Solar Power:Solar energy, Photovoltaic system components, The battery

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Solar Power

This chapter provides an introduction to the components of a standalone

photovoltaic system. The word standalone refers to the fact that the system

works without any connection to an established power grid. In this chapter,

we will present the basic concepts of the generation and storage of photovol-

taic solar energy. We will also provide a method for designing a functional

solar system with limited access to information and resources.

This chapter only discusses the use of solar energy for the direct production

of electricity (photovoltaic solar energy). Solar energy can also be used to

heat fluids (thermal solar energy) which can then be used as a heat source

or to turn a turbine to generate electricity. Thermal solar energy systems are

beyond the scope of this chapter.

Solar energy

A photovoltaic system is based on the ability of certain materials to convert

the radiant energy of the sun into electrical energy. The total amount of solar

energy that lights a given area is known as irradiance (G) and it is measured

in watts per square meter (W/m2). The instantaneous values are normally

averaged over a period of time, so it is common to talk about total irradiance

per hour, day or month.

Of course, the precise amount of radiation that arrives at the surface of the

Earth cannot be predicted with high precision, due to natural weather varia-

tions. Therefore it is necessary to work with statistical data based on the "so-

lar history" of a particular place. This data is gathered by a weather station

over a long period and is available from a number of sources, as tables or

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databases. In most cases, it can be difficult to find detailed information about

a specific area, and you will need to work with approximate values.

A few organizations have produced maps that include average values of daily

global irradiation for different regions. These values are known as peak sun

hours or PSHs. You can use the PSH value for your region to simplify your

calculations. One unit of "peak sun" corresponds to a radiation of 1000 Watts

per square meter. If we find that certain area has 4 PSH in the worst of the

months, it means that in that month we should not expect a daily irradiation

bigger than 4000 W/m2 (day). The peak sun hours are an easy way to repre-

sent the worst case average of irradiation per day.

Low resolution PSH maps are available from a number of online sources, such

as http://www.solar4power.com/solar-power-global-maps.html. For more de-

tailed information, consult a local solar energy vendor or weather station.

What about wind power?

It is possible to use a wind generator in place of solar panels when an

autonomous system is being designed for installation on a hill or mountain.

To be effective, the average wind speed over the year should be at least 3 to

4 meter per second, and the wind generator should be 6 meters higher than

other objects within a distance of 100 meters. A location far away from the

coast usually lacks sufficient wind energy to support a wind powered system.

Generally speaking, photovoltaic systems are more reliable than wind gen-

erators, as sunlight is more available than consistent wind in most places. On

the other hand, wind generators are able to charge batteries even at night, as

long as there is sufficient wind. It is of course possible to use wind in con-

junction with solar power to help cover times when there is extended cloud

cover, or when there is insufficient wind.

For most locations, the cost of a good wind generator is not justified by the

meager amount of power it will add to the overall system. This chapter will

therefore focus on the use of solar panels for generating electricity.

Photovoltaic system components

A basic photovoltaic system consists of four main components: the solar

panel, the batteries, the regulator, and the load. The panels are responsi-

ble for collecting the energy of the sun and generating electricity. The battery

stores the electrical energy for later use. The regulator ensures that panel

and battery are working together in an optimal fashion. The load refers to any

device that requires electrical power, and is the sum of the consumption of all

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electrical equipment connected to the system. It is important to remember

that solar panels and batteries use direct current (DC).

If the range of operational voltage of your equipment does not fit the voltage

supplied by your battery, it will also be necessary to include some type of

converter. If the equipment that you want to power uses a different DC volt-

age than the one supplied by the battery, you will need to use a DC/DC con-

verter. If some of your equipment requires AC power, you will need to use a

DC/AC converter, also known as an inverter.

Every electrical system should also incorporate various safety devices in the

event that something goes wrong. These devices include proper wiring, cir-

cuit breakers, surge protectors, fuses, ground rods, lighting arrestors, etc.

The solar panel

The solar panel is composed of solar cells that collect solar radiation and

transform it into electrical energy. This part of the system is sometimes referred

to as a solar module or photovoltaic generator. Solar panel arrays can be

made by connecting a set of panels in series and/or parallel in order to provide

the necessary energy for a given load. The electrical current supplied by a so-

lar panel varies proportionally to the solar radiation. This will vary according to

climatological conditions, the hour of the day, and the time of the year.

Figure 7.1: A solar panel

Several technologies are used in the manufacturing of solar cells. The most

common is crystalline silicon, and can be either monocrystalline or polycrystal-

line. Amorphous silicon can be cheaper but is less efficient at converting solar

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energy to electricity. With a reduced life expectancy and a 6 to 8% transforma-

tion efficiency, amorphous silicon is typically used for low power equipment,

such as portable calculators. New solar technologies, such as silicon ribbon

and thin film photovoltaics, are currently under development. These technolo-

gies promise higher efficiencies but are not yet widely available.

The battery

The battery stores the energy produced by the panels that is not immedi-

ately consumed by the load. This stored energy can then be used during pe-

riods of low solar irradiation. The battery component is also sometimes called

the accumulator. Batteries store electricity in the form of chemical energy.

The most common type of batteries used in solar applications are

maintenance-free lead-acid batteries, also called recombinant or VRLA

(valve regulated lead acid) batteries.

Figure 7.2: A 200 Ah lead-acid battery. The negative terminal was broken due to

weight on the terminals during transportation.

Aside from storing energy, sealed lead-acid batteries also serve two impor-

tant functions:

· They are able to provide an instantaneous power superior to what the array

of panels can generate. This instantaneous power is needed to start some

appliances, such as the motor of a refrigerator or a pump.

· They determine the operating voltage of your installation.

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For a small power installation and where space constraints are important,

other type of batteries (such as NiCd, NiMh, or Li-ion) can be used. These

types of batteries need a specialized charger/regulator and cannot directly

replace lead-acid batteries.

The regulator

The regulator (or more formally, the solar power charge regulator) assures

that the battery is working in appropriate conditions. It avoids overcharging or

overdischarging the battery, both of which are very detrimental to the life of

the battery. To ensure proper charging and discharging of the battery, the regu-

lator maintains knowledge of the state of charge (SoC) of the battery. The

SoC is estimated based on the actual voltage of the battery. By measuring

the battery voltage and being programmed with the type of storage technol-

ogy used by the battery, the regulator can know the precise points where the

battery would be overcharged or excessively discharged.

Figure 7.3: A 30 Amp solar charge controller

The regulator can include other features that add valuable information and

security control to the equipment. These features include ammeters, voltme-

ters, measurement of ampere-hour, timers, alarms, etc. While convenient,

none of these features are required for a working photovoltaic system.

The converter

The electricity provided by the panel array and battery is DC at a fixed volt-

age. The voltage provided might not match what is required by your load. A

direct/alternating (DC/AC) converter, also known as inverter, converts

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the DC current from your batteries into AC. This comes at the price of losing

some energy during the conversion. If necessary, you can also use convert-

ers to obtain DC at voltage level other than what is supplied by the batteries.

DC/DC converters also lose some energy during the conversion. For opti-

mal operation, you should design your solar-powered system to match the

generated DC voltage to match the load.

Figure 7.4: An 800 Watt DC/AC converter (power inverter)

The load

The load is the equipment that consumes the power generated by your energy

system. The load may include wireless communications equipment, routers,

workstations, lamps, TV sets, VSAT modems, etc. Although it is not possible to

precisely calculate the exact total consumption of your equipment, it is vital to

be able to make a good estimate. In this type of system it is absolutely nec-

essary to use efficient and low power equipment to avoid wasting energy.

