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Solar and wind electricity:

Electricity is a type of energy that may flow from a place to another due to the movement of electrons. Electricity can be responsible for effects such as:

• ionizing a gas (fluorescent bulbs)

• heating a wire (electric stove coil)

• moving a motor (ceiling fan)

Terminology:

Energy is the ability to do work, irrespective of time. In other words, it is what can get some job done. It is measured in Watt-hours, Joules, Calories and others.

Power is using energy in a certain amount of time. We can do the same job (same amount of energy), but if I do it faster then I have more power. Power is mostly quantified in Watts, Horsepower...

Voltage is electric pressure. It is the force of electrons going from one place to another. It is measured in Volts. 

Current is the flow of electrons (electricity) in a specific time and is measured in Amps.

Resistance is the opposition to current. It depends on materials, temperature and cross-sectional area of whatever the electricity is flowing through. Ohms are the units used to quantify resistance

Water analogy:

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Electrical terminology can be understood by considering a reservoir full of water with an attached hose from which water is flowing out. Consider the following scheme for the analogy in terminology.

Formulae:

There are some important ways to relate electrical concepts, in particular energy to power, and power to voltage and current. These relations are expressed by the following formulae:

Energy = Power x Time (E=Pt) _ This formula explains why Watt-hours is an energy unit.

Power = Voltage x Current (P=VA)

Electron flow:

The inner electrons of an atom are said to be bound electrons to that atom due to the fact that they are closer to the positively charged atomic nucleus (composed of positively charged particles called protons and neutral particles called neutrons; electrons are negatively charged). Since opposite charges attract, it is very difficult to remove the inner electrons from the atom. Outermost electrons, on the other hand, are further way from the nucleus, so are more likely to feel a weaker attraction to it and can, generally, be more easily removed. When an outer electron is removed from an atom (due to, for example, heat, light, pressure, chemical or magnetic action), it is called a free electron.

The movement of free electrons through a medium (such as a piece of copper wire) is what forms an electrical current. Since opposite electrical charges attract to each other, electrons flow from an area of high concentration of these particles (more negative) to an area lacking electrons (more positive), called the negative and positive terminals, respectively.

Materials and electricity:

Electrically, materials can be classified as conductors or insulators:

  • Conductors: are materials whose atoms have weakly bound outer electrons. These electrons basically become free to move through the material and conduct electricity. Most electrical conductors, such as metals (for example, copper), are also good heat conductors.

  • Insulators: are materials whose atoms have strongly bound outer electrons. In this situation, electrons cannot escape the atom and, therefore, cannot conduct electricity. Insulating materials include nonmetals such as glass. 

Electrical concepts, in more detail:

Voltage: it is defined as electrical pressure or potential force, and is represented by the letter V. In concrete terms, this means that there is a difference in electrical charge between two points. It is measured in volts, and will cause greater current flow the larger it is. Can push electricity through a conductor but not through an insulating material. The instrument that measures voltage is a voltmeter. It measures the difference in electrical pressure between two points and is used in parallel. Voltage varies for different kinds of equipment, but it is usually 120V in North America. Some electrical appliances have a range of operation – for example between 120 and 220V – which means that any voltage in between that range should be acceptable for the normal operation of the device. A higher voltage could burn out the equipment (higher current flow) and a lower voltage will not be sufficient for the equipment to function properly.

Current: is the movement of electrons through a conducting material, is represented by the letter I, and is measured in amperes. The device that measures current is an ammeter and it is inserted in series in a circuit.

Power: it is measured in Watts (W), and represented by the letter P. One horsepower (also a wayto measure power) is equivalent to 750 Watts. In addition, a kilowatt-hour (kWh, 1000W) is the energy of 1000W working for 1 hour. The amount of energy, or electricity, used or generated by a consumer or a power plant, respectively, is measured in kWh. A watt-meter is the instrument that measures power.

• Resistance: opposes the flow of current, analogously to friction in mechanics, and is measured in Ohms. It changes electricity into some other type of energy, such as heat or light. Every electrical component and circuit have some resistance. The device that measures resistance is called an ohmmeter. The resistance of an electrical conductor depends on its length (the resistance of a longer wire will be greater than that of a shorter but otherwise similar wire), cross-section (a thicker wire has smaller resistance than a thinner but otherwise similar wire), temperature (the higher the temperature, the lower the resistance of a material will be), physical condition and material itself.

Circuits:

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Though electrical circuits can be quite complex, even the simplest ones always carry a source of electricity (such as a battery or a generator), a load (which can be, for example, a light bulb or a motor) and conducting wires that connect the source and the load.

Series and parallel connections :

There are two types of circuit connections:               

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• Series connection – the components are assembled end-to end, such that electrons have only one possible path to flow through the circuit. With this type of connections, the current remains constant, but the voltage is the sum of that at all components in series (all three lamps in the image below).

