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Brainstorming wasted solar energy

I thought I would add some ideas about capturing the wasted solar electricity that I typically have on any given day. If you have ideas about storing excess power OUTSIDE of a system of batteries I would be happy to read your suggestions.

The set of 1.6kW panels usually push my four 6V deep-cycle marine lead-acid batteries to the maximum 29.6VDC level by around 11AM. In my 30-day history the maximum is 2kWh for a day. On average, I'm wasting half that daily.

  • I use the (Kasa app—based) TP-Link HS110/HS100/HS105 devices to remotely turn ON/OFF and schedule AC devices. The HS110 includes a fair amount of power statistics which I find useful.
  • One of these controlled circuits then is a battery charger which charges a single 12VDC deep-cycle marine lead-acid battery. This then is attached to a dedicated inverter with a remote power switch. I use this system to power a grow light for pineapples at the moment. On sunny days outside I then dump power into this system for the pineapples inside (rather than wasting the electricity in the afternoon).
  • I have repurposed several 20-pound steel cylinders from an insulation foaming project. Each get filled with sand and a heating element recycled from a typical 120VAC small space heater. One could alternately purchase or reuse the heating element from a hot water heater. You could dump waste energy into the sand batteries in this way so that they may radiate heat in your home after the sun has gone down.
  • You could dump the waste energy into a well head pump and only operate the pump then under these conditions if the tank isn't otherwise full.
  • You could operate a pump for a drip irrigation system in your garden in the afternoon.
  • You could split water into hydrogen/oxygen reservoirs after the batteries have reached their maximum, gradually creating a stockpile of the separated gases. Connecting the reservoirs to a bank of PEM cells or a fuel cell could then generate power by running a motor from the DC output, for example.
  • You could connect a string of resistors to the back of your solar panels and dump heat to them (with a layer of insulation) on surplus solar days in which the night's weather report calls for snow. (I got at least nine inches of snow accumulation on my nearly-vertical panels just last night alone.)
  • I sometimes setup a DeWalt 20VDC charger and charge my entire set of portable batteries in the afternoon.
  • I used to mine Ethereum (cryptocurrency) on a mining rig. One could do this activity just in the afternoon, for example.

I intend to program a VenusOS Pi computer to itself exercise the TP-Link programming API at a later date to orchestrate some of these projects.

solarfloat
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9 Answers
nickdb avatar image
nickdb answered ·

My home system, is capable of around 42kWh of PV per day, so for this region, a modest amount.

Battery storage is expensive and, that is still the best use of spare capacity.

I have full control of TUYA devices via node red, so when there is surplus, I tend to run pumps longer, or try shift workloads to consume the capacity.

In summer, I run aircon non-stop.

The simplest solution for me is to buy an EV (not a huge fan) and rather offset my spend on a conventional engine.

Ultimately, doubling my battery would be more elegant and cater for weather.

I know you don't want battery options, but simply changing to lithium (same capacity) would immediately unlock a ton of your "lost solar".

I would make that move before getting more creative.

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outsourcedguru avatar image outsourcedguru commented ·
My friend invested $8,000 in lithium batteries and I must say that my own $800 lead-acid set of four outperforms his by probably double. He has to run a 9000W generator almost every morning; I don't even have my own generator setup to do such a thing.

I forgot to mention that I live on the Continental Divide (7000'+ in elevation) and it gets very cold at night. Lithium batteries don't love the temperatures out here. (I had to write a program for my MacBook to exercise the GPU to warm it up so that the battery will charge here; the MacBook will not charge below 26℃ which I find to be ridiculous.)

Next, I live in a house made from straw bails so I'm paranoid about the uncontrollable fires typical of lithium battery malfunctions.

I'd love to hear any other ideas you might have.

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matt1309 avatar image matt1309 outsourcedguru commented ·
Lifepo4 might be a little safer.

Hovever still pricey and sitll have the same temp issues.

