Dump loads with MPP Solar

moosemaster

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Feb 24, 2021
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Hello,

I've been wrapping my head around MPP Solar gear and found the community here seem to have a solid grasp on the subject.

I am considering using 2x MPP Solar 8000W units in parallel, to manage a 16kW array. Providing I don't get a knock-off, these inverters tend to get pretty good reviews. Running without batteries is an impressive feature...

We're at a 58° northerly latitude - 16kW will see us through the short winter days for our basic needs - but kicks out a huge amount in summer.

The plan is to dump summer excess into heating elements - a large hot water thermal store - and a Rayburn range I'm converting; it has a bank of heat storage bricks that can hit 700°C.

I had been looking at keeping everything DC, but using AC from the inverter makes a few things simpler - in terms of selecting standard elements and off the shelf thermostats.

My head gets fuzzy when trying to figure out what size elements to install - and how I might program an inverter to dump to these when the batteries are charged.

For instance, if I attempt to power 16kW of elements (3.3Ω at 230v) - the theoretical maximum of the system - but the PV array is only kicking out 8kW... what happens? I imagine this all depends on the specific inverter. From what I see if the load is too big for the inverter - the inverter shuts down.

These are very cool and might be good to throw into the mix - MPPT controllers for connecting PV direct to heating elements: https://www.polskieprzetwornice.pl/skle ... -3500w-pro

Wondering how best to achieve this goal and perhaps overwhelmed by the number of options I could take...

Any thoughts appreciated!
 

ajw22

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Nov 16, 2018
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My system is a bit complicated, but really good IMHO:

On my grid tied system, I use a Raspberry Pi to log the battery charge status and grid feedback (ie. excess PV production). I have a script that monitor the data and decide when to switch On/Off various Wifi Smartplugs (eg. https://www.amazon.com/TP-LINK-HS103P2-Required-Google-Assistant/dp/B07B8W2KHZ ).

One logic looks just at excess PV power available, and turns on an additional higher priority load, such as my PHEV chargers (ea 200V8A), pond ump, etc. When not enough power is available, one device at a time is turned off.

Another logic looks at the battery charge state, charge rate, and time of day, using is all to estimate whether the battery is likely to get full by the end of the day. If so, it turns on the low priority dump loads, such as my future hot water pre-heater.

PROs:
+ Wifi Smartplugs are easy to use all over the house and garden
+ can start dumping before batteries are full
+ multiple smaller loads to optimize usage of excess energy

CONs:
- SmartPlugs can switch only ~10A, so need to use relays for bigger loads
- A lot of thought and work to configure everything just right
 

daromer

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The inverter cant do what you want. You need external logic. Element is on/off so you need to have many elements or never start it until you have full sun. Like ajw22 wrote.

Like for instance the sun outputs 5kw and you can then have 4kw of elements active. and so forth. Unless you bypass the inverter its AC that is going to be used and when having 8kw sun expect only to get 7kw out at MOST. Also note that running those inverters without battery will work like crap. They are not ment to run full load from sun to a device without battery. Sun fluctuates alot and it relies on using the battery/sun as primary to run the device to. So just a slight cloud and its dead unless you have a battery. Many people have seen this so dont expect it to work off-grid without a battery with only sun.

To me it sounds like you want grid-tie system where you use some of the power to elements? If so look at my bypass video where i forward energy when i have excess around.
Here is one of the videos:

I have later on even done even better ones just using esp8266 controlling it.

Though this require that you can connect it to grid and use it that way but to be honest if you dont plan to have a battery connected this is the only way to go if you want it to work as you think.

And if you ONLY plan to heat water the best option is solar-water heaters instead. Dont even consider going electric because its so inefficient comparing to area vs solar panels for electricity :)

Note going Solar - Electricity - heating water with elements is one of the worst ways doing and should only be done with the rest of the energy so dont design your system to have that as a main thing. If you need heat its better doing that with other sources and cheaper :)

Now im eager to hear your actual thougths. Why 16kWp? Why no Battery? Grid-Tie?

Is it you plan that you have max 16kW needed in the house? or other?
 

moosemaster

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Feb 24, 2021
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Thanks for your replies, lots to chew on!

This is for a self build - we're at a 58° northerly latitude - 16kW will see us through the short winter days for our basic needs - but kicks out a huge amount in summer.

