Riaan Meyer, managing director of GeoSUN Africa – a Stellenbosch University spin-off company, is an expert in solar resource assessment and has been working in this field since 2006. In this industry insight, Meyer gives focus to on-site solar measurement for solar photovoltaic (PV) projects.
On-site ground measurement station
Traditionally, most utility scale solar PV project developers install an on-site ground measurement station for at least one year or longer on their site. The station typically consists of high accuracy solar equipment as well as general meteorological equipment.
The next step is to correlate the ground measurements with multi-year site specific hourly satellite derived data. This method allows the best of both worlds – obtaining the long term inter-annual variation and trends from the satellite derived data as well as pinning down the exact values for the specific site based on the ground measurements.
In certain regions this procedure is not followed. In Europe, a relatively small continent, a high density of well-maintained weather stations exist with publicly available data. In well-established solar markets like South Africa, some of the satellite derived data sets have been verified against a large number of weather stations to the extent that they are trusted by banks.
In most other African countries this is not the case. There is a general lack of good publicly available ground data, which means the satellite derived models are only verified at a small number of locations. The result is a large uptake in on-site ground measurements at PV sites under development in various African countries.
Apart from providing accurate solar data, these stations have various other advantages:
- Site specific wind gust measurements allow for better design of mounting structures.
- The output of a PV panel decreases below its rated output as its temperature increases during operation. This is now more accurately be modeled with available wind speed and ambient temperature values.
- Ambient temperature and other data is also useful in the specification of inverters and other components.
- Precipitation is typically measured with a one minute temporal resolution which allows the better design of onsite storm water infrastructure – useful from high precipitation areas like Rwanda to dry desert areas that experiencing periodic extreme precipitation bursts like Jordan.
Soiling-what is it?
One of the parameters with the highest uncertainty is soiling. It is a site specific parameter that can only be determined by onsite measurements – typically by deploying two similar PV panels or reference cells.
The one panel is cleaned on a regular basis while the other is left to soil. The difference in output from the two panels are a representative indication of not only the soiling rate, but also the effect of soiling on PV production.
West African measurement stations
GeoSUN Africa has various solar measurement stations in northern Nigeria and Ghana, all with soiling measurements.
The soiling effect of the Harmattan, dusty trade wind that blows over the West African subcontinent, is captured at each station.
This north-easterly wind blows from the Sahara Desert into the Gulf of Guinea between the end of November and the middle of March. This corresponds to the dry season of Northern Ghana and Nigeria.
From the ground measurements, it is observed as expected, that the highest soiling rates occur during the Harmattan period with monthly soiling rates as high as 17%.
This means that a PV plant operated in this region will lose up to 17% of its output during a specific month if not cleaned.
By having on-site soiling measurements, the effect of Harmattan can be quantified and more importantly, mitigated by incorporation in the yield calculations, financial model and O&M budget.
Meyer consults developers and operators of solar power plants, investors, financial and governmental institutions. Meyer remains in close collaboration with the University of Stellenbosch and is involved in teaching post graduate modules in renewable and specifically solar energy.