Plus not as fun/cool as the above suggestions

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Al avatar image Al outsourcedguru commented ·

"My friend invested $8,000 in lithium batteries and I must say that my own $800 lead-acid set of four outperforms his by probably double. He has to run a 9000W generator almost every morning; I don't even have my own generator setup to do such a thing."

So your 80% efficient lead acid batteries 'outperform' your friends 96%+ efficient LFP? batteries 200%? I would love to see that.

I think your friend needs more solar, or less loads.

Poor analogy aside, one of the best ways to store excess solar is as hot water in a wet thermal store.

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matt1309 avatar image
matt1309 answered ·

I'm desperate to try the sand battery idea. I did see a company in Finland was doing it on a large scale and claiming it could store heat seaonaly. Imagine they're using more bespoke elements than just old water immersion heaters.


However let me know if you have/built one. Very interested in trying something lke that myself.

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outsourcedguru avatar image outsourcedguru commented ·
I've got about eight of the tanks prep'd but none wired up yet to the system. I've cannibalized a number of AC-based heating elements and sub-elements at the moment. The interesting thing about a 120VAC/1600W+ heating element is that it can be powered with 12VDC, for example, and only pull a fraction of that power in the circuit (maybe less than 20W). This is good in that you can avoid over-taxing your system but it's bad in that it takes a while to heat up the sand this way. You might power a series of ten tanks at 20W each using 12VDC for a total of 200W consumption, though.
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matt1309 avatar image matt1309 outsourcedguru commented ·
Amazing

I was looking at how the Finland company was doing it given they were claiming to store the temperature between seasons. Hot in summer with excess solar. So assumed I'd need some higher temp elements but let me know how your setup goes.

Much easier to get my hands on old immersion heaters

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outsourcedguru avatar image outsourcedguru matt1309 commented ·
Any claims of inter-season heat storage isn't a sand bank, I'd suggest. It sound more like they're actually just doing some kind of heat pump into the ground.
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matt1309 avatar image matt1309 outsourcedguru commented ·

What is a sand battery? — Polar Night Energy

Some bold claims they have, but they are sand batteries. At least that's how i read it.

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ejrossouw avatar image
ejrossouw answered ·

@OutsourcedGuru Hotwater storage is another easy and practical way as it can be used for hotwater as well as heating. I understand sand is more practical for heating and not that viable / efficient for conversion to electricity.

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outsourcedguru avatar image outsourcedguru commented ·
I wouldn't be trying to recapture the heat as electricity. As I might have mentioned I live above 7000' altitude so it's all about radiating the heat at night.
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wilm avatar image
wilm answered ·

My personal way to store wasted solar energy is to put it into batteries. These days LiFePO4-batteries are getting cheap, so I was able to build my next battery with a fancy metal-housing, BMS and active balancer for approx. 100€/kWh. I will end up with 30kWh capacity for a 8kWp system. It's just a MP3000/48, so nothing big.

Investigating into sand-batteries would be nice if I would have the space for more solar-panals, but I am at the limit. Assuming the costs for a sand-battery-system would implode my financial capabilities. Nice for big systems, but not capable for me.

Just my 2 cents,
Wilm


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outsourcedguru avatar image outsourcedguru commented ·

People gift me with their unused project materials so I indicated above that I have eight of those 20# steel tanks for this stuff. The transfer station locally (the dump) will give me used water heaters so I can reclaim the heating elements from those. I'm big into repurposing consumer waste like this.

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Michelle Konzack avatar image
Michelle Konzack answered ·

I am currently working on a Project for my Farm to nearly ZERO the use of my Generator which will be probably only needed in December.


NOTE 1: I live in Estonia and I have the same climate as Canada (+40C to -42C)

NOTE 2: My Farm "Miila Mahe Aed OÜ" was nominated by the Estonian LEADER Program in 2020 as a working Example of an Off-the-Grid Farm.

NOTE 3: My Farm "Miila Mahe Aed OÜ" is considered as an EXPERIMENTAL Farm for Off-the-Grid Solutions.


