Des questions fréquemment posées

Sur cette page, vous trouverez des réponses à certaines des questions les plus importantes concernant le fonctionnement de notre ferme solaire.

 

Questions sur la physique

Qu'est-ce que l'énergie?

C’est une propriété qui permet d'effectuer du travail, une force en action; par exemple: a) un objet en mouvement a une énergie cinétique et peut effectuer du travail en ralentissant; b) l’énergie thermique peut être convertie en action dans un moteur de fusée; c) un champ électromagnétique possède une énergie qui peut être convertie en travail électrique. L'énergie ne peut être transformée que d'une forme à une autre; la quantité totale d'énergie est conservée.

Pourquoi l'efficacité est-elle importante?

L’efficacité nous indique combien d’énergie utile nous pouvons extraire à partir de l’énergie totale collectée. Plus l'efficacité est haute, moins nous avons besoin d'énergie pour effectuer le même travail. Une ferme solaire plus petite mais plus efficace peut fournir l'énergie nécessaire, mais un équilibre entre l’efficacité et le coût est nécessaire.

Quel est l'effet photovoltaïque?

Lorsque la lumière atterrie sur un matériau semi-conducteur, l'énergie des ondes (photons) est transmise aux électrons du matériau. Avec suffisamment d'énergie, les électrons peuvent échapper aux atomes du matériau et ils deviennent libres de se déplacer. Si le matériau est connecté correctement, ces électrons peuvent générer une tension électrique, résultant en la transformation de la lumière solaire en électricité.

Qu'est-ce qu'un panneau photovoltaïque?

Un panneau photovoltaïque, ou panneau solaire, est constitué d’un ensemble de cellules solaires d’un matériau semi-conducteur tel que le Silicium; de nombreux panneaux solaires constituent ensemble une ferme solaire. Les cellules solaires convertissent la lumière du soleil en électricité via l'effet photovoltaïque.

 

Circuitry

Why metal is a good conductor of electricty?

To conduct electricity a material must allow a current to flow. Metals have electrons that are free to move about their structure. These electrons are the charged particles, which when moving in the same direction, produce a current.

Why plastic and rubber are good insulators of electricity?

Both of these materials do not have any charged particles which are free to move. Therefore, no current is able to flow so they insulate against electricity.

AC and DC

Direct current (DC) is when the current only flow is one direction. Alternating current (AC) is when the current changes direction periodically, with a frequency that is often 50 times per second for many mains electricity supplies. Photovoltaic cells produce a direct current used to charge batteries. Batteries always produce a direct current.

Why high voltage (~220V) AC cables can be any length but low voltage (~24V) DC cables must be short?

Cables carrying low voltage DC are subject to higher losses in energy relative to those carrying high voltage AC AC. By keeping cables carrying low voltage DC as short as possible, the energy losses due to resistance are minimised and the best efficiency of a circuit is achieved.

Why thick cables are better than thin cables?

The energy losses in a circuit are related to the resistance of the cables. The larger the diameter of the cables carrying the current, the lower the resistance, and the more efficient the circuit is.

What voltage do I design the system for?

Many solar farms are designed to work at either 12V or 24V or even 48V. Working at a higher voltage requires thinner cables because less current is required to deliver the same power. Both 12V and 24V systems are relatively cheap to put together because of the wide range of devices available at these voltages from the automotive industry. The optimal choice for us was 24V.

How do I ensure my devices receive the correct voltage?

We use voltage regulators to govern the electricity supply from our batteries to ensure that each device we are powering receives exactly the right voltage it was designed to run at. For example, many of our astronomical cameras run a 12V, many communications devices run at 5V whereas the mount that enables the telescope to slew and track is 48V. We use 24V-12V, 24V-5V and 24V-48V voltage converters to accomplish this.

 

Batteries

How does a battery work?

A battery is a store of chemical energy which can be changed into electricity when connected in a circuit. Chemical reaction within the battery cause negatively charged electrons at one terminal to be attracted through the external circuit to the other terminal producing a current. The negatively charged electrons are then attracted to the positive end. But the two sides are separated internally; therefore, the electrons are forced to flow around an externally connected circuit producing a current.

What happens if you connect metal across the positive and negative terminals of a battery?

If just a low resistance cable were to be used - with no other circuit components - we get what's known as a short circuit. A very high current will flow, discharging the battery quickly and producing lots of heat at the same time which can be very dangerous and possibly cause a fire. Fuses are often used to break the circuit safely should a high current begin to flow.

Why lead acid batteries must not be discharged too much?

If a battery is allowed to become too discharged, then sulphate crystals will form at the negative end of the battery - a product from electrochemical reactions when discharging - grow and harden, eventually becoming impossible to reverse through charging. Additionally, some of the active material (substances used in the chemical reactions) can fall off the plates within the battery when the battery is deeply discharged, and drop to the bottom of the battery case. This decreases the amount of available chemicals for producing electricity; and eventually, when enough of these chemicals build up, they can short circuit the battery and destroy it.

 

Safety

What are the dangers of electricity?

Just milliamps of current from a high voltage (>50V) can cause harm to the body because our central nervous systems have their own finely balanced electrical circuitry. However, low voltage (<50V) is less dangerous but no terminal should ever be touched. In addition, cables that are too thin for the current that they have to carry can overheat and lead to fires.

Why use fuses?

A fuse is a small device that interrupts the circuit if a dangerously high current were to start flowing. The use of fuses mean that if a piece of equipment fails and the current in the circuit becomes too high, the fuse will "blow" and break the circuit, thus preventing the situation becoming worse.

What is earth/ground?

A low resistance connection to the earth, ensuring the safe dissipation of any unwanted and dangerous charge build up.

Why is it important to colour code?

Colour coding allows electricians to keep track of whether a cable is positive or negative - it is very important to only connect cables the correct way around otherwise a dangerous short circuit and electrical fire could follow.

Why is water dangerous near electricity?

Water conducts electricty, just as a metal does, raising the risk of a short circuit, and electrocution. All of our cables that are outside weatherproof buildings are contained inside rugged, waterproof, insulating protection that will survive rain and wildlife.

 

How do I begin to build my own?

Basic Ingredients

To collect the solar energy: photovoltaic panels.

To store the chemical energy: batteries.

To connect the above: pairs of cables.

To regulate the above: voltage controller.