04

March

0

Hello everyone, this is the second blog post on technical aspects of solar cells. This article discusses the specifications manufacturers provide in the datasheets of their solar module. If you ever looked at a data sheet and wondered what these terms meant and want to know more, this blog is for you.

Those interested in photovolatics are sure to have seen a solar module datasheet at some point. This blog is to help you understand the different terms in the specifications. These include – “Maximum Power” (P_{max}), “Open Circuit Voltage” (V_{OC}), “Short Circuit Current” (I_{SC}), “Maximum Power Point Voltage” (V_{mpp}), “Maximum power point current” (I_{mpp}).

Of course, there would also be other information such as – component details (dimensions, weight, number of cells, some mechanical characteristics), thermal characteristics (thermal coefficient), system integration parameters etc. These other characteristics will be covered in our forthcoming blogs, but in this blog, we will talk about the different currents and voltages.

The origin of the current in a solar cell was explained in our previous article, “**How does a solar cell work**”. This blog discussed how doped semiconductors can be joined to make a junction, how this junction has a potential drop (or electric field), how energy (in form of heat or light) creates electron-hole pairs and how the junction’s electric field separates them to generate current.

Take such a P-N junction. In the dark, there is only thermal energy. It creates e-hole pairs and junction’s potential drop drives electrons from P to N-side and holes from N to P-side. This is the thermal generation current, I_{gen}. Now the P-side will have more holes and N-side will have more electrons. This concentration difference drives them back (this is the recombination current, I_{rec}). So the recombination current is in opposite direction to the field and will be hindered by it (this dependence of recombination current on the junction’s potential difference will become important once we attach a load).

In an isolated junction, however, externally no current can flow and so the two currents have no choice but to balance each other: I_{rec} = I _{gen}

If light is shone over this junction, the photons in the light will create more electron-hole pairs – let the resulting current from these be called I_{photo}.

**Open Circuit:**i.e. under no external contact where no current can flow externally. So both the photo-generated (I_{photo}) and thermally generated currents (I_{gen}) have to be balanced by the recombination current (I_{rec}). To balance a bigger current, recombination current has to increase. This needs the potential difference of the junction to reduce. This drop is**V**_{OC }or open-circuit voltage.**Short Circuit:**No external voltage is applied, but current can flow.**The Short Circuit current, I**, is the net current of photogenerated, thermal generated and recombination currents (I_{SC}_{photo}+ I_{gen}– I_{rec}) under zero applied voltage.**Realistic Scenario – Under a load:**When a load is connected, some of the photogenerated current (I_{photo}) goes through the external circuit. So there will be a voltage drop across the load. The internal potential difference of the junction is lowered by that voltage drop. So the recombination current is happy – its opposing force has just been lowered. As the I_{rec}increases, the net current (I_{photo}+ I_{gen}– I_{rec}) decreases.So, the solar cells connected under a load can produce both a net current and a voltage drop, both dependent on the load. The graph below plots the interdependence of output current and voltage from a solar cell.

**Peak power:**Power is what solar panels are there to produce. So in practical application, the load is chosen so that the power output (product of current and voltage) is maximized. This is the peak power. The Maximum Power Point trackers (MPPTs) control the external load to maintain peak power under operation.xPeak Power,

**P**= V_{max}_{max}*I_{max}**Efficiency:**This is simply the peak power normalized for the incoming solar irradiance (E) and panel surface area (A). So it is the ratio of electrical output divided by total energy received by the cell.Efficiency = Peak Power / (Irradiance * Panel Area)

**η**= P_{max}/ E*A**STC:**To be able to compare performance of different cells, we need them to be tested under similar conditions. The most widely used standard is called the Standard Test Conditions (STC). Here the irradiance (E) is set at**1000 W/m2**and temperature at**25 °C**. Manufacturers measure the different characteristics I_{sc}, V_{oc}, P_{max}, I_{max}, V_{max}under these conditions and calculate the corresponding efficiency.

** **

**QUICK SUMMARY:**

For those who want to quickly grasp the essence of the different specifications -

- Open circuit voltage (V
_{OC}): The largest voltage you can possibly get out of the cell, but this comes at the cost of zero current. - Short circuit voltage (I
_{SC}): The largest current possible, but comes with zero voltage.

[So under OC and SC conditions, the power output is actually zero.] - Peak Power (P
_{max}): When a load is attached, voltage and current are both lower than V_{OC}and I_{SC}. But their product, Power, is maximized. (Power = Voltage × Current) - Efficiency (η): Peak Power / Irradiance * Panel Area. So it is simply the peak power normalized for the incoming solar irradiance and panel surface area.
- Standard Testing Conditions (STC): 1000 W/m2 of irradiance and 25 °C.

Leave a Reply