I have currently fixed installed:

1) SOPzS Batteries 24V/2940Ah in 4 sets of 735Ah with each one BatteryBalacer and own BMV702

2) SmartSolar MPPT 150/70-Tr VE.Can with 1,86kWp

3) ColorControl GX

4) MultiPlus-II 24/5000

5) 3 SmartShunts 500A

6) 2 SmartShunts 1000A


I will install on my FireWood Storrage this year

7) 2x SmartSolar MPPT 250/100-Tr VE.Can (= 2x 2050Wp)


This produce so much Energy, that in the summer (April-July) I get easy 50kWh a day.


So, my new project which will take probably 2-3 years do to the financial requirements:

8) Building a WINTER GreenHouse with 16,5x7,0m

9) The Roof get 32mm PolyCarbonate (2,1x6,0m) but also 14 Solarpanels 550W (2,28m long) with a Fronius Symo 8.2kW, a Fronius OhmPilot 3x3kW, a SmartMeter and an In-Flow-Heating Element 3x3kW.

10) The Foundation under the Greenhouse is digged out 4m (the floor will be 1 m above the Grass-Level) and put two loops of 200m HDPE Pipe 40x2,4 PN8 (special Earth-Heating Pipes) into it.

11) The Whole thing then is covered with 2m sand.

12) I put another 200m of HDPE 40x2,4mm pipe into it and recover it with 50cm sand, then a Layer of 100mm EPS 200 and drainage membrane 1,2mm and recover it with 4cm of sand. This give a HeatStorrage of approximately 350m3 in which you can store in theory 150kWh/K hence if I have a ground temperature of 10C and pump so much energy in the ground that I reach 50C, then I have 4200kWh of Energy already.

11) Any Energy the Farm/Batteries can not take, will by pumped into the ground using the Fronius OhmPilot and the In-Flow Heating Element.


Once the Greenhouse above is finished, it insolate very well the HeatStorrage and I can also grow things in it.


However, I have my Energy Statistics since 2028 complete and I know, that this System will even September deliver more than 83kWh a day, while the average Power Consumption of the Farm is 15kWh?


Hennce even in September I can charge my Earth-Battery with SolarEnergy.


And believe me, in Estonia it is inutil to use Super Expensive Vacuum SolarCollectors, because it is too cloudy and the they Stop working with the smallest clouds, while my Solarpanels still deliver Energy.


Jeah, SolarCollectors need INFRARED which is blocked by clouds and fog while Solarpanels need simply LIGHT to work, which is also there if you have clouds and fog.


In October I will still get 34kWh a day...


...and give me 15kWh for the Farm and I can EVEN pump 19kWh into the EarthBattery.


However, in October I start slowly heating the house...


...and since my EarthBattery has already 50C I can use it directly in my Underfloor Heating System with a Thermostatic Valve (23-43C). The EarthBattery is Big and Hot enough to provide (in theory) 2100kWh hence if I would need 36kWh a day, nearly 2 month, hence end of November!


Now it comes, the NEWLY build EarthBAttery need around 3 years to become fully operationally because it does not only has the heat inside but build a cloud inside the earth, and the EarthBattery get from 16,6x7,0x3,0m (350m3) more then 20x11x4m (880m3) and over the years even bigger. creating an enormous Heat-Buffer-Zone.


So, How much Energy you need realy to get the EarthBattery to +40C (4100kWh)?


It is a LONG term Energy Storrage and you have to accept an efficiency of only 50%, hence you have to pump the whole year 8,2MWh into it. Any dubts?


In Tallinn we count 1200 Sunhours a year, mean 8200kWh divided by 1200h is ONLY 6,9kW!


The Solarpanels (1,86kWp, 4,1kWp and 8,2kWh) deliver up to 17.000kWh a year.


Oops!


My EarthBattery is too small! ;-)


However, the EarthBattery is already so big, that I can GROW things in my GreenHouse and heat my house until December, because I have also a BrickStove (with WaterBottle) and a WarmWall in my House.