Putting it all together

The complete photovoltaic system incorporates all of these components. The

solar panels generate power when solar energy is available. The regulator

ensures the most efficient operation of the panels and prevents damage to

the batteries. The battery bank stores collected energy for later use. Con-

verters and inverters adapt the stored energy to match the requirements of

your load. Finally, the load consumes the stored energy to do work. When all

of the components are in balance and are properly maintained, the system

will support itself for years.

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217

Regulator

DC/DC

DC load

Solar panels

Inverter

AC load

Battery bank

Figure 7.5: A solar installation with DC and AC loads

We will now examine each of the individual components of the photovoltaic

system in greater detail.

The solar panel

An individual solar panel is made of many solar cells. The cells are electri-

cally connected to provide a particular value of current and voltage. The indi-

vidual cells are properly encapsulated to provide isolation and protection

from humidity and corrosion.

Figure 7.6: The effect of water and corrosion in a solar panel

There are different types of modules available on the market, depending on

the power demands of your application. The most common modules are

composed of 32 or 36 solar cells of crystalline silicon. These cells are all of

equal size, wired in series, and encapsulated between glass and plastic ma-

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terial, using a polymer resin (EVA) as a thermal insulator. The surface area of

the module is typically between 0.1 and 0.5 m2. Solar panels usually have

two electrical contacts, one positive and one negative.

Some panels also include extra contacts to allow the installation of bypass

diodes across individual cells. Bypass diodes protect the panel against a

phenomenon known as "hot-spots". A hot-spot occurs when some of the cells

are in shadow while the rest of the panel is in full sun. Rather than producing

energy, shaded cells behave as a load that dissipates energy. In this situa-

tion, shaded cells can see a significant increase in temperature (about 85 to

100ºC.) Bypass diodes will prevent hot-spots on shaded cells, but reduce the

maximum voltage of the panel. They should only be used when shading is

unavoidable. It is a much better solution to expose the entire panel to full sun

whenever possible.

Irradiance: 1kW / m2

Cell Temperature 25C

75C 50C 25C

800W / m2

600W / m2

400W / m2

200W / m2

Voltage (V)

Figure 7.7: Different IV Curves. The current (A) changes with the irradiance, and the

voltage (V) changes with the temperature.

The electrical performance of a solar module its represented by the IV char-

acteristic curve, which represents the current that is provided based on the

voltage generated for a certain solar radiation.

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219

The curve represents all the possible values of voltage-current. The curves

depend on two main factors: the temperature and the solar radiation received

by the cells. For a given solar cell area, the current generated is directly pro-

portional to solar irradiance (G), while the voltage reduces slightly with an

increase of temperature. A good regulator will try to maximize the amount of

energy that a panel provides by tracking the point that provides maximum

power (V x I). The maximum power corresponds to the knee of the IV curve.

Solar Panel Parameters

The main parameters that characterize a photovoltaic panel are:

1. SHORT CIRCUIT CURRENT (ISC): the maximum current provided by the

panel when the connectors are short circuited.

2. OPEN CIRCUIT VOLTAGE (VOC): the maximum voltage that the panel

provides when the terminals are not connected to any load (an open cir-

cuit). This value is normally 22 V for panels that are going to work in 12 V

systems, and is directly proportional to the number of cells connected in

series.

3. MAXIMUM POWER POINT (Pmax): the point where the power supplied

by the panel is at maximum, where Pmax = Imax x Vmax. The maximum

power point of a panel is measured in Watts (W) or peak Watts (Wp). It is

important not to forget that in normal conditions the panel will not work at

peak conditions, as the voltage of operation is fixed by the load or the

regulator. Typical values of Vmax and Imax should be a bit smaller than the

ISC and VOC

4. FILL FACTOR (FF): the relation between the maximum power that the

panel can actually provide and the product ISC . VOC. This gives you an

idea of the quality of the panel because it is an indication of the type of IV

characteristic curve. The closer FF is to 1, the more power a panel can

provide. Common values usually are between 0.7 and 0.8.

5. EFFICIENCY (h): the ratio between the maximum electrical power that

the panel can give to the load and the power of the solar radiation (PL)

incident on the panel. This is normally around 10-12%, depending on the

type of cells (monocrystalline, polycrystalline, amorphous or thin film).

Considering the definitions of point of maximum power and the fill factor we

see that:

h = Pmax / PL = FF . ISC . VOC / PL

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The values of ISC, VOC, IPmax and VPmax are provided by the manufacturer and

refer to standard conditions of measurement with irradiance G = 1000 W/m2,

at sea-level, for a temperature of cells of Tc = 25ºC.

The panel parameters values change for other conditions of irradiance and tem-

perature. Manufacturers will sometimes include graphs or tables with values for

conditions different from the standard. You should check the performance values

at the panel temperatures that are likely to match your particular installation.

Be aware that two panels can have the same Wp but very different behavior

in different operating conditions. When acquiring a panel, it is important to

verify, if possible, that their parameters (at least, ISC and VOC) match the val-

ues promised by the manufacturer.

Panel parameters for system sizing

To calculate the number of panels required to cover a given load, you just

need to know the current and voltage at the point of maximum power: IPmax

and VPmax.

You should always be aware that the panel is not going to perform under per-

fect conditions as the load or regulation system is not always going to work at

the point of maximum power of the panel. You should assume a loss of effi-

ciency of 5% in your calculations to compensate for this.

Interconnection of panels

A solar panel array is a collection of solar panels that are electrically inter-

connected and installed on some type of support structure. Using a solar

panel array allows you to generate greater voltage and current than is possi-

ble with a single solar panel. The panels are interconnected in such a way

that the voltage generated is close to (but greater than) the level of voltage of

the batteries, and that the current generated is sufficient to feed the equip-

ment and to charge the batteries.

Connecting solar panels in series increases the generated voltage. Connect-

ing panels in parallel increases the current. The number of panels used

should be increased until the amount of power generated slightly exceeds

the demands of your load.

It is very important that all of the panels in your array are as identical as possi-

ble. In an array, you should use panels of the same brand and characteristics

because any difference in their operating conditions will have a big impact on

the health and performance of your system. Even panels that have identical

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221

performance ratings will usually display some variance in their characteristics

due to manufacturing processes. The actual operating characteristics of two

panels from the same manufacturer can vary by as much as ±10%.

Whenever possible, it is a good idea to test the real-world performance of

individual panels to verify their operating characteristics before assembling

them into an array.

Figure 7.8: Interconnection of panels in parallel. The voltage remains constant while

the current duplicates. (Photo: Fantsuam Foundation, Nigeria)

How to choose a good panel

One obvious metric to use when shopping for solar panels is to compare the

ratio of the nominal peak power (Wp) to the price. This will give you a rough

idea of the cost per Watt for different panels. But there are a number of other

considerations to keep in mind as well.

If you are going to install solar panels in geographical areas where soiling

(from dust, sand, or grit) will likely be a problem, consider purchasing pan-

els with a low affinity for soil retention. These panels are made of materials

that increase the likelihood that the panel will be automatically cleaned by

wind and rain.

Always check the mechanical construction of each panel. Verify that the

glass is hardened and the aluminum frame is robust and well built. The solar

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cells inside the panel can last for more than 20 years, but they are very frag-

ile and the panel must protect them from mechanical hazards. Look for the

manufacturer's quality guarantee in terms of expected power output and me-

chanical construction.

