A major challenge facing the solar industry today is thermal dissipation. Photovoltaics (PVs), or solar cells, found in large solar panels convert sunlight into electricity. But during this conversion, PVs mounted on modules generate considerable heat.

Until recently, the solar industry hasn’t given much attention to PCBs as a way to effectively resolve these thermal issues. However, PCBs have a long and well-documented track record for dissipating heat in an endless array of thermal-intensive applications. Given this history, there are compelling reasons for the solar industry to consider adopting PCBs for PV technology.

Fig. 2 Ground Pour Around PCB Circuitry is a Thermal Management Technique

The most popular way to create solar panels today is to use modules to mount hundreds of PVs. Vendors are using a variety of technologies to create the PVs and different materials to make the mounting modules. Consequently, the state of solar technology is in flux with vendors attempting to create the best product at the best cost, but with greater heat dissipation capability.

There are several different ways PCBs can be produced to deal with thermal issues. One is to create a metal-core PCB with aluminum foil laminated to one side. This acts as a broad heat sink that can dissipate the heat. Other methods include creating a chassis-ground, using aluminum shields, attaching heat sinks over the PV cells, increasing the amount of copper on the PCB’s surface, using a copper-clad PCB without a solder mask, deploying high-temperature materials, and, in highly complex solar applications, applying forced air convention or water-based cooling.

Background

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PVs are essentially large-area semiconductor diodes. The sun irradiates these diodes and the photon energy produced is converted into electrical power. The PV cell consists of two or more thin layers of semiconducting material, usually silicon. When the silicon is exposed to light, the electrical charges that are generated can be conducted away as direct current. The electrical output from a single cell is small, so multiple cells are connected together and encapsulated to form a module, normally called a “panel.” The PV module is used to create a PV system and any number of modules can be connected together to give the desired electrical output. PV cells are interconnected into a solar module with different levels of power ranging from 100 to 200 and on to 500 watts or more. In large PV systems, PV modules are produced with typical power of several hundred watts at a time. That high level of power also generates considerable heat. With that intensity of power from a small PV device, certain techniques must be applied to mitigate the enormity of that heat.

A PV module is the basic element of a complete power generation system. That module can have many cells interconnected either in-series or in-parallel. This depends on module configuration, desired voltage, and the current parameters of the solar module. The cells are connected between a polyvinyl plate on the bottom and tapered glass on the top. The cell itself is a combination of alloy and glass. Through different chemistries, these components are connected together with thin contacts on the upper side of the semiconductor material.

Fig. 3 Thieving Balances Copper Density, Plus Dissipates Heat

Heat is dissipated from a PV module interconnect system by a heat dissipating assembly or system. This module consists of a heat dissipation portion on at least one side, the one closest to the heat-emitting diode. Sometimes, the heat-emitting electrical component has a heat sink attached to it. This dissipates the heat by using the sink attached to the heat dissipating device.

This heat-dissipating portion is on top of the cell’s polyvinyl plate so that it comes in either direct contact or in extremely close proximity. The dissipation enclosure, or chassis, is often configured for receiving the heat being dissipated so that the heat dissipation can be done effectively utilizing that chassis or frame. One factor to consider is that heat needs to be dissipated as soon as it is generated so that the PV system can work efficiently.

Deploying PCB techniques

The PV conversion factor from solar energy to electricity is relatively small, between 15 and 20 percent. In lay terms, only a small portion of the sunlight striking a PV cell gets converted into electricity. The other 80 percent is wasted. If that is the case, then the goal is to increase the conversion factor as much as possible.

Solar researchers are attempting to improve the PV cell so it is able to receive considerably more concentrated solar energy. In turn, this means more heat is being created because the PV angles are changed in such a fashion as to be able to capture more of the solar energy. But at the same time, they are generating more heat as a result of increasing the conversion factor.

Regardless, the objective remains to initially increase the conversion factor, even by two to five percent. That can mean a savings of millions of cells and millions of dollars. With this new scenario, there will definitely be a major emphasis on reducing the heat generated by this increased conversion factor.

Thermal issues are common between PV modules and PCBs that are heavy current-based, highly populated with light-emitting diodes (LEDs), or analog designs that require heavy current applications. In this comparison, there is considerable knowhow, expertise, and proven track records solar panel manufacturers can draw from the PCB field to minimize or resolve their thermal management challenges. In particular, techniques used by EMS providers and contract manufacturers for PCB heat dissipation can be applied to solar panel manufacturing.

Heat sinks are the tried-and-true method for dissipating heat in a PCB design, and, in the case of PVs, heat sinks with newer, different, and more closely related materials to PVs can be used to dissipate heat. Further, PVs can be placed horizontally on the board to cover the most area for contact between a PV and the board. But in the scheme shown in Figure 1, the heat sinks are vertically placed. This way, multiple heat sinks can be deployed, which is a similar application used for heat-intense LED PCBs. The downside of using an aluminum-based heat sink is that it increases the weight of solar panels, which are normally mounted on roof top structures.

Also, a common PCB manufacturing method that can be used is to place the PVs flat on a board’s surface to increase the surface area of contact between the PV and the circuit board. Once the area increases, more heat can be dissipated through that increased surface area.

