# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: Chris Wilson on October 10, 2016, 12:51:13 am

Title: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: Chris Wilson on October 10, 2016, 12:51:13 am
Two photo-voltaic panels, side by side in sunlight. One disconnected, the other feeds a load. Which panel has the higher temperature? This seemingly straightforward question on another forum is provoking pages of debate... Thanks.
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: Krytron on October 10, 2016, 02:19:40 am
The unit supplying the load will be warmer.  There are energy losses in electronic devices which are dissipated - which causes heat.
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: Chris Wilson on October 10, 2016, 02:36:21 am
This was my response:

The loaded panel will be warmer as the energy transfer from light to electricity is not, of course, 100% efficient. The working panel will self generate heat as well as current. The unloaded panel will show voltage at its terminals, but until current is drawn it's not producing electrical energy.

But here's a typical alternate view:

nope, because the heat produced from the current comes from the initial solar energy which is converted directly to heat in the unconnected panel.

lets use numbers

numbers are wildly inaccurate and not representative of real life they have been chosen purely to keep things round.

each panel receives 200 Watts of solar energy, panel's reflect 50% of the solar energy

the panel's are 50% efficient at converting light to electricity, and 80% efficient at conducting that electricity internally.

200W falls on unconnected panel 100W reflected 100W absorbed and converted to heat

200W falls on the connected panel, 100W reflected. Of the remaining 100W, 50W are converted to electricity, of that 20% is lost as heat through inefficiency in conductors (10W), the other 50w is turned to heat

which leaves us with unconnected panel receives 100W of heating energy

connected panel only receives 60W of heating energy.
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: Ian.M on October 10, 2016, 02:51:06 am
Noon clear sky solar irradiance is approximately 1KW/m2.  That's the maximum possible power input to the panels and its determined by the sun, its distance and the characteristics of the atmosphere, none of which can be influenced in any way by the load on the panels.

A proportion of the energy available is absorbed and a proportion reflected.   For the panel producing electrical power to run hotter, it would have to absorb more energy proportional to the power it's producing with a constant of proportionality above unity.   If it does not do so, it *MUST* run cooler than the other non-power producing panel as anything else would violate the second law of thermodynamics.
I don't believe the physics of PiN photodiodes supports any mechanism for the surface reflectance to vary significantly with the current.
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: richard.cs on October 11, 2016, 11:39:36 am
Think about the model of the PV cell as a current source (pumped by the incoming photons) shunted by a forward-biased diode (the cell junction). When there's no external load all of the current being produced flows through the diode making it hot (the electrons raised to a higher energy level by incoming photons drop back down inside the cell releasing their energy). When the cell is loaded most of these higher potential electrons flow into an external load and this energy is delivered elsewhere. The cell must run cooler when loaded.

In simple terms it always absorbs the same amount of energy from the sun, most as simple thermal energy and a small fraction as electrical (the electrons which have been pushed to a higher energy level). But if there's no external load then all this energy stays in the cell, it can't go anywhere else, but neither can the cell reject it and not absorb it.

Interestingly it means that a short-circuited cell runs at the same overall temperature as an open circuit cell, just the energy is dissipated in different places, in the former it's resistively in the bulk silicon and the contacts, in the latter in a small region around the junction.
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: Chris Wilson on October 13, 2016, 09:38:33 am
Thanks for the replies, I now see how this rather un-intuitive situation arises, much appreciated!
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: Kleinstein on October 13, 2016, 05:52:23 pm
Where exactly the heat is dissipated in a solar cell is not that simple. There is not just the light absorption and resistive loss. The contacts silicon to metal and P to N doped region have a rather high thermoelectric effect (Peltier-effect). So there are quite some localized heat sources (or sinks if an external current is imposed) at the contacts. So the solar cell in short circuit has local heat sources at the metal to silicon contacts and a local heat sink in the junction region.

I did my university thesis about looking for those heat sources.
Title: Re: Temperature of 2 side by side PV panels, one loaded and one unloaded?
Post by: richard.cs on October 13, 2016, 08:10:58 pm
Where exactly the heat is dissipated in a solar cell is not that simple. There is not just the light absorption and resistive loss. The contacts silicon to metal and P to N doped region have a rather high thermoelectric effect (Peltier-effect). So there are quite some localized heat sources (or sinks if an external current is imposed) at the contacts. So the solar cell in short circuit has local heat sources at the metal to silicon contacts and a local heat sink in the junction region.

I did my university thesis about looking for those heat sources.

That sounds really interesting. Would you be willing to share a copy of your thesis?