Finally, be sure that the manufacturer provides not only the nominal peak

power of the panel (Wp) but also the variation of the power with irradiation

and temperature. This is particularly important when panels are used in ar-

rays, as variations in the operating parameters can have a big impact on the

quality of power generated and the useful lifetime of the panels.

The batter y

The battery "hosts" a certain reversible chemical reaction that stores electri-

cal energy that can later be retrieved when needed. Electrical energy is

transformed into chemical energy when the battery is being charged, and the

reverse happens when the battery is discharged.

A battery is formed by a set of elements or cells arranged in series. Lead-

acid batteries consist of two submerged lead electrodes in an electrolytic so-

lution of water and sulfuric acid. A potential difference of about 2 volts takes

place between the electrodes, depending on the instantaneous value of the

charge state of the battery. The most common batteries in photovoltaic solar

applications have a nominal voltage of 12 or 24 volts. A 12 V battery there-

fore contains 6 cells in series.

The battery serves two important purposes in a photovoltaic system: to pro-

vide electrical energy to the system when energy is not supplied by the array

of solar panels, and to store excess energy generated by the panels when-

ever that energy exceeds the load. The battery experiences a cyclical proc-

ess of charging and discharging, depending on the presence or absence of

sunlight. During the hours that there is sun, the array of panels produces

electrical energy. The energy that is not consumed immediately it is used to

charge the battery. During the hours of absence of sun, any demand of elec-

trical energy is supplied by the battery, thereby discharging it.

These cycles of charge and discharge occur whenever the energy produced

by the panels does not match the energy required to support the load. When

there is sufficient sun and the load is light, the batteries will charge. Obvi-

ously, the batteries will discharge at night whenever any amount of power is

required. The batteries will also discharge when the irradiance is insufficient

to cover the requirements of the load (due to the natural variation of climato-

logical conditions, clouds, dust, etc.)

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223

If the battery does not store enough energy to meet the demand during peri-

ods without sun, the system will be exhausted and will be unavailable for

consumption. On the other hand, the oversizing the system (by adding far too

many panels and batteries) is expensive and inefficient. When designing a

stand-alone system we need to reach a compromise between the cost of

components and the availability of power from the system. One way to do

this is to estimate the required number of days of autonomy. In the case of

a telecommunications system, the number of days of autonomy depends on

its critical function within your network design. If the equipment is going to

serve as repeater and is part of the backbone of your network, you will likely

want to design your photovoltaic system with an autonomy of up to 5-7 days.

On the other hand, if the solar system is responsible for a providing energy to

client equipment you can probably reduce number of days of autonomy to

two or three. In areas with low irradiance, this value may need to be in-

creased even more. In any case, you will always have to find the proper bal-

ance between cost and reliability.

Types of batteries

Many different battery technologies exist, and are intended for use in a vari-

ety of different applications. The most suitable type for photovoltaic applica-

tions is the stationary battery, designed to have a fixed location and for

scenarios where the power consumption is more or less irregular. "Station-

ary" batteries can accommodate deep discharge cycles, but they are not de-

signed to produce high currents in brief periods of time.

Stationary batteries can use an electrolyte that is alkaline (such as Nickel-

Cadmium) or acidic (such as Lead-Acid). Stationary batteries based on

Nickel-Cadmium are recommended for their high reliability and resistance

whenever possible. Unfortunately, they tend to be much more expensive and

difficult to obtain than sealed lead-acid batteries.

In many cases when it is difficult to find local, good and cheap stationary bat-

teries (importing batteries is not cheap), you will be forced to use batteries

targeted to the automobile market.

Using car batteries

Automobile batteries are not well suited for photovoltaic applications as they

are designed to provide a substantial current for just few seconds (when

starting then engine) rather than sustaining a low current for long period of

time. This design characteristic of car batteries (also called traction batter-

ies) results in an shortened effective life when used in photovoltaic systems.

Traction batteries can be used in small applications where low cost is the

most important consideration, or when other batteries are not available.

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Traction batteries are designed for vehicles and electric wheelbarrows. They

are cheaper than stationary batteries and can serve in a photovoltaic installa-

tion, although they require very frequent maintenance. These batteries

should never be deeply discharged, because doing so will greatly reduce

their ability to hold a charge. A truck battery should not discharged by more

than 70% of its total capacity. This means that you can only use a maximum

of 30% of a lead-acid battery's nominal capacity before it must be recharged.

You can extend the life of a lead-acid battery by using distilled water. By us-

ing a densimeter or hydrometer, you can measure the density of the battery's

electrolyte. A typical battery has specific gravity of 1.28. Adding distilled water

and lowering the density to 1.2 can help reduce the anode's corrosion, at a

cost of reducing the overall capacity of the battery. If you adjust the density of

battery electrolyte, you must use distilled water, as tap water or well water

will permanently damage the battery.

States of charge

There are two special state of charge that can take place during the cyclic

charge and discharge of the battery. They should both be avoided in order to

preserve the useful life of the battery.

Overcharge

Overcharge takes place when the battery arrives at the limit of its capacity. If

energy is applied to a battery beyond its point of maximum charge, the electro-

lyte begins to break down. This produces bubbles of oxygen and hydrogen, in

a process is known as gasification. This results in a loss of water, oxidation on

the positive electrode, and in extreme cases, a danger of explosion.

On the other hand, the presence of gas avoids the stratification of the

acid. After several continuous cycles of charge and discharge, the acid

tends to concentrate itself at the bottom of the battery thereby reducing

the effective capacity. The process of gasification agitates the electrolyte

and avoids stratification.

Again, it is necessary to find a compromise between the advantages (avoid-

ing electrolyte stratification) and the disadvantages (losing water and produc-

tion of hydrogen). One solution is to allow a slight overcharge condition every

so often. One typical method is to allow a voltage of 2.35 to 2.4 Volts for each

element of the battery every few days, at 25ºC. The regulator should ensure

a periodical and controlled overcharges.

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Overdischarge

In the same way that there is a upper limit, there is also a lower limit to a bat-

tery's state of charge. Discharging beyond that limit will result in deterioration

of the battery. When the effective battery supply is exhausted, the regulator

prevents any more energy from being extracted from the battery. When the

voltage of the battery reaches the minimum limit of 1.85 Volts per cell at

25°C, the regulator disconnects the load from the battery.

If the discharge of the battery is very deep and the battery remains dis-

charged for a long time, three effects take place: the formation of crystallized

sulfate on the battery plates, the loosening of the active material on the bat-

tery plate, and plate buckling. The process of forming stable sulfate crystals

is called hard sulfation. This is particularly negative as it generates big crys-

tals that do not take part in any chemical reaction and can make your battery

unusable.

Battery Parameters

The main parameters that characterize a battery are:

· Nominal Voltage, VNBat. the most common value being 12 V.

· Nominal Capacity, CNBat: the maximum amount of energy that can be ex-

tracted from a fully charged battery. It is expressed in Ampere-hours (Ah)

or Watt-hours (Wh). The amount of energy that can be obtained from a

battery depends on the time in which the extraction process takes place.

Discharging a battery over a long period will yield more energy compared

to discharging the same battery over a short period. The capacity of a bat-

tery is therefore specified at different discharging times. For photovoltaic

applications, this time should be longer than 100 hours (C100).

· Maximum Depth of Discharge, DoDmax: The depth of discharge is the

amount of energy extracted from a battery in a single discharge cycle, ex-

pressed as a percentage. The life expectancy of a battery depends on how

deeply it is discharged in each cycle. The manufacturer should provide graphs

that relate the number of charge-discharge cycles to the life of the battery. As a

general rule you should avoid discharging a deep cycle battery beyond

50%. Traction batteries should only be discharged by as little as 30%.

