Solar Energy

Solar energy is practically inexhaustible. It is the purest form of energy on Earth, consisting of thermal radiations, visible light waves, radio waves, or any other type, emitted by the Sun. The huge amounts of this energy are the basis of most natural processes on Earth. Nevertheless, it is quite difficult to capture and store it in a certain form (mainly heat or electricity), which could permit its subsequent use.
Solar energy can be used to heat buildings passively, due to their construction (passive houses) or can be stored in thermal collectors as thermal energy. The solar heat can be used mainly to prepare domestic hot water, heat the heat carrier responsible for the ambient temperature of a house, and heat swimming pools. There are also air conditioning systems based on solar heat, the latter representing the main energy necessary for cooling the air.
On a global scale, the use of solar energy represents the most efficient method of providing heat to buildings. Generally, the amount of solar heat which falls on the roof of a building is greater than the total energy consumed in the building.
By simple, constructively effective means, solar energy can be used in order to reduce or even totally replace other energy sources necessary for living in a modern dwelling.

Solar Cells

In 1839, the French physicist Becquerel discovered photovoltaic effect, which consists in the direct transformation of light energy into electrical energy, and in 1930, the American physicist Schottky argued theoretically the photovoltaic effect.
Photovoltaic effect is based on three simultaneous physical phenomena, which are very closely connected among them:

1. The absorption of light by materials;
2. The transfer of energy from photons to charge carriers;
3. The collection of charge carriers.

Solar cells are composed of crystalline or non-crystalline (amorphous) silicon. Solar cells made of other materials, such as GaAs sau CuInSe2 are still being developed.
In the field of low power (mW, µW), for instance, watches and small calculators, amorphous silicon solar cells control the market.
Silicon atoms are not ordered, and consequently, thin silicon films are obtained. Amorphous silicon is used for modules with a power of 30 W. The disadvantage lies in its low efficiency of 5-7%. This is the reason why it is necessary for the surface of solar monocrystalline or polycrystalline modules to be doubled.

Types of Solar Cells

A. Monocrystalline silicon cells (Figure 1.1)

B. Polycrystalline silicon cells (Figure 1.2)

C. Amorphous cells (no Figure)

D. CdTe, CIS, CIGS Cells

Solar Cell Construction

Silicon (Si) is obtained at temperatures of 1800o C from silicon dioxide(SiO2), found in nature as quartz, the main component of sand. As a result of this process, liquid silicon with a purity of 98% is obtained. In order to be used to produce solar cells, this product must be purified up to 99.999999999999 %, which means that for 1,000,000,000,000
1012 ) silicon atoms, only one impure atom of Si can be taken up.
In conclusion, silicon must be transformed into monocrystalline silicon. By adding doping materials when melting, (most often 5-valent phosphorus for negative n-type dopability or 3-valent boron for positive p-type dopability), doped silicon is obtained. The basic doping is always a positive silicon.

Figure 1.3 presents schematically the structure of a solar cell.

Light energy strikes electrons in the p-n junction and releases them. Therefore, a negative electron and a positive hole become available. If the circuit is closed, negative electrons leak into p-layer, and positive photons into n-layer. Hence, the electric current can flow.

Due to the high electric potential of the n-type semiconductor and the space charge region, electrons can follow only the following path:

1. from the n-type layer;
2. by the top contact (the negative pole of the solar cell);
3. by the external consumer;
4. by the bottom contact (the positive pole of the solar cell);
5. to the p-type semiconductor;
6. it recombines with a residual atom, etc.

Solar Cell Connection

In order to obtain larger power, solar cells must be connected to form a module.
Solar cells can be connected in series or parallel, or combinations of cells connected
series and parallel can be realized.
Figure 1.4 presents a module of 18 cells in a series-parallel connection.

Solar Cells Orientation

Maximum power can be obtained from a solar module if the rays of light fall perpendicularly on its surface.
However, this is not always possible if we consider the Sun’s daily and annual movement. Thus, when calculating the efficiency of a photovoltaic plant, these losses must be taken into account.
The ideal angle of installation of a solar celll is determined by the latitude of the locality. However, an angle of installation lower than 15O must be avoided, in order for the auto-cleaning effect to take place in case of rain. An angle of installation of 60O permits the snow to slide down in winter.
In the areas situated to the north of the Equator, the solar modules are positioned towards south, in case of potential shading, and south-west or south-east.

Solar Cell Characteristics

Solar modules construction

After the silicon wafers have been cut, contacts are fixed to the top and bottom side, together with a tinned band. The bottom contact covers the whole cell surface, while the bottom contact is comb-like, allowing light to fall on the silicon surface. Finally, an anti-reflective coating is applied to the top side. This ensures the penetration of as much as possible light on the silicon surface. The final stage is quality control.

