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6kWh of cheap storage - why does no one use it?


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I see there are some basic discussions around the forum, and it seems quite popular in Australia (Google 'PV diverters')- but I see very little on the local market.

Basically, I have a 150L geyser, currently set to 50°C.  If I raise the temperature to 85C, then that is an additional 6.1kWh of energy 'stored' in the geyser.  Of course it is only available as hot water, and is quite a leaky bucket - but it would be available for evening dishes and ablutions, and would certainly reduce the requirements for reheating the next morning.

From what I see on the forum, most solar installations have plenty of headroom (in terms of kWh and even kW) to heat even a 3kW element at most times during the day.

What is required is:

1) a tempering valve (cost ~R900 + installation) to reduce the geyser outlet temperature to a safe level, and

2) a smart thermostat capable of heating the geyser only when there is excess production and battery storage is full.

From what I can find (mostly in Oz), the smart thermostats cost around AUD 1000, but (at least in my case), it should be possible to build something for around R300.

Certainly a lot cheaper than 6.1kWh of batteries.

Am I missing something here? Why do I see so little on this option locally?

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Loadshedding.

Who would want to deplete their batteries when Eskom may start shedding at short notice?

OK... some may have a way of calculating and evaluating that risk, but I like to have full batteries at about 16:30 (they usually are full much later) and then live a low consumption life until the sun comes up the next morning. That way I am insulated against load shedding.

In Australia they seek to pay as little as possible for grid usage. Here we seek to mitigate against a loss of grid power for several hours.

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You do get this locally:

The device intelligence comes in when DC (solar) power becomes insufficient and the unit switches over to AC (grid) power ensuring you always have hot water. It’s a peak saving device for your hot water tank and effectively turns it into a solar battery where the energy generated is stored for later use as hot water.

What this means in simple terms is that DC power from solar panels can be used to directly power the element that you find in your existing hot water tank, effectively converting your 50, 100, or 200 litre hot water tank into a hybrid green heater.

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@JustinSchoeman - I have done exactly this but never calculated the "storage" in terms of kW... I have changed to a 2kW element and heat my geyser to 70 degrees in the afternoon with excess solar.. I dont need to re-heat in the mornings at all unless it is a very cold winters night.  Works like a bomb and far more cost effective (for me) than a solar geyser

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15 minutes ago, Chloe said:

You do get this locally:

The device intelligence comes in when DC (solar) power becomes insufficient and the unit switches over to AC (grid) power ensuring you always have hot water. It’s a peak saving device for your hot water tank and effectively turns it into a solar battery where the energy generated is stored for later use as hot water.

What this means in simple terms is that DC power from solar panels can be used to directly power the element that you find in your existing hot water tank, effectively converting your 50, 100, or 200 litre hot water tank into a hybrid green heater.

Yep - I have seen the Geyswerwise and a few clones, which use a separate DC supply.

I was more thinking about the available headroom on existing solar systems.  I have seen many with substantial over-production, and some for pure sell/buy back schemes.  For most hours of the day, even a 5kW inverter has enough headroom to run a 3kW element, if there is some excess solar generation. So no need for separate DC systems or expensive AC/DC elements - just a relay to switch the geyser over to the inverter (when excess energy is available), and possibly a dimmer to regulate how much energy goes to the geyser.

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I have done the same, switched from  a 3 kW to a 2 kW element, this way I never draw from the grid just use solar pv, but instead of an automated diverter, I keep an eye on the Victron portal, which allows me to use a Sonoff switch controlled from my phone to switch the geyser on/off to try and keep the keep the geyser at its hottest, based on excess pv

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5 minutes ago, JustinSchoeman said:

So no need for separate DC systems or expensive AC/DC elements - just a relay to switch the geyser over to the inverter (when excess energy is available), and possibly a dimmer to regulate how much energy goes to the geyser.

Quite right.  I have a microprocessor that controls my energy system and it also controls both my 3kW geysers with triacs, meaning that I can send any excess energy to the geysers.  The beauty of using analogue control is that you don't have to wait for 3kW to be available - you can balance the system perfectly.

