An OC Overview and Guide to Portable Solar Generators
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Electricity is the lifeblood of modern industrial civilization because most industrial and consumer appliances require electric current to run.
The two most common types of off-grid generators are fuel-powered generators and solar generators. Most of the power in the national electricity grid is obtained from hydro-electric dams.
It is therefore evident that electricity is generated, and according to energy physics and thermodynamics, this involves transformation of energy from one form to another.
Even so, what is energy? What happens when one lacks electrical energy due to a black-out or by virtue of being in an off-grid area?
The answer for the second question is that life without electricity is inconveniencing as one is prevented from using most everyday consumer appliances, and apart from gas cookers, one is otherwise forced to adopt a pre-industrial lifestyle
Simply stated, without electricity, one is cut off from modern life. It is for this reason that one needs to have back-up electricity generators or off-grid electricity-generation systems.
Energy is a thermodynamic quantity that gives a physical system the capacity to perform work.
This means that electric energy allows electrical appliances to perform work such as provide light for lighting appliances, produce heat in heating and cooking appliances, and create mechanical motions in electric motors and rotors.
This process of energy transformation is called energy transduction and the equipment that performs this transduction is called a transducer, or colloquially power transducer.
Therefore, electricity generators are energy transducers that operate as follows. The hydro-electric generators in hydro-power dams convert hydro-power of rapidly falling or running water into electrical energy, while the fuel-powered off-grid generators convert the chemical energy in the fuel into electrical energy.
Likewise, solar generators convert solar energy into electrical energy.
Off-Grid Electricity Generation: Comparing Solar Generators to Fuel-Powered Generators
Hydropower and solar power are clean, renewable sources of energy, while fuel is a non-renewable source of energy that emits smoke and other environmental pollutants when burned in the engines of electricity generators.
As mentioned earlier, solar and fuel-powered generators provide most of the off-grid electric power; with fuel-powered generators being favored when large power outputs are needed due to the high power output-per-unit of fossil fuel as compared to solar energy.
On the other hand, solar generators are favored for personal use because solar power is a clean, renewable and free energy source, as compared to costly fuel which requires one to also handle its storage.
(keeping in mind that it is a combustible fluid that can cause an explosion if it comes into contact with electricity sparks or flames).
Moreover, maintaining solar generating systems is much cheaper as compared to maintenance of diesel- or gasoline-powered generators.
Furthermore, fuel-powered generators are noisy and the finite fuel available (either in the fuel tank or storage tanks) can run out.
In comparison, solar generators are quiet and their energy source is the infinite solar power obtained from the sun (notwithstanding predictive scientific studies that deduce that the sun would lose most of its current energy in 5 billion years!!!).
On the downside, solar generators cost more per unit of electricity produced as compared to fuel-powered generators, with the cost rising steeply if a portable solar generator is used.
This means a solar generator whose output is 1000watts costs much more than a fuel-powered generator of equivalent output.
Another disadvantage that is mentioned above is solar generators have much lower outputs than fuel-powered generators.
Additionally, fuel-powered generators can produce electricity at night, while solar generators cannot but instead have to rely on electrical charge stored in batteries to power electrical appliances.
The other demerits of solar power generators relate to their high repair costs and poor-to-non-existent customizable profile which also makes expansibility difficult.
These demerits are discussed later on.
Portable Solar Generators
As alluded to earlier, the solar generator is designated as such as it generates electrical energy from solar power.
For personal use, not much power is needed, but one needs to be able to move the generator around, especially if (s)he is traveling or is on a camping tour.
For this reason, consumer-grade solar generators need to be portable.
One of the most glaring misconceptions is that portable solar generators can be lifted by a person, and then carried to where it is needed.
This is usually not the case as most generators weigh over 50 pounds (with those fitted with lead-acid batteries weighing more than those with rechargeable lithium batteries); and it is impractical for anyone to carry them around on the shoulder or manually lift and move them around.
Usually, the battery and inverter unit are housed in a single case – with some high-end models featuring a briefcase design (hence the term briefcase solar generators) – which is fitted with 2 wheel as the lower back or 4 (front and back) wheels.
