Energy efficiency EV tube vs Flat plate water heating

[Green Bum]

 

1.       Given relative low tech production requirements of flat panels there quality and therefore efficiency differ a lot. Issues of quality include, reflective glass vs non reflective, the quality of insulation of the thermal box, the quality of thermal absorption plates and connection to the heating pipes normally copper. Like anybody can, I built my own flat plate collectors. I went the whole hog soldering the heating pipes to a copper back-plate spending a lot of time with the insulation etc. etc. how efficient where they? Only in lab conditions would you be able to test and compare.

-          EV tubes require high tech production, quality is relatively safe because it is so expensive to make, only high volumes make them relatively cheap, poor quality is probably not worth the risk.?

-          Life expectancy. The biggest threat to flat plates is corrosion between the copper heating pipes and heat collectors, bad quality aluminium and copper, a bit of moist causes bi-metal corrosion. Bad quality boxes rust, disintegrate, leak moist into the box causing condensation… Most of the abovementioned challenges are not relevant to EV tubes, despite their fragile nature being glass there design and production standards seems able to withstand “natural” challenges of transport, installation and life on the roof in general for much longer than flat plates.

2.       I believe challenges of quality around flat plate collectors and opportunities of global demand for solar water heating created the opportunity for research leading to a superior product at a competitive price given huge production numbers.

3.       Given the same surface comparing ten randomly selected EV tubes systems with ten randomly selected flat palate systems EV tube will be more energy efficient!   

[Janma]

I Green Bum,

Good write-up! I would add one more thing to the list on why EV is more efficient than flat plate. Its got to do with the two ways something can heat up/cool down. There is "ambient heat" which is the temperature of the air and there is the "radiant heat" that we get from the sun. Objects will heat up or cool down if the radiant heat and/or ambient heat of their surroundings are greater/less than that of the object.

The radiant heat is the component that is used when the sun warms up a EV or flat plate collector. The ambient temperature around the collector is normally lower than the collector it self, therefor it will tend to cool it down. This is true for both EV and flat plate. The big difference between the two comes in that the EV has a vacuum in it. Ambient temperature can not propagate through a vacuum, only radiant heat can. The vacuum in the EV tubes is a VERY good isolation for ambient temperature loss.

When you have very cold days a good EV system will always outperform a flat plate of the same size collector, because ambient temperature loss will always be greater in the flat plate than the EV tube system.

Up here in Gauteng where the ambient temperature can drop below zero Celsius the difference in efficiency is more pronounced.

 

[Carl]

Spot on @Janma the efficiency is dominated by the radiation losses for both types of collectors.

See below for a comparison of collectors and how they compare in efficiency. (Picture copied from Soltrain Training Manual)

As expected the Evacuated Tube Collector performs very well and outperforms even a selective coated flat plate collector in most applications. 

But if you look a bit closer you will notice that for domestic hot water systems we are more interested in the left hand side of this graph where their isn't a huge difference between a vacuum tube and good selective coated flat panel.

The fact that the flat plate collector gets less efficient the hotter it gets means that it naturally cools itself down as it heats up and therefore reduces the chances of overheating. 

In my opinion, for our Climate in South Africa, a simple flat plate panel that is painted black is good enough. It is cheap, will not overheat, and will do the job for most of the year.  

 

[Green Bum]

Great guys I learnt something new today!

Thanks

[Transcape Plumber]

I had a conversation with a flat plate seller who explained to me that installing 2 smaller flat plate collectors rather than 1 large one (equaling total surface area) would improve the overall efficiency. He wasn't able to explain it in a way that I understood over the phone. Is this true and why so?

[SilverNodashi]

So, then, why is PV not on the comparison list?

[___]

6 minutes ago, SilverNodashi said:

So, then, why is PV not on the comparison list?

Well, probably because that wasn't the question they wanted to answer. But even if they did, the guys here who argue that PV is "better" don't mean that it makes more hot water... they mean it's more flexible. All that arguing has always been about semantics :-)

[SilverNodashi]

14 minutes ago, plonkster said:

Well, probably because that wasn't the question they wanted to answer. But even if they did, the guys here who argue that PV is "better" don't mean that it makes more hot water... they mean it's more flexible. All that arguing has always been about semantics :-)

I threw a spanner in the works, for a reason

I do feel, however that it's on topic, and even if it's "just caused it's more flexible", doesn't mean it's less efficient in terms of achieving the same goal?

[___]

1 hour ago, SilverNodashi said:

doesn't mean it's less efficient in terms of achieving the same goal?

I believe that as much as efficiency is defined as "this much hot water for a panel this big for this many hours in the sun", PV kan maar gaan doppies blaas...

[ebrsa]

I installed EV tubes as they raise the temperature higher, particularly in winter. PV depends on visible light which is largely locked by cloud cover while there is still infra red getting through to heat both EV and flat panels. PV efficiency and the low efficiency or resistive heating are also negative factors. So yes @plonkster you have a very valid point in my view. Of course one can go all the way and spend a lot by installing a lot of PV panels and a heat pump. Depends on your age as the repayment period will be a while.

