Sand Battery Experement

[GXMnow]

or 7-8 in winter, like we do.

No, not even close.
You may have 7-8 hours of daylight, but the sun is at such a low angle, the energy is much lower.

One "Sun Hour" is an amount of sunlight that will produce 100 watt hours from a 100 watt solar panel, but it may take 4 hours to get it. I am in a very sunny area, and in summer, we rarely hit 7 sun hours a day. And in the winter, it can fall under 3 sun hours even on a clear day. Add in clouds, and it falls to just 1 sun hour all day.

Look up "Solar irradiance at my location" and you should be able to find a chart or calculator to find this data. I have used this one and it is a pretty good estimate.

Solar Irradiance Calculator

www.solarelectricityhandbook.com

I typically get 80 to 90% of the estimate from this site each month. I think most of that error is that it does not allow exact panel angles, and I have some shading in both the early morning and the late evening. And my power reading is after the inverter which has some loss and also clips a little near noon on sunny but cold days.

For the whole year, my 4,800 watts of solar panels produce right about 8,000 kilowatt hours. That is an average of just over 4.5 sun hours a day.

 

[MyK3y]

Cloudless Wh per m2 at my location, middle of winter. To the left, cumulative kWh for the day per m2 of collector. Further left, average Wh per m2 over the month

I don't need your ad site - besides which, it's broken

 

[Ceefiveceefive]

No, not even close.
You may have 7-8 hours of daylight, but the sun is at such a low angle, the energy is much lower.

One "Sun Hour" is an amount of sunlight that will produce 100 watt hours from a 100 watt solar panel, but it may take 4 hours to get it. I am in a very sunny area, and in summer, we rarely hit 7 sun hours a day. And in the winter, it can fall under 3 sun hours even on a clear day. Add in clouds, and it falls to just 1 sun hour all day.

Look up "Solar irradiance at my location" and you should be able to find a chart or calculator to find this data. I have used this one and it is a pretty good estimate.

Solar Irradiance Calculator

www.solarelectricityhandbook.com

I typically get 80 to 90% of the estimate from this site each month. I think most of that error is that it does not allow exact panel angles, and I have some shading in both the early morning and the late evening. And my power reading is after the inverter which has some loss and also clips a little near noon on sunny but cold days.

For the whole year, my 4,800 watts of solar panels produce right about 8,000 kilowatt hours. That is an average of just over 4.5 sun hours a day.

Here are my results with the panels flat. I however adjust them every month & I do not have shading.
My guess is that I will have at least 5-6 hours per day in the winter.

 

[GXMnow]

Here are my results with the panels flat. I however adjust them every month & I do not have shading.
My guess is that I will have at least 5-6 hours per day in the winter.

I am 30 miles north of Los Angeles, so our numbers should be pretty close, but AZ typically has a bit less clouds, and you are a little further south than me, so you will do a little better. And adjusting the panel angle every month really helps. You also selected Due South, my main array is about 25 degrees west.

If you click on the "Adjusted Throughout the Year", it will give you the optimum for each month.

For my Los Angeles location, going due south, and adjusted each month, the prediction does go up to 4.85 for Dec., 4.8 for Jan., and 4.95 for Feb.
Doing the same for Chandler, AZ it is a bit better at 5.28 for Dec., and 5.29 for Jan. That has the panels way up at 48 degrees (42 from vertical). I did not expect it to go up that much. That is truly excellent winter production. To get that much, there has to be nothing around to create shadows. In winter here, I get shadows from all my neighbors trees with the low sun angle. I make enough power/energy in winter, so it is not a big deal for me, the 3 sun hours I get is enough. And I do think that web site is also a little optimistic. I am only hitting about 90% of what they predict. But still, those are good numbers.

Location and panel angle means a lot for energy production. Too many people seem to assume a solar panel just makes it's rated power whenever there is daylight, and that simply does not happen. For 90% of the year, my panels top out at under 80% of the STC rating. And you also need to accept the losses when the panels get hot. This will certainly be a factor in AZ. My panels exceed 140 degrees F (about 60C) on hot summer days here. And that has a big effect on the panel output voltage. My panels lose about 0.3% per degree C over 25. This is 35 degrees C hotter. 35 x .3 = 10.5% drop in output power. That makes my 300 watt panels into 268 watt panels just from the heat. Some newer panels can get the temp coefficient down to 0.22% per degree C, but they are more expensive panels.

