Rotating Shadowband Radionometer (RSR) are a cheap and precise way to measure the solar energy resource (global, diffuse and direct radiation) with a silicon photodiode radiometer and a shadow band.
This system was developed by Dr. Edward Kern, MIT professor who I personally met few years ago, who founded in 2003 Irradiance, Inc. the company which manufacture the Rotating Shadowband Radionometers (RSR2).
Rotating Shadowband Radionometer (RSR2),Image from Irradiance
The Irradiance RSR2 head unit uses a rotating curved band and a single, fast-response, photodiode sensor to measure global and diffuse sunlight. Direct sunlight is calculated by Irradiance’s computer program onboard the Campbell Scientific data logger. It includes a head unit, motor controller, temperature/relative humidity sensor, data logger, PV/battery power supply, cellular modem for remote data access, and stable, light-weight tripod.
The RSR system could offer a cost-effective solution to obtain GHI, DHI, and DNI data at the project location with however a low decrease of the quality. But, since the project designs may incorporate either fixed tilt or tracking technology, this provides the flexibility to accommodate either solution. The ability of the RSR system to measure multiple irradiance components will allow the calculation of irradiance on a specified plane of array for specific project design while maintaining a relatively low measurement uncertainty. If measurement equipment is installed to measure strictly GHI or irradiance on a given plane at a given tilt angle, there may be additional risk assumed due to the potential of project design change. This risk is mitigated somewhat through the use of a system to measure multiple irradiance components.
The optimal solution would be the installation of a tracking pyrheliometer coupled with a RSR or other pyranometers to collect high accuracy DNI data. However, it should be analyzed the cost and required maintenance due to the incursion of additional equipment. So this is mainly dependent on the site accessibility and the frequency of maintenance.