This allows one to wheel the generator to site, and hence the designation portable as it can be moved around.
This portable solar generator is functionally an integrated system that combines energy transduction and voltage inversion into a single unit.
Component-wise, this multi-component system has 4 principal components/parts: solar panel(s), battery charger, rechargeable battery, and an inverter.
This multi-component system can therefore be divided into 2 functional systems:
1. Energy Transduction System – Solar Panels
The main component here is the solar panel. A solar panel is basically a solar module made of an assembled grid of solar cells.
These solar cells are oriented on a single plane, and functionally integrated to work as a unit that can generate electricity from solar energy.
This also allows for a protective glass sheet to cover the entire module, which gives solar panel their characteristic glass surface on its sun-facing side.
Each solar cell is basically a photovoltaic wafer made of crystalline silicone semiconductor that can generate electricity through the photovoltaic effect.
The photovoltaic effect describes the phenomenon that occurs when solar light (energy) is absorbed by a material with a subsequent energy transfer into an electron which causes it to move to a higher-energy state called electron excitation, which supports electron ejection and flow through a conductive material.
This flow of electrons is called electric current, and it is the hallmark of electricity generation. This makes a solar cell is a photovoltaic cell capable of generating electricity. This also means that the solar panel is a photovoltaic module whose power output is the sum total of the individual power output of its cells.
Because the electron flow is directional, the current gains a polarity and becomes direct current (DC), and the panel thus features positive and negative terminals.
Thin-film technology produces more power efficient solar cells with higher power output and less mass as compared to c-S technology.
This is because thin-film technology allows solar cells to be built using amorphous silicone, or non-silicone materials such as cadmium-telluride and copper-indium-gallium-selenide (CIGS).
There are also smart solar cells that feature embedded electronic that track power output and support power-point tracking that allows the panel to track the movement of the sun.
This ensures that its sun-facing side changes position to reflect the sun movement so that sunlight always falls on the entire panel during sunny days.
Most solar panels can therefore be carried around as off-grid transducer systems. This also allows one to position the solar panel outside and far-away from the indoor location of the power generation unit.
Moreover, this allows one to place as many panels outside so as to maximize the use of free space to generate electricity.
The solar panel is usually cased (clothed) in a foldable fabric, and depending on the size (especially the length) of the fabric case, two or more solar panels can be fitted side-by-side to create a multi-panel array when the fabric is fully unfolded.
Using more than two solar panels allows one to regulate the recharging time, as more solar panels deliver more electricity to the rechargeable battery hence reducing the time it takes for it to become fully charged.
Even so, there are foldable CIGS panels that feature a plastic backing, while some c-S panel come inbuilt into the external surface of the generator case.
This unit is made up of two integrated systems: the energy storage system and the voltage inversion system.
These systems are housed in a single (usually wheeled) encasement that needs to be compact, sturdy and durable so as to be able to withstand wear, tear, scratches, and dents.
Energy Storage System
This is made up of a rechargeable battery and the battery charger.
i. Battery Charger
The electricity that flows from the solar panels first enters the battery charger which serves to regulate how much current flow into the rechargeable battery so as to prevent overcharging or improper battery charging.
This charger comes with a hard-coded charging protocol that is synced to the charge capacity of the battery.
This protocol determines how much current and voltage flows to the battery during charging, and for how long, as well as determines when charging is complete and disconnect the battery from the solar panel.
The charger may also feature embedded electronics that operate as current-sensing, voltage-sensing, and temperature-sensing circuits.
This can also include microprocessor controllers for adjusting charging current depending on the state of battery charge, that is, allow more current to flow when battery is fully discharged and gradually reduce current flow as battery nears full charge.
This are usually included in three-stage chargers where the initial fast charging rate allows maximum current flow through the charger till the battery cells reach their outgassing voltage (when they release large amounts of hydrogen gas at 2.22 volts).
