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Solar Irradiance Measurements

Irradiance refers to the power of solar radiation per unit area on a surface. Global irradiance on a horizontal surface is comprised of direct irradiance and diffuse irradiance. On a tilted surface, the reflected irradiance from the ground also contributes to the total global irradiance. Generally, solar irradiance is called insolation.

Solar radiation affects many systems in the house and can vary considerably within the same town. On-site, solar irradiance is a particularly useful measurement if there is a photovoltaic (PV) system or solar thermal system installed at the field test location. Also, if space conditioning is a focus of the field test, solar irradiance is an important measurement as solar radiation has a large effect on heating and cooling load requirements, electrical lighting demand (daylighting), and envelope performance.

There are three types of sensors can be used to measure irradiance, all with a specific purpose: Pyranometers, Pyrgeometers and Photometers.

Pyranometer

A pyranometer is a device used to measure solar irradiance, or insolation, both direct and diffuse on a planar surface. Pyranometers are typically passive devices, requiring no power to operate. Pyranometers use a black-coated thermopile that will absorb all solar radiation across a wide wavelength range. A glass dome limits the radiation to the short wave range only. The thermopile generates a voltage signal that is proportional to the incident solar radiation. Alternatively, a silicon photovoltaic detector can be used to measure the incoming radiation for wavelengths, which generates a current signal that is converted to a voltage signal using a potentiometer. The variable resistance of the potentiometer is used to calibrate the sensitivity for the photovoltaic sensors.

Photo of a thermopile-type pyranometer shown with a field of solar panels in the background. The device is cylindrically-shaped, with a small glass dome on the top in the middle, above a surface that slopes downward. Photo of a Photovoltaic detector-type pyranometer shown attached to a round surface. The sensor in the middle is black with a small round white area in the middle and have a cord protruding from the back.

Thermopile-type pyranometer
PIX #15537

Photovoltaic detector-type pyranometer.
Lieko Earle

Pyranometers are a common sensor for field tests, especially when photovoltaic (PV) panels are installed at the house, or cooling loads are a main subject of the field test. Typically, two pyranometers will be installed: one mounted on a horizontal surface and a second affixed to one of the PV panels (if applicable) in order to measure the available short wave radiation in the plane of the collector. The horizontally mounted pyranometer should be leveled using adjustable feet and the level sensor (shown in the photo above, right). Pyranometers can be used to determine the installed efficiency of PV systems, since the available solar radiation in the plane of the PV panels will be measured. Additionally, insolation can have a large effect on the cooling and heating loads in the house and capturing the on-site insolation, in addition to outdoor temperature and humidity, will help complete the picture of the environmental effects on the home's HVAC system.

Pyrgeometer

To measure radiant heat transfer, such as night-sky radiation or the sun hitting a solar thermal system, use a pyrgeometer. A pyrgeometer is a passive device used to measure far infrared radiation. A thermopile sensor detects infrared radiation, but a coated silicon shield over the sensor blocks the shortwave radiation. Shortwave radiation is measured with a pyranometer. Pyrgeometers output a voltage signal that is proportional to the radiation exchange between the sensor and the sky. They provide an averaged measurement across the field of view, which would include effects of partial cloud cover, etc.

Photometer

Photometers, also referred to as light meters or lux meters, are used to measure light intensity inside a building. Hand held photometers are often used to spot check the light levels in a room or on a particular surface. Photometers can also be installed as a part as a lighting control system to automatically dim the lights based on the amount of daylight.

Photo of a handheld lux meter. Two pieces connected by a chord include the sensor and a digital display. Photo shows a buildings researcher standing with a photo meter pointed at a ceiling to measure light.

A handheld lux meter
Tilt u, Wikipedia Commons

A researcher checks the quantity of daylight and the
accuracy of the lighting sensors
NREL/PIX #05171

Photometers can be used in a variety of ways in a field test. Photometers could be installed as a means of detecting occupancy. Photometers could be installed in several rooms in the house and when lights are turned on, occupancy can be inferred. For that application, the light meters should be installed on a wall without direct sunlight, changes in natural light will not trigger the light sensor.

Another application for light meters in a field test is for verification of a lighting control system. Sophisticated lighting control systems use photometers to adjust room lighting levels as the natural light entering the room changes.  To verify that the lighting control system is working as intended, the photometers for the field test should be installed by the researchers next to the photometers used for control. If the appropriate lighting levels (as prescribed by the lighting control system) are being maintained, the control system is working as intended.

Photo shows a closeup of a light sensor. Photo shows a closeup of a field-test grade photo meter.

Light sensor for daylighting in NREL's
Research Support Facility
Dennis Schroeder

Field-test grade photometer

The handheld photometers generally do not produce an analog output. They could be used for a quick verification of a lighting control system, but would not be suitable for occupancy detection, which would require a long-term installation and an output signal. There are other options for photometers that send an analog signal that can be read by a data logger. The photo above, right, shows a photometric sensor that uses a silicon photodiode that produces a μA signal that is proportional to the incident light intensity.