Answers to most frequently asked questions

 1 - How is electrical energy produced from the sun?

 2 - What is the global energetic consumption? How much solar energy reaches the Earth's surface?

 3 - What is a photovoltaic or wind system and how do they operate?

 4 - What are the applications of solar photovoltaic energy?

 5 - Is photovoltaic energy cost effective?

 6 - Can photovoltaic energy be used for heating homes or heating water?

 7 - What is the effective life of a photovoltaic module?

 8 - Can PV modules be broken easily?

 9 - What are the maintenance requirements of a photovoltaic system?

10 - What is the environmental impact of photovoltaic solar energy?

11 - What is the peak power of a PV module?

12 - What are the elements of a PV module?

13 - What is the difference between monocrystalline and policrystalline PV modules?

14 - Can PV modules operate on cloudy days?

15 - What factors affect the efficiency of a PV module?

16 - Does the use of a tracking system improve the photovoltaic conversion efficiency?

17 - What is the function of the diodes in a photovoltaic installation?

18 - What are the characteristics that define the behavior of a battery?

19 - What is the composition of a lead-acid battery?

20 - What are the differences between lead-acid and nickel-cadmium batteries?

21 - What is 'sulfation' of a lead-acid battery?

22 - How can the state of charge of a battery be found out?

23 - Can the batteries become frozen? At what temperature?

24 - What are the effects upon the capacity and the voltage of serial or parallel connection of batteries?

25 - What is the effect of fast discharge of a battery?

26 - What are the effects of heat in a battery?

27 - Where should the batteries be installed?

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

29 - What are the most common causes of battery sulfation?

30 - What are the symptoms that show that a battery is sulfated?

31 - What kind of water should be used to fill up the batteries?

32 - Is the use of a charge controller in a photovoltaic installation necessary? In what cases can the charge controller be avoided?

33 - What is the difference between the different kind of inverters? What is the most suitable inverter for each application?

34 - How to size the inverter

35 - What is the difference between resistive and inductive loads?

36 - What kind of lighting elements are recommended to be installed in photovoltaic systems?

37 - What is the difference between a direct photovoltaic pumping system and a conventional one?

38 - What is a fuel cell?

1 - How is electrical energy produced from the sun?

Photovoltaic effect in a solar cell

        The energy production is based on a physical phenomenon called "photovoltaic effect", this is basically the conversion of  solar light into electrical energy using semiconductor devices called photovoltaic cells. These cells are manufactured from silicon (one of the most abundant elements, main component of sand) with the addition of impurities of some chemical elements (boron and phosphorous), and they are each able to produce a current of between 2 and 4 Amps at a voltage of 0.46 to 0.48 Volts using luminous radiation as the source. The cells are mounted in series on the photovoltaic (PV) modules in order to reach a voltage suitable for operating normal loads. Some of the incident radiation is lost by reflection (turn back) and by transmission   (cell cross over). The rest is able to make electrons jump from one state to another, thereby creating an electrical current proportional to the incident radiation. The antireflective coating increases the cell efficiency.

2 - What is the world energetic consumption? How much solar energy reaches the Earth's surface?

View of the Earth from space

        The sun produces a great amount of energy: approx' 1.1 x 1020 kW hours every second (1 kW hour is the energy necessary to operate a 100 W lamp for 10 hours). The outer atmosphere scatters approximately half of a billionth part of the energy generated by the sun, or approximately 1.5 trillion (1,500,000,000,000,000,000) kW hours a year. Nevertheless, due to the reflection, dispersion and absorption produced by the atmospheric gases, only 47% of this energy, or approximately 0.7 trillion (700,000,000,000,000,000) kW hours reach the Earth's surface.

        This energy is the basis of the life cycles of the Earth. It heats the atmosphere, the oceans and the continents, is the origin of the winds and the water cycle, it makes plants grow, provides animal food, and even (over a large period of time) produces fossil fuels. We depend on the plants' energy, water, wind and fossil fuels to operate our industries, heat or cool our homes and propell our transportation systems.