• Parallel connection – the components of the circuit are assembled such that there are multiple paths for electrons to flow through. Parallel wiring is opposite of series wiring in that the voltage remains constant, but the total flow of current is the addition of the current flowing through each component of the circuit (all three lamps in the image below).

Direct vs alternating current:

When current always flows in the same direction in a circuit, the positive and negative terminals of a power source (such as a battery or a solar panel) are always, respectively, positive and negative. In this case, the current is said to be direct current (DC).

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Conversely, when the direction of the current reverses, it is called alternating current (AC), resulting in a change of polarity of the terminals. In the United States, the current reverses (or alternates) its direction 60 times in a second (60Hz). In other places, such as in Europe, the current alternates at 50Hz (50 times per second).

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Components of a solar system:

The components for a Global Model Earthship solar system and their connections are described below:

Solar/Photovoltaic (PV) Panel – produces Direct Current (DC) electricity from sunlight.

Battery Bank – Provides DC power storage for electricity produced by the PV panels. Typically, lead-acid batteries are used in Earthships. This type of batteries should not be used more than one third of their capacity to protect the batteries and enhance their durability.

• PV Combiner – Houses lightening arrestor, PV breakers and provides a PV disconnect.

• Charge Controller – Monitors battery voltage, determines necessary charge voltage and amperage and prevents overcharge of the batteries. The type of charge controller required for a given system is determined by the total Voltage and Current output of the panel array and the configuration of the battery bank (12V, 24V, 48V, ...).

• Load Center – Houses AC and DC breakers and disconnects involved in electricity production and power storage.

• Inverter – Converts DC power from the battery bank into AC electricity. The required inverter is determined from the requirements of the AC appliances.

• AC breaker box – Distributes AC electricity throughout the house for lighting and electrical outlets.

• DC breaker box – Distributes DC electricity throughout the house for lightening, refrigeration, Water Organizing Module (WOM) and grey water pump.

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Types of solar panels: 

There are three types of solar panels:

• Mono-crystalline – these panels are more efficient in ideal conditions (when the panel is oriented perpendicularly to the Sun). However, their performance drops down dramatically in non-ideal circumstances.

• Poly-crystalline – are less efficient than mono-crystalline panels in ideal conditions, but do much better in non- ideal circumstances (such as a cloudy day).

• Amorphous thin films – are the least efficient panels, but are cheaper, lightweight and easy to mount.

Wiring solar panels and batteries:

Wiring solar panels and batteries in parallel and series is similar to any other circuit (for, say, a solar panel, in a series connection the voltage is the aggregate of each of the panels' voltages while the current stays constant, and in a parallel connection the voltage remains the same as having a single panel while the current is added for each panel in the array).

Solar panels:

Consider an array of four 12V, 5A solar panels. The resulting voltage and current for a given wiring type is the following:

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Series wiring: net result of 48V, 5A.

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Parallel wiring: net result of 12V, 20A

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Parallel series wiring: Series wiring of an array yields 48V, 5A; Parallel wiring of two series arrays yields a total result of 48V, 10A.

Batteries:

Consider a 12V, 100Ah battery. The resulting current and voltage for a given wiring type is:

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• Series connection of two batteries: the net result is 24V, 100Ah.

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  • Parallel connection of two batteries: the net result is 12V, 200Ah.

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  • Parallel connection of two sets of batteries connected in series: the net result is 24V, 200Ah.

Deciding on the DC system voltage:

When setting up your DC system, it is crucial to decide what voltage it will operate at (normally either 12, 24 or 48V). The advantages and disadvantages of each are as follows:

  • 12V: a lot of DC appliances exist for a 12V DC system. This is mainly because cars, RVs and boats operate on 12V DC systems.  For this type of system, 6V batteries should exist in groups of two (to achieve the required voltages according to the wiring schemes described in the previous section).

  • 24V / 48V: can still invert to 120V AC, and the solar panels can be further away from the house, with smaller (less expensive) wire connecting the two. However, the number of 24V appliances is limited compared to 12V, and is even more restricted for 48V systems. In a 24V system, solar panels have to exist in groups of two and 6V batteries in groups of four. In a 48V system, solar panels should exist in groups of four and batteries in groups of eight.

Nonetheless, all 12V, 24V and 48V DC power can be converted into AC by an appropriate inverter, and the AC circuits can be wired similarly to those in a conventional house.


System sizing:

One of the crucial aspects of a solar electrical system design is figuring out how many panels and batteries are necessary for a particular household. The number of these components will vary depending on the needs of the household (such as the number of people living in the space), insolation (number of full hours a day that the sun shines at a particular location), the climate (variation of insolation throughout the year) and the specifications of the parts (such as the amount of energy the batteries can store or their type) themselves. In more detail, system sizing is done by considering all these aspects:

  • Needs of the household: because system sizing depends on the number of people using it and their personal needs, a list of appliances should be made, as well as the power of each appliance and the number of hours of usage per day. This allows us to calculate how much energy (remember that Energy = Power x time) the solar panels have to be capable of producing and the batteries required to store in a day. An example list is given in the table below (that you can fill according to your personal needs for practice).

