Solar Energy: Answers to Common Questions

What is the world energy consumption? What amount of solar energy incident on the earth?

Earth as seen from space

The sun produces an enormous amount of energy: about 1.1 x 1020 kW hours every second (1 kW-hour is the energy needed to light a 100-watt bulb for 10 hours). The outer atmosphere intercepts approximately about 1.5 trillion (1.500.000.000.000.000.000) kilowatt hours per year. However, due to reflection, scattering and absorption caused by atmospheric gasses, only 47% of this energy, or about 0.7 trillion (700.000.000.000.000.000) kW hours reach the surface of the earth.

This energy is what launches the “machinery” of the Earth. Warms the atmosphere, oceans and continents, it generates winds, moves the water cycle, makes plants grow, provides food to animals, and even (over a long period of time) produces fossil fuels.

The world each year is about 85 trillion (85,000,000,000,000) kilowatt hour. This is what can be measured, ie the energy that is bought, sold or traded. There is no way of knowing exactly how much non-commercial energy each person (eg how much wood is burned, or that amount of water is used in small waterfalls to produce electricity). According to some experts, this non-commercial energy may be as much a fifth of the total energy consumed. Even if this is the case, the total energy consumed by the world would mean only 1 / 7,000 of the solar energy that strikes the surface of the earth every year.

What is it and how a photovoltaic or wind system work?

A photovoltaic system is a device that, from solar radiation, produces electricity in a position to be exploited by man. The system consists of the following elements (see diagram):

A solar generator, consisting of a set of photovoltaic panels, which capture light radiation from the sun and transform it into direct current at low voltage (12-48 V).

If instead of a solar panel installed a wind turbine system is called the wind. If installed both be a mixed system. In this case, each one should carry his own regulator.

Is photovoltaic solar energy profitable?

The answer to this question depends on where in the world where we are. A large part of humanity in developing countries have no access to electricity for lacking a basic electrical infrastructure. In these countries solar photovoltaic energy turns out to be the most profitable for electricity source, and in some places only.

In developed countries, where there is a large electric infrastructure, the question is different. In this case, in purely economic terms, photovoltaic systems are profitable only in places far from the conventional network. However, the question will change a lot if, in addition to cost effectiveness, we also consider the environmental cost of each energy source.

Can you use photovoltaic solar energy for heating or for heating water?

Although technically it would be possible, from an economic point of view makes no sense. To produce hot water is best to use a solar thermal system, which uses collectors are filled with water and absorb heat. As for heating, the only possibility to implement solar energy is to use a solar thermal system with underfloor heating.

What is the lifespan of a photovoltaic solar panel?

Given that the panel has no moving parts and that cells and contacts are encapsulated in a robust synthetic resin with very good reliability is achieved with a long service life of the order of 30 years or more. Also, if one of the cells fails, this does not affect the operation of the others, and the current and voltage produced can be easily adjusted by adding or removing cells. 

What maintenance requires a photovoltaic system?

Photovoltaic systems require minimal and simple maintenance, which is reduced to the following:

Panels: require no or very little maintenance due to it is advisable to make a general inspection 1 or 2 times a year to ensure that connections between panels and the regulator are tight and free of corrosion. In most cases, the action of rain eliminates the need for cleaning of the panels; if necessary, simply use water and a mild detergent.

Regulator: the simplicity of control equipment substantially reduces maintenance and makes faults are very slim. The operations that can be performed are: visual observation of the condition and operation of the regulator; check wiring and wiring equipment; observation of the instantaneous values of the voltmeter and ammeter provide an indication of the behavior of the installation.

Accumulator: is the element of the installation that requires further attention; their correct use and good maintenance depend largely on its duration. The usual operations to be performed are as follows:

Check the electrolyte level (approximately every 6 months) should be maintained within the range of brands of “Maximum” and “Minimum”. If no such marks, correct electrolyte level is 20 mm above the protector of separators. If a lower level is observed in any of the elements should be filled with distilled or demineralized water. It must never be filled with sulfuric acid.

By performing the above operation it must also check the status of the battery terminals; It must be cleaned of possible sulfate deposits and neutral Vaseline all connections.

 

What environmental impact of photovoltaic solar energy?