Gold can be used as a surface finish on PCBs because it has a greater capability for dissipating the heat compared to HASL. The ability of gold to efficiently transfer heat and electricity makes it indispensable for use in the semiconductor industry. Not only is its melting point more than 1,000 degrees C, but gold also offers high corrosion resistance and is resistant to oxidation, thereby ensuring a good electrical connection. Hence, a correct surface finish can be part of a mitigating strategy for relieving the heat trapped in the board.

Another key technique that can be leveraged from the PCB industry is to increase the surface area of copper on the board needed in solar applications. This means pouring more copper on the board’s surface, allowing more area to dissipate the heat generated by PVs. This is commonly known in PCB circles as a “ground pour copper.” (See Figure 2.)

The technique of “thieving” is closely related to ground pour copper, as seen in Figure 3. Thieving is used to balance copper on the board when it is fabricated and is commonly referred to as a dot pattern that helps to minimize printed circuit warping. These days, for PCBs, a semi-automated process takes place where the software can be instructed on how to add thieving without compromising the integrity of the layout. But for PVs, thieving is used to connect PVs to each other. In this manner, heat is dissipated through the greater surface area of a circuit board.

Another method for dissipating heat is to link the PV cell to the chassis, cage, or the enclosure containing those PV systems. This enables the surface area to increase, allowing heat to spread. Heat will transfer from the PV cell to the chassis and then radiated throughout the atmosphere, using this enlarged area. When connecting to the chassis, it is important to make sure the connection is sufficiently solid, wide, and strong enough to take all the heat away from the PV cell.

As shown in Figure 4, thermally conductive grease can also be applied to the surface of a circuit board to further dissipate heat. Thermal grease, also known as heat transfer compound or heat sink compound, is a fluid substance that increases thermal conductivity. It is normally used in conjunction with a heat sink to more efficiently dissipate heat. Thermally conductive paste improves the efficiency of a heat sink by filling air gaps that occur when the irregular surface of a heat generating component is pressed against the irregular surface of a heat sink, air being approximately 8,000 times less efficient at conducting heat.

V-shape troughs

In a concentrator PV module, solar cells are integrated in V-shape troughs designed for better heat dissipation. In this scheme, all the channels in the V-troughs are made using a singular aluminum sheet to achieve better heat dissipation from the cells under concentration. These concentrator PVs are the ones that trap solar energy and generate more heat.

Six module strips, each containing a single row of six monolithic, mono-crystalline cells, are fabricated then mounted on six V-trough channels to get the modules into 36 cells, maximizing the amount of generated power and heat dissipation from the cell. Compared to conventional PV modules, a V-shape trough module design generally provides three to four times more heat dissipation area and capability through the walls not using the cooling methodology.

Cooling methodologies can be forced air, like that used in computer applications. In this example, there is a heat sink on top of another heat sink with a cooling fan, very much like a motherboard and a CPU. While these are two known ways to dissipate heat in PVs, a cooling scheme has not yet been designed. It is only in the conceptual stage.

Fig. 4 Thermal Grease is a Fluid Substance That Increases Thermal Conductivity

Another way is forced liquid cooling, where water or another coolant runs through a closed loop and dissipates the heat. Right now, there are no PV cooling methodologies that are well developed for commercial use.

A key consideration is that PV cells suffer an efficiency drop as the operating temperature increases. This is especially true if exorbitant insulation levels are used. If a forced air-flow or another cooling method is not used, and since PVs are subjected to heat all day long, the extraction of the heat by air circulation is limited. Forced air can be a mitigating strategy, but it definitely needs to be augmented to completely dissipate the heat. A heat dissipation system like this has to take into account daily temperature ranges so that it is capable of taking different fluctuations without deteriorating the cooling system itself.

However, in practice, to maximize a PV cell’s yield during the day, an exposure or tracking system is needed. Such a system would adapt to the changing variations of the solar irradiation conditions. A solar electro-mechanical system based on reflectors and concentrators, for example, would track the direction and origin of solar rays. This is where PCB technology can pay solar-energy companies large dividends. Electro-mechanical devices can track the sun’s direction and that is best achieved with circuitry on a printed circuit board. By taking this route, solar panel makers can collect the maximum amount of energy, and that is only possible by tracking the sun as it changes its path through the day.

Conclusion

A word of caution. There are many PV and solar panel technologies now in the works. There’s no one way of mitigating the heat, although there are a large number of companies working to resolve the issue. Yet, they are still far from achieving a standard or solution that is acceptable.

Right now, cost is the most important heat dissipation issue. Some of the best materials for heat conduction are inordinately expensive. Add to that the existing cost of producing solar cells, and total cost skyrockets since a heat dissipation solution like this becomes economically unfeasible.

That’s why PCBs, with their tried-and-proven methods for dissipating heat, are frontrunners for being integral to PV and solar panel growth. At this point in time, the jury is still out and it cannot yet be concluded that PCBs will be the ultimate winner. But it is highly probable that current R&D will soon produce a low-cost material that is used as a heat sink or a device that is going to dissipate the heat using those special materials.