Hi .

Just wondering if any one is using pv   panels connect directly to the electric geyser .

I read at sometime some were that they use 2 250w 40v panels connect in series and work great at getting the geyser up to temp through the day .

We just 2 in the house and using gas water  warmer and run the house from an 3 kw inverter with 2050w panels and 100ah lithium and don't need anything bigger but would be an  saving if we can use the sun to heat the electric geyser on sunny days and use  gas warmer  an rainy days.  

Edited March 26, 20231 yr by GMAC

1 hour ago, GMAC said:

Hi .

Just wondering if any one is using pv   panels connect directly to the electric geyser .

I read at sometime some were that they use 2 250w 40v panels connect in series and work great at getting the geyser up to temp through the day .

We just 2 in the house and using gas water  warmer and run the house from an 3 kw inverter with 2050w panels and 100ah lithium and don't need anything bigger but would be an  saving if we can use the sun to heat the electric geyser on sunny days and use  gas warmer  an rainy days.  

These 2 x 250W panels will on average only provide 2500Wh of power to the geyser to heat the water. A 150L geyser needs 7000Wh to heat from 20 to 60 degrees C. If no hot water is used in 24hrs you need about 2000Wh just to replace the normal heat loss. This leaves only 500Wh to actually increase the water temperature. The above based on the average geyser say 4 yrs ago. Currently there are geysers with less heat loss per day. 

16 hours ago, GMAC said:

Hi .

Just wondering if any one is using pv   panels connect directly to the electric geyser .

I read at sometime some were that they use 2 250w 40v panels connect in series and work great at getting the geyser up to temp through the day .

We just 2 in the house and using gas water  warmer and run the house from an 3 kw inverter with 2050w panels and 100ah lithium and don't need anything bigger but would be an  saving if we can use the sun to heat the electric geyser on sunny days and use  gas warmer  an rainy days.  

Probably not a good idea to connect them directly. If the place burns down insurance will not pay out is one reason not to do it. The power from solar panels are DC and geyser elements are AC. Not a good idea to mix and match them. There are also power factor considerations, etc. So yeah, don't do it. There are specialized kits available like the Geyserwise kit that uses PV to charge your geyser.

Okay thanks will look into it further but not much info on Google about this topic 

1 hour ago, GMAC said:

Okay thanks will look into it further but not much info on Google about this topic 

As suggested look at the Geyser wise kit. 

I still prefer to have the geyser heating via an inverter with low wattage element if one goes to a larger system. 

1 hour ago, Scorp007 said:

As suggested look at the Geyser wise kit. 

I still prefer to have the geyser heating via an inverter with low wattage element if one goes to a larger system. 

thats always going to be first prize !

Just now, BritishRacingGreen said:

thats always going to be first prize !

My 1st prize will remain a good heat pump. 

8 minutes ago, Scorp007 said:

My 1st prize will remain a good heat pump. 

$$$ and maintenance. 

3 minutes ago, BritishRacingGreen said:

$$$ and maintenance. 

In nearly 10yrs only had to change the pump(R1200) . Installer never put in a strainer. 

Never had it maintained as I saw what they do annually. I blow out the unit with air hose. Never seen any debris in my unit. 

I have heard other people also say not to use DC on a geyser element.  Frankly, I am too stupid to understand why.  The geyser element is resistive and should not care about AC or DC.  It is also fully isolated, so it can't be a matter of some galvanic electrode corrosion.

Come to think about it, the reason might be the thermostat which effectively is an electro-mechanical switch.  DC will cause far worse arcing and the contacts will burn.  This simply means one would need to control the geyser with a MOSFET or SCR.  Or fit one of those self-regulating PTC elements and leave the solar panels connected indefinitely.

So warn people not to use DC on a standard geyser fitted with conventional thermostat.  The geyser element itself should be perfectly fine.

One more thing, if you are going to use DC on an AC element, the DC equivalent voltage is 163V. 

DC voltage (RMS voltage) = AC Voltage / sqrt(2)

For the thermostat you could use a relay to help with the DC arc situation. Use AC to drive the thermostat that powers the relay coil, then the relay does the DC switching. You must make sure you get a decent DC rated relay, otherwise it will be a serious fire hazard.

