Greentherm supply Solar P.V. and Solar Thermal Systems to the Irish Market. Solar Thermal has been discussed before, but what about Solar P.V.? It seems like magic, that a simple flat sheet of silicon can create electricity from the sun. So how does it happen?
The majority of Solar P.V. Panels on the market today are manufactured from various forms of Silicon. This is the same basic Silicon that is used to manufacture computer chips – in fact, many of the early production processes are shared between the two. Silicon is what’s known as a semiconductor. It’s not quite an insulator like plastic. It’s not a conductor like copper either. It’s crystal structure gives it unique properties. When exposed to an external source of energy – such as heat – it’s ability to carry current – it’s conductivity – increases. Also, the conductivity and electrical characteristics can be precisely controlled through the use of special manufacturing techniques such as doping
By adding other materials to the Silicon crystal, it’s current carrying characteristics can be precisely changed, making it more susceptible to current flow. Positively Doped (P-type) Silicon has been altered so that it has a slight positive charge. Negatively Doped (N-type) silicon has been doped to have a slight negative charge.
When P-type and N-type silicon are combined together, they form what is known as a P-N Junction. A natural barrier forms between the pair that will only allow current to pass in one direction, and only when a certain switch-on voltage has been reached. Think of it like a dam in a river. Only when the river level rises above the level of the dam wall will water be able to flow over . Also, a dam blocks water from flowing back up the river – in the event of a tidal flood or similar event. A P-N Junction connected in reverse will block current flowing backwards.
The P-N Junction forms the core of most modern electronics, the most basic use being in a Diode – a device normally used to limit the flow of electric current to one direction.
Semiconductors don’t behave like normal electrical devices. They have a number of strange characteristics. Once a diode has ‘switched on’, for example, it’s effective resistance drops to almost nothing. This means that – even for small increases in voltage – a rapid increase in current will be experienced. This switching function makes them very useful for modern electronics hardware – such as computer processors.
The ‘switch’ however, doesn’t necessarily have to be the voltage across the junction. It can come from any source. A Photodiode is a diode that is switched on by light-energy for example.
But, how do we use this to generate electricity?
The Photoelectric Effect
Light is a form of energy. When light strikes an object, it has the potential to knock electrons loose within that object, generating a small voltage. This is the photoelectric effect. Some substances are more prone to it than others. However, due to the nature of most substances, these voltages tend to rapidly dissapate unless some sort of barrier exists between charges.
Like a p-n junction.
The p-n junction in a solar cell prevents these charges from dissipating with in the cell, allowing them to build up. This creates a voltage across both terminals of the cell. This voltage is limited by the characteristics of the solar cell – including its size and the quality of the p-n junction seperating the charges. The lower the quality, the easier it is for charge to leak back across. Eventually, this leakage matches the rate of charge production, and creates the Open Circuit Voltage of the panel.
If these terminals were connected with a piece of cable however, current would begin to flow.
A Current Source
Under ideal circumstances, a Solar Cell functions as what’s known as a Current Source. This means that, when connected to any load it will attempt to push a fixed current through that load – say 3 Amps. It will increase the voltage to whatever is required to push that amount of current through the circuit. The amount of current the cell attempts to deliver is directly related to the amount of solar energy falling on it.
The more sun, the higher the amount of current that will be delivered. It’s that simple.
There is, however, a limit. Remember above, the Open Circuit Voltage?
If the resistance to current flow through the load is high enough, then the voltage required to drive current through the load will get so high that more and more of the charge will begin to dissipate within the panel. The voltage will slowly increase to the Open Circuit Voltage, while the delivered current will drop off rapidly. A side effect of this is that the dissipation of energy through the panel will cause it to begin to heat up.
This heating reduces the performance of the panel somewhat by making it easier for charge to leak across, limiting voltage and current further.
The Power of the Panel
The I/V characteristic Curve for the Viridian Clearline range of Solar Panels Greentherm offers, is shown below. It shows curves for the 250W, 300W and 500W PV panel models. It can be seen that the maximum operating current off all three models of panel remain the same – the only change is the maximum voltage this current is supplied at. It’s also clear that the open circuit voltage of the 500W panel, is double that of the 250W panel. This is due to how the panels are manufactured – from a whole bank of individual solar cells connected together in series, such that their open circuit voltages are added to each other.
Effectively, these lines represent the operating power of the panel, for various loads. The delivered power through a load can be easily calculated from these graphs for any attached resistive load. The Maximum Power Point of the panel – where it is operating at its most efficient – is achieved when the voltage and current multiple (VxI) is at its maximum. This happens right as the current begins to decrease, and is marked on the graph above with a red line. These lines represent the Maximum Power Voltage and Current on the datasheet. This point, is where the panel achieves it’s rated power output, or Watts-peak (kWp, or Wp).
Maximum Power Point Tracking (MPPT)
If a load is directly connected to the panel, it’s unlikely that the panel will be operating at it’ maximum power point. For example, if a battery to be charged were connected directly to the panel, the voltage in the system will be clamped down to the battery voltage. The maximum current delivered will always be limited by the characteristics of the panel – in the case of the above panel, approximately 8 Amps. With a 12 Volt battery connected, this would limit the maximum delivereable power to the battery to less than 96W – less than half of the rated power output.
The same principal applies to other electrical loads. Attaching a simple resistive immersion heating, or a basic chanrge controller to the PV panel will pull it outside of it’ most efficient operating regime. This also happens if an individual panel is shaded in an array, or otherwise malfunctions – the output of the array is clamped down to that shaded panel’s maximum voltage or current capability (Depending on how they are connected together)
The get the best out of a Solar PV Panel, the load has to be matched to the Maximum Power Point of the panel.
The Maximum Power Point is not necessarily a fixed point on the graph. It will vary according to the amount of sunlight being provided to the panel. The maximum output current of the panel is directly related to the amount of sunshine received by the panel. Shading of individual panels or modules within the PV system will also effect the maximum power point.
It becomes necessary to have an electronic device – either a charge controller, or inverter for grid-tie connection, with Maximum Power Point Tracking capability. Devices with MPPT capabilities can adjust their electrical characteristics to ensure that the connected Solar Panels are always operating close to their regime of peak efficiency, regardless of what load is connected to them.
Greentherm sell charge controllers and inverters with this capability.
For more Information
Contact us for more information on the designing, specification and installation of Solar PV systems, or to arrange a no-obligation quotation or consultation.