Solar as a Thermal Shield

Wednesday, November 25, 2020
Training

Are there any additional benefits to installing a roof mounted solar panel array, beyond the energy generation?

What solar panels usually do...

When a business starts investigating the viability of solar it is usually along the lines of pay back period, return on investment, basically how much they can save on their electricity bills. We all understand that panels capture some of the abundant energy from the sun and convert it into the power that runs your home or business

But, roof mounted solar panels can actually do more than that.

In addition to utilising those rays of the sun to convert to usable energy, the physical nature of the panel also acts as a thermal shield in roof mounted situations. It effectively acts as a barrier between the sun and the roof. Let's explore that a bit more...

Solar panels as a thermal shield

This thermal shield effect results in a cooling effect for your business (or home), which reduces the HVAC (Heating Cooling Air conditioning Ventilation) loads of the building. The process of heating and cooling a building is often one of, if not THE, biggest contributors to a buildings energy expenditure. According to the HVAC section over at energy.gov.au:

"HVAC systems typically account for as much as 30% of energy use and costs in commercial buildings and office spaces. Several technologies for heating and cooling exist, including reverse cycle air conditioners, electric space heaters, wood-burners and ducted gas heating. The energy efficiency and emissions intensity of these technologies can vary substantially."

The evidence

In a study conducted by Jan Kleissl, a professor of environmental engineering at the UC San Diego Jacobs School of Engineering, the professor and his team published what they believe are the first peer-reviewed measurements of the cooling benefits provided by solar photovoltaic panels. 

Using thermal imaging, the team of researchers determined that during the day, a building's ceiling was up to 5 degrees Fahrenheit (or ~2.78°c) cooler under solar panels than under an exposed roof. At night, the panels help hold heat in, reducing heating costs in the winter.

The data

Data for the study was gathered over three days in April on the roof of the Powell Structural Systems Laboratory at the Jacobs School of Engineering with a thermal infrared camera. The building is equipped with tilted solar panels and solar panels that are flush with the roof. Some portions of the roof are not covered by panels.

What they found was for the building researchers analysed, the panels reduced the amount of heat reaching the roof by about 38 percent!

Thermal infrared image of the ceiling underneath a solar panel array. (UC San Diego)

But... don't solar panels generate heat?

As far as your building is concerned, the short answer is "no".

Instead, solar panel roof arrays block heat from being absorbed by your roof— and eventually your home or business— and actually prevent extra heat from being generated.

When there is a gap between the bottom of the panel and the roof, air can circulate, reducing roof temperatures even further. 

How do they keep the roof cool?

According to the solar nerd there are three ways this can be achieved:

  1. The conversion of energy
  2. Reflection
  3. Emittance

Energy conversion

Domestic solar panels have an efficiency between 16 - 20% and this means that 16 - 20% of the energy is absorbed by the panel and converted into electricity. Basically solar panels not only reduce the amount of heat striking the roof surface but also power loads on your site.

Reflection

Ideally solar panel design reduces reflection to a minimum for obvious reasons but there is a certain amount of sunlight that is reflected away. In addition the aluminium frame also plays a role in reflecting away a certain percentage of the light energy striking its surface.


Emittance

Solar panels also absorb energy causing it to heat up, and some of that energy gets re-emitted back up to the sky. Unfortunately, some of that heat also gets emitted downward toward the roof surface, where it can potentially heat up the building but this is negated by the wind flow that is channelled through the gap between the underside of the panels and the roof.

Other benefits

An additional benefit not immediately apparent is a decrease in the amount of UV light and heat that can accelerate the aging of your roof, so by keeping this part of the building structure shaded and cooler, solar panels will help prolong the life of your roof.

Conclusion

The decision for your business to go down the solar road must be built on a careful assessment of the financial viability of such an endeavour. This is not just an assessment from the point of view of reduced electricity consumption from the grid but also from the additional benefits that solar panels offer in the way they act as a thermal shield, therefore reducing the amount of sunlight striking the roof space leading to reductions in HVAC loads.

Remember a commercial solar system will negate energy from the grid and will save you money and allow, by default, investment in other areas of your business. Exercise due diligence, be prudent in your cost benefit analysis of the options before you make the best decision for you and your business.


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