· Useful Capacity, CUBat: It is the real (as in usable) capacity of a battery. It

is equal to the product of the nominal capacity and the maximum DoD. For

example, a stationary battery of nominal capacity (C100) of 120 Ah and

depth of discharge of 70% has a useful capacity of (120 x 0.7) 84 Ah.

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Measuring the state of charge of the battery

A sealed lead-acid battery of 12 V provides different voltages depending on

its state of charge. When the battery is fully charged in an open circuit, the

output voltage is about 12.8 V. The output voltage lowers quickly to 12.6 V

when loads are attached. As the battery is providing constant current during

operation, the battery voltage reduces linearly from 12.6 to 11.6 V depending

on the state of charge. A sealed lead-acid batteries provides 95% of its en-

ergy within this voltage range. If we make the broad assumption that a fully

loaded battery has a voltage of 12.6 V when "full" and 11.6 V when "empty",

we can estimate that a battery has discharged 70% when it reaches a volt-

age of 11.9 V. These values are only a rough approximation since they de-

pend on the life and quality of the battery, the temperature, etc.

State of Charge

12V Battery

Volts per Cell

Voltage

100%

12.7

2.12

90%

12.5

2.08

80%

12.42

2.07

70%

12.32

2.05

60%

12.2

2.03

50%

12.06

2.01

40%

11.9

1.98

30%

11.75

1.96

20%

11.58

1.93

10%

11.31

1.89

10.5

1.75

According to this table, and considering that a truck battery should not be

discharged more than 20% to 30%, we can determine that the useful capac-

ity of a truck 170 Ah truck battery is 34 Ah (20%) to 51 Ah (30%). Using the

same table, we find that we should program the regulator to prevent the bat-

tery from discharging below 12.3 V.

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Battery and regulator protection

Thermomagnetic circuit breakers or one time fuses must be used to protect

the batteries and the installation from short circuit and malfunctions. There

are two types of fuses: slow blow, and quick blow. Slow blow fuses should

be used with inductive or capacitive loads where a high current can occur at

power up. Slow blow fuses will allow a higher current than their rating to pass

for a short time. Quick blow fuses will immediately blow if the current flowing

through them is higher than their rating.

The regulator is connected to the battery and the loads, so two different kinds

of protection needs to be considered. One fuse should be placed between

the battery and the regulator, to protect the battery from short-circuit in case

of regulator failure. A second fuse is needed to protect the regulator from ex-

cessive load current. This second fuse is normally integrated into the regula-

tor itself.

Figure 7.9: A battery bank of 3600 Ah, currents reach levels of 45 A during charging

Every fuse is rated with a maximum current and a maximum usable voltage.

The maximum current of the fuse should be 20% bigger than the maximum

current expected. Even if the batteries carry a low voltage, a short circuit can

lead to a very high current which can easily reach several hundred amperes.

Large currents can cause fire, damage the equipment and batteries, and

possibly cause electric shock to a human body

If a fuse breaks, never replace a fuse with a wire or a higher rated fuse. First

determine the cause of the problem, then replace the fuse with another one

which has the same characteristics.

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Temperature effects

The ambient temperature has several important effects on the characteristics

of a battery:

· The nominal capacity of a battery (that the manufacturer usually gives for

25°C) increases with temperature at the rate of about 1%/°C. But if the

temperature is too high, the chemical reaction that takes place in the bat-

tery accelerates, which can cause the same type of oxidation that takes

places during overcharging. This will obviously reduce the life expectancy

of battery. This problem can be compensated partially in car batteries by

using a low density of dissolution (a specific gravity of 1.25 when the bat-

tery is totally charged).

· As the temperature is reduced, the useful life of the battery increases.

But if the temperature is too low, you run the the risk of freezing the elec-

trolyte. The freezing temperature depends on the density of the solution,

which is also related to the state of charge of the battery. The lower the

density, the greater the risk of freezing. In areas of low temperatures, you

should avoid deeply discharging the batteries (that is, DoDmax is effec-

tively reduced.)

· The temperature also changes the relation between voltage and charge. It

is preferable to use a regulator which adjusts the low voltage disconnect

and reconnect parameters according to temperature. The temperature

sensor of the regulator should be fixed to the battery using tape or some

other simple method.

· In hot areas it is important to keep the batteries as cool as possible. The

batteries must be stored in a shaded area and never get direct sunlight. It's

also desirable to place the batteries on a small support to allow air to flow

under them, thus increase the cooling.

How to choose a good battery

Choosing a good battery can be very challenging in developing regions. High

capacity batteries are heavy, bulky and expensive to import. A 200 Ah battery

weights around 50 kg (120 pounds) and it can not be transported as hand

luggage. If you want long-life (as in > 5 years) and maintenance free batter-

ies be ready to pay the price.

A good battery should always come with its technical specifications, including

the capacity at different discharge rates (C20, C100), operating temperature,

cut-off voltage points, and requirements for chargers.

The batteries must be free of cracks, liquid spillage or any sign of damage,

and battery terminals should be free of corrosion. As laboratory tests are

Chapter 7: Solar Power

229

necessary to obtain complete data about real capacity and aging, expect

lots of low quality batteries (including fakes) in the local markets. A typical

price (not including transport and import tax) is $3-4 USD per Ah for 12 V

lead-acid batteries.

Life expectancy versus number of cycles

Batteries are the only component of a solar system that should be amortized

over a short period and regularly replaced. You can increase the useful life-

time of a battery by reducing the depth of discharge per cycle. Even deep

cycle batteries will have an increased battery life if the the number of deep

discharge (>30%) cycles is reduced.

If you completely discharge the battery every day, you will typically need to

change it after less than one year. If you use only 1/3 of the capacity the bat-

tery, it can last more than 3 years. It can be cheaper to buy a battery with 3

times the capacity than to change the battery every year.

The power charge regulator

The power charge regulator is also known as charge controller, voltage regu-

lator, charge-discharge controller or charge-discharge and load controller.

The regulator sits between the array of panels, the batteries, and your

equipment or loads.

Remember that the voltage of a battery, although always close to 2 V per

cell, varies according to its state of charge. By monitoring the voltage of the

battery, the regulator prevents overcharging or overdischarging.

Regulators used in solar applications should be connected in series: they

disconnect the array of panels from the battery to avoid overcharging, and

they disconnect the battery from the load to avoid overdischarging. The

connection and disconnection is done by means of switches which can be

of two types: electromechanical (relays) or solid state (bipolar transistor,

MOSFET). Regulators should never be connected in parallel.

In order to protect the battery from gasification, the switch opens the

charging circuit when the voltage in the battery reaches its high voltage

disconnect (HVD) or cut-off set point. The low voltage disconnect (LVD)

prevents the battery from overdischarging by disconnecting or shedding

the load. To prevent continuous connections and disconnections the regu-

lator will not connect back the loads until the battery reaches a low recon-

nect voltage (LRV).

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Typical values for a 12 V lead-acid battery are:

Voltage Point

Voltage

LVD

11.5

LRV

12.6

Constant Voltage

14.3

Regulated

Equalization

14.6

HVD

15.5

The most modern regulators are also able to automatically disconnect the panels

during the night to avoid discharging of the battery. They can also periodically

overcharge the battery to improve their life, and they may use a mechanism

known as pulse width modulation (PWM) to prevent excessive gassing.

As the peak power operating point of the array of panels will vary with tem-

perature and solar illumination, new regulators are capable of constantly

tracking the maximum point of power of the solar array. This feature is known

as maximum power point tracking (MPPT).