 

But again, I doubt your house will pass a COC inspection with this stuff in place. If the place burns down, insurance is not going to pay out.

Edited March 28, 20231 yr by HendrikBigChief

@HendrikBigChief  No, you have the AC and DC swopped.  Your equation should be:

Vrms = Vp / sqrt(2)       (where Vp = the peak AC voltage)

we measure and quote an AC voltage by it's true RMS (root means square) value.  A 230V dc voltage will thus produce the same power in a resistive load than 230VAC.

The above is the reason why, if you rectify 230VAC you will get 325V DC.  The smoothing capacitor will charge to the peak of the rectified AC waveform.

If someone wants to connect solar panels directly to a geyser element he cannot, or should not, aim for 230VDC.  The applied DC voltage should be a fair bit lower.  Most likely in the 140V range, but this needs to be calculated based on the geyser element power rating and the total solar panel voltage and wattage.
 

7 minutes ago, Modina said:

@HendrikBigChief  No, you have the AC and DC swopped.  Your equation should be:

Vrms = Vp / sqrt(2)       (where Vp = the peak AC voltage)

we measure and quote an AC voltage by it's true RMS (root means square) value.  A 230V dc voltage will thus produce the same power in a resistive load than 230VAC.

The above is the reason why, if you rectify 230VAC you will get 325V DC.  The smoothing capacitor will charge to the peak of the rectified AC waveform.

If someone wants to connect solar panels directly to a geyser element he cannot, or should not, aim for 230VDC.  The applied DC voltage should be a fair bit lower.  Most likely in the 140V range, but this needs to be calculated based on the geyser element power rating and the total solar panel voltage and wattage.
 

Thanks, my bad

11 hours ago, Scorp007 said:

My 1st prize will remain a good heat pump. 

Well, with an existing inverter and solar system, you could either add a heat pump for around R25k or a couple of extra panels that drives the standard geyser element, also costing around R25k. Which is better now?

3 minutes ago, HendrikBigChief said:

Well, with an existing inverter and solar system, you could either add a heat pump for around R25k or a couple of extra panels that drives the standard geyser element, also costing around R25k. Which is better now?

A heat pump is better, as you can run it off a battery more easily. Or put another way: if you calculate the cost difference with the requirement to run off battery, the heat pump is much cheaper.

3 minutes ago, P1000 said:

A heat pump is better, as you can run it off a battery more easily. Or put another way: if you calculate the cost difference with the requirement to run off battery, the heat pump is much cheaper.

I have a geyser timer that switches the geyser on midday so it has the maximum chance to use solar instead of Eskom and battery, but on cloudy days it might chow your battery a bit more if you don't have a heat pump and batteries are very expensive. Good point.

7 minutes ago, HendrikBigChief said:

I have a geyser timer that switches the geyser on midday so it has the maximum chance to use solar instead of Eskom and battery, but on cloudy days it might chow your battery a bit more if you don't have a heat pump and batteries are very expensive. Good point.

If in need I can run my heat pump from a 24V system. During peak PV periods panels can help to have a lower draw from battery while my PV is below the load of the heat pump. 

As said the lower load of a heat pump and heating water in the same time as an element is a major plus. 

Well, there are DC geyser elements as well…

Edited March 28, 20231 yr by PowerUser

2 hours ago, P1000 said:

A heat pump is better, as you can run it off a battery more easily. Or put another way: if you calculate the cost difference with the requirement to run off battery, the heat pump is much cheaper.

Yes. I run my heat pump on the backed up side of the DB. Most days it is effectively powered by the sun, though we have double the battery that @GMAC has (I'm assuming he has 50ish V battery). We run it twice a day, first run is 6:00 (this gets pushed to 5:00 when I'm going to the office) and then again at 12:00. With good insulation on the geyser and hot water pipes we still have hot water at 20:00. On overcast days it still runs from the battery, and I have to recharge the battery, but I'd have to do that anyway.

