Irradiance Monitoring for Small Commercial Solar Farms

Solar power cells

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The role of photovoltaic monitoring systems has never been more important with the number of solar photovoltaic (PV) installations rapidly increasing across the globe. Besides large solar power facilities, a sizable proportion of the global photovoltaic output is from small-scale rooftop installations. With this arises the need for a reliable, affordable, and simple-to-use panel monitoring system.

This article discusses the current trends in the adoption of rooftop solar PV systems and the significance of panel monitoring.

Rooftop Solar Installations are the Future

Rooftop solar is on the rise. Although extensive adoption of feed-in tariffs in the early 2000’s fueled early uptake of solar technology by reducing the financial burden to investors, developments in photovoltaics are increasingly producing efficient solar cells at ever lower costs. The rate of solar uptake is increasing faster than for any other energy source. For instance, in 2016, new global solar PV capacity increased by more than 50% and exceeded 74 GW.1

For many consumers, the possibility of generating power from a rooftop solar setup is an increasingly appealing prospect, providing long-term profitability, short-term affordability, and much-needed movement away from fossil fuels. The prospect is particularly attractive to owners of office buildings, warehouses, or flats or blocks with large, unused roof areas. Recently London’s “Walkie Talkie” skyscraper was added to the long list of skyscrapers which make use of their roof space with solar PV. It is now housing a 50 kWp (Kilowatt peak) solar installation, while the average warehouse in the US is estimated to house a few hundred kilowatts of solar capacity on its roof.2,3,4

There is a huge capacity to exploit the existing roof space for solar power: recent estimates from the USA’s National Renewable Energy Laboratory propose that by using rooftop solar installations alone, the US could generate 39% of its current annual electricity consumption, while sunnier states could make 74% of their yearly consumption.5

With the advancement in technology and increased interest, the cost of solar PV technology is falling. For instance, in the UK, the cost of solar PV technology decreased by 70% between 2011 and 2016, and is expected to further decrease by 35% by 2020 [solar roofs guide]. In 2015, Chinese modules sold for less than €0.65 per Watt of capacity, and European PV modules for less than €0.80 per Watt of capacity.6

Why PV Panel Monitoring is Important

Although the costs are lower than ever before, installing a rooftop solar PV system continues to be a considerable investment, and as with any investment, effective monitoring of solar PV systems is vital to their performance.

There are several advantages of solar PV monitoring. Users can maximize the efficiency of their system by measuring the energy production throughout the day- for instance, by timing the use of high-load appliances or machinery to coincide with periods of high solar irradiance. Maximizing the self-use proportion in this way guarantees the best economic outlook as the avoided cost of purchasing electricity from the grid far exceeds the potential earnings from feeding energy into the grid.5

Monitoring can also inform users about the incorrect functioning of cells. For most smaller PV installations, this monitoring is typically done by comparing the output of one PV panel with another. Although this is useful, it doesn’t actually provide information on the level of performance of the solar installation. Some measure of solar irradiance is needed to accurately gauge if the system is functioning optimally .7

This is where the pyranometer comes in – a device which measures solar irradiance. Monitoring solar irradiance (and therefore the solar energy available to a PV installation) offers essential knowledge to make crucial decisions on performance, efficiency, future energy yield, and maintenance. Measurement of the solar energy incident on a PV panel is needed to determine the performance ratio, and thus the return on investment of any solar PV project.8

Affordable Irradiance Monitoring for Small Solar Installations

Although solar irradiance monitoring is common in larger solar PV installations, this usually involves expensive solutions that demand careful calibration. As PV gains popularity, there is an increasing demand for an affordable and user-friendly pyranometer for rooftop installations.

Taking this into account, meteorological instrument manufacturer Kipp & Zonen developed the RT1 rooftop monitoring system. It is designed to easily fit to the corner of any commercial PV panel without screws or tools. This compact device houses electronics and a silicon pyranometer, with a plug-in temperature sensor that can be attached to the back of the PV panel.

The RT1 from Kipp & Zonen

The RT1 from Kipp & Zonen

Although, the RT1 is designed to be set up and used as easily as possible, it still provides high levels of accuracy. The device can measure plane-of-array (POA) irradiance from 0 to 2000 W/m2, to within ±1 W/m2. The device can be accurately positioned: as it directly fits onto a panel, it is automatically positioned to measure POA irradiance.

This information allows rooftop solar users to optimize the position of their solar array themselves. The temperature sensor is thermally isolated from the air, and is coupled to the back of the panel using special heat-conducting 3M tape.9

Download the Brochure for More Information

The device is adequately hard-wearing to suit almost any rooftop application: it is built to operate at temperatures as high as 80 °C and as low as -40 °C, and the sensors and cables are waterproof and durable.

Recalibration is not required for 2 years once it is installed; the RT1 also comes with self-adhesive mounting pads and cable ties. All of this results in a user-friendly and inexpensive system for accurate monitoring of rooftop PV system vitals.

References and Further Reading

  1. International Energy Agency – Renewables 2017
  2. EvoEnergy finishes solar power installation on London skyscraper
  3. The Typical Warehouse
  4. BRE National Solar Centre - Solar PV on Commercial Buildings: A guide for owners and developers
  5. Rooftop Solar Photovoltaic Technical Potential in the United States: A Detailed Assessment, Pieter Gagnon et al, 2016, National Renewable Energy Laboratory
  6. WACC the dog: The effect of financing costs on the levelized cost of solar PV power, Janosch Ondraczek et al, 2015, Renewable Energy 75:888-898  
  7. Kipp & Zonen – Solar Irradiance Monitoring in Solar Energy Projects,
  8. Kipp & Zonen Newsletter 43, 2018
  9. Kipp & Zonen RT1

Kipp & Zonen logo

This information has been sourced, reviewed and adapted from materials provided by Kipp & Zonen.

For more information on this source, please visit Kipp & Zonen.


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