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carlysun

DC geyser elements

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HI All

 

Does anyone have any expereince with retrofitting DC geyser elements that run directly off solar panels?

There are a number of solutions out there, an interesting one is and AC/DC solution which will use mains at night or on overcast days.

 

Looking for any opinions out there on this as it looks like a viable alternative to installing solar evac tube geysers

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Hi 

I do not have any experience in fitting dc elements however dc elements and ac elements will work the same as it a resistive load and does not change with dc if the voltage and current is the same. You do not have to change your element. This means that you have to match the output of the solar panels to a voltage of about 230v dc and a current of about 8.7amps for a 2kw element and 13amps for a 3kw element. (eg: 6 X 330w panels in series will give you about (37.2vdc x 6) 223,2dc volts and can deliver 8,9 amps ). This at optimum sunlight conditions which means you could go slightly higher.(add 1more panel to give 260vdc and 8,9amps)

There is another problem which is the thermostat  that is commonly used as it does not work well with dc switching. You could use the thermostat to switch a dc contactor that can handle the dc current during temperature control. You need the thermostat so that you do not boil the water or go above about 70 degrees Celsius.

It has the same problem that with no sunlight it will not heat the water.

You can with using the above method using an appropriate dc switch, switch from solar panels to mains when there no sunlight as you would use the same element.

I have a system which uses vacuum tubes to heat my water in a normal geyser which then is  fed to a gas geyser. It has also the option to be heated by electricity if I do not have gas and the tubes have not heated the water enough. I set my water temp at 50 degrees Celsius.

 

 

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I've looked into this in the past but have still not done it, I'm interested in potentially going this route still so curious to see what answers come up.

While there is the geyserwise system, it is in my opinion massively overpriced, last I checked (maybe its changed) its around R5000 for the MPPT portion alone and then there are still other parts to the system, an expensive new heating element for a further R2000 and so on.
While it maybe works well at almost R10000 to R12000 (or maybe more if you need to install the panels as well) it IMO doesn't really make that much sense from a cost perspective.


Other local options include:
geyserworx

geyserrobot

elon 100


There are some good and affordable solutions on the international market, but most of them require a geyser that can take two elements (one for the AC side of things, one for the DC side)
unless you have such a geyser they aren't really an option.

If you only want it to run on DC and not also AC (like most of the above solutions allow) - or have a geyser that can take two elements, then there are various mppt controllers on the market (international and local) that can do it, some of them aimed specifically at geysers (microcare geyser mppt) and others just configurable in such a way that they can work with a geyser.
 

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Interesting topic.

Agree with Peter that a "DC" element should not be needed if enough panels are added in series.

Another possible alternative may be to use a device that will relay any extra energy generated, that would otherwise be exported to the grid (if grid connected) to the geyser.

Below are links two such units, both of which has been available in SA before, not sure if they still are:

https://www.earthwiseproducts.co.uk/solic-200/

https://www.apollosolarelectric.co.uk/

The 2nd unit is more pricey but include a timer function and thermostat and should be suitable for a single element geyser, which may be an issue with the 1st unit that only has a manual "boost" function as far as I am aware, which will still word but will require the "boost" button to be pressed if the water needs additional heating.

Edited by WeNotGood
Add info

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From what I gather, elements sold as DC elements have different resistances for DC and AC, allowing the same element to be used with 220VAC or 72VDC.

Usually, with the amount of panels needed to heat water, it is cost-effective to install a grid-tied solution for your entire home. And it is even more cost effective to install evacuated tubes.

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Hi 

There are many other insights if you read other posts n this section.

To add to my suggestion above I would use a dc solid state relay for temp control using the existing thermostat and a normal 30amp switch to  switch from dc to ac and only switch when there no direct light to avoid arching problems.

 

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Thanks for all the great answers. I also found that the dc geyser controllers to be close to the price of a hybrid inverter!!!

Peter topps solution of dc solid state relay (http://www.elexco.co.za/index.php?route=product/search&search=solid state relay dc) with existing thermostat and switch looks the most cost effective. FWIW i found reasonably price elements here http://www.elexco.co.za/index.php?route=product/product&path=66&product_id=909

So all in all, not horrendously expensive, I might use this as an assist on a solar evac tube geyser that struggles a bit on cloudy days

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1 minute ago, carlysun said:

So all in all, not horrendously expensive, I might use this as an assist on a solar evac tube geyser that struggles a bit on cloudy days

Evac tubes are a lot more efficient than PV panels on cloudy days (well, it's worse on cloudy days)

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Just got a quote on the geyserrobot kit so that there is another comparison point here, the full kit (excluding panels of course) is about R5500 (vat inclusive)

Kit includes:
Controller
Separate digital display
Thermocouple
Thermostat
Cable and connectors

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3 minutes ago, P1000 said:

Evac tubes are a lot more efficient than PV panels on cloudy days (well, it's worse on cloudy days)

Thanks p1000, so my setup is evac tube geyser feeds 1 bathroom directly and feeds another standard geyser. Standard geyser element is normally switched on for an hour a day.  Was looking to either replace element on standard geyser with dc/ac option or boost evac geyser in cloudy weather.