Having lived with the heatpump for a decade, I like it (and I did before we got PV). Cheaper to run than an element, not so bothered by overcast days as a solar geyser, you can run it outside of sunlit hours, and if you have PV it doesn't load the battery too heavily.

@Bobster. & @Scorp007 I never investigated the tech and prices of aircons or heat pumps so I am not up-to-date with what is available on the market these days.  I remember many years ago, attending some home expo, where someone demonstrated the latest model aircon and the the selling point was that that particular model had plumbing to connect to the geyser.  So in summer it would cool the house and at the same time heat your water.  I presume one could take this one step further and design a heat pump that had enough capacity to do room heating AND water heating in winter.  I am sure this tech is available in cold countries.  Have these sort of systems found there way into SA?

1 hour ago, Modina said:

@Bobster. & @Scorp007 I never investigated the tech and prices of aircons or heat pumps so I am not up-to-date with what is available on the market these days.  I remember many years ago, attending some home expo, where someone demonstrated the latest model aircon and the the selling point was that that particular model had plumbing to connect to the geyser.  So in summer it would cool the house and at the same time heat your water.  I presume one could take this one step further and design a heat pump that had enough capacity to do room heating AND water heating in winter.  I am sure this tech is available in cold countries.  Have these sort of systems found there way into SA?

Heat pumps are used in the UK to heat water that runs through radiators in the house, but not that common. They have cheap north sea gas, so they mostly heat their houses with gas boilers that heat water that is circulated through their houses. The issue is the outside ambient temperatures are much lower than here, so heat pumps are not that efficient over there and they have relatively expensive electricity.

Edited March 28, 20231 yr by HendrikBigChief

5 hours ago, PowerUser said:

Well, there are DC geyser elements as well…

One issue I see with those is the high current they need, so you will need some beefy cables to run up to the geyser.

1 hour ago, HendrikBigChief said:

One issue I see with those is the high current they need, so you will need some beefy cables to run up to the geyser.

I don't think that is valid.  I would expect any element marked as DC to be intended for low(ish) power over an extended time.  The idea being that you do not heat your water as quickly as possible, but rather over a much longer duration during sunshine hours.  I would expect the dual AC/DC elements that Geyserwise sell to fit that bill, but I might be wrong.

This would mean that for large families where everyone want's to bath/shower within a short time-window, DC heating would not really be suitable.  However, with Geyserwise having a second 230VAC element that would/should not apply.  There is really not enough published information on these systems, so one can only speculate.   It seems that companies only market to the normies that have absolutely no clue about even the basic concepts of science.  -  Trust us, it works.  You don't need to know how.

Tell me the GW dual element consists of a 17.5ohm AC and a 35ohm DC element and it's max permitted power dissipation is x and then I could tell you exactly what is what.

I could further speculate that the GW dual element might have two IDENTICAL heaters.  Any which one could be used for 230VAC and the second intended for much lower DC voltages?   Well, why would you use two heaters then, could you not switch between AC and DC with a relay?  Yes.  And No.  A dual element would allow the solar panels to push whatever power they can into the DC heater.  The AC heater would only be use for boosting.  With only one element, its either or.

On 2023/03/27 at 1:00 PM, HendrikBigChief said:

Probably not a good idea to connect them directly. If the place burns down insurance will not pay out is one reason not to do it. The power from solar panels are DC and geyser elements are AC. Not a good idea to mix and match them. There are also power factor considerations, etc. So yeah, don't do it. There are specialized kits available like the Geyserwise kit that uses PV to charge your geyser.

Please indicate what are the power factor problems feeding DC in a pure resistive element? 

I think that heating the geyser must run on the kiss principle.

Mine has a Geyserwise installed to control it - this makes it remarkedly easy to use (especially for our companions 😀)  

It is connected to the 8kw Sunsynk db and runs max 3 hours per day. On a 2000w element (cost me R175) it increases the temperature by 14 deg C per hour. It is set to go to 65 deg C. 
BUT it is not used for a bathroom - I have a gasgeyser for that. I would love to preheat the water for this but the cost for a new geyser is exorbitant. 
And it shares the power output with all the other electricity guzzlers.