Wow. 4 x 400w panels at 40V each should thus give UP TO 1600W… multiple by about 4 to get about 6.4kwh per day from R8800’s panels - per geyser. Why not?
I looked at the geyserwise dual element, but these are the specs:
GEYSERWISE PTC Element, 2.0kW (230V AC), 900W (48-72V DC) … the DC element looks way too small…?
No, not really. We already have established that this layout is inadequate.
I can see that this layout is not optimum.
But, I really need to see what options are available to you, to see if we can do something about it.
So the answers to my questions are important.
Are you totally off grid? Why not just use some the grid to heat the geyser(s) that gets used in the morning early. Remember that the hotter your geyser gets the faster it loses heat too. It takes the same amount of energy to heat from 20 to 50 than from 50 to 80 degrees, but it will cool from 80 to 50 much faster than from 50 to 20. What I’m saying is the gain on your investment is much lower and it’s going to be a hell of a long time to recoup your investment, if ever.
If you’re totally off grid I’d add a Rennai or Paloma gas geyser downstream from the other geysers. They’re very efficient and can only “top up” the water’s temperature to what’s required. In summer they won’t even switch if the water is hot enough already.
True. One now gets B+ rated geysers, they hold their temp even better. Our geyser is outside and on a cold rainy night, the outside feels ice cold with the geyser at 60 deg.
Titbit: Interestingly though, being a vertical one (wanted to try one), although the geyser after use would show say like 21deg, you still have to add cold water if you are a male.
That’s because the Thermostat is at the bottom with stratification and all there could be several layers of different temp water, depending how much cold water replaced the (hot) water covering the Thermostat.
This. If it is a case of wanting to use a little bit less of Eskom, probably not worthwhile investment of effort and money.
Eskom heats my geyser between 4am and 6am. Uses 2-3kWh per day. To take that “off grid”, I’d need to purchase another battery and hope that my PV gets to charge it full during the day, which will not be the case every day, but probably most.
The above is why I’ve made the decision to wait for my one very old geyser (two plumbers already told me I probably don’t have more than two years on it) to go. At least then I get some insurance money, can pay to change my plumbing a bit so that I only have one large geyser instead of two smaller ones, and then put a heatpump on the lot.
With a heatpump the 1kW or so it’ll take to topup my geyser for our morning showers will be fine to run off the batteries I have currently. During the day I’d still be able to heat my geyser higher than the heatpump can go with some excess PV and the element in it so that the morning topup would at least be a bit less.
I haven’t owned a vertical geyser but I believe they are more efficient?
What I have noticed however with these is the huge difference between the Gyserwiswe thermostat temperature and the water temp at the outlet.
The horizontal geysers don’t have as much of a discrepancy.
@Ironman, if you are not going to tell me which directions things are facing, I am going to assume things are facing North.
This is great if you want to shower at lunchtime, not so good if you want to shower in the morning, and not so good if you want to make heat in the late afternoon.
So first things first, before we even think about optimizing the PV.
I can’t give you options, because I don’t know what options are available to you.
You haven’t even said which geysers supply what showers at what times.
I can tell you what to think about though.
That 4 am shower is too early to get heat from the sun in the morning, if it’s thermal it should face West to maximize the afternoon sun.
Later morning showers can face East they can benefit from the early morning sun.
The next thing to consider is the plumbing. You may have hot water all day long in a geyser that no one is using. How about changing the system so that the geysers feed each other.
That way you can set them up so that they are not topped up with cold water but with warmer water. So consider times of use and you can set them up so that remaining unused hot water from one geyser is available to another or both of the others.
Edit: Yes, by the way, I have repositioned tubes from North to East before and achieved benefits, and also plumbed a solar geyser to feed an electric one.
Sorry - had the extended family over yesterday. The property is about 15 degress from north. Meaning that the solar peak shifts from about 12h00 to 14h00, and that is when the shading on the evac tubes starts.
I’m not off grid, and use eskom in the mornings to re-heat the geysers because my batteries are flat at that time.
Unfortunately not - I have multiple hot water endpoints distributed throughout.
Cool idea - but I have one geyser on borehole water for showers only, and another on council water for the kitchen.