Module assembly

First of all, conductors are fastened to the the solar cell contacts. Depending on power and voltage, they will be connected in series or parallel. Silicon is brittle and breaks easily, that is the reason why the solar cells will be deposited onto a plastic substrate. This plastic must not age and must be resistant to degradation by ultraviolet rays.
Similarly, the covering glass must comply with certain conditions. The glass, the back of the module and the plastic will finally form a single unit. Therefore, the cells can no longer be separated without being destroyed. As a final stage, the whole module is fitted into an aluminium frame, and the module junction box is installed.

Module shading

Problems may arise if, when connecting the module cells in series, one them is shaded, for example, by a leaf.
Let us analyse the most critical case, the module provides current to the accumulator. The shaded cell becomes a consumer of energy. Those cells which are still free, direct the current through the shaded cell. Consequently, the energy made available by the module is converted into heat by the shaded cell. This energy may destroy the cell.
This inconvenience can be avoided by connecting anti-parallel a bypass diode. It does not allow the current to flow through the shaded cell. Ideally, each cell would be connected to a bypass diode.
In practice, it is sufficient to connect one diode to 15-20 solar cells. This diode is integrated into each module junction box. Connecting two bypass diodes protects from the danger of shading the cells.

Modules connection

Interconnecting cables must be resistant to ultraviolet rays and humidity (water). The decrease in voltage at the inverter must not exceed 1 % -3 %. The problem is whether the converter can function correctly even in case of a voltage drop. The cables to the inverter must be protected against short-circuit. Simple or double special solar cables are usually used.
The general problem of solar generators is that the short-circuit current is only 10 % higher than the rated value. In this case, a common fuse will not be activated, and consequently, the electric arc will be able to continue to burn.

Solar Generators

In order to construct larger photovoltaic systems, more solar modules are connected in series or parallel, thus obtaining a solar generator.
In order to obtain a particular voltage of the system, more modules are connected in series.
By connecting in parallel more modular systems connected in series, the desired power of the system is obtained.
Figure 1.5 presents a solar generator.

Solar Enegy Conversion

In order to use solar energy, it is necessary to convert it into other forms of energy, such as:

Photothermal conversion

Photothermal conversion (thermoconversion) consists in energy transfer from the Sun’s rays to water, vapour, hot air, and other media (liquid, gaseous or solid). The heat thus obtained may be used directly or converted into electrical energy, in thermoelectric stations or by thermionic effect. It can also be used for thermochemical transformations or can be stored in various solid and liquid media. Photothermal conversion is very important to industrial applications, space heating, preparing domestic hot water, drying the materials, distilling water, etc.

Photomechanical conversion

Photomechanical conversion is important in spatial energetics, in which conversion based on light pressure produces the “solar sail” engine, used for spacecraft flights. Photomechanical conversion refers to equipping the spacecraft intended for long, interplanetary voyages with the so-called “solar sails.” Due to the interaction between the photons and the large reflective area, which takes place after the spacecraft has reached the cosmic vacuum, the spacecraft is propelled by means of the impulse emitted by the photons at the moment of interaction.

Photochemical conversion

Photochemical conversion can be divided into two categories: one refers to the direct use of the Sun by the luminous excitation of the molecules of a body, and the other, to the indirect use by means of plants (photosynthesis) or the transformation of animal waste. Photochemical conversion is used to obtain fuel cells by means of the above-mentioned processes.

Photoelectric conversion

Direct photoelectric conversion is realized by using the properties of the semiconductor materials, which the photovoltaic cells are made of.

Solar Energy Capture

In order to capture sunlight, and for photothermal conversion, several systems are used:

1. Central Receiver System – these systems concentrate sunlight towards a central collector by means of a mirror positioned radially;
2. Trough Systems – troughs are long, made up of curved mirrors, which concentrate the sunlight on pipes filled with a liquid. This liquid can reach very high temperatures (up to 400°C);
3. Parabolic Dish Systems- use a parabolic dish, which concentrates solar radiation towards a collector fitted in its focal point.

Solar Collectors

The main types of solar collectors used in usual applications are:

1. Flat-plate collectors;
2. Evacuated-tube collectors

Flat-plate collectors (figure 1.6) consist of a network of pipes made of a thermoconductor material (copper), with metal sheets to increase capture area. The unit is put into a very well themally insulated box. This box is placed with its transparent wall in sun (made of glass with high transparency), and the surface of both the pipe and the metal sheets is covered with a layer of a material which facilitates the absorption of solar radiation and, at the same time, limits its reflection.
The efficiency of this type of collectors is lower than that of evacuated-tube collectors, for similar capture areas. They have a relatively low price.

