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Solar thermal panels vs Photovoltaic panels

Next we will make a distinction between photovoltaic solar panels and thermal solar panels, so that you understand exactly the difference between both types of panels.

Solar collectors or photovoltaic panels? What solution to choose?

Solar collectors and photovoltaic cells are often confused, although the principle of operation is completely different. What they have in common is that they are used to capture solar energy and transform it into a usable form (heat or electricity). From a practical point of view, this brings about some very important similarities.

Sun energy: a common part

The amount of energy that we obtain from a solar installation depends on several factors. They are common to both systems based on  solar collectors  and  photovoltaic panels  .

The intensity of solar radiation.

It depends on the season and the level of cloudiness. The sometimes heard opinion that a photovoltaic installation can provide the same amount of energy, regardless of the season, is a nonsensical myth. We have no influence on changes in radiation intensity in the annual cycle. 

Unfortunately, without strong sunlight, we cannot count on large amounts of energy. This is the reason for the low efficiency of solar installations in the winter season. There is no sun, there is no energy. In winter, the sun shines not only weakly, but also only for a few hours a day.

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Unfortunately for us, in winter, when the consumption of heat and electricity is highest, the sun provides us with less. Furthermore, a large part of solar energy is then diffuse radiation, which is more difficult to use effectively. Collector users know this, as their efficiency drops significantly even on cloudy summer days. 

Even if the collectors were 100% efficient, that is, they would absorb all the energy that reaches them without losses (which of course is impossible) and their surface were large, for example 10 m2, due to very little sunlight, they would not they would give a lot in the middle of winter. In December, the maximum energy yield of such an installation would be 5 kWh per day, which corresponds to the operation of a small 10 kW boiler for half an hour.

Collectors or panels surface

By having more of them, we will use the solar radiation that falls on a larger area. Obviously, we get more energy. However, increasing the collector area does not necessarily make sense. First, it increases costs. Second, an  over-enlarged system with liquid collectors causes problems in summer if we cannot take advantage of the large amount of heat that is produced at that time. Third, a large installation implies higher investment costs. Oversizing can “kill” the economic sense of solar energy.

Alignment with the directions of the world

It is optimal to orient the collectors and panels to the south. However, a fairly significant deviation to the east or west is acceptable, which can be compensated for by increasing the area. When you lean 50 ° west, the area should be increased by 10%, by 50 ° east, and the correction should be 20%.

It makes no sense to place such devices from the north or in constantly shady places. In practice, this is a significant problem, because collectors and panels are most often installed on roofs, the arrangement of which may be far from optimal and is the result of basic conditions such as the shape of the terrain, the shape of the building and the Entrance orientation. Dormers and roof windows are other limiting factors.

Often times, there simply isn’t a good spot on the roof.

Inclination angle

The sun’s rays should hit the absorbent surface at right angles. The problem is that this angle of incidence varies with the season. In winter, the sun is low above the horizon and the best option would be to place the collectors and cells almost vertically. In summer the opposite happens. Therefore, a universal fit of about 45 ° is generally selected. This is roughly the most common roof pitch angle in single-family homes.

Good design and execution of the entire installation

A poorly configured installation will not work well, even if its components are of the highest quality. The ideal scheme for solar systems is simple, but its proper configuration in practice is no longer. Hardly anyone realizes how much it complicates the situation, for example, the placement of collectors or panels on roof slopes with different orientation. When some are in the sun and the facility is supposed to start operating, the rest are in the shade. In the case of collectors, separate circulation pumps and controllers should be used for them in such a situation. In turn, the panels must have separate investors (investors).

Solar panels

The collectors and the associated installation use solar radiation to heat the water. This process takes place in several stages:

  • the collector absorbs and converts solar radiation into heat;
  • heat collects the working medium (most often antifreeze);
  • The working medium flows through the pipes to the coil placed in the water tank and transfers its heat.
The biggest problem with vacuum collectors is the sealing of the connection between the vacuum tube and the tube that collects the heat from its interior.

To use the obtained heat effectively, the collector cannot lose too much to the environment. The absorbent’s special coating (most often based on titanium oxide) makes it absorb solar radiation very well and, when heated, it radiates little heat.

To limit the remaining heat losses (by conduction of air and convection), the housing of the flat plate collectors is insulated with mineral wool. However, in vacuum collectors, the insulator is precisely the vacuum: in a space without air there is no conduction or convection, only the loss of radiation remains.

The heat from the heat absorber must be transmitted. In flat collectors, a pipe through which the working medium (antifreeze) flows contacts it from the bottom. In turn, in the vacuum collectors in each vacuum tube there is a tube through which the working medium flows, or what is called  a heat pipe (tube with a liquid at low evaporation temperature). The individual heat pipes must be connected to the outside, apart from the vacuum pipes, by a “busbar” through which the working medium flows.