Regulator Parameters

When selecting a regulator for your system, you should at least know the

operating voltage and the maximum current that the regulator can handle.

The operating Voltage will be 12, 24, or 48 V. The maximum current must be

20% bigger than the current provided by the array of panels connected to the

regulator.

Other features and data of interest include:

· Specific values for LVD, LRV and HVD.

· Support for temperature compensation. The voltage that indicates the state

of charge of the battery vary with temperature. For that reason some regu-

lators are able to measure the battery temperature and correct the different

cut-off and reconnection values.

· Instrumentation and gauges. The most common instruments measure the

voltage of the panels and batteries, the state of charge (SoC) or Depth of Dis-

charge (DoD). Some regulators include special alarms to indicate that the

panels or loads have been disconnected, LVD or HVD has been reached, etc.

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231

Conver ters

The regulator provides DC power at a specific voltage. Converters and invert-

ers are used to adjust the voltage to match the requirements of your load.

DC/DC Converters

DC/DC converters transform a continuous voltage to another continuous

voltage of a different value. There are two conversion methods which can be

used to adapt the voltage from the batteries: linear conversion and switch-

ing conversion.

Linear conversion lowers the voltage from the batteries by converting excess

energy to heat. This method is very simple but is obviously inefficient.

Switching conversion generally uses a magnetic component to temporarily

store the energy and transform it to another voltage. The resulting voltage

can be greater, less than, or the inverse (negative) of the input voltage.

The efficiency of a linear regulator decreases as the difference between the

input voltage and the output voltage increases. For example, if we want to

convert from 12 V to 6 V, the linear regulator will have an efficiency of only

50%. A standard switching regulator has an efficiency of at least 80%.

DC/AC Converter or Inverter

Inverters are used when your equipment requires AC power. Inverters chop

and invert the DC current to generate a square wave that is later filtered to

approximate a sine wave and eliminate undesired harmonics. Very few

inverters actually supply a pure sine wave as output. Most models available

on the market produce what is known as "modified sine wave", as their volt-

age output is not a pure sinusoid. When it comes to efficiency, modified sine

wave inverters perform better than pure sinusoidal inverters.

Be aware that not all the equipment will accept a modified sine wave as volt-

age input. Most commonly, some laser printers will not work with a modified

sine wave inverter. Motors will work, but they may consume more power than

if they are fed with a pure sine wave. In addition, DC power supplies tend to

warm up more, and audio amplifiers can emit a buzzing sound.

Aside from the type of waveform, some important features of inverters include:

· Reliability in the presence of surges. Inverters have two power ratings:

one for continuous power, and a higher rating for peak power. They are

capable of providing the peak power for a very short amount of time, as

when starting a motor. The inverter should also be able to safely interrupt

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itself (with a circuit breaker or fuse) in the event of a short circuit, or if the

requested power is too high.

· Conversion efficiency. Inverters are most efficient when providing 50% to

90% of their continuous power rating. You should select an inverter that

most closely matches your load requirements. The manufacturer usually

provides the performance of the inverter at 70% of its nominal power.

· Battery charging. Many inverters also incorporate the inverse function:

the possibility of charging batteries in the presence of an alternative source

of current (grid, generator, etc). This type of inverter is known as a charger/

inverter.

· Automatic fall-over. Some inverters can switch automatically between

different sources of power (grid, generator, solar) depending on what is

available.

When using telecommunication equipment, it is best to avoid the use of DC/

AC converters and feed them directly from a DC source. Most communica-

tions equipment can accept a wide range of input voltage.

Equipment or load

It should be obvious that as power requirements increase, the expense of the

photovoltaic system also increases. It is therefore critical to match the size of

the system as closely as possible to the expected load. When designing the

system you must first make a realistic estimate of the maximum consump-

tion. Once the installation is in place, the established maximum consumption

must be respected in order to avoid frequent power failures.

Home Appliances

The use of photovoltaic solar energy is not recommended for heat-exchange

applications (electrical heating, refrigerators, toasters, etc.) Whenever possi-

ble, energy should be used sparingly using low power appliances.

Here are some points to keep in mind when choosing appropriate equipment

for use with a solar system:

· The photovoltaic solar energy is suitable for illumination. In this case, the

use of halogen light bulbs or fluorescent lamps is mandatory. Although

these lamps are more expensive, they have much better energy efficiency

than incandescent light bulbs. LED lamps are also a good choice as they

are very efficient and are fed with DC.

· It is possible to use photovoltaic power for appliances that require low and

constant consumption (as in a typical case, the TV). Smaller televisions

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233

use less power than larger televisions. Also consider that a black-and-white

TV consumes about half the power of a color TV.

· Photovoltaic solar energy is not recommended for any application that

transforms energy into heat (thermal energy). Use solar heating or butane

as alternative.

· Conventional automatic washing machines will work, but you should avoid

the use of any washing programs that include centrifuged water heating.

· If you must use a refrigerators, it should consume as little power as possi-

ble. There are specialized refrigerators that work in DC, although their con-

sumption can be quite high (around 1000 Wh/day).

The estimation of total consumption is a fundamental step in sizing your solar

system. Here is a table that gives you a general idea of the power consump-

tion that you can expect from different appliances.

Equipment

Consumption (Watts)

Portable computer

30-50

Low power lamp

6-10

WRAP router (one radio)

4-10

VSAT modem

15-30

PC (without LCD)

20-30

PC (with LCD)

200-300

Network Switch (16 port)

6-8

Wireless telecommunications equipment

Saving power by choosing the right gear saves a lot of money and trouble.

For example , a long distance link doesn't necessarily need a strong amplifier

that draws a lot of power. A Wi-Fi card with good receiver sensitivity and a

fresnel zone that is at least 60% clear will work better than an amplifier, and

save power consumption as well. A well known saying of radio amateurs ap-

plies here, too: The best amplifier is a good antenna. Further measures to

reduce power consumption include throttling the CPU speed, reducing

transmit power to the minimum value that is necessary to provide a stable

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link, increasing the length of beacon intervals, and switching the system off

during times it is not needed.

Most autonomous solar systems work at 12 or 24 volts. Preferably, a wireless

device that runs on DC voltage should be used, operating at the 12 Volts that

most lead acid batteries provide. Transforming the voltage provided by the

battery to AC or using a voltage at the input of the access point different from

the voltage of the battery will cause unnecessary energy loss. A router or

access point that accepts 8-20 Volts DC is perfect.

Most cheap access points have a switched mode voltage regulator inside

and will work through such a voltage range without modification or becoming

hot (even if the device was shipped with a 5 or 12 Volt power supply).

WARNING: Operating your access point with a power supply other than the

one provided by your manufacturer will certainly void any warranty, and may

cause damage to your equipment. While the following technique will typically

work as described, remember that should you attempt it, you do so at your

own risk.

Open your access point and look near the DC input for two relatively big ca-

pacitors and an inductor (a ferrite toroid with copper wire wrapped around it).

If they are present then the device has a switched mode input, and the

maximum input voltage should be somewhat below the voltage printed on the

capacitors. Usually the rating of these capacitors is 16 or 25 volts. Be aware

that an unregulated power supply has a ripple and may feed a much higher

voltage into your access point than the typical voltage printed on it may sug-

gest. So, connecting an unregulated power supply with 24 Volts to a device

with 25 Volt-capacitors is not a good idea. Of course, opening your device

will void any existing warranty. Do not try to operate an access point at higher

voltage if it doesn't have a switched mode regulator. It will get hot, malfunc-

tion, or burn.

