How to calculate how many solar modules you need |

March 31, 2020

How much solar power do you need?

How many modules are needed for this?

What is the efficiency?

How would you calculate?

Solar Power Professional Development Course for Engineers Sizing a PV System Calculate the required PV power capacity based on – the energy consumption of the load and – the number of peak sun hours per day (at 1000W/m2) Example : Annual energy consumption of the load = 9125 kWh/year = 25 kWh/day Let’s say 50% of energy consumption will be provided by PV PV system energy output = 12.5 kWh/day Now determine the number of peak sun hours available per day at the installation location. There are two ways of doing this…

First option: Look up the number of peak sun hours available per day on a map or chart such as the ones shown below. Use the value of the worst month of the year for critical applications. World Insolation Map: This map the amount of energy in hours, received each day on an optimally inclined surface during the worst month of the year, Sun Hours Available Per Day (Canada) The table shows the number of sun hours at peak solar irradiance (1000 W/m2) during the summer, winter and over the year (average). It is more difficult to produce electricity during the winter because of shorter days, increased cloudiness and the sun’s lower position in the sky. If you are using your system primarily in the summer, use the summer value. If you are using your system year-round, especially for a critical application, use the winter value. If you are using the system most of the year (spring, summer and fall) or the application is not critical, use the average value. Sun Hours Available Per Day (USA)

Second option: Calculate the number of peak sun hours available per day based on the Global Horizontal Irradiation map. This will give you the yearly average value. GLOBAL HORIZONTAL IRRADIATION (GHI) Montreal – GHI (daily) = 3.5 kWh/m2*day x 1000W/kW x 1 m2/1000W = 3.5 hrs/day GHI (annual) = 1275 kWh/m2*year x 1 year/365 days x 1000W/kW x 1m2/1000W = = 3.5 hrs/day NOTE: We obtained the same result as the average value listed in this table using the Global Horizontal Irradiation map. We will use the number of sun hours available per day in Montreal during the winter as indicated in the table. PV system energy output = 12.5 kWh / day # of hours of peak sunshine / day (Montreal, winter) = 2 3 h PV Power required = 12.5 kWh/day x 1 day/23 h = 5.4 kW #2: Take into account the losses found in the overall system, including those due to the conversion of direct current into alternating current.

Derating Factors Pmax Multiplier PV module nameplate DC rating = 0.95 Tilt Factor / Orientation Adjustment = 1.00 Inverter and Transformer = 0.95 Mismatch = 0.98 Diodes and connections = 0.99 DC wiring = 0.98 AC wiring = 0.99 Soiling = 0.95 System availabilty = 0.98 Shading = 0.95 Sun-tracking = 1.00 Age = 0.99 Overall Pmax Multiplier = 0.74 This means that a module having a rated capacity of 100W, for example, would in reality produce 74W because of the losses found in the system. PV installed capacity = Power required/Derating Factor = 5.4 kW/0.74 = 7.3 kW #3: Choose a module. Correct Pmax of the solar module for the actual operating temperature Sample Commercial Module Specification (Canadian Solar) Nominal Max Power (Pmax) = 285W Measured at 25 degrees C TEMPERATURE CHARACTERISTICS Temperature Coefficient (Pmax) Nominal Module Operating Temperature (NMOT) These coefficients allow us to calculate Pmax under the operating conditions, namely, Module Temp. = NMOT (Nominal temperature) The power output of a solar module can be estimated using the following equation delta T = Temperature increase compared to standard conditions, 25 degrees C a = Temperature coefficient Pmax (%/degree C) Temperature coefficient (Pmax) = -039 % / degree C Nominal temperature (NMOT) = 43C Pmax (at 43C) = 265 W per module A few words on the NMOT…

In reality, the modules operate at higher temperatures and slightly lower solar irradiation conditions than those used in standard tests. (Namely, 25C and 1000 W/m2) And as we have just seen, the NMOT is used to determine the power output of the solar module. The NMOT is measured under the following conditions, Solar irradiance = 800 W/m2 Air temperature = 20C Wind speed = 1 m/s Mounting open back side. This definition tells us that the the cell varies according to:

  • solar irradiation,
  • ambient temperature,
  • module cooling The graph on the right shows the difference between the cell temperature and the ambient temperature (see y-axis) at different irradiation levels for a well cooled module, a poorly cooled module and a typical module.

The temperature of the module will be lower than shown in the figure if the wind speed is 1 m/s and higher under calmer conditions. The following formula can be used to estimate the temperature of the cell NOCT = Normal Operating Cell Temperature in degrees Celcuis, S = irradiation in mW/cm2 Red line = Plexiglass with air gap Black line = Typical Average Module Blue Line = Aluminum Finned Substrate Now back to our example… #4 Calculate the number of PV modules required Number of modules = PV Installed Capacity / Pmax (T=Actual Operating or NMOT) = 7 300 W / 265 W per module = 28 modules The number of modules will decrease if the efficiency increases thanks to a higher Pmax. The size of the installation will become smaller if the efficiency increases thanks to a smaller collector area.

Researchers are constantly working to improve the efficiency of solar cells. Each new wave of solar cells gives rise to a new generation of photovoltaic devices. Today, there exists three generations. Now that we’ve seen some basic calculations and understand efficiency We think the time has come to ask the question What different types of cells exist today and how efficient are they?


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