Option 1

In bad weather, everyone uses the standard geyser (AC heated and some pv during the day)...in good weather we use the evac geyser and the standard geyser would be boosted by pv so no need for ac.

Option 2

standard geyser now obsolete and disconnected..all hot water from AC/pv boosted evac geyser 

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11 minutes ago, Richard Mackay said:

How is the comparison between solar evac tubes and PV made?

In terms of energy output vs insolation? Neither of them are really useful under solid cloud cover, but evac tubes have a slight upper hand here. I would not rely on either for cloudy days.

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I'm not sure I agree with the sentiment that evacuated tubes are better/cheaper. Using geyserrobot as an example here but any other similar product the same logic applies.

At R5500 for a geyserrobot plus say R8000 worth of panels it seems you could probably have a reasonable install for between R15000-R20000

A decent evacuated tube retrofit (or new install) costs easily in the same region, with various downsides:
1) Much easier to have a bad install that then has endless problems
2) Additional moving parts that can break
3) Shorter lifespan/guarantee
4) Potential health risks if not installed/operated properly

This assumes installing PV just for the geyserrobot, if you have existing PV with additional capacity you are looking to use up then the price for the geyserrobot install is even less.
Additionally the PV route can serve as the basis for further upgrades to a larger PV system, potentially provides you some excess PV at times that could be theoretically harnessed and so on.

PV panel prices are still on the decline while evacuated tube prices are stationary, so the PV option is going to become more and more competitive.

I do think all of these sorts of products are still a bit 'dissapointing'/'lacking' in terms of the options they offer for operating with an existing array, and that this stunts their competitiveness a bit - but on paper at least they do already seem competitive right now.

Edited by mmacleod
Add some additional thoughts

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3 minutes ago, mmacleod said:

I'm not sure I agree with the sentiment that evacuated tubes are better. Using geyserrobot as an example here but any other similar product the same logic applies.

...
 

I agree evacuated tubes have their downsides. I am just saying that PV panels are not a solution for rainy days.

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3 hours ago, WeNotGood said:

Agree with Peter that a "DC" element should not be needed if enough panels are added in series.

It's not quite that simple. It needs to be impedance matched, so if a 3000W resistive element is meant to run at 230VAC and 13 ampere, then you need to size your PV array so that you can get 13 amps out of it, and put enough of them in series to get the voltage up high enough.

Then what will happen in practice, if you use the usual 320W (ish) module, is it will max out at a more common 9 ampere. 9 Ampere across the 230/13 = 18Ω means the voltage is drawn down to 170V or so, and overall power is down to maybe 1.5kw. Adding more panels (above 200V or so) would be pointless.

Matching panels would require matching those amps. Hence the MPPT used by many such systems.

3 hours ago, P1000 said:

From what I gather, elements sold as DC elements have different resistances for DC and AC, allowing the same element to be used with 220VAC or 72VDC.

That's the trick with the DC (PTC) elements. Apparently they are better matched for PV. Or so I heard.

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2 hours ago, mmacleod said:


This assumes installing PV just for the geyserrobot, if you have existing PV with additional capacity you are looking to use up then the price for the geyserrobot install is even less.
Additionally the PV route can serve as the basis for further upgrades to a larger PV system, potentially provides you some excess PV at times that could be theoretically harnessed and so on.

PV panel prices are still on the decline while evacuated tube prices are stationary, so the PV option is going to become more and more competitive.

I do think all of these sorts of products are still a bit 'dissapointing'/'lacking' in terms of the options they offer for operating with an existing array, and that this stunts their competitiveness a bit - but on paper at least they do already seem competitive right now.

Does the geyserrobot need to have panels dedicated to it or can the panels stay connected to your inverter/charger?

How does grid tied or hybrid compare? In that case I would assume you don't even need the geyserrobot?

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10 minutes ago, gooseberry said:

How does grid tied or hybrid compare? In that case I would assume you don't even need the geyserrobot?

On the surface, for a large grid tie/hybrid system there is as you say not that much need for something like a geyserrobot. Although it could potentially still make sense, I'll come back to this just now.

Such devices I think have 3 potential use cases:
1) As an initial step into PV for those who want to go full PV but can't afford the entire system in one go.
Start with some panels on the roof and a geyser robot, maybe have the same panels also power your lighting or something small. Its cheap because no batteries/inverter etc. to worry about, and ROI should be reasonably fast. You can then upgrade to grid tie or whatever at a later point.