Equipment based on traditional Intel x86 CPUs are power hungry in compari-

son with RISC-based architectures as ARM or MIPS. One of the boards with

lowest power consumptions is the Soekris platform that uses an AMD

ElanSC520 processor. Another alternative to AMD (ElanSC or Geode

SC1100) is the use of equipment with MIPS processors. MIPS processors

have a better performance than an AMD Geode at the price of consuming

between 20-30% of more energy.

The popular Linksys WRT54G runs at any voltage between 5 and 20 volts

DC and draws about 6 Watts, but it has an Ethernet switch onboard. Having

a switch is of course nice and handy - but it draws extra power. Linksys also

offers a Wi-Fi access point called WAP54G that draws only 3 Watts and can

run OpenWRT and Freifunk firmware. The 4G Systems Accesscube draws

Chapter 7: Solar Power

235

about 6 Watts when equipped with a single WiFi interface. If 802.11b is suffi-

cient, mini-PCI cards with the Orinoco chipset perform very well while draw-

ing a minimum amount of power.

Equipment

Consumption (Watts)

Linksys WRT54G

(BCM2050 radio)

Linksys WAP54G

(BCM2050 radio)

Orinoco WavePoint II ROR

(30mW radio)

Soekris net4511

1.8

(no radio)

PC Engines WRAP.1E-1

2.04

(no radio)

Mikrotik Routerboard 532

2.3

(no radio)

Inhand ELF3

1.53

(no radio)

Senao 250mW radio

Ubiquiti 400mW radio

The amount of power required by wireless equipment depends not only on

the architecture but on the number of network interfaces, radios, type of

memory/storage and traffic. As a general rule, a wireless board of low con-

sumption consumes 2 to 3 W, and a 200 mW radio card consumes as much

as 3 W. High power cards (such as the 400 mW Ubiquity) consume around 6

W. A repeating station with two radios can range between 8 and 10 W.

Although the standard IEEE 802.11 incorporates a power saving mode (PS)

mechanism, its benefit is not as good as you might hope. The main mecha-

nism for energy saving is to allow stations to periodically put their wireless

cards to "sleep" by means of a timing circuit. When the wireless card wakes

up it verifies if a beacon exists, indicating pending traffic. The energy saving

therefore only takes place in the client side, as the access point always

needs to remain awake to send beacons and store traffic for the clients.

Power saving mode may be incompatible between implementations from

different manufacturers, which can cause unstable wireless connections. It is

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nearly always best to leave power saving mode disabled on all equipment, as

the difficulties created will likely outweigh the meager amount of saved

power.

Selecting the voltage

Most low power stand-alone systems use 12 V battery power as that is the

most common operational voltage in sealed lead-acid batteries. When de-

signing a wireless communication system you need to take into consideration

the most efficient voltage of operation of your equipment. While the input

voltage can accept a wide range of values, you need to ensure that the over-

all power consumption of the system is minimal.

Wiring

An important component of the installation is the wiring, as proper wiring

will ensure efficient energy transfer. Some good practices that you should

consider include:

· Use a screw to fasten the cable to the battery terminal. Loose connections

will waste power.

· Spread Vaseline or mineral jelly on the battery terminals. Corroded connec-

tion have an increased resistance, resulting in loss.

· For low currents (<10 A) consider the use of Faston or Anderson power-

pole connectors. For bigger currents, use metallic ring lugs.

Wire size is normally given in American Wire Gauge (AWG). During your cal-

culations you will need to convert between AWG and mm2 to estimate cable

resistance. For example, an AWG #6 cable has a diameter of 4.11 mm and

can handle up to 55 A. A conversion chart, including an estimate of resis-

tance and current carrying capacity, is available in Appendix D. Keep in

mind that the current carrying capacity can also vary depending on the type

of insulation and application. When in doubt, consult the manufacturer for

more information.

Orientation of the panels

Most of the energy coming from the sun arrives in straight line. The solar

module will capture more energy if it is "facing" the sun, perpendicular to the

straight line between the position of the installation and the sun. Of course,

the sun's position is constantly changing relative to the earth, so we need to

find an optimal position for our panels. The orientation of the panels is de-

termined by two angles, the azimuth a and the inclination or elevation ß.

The azimuth is the angle that measures the deviation with respect to the

Chapter 7: Solar Power

237

south in the northern hemisphere, and with respect to the north in the south-

ern hemisphere. The inclination is the angle formed by the surface of the

module and the horizontal plane.

Azimuth

You should have the module turned towards the terrestrial equator (facing

south in the northern hemisphere, and north in the southern) so that during

the day the panel catches the greatest possible amount of radiation (a = 0).

It is very important that no part of the panels are ever under shade!. Study

the elements that surround the panel array (trees, buildings, walls, other

panels, etc.) to be sure that they will not cast a shadow on the panels at any

time of the day or year. It is acceptable to turn the panels ±20º towards the

east or the west if needed (a = ±20º).

Inclination

Once you have fixed the azimuth, the parameter that is key in our calcula-

tions is the inclination of the panel, which we will express as the angle beta

(ß). The maximum height that the sun reaches every day will vary, with the

maximum on the day of the summer solstice and the minimum on the winter

solstice. Ideally, the panels should track this variation, but this is usually not

possible for cost reasons.

In installations with telecommunications equipment it is normal to install the

panels at a fixed inclination. In most telecommunications scenarios the energy

demands of the system are constant throughout the year. Providing for suffi-

cient power during the "worst month" will work well for the rest of the year.

The value of ß should maximize the ratio between the offer and the demand

of energy.

· For installations with consistent (or nearly consistent) consumption through-

out the year, it is preferable to optimize the installation to capture the maxi-

mum radiation during "the winter" months. You should use the absolute value

of the latitude of the place (angle F) increased by 10° (ß = | F | + 10 °).

· For installations with less consumptions during winter, the value of the lati-

tude of the place can be used as the solar panel inclination. This way the

system is optimized for the months of spring and autumn (ß = | F |).

· For installations that are only used during summer, you should use the absolute

value of the latitude of the place (angle F) decreased by 10° (ß = | F | - 10°).

The inclination of the panel should never be less than 15° to avoid the accu-

mulation of dust and/or humidity on the panel. In areas where snow and ice

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occur, it is very important to protect the panels and to incline them an angle

of 65° or greater.

If there is a considerable increase in consumption during the summer, you might

consider arranging for two fixed inclinations, one position for the months of

summer and another for the months of winter. This would require special support

structures and a regular schedule for changing the position of the panels.

How to size your photovoltaic system

When choosing equipment to meet your power needs, you will need to de-

termine the following, at a minimum:

· The number and type of solar panels required to capture enough solar en-

ergy to support your load.

· The minimum capacity of the battery. The battery will need to store enough

energy to provide power at night and through days with little sun, and will

determine your number of days of autonomy.

· The characteristics of all other components (the regulator, wiring, etc.)

needed to support the amount of power generated and stored.

System sizing calculations are important, because unless the system com-

ponents are balanced, energy (and ultimately, money) is wasted. For exam-

ple, if we install more solar panels to produce more energy, the batteries

should have enough capacity to store the additional energy produced. If the

bank of batteries is too small and the load is not using the energy as it is

generated, then energy must be thrown away. A regulator of a smaller am-

perage than needed, or one single cable that is too small, can be a cause of

failure (or even fire) and render the installation unusable.

