Electricidad y energía

Electricidad parece que está por todas partes: fluye en nuestros cuerpos para alimentar nuestros corazones, los rayos constantemente caen sobre la tierra a aproximadamente 100 descargas por segundo y la mayoría de las fabricas y oficinas del mundo funcionan con electricidad. Pero solo parece que está en todas partes, aún hay casi mil millones de personas sin acceso a la electricidad en todo el mundo, mayormente en comunidades rurales. El problema principal siempre ha sido que la electricidad siempre se producía en unos pocos lugares seleccionados y luego se distribuía desde allí en una red eléctrica a hogares y negocios, cuya expansión puede ser muy costosa. A pesar de que ni siquiera llega a todos en el planeta, la red eléctrica provoca importantes daños ambientales que afectan a todos.

Sistemas FV pueden ayudar a solucionar ambos problemas: aceso a la energía y la contaminación que produce las fuentes de energía non-renovables como carbón. Sistemas FV conectados a la red se puede utilizar para ayudar a reducir los impactos ambientales del uso de electricidad y sistemas FV autónomos puede ayudar a proporcionar energía a áreas donde la red no llega, ya que pueden producir, almacenar y proporcionar energía en forma de electricidad incluso en los lugares más remotos.

Right - Most power grids rely on centralized forms of generation (coal, natural gas, nuclear, large scale hydro to produce electricity that is distributed to homes through the transmission and distribution lines.
Left - Off-grid PV systems are independent of this system.

A PV system needs to be designed to match the characteristics of the electrical system in an area and the energy needs of the end-user. Not just designers and installers of off-grid systems need to understand electricity and energy thoroughly, but also users to make sure that they do not damage their system by using it beyond its capabilities. The main concepts that are necessary to understand are:

• Current
• Voltage
• Resistance

These are the building blocks of even the most complex electrical systems.

What is electricity?

A cross-section of a copper wire with its atoms enlarged. The electrons are flowing from atom to atom on their way from areas of high concentration to areas of low concentration.

Electricity is a force created from the basic building block of all matter - atoms. All atoms are composed of three core components - neutrons (no charge), protons (positive charge) and electrons (negative charge). Out of these three, the only one that is able to freely move from atom to atom is the negatively charged electron. Electrons can build up in higher concentrations in some locations and create a negative charge. Or there can be a lack of electrons, which create a positive charge. Electrons desire to flow from areas of high electron concentration to areas of low electron concentration. Not all atoms or materials have free electrons that can move around easily - most do not like wood, plastic rocks - we call these insulators. Metals and copper are good conductors as they have abundant free electrons.

The small static electric shocks that we receive from our cloths are the result of a difference in electrons from your body to that item - this difference is voltage. As the electrons pass from your body to that item of clothing a current is created.

Circuits

Static electricity and lightning are not useful to humanity as they are not in controlled systems. Electricity needs to be contained within an electrical system comprised of circuits for it to be used properly and safely. A basic electrical circuit is a closed loop built out of the following: 1. An energy source that has or can create an imbalance of electrons between to two points, which is voltage. 2. Conductive material, like wires, that allows electrons to flow from areas of high concentration to areas of low concentration. This flow is current. 3. A load or some means of constraining electron flow. Without a load or some kind of way of restraining electron flow, the electron difference created by the energy source will quickly reach zero.

Circuits can be in various states:

• Closed: connected, on, functioning. A properly connected circuit with a load which has current flowing.
• Open: disconnected, off, disabled. A circuit that is not connected or switched off which has no current flowing.
• Short: fault, improper low resistance connection. A circuit that has been improperly built without sufficient resistance - like a load - to constrain the flow of current. A circuit in a short circuit state will allow as much current to flow as possible until the power source is exhausted. If a load is connected in parallel with a short-circuit, like in the diagram, the load may stop functioning due to insufficient voltage/current.

• 

  Closed circuit:
  A properly functioning. Electrons are flowing and the light bulb is lit up.

• 

  Open circuit:
  A circuit that is disconnected - possibly due to a switch - with no electrons flowing.

• 

  Short circuit:
  An improperly functioning circuit with a low-resistance path for current to flow. Voltage drops nearly to zero as electrons take the low resistance path.

Characteristics of electricity

Electricity is almost always invisible, but flowing water enables us to create good comparisons and make the concept understandable. A circuit with a battery - like in the previous graphic - operates at a certain voltage and current, similarly a basic hydraulic system operates with a certain pressure and volume.