Photovoltaic solar energy, like other renewable energy sources, is compared to fossil fuels, an inexhaustible source, contributes to national energy self-sufficiency and is less damaging to the environment, avoiding the effects of direct use (air pollution, waste , etc) and derivatives of his generation (excavations, mines, quarries, etc.).

The effects of photovoltaic energy upon the main environmental factors are:

Climate: generation of electricity directly from sunlight does not require any kind of combustion, so no thermal pollution or CO2 emissions that favor the greenhouse effect.

Geology: Photovoltaic cells are made from silicon, element obtained from sand, very abundant in nature and that no significant quantities are required. Therefore, in the manufacture of photovoltaic modules no alterations in the lithological, topographical or structural characteristics of the terrain.

Floors: to not occur or contaminants, or discharges, or earthworks, the impact on the physical and chemical soil characteristics or credibility is zero.

surface and underground water: no alteration of aquifers or surface water or by consumption or pollution by waste or spillage occurs.

Landscape: solar panels have different possibilities for integration, which makes them easy to integrate and harmonize element in different types of structures, minimizing their visual impact. Moreover, as autonomous systems, not the landscape with poles and power lines altered.

Noises: the photovoltaic system is absolutely silent, which represents a clear advantage over motor generators in remote homes.

Moreover, photovoltaic solar energy is the best solution for those places that you want to provide electricity while preserving the environmental conditions; as is the case for example of Protected Natural Areas.

What is the peak power of a panel?

It is the output power, in watts, producing a photovoltaic panel under conditions of maximum solar lighting, with a radiation of about 1 kW / m2 (which occurs on a sunny day at solar noon).

As a photovoltaic panel manufactured?

A photovoltaic panel is formed by a set of solar cells electrically connected together in series and parallel to the required voltage for use.

Cross section of a photovoltaic panel

This group of cells is surrounded by elements that confer protection against external agents and rigidity to engage the structures that support them. The elements are:

Encapsulation, consisting of a material should have good radiation transmission and low degradability to the action of sunlight.

Tempered glass outer sheath, which, apart from providing maximum light transmission, must withstand the most adverse and sudden changes in temperature withstand weather conditions.

Rear cover, normally made up of several opaque layers that reflect light that has passed between the interstices of the cells, causing again influence on them again.

Setting of metal, usually aluminum, which ensures rigidity and tightness joint, and carries the elements needed (usually bores) for mounting the panel on the support structure.

Terminal box: incorporates the terminals for connecting the module.

Protection diode: preventing damage by partial shadows on the panel surface.

What is the difference between monocrystalline and polycrystalline panels?

Photovoltaic panels are composed of photovoltaic cells of monocrystalline or polycrystalline silicon. The difference between them lies in the manufacturing process. The monocrystalline silicon cells are obtained from very pure silicon, which is remelted in a crucible together with a small proportion of boron. Once the material is in liquid state it is inserted a rod with a “seed crystal” silicon, which is becoming regrow new atoms from the liquid, which are arranged following the crystal structure. Thus, a doped single crystal, which is then cut into wafers about 3 tenths of a millimeter thick is obtained. These wafers are then introduced in special furnaces, into which phosphorus atoms are deposited on one side and reach a certain depth in the surface spread. Subsequently, and before screen printing for surface interconnections, they are coated with an antireflective treatment of titanium dioxide or zirconium.

In polycrystalline cells, rather than from a single crystal, it is allowed to slowly solidify the paste on a silicon mold, whereupon a solid formed by many small silicon crystals, which can then be cut into thin multi crystalline wafers is obtained.

solar energy

solar energy

In what factors affect the efficiency of a photovoltaic panel?

Fundamentally the intensity of light radiation and temperature of the solar cells.

Current and voltage variation with radiation and temperature according to nominal power

The current intensity generated by the panel increases with radiation, remaining approximately constant voltage. In this connection it is very important placing panels (its orientation and inclination to the horizontal), as the radiation values vary throughout the day depending on sun angle relative to the horizon.

The temperature increase in the cells an increase in the current, but also a much greater decrease proportionally voltage. The overall effect is that the panel power decreases with increasing temperature of the same. A radiation 1000 W / m2 is capable of heating a panel about 30 degrees above the temperature of the surrounding air, which reduces the voltage at 2mV / (cell * degree) * 36 * 30 degrees cells = 2.16 Volts and thus the power by 15%. It is therefore important to place the panels in a place where they are well ventilated.

