Innovations are carried out every day to design better, more efficient and lower cost technologies in the solar energy industry.
Analyzing the advantages and disadvantages of these technologies can give us a hint of possible new market opportunities and developments for our needs.
Let’s take a look at each one of them.
Concentrated Photovoltaics (CPV)
Concentrated Photovoltaics (CPV) is an alternative to flat panels.
These systems consist of replacing a flat surface of solar cells with optical elements which increase the solar radiation (in W/m2) and project it onto the special cells whose area is much less than the capturing surface.
To get the idea of it, take a look at the Amonix system (above) in Las Vegas, USA that consists of thousands of small Fresnel lenses, each focusing sunlight to ~500X higher intensity onto a tiny, high-efficiency multi-junction solar cell.
A Tesla Roadster is parked beneath for scale.
CPV is not a new technology. Actually, developments on this area started back in 1975, but it is only recently that the market for CPV started to expand due to the price drop.
The former limitation to the expansion was related to the design of high-efficiency multi-junction cells that could compete with the flat solar panels. Today, the scenery has changed.
What is CPV?
It is logical to think that the sunniest places are the most profitable for solar PV installations, and that is true.
But if you consider the use of high concentration of solar energy onto a single spot, then the unit price of the solar cells can be reduced.
The key lies in a variable known as the concentration factor (C). If increased, C drives down the unit price of solar cells and therefore, the overall costs of the system.
Changing and increasing this factor is what makes CPV a market of opportunities.
Do not get confused!
In some cases, a system uses the term concentrators to refer to the use of flat mirrors that intensify the light onto conventional panels. It is important not to confuse this with CPV.
How CPV Works
The concept of CPV is based on semiconductor materials which collect sunlight and convert it into electricity as in any ordinary PV module.
The main difference with CPV is the amount of material needed.
Despite the fact that the price of PV cells has decreased at an accelerated rate, one of the manufacturing components that have higher capital costs is the silicon material used.
CPV technology aims to reduce the costs of the conventional PV cell by using less silicon.
The idea lies in using mirrors and lens that concentrate the sunlight beams into a receiver that acts as a focal point and reflects the concentrated light into a high-efficiency PV cell.
As you can imagine, the balance between the design of a conventional PV module full of silicon cells and the design of a CPV system, focuses on the reduction of semiconductor materials, substituting a set of mirrors and lens instead, which costs much less.
Here is a basic diagram function of a CPV system:
This is the example of parabolic CPV system:
System Components And Limitations
Regarding the system components of a CPV system, there are several elements that distinguish this technology:
- PV cell
- Solar tracking systems
- Cooling system
Concentrators are the name given to the mirrors that focus the sunlight into the receiver. They also establish the concentration factor discussed above, which reduces the overall cost of the system.
On the other hand, the PV cell is located on the focal axis right below the receiver. The idea to reduce costs in material and reduce the space needed for PV cells makes it necessary to consider high-efficiency materials in order to obtain the same or more power output than a conventional silicon PV cell could achieve.
Thus, CPV cells are made of a multi-junction composition of three materials (Gallium Indium Phosphide, Gallium Indium Arsenide and Germanium) all of which have a higher efficiency than silicon cells and can absorb a wider range of light from the solar radiation spectrum.
There are of course, limitations in this technology.
As you may know, there are days when the sky is clear and the solar radiation directly impacts the surface of the panels (direct irradiance).
However, there are other days where the sky is clouded and no direct solar beams can be seen, even so, we can still see light, because the solar beams are reflected by the atmosphere and the ground, which allows the conventional PV cells to keep producing energy (although lower amounts) when the sky is clouded, this is called diffused irradiance.
The first and most important limitation of CPV systems is that they are incapable of absorbing diffused irradiance, so when the sky is clouded the concentrators won’t be generating at all.
CPV configurations need the presence of direct sunlight beams in order to produce electricity and to be efficient.
The second limitation arises from that fact.
The CPV technology needs a solar tracking system.
While on conventional PV arrays the solar tracking system is just an option that can be considered to increase efficiency, on CPV technology, it is a must.
The reason is completely related to the description above, they need the direct beams from sunlight and the best way to achieve this is to track the sun’s position.
The necessary presence of a tracking system increases costs, which is why these configurations are only financially profitable when the system is over 10 kW, assuming, of course, high direct irradiance levels (DNI) and clear sky conditions like in Australia or in the Middle East.
The third intrinsic limitation is related to temperature.
When you concentrate several solar beams into one spot the temperature increases considerably.
An increase in temperature in PV cells is related to lower voltages and lower efficiency, which is why these PV cells must work at the appropriate operating temperature.
The fact is that when you concentrate all the heat into one cell (CPV case), then the resultant temperature value widely exceeds this appropriate operating temperature.
That is why it is necessary to include a cooling system with a heat transfer fluid like water that takes all the excess heat from the PV cell in order to keep it within the established temperature values.