        The amount of energy consumed around the world at this time is approximately 85,000 billion (85,000,000,000,000) kW hours. This is the energy consumption that can be measured, that is to say the energy that is sold, bought or commercialised. There is no way of knowing the exac amount of non commercial energy consumed by humanity (for example, the amount of wood, or the amount of water used in small hydraulic stations to produce electrical energy). According to data compiled by experts, this non commercial energy may be as much as 1/5 of the total amount of energy consumed. In any case, global energy consumption is only equivalent to aproximately 1/7,000 of the solar energy incident upon the Earth's surface every year.

3 - What is a photovoltaic or wind system and how do they operate?

        A photovoltaic system is a system that produces electrical energy using solar radiation as the source. The system has the following elements (see diagram):

        - A photovoltaic generator. Composed of photovoltaic modules which receive the solar radiation and transform it into direct (DC) electrical current at a low voltage (12 to 48 V).

        - An accumulator or battery, that stores the energy produced by the generator and allows the supply of electrical current in the absence of daylight or on cloudy days.

        - A charge controller, whose mission it is to avoid overcharges or over discharges of the battery - problems which will produce irreversible damages - and to ensure that the system always operates at the point of maximum efficiency.

        - An inverter (optional), that transforms the DC current stored in the battery to 230 V AC current.

A photovoltaic installation without inverter, consumption at DC

A photovoltaic installation with inverter, consumption at 230 V AC

        Once the energy is stored in the battery there are two options: to obtain the energy directly from the battery for use in lighting systems and DC loads (first diagram) or to transform the direct current into alternating current using an inverter (second diagram).

        If, instead of the photovoltaic generator a wind generator is installed, the system is called a wind system. If both generators are installed we will have a hybrid system. In this case each generator should have its own charge controller.

4 - What are the applications of solar photovoltaic energy?

        Practically every application that requires electricity to operate can by powered using a correctly sized photovoltaic system. The only limitation is the economic cost of the equipment and, in some cases, the size of the photovoltaic array. Nevertheless, in remote places, far away from the electrical grid, the most cost effective solution tends to be the installation of photovoltaic energy rather than a costly connection to the electrical grid.

        Amongst the main applications can be found: home electrification, water pumping systems and irrigation, outdoor lighting, radio and TV repeaters, etc...

5 - Is photovoltaic energy cost effective?

      The answer to this question will depend on the location of the proposed installation. A large proportion of humanity, particularly in those countries which are undergoing a process of development, does not have access to electricity due to the lack of basic electrical infrastructures. In these countries solar photovoltaic energy can be the most cost effective solution for the production of electricity, and indeed in many places, the only solution.

        In those countries in which the production and distribution infrastructures have been fully developed, the question is quite different. In these cases, economically speaking, photovoltaic systems are only cost effective in remote places, far away from the conventional electrical grid. Nevertheless, this question will change a lot if, apart from cost effectiveness, we consider the environmental costs of each source of energy.

6 - Can we use photovoltaic energy for heating homes or heating the water of a domestic swimming-pool?

        Even though this is technically possible, from an economical point of view it loses all sense. To heat water the best option is to use a solar thermal system, these systems use thermal collectors that absorb heat and transfer it to water or to another suitable fluid. To heat homes, the best option for the application of solar energy is to use a thermal system with a radiant floor installation.

7 - What is the effective life of a photovoltaic module?

        If we consider that a photovoltaic module does not have mobile parts and that the cells and contacts are encapsulated in a robust resin, high reliability and a long life are achieved, the efective life of most modules being about 30 years or more. Also, if one of the cells fails, this does not affect to the operation of the others, and the current and voltage produced can be easily adjusted, thereby increasing or decreasing the number of cells.

8 - Can the PV modules be broken easily?

        The external surface of a photovoltaic module is protected with glass which is able to withstand very severe meteorological conditions such as ice, abrasion, sharp changes in temperature, or even impacts produced by hailstones. One of the standard qualification tests is an ice ball impact test, in which ice balls of a defined size and consistency are fired at various points of the glass using a pneumatic gun.

9 - What are the maintenance requirements of a photovoltaic system?