Never forget that the ability of the photovoltaic energy to produce and store

electrical energy is limited. Accidentally leaving on a light bulb during the

day can easily drain your reserves before nighttime, at which point no addi-

tional power will be available. The availability of "fuel" for photovoltaic sys-

tems (i.e. solar radiation) can be difficult to predict. In fact, it is never possi-

ble to be absolutely sure that a standalone system is going to be able to

provide the necessary energy at any particular moment. Solar systems are

designed for a certain consumption, and if the user exceeds the planned

limits the provision of energy will fail.

The design method that we propose consists of considering the energy re-

quirements, and based on them to calculate a system that works for the

maximum amount of time so it is as reliable as possible. Of course, if more

Chapter 7: Solar Power

239

panels and batteries are installed, more energy will be able to be collected

and stored. This increase of reliability will also have an increase in cost.

In some photovoltaic installations (such as the provision of energy for tele-

communications equipment on a network backbone) the reliability factor is

more important that the cost. In a client installation, low cost is likely going to

be a the most important factor. Finding a balance between cost and reliability

is not a easy task, but whatever your situation, you should be able to deter-

mine what it is expected from your design choices, and at what price.

The method we will use for sizing the system is known as the method of

the worst month. We simply calculate the dimensions of the standalone

system so it will work in the month in which the demand for energy is

greatest with respect to the available solar energy. It is the worst month of

the year, as this month with have the largest ratio of demanded energy to

available energy.

Using this method, reliability is taken into consideration by fixing the maxi-

mum number of days that the system can work without receiving solar radia-

tion (that is, when all consumption is made solely at the expense of the en-

ergy stored in the battery.) This is known as the maximum number of days

of autonomy (N), and can be thought of as the number of consecutive

cloudy days when the panels do not collect any significant amount of energy.

When choosing N, it is necessary to know the climatology of the place, as well

as the economic and social relevance of the installation. Will it be used to illu-

minate houses, a hospital, a factory, for a radio link, or for some other applica-

tion? Remember that as N increases, so does the investment in equipment

and maintenance. It is also important to evaluate all possible logistical costs of

equipment replacement. It is not the same to change a discharged battery from

an installation in the middle of a city versus one at the top a telecommunica-

tion tower that is several hours or days of walking distance.

Fixing the value of N it is not an easy task as there are many factors in-

volved, and many of them cannot be evaluated easily. Your experience will

play an important role in this part of the system sizing. One commonly used

value for critical telecommunications equipment is N = 5, whereas for low

cost client equipment it is possible to reduce the autonomy to N = 3.

In Appendix E, we have included several tables that will facilitate the collec-

tion of required data for sizing the system. The rest of this chapter will explain

in detail what information you need to collect or estimate and how to use the

method of the "worst month".

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Chapter 7: Solar Power

Data to collect

· Latitude of the installation. Remember to use a positive sign in the

northern hemisphere and negative in the south.

· Solar radiation data. For the method of the "worst month" it is enough to

know just twelve values, one for every month. The twelve numbers are the

monthly average values of daily global irradiation on horizontal plane

(Gdm(0), in kWh/m2 per day). The monthly value is the sum of the values of

global irradiation for every day of the month, divided by the number of days

of the month.

If you have the data in Joules (J), you can apply the following conversion:

10-7 kWh

1 J = 2.78

The irradiation data Gdm(0) of many places of the world is gathered in tables

and databases. You should check for this information from a weather station

close to your implementation site, but do not be surprised if you cannot find

the data in electronic format. It is a good idea to ask companies that install

photovoltaic systems in the region, as their experience can be of great value.

Do not confuse "sun hours" with the number of "peak sun hours". The number

of peak sun hours has nothing to do with the number of hours without clouds,

but refers to the amount of daily irradiation. A day of 5 hours of sun without

clouds does not necessary have those hours when the sun is at its zenith.

A peak sun hour is a normalized value of solar radiation of 1000 W/m2 at 25

C. So when we refer to 5 peak sun hours, this implies a daily solar radiation

of 5000 W/m2.

Electrical characteristics of system components

The electrical characteristics of the components of your system should be

provided by the manufacturer. It is advisable to make your our own meas-

urements to check for any deviation from the nominal values. Unfortunately,

deviation from promised values can be large and should be expected.

These are the minimum values that you need to gather before starting your

system sizing:

Panels

You need to know the voltage VPmax and the current IPmax at the point of

maximum power in standard conditions.

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241

Batteries

Nominal capacity (for 100 hours discharge) CNBat , operational voltage VNBat ,

and either the maximum depth of discharge DoDmax or useful capacity CUBat .

You also need to know the type of battery that you plan to use, whether

sealed lead-acid, gel, AGM, modified traction etc. The type of battery is im-

portant when deciding the cut-off points in the regulator.

Regulator

You need to know the nominal voltage VNReg, and the maximum current that

can operate ImaxReg.

DC/AC Converter/Inverter

If you are going to use a converter, you need to know the nominal voltage VNConv,

instantaneous power PIConv and performance at 70% of maximum load H70.

Equipment or load

It is necessary to know the nominal voltage VNC and the nominal power of

operation PC for every piece of equipment powered by the system.

In order to know the total energy that our installation is going to consume, it

is also very important to consider the average time each load will be used. Is

it constant? Or will it be used daily, weekly, monthly or annually? Consider

any changes in the usage that might impact the amount of energy needed

(seasonal usage, training or school periods, etc.)

Other variables

Aside from the electrical characteristics of the components and load, it is

necessary to decide on two more pieces of information before being able to

size a photovoltaic system. These two decisions are the required number of

days of autonomy and the operational voltage of the system.

N, number of days of autonomy

You need to decide on a value for N that will balance meteorological condi-

tions with the type of installation and overall costs. It is impossible to give a

concrete value of N that is valid for every installation, but the next table gives

some recommended values. Take these values as a rough approximation,

and consult with an experienced designer to reach a final decision.

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Chapter 7: Solar Power

Available

Domestic

Critical

Sunlight

Installation

Very cloudy

Variable

Sunny

VN, nominal voltage of the installation

The components of your system need to be chosen to operate at a nominal

voltage VN. This voltage is usually 12 or 24 Volts for small systems, and if the

total power of consumption surpasses 3 kW, the voltage will be 48 V. The

selection of VN is not arbitrary, and depends on the availability of equipment.

· If the equipment allows it, try to fix the nominal voltage to 12 or 24 V. Many

wireless communications boards accept a wide range of input voltage and

can be used without a converter.

· If you need to power several types of equipment that work at different

nominal voltages, calculate the voltage that minimizes the overall power

consumption including the losses for power conversion in DC/DC and DC/

AC converters.

Procedure of calculation

There are three main steps that need to be followed to calculate the proper

size of a system:

1. Calculate the available solar energy (the offer). Based on statistical

data of solar radiation, and the orientation and the optimal inclination of

the solar panels, we calculate the solar energy available. The estimation

of solar energy available is done in monthly intervals, reducing the statis-

tical data to 12 values. This estimation is a good compromise between

precision and simplicity.

2. Estimate the required electrical energy (the demand). Record the

power consumption characteristics of the equipment chosen as well as

estimated usage. Then calculate the electrical energy required on a

monthly basis. You should consider the expected fluctuations of usage

due to the variations between winter and summer, the rainy period / dry

season, school / vacation periods, etc. The result will be 12 values of

energy demand, one for each month of the year.

Chapter 7: Solar Power

243

3. Calculate the ideal system size (the result). With the data from the

"worst month", when the relation between the solar demanded energy

and the energy available is greatest, we calculate:

· The current that the array of panels needs to provide, which will

determine the minimum number of panels.

