CREST investigates the benefits of AirShield® DNI in a less obvious climate
Published: Thursday, January 10, 2019
We, the Centre for Renewable Energy Systems Technology (CREST) of Loughborough University, were one of the first to install the AirShield® DNI for pyrheliometers since Kipp & Zonen redesigned it and started manufacture. The Applied Photovoltaics group at CREST specializes in improving the performance and reliability of PV systems. Having reliable and long term solar irradiance data is invaluable for our work.
The CREST research centre has measured irradiance outdoors at Loughborough in the East Midlands region of the United Kingdom for 15 years. The main focus of the group is to improve the accuracy of solar photovoltaic performance monitoring. There are two aspects to this, one is to characterize the performance of nearby PV panels under test and the second is to improve the way irradiance data is processed for use in system design and performance prediction.
For example, we recently published a paper addressing the bias which is introduced when data is interpolated between geographical locations and when data is averaged into hourly datasets. This work demanded that we have high temporal resolution of direct sun and diffuse sky irradiances to validate our new methodology.
Having our own irradiance monitoring system means that we can measure at very high temporal resolution (up to 4Hz), whereas the fastest publicly available data is at 1 minute (0.16Hz). This high temporal resolution is important for photovoltaic applications since solar cells change their performance instantaneously in response to clouds and shading. Thermal lag of thermopile sensors must obviously be taken into consideration.
Pyheliometer soiling issues
CREST operates two pyrheliometers and two pyranometers on a SOLYS2 precision sun tracker; to measure direct, diffuse and global irradiances. The pyranometers are fitted with ventilation units and our experience is that the outer window on a pyrheliometer tends to become soiled much more quickly than the domes of our pyranometers. The pyranometers are exposed to rain, sleet and snow which have a cleaning effect; whereas the pyrheliometers have a shield that protects the window from the precipitation. In addition, the pyrheliometers are pointed slightly downwards at night by the sun tracker and the windows are not exposed.
Our equipment is located on an exposed rooftop location to minimize shading effects, but this means that daily cleaning can be challenging to achieve safely at certain times of the year, so CREST is investigating ways to reduce the cleaning requirements for our systems.
We understand that the AirShield was originally designed for dusty climates, but we thought it would be interesting to try it and to measure the difference in dirt accumulation on pyrheliometers with and without the AirShield. We will also be checking whether the AirShield makes any reduction in condensation or frosting of the windows. We may experiment in the future with adding a small heater at the fan intake as is done in the pyranometer ventilation units.
We like the design of the AirShield because standardized field-replaceable components have been used for the air supply unit and hose. The system was easy to install and was performed in two hours by two technicians. The 12 volt blower fan and separate power supply meant that we had the option of supplying the system from a 230 VAC or 12 VDC supply, depending on safety and other design requirements. No modifications to the AirShield were required to achieve a robust installation.
If the trial is successful, then we will consider installing a second AirShield DNI for our other pyrheliometer.
For more information on the group’s research go to:
www.lboro.ac.uk/crest
AirShield DNI installed and almost ready to operate
By Brian Goss, Ian Cole and Patrick Isherwood, Centre for Renewable Energy Systems Technology (CREST), Loughborough University, UK