Do solar inverters dump excess generated energy as heat?

Question

I have a off-grid solar system and it got me curious, how is my inverter able to know how much excess energy is being produced? I could not show the actual graph from the app but it's really close to this one:

Looking at it with my limited electronics experience I could not come up of an explanation that makes sense on how it is able to know how much excess energy is produced.

So the way to track power consumption is measuring voltage and current. Those are really what you fundamentally need. Measuring voltage is pretty easy. Current on the other hand is not as straightforward. Assuming there is very little load and very high photovoltaic generation there should be very little current passing into the tracking sensors.

The only possible explanation I could come up with is the surplus energy is being dumped as heat, the problem with that is the inverter is tiny and compact and I do not think it could dissipate the full 5 kW of power during the middle of the day for hours.

Answer 1

how is my inverter able to know how much excess energy is being produced?

I can't say how your inverter does it, but one method used is to calculate potential power available from the actual power drawn and the duty cycle.

In an MPPT (Maximum Power Point Tracking) system the controller periodically adjusts its duty cycle to get the combination of panel voltage and current that produces the most power, if the load can absorb it. If the load cannot absorb the full panel power then the controller has to lower the duty cycle to reduce the output power. If it knows how much power is produced at the maximum power point then it can estimate how much power would be produced if it didn't have to lower the duty cycle.

The other question is what happens to the power produced by the panel when it is not drawn off by the inverter. The answer is that the electrons kicked out of their 'holes' by sunlight recombine with the 'holes' inside the panel, releasing the energy they absorbed as heat. In silicon solar cells the primary mechanisms are Auger recombination (where the electrons 'bump into' other particles and transfer their kinetic energy to them) and surface recombination caused by defects that 'trap' charge carriers in the surface layer.

Answer 2

No, the solar cells simply warms up and radiate away any incoming sunlight which isn’t captured as electricity.

Due to fairly low efficiency of solar cells, this effect isn’t very pronounced to compare 100 % versus 85 % (with 15 % efficiency of a solar cell) solar irradiation converted to heat, so if you have two identical panels next to each other, one unloaded and one with perfect MPPT load, the MPPT one won’t be cold while the unloaded is scolding hot, just some temperature difference between them.

I’m ignoring any reflection going on to simplify my example.

Answer 3

The surplus of power is not converted to electrical power. When more power is generated than consumed, the output voltage of the inverter increases. Above a specific voltage threshold the inverter is not operating at the MPP but at lower power operating points. It should still be possible to calculate the power that could be provided using the MPP with the I/V characteristic of the solar module with the voltage and current measurement of the lower power OP.

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But the I/V characteristic also depends on temperature. And the operating point also has an impact on the cell temperatures since available energy that is not converted to electrical energy will be dissipated in the solar cells. The cell temperatures are usually also measured.

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I would assume that most inverters only approximate the power surplus with a relatively low accuracy so simpler approaches might be used. Maybe the manual/datasheet of your inverter provides some insight.

Answer 4

There is no need to dump the energy on the inverter side. Let's suppose that extracted energy from panel is lower than produced from it, then the excess energy is converted to heat. This will cause little bit more wear of panel, but when compared to overall efficiency, the temperature rise is so small that could only be measured with scientific instruments.

On the other hand, more serious condition happen if the panel is just partially shaded. In that case, the shaded cells (or single cell) would start to overheat. The temperature rise is such that can be seen even with a cheap thermal camera.

Answer 5

Current sourced PV’s do not shunt excess current as internal power. When unloaded they just store an open circuit Voc charge voltage and cutoff down to the leakage current as reverse biased diodes with no load.

Answer 6

Lars has a good reply to your question. I will add some thoughts from my experience in designing MPPT converters back in years.

As you may have noticed, a given PV Panel(a.k.a Solar Panel) produces a certain voltage across its terminals when bombarded with light photons. If 1000w/m2 of light is incident on a panel, a small percentage of this actually gets converted into electrical energy, the other just stays in the form of heat(so a panel would normally heat up just as much as any other item lying in the sun).