        A photovoltaic installation requires minimal and very easy maintenance which can be reduced to the following procedures:

        - PV modules: they need very little or no maintenance due to their configuration; they have no mobile parts and both the cells and their internal connection are encapsulated in various coatings of protective materials. It is adviseable to carry out a general inspection once or twice a year: check the connections between PV modules and between the PV array and the charge regulator, these should be tight and free of corrosion. In most cases, the rain eliminates the need for cleaning the modules, but if necessary they can be washed using water and non abrasive detergent.

        - Charge controller: its simplicity reduces the need for maintenance substantially and makes faults very rare. The main actions to be performed are the following: visual inspection of the state and operation of the charge controller; checking of connections and cabling and checking of the instantaneous values of the voltmeter and of the ampmeter which provide an indication of the behavior of the installation.

        - Batteries: ths is the element of the installation that needs more attention; their effective life will be dependent upon their correct use and maintenance. The usual procedures to be performed are the following:

10 - What is the environmental impact of photovoltaic solar energy?

        Photovoltaic energy, like other renewable energy sources and unlike fossil fuel based energies, constitutes an inexhaustible source of energy, it contributes to the national energy supply and is less damaging for the environment, avoiding the negative effects caused by the direct use of conventional fuels (atmospheric pollution, residues, etc) and their generation derivates (excavations,  mines, quarries, etc).

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

        Climate: the generation of electrical energy directly from solar light does not require any kind of combustion, neither thermal pollution nor CO2 emissions are produced and so the contribution to the greenhouse effect is inexistent.

        Geology: photovoltaic cells are manufactured from silicon which can be obtained from sand, it is extremely common and large amounts are not required. For this reason, in the manufacturing process of photovoltaic modules no alterations are produced in the lithology,, topography or structure of the Earth's surface.

        Ground: as neither pollutants, nor wastes, nor earth movements are produced, the impact upon the physico-chemical characteristics of the ground is null.

        Surface and underground water: no alteration of aquifers or surface water resources is produced.

        Flora and fauna: the incidence on vegetation is null, and photovoltaic systems do not require grid cabling (birds are not harmed by collision with cables).

        Environmental integration: PV modules have different possibilities for integration. They are an easy element for integration and harmonization in different types of structures, thus minimizing their visual impact. Also, in stand alone systems, the countryside is not modified with electricity pylons or electrical cabling.

        Noise: photovoltaic systems are totally silent, that is clearly an advantage over the use of motor driven generators in remote homes.

        Social media: the ground occupied by the installation of a photovoltaic system is insignificant and therefore does not produce a great impact. Furthermore, in most of cases, the PV arrays can be integrated into the roofs and structures of buildings.

        In other words, photovoltaic energy represents one of the best solutions for those places where there is a need for electrical energy whilst preserving the environmental conditions, as is the case in protected natural parks or areas.

11 - What is the peak power of a PV module?

        Is the power output, in Watts, that a PV module produces in standard test conditions (STC) which are 1000 W/m² of irradiance (this is approximately the irradiance on a sunny day around noon), module temperature of 25 ºC and solar spectrum AM1.5G.

12 - What are the elements of a PV module?

        A photovoltaic module is composed of solar cells which are electrically connected in series and in parallel, thereby reaching a suitable voltage for their utilization.


Transversal view of a photovoltaic module

        The cells ensemble is enclosed in materials and elements that provide the necessary protection against the environmental agents and the rigidity to be installed in the support structures. The elements are the following:

        - Encapsulate, composed of a material with good solar radiation transmission characteristics and low degradation against the action of sunlight.

        - External cover of temperate glass, which apart of facilitating the luminous transmission, will withstand the most adverse climatic conditions and sharp temperature changes.

        - Back surface, usually composed of an opaque layer that reflects the light that has crossed over the cell separations, thus causing it to incide again upon the cells.

        - Metallic frame, usually made from aluminum, that ensures the rigidity and the water tightness of the ensemble and provides the necessary elements (usually drilled holes)  to install the module on the support structure.

        - Connection box: incorporates the terminals for module interconnection.

        - Protection diode: to avoid damage by partial shadows on the module surface.

13 - What is the difference between monocrystalline and policrystalline PV modules?