 

The incorporation of a solar tracking system does it improve the performance of photovoltaic capture?

It depends on the weather and the type of application. Ideally, system performance can be improved up to 40%, but the higher cost of not offset the increase is achieved. Its application is limited to cases where the highest performance coincides with increased demand (in the case of pumping systems for livestock in very dry regions).

 

What characteristics define the behavior of a battery?

Serves, in a photovoltaic system, how long the system can operate without light radiation recharge the batteries. This measure of the days of autonomy is one of the important parts in the design of the installation.

Theoretically, for example, 200 Ah battery can supply 200 A for an hour, or 50 A for 4 hours, or 4 A for 50 hours, or 1 A for 200 hours.

However, this is not exactly true, since some batteries, including automotive, are designed to produce fast downloads in short periods of time without damage. However, they are not designed for long periods of low discharge. This is why automotive batteries are not the most suitable for photovoltaic systems.

 

Factors that can vary the capacity of a battery:

Loading and unloading ratios. If the battery is charged or discharged to a different specified rate, the available capacity may increase or decrease. Generally, if the battery is discharged at a slower pace, its capacity will increase slightly. If the pace is faster, the capacity will be reduced.

 

Temperature. Another factor that influences the capacity is the temperature of the battery and its environment. The behavior of a battery is classified at a temperature of 27 degrees. Lower temperatures reduce capacity significantly. Higher temperatures produce a slight increase in capacity, but this can increase water loss and decrease the number of cycles of battery life.

Depth of discharge:

Batteries “shallow cycle” are designed to download 10 to 25% of its total capacity in each cycle. Most batteries “deep cycle” manufactured for photovoltaic applications are designed for downloads of up to 80% of capacity, without damage. Battery manufacturers NiCad say they can be fully discharged without harm.

The depth of discharge, however, even affects deep cycle batteries. The greater the discharge, the smaller the number of charge cycles the battery can have.

What is the composition of a solar lead acid battery?

These batteries are composed of several lead plates in a sulfuric acid solution. The plate consists of a grid of lead alloy with a lead oxide paste on the grid embedded. The solution of sulfuric acid and water is called electrolyte.

Construction of a monoblock battery (VARTA)

The grid material is an alloy of lead because the lead is a pure physically weak material and could break during transport and battery service.

Normally the alloy is lead with 2-6% antimony. The lower the antimony content is less resistant battery during the charging process. The lower amount of antimony reduces the production of hydrogen and oxygen during charging, and thus water consumption. Moreover, a higher proportion of antimony allows deeper discharges without damaging the plates, which implies a longer life of the batteries. This lead-antimony are the type of “deep cycle”.

Cadmium and strontium are used instead of antimony to strengthen the grid: offer the same advantages and disadvantages of antimony, but also reduce the rate of self-discharge the battery suffers when not in use.

Calcium also strengthens the grid and reduces the self-discharge. However, calcium reduces the recommended depth of discharge to no more than 25%, so the lead-calcium batteries are the type of “shallow cycle”.

The positive and negative plates are immersed in a solution of sulfuric acid and subjected to a load of “training” by the manufacturer. The direction of this loading results in the paste to the grid of the positive plates is lead dioxide transforms. Paste negative plates become spongy lead. Both materials are highly porous, allowing the sulfuric acid solution penetrates freely into plates.

The plates alternate in the battery, with separators between them, which are made of a porous material that allows the flow of electrolyte. electrically nonconductive. May be mixtures of silicone and plastics or rubbers.

The spacers may be individual sheets or “envelopes”. Envelopes are sleeves, open at the top, which are placed only on the positive plates.

Regardless of the size of the plates, a cell only a nominal supply voltage of 2 volts (for lead acid). A battery consists of several cells connected in series or elements, internally or externally, to increase the voltage to normal values to electric appliances. Therefore, a battery of 6 V is composed of three cells, and a 12 V 6.

Positive on one side and the negative plates on the other are interconnected by external terminals at the top of the battery.

 

 

What is the sulfation of battery Lead-acid?

If lead-acid battery is left in a state of deep discharge for a long period of time, sulphation will occur. Sulfur acid part will be combined with lead from the plates to form lead sulfate. If the battery is not filled with water periodically, part of the plates will be exposed to air, and the process will be accelerated.