Now coming to your question, How does your inverter know how much power is available and what is the best ‘operating point’. Depending on the topology, Most solar inverters have a Boost, Buck-Boost or a buck converter at the initial stage. This converter senses input current and voltage, and within converter’s software, these measurements are used to contineously calculate input power.

The interesting bit comes next, the inverter can control the total load seen by the panel. If I were to simplify, a panel has no clue if you have connected it to a resistive load(e.g bulb) or a more sophisticated converter. The converter’s processor can ‘play around’ with the pulse width of signal being sent to transistors and keep an eye on total power to identify the ideal operating point where maximum power is harvested. The mechanism of this ‘play around’ varies depending on the MPPT algorithm. For starters, a controller would start with a high/low pulse width and keep changing it to see at what load maximum power can be obtained from the panel. Normally good converters keep ‘perturbing’ the pulse width back and forth slightly to see if the maximum power point may have drifted and as a result can always keep harvesting maximum power irrespective of the time of day/external conditions).

Now lets come to the second part of your question, if the downstream load(e.g a battery) no longer needs power, how can the MPPT converter still continue to harvest maximum power. Short answer: It cant. If the maximum power that can be harvested is 100w while the battery/load needs no more than 50w, then simply put, the converter can no longer track the power point. I mentioned earlier that the MPPT converter keeps an eye on input voltage and current, it keeps an eye on the output bus voltage aswell. If the output voltage rises beyond maximum allowed bus voltage, the controllers software prevents it from trying to increase input load any further. The output bus voltage essentially overrides the MPPT algorithm’s wishes and your panel is just producing less energy then it could.

Lets come to the last part, if your converter is harvesting less power from the panel. Where does the remaining power go, the answer is: it doesnt even get produced. When a solar panel is just lying in the sun with no load connected to it, the panel’s cells create a voltage and wait for someone to apply a load, if there isnt enough load, no actual power generation happens and no additional heating should be produced(other then unnoticable effects from leakage currents).

Hope this helps.

Answer 7

Two types of controllers:

PWM and MPPT.

What PWM does: it plays on/off for the solar panel. Say, 1000 or 10000 timer per second. The on and off times are regulated in a fashion that the total produced power matches the load. When the circuit is in the "on" state, one can measure both current and voltage and know exactly what the panel can produce (at this load profile and not in general, this is a limitation of the PWM controllers).

What MPPT does:

Changes the load presented to the panel (voltage or current) constantly in order to find the point where the panel produces the most. There are quite a few algorithms fot MPPT, but most of them boil down to:

1. Constantly check a small range around the currently known best point in order to move it if the conditions changed.
2. (optional) Once in a while, say, 1 or 10s sweep the whole range of voltages in order to find if a new, better local maximum in the panel array volt/watt curve did emerge. Aged panels, physically damaged panels, partially shaded panels or panels of different characteristics wired together may have more than one local maximum.
3. At partial load: once the best point is known, shift away from it towards either higher or lower voltage in order to produce less than the maximum power. In effect, this lowers the PV panel efficiency and the unused energy ends up as heat in the panel.

In all these cases, the power produced fluctuates somewhat. Big capacitors, as well as the system batteries (if present) or the whole public power grid (if connected) are used to smooth out the fluctuations.

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There are PV systems that use the concept of a "dump load" - they always produce the maximum. The excess electrical power that is not used is "dumped" at a deliberately connected "dump" load.

It may be a supplemental heater element in the water heater, water pump, basement dehumidifier, a combination of them, whatever - that can consume some power usefully or just dispose it off safely.

Dump loads are usually used with wind power (where one has to prevent the generator from spinning too fast) and less frequently with solar (where panels don't care what the other end of the wire does).

Answer 8

This question needs a short answer. When you have more power available than you need, the inverter changes the duty cycle. It will still draw nearly maximum power from the solar cells, but only for x% of the time.

power_drawn * 100/x is the total amount of power that was available.