        Photovoltaic modules are composed of monocrystalline or policrystalline silicon photovoltaic cells. The difference between them is the manufacturing process. Monocrystalline silicon cells are obtained from very pure silicon which is melted in a crucible along with a small proportion of boron. Once the material is in its liquid state, a silicon seed is introduced and is then very slowly withdrawn, thereby incorporating the new atoms  from the liquid that take up positions in the crystalline silicon structure. In this way a doped single crystal is obtained which is cut into wafers of about 300 micrometers in width. These wafers are introduced into special furnaces where the phosphorus atoms are diffused into one face. After this, and before doing the screen printing for the surface interconnections, the cells are provided with an anti reflective coating of titanium or Zirconium dioxide.

        In policrystalline cells, as the name suggests the origin is not a single crystal. Molten silicon is solidified slowly in a silicon paste mould, in this way a solid is obtained composed of many small silicon crystals, this solid can again be cut into thin wafers.

14 - Can PV modules operate on cloudy days?

        Photovoltaic modules produce electricity even on cloudy days, nevertheless their efficiency decreases. The production of electricity varies linearly with the incident light and so on an overcast day when the incident light is equivalent to approximately 10% of the total intensity of the sun, the efficiency of the PV modules decreases in proportion to this value.

15 - What factors affect the efficiency of a PV module?

        Basically the luminous radiation intensity and the cell temperature.


Current and voltage variation in function of the solar radiation and temperature

        The current generated by the module increases with the radiation, and the voltage remains more or less constant. In this sense the position of the modules (tilt angle and orientation) is very important due to the fact that the radiation values vary throughout a day as a function of the tilt angle of the sun with respect to the skyline.

        Temperature increase of the cells gives an increase of generated current, but at the same time a reduction (greater than the current increase) of the voltage. The global effect is that the module power decreases when temperature increases. When the module operates at 1000 W/m2 the module temperature can reach about 30 degree above the ambient temperature, and the voltage is reduced about 2 mV/(cell * degree) * 36 cells * 30 degrees = 2.16 Volts and thus the power is reduced by 15%. For this reason it is very important to place the PV modules in a well ventilated place.

16 - Does the use of a tracking system improve the photovoltaic collection efficiency?

        This will depend on the climate and the type of application. At best the efficiency of the system can be improved by 40%, but the higher cost that the use of trackers supposes does not compensate the efficiency increase obtained. The applications are generally limited to those cases where the efficiency increase occurs at the same time as the increase in energy demand (such as in the case of water pumping systems for livestock in very dry regions).

17 - What is the function of the diodes in a photovoltaic installation?

        Diodes are electronic components that allow the flux of current in one direction only. In photovoltaic systems they are generally used in two ways: as blocking diodes and as bypass diodes.

Diode connection diagrams

        Blocking diodes avoid the discharge of the battery through the photovoltaic modules when there is no sun light. They also avoid the current flow becoming inverted between the different parts of a parallel connected array, when in one or more of them a shadow is produced.

        Bypass diodes individually protect each module from possible damages caused by partial shadows. They should be used when the modules are connected in series but are generally not necessary in systems operating at voltages lower than 24 Volts.

        While blocking diodes avoid the flow of current from a group of serially connected modules through another group connected in parallel to the first one, bypass diodes avoid each module individually dissipating current arising from others connected in series, if a shadow is produced in one or more of the modules of the group.

18 - What are the characteristics that define the behavior of a battery?

        There are basically two: capacity in Amp hours and depth of discharge.

        Capacity in Amp hours:

        The Amp hours of a battery are simply the number of Amps that a battery can provide multiplied by the number of hours that this current is available.

        It serves to determine, in photovoltaic installations, how long the system can operate without solar radiation to recharge the batteries. This number of days in which a system can operate autonomously (in the absence of a charging current from the modules) is one of the important aspects in the design of an installation.

        Theoretically, for example, a battery of 200 Ah can provide 200 A for one hour, or 50A for 4 hours, or 4 A for 50 hours, or 1 A for 200 hours.

        Nevertheless, this is not exactly true, some batteries, such as those used in cars, are designed to produce fast discharges in short periods of time  without suffering any damage. They are not designed to operate for long periods of time with a low state of charge. For this reason, car batteries are not suitable for photovoltaic systems.