(You can click on this table below to download it as a pdf)

(You can click on this table below to download it as a pdf)

Finally, a correction should be applied to account for losses:

ETOTAL CORRECTED = ETOTAL x 1.2

where 1.2 is the loss multiplier and ETOTAL CORRECTED is the amount of energy required in a day (in Watt-hours).

  • Insolation: panels can output more electricity when the sunlight shines close to perpendicularly at them. So, during early morning and late afternoon the angle between the sunlight and the panels prevents the latter from outputting significant voltages.  Furthermore, the sunlight from the early morning and late afternoon  is less intense than at around midday due to the fact that it has to go through less atmosphere at the latter time. For these reasons, the beginning and end of the day are not considered full sun hours and not considered towards insolation times (even though the sun is up for 12 hours per day) for a given location. Hence, the most productive hours of sunshine for solar electricity production occur between 9am and 3pm. Obviously, the more full sun hours there are in a given location, the more energy a solar panel can convert to electricity in a single day.

  • Climate: even if the average yearly insolation is very high at a certain place, there can be large variations on the number of full sun hours a day if winter and summer insolations are considered separately (due to, for example cloudy winter climates or relative position of the sun in the sky affecting the intensity of sunlight reaching the panels). In these cases, it may be more appropriate to use the winter insolation values to determine the system sizing.

  • Parts specifications: consider types of panels (for example mono- vs poli-crystalline) and batteries (usually lead-acid batteries are used in Earthships) and their wattage, and/or voltage, current and energy specifications.


Consider the following example on how to calculate the required number of panels and batteries for a given system:

You use 6000Wh of energy per day in your home. You live in a place with 4 good hours of sun per day. You buy batteries that are rated at 6V and 400Ah. Your panels are 235W output each. 

How many batteries will you need?

The energy (E) stored in a single battery is 

EBattery = Power x time = 6V x 400Ah = 2400Wh.

Since lead-acid batteries should only be used to 1/3 of their capacity (to maximize their durability), the total energy available for a single battery is actually only

EBattery = 2400Wh / 3 = 800Wh.

So, the total number of batteries that are necessary to provide the total energy required in a day (ETotal = 6000Wh) is given by

ETotal  / EBattery = 6000Wh / 800Wh = 7.5

or rather, we need a total of 8 batteries (assuming a 12V system).

How many panels will you need?

The amount of energy each panel can output in a day (given 4 hours of sunlight each day) is

EPanel = 235W x 4h = 940Wh.

But the total amount of energy necessary per day is

ETotal = 6000Wh x 1.2 = 7200Wh

where 1.2 is the factor that accounts for the electrical losses in the system.

So, the total amount of panels necessary is given by

ETotal / EPanel = 7200Wh / 940Wh = 7.7

This means that we need 8 panels in total.


Adding wind power:

Different parts of the Earth surface is heated differently by the Sun. This creates pressure differentials in the atmosphere, which causes air movement (from high to low pressure regions) – wind.

The power of wind flowing through an area of interest depends on three factors:

  • quantity of air (volume).

  • speed of air (velocity).

  • mass of air (density).

The larger any of those quantities is, the larger is the wind power. It is not possible, however, for the full power of the wind to be captured by a turbine. Instead, an ideal wind turbine is limited to capture a maximum of 59% of the total wind power – this is typically called the Betz limit. In reality, wind turbines capture less than the Betz limit due to design and physical limitations (such as friction). 

Wind turbines are defined by:

  • Cut-in speed, which is the wind speed at which the wind turbine can overcome the battery voltage and begin production of electricity; the cutting speed should be as low as possible.

  • Rated speed, or the wind speed at which the turbine will start to generate its designated rated power.

  • Cut-off speed, that is the wind speed (voltage) at which the turbine shuts down. This is a safety feature, mostly achieved by brakes, that prevents the turbine from over-spinning and damaging itself.

The graph below shows a schematic power output curve of a typical wind turbine depending on wind speed.

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There are two main types of wind turbines:

  • Vertical Axis Wind Turbines (VAWT) – although they typically are less efficient than vertical axis turbines, they tend to have less down time (require less maintenance), be more silent and cheaper.

  • Horizontal Axis Wind Turbines (HAWT) – are the most common wind turbines. They are more efficient than HAWTs, and typically have a gear box that converts slow rotations into faster ones, more suitable for electrical generators.

Adding wind power generation to an off the grid household with solar requires, besides a turbine, an extra charge controller (separate from the solar) with the capability of dealing with wind generating devices, as described below.

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