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jasonvanwyk

Solar power vs PV Power

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Hi Forum,

I have an interesting question.

We know that PV panels use light energy and vacuum tube solar geysers use solar heat energy. Both sources are from the sun. I presume that there will be different inefficiencies within the two models, but I was just wandering...

We have had a lot of rain lately, so my PV panels as well as my vacuum tube geysers have not been able to harvest as much energy as they usually do (it's been cold and light is poor). I have a backup electrical geyser element (2kw) in my vacuum tube geyser for days like this, regulated with a thermostat (as many people do). I usually don't have to use it 90% of the time. I also have it connected to a timer which I can set in many different ways. so my question is, If my panels are producing 2500w on a overcast cloudy and reasonably cold/cool day (and the weather has been like this for a week, so my geysers are not getting as hot as they usually do), and my electrical Element is 2000w, could i make a reasonable assumption (all things being equal - ignoring the rest of the house's load for this example) that I would not have to put my element on at all since the power of the PV is roughly equivalent to the power that would be used by the element to heat the water to say 55-60 c? In other words 2500w of power is being output on the same geographical square area, whether I harvest is via solar or PV (I know PV is roughly only 15-20% efficient can capturing light energy from the sun p/sqm). I feel like I'm talking in circles to try and explain myself, but I'm going to try one more time...

If I see that my pv power is 2500w can I assume that my water is also being heated by at least the same amount of energy, so I don't have to worry about putting my element on?   

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11 hours ago, jasonvanwyk said:

wandering

A wise man once said that not all those who wanders are lost...

11 hours ago, jasonvanwyk said:

I feel like I'm talking in circles

... but maybe you are 🙂

Aaaanyway, jokes aside. You want to know if there is a way to estimate the power harvested by the EV tubes by looking at the power produced from the PV... and I would think that yes there is... but I'm not sure about some of the "fudge factors".

If you know how many square meters of PV you have, and the efficiency (say, 15%), you can of course work out the insolation per square meter from that. Then, if you also know how many square meters of EV tube you have (difficult one, the round shape of the tube means only a small part counts), and you know those things are around 60%-70% efficient, then you can work out the equivalent kWh that's going into the water. That needs to be more than 2kWh or you're essentially getting no heating at all (because of the standing loss of the tank).

The fudge factor I am not certain of, is working out the size of the EV tube collector "mesh". If we say that the half-round surface is equivalent to 70% of the straight-on surface (probably not a bad estimate, then you can calculate length times diameter times number of tubes times 0.7 (fudge factor), times 70% efficiency in absorption... well... I'll leave that as an exercise for the reader.

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12 hours ago, plonkster said:

A wise man once said that not all those who wanders are lost...

... but maybe you are 🙂

Aaaanyway, jokes aside. You want to know if there is a way to estimate the power harvested by the EV tubes by looking at the power produced from the PV... and I would think that yes there is... but I'm not sure about some of the "fudge factors".

If you know how many square meters of PV you have, and the efficiency (say, 15%), you can of course work out the insolation per square meter from that. Then, if you also know how many square meters of EV tube you have (difficult one, the round shape of the tube means only a small part counts), and you know those things are around 60%-70% efficient, then you can work out the equivalent kWh that's going into the water. That needs to be more than 2kWh or you're essentially getting no heating at all (because of the standing loss of the tank).

The fudge factor I am not certain of, is working out the size of the EV tube collector "mesh". If we say that the half-round surface is equivalent to 70% of the straight-on surface (probably not a bad estimate, then you can calculate length times diameter times number of tubes times 0.7 (fudge factor), times 70% efficiency in absorption... well... I'll leave that as an exercise for the reader.

thanks plonk - I'm intrigued, so I think this will be an interesting little experiment

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The two work on a different part of the light spectrum so AFAIK won't have the same performance characteristics in cloudy weather, as clouds block/reflect/bounce different parts of the spectrum differently.
e.g. Think of how you can go hiking on a cloudy day and not feel any 'warm' sensation on your skin and yet still get massive sunburn.

Its my understanding (though I can't find a good link on it right now) that despite overcast weather having a negative impact on both; PV systems fare much better than solar water heaters in various forms of overcast conditions because the part of the spectrum they rely on is closer to the 'burn your skin' side while the part that solar water heaters rely on is closer to the 'warm feeling on your skin' side.

With this in mind I don't think (other than in sunny weather) that your assumption of looking at your PV array output as a measure of how your hot water heater is also performing will hold up all that well.

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39 minutes ago, mmacleod said:

Its my understanding (though I can't find a good link on it right now) that despite overcast weather having a negative impact on both; PV systems fare much better than solar water heaters in various forms of overcast conditions because the part of the spectrum they rely on is closer to the 'burn your skin' side while the part that solar water heaters rely on is closer to the 'warm feeling on your skin' side.

I heard the reverse.

Basically, the burn your skin side of the spectrum is the UV side, and the warm feeling on the skin side is the IR side. EV tubes can apparently heat water with UV, and conversely, PV modules can absorb IR.

But it doesn't actually matter which one of us is right, you make a good point: Different parts of the spectrum. I still suspect you could make an estimate of some kind, but this is another fudge factor I just don't know.

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5 hours ago, mmacleod said:

With this in mind I don't think (other than in sunny weather) that your assumption of looking at your PV array output as a measure of how your hot water heater is also performing will hold up all that well.

thanks mmacleod - I was just wondering If I could make an estimate within a certain day temperature range. But You guys are right I think that there are so many other factors to consider as to the make the "estimate" unreliable" - still would be a good study/experiment.

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Another point to keep in mind is that PV Panels generate optimal power in sunny and cold conditions, whereas solar water panels work best when it is sunny and hot.

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Posted (edited)
5 hours ago, plonkster said:

I heard the reverse.

Basically, the burn your skin side of the spectrum is the UV side, and the warm feeling on the skin side is the IR side. EV tubes can apparently heat water with UV, and conversely, PV modules can absorb IR.

But it doesn't actually matter which one of us is right, you make a good point: Different parts of the spectrum. I still suspect you could make an estimate of some kind, but this is another fudge factor I just don't know.

You are right I think, I have it backwards. My main point was that they differ in spectrum, and in my eagerness to try demonstrate this in a simple analogy I ended up a bit far from the truth.
This stuff (light spectrums) is way out of my comfort zone in terms of knowledge, I only know that there is definitely a spectrum difference and that this alone probably makes any extrapolation between the outputs of the two 'tricky' at best.

It would be nice to have a better mental model of which parts of the spectrum the different types of PV panels/heating systems on the market actually rely on - and what the given energy in each of those spectrums tends to be for various types of common weather conditions. Something that is easily digestible by us laymen, I tried a brief search on the subject now but theres no super great resources on it to be found. :)

Edited by mmacleod

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