As a past installer of both solar thermal and PV electric systems, I can say that they each have their pros and cons. Hot water solar systems are tricky to put together, they can leak, need pressurizing, antifreeze adding, etc. In short, installation is a hassle.
Modern systems generally need a circulating pump and regular maintenance is essential. Saying that, they work really well. If the system is sized correctly, it will provided lots of hot water for years.
On the other hand, solar PV panels are really easy to install. There are no moving parts, so once it’s all connected up there isn’t a lot of attention required to keep the system running smoothly. Wipe down the panels twice a year, keep them out of the shade and you’re good to.
Hybrid solar panels are also known as PVT, or PhotoVoltaic Thermal Panels, a relatively new technology which combines two solar energy conversion principles into one unit. A solar hybrid PVT panel converts sunlight into electricity and also heat in the form of hot water.
What is solar PVT?
Solar thermal panels were all the rage 10 years ago but lately haven’t been doing so well. This is surprising, because it’s a much more efficient process than solar PV. The only downside is that the energy can only be used as hot water and isn’t flexible like electricity.
Hybrid solar thermal and solar PV is an interesting technology combination that improves the output from your solar PV from a mere 20 percent to over 80 percent energy conversion. Solar PV is a great technology that has grown leaps and bounds, particularly over the last two decades, but its efficiency remains around 20 percent, even in premium panels.
How efficient are PV solar panels?
So only 20 percent of the energy intercepted by the solar panel is converted into electricity. The rest of the 80 percent is lost. Now this may not be important for people that have large roof areas for installing solar panels, but for people with limited roof space like in an urban setting, that 80 percent lost energy means a lot more.
Even a cursory look at the solar PV technology tells us that it is approaching a point of maturity. Technology development is adding extra power to the panels but only in small increments. In recent years we have seen cut-cell, PERC and bi-facial solar technology has improved the gain of panels.
We are now coming to a point where, in order to squeeze more out of the PV panels, we have to look either towards the prohibitively expensive multi-junction solar cells used in satellites or perovskite cells. Perovskite has the potential to go further than crystalline silicon cells but at present it remains unstable and degrades quickly.
How to make solar panels more efficient?
Discover your solar saving potential
Is there a way to convert more energy out of solar irradiance?
Yes, there is. It is by using a solar PVT panel, or in other words a photovoltaic thermal hybrid collector. As the name suggests, it is a combination of solar thermal collector and photovoltaic cells that maximize the solar gain with the simplest of methods.
Video and transcript by kind permission Synergy Files
It provides both electricity and heat as an output and the best thing about it is that the two technologies complement each other and work symbiotically. If we look at solar thermal collectors on their own, we find that they are very efficient devices. In fact, the evacuated tube collectors have been recorded with energy efficiency of over 90 percent, meaning they can convert nearly all of the incident sunlight into heat.
Does heat affect solar panels?
Solar PV, on the other hand, can only work with photons in the sunlight of particular energy levels and the rest of the photons simply pass through and get absorbed by the back layer. They end up producing heat in the cells which is undesirable. In fact, this generated heat reduces the output of the solar cells.
The thermal coefficient of the PV panel, a value that is provided with its specification sheet, tells us precisely the drop in performance of the panel with rising temperature. In desert climates the PV panel temperatures are known to reach above 70 degree Centigrade. To cool the panels down options like cooling jackets are used.
In solar PVT panels the photovoltaic cells are placed on top of a solar thermal collector. The excess heat that builds up is removed by water running through the thermal collector. It has been claimed that hybrid panels can have efficiency as high as 85 percent and can generate four times the energy produced from the same surface area for only 25 percent increase in cost.
In cold climates biggest energy use is heat
In Europe and many other cold climate countries over 52 percent of the total energy used goes into space and water heating. Heat remains the biggest energy and use studies have shown that active heat removal systems can improve the PV panel life from 30 years to 50 years.
Heat removal not only improves the instantaneous performance of the solar PV cells but also adds longevity to their life. 20 percent higher annual output of electricity has been reported for PV alone in a PVT system, compared to a non-pvt system.
Well, one may ask that if this is the case then what is the hitch? Why is this technology not so prevalent? Why is it that only a small number of companies are producing it?
Image courtesy Solar2Power
How to use solar hot water
It turns out that PVT technology is not simple plug and play as PV technology is. Furthermore, in summer time one can end up with large quantities of water that is 35 to 40 degree centigrade in temperature and has nowhere to go. For this reason people with swimming pools are the ones that are opting to install them for now.
There is a way around it. The excess heat can be dumped by passing the hot water through outdoor convectors when it’s not needed. The fact remains that when we are using both electric and thermal outputs of the PVT we are essentially saving double the amount of money.
In many places electricity prices are three times the price of gas and this is exactly the ratio of output you get from a PVT solar panel, that is three units of heat with one unit of electricity, thus doubling your savings. If Tesla went this route of the PVT rather than for the solar roof tiles, it will be much more beneficial for people who don’t have the roof space enough to accommodate several kilowatts of PV panels.
What does PERC mean in solar panels?
PERC means Passivated Emitter and Rear Cell and is a variation on the standard architecture of crystalline silicon solar cells. An extra layer reflects light energy back into the cell structure where it can be converted into extra energy, bringing 1 to 2% extra efficiency.
What are half cut solar panels?
A typical industrial solar PV panels has 60 cells. Half-cut solar panels have their cells cut in half, so they have 120 cells per panel. Advantages include more electrical paths for supplying electricity to inverters and less reduction in power output due to shading.
How do bifacial solar cells work?
Bifacial solar cells work by allowing more light to pass onto the solar cell structure so more energy is converted. This is achieved by replacing the traditional back cover with glass or other transparent material.