[Guest]

46 minutes ago, ebrsa said:

Depends on your age as the repayment period will be a while.

Told my parents but the panels will last 20 years! Yes they said, with a smile, we won't. I could not justify anything after that quip. 

Have posted before the numbers on PV versus EV or flat panel. EV tubes win hands down.

BUT ...

We all agreed back then, I believe, that if you install a solar PV system, there probably will be spare so use that for the geyser. JDP also made the case for heatpumps on solar.

Again, each one has to ensure their own particular needs are met but from all posters on this forum, tubes/flat panels are probably 2nd price if you have a large enough array to heat geyser direct or with a heatpump.

If your array is not enough, or you live in the cape, EV tubes, not flat panels, EV tubes, as ebrsa also points out, are more efficient with cloudy winters, for then PV is just about near useless for a 2kw element.

[SilverNodashi]

Just now, The Terrible Triplett said:

Told my parents but the panels will last 20 years! Yes they said, with a smile, we won't. I could not justify anything after that quip. 

Have posted before the numbers on PV versus EV or flat panel. EV tubes win hands down.

BUT ...

We all agreed back then, I believe, that if you install a solar PV system, there probably will be spare so use that for the geyser. JDP also made the case for heatpumps on solar.

Again, each one has to ensure their own particular needs are met but from all posters on this forum, tubes/flat panels are probably 2nd price if you have a large enough array to heat geyser direct or with a hear pump.

If your array is not enough, or you live in the cape, EV tubes, not flat panels, EV tubes, as ebrsa also points out, are more efficient with cloudy winters, for then PV is just about near useless for a 2kw element.

The deciding factor for me, with the current water shortage and water shedding in some parts, if / was the fact that on a solar geyser (evac or flat panel) you don't have much control over the wasted water. You could probably catch it in something, that could sustain the 100+ degrees water, at that time, but a lot is lost in evaporation. 

One day, when I'm big, or when the geyser burst, I'll replace it with a heatpump. OR see what a couple extra panels and new geyser will cost at the time and decide which is more economical to replace. 

[ebrsa]

TTT your folks could save a lot on electricity charges by installing EV tubes. I know I do as for October my electricity consumption by the geyser was 12.5Kwh according to the Owl meter that measures only geyser consumption. I installed a 16 tube unit and the geyser is a 200 liter Xtream sitting on top of my roof which is flat concrete. Right now (16:30) the top of the tank is 58.6C and the bottom 55.8C. I have to add 6C to these as the probes are attached to the inlet and outlet pipes and 6C is the difference that I measured at a tap in the our en suite bathroom with a short pipe length to the geyser. Pipes are Pex and insulated for about 50% of the length. Highest temperature last summer was about 73C on the controller which is an Arduino cobbled together as described on www.nateful.com. I modified the code a bit but it works real well for me. The whole setup is self contained with its own 20W solar panel, battery and PWM charge controller. I did save money by installing it myself but still figure I will recover the capital cost in not more than 18 months so even at my age it makes sense. Besides it was a lot of fun and very gratifying to see the water temperature rise during the day.

I believe those who have problems with overheating, installed units that are too large but maybe they need the volume of water. It is just my wife and I in the house. Sure if you have plenty of solar power then a heat pump is the answer. In my case the economics just did not make sense. 

[Arandoza]

I am running glass ev tubes and it won't overheat, only if we don't use the geysers at all.

We then catch it in the rain harvesting tank and reuse.

If we go away for a few days, I simply shade the ev tubes with high density shade cloth, which means the geysers remain hot but far less likely to overheat and waste water, however our roof lends to easily getting to the tubes, flat roof, where the geysers are.

[Guest]

1 hour ago, ebrsa said:

TTT your folks could save a lot on electricity charges by installing EV tubes.

Believed so too.

The are very efficient with hot water and are not around 3 months of the year.

And their geysers is on for maybe 2 hours a day - .R4.56 per day = R1231.20 a year. 

They don't want to budge. I tried. 

Ours on the other hand, as yours, was paid also for in less than 24months, but a very long time ago. We switch to Eskom maybe wot +-30 times a year for not even 2 hours at a time, rest of the time we rely on the sun.

BUT, I must add, the house is following a very nicely layed out guideline of do's and dont's, degined to save not only electricity, but more important than that, water. (very evil grin)

[Wetkit]

I also believe EV is better than flate plate....

But, you do get diffrent types of flate plate!!

I currently have 2 flate plate units from Austria. They work way too good in summer, so they completely covered in summer and I only open them in winter.

They have a 22mm pipe at the bottom and top that pass strait through the unit. There is only a single 8mm pipe that runs in a zig zag from the bottom to the top of the plate.

The water can be 20 degrees at the bottom, but once it gets to the top it is at 60

Problem is you need at least 3 of these for a system, but it is more geared for low flow and high heat. A 1000L geyser will work well with these