The biggest problem with vacuum collectors is the sealing of the connection between the vacuum tube and the tube that collects the heat from its interior. Even a slight inaccuracy is enough and air enters, and then the pipe no longer has insulating properties.

There is no clear answer to the question of whether flat or vacuum collectors are better.

A flat collector  with the same footprint as the   vacuum tube collector has a larger absorption surface (which absorbs radiation), but poorer thermal insulation. As a result, the flat plate collector gets more heat in summer, when sunlight is strong and heat losses are relatively low. However, in autumn and winter  , the  vacuum  tube  collector  will have an advantage  , because it will lose less heat to the environment.

Solar panels – principle of operation

Photovoltaic (PV) panels  produce electricity. Due to the high price, its use in installations that provide at least a few kW of power, for example, for the needs of a single-family house, is still rare. However, the simple act of using individual cells or their batteries, called  photovoltaic  (solar, solar) panels, is nothing new. They are used, for example, in calculators and to power street lights.

We will not describe in detail the physical basis of how solar cells work here. Those interested will find this information easily in any encyclopedia. Finally, to explain the so-called  photoelectric effect – Albert Einstein was awarded the Nobel Prize. Suffice it to say that a solar cell is a semiconductor element that forms a pn junction. Under the influence of solar radiation (photons) falling on it, an electromotive force is created, because the electrons move towards the area n, while the so-called  holes to the region P. The result of this movement of charges is the creation of a potential difference, that is, electric current.

The appearance of photovoltaic panels is very similar to that of flat plate collectors (photo: Braas)


Why are cells grouped into panels?

Because a single cell of the most popular type, which is made of silicon, gives a voltage of only about 0.5 V. By connecting them in series, we get a correspondingly higher useful value.

From a practical point of view, the most important thing is to know some basic rules that govern the operation of photovoltaic installations:

  • the intensity of the electrical current obtained, that is, the amount of energy, depends on the intensity of the solar radiation;
  • The efficiency of converting solar energy into electricity varies widely, from a few to more than 20%, depending on the structure of the cells. However, their prices are also very different, so it is better to convert the price into the electrical energy obtained (PLN / kW);
  • we draw direct current from the panels, and the vast majority of appliances are powered by alternating current. Therefore, it must be processed, which requires additional devices (inverter);
  • Storing electricity in batteries is expensive and impractical, so the most desirable solution is the ability to resell surplus energy to the grid.

Electricity is a much more universal form of energy than the heat obtained in solar collectors. It can be used to power all electrical appliances, but also to heat or prepare hot water.

Who are the collectors, who are the photovoltaic panels? – Resume

The choice of   liquid  solar collectors seems obvious if we are mainly concerned with the preparation of domestic hot water, since from spring to autumn the collectors cover most of the demand. Almost entirely in summer, partially in other seasons. However, even in the coldest season, when the collectors heat the water in the tank to, say, 20 ° C, it cannot be said that they are useless. The boiler, heating it up to 40-50 ° C, will consume less fuel. This is an argument for those who want to reduce the costs of preparing domestic hot water, because, for example, they use expensive heat carriers (electricity, fuel oil, LPG).

There is also a second group for which collectors can be very attractive. They are users of solid fuel boilers. In your case, the solar installation does not require the boiler to be turned on in summer. Anyone who has experienced this knows how valuable this advantage is. Also, in practice, in order not to burn in the boiler, the tank is often heated with an electric heater in the summer. And this is the use of the most expensive form of energy. There is also an ecological argument. It is not just the fact that the boiler is not working for part of the year, it obviously pollutes the environment less. Currently popular fuel-fed boilers operate almost exclusively in the keep-warm mode in the summer. And then they emit a lot of pollutants, because the combustion process is far from optimal. Furthermore, by operating in this way, the boiler uses more fuel to maintain operation,  than just to heat sanitary water. In this situation, the real cost of preparing domestic hot water increases significantly.

When is it worth recommending the choice of photovoltaic panels? Its undeniable advantage is its versatility. We obtain electricity, that is, clean energy that can be used in any way. From the lighting, through the power supply of various electrical appliances, through the heating and finally the preparation of hot water. In terms of mounting, they are also more convenient in many respects. Nothing prevents the panels from being placed at a considerable distance from the house, for example on the roof of a farm building or on the ground. If we only use a cable with a large enough cross section, the transmission energy losses will be small. The same cannot be said for manifold installation. The main problem is the still high price. Even taking into account prime loans and guaranteed rates, investing requires careful financial calculation.


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