At this point the charging voltage is kept constant at about 2.4volts while the current is gradually limited till the battery is fully recharged, upon which the third stage – the trickle charging stage – kicks in.
Some chargers also support trickle charging which supplies relatively low current to a fully-charged battery so to counter self-discharge.
Battery chargers have specification such as voltage rating, current rating, and power rating. Most importantly, the battery charger determines the charging time of the battery.
It also allows for efficient charging of multiple batteries in high-capacity solar generators.
Regarding customization and expansion, it is the battery charger that limits how much one can expand the battery capacity, or increases the output of the transducer by either replacing the solar panel with a larger panel or adding more panels to the existing panel.
This is because it determines the charging rate, charging voltage, and charging current; as well as ensures that the battery used is compatible with the solar panel.
Therefore customization that seeks to increase the charging current and/or voltage, or reduce the charging time beyond what the charger supports is going to disrupt the smooth operation of the generator.
For this reason, most portable solar generators do not support customization, and limit how much one can expand the existing components with their compatible counterparts.
The Rechargeable Battery
This is the main component that determines the power output of the solar generator. It serves to store the electrical charge produced by the solar panel(s).
This secondary battery can be a standard flooded lead-acid battery, a sealed lead-acid (SLA) battery, sealed valve-regulated lead-acid (VRLA) battery, or a lithium-polymer (high capacity lithium-ion) battery.
The VRLA battery can either be a gel battery or an absorbed glass-mat (AGM) battery. Usually, for lead-acid batteries, the deep-cycle batteries are favored over starter batteries.
Most average solar generators have a single rechargeable battery, with the cheaper heavier models featuring lead-acid batteries while their more expensive counterparts contain a large lithium battery.
Depending on the battery size and (battery) capacity, as well as functional design of the battery charger, a generator can have multiple batteries; and these are usually lithium-ion batteries.
The five most important considerations about these rechargeable batteries are:
Rate of discharge: This is usually referenced as the C rate, and it is the discharge rate that would drain a fully-charged battery of its charge within a single hour. In terms of electric current, it is the maximum current draw that the battery can sustain for an hours before it is fully discharged.
Cell Reversal Damage: If a cell over-discharges rapidly, then the polarity of its terminals can be reversed – a phenomenon described as cell reversal; and it can cause irreparable damage. Therefore, the battery needs to be protected from over-discharging
Depth of Discharge (DoD): This is the percentage of how much the battery can be discharged from a fully charged state. It is related to state of charge (SoC) in an inverse proportional relationship, that is, if one is high, the other is low. Basically, a high DoD equate to a low SoC. Even so, repeated charging cycles alters the DoD for full discharge; and high-quality batteries can tolerate high DoDs even after repeated charging cycles.
Cycle Stability and Lifespan: As the battery is charged and discharged repeatedly, it loses its capacity to store electric charge; and after a number of charge-and-discharge cycle, it can no longer store charge, and is considered non-functional – that, the end of the battery lifespan has been reached. High-quality batteries have high cycle stability, and can support far more charge-and-discharge cycles as compared to low quality batteries.
Recharging time: This is regulated by the battery charger, and it ensures that the charging rate does not exceed the level the battery can support as an arbitrarily high charging rate rapidly increases the internal resistance in the battery cells which results in heat generation and overheating which can result in explosions.
The most important rating of the battery is its capacity, which is the quantity of electric charge it can provide at a specific voltage.
High capacity batteries of the same functional design and construction feature more electrode material as compared to low-capacity batteries.
The relationship between battery charge and current draw is as follows:
Battery charge (in Ampere-hour) = current draw (in amperes) X time (in hours)
Relatedly, Ohm’s Law defines the relation between voltage and current as:
Current (in amperes) = Voltage (in volts) ÷ Resistance (or electric load in Ohms)
And because power is calculated as
This also allows one to calculate how much power the battery can provide:
Power (in watts) = Voltage X Current
= (Battery charge X voltage)/time
Therefore, a 120A.h battery can provide:
(120 X 12)/1 = 1440 watts in one hour.