But In November I will get only 9,6kWh of SolarEnergy which is not enough to keep the Farm running. Hence requires around 3kWh of Energy a day from the Batteries which will be empty in 15 days and I have to run my LPG Generator.


In december I get estimated 5,5kWh Energy a day, which kake things worse.


This is why I want to install a second Fronius Symo 8.2 with 21 Solarpanels 410Wp which will give me in November enough Energy to avoid running the Generator. This Additional System cost around 8000€ and the panels are installed Vertical side by side on a ground based Shelf for easier cleaning in the Winter and also in an angle of 70deg due to the flat sun we have.


However, this is ONLY the first EarthBattery I want to build. because under the second Solarpanel Set I will install the second EarthBattery with 20m lenght, 8m width and 4m depth. and shoud get heated by the second Fronius Symo 8.2 with OhmPilot.


This should be then also enough, to give me even in December 9kWh a day, hence 6kWh have to come out of the Batteries and I have to use my LPG Generator all onec a week.


In January I get already twice the Energy from December and the Generator is not more needed.


So, as you can see, bigger Batteries are not always better.


They are expensive and have to be maintained and replaced regulary.


I have already dropped the Idea of increasing my SOPzS Batteries from 24V/2940Ah (70,5kWh)) to 24V/7320Ah (175kWh) because they cost 29.000€ while a ground bases Fronius Symo 8.2 with 21 Solarpanels of 410Wp cost only 6000-7000€.


For 29.000€ I could install 4 Fronius Symo 8.2 Inverter and have not more to run my Generator at all...


However, then I would need a Grid Connection to get rid of the Summer Energy and get an additional pension... ;-) (4 additional Fronius and the Grid is considered a JOKE)


Any comments and private messages welcome (also per EMail)

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Michelle Konzack avatar image Michelle Konzack commented ·
I have forgotten one thing to mention:


I calculated only the SolarEnergy, but NOT the WindEnergy (3x iSTA-Breeze i2000/48, 1x iSTA-Breeze i700/48 and 1x iSTA-BREEZE i700/24) which produce also more then 1,5MWh a year together.


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outsourcedguru avatar image outsourcedguru commented ·
You and I think alike. I'm in the process of building my greenhouse. I've dug down as you've indicated. The soil there is already about 50% clay and 50% sand by the way. The northern wall is completed. It uses 40 spare tiles filled with sand with a diagonal wall sloping away from that end. It will be the lowest spot so that rainwater will be caught in this direction.

The other three walls will be constructed with (or framed with) logs like a log cabin style. I currently have a pile of 30 of those at the moment ready. The sweeping southern wall will have six tall windows (purchased from the Habitat for Humanity site at 50% their retail cost since they were donated to charity). The roof on the south side will be the tallest to allow in lots of light.

Now here's where things get interesting. Tiny glass electronic diodes will produce voltage across them in the light. (Solar cells are diodes in case you didn't know.) So imagine installing hexagonal mesh chicken wire inside each of the windows. At each vertical junction of the chicken wire you cut it and solder or spot-weld into place a properly-oriented diode. The parallel vertical runs of the diodes then are resistant to shading and contribute to added current. The height of the grid determines the voltage. Expect each diode to generate about 0.7VDC. If it's 1/2" wire mesh then you ought to have a vertical diode density of 144 diodes in a 6' tall pane which should be 100VDC. Assuming that the window width is 4' then that should be 96 times the average current per diode produced. I would have to do some measurements but I anticipate that a 6'x4' mesh of diodes in full sunlight should be comparable to 800W in solar panels. In bulk, you should be able to purchase the diodes for around $0.06 each.

By my calculation in this moment that's about $800 worth of diodes. Youch. Admittedly, this might not be feasible from a cost standpoint. My intention was to create a see-through solar panel, both allowing the plants to have their light and to generate the power necessary for pumping the water around in a hydroponic system.

