It happens again and again that even well-educated representatives of the Californian media equate or even confuse solar cells and solar collectors (popularly called solar collectors) in their reporting. The difference between solar cells and solar collectors is often not known. One reason for this, for example, is that many pictures of photovoltaic modules (which consist of individual solar cells) can be found in image banks, while suitable pictures of solar heating systems are often difficult to find.
What is a solar collector? And what is a solar module? What are the differences between a solar collector and a solar cell – and what do they have in common? This article explains the similarities and differences between solar collectors and solar modules – because we know from experience: There is danger of confusion!
Solar is “something with sun” – that is already clear!
First, let’s talk about what solar collectors and solar cells have in common: Both are parts of what is known as a solar system. “Solar” means “something with the sun” or “relating to the sun.” Consequently, a solar system can also be called a solar system.
A solar system is a technical system in the field of solar technology that generates usable energy from solar energy, i.e., the energy of the sun (solar energy). The sun sends us energy in two forms with its rays: light energy and thermal energy.
Before we look at the difference between solar cells and solar collectors, let’s look at photovoltaic and solar thermal systems. Both systems obtain their energy by using solar energy. Photovoltaic energy produces electricity, while solar thermal energy produces hot or warm water. The so-called solar collectors are responsible for this. These capture the sun’s energy, so to speak, hence the name COLLECTOR. The term is used only in connection with solar thermal energy. More precise forms of collectors (flat plate collectors and tube collectors) will be clarified in the course of the article.
So what is the difference between solar cells and solar collectors? In the following paragraphs, we will first explain the different types of solar collectors.
Evacuated tube collectors
Vacuum tube collectors are essentially airless, highly resistant glass tubes that are highly efficient at capturing sunlight. Inside the glass is a so-called copper absorber, which is heated by sunlight, and a heat-conducting tube inside it. Cold water flows through this structure and absorbs and transports the generated heat (up to 100 degrees). The big advantage: because the vacuum cannot dissipate heat, the efficiency is very high, since virtually no solar energy is lost. Vacuum tube collectors are thus very efficient in terms of high efficiency, but this is also reflected in a higher price. One reason for the high production cost: the production of vacuum tube collectors requires a special glass to avoid the so-called “dishwasher effect.”
If glass is often in contact with water, weathering occurs. This can be seen, for example, in old glass in households, which is already very cloudy. When used in solar thermal energy, this would lead to major problems, which is why it is important to counteract this and develop our own materials. This is a difference between solar cells and solar collectors, as this effect does not occur in solar cells.
Flat plate collectors are installed more often, despite their lower efficiency compared to vacuum tube collectors. As a rule of thumb, 9 out of 10 installed solar thermal systems consist of flat plate collectors. Flat plate collectors consist of a black, metal absorber with a glass pane above it. Below the absorber is water, which is heated by the absorber by absorbing heat energy from the sun. One drawback: flat-plate collectors were considered inefficient, in the sense that on the one hand they absorb a lot of heat, but on the other hand they release an equal amount of heat, which in turn is lost to energy generation. To counteract this problem, a cover layer was developed that lies directly over the absorber. This means that the absorber still absorbs the heat, but the loss is in the order of 5%. This results in potential water temperatures of 60 – 80 degrees.
There are also so-called hybrid collectors. These can generate both hot water and electricity. In this context, we also speak of photothermal or thermovoltaic energy. They are constructed as follows: as with flat-plate collectors, photovoltaic cells that are responsible for generating electricity are located under a glass surface. Underneath them, in turn, is the absorber, which absorbs the incoming thermal energy to provide hot water. However, the solar cells are not distributed over the entire surface, creating “gaps” through which the sun hits the absorber. However, this also has drawbacks. For example, in the summer, when it is very hot, you need less solar thermal energy and more photovoltaic energy (less heat and more electricity).
Covered and non-covered hybrid collectors
Non-covered hybrid collectors are primarily intended for the production of electricity. The difference with covered hybrid collectors is that there is air under the window, which is more efficient in generating heat, but at the same time it limits the efficiency of the solar cells for generating electricity. So, covered or uncovered does not describe the glass panel itself (since both forms of glass are covered), but how the technology underneath is constructed. If you look at the suppliers, you will see that most of the hybrid collectors in their range are uncovered. These are probably better in terms of energy output and production costs.
Sounds all in all genius, what’s the catch?
Logically, one advantage is the combination of the systems, which is especially evident when space is at a premium. A disadvantage, namely a forced increase in heat production in summer, has already been mentioned above. In addition, there is a significant difference between solar cells and solar collectors when it comes to the temperatures that affect the system. Flat plate collectors and vacuum tube collectors can get very hot (>80 degrees), a vacuum tube collector much more so. An important difference between photovoltaic modules and solar collectors is that solar cells work better in cold outdoor temperatures than in very hot ones. The optimum temperature for solar panels is 25 degrees. Therefore, systems that cool the solar cells should be installed in hybrid collectors.
Polycrystalline solar cells
In contrast to monocrystalline solar cells, polycrystalline solar cells are produced by a casting process, which makes the crystal structure more uneven or inhomogeneous. This technology was developed in the early 1960s. The actual semi-material used in production is also silicon. Due to the inhomogeneous structure, the efficiency of polycrystalline solar cells is lower than that of monocrystalline ones (15 – 18 %). However, they are cheaper to produce because the energy required for production is not as high. This leads to a lower purchase price. If we look at the overall environmental impact, polycrystalline solar cells get better marks because they require less energy and fewer resources and thus produce less waste. It can be said that the majority of installed photovoltaic modules are polycrystalline solar cells.
Thin-film modules will only be briefly outlined below, as they are not used in practice today (at least in California), if at all. In simple terms, they involve a sheet of glass covered with a conductive, photovoltaic material using vapor. The advantages are that they are very easy to manufacture and therefore have the lowest production costs. The temperature coefficient is even lower than that of polymer modules, which means that they work well in very intense and prolonged sunlight. However, they also have the disadvantage that they require, for example, almost three times as much surface area as monocrystalline ones to generate the same amount of energy.
Solar collector or solar panel – a difference you can see too now!
If you are out and about and looking on rooftops, you will see solar collectors and solar modules. But which one is which? Solar installers in California know the difference between them.
Striped or checkered. The striped pattern (longitudinal stripes) indicates that the collectors are side by side, while the checkered pattern is the result of assembling the solar cells into modules.