As PV is so affordable these days the cost of covering the entire roof (approximately 100m2) with PV and GSE mountings is comparable to conventional roofing - slate or metal.

I do plan on using a battery (these look nifty - https://www.bimblesolar.com/batteries/mega-long-life-lto-lithium-batteries) I was just commenting on the fact the inverter is quite unique with the battery-less feature.

I am planning to be off-grid - the money we would get for selling electricity back would be negligible here. I'm looking at ways of storing the excessive summer PV in the form of heat - in a large thermal store for hot water - and in heat storage bricks in a Rayburn cooker.

I realise the inverter won't be able to do that by itself, I'm wondering if I can use its "Dry Contact" to operate a relay when the battery is charged - switching the PV over to these units that are designed to control the power to heating elements - https://www.polskieprzetwornice.pl/sklep/przetwornica-solarna-eco-solar-boost-mppt-3000-3500w-pro

People who have Midnite Classics play about with operating dump loads by operating solid state relays with either Aux 1 or 2, I'm wondering if the Dry Contact on an MPP Solar inverter/charger can act similarly.
 

daromer

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Just hook up a unit that monitors output properly and then engage 1 or 10 relays/ssrs for powering external heaters or what not. Get alot small ones like 1kW each so you can vary the load based on what you got

You can add a diversion unit instead that do it. Switching over the pv to other unit is generally not worth it. Better to have a pv diversion unit or build one that use the output on the AC side. its easier and cheaper.
 

OffGridInTheCity

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Its not clear to me - but it sounds like your house can consume more than you produce. I have this situation and I'm off-grid (don't sent any power back to the grid).

What I do is use the whole house as the 'dump load' instead of configuring individual dump loads. Its OK to me that solar only runs the house 6hrs/day or 12hrs/day or 20hrs/day depending on the PV production. So instead of running a portion of the house 24hrs/day + optional dump loads, the whole house is running X percent of the day that varies day by day depending on PV production (e.g. time of year, sun/cloudy, etc).

In an emergency, with the whole house is wired to run - I could choose what to run and not run to extend PV power by simply turning off breakers (such as the dryer, auxiliary refrigerator, etc) or unplugging individual appliances.

The system is implemented by turning inverters on/off by battery bank voltage (pretty simple / single point of control) which activates the ATSs between inverter and grid automatically. Things operate without any manual intervention and some days I don't even think about it. It does required a large enough battery bank to buffer/store the PV power produced to allow the house to consume excess PV power for that day thru the night and the PV system cannot grow beyond what the house can consume without wasting PV power.
 
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moosemaster

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Just hook up a unit that monitors output properly and then engage 1 or 10 relays/ssrs for powering external heaters or what not. Get alot small ones like 1kW each so you can vary the load based on what you got

You can add a diversion unit instead that do it. Switching over the pv to other unit is generally not worth it. Better to have a pv diversion unit or build one that use the output on the AC side. its easier and cheaper.

Had always thought diversion units had to have grid connection, but after a search seems that's not so!

An Australian unit - https://www.ecocool.com.au/products/hot-water/plasmatronics-jackal-hot-water-system-power-diverter/

And one in the UK that apparently works off-grid - https://www.earthwiseproducts.co.uk/solic-200/

Just finding out if these units can work in multiples....
 

daromer

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Correct that basically sitt in between. :) But as Said think twice. IF you aint going grid an Arduino can do the jobb with some relays :)
 

moosemaster

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Correct that basically sitt in between. :) But as Said think twice. IF you aint going grid an Arduino can do the jobb with some relays :)
Never played with Arduino's but could be persuaded! Do you happen to know of any existing projects I could copy?
 

daromer

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I linked the rasp above. But it can be replicated to an mcu. My esp8266 project to mpp can be converted to controll output but it Will demand coding skill :)
 

daromer

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@hbpowerwall have a diversion installed but its installed on a grid-tie system if not mistaken.
 

moosemaster

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Still weighing up what approach to take.

If using these off the shelf components, diverting to the Polish MPPT heater controllers actually looks to be cheaper. It would also mean the main inverter is used far less which is surely a good thing:


The little Australian units seem simpler, but pricier - and all the loads will be running through the main inverter:


I shall keep pondering.
 
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