A further advantage is that you now really don't need extra inverter capacity for your geysers - the system monitors for overload conditions and switches off/reduces the geyser power instantly if too much other loads come on line.

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21 minutes ago, JustinSchoeman said:

I was more thinking about the available headroom on existing solar systems.  I have seen many with substantial over-production, and some for pure sell/buy back schemes.  For most hours of the day, even a 5kW inverter has enough headroom to run a 3kW element, if there is some excess solar generation. So no need for separate DC systems or expensive AC/DC elements - just a relay to switch the geyser over to the inverter (when excess energy is available), and possibly a dimmer to regulate how much energy goes to the geyser.

Indeed! But as I have tried to get my head around this challenge it's a not so easy.. 

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I’ve also been thinking of doing this but looking at going 1 step further. I currently just heat my geysers to max when I have excess power available and am going to also connect a 2kw element into my solar geyser. 
 

But what I would really like to do is get some sort of water to air heat exchanger so that I could use hot water for space heating in winter. Around 25-30 percent of my winter bill comes from space heating so would be great to be able to store energy in water for use at night. 
 

It’s used a lot in Europe and US probably because winter are a lot colder and last longer than ours. 

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40 minutes ago, Tariq said:

@Calvin, I am a not computer expert, can you describe your system in a bit more detail, seems exactly what I would want to do.

It is quite simple in principle, but as is often said the devil is in the detail.

Essentially the microcontroller measures the available incoming PV, the outgoing AC (to loads) and DC (to the battery) and estimates the system losses in order to calculate what spare power is available.  It then adjusts the power to the geysers every second to use exactly this excess power.

The only problem with it is that it is complicated (it does many other things) and home-grown - I used to do this sort of thing professionally and now that I am retired it is my hobby.  Unless you are a competent programmer (in C/C++) and handy with electronics you would probably need to wait until somebody commercialises something like this.

Below is the diagram of the entire system.  The microcontroller is the blue box labelled "Arduino Due".  The DS18B20s are temperature sensors.  The triacs (and supporting circuits, not shown) could be replaced by suitable solid state relays.

SolarSystem.png

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So it seems this is more common than I thought - but not really commercially available locally - all seem to be DIY solutions or imported.

It seems quite strange, as this is by far the cheapest way to store solar energy...

I will add the code and designs to my github page as I go, it anybody else wants to tinker with it.

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9 hours ago, JustinSchoeman said:

So it seems this is more common than I thought - but not really commercially available locally - all seem to be DIY solutions or imported.

It seems quite strange, as this is by far the cheapest way to store solar energy...

I will add the code and designs to my github page as I go, it anybody else wants to tinker with it.

Don't you have a Deye or a Sunsynk inverter? As far as I know that inverter has a generator input output port that you can theoretically connect a geyser element to and divert excess power to the element during peak times would that not give you the same result?

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1 hour ago, Anonymous said:

Don't you have a Deye or a Sunsynk inverter? As far as I know that inverter has a generator input output port that you can theoretically connect a geyser element to and divert excess power to the element during peak times would that not give you the same result?

On the most recent firmware it could work (added the always on on grid option), but you still need some sort smart thermostat to handle the boost temperature.

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To me the simplest solution seems to be 2 geysers. The primary being set to 45 degrees with element connected to the grid. The secondary feeds the primary, its configured to used excess solar power. (easy enough to setup with automation) and it is setup to go as high as the tank can take. 

This way you never need to worry about an automation failing of having cold water. Spare geysers are cheap. 

 

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3 hours ago, calypso said:

To me the simplest solution seems to be 2 geysers. The primary being set to 45 degrees with element connected to the grid. The secondary feeds the primary, its configured to used excess solar power. (easy enough to setup with automation) and it is setup to go as high as the tank can take. 