A failure in the cooling system could actually lead to burning the cell, which is why it is crucial to design a good control system.
The bright side of this limitation is that this excess heat can be used to store thermal energy in storage tanks outside the CPV system. This is profitable when considering large systems.
Another great breakthrough in the solar industry is the development of Bifacial Modules.
This type of technology aims to produce electricity using not only the direct and diffuse irradiance on the front side of the module but also using the diffuse sunlight reflected on the ground according to the albedo factor (reflection factor). The direct diffuse sunlight coming from the atmosphere impacts the backside of the panel.
Let’s take a look at a basic principle of bifacial modules:
There are many important benefits this technology can bring to the market. The most relevant is the higher efficiency and power output that can be obtained, especially in locations where the direct irradiance is not that high or where there are long cloudy seasons as well.
However, in order for the system to be profitable, the albedo (ground reflection factor) needs to be high enough to reflect as much irradiance as possible for the module’s backside to produce the optimum amount of energy.
Albedo and ground factors depend mainly on the type of surface located behind the modules such as grass, dry or wet asphalt, concrete, red tiles, aluminium or copper. Follow this link to learn more.
Bifacial vs Traditional PV Modules, Are They Worth It?
Studies have been carried out for bifacial modules to determine the possible power outputs taking into consideration three types of surfaces: grass, sand and white-painted.
The study proved that energy gains could be obtained for all types of surface. For grass, the energy gain was 5.2 %, for sand 10.79% and white-painted 21.9%.
As you can see, the more white and clean the surface is, the higher the power output that can be obtained from the bifacial modules.
Moreover, when a solar tracking device was added, energy gains were reported to be even higher (grass – 10.57%, sand – 24.42%, white-surface – 33.2%), facts that can be considered crucial for large commercial or utility-scale projects.
Furthermore, these values have been set as a reference, but other studies point out that the enhanced power obtained from the bifacial modules can be up to 50%!
There are several designs for bifacial modules, but most of them are made of monocrystalline silicon as this is more efficient than polycrystalline, although you can find polycrystalline bifacial modules as well.
A comparison between a traditional monocrystalline PV module and a monocrystalline bifacial module can be made in order to have an idea of the advantages of this technology:
As you can see, the benefit is evident, with more power output using the same available area!
So if the bifacial modules are so good, why is it that the market share of this technology is so small?
The reasons are related to commercial restraints, as only a limited number of manufacturers produce bifacial modules.
In order to overcome this market limitation the following challenges must be addressed:
- A standardized rating of nameplate power. No reliable and international standard method has been established to measure the power output of a bifacial module, making it unattractive for designing purposes.
- Manufacturing costs.
- High dependence on ground reflection values makes it unpredictable under snow, rain or ageing of the surface. Further technical simulations must be made.
Bifacial modules have several applications.
As traditional PV modules, they can be installed on a roof or as mounted type configurations and are applicable for residential, commercial or utility purposes.
Moreover, as thin-film technologies, bifacial modules can be integrated into buildings as well.
And particularly, a very unique and interesting feature of bifacial modules is that they can be vertically installed.
The idea bases on the fact that bifacial modules can obtain power from both sides and catch the solar energy reflected from the ground as well.
Therefore, a vertical array design could take advantage of this and add considerable benefits like:
- reduced soiling/snow and inclination losses
- align production with energy demand across the day
- eliminate the variable tilt configuration.
Here is an example of a vertical-mounted bifacial module:
We have examined two innovative technologies with low penetration in the market but with potential benefits for the solar industry.
After all, efficiency and costs are a big deal in the PV market, so in order for these alternatives to having a bigger market share, the cost and manufacturing process barriers must be overcome.
Particularly, a big issue of CPV is that the technology is unable to collect energy from solar diffuse irradiation and requires a very precise control system in order to ensure the lifespan of the system, making it inappropriate for some regions like Northern Europe, Russia, Canada, Indonesia, Japan, Alaska, South Korea and others.
Bifacial modules do not have such problems and actually, on the contrary, they take wide advantage of diffuse irradiance, making it very attractive for many regions across the world.
Only time will tell if the technology can take over some market share from conventional PV panels.
Particularly for Australia, given the direct normal and diffuse radiation here, both options (concentrating PV and bifacial modules) are great choices to consider for the solar industry.
If you want to see how much solar or battery storage could save you over the next 5 years, then take our solar saving calculator quiz below!
Or talk to an Instyle Solar expert about the best solutions for home energy storage or PV-panels.
Otherwise, head back to the solar blog to find even more great educational content.
Photo credit: Wikipedia, EPRI, BiFiPV Workshop, Zenith Solar, Platinum Solar, A. Luque and S.Hegedus. Handbook of Photovoltaics Science and Engineering. John Wiley and Sons, 2011