· The necessary energy storage capacity to cover the minimum

number of days of autonomy, which will determine the required

number of batteries.

· The required electrical characteristics of the regulator.

· The length and the necessary sections of cables for the electrical

connections.

Required current in the worst month

For each month you need to calculate the value of Im, which is the maximum

daily current that an array of panels operating at nominal voltage of VN needs

to provide, in a day with a irradiation of Gdm for month "m", for panels with an

inclination of ß degrees..

The Im(WORST MONTH) will be the largest value of Im, and the system sizing

is based on the data of that worth month. The calculations of Gdm(ß) for a cer-

tain place can be made based on Gdm(0) using computer software such as

PVSYST (http://www.pvsyst.com/) or PVSOL (http://www.solardesign.co.uk/).

Due to losses in the regulator and batteries, and due to the fact that the pan-

els do not always work at the point of maximum power, the required current

ImMAX is calculated as:

ImMAX = 1.21 Im (WORST MONTH)

Once you have determined the worst month, the value of ImMAX, and the total

energy that you require ETOTAL(WORST MONTH) you can proceed to the final

calculations. ETOTAL is the sum of all DC and AC loads, in Watts. To calculate

ETOTAL see Appendix E.

Number of panels

By combining solar panels in series and parallel, we can obtain the desired

voltage and current. When panels are connected in series, the total voltage is

equal to the sum of the individual voltages of each module, while the current

remains unchanged. When connecting panels in parallel, the currents are

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Chapter 7: Solar Power

summed together while the voltage remains unchanged. It is very important,

to use panels of nearly identical characteristics when building an array.

You should try to acquire panels with VPmax a bit bigger than the nominal volt-

age of the system (12, 24 or 48 V). Remember that you need to provide a

few volts more than the nominal voltage of the battery in order to charge it. If

it is not possible to find a single panel that satisfies your requirements, you

need to connect several panels in series to reach your desired voltage. The

number of panels in series Nps is equal to the nominal voltage of the system

divided by the voltage of a single panel, rounded up to the nearest integer.

Nps = VN / VPmax

In order to calculate the number of panels in parallel (Npp), you need to divide

the ImMAX by the current of a single panel at the point of maximum power

Ipmax, rounded up to the nearest integer.

Npp = ImMAX / IPmax

The total number of panels is the result of multiplying the number of panels in

series (to set the voltage) by the number of panels in parallel (to set the cur-

rent).

NTOTAL = Nps x Npp

Capacity of the battery or accumulator

The battery determines the overall voltage of the system and needs to have

enough capacity to provide energy to the load when there is not enough

solar radiation.

To estimate the capacity of our battery, we first calculate the required energy

capacity of our system (necessary capacity, CNEC). The necessary capacity

depends on the energy available during the "worst month" and the desired

number of days of autonomy (N).

CNEC (Ah)= ETOTAL(WORST MONTH)(Wh) / VN(V) x N

The nominal capacity of the battery CNOM needs to be bigger than the CNEC as

we cannot fully discharge a battery. To calculate the size of the battery we need

to consider the maximum depth of discharge (DoD) that the battery allows:

CNOM(Ah) = CNEC(Ah) / DoDMAX

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245

In order to calculate the number of batteries in series (Nbs), we divide the

nominal voltage of our installation (VN) by the nominal voltage of a single

battery (VNBat):

Nbs = VN / VNBat

Regulator

One important warning: always use regulators in series, never in parallel. If

your regulator does not support the current required by your system, you will

need to buy a new regulator with a larger working current.

For security reasons, a regulator needs to be able to operate with a current

ImaxReg at least 20% greater than the maximum intensity that is provided by

the array of panels:

ImaxReg = 1.2 Npp IPMax

DC/AC Inverter

The total energy needed for the AC equipment is calculated including all the

losses that are introduced by the DC/AC converter or inverter. When choos-

ing an inverter, keep in mind that the performance of the inverter varies ac-

cording to the amount of requested power. An inverter has better perform-

ance characteristics when operating close to its rated power. Using a 1500

Watt inverter to power a 25 Watt load is extremely inefficient. In order to

avoid this wasted energy, it is important to consider not the peak power of all

your equipment, but the peak power of the equipment that is expected to op-

erate simultaneously.

Cables

Once you know the numbers of panels and batteries, and type of regulators

and inverters that you want to use, it is necessary to calculate the length and

the thickness of the cables needed to connect the components together.

The length depends on the location of your the installation. You should try to

minimize the length of the cables between the regulator, panels, and batter-

ies. Using short cables will minimize lost power and cable costs.

The thickness is chosen is based on the length of the cable and the maxi-

mum current it must carry. The goal is to minimize voltage drops. In order to

calculate the thickness S of the cable it is necessary to know:

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Chapter 7: Solar Power

· The maximum current IMC that is going to circulate in the cable. In the case of

the panel-battery subsystem, it is ImMAX calculated for every month. In the

battery-load subsystem it depends on the way that the loads are connected.

· The voltage drop (Va-Vb) that we consider acceptable in the cable. The

voltage drop that results of adding all possible individual drops is ex-

pressed as a percent of the nominal voltage of the installation. Typical

maximum values are:

Voltage Drop

Component

(% of VN)

Panel Array -> Battery

Battery -> Converter

Main Line

Main Line (Illumination)

Main Line (Equipment)

Typical acceptable voltage drops in cables

The section of the cable is determined by Ohm's Law:

S(mm2) = r( mm2/m)L(m) ImMAX(A)/ (Va-Vb)(V)

where S is the section, r is resistivity (intrinsic property of the material: for

copper, 0.01286 mm2/m), and L the length.

S is chosen taking into consideration the cables available in the market. You

should choose the immediately superior section to the one that is obtained

from the formula. For security reasons that are some minimum values, for the

cable that connects panels and battery, this is a minimum of 6 mm2. For the

other sections, that minimum is 4 mm2.

Cost of a solar installation

While solar energy itself is free, the equipment needed to turn it into useful

electric energy is not. You not only need to buy equipment to transform the

solar energy in electricity and store it for use, but you must also replace

and maintain various components of the system. The problem of equipment

Chapter 7: Solar Power

247

replacement is often overlooked, and a solar system is implemented with-

out a proper maintenance plan.

In order to calculate the real cost of your installation, we include an illustra-

tive example. The first thing to do it is to calculate the initial investment costs.

Description

Number

Unit Cost

Subtotal

60W Solar panel

$300

$1,200

(about $4 / W)

30A Regulator

$100

Wiring (meters)

$1 / meter

$25

50 Ah Deep cycle bat-

$150

$900

teries

Total:

$2,225

The calculation of our investment cost is relatively easy once the system

has been dimensioned. You just need to add the price for each piece

equipment and the labor cost to install and wire the equipments together.

For simplicity, we do not include the costs of transport and installation but

you should not overlook them.

To figure out how much a system will really cost to operate we must estimate

how long each part will last and how often you must replace it. In accounting

terminology this is known as amortization. Our new table will look like this:

Description

Unit Cost

Subtotal

Lifetime

Yearly

(Years)

Cost

60W Solar panel

$300

$1,200

$60

30A Regulator

$100

$20

Wiring (meters)

$1 / meter

$25

$2.50

50 Ah Deep

$150

$900

$180

cycle batteries

Total:

Annual Cost:

$2,225

$262.50

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Chapter 7: Solar Power

As you see, once the first investment has been done, an annual cost of

$262.50 is expected. The annual cost is an estimation of the required capi-

tal per year to replace the system components once they reach the end of

their useful life.

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