 

[GXMnow]

Cloudless Wh per m2 at my location, middle of winter. To the left, cumulative kWh for the day per m2 of collector. Further left, average Wh per m2 over the month

I don't need your ad site - besides which, it's broken

I took the numbers from your chart. If you add them up, you get a grand total of 6,056 watt hours per square meter. That would calculate to 6.056 sun hours. BUT... What surface was that measured on? I am guessing it is total from a tracked panel, always facing directly at the sun. Are you going to use a tracker? And even with a tracker, those numbers still seem very optimistic. Where is that data from? Is that a spread sheet available on the web? I would be curious to see what it produces for the Los Angeles area of California. How close are the numbers to what I am getting from real solar panels?

That site I linked is not an ad site, I have nothing to do with it, it is just one I found that seems to work fairly well. You can search the web and find another, there are many on the web. The calculator is not perfect and it does sometimes error out, especially if you try to change entries after the initial choices. All I do is click "home" at the top, then click "online calculators", and bottom left is the "irradiance calculator". Load the table and again and then re-enter the data. I did it just now for Aukland, NZ. I do not know how far that is from you as I don't know NZ very well. I guess I could go to a map and find the closest city to you, but I do not care enough at this point.

With the panels facing straight north, and the tilt angle at best for winter, the May results are coming in at just 3.64 sun hours. I have real panels, and I measure the real output, and this site is predicting a bit more than I get most of the time. A few days, when the air is cool, I am able to match it and even did beat it a few days, but on average, I get about 10% less than the prediction. Unless you have a tracker following the sun, I would not expect you to get over this 3.64 sun hours a day in May, unless you are much closer to the equator than Aukland, but there is not a lot more of NZ to the north from there. You might do better, but if you do, I would call that a bonus. And the 6 sun hours from your table seems too far off to be a good prediction.

I am not telling you this to say you are wrong or to make you mad. I am doing this to give you a better idea of what to expect in the real world. If you spend a lot of money on this project, expecting to get 6 sun hours in May, and you only get 4, you may be very upset. But if you expect just 3.64 sun hours, and you do get 4, then you are over producing. And that tends to make people a bit happier. If you go in expecting too much, you won't be happy. The calculator I linked is not the best or the only one around, and I don't care if you never go to the page. But when you do put up PV solar panels, and you end up getting only 60% of the energy you expected, don't be mad at me.

 

[MyK3y]

View attachment 167184Here are my results with the panels flat. I however adjust them every month & I do not have shading.
My guess is that I will have at least 5-6 hours per day in the winter.

Hey, I used to live in Chandler! Worked for a certain semiconductor company in your neck of the woods. Back in 2000.

Interesting that that recommends 57deg as your optimum year-round. At 33 deg latitude I would have thought 35-40 would be the best for year-round.

I have a contact in Arizona - his year-round collection numbers are staggering. We get it pretty sunny most of the year with an average of over 2100 bright sunshine hours, but Chandler gets around 3500. Not sure your USGS is calculating them the same as we are, though. ou sure don't get twice as much UV as we do - we have a massive ozone hole and you can burn badly in 20 minutes.

 

[MyK3y]

e. But when you do put up PV solar panels, and you end up getting only 60% of the energy you expected, don't be mad at me.

This is not my first rodeo. I've been an EE for over 40 years and have been involved in solar projects for decades.

This is my first go at a solar battery, though, and my first go with RVs over 20% efficency.

I wasn't really asking for help, but answering a question asked by another poster. Why would I be mad at you? That would require me using you as a resource.

 

[OzSolar]

Sand batteries are sort of experimental also in here, one(1) unit in use.
Whereas large 2-3m3 water tanks are rather common and have been in use at least last 40 years.
I know half a dozen relatives and friends houses that have such a system installed. Forum member upnorthandpersonal also has one.

I'm starting my 23rd trouble free winter with my homebuilt 1200g storage hot water storage tank. I've installed and continue to maintain several dozen that are also doing quite well.

I'll try to offer a few words of caution even knowing that it will likely fall on deaf ears. I've been at this for a bit over 30 years and have never seen a successful storage media for heat other than water. From what I've seen sand batteries and seasonal heat storage have always had a bright future.... and always will.

Or, you can go and tell the Finns they're doing it all wrong and need to convert their municipal sand batteries to water?

They aren't using sand batteries in Finland. AFAIK there's only ONE sand battery in Finland. It's a demonstration project paralleled into an existing water system so there's really no clear data as to how well it is doing and likely there never will be. It's worth mentioning that's it is done by a startup looking for venture capital. Every other municipal heat storage system in Finland and Denmark is still 100% water based.

 

[Ceefiveceefive]

I'm starting my 23rd trouble free winter with my homebuilt 1200g storage hot water storage tank. I've installed and continue to maintain several dozen that are also doing quite well.