The lead sulfate covering the plates so that the electrolyte can not penetrate them. This represents an irreversible loss of battery capacity that even with the addition of water, can not be recovered.

 

How can you find out the state of charge of a battery?

A hydrometer (unassembled) from those used in stationary batteries

Density is weight of the electrolyte as compared with the same amount of water and is measured with a densimeter or hydrometer. The most common densimeter is used for automotive, indicating charging in percent. It has the disadvantage that it is calibrated for the electrolyte used in starter batteries and non-stationary, so mark always less than the real (50% for a fully charged battery stationary).

 

Can I get to freeze the accumulators? What temperature?

Since lead-acid batteries use an electrolyte that carries water, they may freeze. However, sulfuric acid leading acts as an antifreeze. The higher the percent acid in water, lower the freezing temperature. However, even a fully charged battery to an extremely low-temperature freeze.

As shown in the table, an accumulator lead-acid, 50% load, it will freeze at a temperature of about -25 degrees.

As can be seen, the battery must be kept above -10 degrees, if you will be fully discharged. If you will not be able to keep a higher temperature, keep it charging status at a high enough level to prevent freezing. This can be achieved automatically with a charge controller capable of disconnecting consumption when the battery voltage falls below a preset level.

  • State Density Volts / glass Volts / set Freezing
  • Steeped 1,265 2,12 12,70 -57 ° C
  • Steeped 75% -38 12.60 2.10 1.225
  • Steeped 50% 1,190 2,08 12,45 -25 ° C
  • Steeped 25% 1,155 2,03 12,20 -16 ° C
  • Unloaded 1,120 11.70 1.95 -10

State of charge, density, voltage and freezing point of a lead-acid battery

 

What is the effect of rapidly discharging a battery?

First, all the energy that is capable of providing the battery is not obtained. For example, a discharged battery within 72 hours returns approximately twice the energy that if discharged in only 8 hours.

 

Furthermore, discharges produce fast deformation and premature disintegration of the plates of the elements, which are deposited on the bottom of containers in powder form to reach shorting both plates, disabling the battery.

What effect does heat in batteries?

The temperature rise is extremely detrimental to the batteries. If the temperature of the containers is greater than about 40 degrees, it is necessary to decrease the charging rate.

 

Where be the batteries installed?

Must be sought a place where the temperature is warm, avoid cold or exposed to low temperatures places. It should also avoid temperatures below 0 degrees because then the internal resistance of the batteries increases greatly.

 

What is the danger of leaving a battery discharged for a long time?

The lead sulfate covering the plates hardens when the battery is discharged; pores clogged, leave penetrate the electrolyte and therefore can not act on the active elements of the plates, reducing the effective capacity. This also makes it very difficult to recharge a battery that has been left sulfated.

What are the most common causes of battery sulfation?

The most common causes of sulfation of a battery are:

 

  • Leave it empty for a long time.
  •  Pure Add acid electrolyte.
  • Overloading too frequent.
  • Not have added distilled water at the right time.
  • The transfer of electrolyte few glasses other.

 

What are the symptoms of a battery element is sulfated?

The most obvious symptoms are:

  • The densimeter always registers a low density of the electrolyte, even though the element always subjected to the same charge as the other elements.
  •  The voltage is lower than other elements during discharge and during charging top.
  • It is impossible to charge the battery to full capacity.
  • The two plates, positive and negative, have a clear color.
  •  In extreme cases, one of the terminals protrudes more than normal due to the deformation of the plates.

 

What is and how a fuel cell works?

A fuel cell is an electrochemical device that generates electricity directly from chemical energy. Its construction is very similar to the accumulators consist of an electrolyte (which may be alkaline, phosphoric acid, molten carbonate or solid oxide) and two electrodes. The anode is fed with fuel (typically hydrogen) and the cathode with the oxidant (usually oxygen). To increase the effective area of the electrodes they are constructed with these porous materials. Furthermore, high pressures and high temperatures are used to promote the reaction. The byproduct of the chemical reaction is steam. Fuel cells phosphoric acids are approximately 40% yield and a working temperature of 200C. Currently, they manufactured in units of about 200kW. To learn more about this technology click here.

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