2) For people (like myself) with smaller off grid systems designed to meet e.g. 20% or 30% of their requirements.
i.e. only powering a few devices and not grid tie/hybrid (which also happens to be the type of install with the shortest ROI)
For such an install on many days your batteries end up full and then any excess generation is 'wasted', for such a system something like the geyserrobot is a tempting option where potentially I could probably generate my hot water for free 70% (thumb suck) of the time using excess capacity that otherwise goes to waste.

3) Large grid tie. On the surface this sounds silly, why would you want this for grid tie when you can just have the geyser run from the inverter?
Well a lot (or at least some) people deliberately 'oversize' their arrays such that even in relatively overcast weather the array can still produce enough to max out the inverter.
What this means is that a lot of the time they also have excess capacity that their array could be producing that is instead going to waste - in such a scenario by introducing such a device they could in theory also 'gain back' some of this excess capacity for hot water heating, and export extra from the inverter instead of using the inverter to do the job.

So can it make sense even for grid tie/hybrid, I guess, maybe, it depends...
 

10 minutes ago, gooseberry said:

Does the geyserrobot need to have panels dedicated to it or can the panels stay connected to your inverter/charger?

And this is what I was basically alluding too when I said "I do think all of these sorts of products are still a bit 'dissapointing'/'lacking' in terms of the options they offer for operating with an existing array" - In theory there is no reasons devices like this should have to have the panels dedicated to it.
It should be possible for them to share, either via a smart controller/relay that decides whether to divert to the 'geyserrobot' or to the batteries, or simply by sharing the panels with something else.
None of the devices that I've looked into so far seem to have a reasonable out of the box way to support this though, and this is the main reason I haven't yet bought one. Though I've not looked closely.
 

 

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This is my experience with evac tubes..

I do not have the most efficient tubes. I know there are better ones out there. In winter and cloudy days they are useless. My wife and  I also like to shower in the morning and by this time the water in the geyser has cooled to below 30 degrees and I need to use the gas geyser to heat the water. If I want to get the best out of the tubes I  will have to change my habits which I am reluctant to do. I suppose the most efficient way all the time is to have a geyser that is super isolated or use gas. I use the gas geyser as this only heats the water when I need it. Gas per watt is more expensive than electricity however their are no losses as with a electric  geyser that continually needs to be heated up as the water cools. Lucky for me I have combination of all 4  choices (mains, evac tubes, gas and inverter with batteries and pv panels) each  which can be activated at any time except the evac tubes which are permanently in use.

Edited by Peter Topp

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49 minutes ago, Peter Topp said:

Gas per watt is more expensive than electricity however their are no losses as with a electric  geyser that continually needs to be heated up as the water cools.

I suppose it depends on where you live. In Cape Town gas and electricity has a very similar cost per kWh, but the electric heater is more efficient at getting the heat into the water. On the other side of the equation, the electric heater has that standing loss that comes from keeping the water heated all the time, which the gas heater (or any other point of use heater) doesn't.

I have recently changed my mind a bit about heating water with gas. Because of that 2kWh standing loss that is recovered, there will be a net gain in efficiency. This will hold for some minimum amount of water, but for larger volumes the electric heater is going to win. I have yet to come up with some kind of calculation.

Just off the top of my head, it takes 1.16Wh to heat one liter of water by 1°C, so assuming a heating delta of 40°C, that 2kWh translates to 43 liters of hot water. So if the heating efficiency was the same (which it isn't), you'd get 43 liters of extra hot water for the same money (assuming similar electricity cost).

So while I have changed my mind about gas... I still don't really think there is a significant saving.

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Gas geysers have become more efficient lately as they now has forced fan and smart gas control so that you can set then on a specific temperature. I suppose they will be still more expensive per watt for heating however and they are great for load shedding. I think this is that is what people on this forum are most interested in and looking for an alternative to mains power because of load shedding and power outages which seems to be our lot in South Africa and will continue for some time to come.

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10 minutes ago, Peter Topp said:

they are great for load shedding

Absolutely. I moved house. The new house has gas geysers. It is lovely.

Well, there is one stupid thing it does. A few minutes into the shower it seems the water goes cold. Then you need to swivel the mixer full-on to the hot side and wait again for hot water to arrive. So while I don't have temperature problems, I have another stupid flow issue, likely related to the fact that I prefer to shower in a drizzle and not in a downpour 🙂 But that's a tad off-topic I guess.

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This is typical when a gas geyser does not get enough water flow and it will switch off. This can be due to low mains pressure or someone else using the cold water at the same time. I have a booster pump to help with this as we had a problem with municipal supply. 

So if anyone wants to install a gas geyser it is advisible to check your water flow before considering to install one.

 

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