        There are various factors which can affect the battery capacity:

        - Charge and discharge rates: if the battery is charged or discharged at different rates, the capacity available can increase or decrease. Generally, if the battery is discharged slowly, its capacity will increase a little. If the discharge is fast, the capacity will decrease.

        - Temperature: another factor that affects the capacity is the battery temperature and the ambient temperature. The battery behavior is catalogued at 27 ºC. Lower temperatures reduce its capacity significantly. Higher temperatures produce a small increase of its capacity, but this can increase the water losses and decrease the number of cycles in the battery life.

        Depth of discharge:

        The depth of discharge is the percentage of the total battery capacity that is used in charge/discharge cycle.

        "Shallow cycle" batteries are designed for discharges from 10 to 25% of their capacity in each cycle. Most "deep cycle" batteries manufactured for photovoltaic applications are designed for discharges of up to 80% of their capacity without suffering any damage. Nickel-cadmium battery manufacturers affirm that they can be fully discharged without any damage.

        The depth of discharge even affects deep cycle batteries. The deeper the discharge, the lower the number of charging cycles that the battery can produce.

19 - What is the composition of a lead-acid battery?

        Lead-acid batteries are composed of lead plates in an sulfuric acid solution. The plate consists of a lead alloy grid covered by a  lead oxide paste. The solution of sulfuric acid and water is called electrolyte.

Monoblock battery structure (VARTA)

        The grid material is a lead alloy due to the fact that pure lead is soft and fragile and could be broken during transportation or battery operation.

        The lead is usually alloyed with 2-6% of antimony. The lower the antimony content, the less resistant the battery will be in a charge process. A reduced amount of antimony reduces the hydrogen and oxygen production in a charge process, and thus the water consumption. In other words, a higher proportion of antimony allows deeper discharges without any plate damage, resulting in a longer battery life. This kind of lead-antimony batteries are "deep cycle" type.

        Cadmium and strontium are used in place of antimony to strengthen the grid. These materials offer the same advantages and inconveniences as antimony, but furthermore reduce the percentage of auto discharge that the battery suffers when it is not in operation.

        Calcium also strengthens the grid and reduces the auto discharge. Nevertheless, calcium reduces the recommended depth of discharge to no more than 25%. Lead-calcium batteries are therefore of "shallow cycle" type.

        The positive and negative plates are immersed in a sulfuric acid solution and are subjected to a "growth" charge by the manufacturer. The direction of this charge gives rise to the transformation of the paste on the grid into porous lead dioxide. Both materials are highly porous, thus allowing the sulfuric acid solution to cross over the plates.

        The plates are alternated in the battery and separators are positioned between them. The separators are manufactured from a porous material that permit the flux of the electrolyte. They are electrical isolators and can be a mixture of silicone and plastics or rubbers.

        The separators may be individual sheets or envelopes. The envelopes are opened along their upper edge and are placed over the positive plates.

        A group of positive and negative plates, with separators, form a battery element. An element inmersed in electrolyte in a container forms a battery cell.

        Bigger plates, or a large number of them, increase the amount of Amp hours that the battery can supply.

        Independently of the plate size, a cell will supply a nominal voltage of 2 Volts (for lead-acid). A battery is formed by many cells or elements connected in series either internally or externally, in order to obtain a suitable voltage for normal electrical applications. For this reason, a battery of 6 Volts is composed of three cells, and one of 12 Volts of 6 cells.

        The positive plates on one side, and the negative plates on the other, are interconnected by means of external terminals on the top of the battery.

20 - What are the differences between lead-acid and nickel-cadmium batteries?

        Nickel-cadmium batteries have a similar physical structure to lead-acid batteries. Instead of lead, nickel hydroxide is used for the positive plates and cadmium oxide for the negative plates. The electrolyte is potassium hydroxide.

        The nominal voltage of a Ni-Cd battery cell is 1.2V, in place of the 2V of the lead-acid battery cells.

        Ni-Cd batteries can support freezing and defrosting processes without any negative effects on their performance. High temperatures have less influence than on lead-acid batteries. The auto discharge values are in the range of 3% to 6% a month.

