1 hour ago, phil.g00 said:

don't doubt, the point I am trying to make is, is this a storage volume issue?

If he had 200L of hot water stored by the evening, instead of 150L, would that resolve things in a more efficient manner?

I also have no doubt that increasing my storage volume to 200L would definitely help since I would be storing more thermal energy from my flat plate collector.

So I would guess instead of having a geyser temp of 30-34C in the morning I would probably have 40C? (Pure speculation I would need to log some temp data to confirm)

Thus I only need to raise the temp by 10C vs 20C

But now I would need:

- 4180 Joules x 10C x 200L = 2.32 kWh 

-4180 Joules x 20C x 150L = 3.48 kWh

3.48 kWh - 2.32 kWh = 1.16 kWh saving

Or if I added that 3KW heat pump, assuming a COP of 3:

3.48kWh / 3KW = 1.16 hours x 1000W = 1.16 kWh of energy required.

3.48 kWh - 1.16 kWh = 2.32 kWh saving.

 

@PJJ,  At the risk of laboring a point:

If you want to use 200L of hot water before the sun shines again, and you want to use the sun to heat it, be able to store 200L of hot water.

18 hours ago, Jaco de Jongh said:

 

You can have a good heat-pump installed for roughly R16-R18k, then it will use 1.25kwh for every hour to add the same heat as a 3kw element will do over an hour. 

So theoretically (If you leave the 3kw element intact) and you chose to add panels, you should add at least kwp 3.7kwp (Losses included) to do the same. Now to add 3.75 kwp to your system will cost about R26k for 10 x 400 watt panels, plus the mounting structure plus the labor. 

Those panels can help to power other loads while the sun shines and heating up the geyser will be limited to sunlight hours, and lets not forget, your inverter should be able to handle this extra load as well. 

The heat pump on the other hand, uses almost 2/3 less energy to do the same work, whether the sun is shining or not, The COP is effected by very low ambient temperatures, but in most areas of South Africa, it remains the most energy efficient method of heating water. 

Let's do the maths on the above 3kW element example.

With the heatpump you are saving 1.75kWh per hour of operation (3kW element minus 1.25kWh consumption). With the 3.7kWp extra PV you will be generating an ave. of 16.65kWh/day (North-facing in Cape Town) less the 3kWh to heat geyser for 1 hour = 13.65kWh/day which can be used for other loads or stored in battery.

Heatpump saving: 1.75kWh/day

PV saving: 13.65kWh/day

Cost of PV is about double but life is 25 years vs 10 years for heat pump so that evens out.

Running costs will be zero with the PV with an extra 13.65kWh in the bank daily vs 1.25kWh x tariff x 25 years (+ 1x replacement of heatpump) for the heat pump.

I know this is over simplified as you won't always have sufficient sun + you may well need to use Eskom/battery to get morning showers hot + you might already have excess PV you could use to power heatpump.

If you don't shower often in the evenings the PV will be more cost-effective over life of system. If you empty geyser every night it becomes more even.

The EV tubes will use no power whatsoever in summer (provided you don't empty geyser completely at night) and reduce consumption in winter and grey days if over specced - but once the water is hot they stand idle, hence my preference of the PV solution.

But as I stated when I started out - it depends. It depends on when you shower, how much you shower, available roof, available cash upfront, priority of low running costs over cost of system etc.

16 minutes ago, PeterP said:

Heatpump saving: 1.75kWh/day

Based on an assumed 1 hour run-time. We dont know what the OP's precise usage are, we also dont know how well the Geyser is isolated and what the standing losses are, so its not accurate to allow for only one hour running time. 

19 minutes ago, PeterP said:

PV saving: 13.65kWh/day

This again is based on an assumption that the OP has enough loads to use this extra production. If his system was designed for his current loads, these extra panels will just lie idle till its needed the next day. 

I fully agree with your theory, if the household can limit the usage to fit in with the sunlight hours, but the OP has already stated in his first post that he needs the hot water for evening and morning showers. (No amount of extra panels will help with heating the water during night time) 

28 minutes ago, PeterP said:

it depends

On so many factors, that not one answer alone is correct, not mine, not yours............. it depends........... 

Whilst researching for something else I came across this product; (The name caught my eye...like all hotties do.🙂)

www.solarhotty.com.

Claims to use PV to use existing geyser (upto 4kw) without modifications (dc to the element?).

Would this perhaps be a viable (cost effective) solution for water heating with PV? (They also claim use for cooking?)

The only people I could find selling it is sustainable.co.za at R4192 on "special".

I did post about this on the "Product Opinions & reviews" forum but on reading this thread I thought it might be pertinent. Please excuse me if it isn't!

Rgards to all.

 

Good evening guys. In our previous home, we had a 300l Suntank geyser which was connected in series with a Paloma 26l gas geyser. The flat panels on the Suntank didn’t work well in winter as the water only heated to 40deg. The controller on the Paloma was set to 46deg so the Paloma only needed to heat the water by 6deg which in turn uses far less gas. 
 

When the water in the solar Geyser was at 65deg, I switched the gas geyser off or else it fired up for a short while to heat the cold water in the pipe between the solar Geyser and the gas geyser which in turn wastes gas. 
 

I also had a Goodwe 4.6 ES system and when I had spare power during the day, I used that power to run the 3kw element in the solar Geyser just so that it didn’t go to waste.

We managed this for 2 years and I only used max 20kwh from Eskom per month and a 48kg cylinder lasted 13 months. The entire home ran on the Goodwe inverter but we were limited to 5kw at any given time on the backup side. 

On 2019/10/14 at 8:26 PM, PJJ said:

I also have no doubt that increasing my storage volume to 200L would definitely help since I would be storing more thermal energy from my flat plate collector.

So I would guess instead of having a geyser temp of 30-34C in the morning I would probably have 40C? (Pure speculation I would need to log some temp data to confirm)

I have a 200ltr ev tube geyser and see average temperatures at 16h00 of 60 to 84 degrees. It is more than enough for 4 people. But I do switch it on during the night if someone over-showered especially my 10year old who have to be reminded to get out of the shower.

My problem is if no one uses hot water for the evening and it sits above 60’s the next day it gets to upper 80’s than it blows boiling water down my roof. I am now considering fitting some socks partially over the tubes to prevent the wasted wster and energy, But I think the money this geyser saves me per month definitely makes it worth it.

I have a 150 ltr EV tube geyser for 3 people. Getting back from holiday in December, it sits above 90degrees 😳. The pressure reducing valve only drips a little bit to keep the pressure build-up down. I am also thinking of covering it this December.

This was now the first winter with the house on PV. I changed the EV tube geyser element to a 2kw one and set the Geyserwise to heat it up to 60 degrees midday with excess PV. Also the geyserwise was set to heat in the mornings for a maximum duration of 30minutes which gave me about 55 degrees. A normal geyser on PV could work if you have extra PV, but the EV one for me works great. EV works great in the summer, but needs some "assistance" in the winter.