How to value dust measurements
Measuring dust is a new area with new challenges. Below are a few of them explained.
Introduction
DustIQ is not an indicator of dust, but a real accurate measurement device, in line with all Kipp & Zonen instruments. The
DustIQ measurement technology is unprecedented and new to the market. Therefore some extra words for the user of the
measurement information.
One should understand that measuring physical quantities needs careful interpretation. Measuring voltage or velocity
can be done very precise, and the answer is always very close to the truth. Influenced only by the accuracy of the used
instrument. Much more difficult it gets for instance with precipitation or solar radiation measurements; there is an
intrinsic limit as to how precise one can measure it and with which uncertainty.
Measuring dust / transmission loss /soiling ratio is a new field of science. There are many opinions about it, but there is
no large history of data and experience in measurements. Although the spot measurement of DustIQ is very precise, it is
wise to do interpretation and use common sense when extrapolating the measurement value to the whole PV plant, or a
part of it, for decision-making. The coming years, Kipp & Zonen will build up this expertise together with you and our other
customers and scientific users. Visual inspection seems the most logical validation for understanding and extrapolating
the dust measurement information. Before decision making and taking cost intensive cleaning actions, it is recommended
to do visual inspection: is the DustIQ device soiled in the same way as the average soiling on the PV modules? What helps
in data validity, is the fact that the DustIQ is designed to handle local irregularities of soiling, and that the DustIQ has two
measurement sensors, so one can compare the values.
For larger areas, it is wise to mount several DustIQs, just like you now mount several pyranometers across a PV plant.
Again: for DustIQs, one can trust the measurement value to be correct, the only judgement you have to make, is, whether
this value is representative for the whole PV park.
Solar angle dependency
The actual transmission loss and soiling ratio have a solar angle dependency. The DustIQ is not aware of the solar angle,
but based our test results and literature research we have two simple formulas to calculate the daily SR and TL averages.
SR
DAILY_AVERAGE
= 1.2 x SR
DUSTIQ
– 20
and
TL
DAILY_AVERAGE
= 1.2 x TL
DUSTIQ
Background
: For the customer a very useful value is to know the weighted average value for the Soiling Ratio over a day.
Such a weighted average value is also prescribed by the IEC 61724-1 norm (page 30). We have chosen a weighting by PV
power output over the course of the day.
For this calculation, we have chosen multiple locations:
1)
Close to the Dubai Mohammed bin Rashid Al Maktoum Solar Park in Dubai.
2)
Close to planned new Solar Park in Baganuur near Ulaanbatur in Mongolia.
Dubai Mohammed bin Rashid Al Maktoum Solar Park
Hypothetical PV AC output at the 8th of august 2017 in Dubai
We assumed the panels are mounted at 22° degrees in Dubai
and soiling ratio (SR) over the day