        They are less sensitive to overcharges, they can be fully discharged without suffering any damage, they do not suffer sulfation and their ability to accept a charge cycle is temperature independent.

        The cost of a Ni-Cd battery is much higher than the cost of a lead-acid battery, nevertheless they have lower maintenance requirements and a longer life. Ni-Cd batteries are recommended for remote places or where access is dangerous or problematic.

        The test results of Ni-Cd batteries are not as reliable as those of lead-acid batteries. Therefore, if it is necessary to control the level of the charge, Ni-Cd batteries are not the best option.

        Ni-Cd batteries present the “memory effect”: the battery “remembers” the depth of discharge and reduces its effective capacity. This is due to the formation of a chemical compound on a charged plate which tends to chrystalise, for this reason, if left unused for a certain amount of time the battery will lose capacity. This process is not irreversible but its reversion is difficult.

21 - What is 'sulfation' of a lead-acid battery?

      If a lead-acid battery remains in a state of deep discharge for a long time, it will suffer a sulfation process. Some of the sulfuric acid will combine with the lead from the plates to form lead sulfate. If the battery is not topped up periodically with distilled water, part of the plates will remain exposed to the air and the process will be acelerated.

        The lead sulfate covers the plates in such a way that the electrolyte cannot penetrate them. This means an irreversible loss of battery capacity that even with the addition of water, cannot be recovered.

22 -  How can the state of charge of a battery be found out?

A hydrometer (in pieces) used in stationary batteries

        The easiest way is through the measurement of the density (or specific gravity) of the liquid inside the battery (the electrolyte). The density shows the weight of the electrolyte in comparison with the same amount of water, and is measured using an hydrometer. The most common hydrometers are those used for cars, which show the charge as a percentage. However, the disadvantage of these devices is that they are calibrated for the electrolyte used in starting batteries and not for stationary batteries, for this reason they will show less than the real value (50% for fully charged stationary batteries).

Typical values of Density and voltage for a cell in a lead-acid battery

        The higher the density of the electrolyte, the higher the state of charge of the battery. The voltage in each cell, and thus the battery voltage, is also higher. The density measurement in a discharge process will be a good indicator of state of charge of the battery. During a charging process, the density will delay the measurement of the state of charge because the complete mixture of the electrolyte will not take place until the gassing process begins, near the end of the charging process (see graph). In any case, this should not be considered as an absolute measurement of the battery capacity and should be combined with other techniques.

23 - Can the batteries become frozen? At what temperature?

        As the lead-acid batteries use an electrolyte containing water, it may freeze. Nevertheless, the sulfuric acid act like an antifreeze. The higher the percentage of acid in the water, the lower the freezing point of the electrolyte. Nevertheless, even a fully charged battery operating at extremely low temperatures may freeze.

        As shown in the following table, a lead-acid battery at a state of charge of 50% will freeze at about -25ºC.

        As can be seen, the battery should be kept above -10 ºC, if it is to remain fully discharged. If a higher temperature can not be maintained, the state of charge should be at a high enough level to avoid freezing. This can be achieved automatically using a charge regulator which is able to disconnect the load when the battery voltage decreases below a defined level.


State Density Volts/cell Volts/battery Freezing point
Charged 1,265 2,12 12,70 -57ºC
Charged 75% 1,225 2,10 12,60 -38ºC
Charged 50% 1,190 2,08 12,45 -25ºC
Charged 25% 1,155 2,03 12,20 -16ºC
Discharged 1,120 1,95 11,70 -10ºC
 State of charge, density, voltage and freezing point for a lead-acid battery

24 - What are the effects upon the capacity and the voltage of serial or parallel connected batteries?

        The batteries can be connected in series in order to increase the voltage, or in parallel to increase the capacity, in Amp hours, of the battery system.

        When connecting a group of batteries in series and parallel, both voltage and capacity are increased.

Batteries connected in parallel, in series and in series and parallel

25 - What is the effect of the fast discharge of a battery?

        Firstly, not all the energy that a battery can supply is obtained. For example, a battery which discharges in 72 hours can supply about twice the energy that it would supply if discharged in only 8 hours.