For you, though, I would strongly suggest that you add an anti-rabbit "corral" of chickenwire under and around the sides of your underground sand bank. Once you heat the ground like you've suggested you will attract LOTS of underground animals who will wish to live there in the comfortable space you've created. Second, I'd suggest that you install some sunken 50-gallon plastic barrels into the sand pit. Your heating strategy will prevent the water from freezing. The water itself will store heat and you know you'll need the water for irrigation inside the greenhouse.

Good luck.

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Michelle Konzack avatar image Michelle Konzack outsourcedguru commented ·
The walls of the sand battery are from FIBO blocks, outside insulated with 100mm EPS and a drainage membrane. Since it is INSIDE a fenced up apple farm (the rabbits would eat up my appel trees in the winter), no chance to get in and mice do not dig this deep. They stop at 20-30cm.
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Michelle Konzack avatar image
Michelle Konzack answered ·

Oh, I forgot one thing: A BIG SUGGESTION FOR VICTRON ENERGY


Fronius has this nice OhmPilot but I would like to see something in the size of the SmartSolat MPPT 250/100 where you can connect the Battery, VE.Bus, VE.Direct or VE.Can to your GX Device and connect a 12/24/48V Heating Element to it which is PWM Controlled. Output should be of course 100A.


This would allow people which are purely on 12/24/48V without 230V to use excessive Energy for Heating a Boiler or HeatingBuffer.


Heating Elements 12V exist up to 300W, for 24V up to 1000W and 48V up to 2000W.

This are physical limitations do to the Energy Density.


I use the 48V/2000W for my Windmills as Breaks, four 24V/3x300W in my 500l HeatingBuffer and 24V/2x500W in my 200l Boiler

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outsourcedguru avatar image outsourcedguru commented ·
For the moment, you might look into the VenusOS they provide running on a Raspberry Pi computer. You would need either a VE.Direct or VE.Bus USB adapter so that the Pi can talk to your charge controller to know when you're going into "Float", for example. After this, add any of the Raspberry Pi "hats" with one or more relays like the PiFace Digital 2. You can then programmatically toggle on/off your heater with either of the two relays. This requires some setup of the VenusOS, the Raspberry Pi OS and a little Python programming.
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Michelle Konzack avatar image Michelle Konzack outsourcedguru commented ·
If I am in float, then it is already to late.


You need to get already the energy WHILE IN ABSORBTION, without disturbing the absorbtion process.


Currently I switch on my heating elements when I reach 28,1V because the SmartSolar MPPT 150/70-T VE.Can can deliver 2000W @28V. and depending on the time, I have 2 Heating elements with each 3x300W. Hence I simply increase the Power slowly over 2 hours which mostly works.

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max-payne avatar image
max-payne answered ·

Just get a pool and dump all extra Joules in there.
Use saltwater then you also do Chlorine production cycle with the extra energy.

No bought Chemistry and warm water.

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outsourcedguru avatar image outsourcedguru commented ·
I absolutely follow your logic and the chemistry involved. If I were in a warmer climate or had kids I'd consider that route. (I was daydreaming water filtration solutions in bed last night before falling to sleep.)
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nickdb avatar image
nickdb answered ·

With such a small system, I don’t see how anything can be done, economically, to harness the modest amount of unused capacity.

This only makes sense with some degree of scale.

As a science experiment, sure, but in a practical sense it seems hopelessly unobtainable.

This is a bit off topic for Q&A, moving to mods..

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Michelle Konzack avatar image Michelle Konzack commented ·

With small systems up to 5600Wp I would say, it make only sense to heat a boiler.


However, with a 500m3 sandbattery (roughly 20x9x3m = 540m3) and two Fronius Symo 8.2kW you can easyly pump the 20MWh the whole year with two Ohmpilots 9kW into the ground and and take it out in October-December without using a HeatPump because the ground temperature will be roughly 50C and the Underfloor Heating System need only 28-35C.


Since it is a long term HeatStorrage you get only 50% of the Energy out, and of course, from this only the half, because you use only the 50-30C and then the 30-10C must be taken out using the Greenhouse (will never go under 10C) or using a HeatPump but this is contradictonary to Off-the-Grid Living.


