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When operating in a mobile outdoor environment, there are many factors that impact ability to gather solar energy. Some of the larger factors are:
- Solar Resource – On a bright sunny day, there can be approximately 1000W/m2 coming from the sun. In high altitude environments, this can go as high as 1200W/m2 while an overcast day can bring down the power significantly.
- Orientation to the sun – To maximize energy harvest, the solar module must be pointed within 20 degrees of the sun. At angles further away from +/-20, the power rolls off as a function of the cosine law (power = max power * cosine (angle))
- Behind glass – When behind either a standard window or windshield, the amount of solar power is reduced.
Based on these factors, Alta has selected 4 common use conditions that are used in the energy calculator.
- Direct sun – the device is placed in direct, bright, outdoor light and angled toward the sun. We assume 1000W/m2 in this instance.
- Indirect sun – the device is outdoors but is not actively pointed toward the sun. The device may be on a backpack, a belt, a table or otherwise moving. We assume an average of 500W/m2 for this condition.
- Window sill – the device is placed flat on a window sill in direct sun. No consideration is made to point the device to the sun. This results in an average of 200W/m2.
- Near window – the device is within a few feet of a window and will experience some direct sun during the day. Once again, no consideration is made to point the device at the sun. The average energy is estimated at 100W/m2.
These common use conditions are then used to calculate an estimate of how much solar power is incident on the solar material across a single day. For instance, if the user states they are an outdoor worker, then the total irradiance seen during the day is 1 hour at 1000W/m2 and 5 hours at 500W/m2 or 3.5kWh/m2. This number is used with the efficiency of the Alta material and the active area to determine the quantity of watt hours generated.