        Also, fast discharge of a battery produces deformation and premature disintegration of the cell plates, which disintegrate, giving rise to a sediment in the bottom of the battery containers. This sediment can cause a short circuit between the plates, thus destroying the battery.

26 - What are the effects of heat on a battery?

        An increase of temperature is highly damaging for batteries. If the container temperature is higher than 40 ºC, it is necessary to decrease the charge regimen.

27 - Where should the batteries be installed?

        A warm place should be found, avoiding very low temperatures. It is also necessary to avoid temperatures lower than 0 ºC because the internal resistance of the battery increases a lot.

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

        The lead sulfate that covers the plates will become hard when the battery is left discharged; the pores become obstructed and do not allow the electrolyte to flow, the active elements of the plates therefore cannot function, thus reducing the effective capacity of the battery. The end result is that a sulfated battery is more difficult to charge.

29 - What are the most common causes of battery sulfation?

        The most common causes of battery sulfation are:

        - A battery which remains discharged for a long period of time.
        - The addition of pure acid to the electrolyte.
        - Frequent overcharges.
        - Not filling up with distilled water at the right time.
        - The transfer of electrolyte from one cell to other.

30 - What are the symptoms that show that a battery is sulfated?

        The symptoms are:

        - The hydrometer always measures a low density, even though the cell is charged in the same way as the other cells.
        - The voltage is always lower than the voltage of normal cells.
        - It is not possible to recharge the battery to its full capacity.
        - The sulfated cell does not allow the usual intensity flow due to a high increase in its electrical resistance.
        - Both positive and negative plates are very light in color.

31 - What kind of water should be used to fill up the batteries?

        Distilled water or rain water, which should be stored in very clean glass containers. The rain water is the best, but it should be collected in such a way so as to avoid contact with metals (zinc roofs, etc) as this will cause the incorporation of impurities. Collection on a roof with ceramic tiles or canvas, for example, are very good options.

32 - Is the use of a charge controller in a photovoltaic installation necessary? In what cases can the charge controller be avoided?

        The primary function of a charge controller in a photovoltaic system is to protect the battery form overcharges or overdischarges. Almost all installations require a charge controller. Systems without charge controllers will reduce the battery life and the load availability.

        Systems with small, predictable and continuous loads can be designed to operate without charge controller. If the system has an over-sized battery and the discharge regimen will never go below the critical depth of discharge of the battery, the charge controller can be avoided.

33 - What is the difference between the different kind of inverters? What is the most suitable inverter for each application?

        Inverters transform DC current into AC current. DC current has a current flux which only flows in one direction, while AC current changes the current flux very fast from one direction to the other. The frequency of AC current in Spain is usually 50 cycles per second (Hertzs). Each cycle involves the movement of current in one direction and then in the other. This means that the direction of the current changes 100 times every second.

Different wave forms for AC current (50 Hz)

        AC current supplied by an electrical company or by a diesel generator is (or should be) as shown in the figure in black. The magnitude changes of the voltage follow a sinusoidal law, in such a way that the current can also be represented as a sinusoidal wave.

        The conversion of DC current to AC current can be performed in different ways. The best way will depend on how close it should be to an ideal sinusoidal wave in order to correctly operate an AC load:

        Square wave inverters: most inverters work by making the current flow through a transformer, first in one direction and then in the other. The commutation device that changes the current direction should act very fast. As the current goes through the primary side of the transformer, the polarity changes 100 times every second. In consequence, the current leaving the secondary coil of the transformer alternates at a frequency of 50 cycles per second. The current flow through the primary side of the transformer changes very sharply, in such a way that the wave form of the secondary is "square", as shown in the figure in pink.

        Square wave inverter are cheaper, but they are also usually less efficient. They produce harmonics and interference (noises) and are not suitable for induction motors.

        If AC current is required to operate a TV, a computer or a small electric device, this kind of inverter can be used. The power will depend on the nominal power of the load (for a 19" TV a 200 W inverter is sufficient).