The principle of energy conservation applies here as the inverter does not generate any power but draws all the power for the connected electrical appliances from the battery.
This also explains why DC batteries do not provide enough power to run consumer-grade electronics for long. For example, a 1500-watts toast roaster can drain a 120A.h battery within an hour.
Moreover, the maximum DoD varies depending on battery designs with flooded lead-acid capping at 70 % while lithium polymer support upto 90% DoD.
Therefore, a 120A.h cannot power the roaster for an hour, while an equivalent lithium-polymer battery would barely last beyond 50minutes.
The main component here is the inverter which serves to convert the direct current of the battery to alternating current (AC), as well as stepping up the voltage from 12volts to 110/120volts or 230/240volts which can be supplied to 110V AC appliances or 240V AC appliances.
The process of converting direct current to alternating current is called inversion, and because it changes DC voltage into AC voltage, it is also described as Voltage Inversion.
The inverter circuit comprises of an inversion circuit and a voltage step-up circuit which converts DC voltage to AC voltage of a specified (output) frequency (usually 50 or 60Hz).
The power output of the inverter is dependent on the power input from the battery; and most inverters allow one to run them suing multiple batteries. This runtime supports both series and parallel configuration.
In series configuration, multiple batteries are daisy chained together and then connected to the inverter. However, if one battery fails, then the entire setup is rendered non-functional.
This can be avoided by connecting the positive terminals of all the batteries to the positive terminal of the inverter, and the same is done for the negative terminal so that achieves a parallel configuration.
In series configuration, the amount of voltage going into the inverter (inverter input voltage) is increased as it is the sum of the individual output voltages of the individual batteries.
On the hand, parallel configuration increases the inverter input current, as well increases the overall ampere-hour rating of the generator.
The inverter does output the AC voltage in either of 2 main output waveforms; square wave or sine wave. The sine wave can further be smoothened and modulated depending on the inverter circuit into a pure sine wave, a modified sine wave, a pulsed sine wave, or a pulse-width modulated wave.
The square wave is for low-sensitivity appliances as it causes considerable hum in sensitive electronics.
Inverters that produce sine wave AC output are called sine-wave inverters with pure-sine wave inverters producing the ideal AC waveform for sensitive electrical appliances. As expected, pure sine wave inverters are more expensive than square-wave inverters.
Another important consideration about inverters is their power output, which is the maximum current draw that they can support at either 110/120volts or 220/230/240 volts.
Usually, the power rating of the inverter should be matched with than of the battery or battery pack if it is connected to multiple batteries.
This is to ensure that the inverter produce far less power than the generator can produce as this causes the solar generator to underperform.
Now, that all the components of the solar generator have been described, it is time to address the misconceptions about these off-grid supplies of electricity.
In SEGS, parabolic mirrors are used to reflect and concentrate sunlight onto a central tube that contains synthetic oil.
This causes the oil to be heated up and flow upwards through the pipes to the water tanks where it transfers its heat to the water, hence causing the water to boil and produce steam which drives steam turbines.
Evidently, SEGS neither use solar panels nor batteries; nor have a much more complex operation and functional attributes as compared to solar generators
As mentioned earlier, solar panels can be wheeled around but it is impractical for one to carry it on the shoulders.
This is determined by the quality of solar panel, battery charger, battery, and inverter used in the solar generator.
Therefore, no 2 solar generators can be compared to each other based on their power efficiency ratings alone.
Solar generators cannot run multiple power-hogging electronics at the same time as most people are made to believe.
A solar generator with an output power rating of 2000watts can run three 2,000watts electrical appliances simultaneously for only 20 minutes before it is drained out.
Why It Is Inadvisable to Connect D.C Electrical Appliances Directly to the Solar Panel?
Sometimes, one can desire to connect DC appliances to the solar generator, and most inverters feature a separate DC power outlet which allows one to draw DC power.
Even so, one can be tempted to connect the appliance directly to the solar panel; and this is wrong because the panel delivers electric current that fluctuates widely depending on how much sunlight it receives, and this fluctuating current can damage the appliance.