Natural Ventilation and Effect of Temperature on Solar Roofs

Mitigating energy demands in buildings will substantially curtail the required supply of energy and, hence, minimise greenhouse gas (GHG) emissions. Heat generation issues associated with the design and implementation of BIPV systems are fundamental problems that require systematic mitigation and development. When the surface temperature of your solar panels gets too high, solar panel efficiency can decline somewhat. Let’s investigate the effect of temperature on solar roofs.

PV thermal basics

During the operation, PV modules absorb incident solar radiation to generate electricity. Only 15%-20% of solar radiation is converted to electricity and the other staggering approx. 80% of incoming solar irradiation is absorbed by the PV panel and transferred via thermal radiation and heat convection to nearby surfaces. This represents a heat load that needs to be removed from the PV panel in order to increase performance and decrease cooling-loads. A BIPV module is always mounted close to a surface and an increase in temperature will occur due to constrained airflow around the module and reduction in heat loss by radiation because of the presence of surrounding warm surfaces.

Essentially, there are three negative effects:

1. solar radiation increases solar panel temperature, thus reducing efficiency,

2. some of the solar radiation temperature increase is transferred to the building roof and attic by natural convection and radiation,

3. badly designed and ventilated BIPV systems may become a fire risk.

Overheating of PV modules and transferring this heat into the building can inadvertently increase the cooling load and increase the power consumption for cooling equipment. Therefore it is important to take precautions so unnecessary overheating of modules can be avoided.

Factors affecting building-integrated solar (BIPV) systems

Building materials and house constructions are different from one country to another, but the photovoltaic technology is almost similar and international. PV panels have limited overall efficiency and factors that affect BIPV systems are solar radiation, PV panel size, humidity, design, placement, air-gap, wind speed, and roof ventilation strategy. In hot and humid climates, PV modules experience changes in the moisture content which will eventually have a harmful effect on the module performance. If moisture begins to penetrate the polymer and reaches the solar cell, it can weaken the interfacial adhesive bonds, resulting in delamination and increased numbers of ingress paths, loss of passivation, and corrosion of solder joints.

BIPV should maintain technical and economic requirements and aesthetical aspects prior to integrating into the building envelopes to fulfill the necessary factional requirements.

Natural ventilation of solar panels

During the summer months, the cell temperature could reach as high as 70 °C and will lead to a reduction of conversion efficiency by approx. 22.5% from standard test conditions. One method to mitigate the solar radiation load is directed natural ventilation underneath the PV. Providing the module with an air gap that allows air to flow behind the module decreases solar panel temperature and increases the performance of BIPV. Heat is transferred by convection to the air and transported away by the airflow.

Buoyancy (heat) and the wind-induced pressure difference between the top and bottom of the air gap drive the air. Higher flow rate in the air gap minimises the cell temperature. Makes sense, right? The air gaps should be designed to maximise the airflow while maintaining an evenly distributed and stable airflow. Not uniformly distributed airflow can lead to the occurrence of “hot spots”. Instability of the flow may lead to partly reversed flow.

If the air gap is small, the temperature remains high due to flow resistance inside the air cavity. As the air cavity depth increases, the temperature of surrounding air and solar panels drops. Studies have found that air gap between 10-12,5 cm is optimal to provide the lowest cell temperature. As the wind speed increases, the average cell temperature decreases and there can be noted significant drop when the wind speed is higher than 1 m/s.

Solarstone®’s approach to reduce solar roof temperature

In terms of natural ventilation, it comes down to the system design and air gap. Solarstone always advises builders to install a watertight breathable underlay which evacuates all moisture out of the building structure. High quality underlay should be used (in accordance with harmonised European Standard BS EN 13859-1:2014) with a high resistance and stiffness against wind uploads. For improved ventilation Solarstone recommends using following batten specifications.

• Ventilation Battens: 45…53 x 45…53mm

• Counter Battens: 45…53 x 95…103mm

After fixing the starter clamps, a ventilation profile should be installed which acts as eaves filler anti-bird comb. The profile prevents the entry of birds and vermin into the batten cavity and allows airflow into the underlay and batten cavity.

Sources:

1. Simulation Analysis of a Ventilated Building Integrated-Photovoltaics Air-Gap Duct System for Natural Ventilation of a Building

2. State-of-the-Art Technologies for Building-Integrated Photovoltaic Systems

3. Effect of Temperature and Humidity on the Degradation Rate of Multicrystalline Silicon Photovoltaic Module

4. Cooling Of Building Integrated Photovoltaics By Ventilation Air

5. An Investigation on Ventilation of Building-Integrated Photovoltaics System Using Numerical Modeling