You need to describe in detail how you use the excess solar power that's available.. (In my mind this isn't as easy as you make out)

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29 minutes ago, Richard Mackay said:

You need to describe in detail how you use the excess solar power that's available.. (In my mind this isn't as easy as you make out)

I suppose that depends on your system... If you have a Sunsynk/Deye inverter, just connect the secondary geyser to the Smart Load output and configure the trigger parameters.

For a more universal solution you can use a PV diverter to power the secondary geyser from the grid side of a hybrid inverter.

Otherwise you need some sort of an automation setup to do it.

I like the two geyser setup idea, as it increases the energy storage capacity significantly. (Although for my case I will take the cheapie option and build a smart thermostat.)

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59 minutes ago, JustinSchoeman said:

I suppose that depends on your system...

Indeed! I'm amazed at how inverters are so different. 

One needs to understand the capabilities of each one to:  A: know what it's capable of and B: know how to configure that device to get what you require from it..

(There's got to be a business opportunity here...)  

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1 hour ago, Richard Mackay said:

You need to describe in detail how you use the excess solar power that's available.. (In my mind this isn't as easy as you make out)

If you are using an Axpert, Goodwe or Victron then you could go the automation route.  By setting up a Home Assistant server you could integrate the data from your inverter. 

By using the solar production and Battery SoC data you could set a smart Thermostat to heat the geyser to 55 irrespective of the battery SoC and Solar load, then once it reaches the set temperature you create a second automation to further heat the Geyser to a higher temperature but only if the Battery SoC is 100% and the available solar exceeds the element's load

 

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I got a geyserwise thermostat this afternoon for R250 from the local plumbing shop. Removing the existing sensor is pretty easy but the sonoff th16 sensor is too fat to fit into the geyserwise pocket. Original geyserwise is 5mm diameter while sonoff has a 6mm thick sensor. 
 

Also got a new 2kw element for a 200l solar geyser.
 

Will probably cut the sonoff cable, feed it through the geyserwise pocket and then rejoin. 

71788064-2A76-47E7-80B3-D80F79E64C5F.jpeg

162364A9-EE52-420D-98E6-E9D05E48BE29.jpeg

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16 hours ago, Richard Mackay said:

You need to describe in detail how you use the excess solar power that's available.. (In my mind this isn't as easy as you make out)

I speak all in theory as while I have a fully automated house, i don't have any solar or batteries (I just dont want to spend the money if I don't have to). But for the most part you can compare load/solar/grid to see where the power is coming from and then adjust the load to get the most it can out of the panels. 

 

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19 hours ago, Richard Mackay said:

In my mind this isn't as easy as you make out

3 hours ago, calypso said:

I speak all in theory as while I have a fully automated house, i don't have any solar or batteries (I just dont want to spend the money if I don't have to). But for the most part you can compare load/solar/grid to see where the power is coming from and then adjust the load to get the most it can out of the panels. 

 

I have done this, and it is indeed not trivial if you want to do it well.

The biggest challenge turned out to be finding how much excess PV is in fact available.  When the battery is charging at the maximum allowed current (or is full) the SCC will clip it's output to what the system can use.

First I tried looking at the MPPT voltages - the logic being that if they are higher than the maximum power point, the SCC has pushed them up to reduce power, so there must be spare PV out there.  This worked OK but not fantastically - perhaps complicated by the fact that I have 3 inverters in parallel but with each SCC doing it's own thing.

Next I tried the approach of slowly increasing power to the geysers until the batteries started discharging.  This worked well when loads and sunshine were consistent, but otherwise badly.

Finally I started measuring the solar irradiance directly (using a separate small panel). Even this is no exact answer, as my Axpert's SCC reacts much more slowly than my home-made pyranometer to intermittent sun.  Perhaps other manufacturers' SCCs are better.

My eventual solution uses a combination of all 3 methods.  It works fine, but it is not simple.

In my case I can control the power to the geysers exactly.  If you have simple on/off control you face the additional decision: Do you run the geyser when there is say 1kW of extra PV available, taking the rest from battery / grid?  Given that neither is a cost-free option, not a trivial decision.

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