Pyrheliometer research

[originalfoo]

There will need to be at least 2 sensors for pyrheliometer - one with a collimator, one without.

GHI = DNI + DIF

Where:

• GHI = Global Horizontal Irradiance
• DNI = Direct Normal Irradiance
• DIF = Diffuse Horizontal Irradiance

Output units: W/m2 or kWh/m2/day

The total amount of solar energy that falls over a given time is called the insolation. Insolation is a measure of energy. It is the power of the sun added up over some time period. – source

Global Horizontal Irradiance is the total amount of shortwave radiation received from above by a horizontal surface. This value is of particular interest to photovoltaic installations and includes both Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DIF).

Direct Normal Irradiance is the amount of solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky. Typically, you can maximize the amount of irradiance annually received by a surface by keeping it normal to incoming radiation. This quantity is of particular interest to concentrating solar thermal installations and installations that track the position of the sun.

Diffuse Horizontal Irradiance is the amount of radiation received per unit area by a surface (not subject to any shade or shadow) that does not arrive on a direct path from the sun, but has been scattered by molecules and particles in the atmosphere and comes equally from all directions.

source

In terms of sensors, we'd be getting GHI and DNI (collimated sensor) then calculating DIF.

DIF = GHI - DNI

The collimated sensor (DNI) will need to be on the heliotropic platter so it can be pointed at the sun.

Does the GHI measurement need taking at the same time? Unsure. Assuming several readings will be taken over a period of time (need to investigate) and averaged, simultaneous readings might not be critical.

Assuming readings don't need taking at same time, the best location for the GHI sensor is in the middle of the heliotropic platter on the top side with solar panels. The GHI sensor can't go on the same side as the DNI because the camera lens will be blocking light from at least some directions, leading to flawed measurements. Similar issues with putting it on the outer rim. Only feasible location is the centre of the top side of the platter. The platter would need to be perfectly level when taking GHI measurement. The GHI sensor will need to be wide-angle (ambient light sensor) and not obstructed by solar cells.

[originalfoo]

source

The SI unit of irradiance is watt per square meter (W/m2).

An alternate unit of measure is the Langley (1 thermochemical calorie per square centimeter or 41,840 J/m2) per unit time.

The solar energy industry uses watt-hour per square metre (Wh/m2) per unit time. The relation to the SI unit is thus: 1 kW/m2 = 24 kWh/m2/day = 8760 kWh/m2/year.

Irradiance can also be expressed in Suns, where one Sun equals 1000 W/m2 at the point of arrival.

[originalfoo]

Technically, it's the pyranometer that measures GHI