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electrodacus electronics solar off-grid
electrodacus electronics solar off-grid

electrodacus's posts

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One of the SBMS100 users ( Paul Alger ) has made an Ajax Web display for the SBMS so that you can remote monitor your SBMS over the Internet.
You can see a link to his SBMS live data and the link to the source and install notes.

View demo:

If you make improvements to this please make sure to share with others :)
Just send me an email and I will post here.

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Please check the DMPPT presentation I worked on for the last few days.
Your feedback is highly appreciated.
PS: Happy new year!!!!

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I build two DMPPT450 samples yesterday and took some photos today.
There is just one photo of the second sample you can notice the different power mosfets transistors and not all of them are mounted since I need to test them in different configuration. 
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Just got the PCB for the DMPPT450 prototype and took a few photos for you :)
All seems fine at first inspection will try to build one or two units this week.
But I will need to start writing some software in order to realy test all the functionality.
There is a lot of gold plated pad surface so I guess they will need quite a bit of solder paste compared to SBMS. 
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Here is a comparison of 3 different heating system for my house.

The DMPPT450 with PV panels is compared against thermal solar and natural gas.
Please let me know what you think this is just a draft did not even had time to read what I wrote so there may be a lot of grammar mistakes.
I'm more interested in what you think about the comparison and if there is something missing from there.
Is maybe something better to compare against. The natural gas seems to be the best option and I do not thing even wood, pellets or anything else can compete with that since the cost here at this time is just 1.87 cent US/kWh so extremely low (the lowest ever based on historical data). Curios is you have natural gas what it costs at your location since this to me seems extremely low.
In 2008 the cost was much higher at 4.26 cent US /kWh
In any case it seems that world wide natural gas is the most preferred energy source for heating with 50% of the US house holds using natural gas for heating.
Heat pumps and grid electricity can not compete with this for sure even if you take just 10cent/kWh rate in some low cost location and an extreme average COP of 5 (not even COP of 3 is possible in cold climates) In fact not possible at all at my location and it will require geothermal ground loops that will just explode the price.

I need some sleep now :) 

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As promised here is the DMPPT450 battery charging ability.

In this example a 200Ah 24V battery is used with both a fixed 6x 260W PV array and SBMS in blue and also the DMPPT450 with 39x 260W PV array used for both heating and battery charging.

The DMPPT450 will prioritize the battery then all the remaining energy if any will be stored in thermal mass as seen the the earlier post I made.

A 200Ah 8s LifePO4 battery should not be charged above 0.25 to 0.3C for long life that is why 6x 260 PV panels where used in total so charge current will never exceed 60A.

The DMPPT450 will of course limit the current to the same level but since it has access to a much larger array it can select how many panels to redirect to battery charging so that to maximize the charging while keeping the charge current below 60A.

The PV panels on the 6 PV inputs on the DMPPT are set as follow 1,2,4,8,12,12 that way 39 total steps are possible from that only 34 steps are usable in this example since a 200A battery is used and a minimum of 6 panels will be needed at any time for battery charging as long as the battery is not fully charged of course. In that case all panels will be used for heating/cooling.

The DMPPT can be used
a) as stand alone just for heating so just PV array + DMPPT + resistive heat elements and thermal mass.
b) just for battery charging useful in some cases to reduce the battery capacity needed for a certain consumption.
c) the most advantage is when both heating/cooling is needed and electricity. In this case you can potentially use the entire energy available form the PV array have low cost heating and reduced battery capacity.

To be able to use as much electricity as in this example with a 200Ah 24V battery and DMPPT you will need at least a 400Ah battery and still have a limited number of backup for cloudy days while with DMPPT almost unlimited number of cloudy days are posible with half the battery capacity because of the over-sized heating PV array can be used safely for battery charging.