Some more info on the geysers:
So this one basically has to be electrically heated and is a distance away, so you don’t want to feed it with hot water because it will lose heat in the pipes.
Do you need to heat 200l? Or could you swap it out for a smaller volume and reuse it elsewhere? Can rather you feed this one with the borehole, feeding it with cold water won’t lose heat. I assume being in JHB, water restrictions aren’t your problem, just the cost of water.
I assume these two are the ones in the photo with evac tubes facing 15 degrees off North.
One fed by borehole and one fed by the council. (There will probably be hygiene regulations prohibiting mixing supplies, check this).
These are the ones you want to put in series off the same water supply to feed the kitchen and the local bathrooms.
It also appears as if you have close by options for at least 12 tubes from either East or West or both on that roof?
Ideally, the geyser that first feeds everything should be East, the other geyser that feeds the first one can probably stay North, or alternatively West.
You want to feed the first geyser with the hottest water depending on your usage pattern.
The idea with energy is to make as you use it, every time to convert it from one form to another you lose.
For example, the round trip PV energy has to take to heat water via a battery is:
Light energy to DC electrical energy to chemical energy to DC electrical energy to AC electrical energy back to heat. You are the loser at every single exchange.
If you store it you also lose out through energy leaks as well. Whether in batteries or hot water tanks you lose out.
Combining the hot water volumes gives you similar flexibility akin to having a big battery bank instead of two small ones. If you have two small banks and one goes flat everything that it feeds turns off, even if the other one is full.
Things to think about.
Thanks for your input - there is no easy gain to be had from this setup. I have a lot on my plate right now - will revisit in a while.
The plumbers I spoke to, says they are more efficient yes.
It is all about the the separation area for the warm/cold water.
In a geyser the warm water moves to the top, while cold water settles at the bottom. Thermosiphon geysers use this process to get water flow without a pump.
If your geyser is installed horizontally the separation area between warm and cold would be the width x length of the geyser.
While if you mount the geyser vertically the area of separation is the circle cut (π x r x r) which is much smaller. This means there are less warm water touching cold water that result in less heat transfer.
Note: this is a simplification as the area of the rectangle change the more hot/cold water is and there is also not a fix line to say below is the cold and above the warm. So it is a lot more complex, but that is the basics of the theory.
OK, but what kind of efficiency are we talking about? Are we talking about reducing the standing loss? In that case it makes sense to me. A smaller layer of hot water touching a smaller part of the cylinder should reduce the standing loss.
On a thermosiphon system I can also see that larger separation means colder water at the bottom. Newton’s law of cooling says the rate at which you absorb/emit heat is proportional to the delta between that and ambient, so colder water into the heating panel means better absorption of ambient heat. So there too it makes sense that there is an improvement in efficiency.
Outside those two, I would think the efficiency of electrically getting heat into the water would be exactly the same. And with a flat tank I’d probably have more hot water for a shower while with an upright tank the uppermost hottest bit would be of a smaller volume.
I think the standing loss would be a wash, because the smaller layer of hot water touching the cylinder is cancelled by the top layer of hot water being hotter than in the horizontal geyser. Just my first year university understanding of thermodynamics says that the total in == total out in a closed system and orientation can’t make a difference.
You are basically right it all has to do with stratification.
Stratification occurs horizontally.
There is also another aspect to the energy efficiency, that I suppose most South African are unaware of due to never having seen it.
You only have to heat the water you need with a vertical geyser. That sounds a bit puzzling.
But think where the elements are in the tank. A horizontal geyser ( at least any have seen) have their elements down the centre longitudinally. A vertical geyser is the same, just flipped vertically. ( element in on the top).
Now here is the but.
Where I live in Ireland, domestic geysers (immersions here), are all vertical and equipped with two elements, bath and sink. The sink element is a short element.
It heats up a far smaller amount of water if that is all you use. Effectively, it gives you a two size geyser option.
Due to the horizontal nature of stratification, a short element would still have to heat up the same volume of water as a long one in a horizontal geyser.
It isn’t a closed system, otherwise a geyser would stay hot forever.