        Modified square wave inverters: are more sophisticated and expensive, and they use pulse width modulation techniques (PWM): the width of the pulse is modified so as to be as close as possible to a sinusoidal wave form. The output is not quite a real sinusoidal wave, but it is very close. The harmonic contain is lower than with a square wave. In the figure it is represented in blue. These inverters present the best ratio quality/price for lighting, TV or frequency converters connection. An example of this kind of inverters is the SM-1500.

        Sinewave inverters: with more elaborated electronics a sinewave can be obtained. Until recently these inverters were big, expensive and of low efficiency (sometimes only 40%). Recently, new sinewave inverters have been developed with an efficiency of 90% or more, in function of the power, for example the S-1200. The use of state-of-the-art microprocessors has permitted the incorporation of other functions such as remote control, energy counting, battery selection,... Nevertheless, their cost is higher than that of less sophisticated inverters.

        As only induction motors or the most sophisticated devices or loads require a sinusoidal wave form, it is usually preferable to use a less expensive and more efficient inverter. In the near future the cost of sinewave inverters will decrease, thereby increasing their utilization.

34 - How to size the inverter

        The inverters should be sized in two ways. The first takes into account the electrical power (measured in Watts) that the inverter can supply during normal and continuous operation.

        The inverters are less efficient when operating at a low percentage of their capacity, for this reason it is not advisable to oversize the inverters, it should be selected with a power as close as possible to the load.

        The second way is to size the inverter using the starting power. Some inverters can supply more than their nominal capacity for short periods of time. This surge capacity is important when using motors or other loads that need from 2 to 7 timers more the power to start-up than to remain in operation (induction motors, high power lamps...).

35 - What is the difference between resistive and inductive loads?

        A load is any device that consumes energy in an electrical system. Appliances, and electrical devices in general, are divided into two big groups of loads: resistive and inductive. Resistive loads are simply those in which the electricity is used to  produce heat and not movement. Typical loads of this kind are incandescent lamps or electric heaters.

        Inductive loads are generally those in which the electricity flows through coils. They are usually motors, such as fans or refrigerators; or transformers, that are present in most electronic devices, such as TVs, computers or fluorescent lamps.

36 - What kind of lighting elements are recommended to be installed in photovoltaic systems?

        Due to the intrinsic characteristics of photovoltaic systems, in which the capacity of energy storage is limited, the lighting devices should be of high efficiency and low consumption in order to maximize the usage time. The best are electronic lamps, these have the same luminous capabilities as conventional incandescent lamps but their energy comsuption is approximately 80% lower and their life-time 8 times higher. This is due to the fact that 95% of the energy consumed by incandescent lamps is transformed into heat and not into light, while electronic lamps irradiate much less heat and transform about 30% of the consumed energy into light. Also, conventional fluorescent lamps can be used but the should always be accompanied by electronic reactances.

37 - What are the differences between a direct photovoltaic pumping system and a conventional one?

        A conventional solar water pumping system is composed of PV modules, batteries, an inverter (if needed) and the pump. In direct solar water pumping there is no charge controller nor batteries, and a cheaper inverter is used in place of a conventional one. This reduces the installation costs and the maintenance requirements. On the other hand, it can only pump water during the daytime. In some installations it is necessary to store the water in a tank or reservoir, the function of these deposits is very similar to that of the batteries, they are used to store the water for use when the pump is not operational. If the direct pumping system uses a positive displacement pump the energetic efficiency will be almost twice that of a conventional pumping system, thereby reducing the number of photovoltaic modules required and, in consequence and despite the higher cost of the pump itself,  the price of the installation.

38 - What is a fuel cell?

        A fuel cell is an electrochemical device that generates electricity from chemical energy. Its structure is very similar to that of the accumulators commonly used in solar energy installations: it is composed of an electrolyte (alkaline, phosporic acid, molten carbonate or solid oxid) and two electrodes. The anode is inmersed in the fuel (normally hydrogen) and the cathode in the oxidant (normally oxygen). Both electrodes have very porous surfaces, and, in combination with the high pressures and temperatures required, maximize the reaction. The residue of the reaction is hot water. The fuel cells made with phosporic acid have an efficiency of 40% and a working temperature of 200ºC. The most common units generate about 200 kW.