microprocessors for computers in California in the 1980's and 90's creating the name 'Silicon Valley.'