Evacuated-tube collectors (figure 1.7) have each of the tubes made up of two concentric borosilicate glass tubes (very resistant and with a high degree of transparency), welded together.
The space between the two tubes is evacuated, and the interior surface of the interior tube is covered with a selective coating with excellent properties of absorbing solar radiation (>92%) and a very low reflection (<8%).
The heat is transferred to the thermal agent, directly, or by means of a thermal tube. The vacuum between the two tubes forms a kind of a “thermos” so that – although the temperature inside reaches 150°C – outside, the tube is cold.
This property makes it possible for the plant to be used in areas with a very cold climate, tube collectors being more efficient than classical flat-plate solar collectors are. Their price is higher than that of flat-plate collectors.

Solar Energy Storage

Solar energy at the level of the Earth’s crust is an energy source dependent on Earth rotation and atmospheric conditions.
Similarly, the energy requirements are variable in time and depend on the number of consumers connected, at a certain moment in time. Consequently, to supply certain consumers with energy coming from sunlight, it is necessary for them to be fitted with appropriate elements of energy storage (accumulation).

The characteristics that a solar energy storage unit must fulfil (according to the field of application) are the following:

1. the unit of storage must be able to receive the energy with a maximum of speed without excessive thermodynamic forces (for example, differences in temperature, pressure, potential, etc.);
2. the unit of storage must deliver the energy with maximum of speed (depending on the purpose of the plant) without using excessive thermodynamic forces;
3. the unit of storage must have low losses (low self-discharge);
4. the unit of energy storage must be capable to allow a high number of charging –discharging cycles, without a substantial reduction of its capacity;
5. last but not least, the unit of storage must be cheap.
   

In stand-alone electric systems, energy storage is ensured by storage batteries, and the most common in use are lead-acid batteries.
They are of two types:

1. flooded (wet) batteries;
2. absorbed glass mat batteries

Figure 1.8 presents a storage battery.

Additional equipments

Additional equipments are necessary for the proper use of solar energy. The most common in use are charge controllers and static converters. Protections against lightning discharge, switches, and cartridge fuses are also used.

Charge Controllers

Charge controllers control the flux of energy and protect the battery from overcharge and the consumer from accidental discharging. Charge controllers also ensure the supervision and safety of the plant.

In photoelectric systems, several types of regulators are used, such as:

1. series regulator;
2. shunt regulator;
3. maximum power point tracker.

Series regulators contain a switch between the solar panel and the storage battery.
When the battery is charged, the static switch opens and therefore, it protects the storage battery from overcharging. The schematic of a series regulator is presented in figure 1.9. The schematic also contains a switch which disconnects the charge (the consumer) from the battery.

Fig. 1.9

Shunt regulators short circuit the solar panel after the storage battery has been charged. While charging, the solar panel is connected directly to the storage battery. After the storage battery has been charged, the static switch closes and the solar panel is in short circuit. The isolation diode has the role of protecting the storage battery from short-circuiting. The schematic also contains a switch, which disconnects the charge (the consumer) from the battery. The schematic of a parallel regulator is presented in figure 1.10.

Fig. 1.10

Maximum power point trackers permit the maximum extraction of power from the panel of solar cells.

Static Converters

Static converters adapt direct-current power supllied by the solar panels to the demands of the charge. There are two types of static converters, namely:

d.c.-d.c. static converters– adapt direct-current voltage obtained from solar panels to the charge voltage;

d.c.-a.c. static converters – transform direct-current voltage into alternating voltage. They are also called inverters.

There are two types of d. c - d. c. static converters, namely, step-up and step-down voltage converters.

Figure 1.11 represents the schematic of a d. c. - d. c. static converter. Their usual efficiency is of 70 %, but high performance versions may reach 85-90 %.

Fig. 1.11

Fig. 1.12

Solar Energy Use

Solar energy has multiple applications and uses, such as:

1. Supplying industrial and domestic consumers with electrical energy;
2. Solar ovens;
3. Solar drying plants;
4. Solar toys;
5. Solar distilleries;
6. Solar desalination plants;
7. Solar-powered satellites;
8. Solar-powered space robots;
9. Solar-powered interplanetary spacecrafts;
10. Air-conditioning plant in summer;
11. Central-heating plant in winter;
12. Domestic water heating;
13. Solar cells;
14. Solar cookers;
15. Solar fridges;
16. Houses supplied with solar energy and heat;
17. Pools supplied with solar-heated water;
18. Solar lamps, which charge by day and light up at night.
19. Solar automobiles.

Solar Automobile

The mode of operation of a photoelectric-powered automobile consists in supplying a storage battery with the solar energy collected by the solar panels of the automobile. The electrical energy supplied by the accumulators is transmitted to a direct current electric engine which propels the car.
If the motion takes place in a sunny area, the energy supplied by the panels may serve directly to the propulsion, the accumulators being activated only in shaded areas, or when the vehicle goes up a steep slope.
For an automobile, the electrical energy is necessary, apart from supplying the engine, to the block of lights and a multitude of elements electrically controlled and actuated.

Figures 1.13 and 1.14 present prototypes of solar-powered automobiles

Fig. 1.13