From in total 22kWp I get even in December with clean Solarpanels 6-9kWh a day.


And if I can save 5-6m3 of Firewood, over the next 20 years, the EarthBattery become very well rentable.


But installing 54 Solarpannels is not every ones option and it must be always clean in the Winter.

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wilm avatar image wilm Michelle Konzack commented ·
Michelle,

from a economical point of view, the sand-battery won't work despite you have your own sand-mine and a construction-company...

Let's compare:

6m³ dried firewood costs in Germany appox 600€. So 20 years this would mean 12k€.

If you can build such a sand-battery, with all that concrete, masses of sand and the additional heating elements, when do you expect the ROI ?

It would make more sense to wait some years until the development for green hydrogen is at the point. Then put you energy left over into hydrolysis and enjoy the stored energy during the winter. This might be cheaper than a monster-sand-battery.

Just my 2 cents,
Wilm

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Michelle Konzack avatar image
Michelle Konzack answered ·

A ring (16,5x7,2m) of FIBO Blocks is 98 blocks roughly and since they are supported from both sides from sand/limestone, We do not need any cement for it.

Hence I need for 2m height 980 blocks which cost me 2500€.

The next 7 layers are made from Columbia Blocks and I will need 840 blocks which cost me around 2000€ plus 360 bags Sakret BE which cost me 1300€ plus the steel armatures.

Outside insulation is 100mm EPS 80, hence I need 160m2 (32 packs) which cost me 900€. Then the drainage membrane outside two rolls of 3m width is 200€.


The 3x 250m HDPE Pipe 40x2,4 PN8 cost roughly 750€


Digging out and refilling this hole is roughly 3 days with an 8t Excavator and cost including driver 40€/h hence 1000€


Hence the base constuction is around 8650€.


If I insulate the top with 100mm, then it is 1200€ more, hence 9850€ in total.


Now it comes, because where I wanted to dig out my 1500m3 pond we discovered some difficulties, because there was no clay of limestone at all, but a huge sandy spot, hence we do not need to get sand. ;-)


Since I build a 16,5x7,2m WINTER Greenhouse on top of it, which would need roughly 6m3 Firewood for minimal heating a month and my house need 2m2, it become minimal 8m3 a month or 40m3 for a singel winter.


We use what we have, speak Fichten (spruce), Erlen (Lepp) and Birken (Birch) and how much do you think, you can get from 1,3ha?


I get from the (remaining) Forest (one of the tree owners of our 13ha died and the Kids and the second owner wanted MONEY, hence we left with 1,3ha Forest and 3,4ha Farmland) and I am happy, if I get CURRENTLY 18m3 a year out, unfortunately most spruce which you can not use for heating in a House in the Brickstove.


Buying 28m3 Firewood a year is no option at all even if I pay only 65-70€/m3 hence 2000€ a year.


The 10000€ for the Earth-Battery is VERY fast payed off. because it can store roughly 4800kWh of Energy and need only 9600kWh to charge it (long term storrage with 50% efficiency).


8,6kWp Solarpanels (21x 410Wp) with 1200 Sunhours a year (Tallinn) produce 10.000kWh a year and Since We have a second System with 7,7kWp plus DC coupled 1,86+4,1kW, the Earth Battery is easily charged with two Fronius Symo 8.2 + Ohmpilot 9kW.


From November to February the Solarpanels are switched 100% to Support the Electricity to the House and Greenhouse and with this system it is only neccesary to use the Batteries in December plus 2 times charging from Generator with each run 30-40 kWh.


Another 8,6kWp Solarsystem would allow me to get rid of the generator, but it does not justify the 7000€ extra (I import the Solarpanels myself from China). Would make only sense, IF I GO ONE DAY on-Grid and purely sell back, but then I would add a whole pallet (36pcs = 15kWp) of the Solarpanels.


Never get Electricity from the Grid and direct the earnings to my pension fond.


In summary:


If you have a Farm with a Winter Greenhouse and CAN dig a 3-4m deep hole without hitting the groundwater it IS rentable

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