• The voltage in the electrical circuit is similar to the pressure in the hydraulic system.
• The current in the electrical circuit is similar to the flow in the hydraulic system.
• The wires in the electrical circuit and the load create resistance. The pipes and sprinkler in the final graphic also create friction.

Voltage

Voltage is the force that moves electrons in a circuit and is measured in volts (V). It can be thought of as electrical pressure and in a circuit with a battery the voltage is determined by the amount of energy stored in the battery. Voltage is similar to the pressure created in the hydraulic system. It depends upon the amount of water in the water that it holds.

Current

Current is the flow of electrons in a circuit and is measured in amperes or amps (A). Current is similar to the volume of water flowing in the hydraulic system. It depends upon the amount of water permitted to flow by the valve and upon the pressure in the system.

Resistance

Resistance (R) is a resistance to current that is present in all materials and all electrical systems and it is measured in Ohms (Ω). If the wires in an electrical circuit are too small for the amount of current that they need to carry, it will create friction and heat. Voltage is lost as a result. Similarly, the pipes through which the water flows in the hydraulic system can create friction if there is too much pressure or volume trying to pass through them.

• 

  Voltage:
  Left - A full water tank has a lot of pressure.
  Right - An empty water tank has no pressure.

• 

  Current:
  Left - A valve that is completely open allows a high volume of water to flow.
  Right - A valve that is nearly shut allows a low volume of water to flow.

• 

  Resistance:
  Left The valve part way open with a small pipe creating friction and reducing volume.
  Right - The valve part way open with a large pipe allowing the volume to easily pass through.

Power: watts

Power (P) is a measurement of work done in a unit of time. How much electricity is being consumed, which is power, in an electric circuit depends upon both the voltage and the current of the circuit. In electrical systems power is measured in watts (W) A watt is a measure of the energy produced or consumed in one second. Power is also commonly expressed in kW (1 kW = 1000 W) and MW (1 MW = 1,000,000 W) in larger systems. Similarly, if water flowing from the hydraulic system is used to perform work, like spinning a wheel, the power that is used will depend upon both the volume and the pressure of the water supplied. An inefficient load in an electrical system or hydraulic system will consume more power than an efficient one.

• 

  Power:
  Left - Higher pressure and volume has more power and will cause the wheel to spin faster.
  Right - Low pressure and volume will cause the wheel to spin slowly.

• 

  Efficiency:
  Left - An inefficient load will consume a lot of power.
  Right - An efficient load will consume little power.

The formula for calculating power in an electrical system is:

Power (P)	= voltage (V) × current (I)

The same amount of power can be generated with by using varying amounts of voltage and current. For example:

• 1000 watts = 10 volts × 100 amps
• 1000 watts = 100 volts × 10 amps

All appliances should have a label with their rated power consumption on them. Often times it is in volts and amps rather than watts. You can easily calculate watts from these two values.

The equation can also be rearranged to solve for missing variables. If you have any two of the three variables (P, V, I), then you can solve for the third. For example:

Example 1: A cell phone is plugged into to charge. It is connected to a 12V battery and there is 1A of current flowing. How much power is being consumed?

• P = 12 V × .5 A

W = 6 W

Example 2: A television is using 48 W of power. The battery that it is connected to has a voltage of 12 V. How much current (I) is flowing?

• 48 W = 12 V x I

I = 48 W ÷ 12 V

I = 4 A

Example 3: A small water pump is being used to fill a tank. The pump is a 440 W pump and there are 2 A of current flowing. What is the voltage of the system?

• 440W = V × 2 A

V = 440 W ÷ 2 A

V = 220 V

Energy: watt-hours

A typical electricity meter for a grid connection. Most meters measure energy in kWh. If you are connected to the grid, you will be charged a price per kWh consumed.

Power is a quick look at how much energy is being consumed or produced. For an electrical system this is an important value, but it is equally important to understand power consumption over time. Energy consumption over time is measured in watt-hours (Wh) or kilo-watt-hours (kWh). A watt-hour is the consumption of 1W of power for 1 hour. The formula for calculating Watt-hours is simple:

Watt-hours (Wh)	= power (P) × time in hours (t)

• Time in hours can be a fraction or percentage if necessary.

Example 1: A radio is plugged in and plays music for 3 hours. The radio says on the back that it consumes 7 W of power.

• Wh = 7 W × 3 hours

Wh = 21 Wh

Example 2: The motor on a fan says that it requires 60 W. The fan is left on during the night for 12 hours.

• Wh = 60 W × 12 hours

Wh = 720 Wh