A worst cloudy/overcast day is possible than the one used as example in day 2 and in that case the gain vs a fixed 6x 260W PV array will be even higher with 39x 260W and DMPPT up to 550% more energy in such a bad day 6.5x more (39/6) 

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Here is a graph that shows the efficiency of DMPPT vs a simple resistive heater with fixed value.
First graph is for a sunny day and the second one for a typical cloudy /overcast day.
The solar data are real data from my 3x 240w PV system collected last year.

You can see ad download this raw data from my website where if you scroll at the end of the page you will see the 7 day's of data and if you click on that graph you can download the raw CSV data.
This days where at the end of February here when is still winter with short and cold days so that panels will output more than STC rated power.
I added a multiplier to the current data of 14.190 so that the 720W PV array will translate in to a 10.2kW PV array to have a more realistic DMPPT example.
The DMPPT450 will be able to support a max 14kW STC PV array.

I considered the max power point voltage fixed at 30v for this example to make the calculation easier and so the peak output power from that 10.2kW STC array was 11.16kW in the sunny day at peak the y axis is power in Watt.
Based on that I selected the best possible fixed resistor value as 11.16kW / 30V = 372A and 30V / 372A = 0.080645Ohm
For the DMPPT 6 resistor where needed and formula gets a bit more complicated where the first 3 heat elements will have the same value X in Amp and then you have a 1/2X an 1/4X and an 1/8X and so X= 372A / 3.875 = 96A
So first 3 resistor values are 30V / 96A = 0.3125 Ohm then you have 0.3125 x 2 = 0.625 Ohm next is 0.3125 x 4 = 1.25 Ohm and last 0.3125 Ohm x 8 = 2.5Ohm
You can see all this in the screen-shot image.
All this 6 resistor values when connected will be parallel of course and will add up to the exact same value as the fixed resistor value used as comparison 0.080645 Ohm.

Do not worry to much about the math I will provide a simple calculator tool where you just introduce the STC PV array size you want to use and it will output the 6 resistive values you need and also recommend the type of cable to use a heat element and exact length you need.

As you can see surprisingly the fixed resistor did not do to bad in the sunny day with an 80% efficiency this has to do a bit with the fact that is a short winter day when solar power ramps up quite fast to max and then also drops fast so the bell shaped curve is quite steep in summer the efficiency with fixed resistor will be lower. Then next in the cloudy day the efficiency drops significantly to just 31.8% and this is because the resistor was designed for the max output power of the PV array and the power in a cloudy day is always significantly lower. So I guess the average considering a mix of cloudy and sunny day's will be around 55%

With the Digital MPPT thermal controller things are of course much better :) because it has the ability to change the load with 31 equal steps (32 if you include zero) and that way it can always be really close to max power point by selecting the appropriate load level. The efficiency is still highest in the sunny day with 97.8% efficiency and a bit worse for the cloudy day but still an extremely decent 93.5% thus the average for a mix of sunny and cloudy day will be 95.6%
Even the most efficient DC-DC converter will have a hard time getting to this level of efficiency and you can imagine the cost, size and weight of a 14kW capable DC-DC converter not to mention it will be less reliable.

Sorry if I seem to exited about this :) but I really think this is the best technical solution for PV heating or possibly cooling and combining this with the ability to charge a small LiFePO4 for electricity is just great.
I need to make also a graph showing how a battery will be charged again in this two typical days from the large PV array keeping the charge rate below 0.3C
This PV heating size in this example will be almost exactly what I need to install 9 to 10kW and I will be using my 100Ah GBS battery but maybe considering the size I should use a 200Ah battery in this example not really sure.
The 100Ah will look better since the array size will be huge but 200Ah is probably more appropriate for this heating PV size array.
The 200Ah battery should be as good as a 400Ah with just a fixed PV array size and SBMS when heating PV array is included and the DMPPT saving about $2000 on battery cost that can be used for the larger PV array.

Let me know what you think and if the graphs are easy to understand. You will need to watch the photo on full HD screen or zoom in to see the effect of those 31 steps on the sunny day.

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