I'll try to offer a few words of caution even knowing that it will likely fall on deaf ears. I've been at this for a bit over 30 years and have never seen a successful storage media for heat other than water. From what I've seen sand batteries and seasonal heat storage have always had a bright future.... and always will.


They aren't using sand batteries in Finland. AFAIK there's only ONE sand battery in Finland. It's a demonstration project paralleled into an existing water system so there's really no clear data as to how well it is doing and likely there never will be. It's worth mentioning that's it is done by a startup looking for venture capital. Every other municipal heat storage system in Finland and Denmark is still 100% water based.

Thanks for your comment! I am however looking for info on sand batteries, not water.

 

[Eber]

Thanks for your comment! I am however looking for info on sand batteries, not water.

Me too, only I am planning on the size of a fuel oil tank or about 300 gal steel tank.
so good info, keep it coming.

 

[OzSolar]

Thanks for your comment! I am however looking for info on sand batteries, not water.

You're very welcome! I'm looking forward to hearing all about your journey into sand battery building.

Some helpful info I could offer is that you should first calculate how many BTU's you can generate with your ten 240/250 watt panels on a winter day. Once you gain a better understanding of that you can then start to play around how to get those BTU's into AND out of your sand battery.

 

[Turnermjt]

Wondered if someone could help me with some Maths please.

If I have an insulated container of sand approx 2.5 x 2.5 x 2.5 mtrs and over the summer months I believe I can heat it to 750c evenly using excess solar power.

If I then take the heat back out in winter by blowing air through a coiled tube In the sand.

How many hours/days would I get from that amount of sand at 750c if I was withdrawing 3 kws an hour??

Thanks Mark

 

[MattiFin]

Wondered if someone could help me with some Maths please.

If I have an insulated container of sand approx 2.5 x 2.5 x 2.5 mtrs and over the summer months I believe I can heat it to 750c evenly using excess solar power.

If I then take the heat back out in winter by blowing air through a coiled tube In the sand.

How many hours/days would I get from that amount of sand at 750c if I was withdrawing 3 kws an hour??

Thanks Mark

2.5^3 = 15,6m3
sand density 1600kg/m3*15,6 = 24960kg
specific heat capacity 800..900J/kg*Cel, lets take average 850J/kgCel
Assuming you get usable heat down to 50cel sand temperature you get
24960kg*850J/kgCel*700Cel=14851200000 Joules
1 watt is 1 joule per second. 1 watthour is 3600 joules. 1kWh is 3600kJ
14851200kJ/3600= 4125 kWh.
at 3kW constant load this would be 1375 hours or 57 days.

BUT even if you add 500mm of insulation wool the heat loss is going to be huge
fiber wool insulation lambda value rises with higher temperatures and losses go up really fast

Insulation wired mat - up to 660°C

Mat quilted to a wire mesh. Wired mat is particularly suitable for the insulation of pipelines and technological equipment. With temperatures exceeding 660°C, according to AGI Q 132, a mat quilted to a galvanized mesh with a stainless steel wire or a mat

www.gteek.com

2.5m cube insulated with 500mm lambda 0.15 high temperature wool has thermal resistance of 0.06K/W
put in other way your losses trough the insulation would be 700/0.06= 11700W or nearly 12kW losses

 

[Eber]

I am curious about that 3kw load. How is/was that determined? I am thinking it is current use?

I have to say, this information could be quite useful as I am planning on dumping some solar into a
1 or 2 - 150gal sand barrels and 2 or 3 days of heat when there is no sun, sounds good to me.

 

[MattiFin]

Similarly sized water tank with temperature range 30...95Cel (floor heating optimized for low temperature source)
temperature delta 65Cel
stored energy 1183 kWh =16 days @3kW
Insulated with wool/PIR-foam: losses 250W

Now the losses are less than 10% of targeted power level, unlike of losing all the energy trough the losses in previous high temperature sand battery example.
High temperature sand battery is just not feasible for long-term energy storage at this size. It could work for shorter time spans and in situations where you can use the heat losses directly. Quite easy to see why water is used in 99% of domestic and commercial applications.

 

[Eber]

I use heat pump water to air for heat. I do not use anything close to 3kw to heat. In fact, less than half that.
I was just curious as to where the 3kw would be coming from and how it is computed. I make so much extra power on sunny days that a simple cheap short term system will be fine for me.

 

[Johneb]

This is what I need help with in my calculations.

I have a total of at least ten 240W & 250W panels to use for this project. I'm not sure if I need to combine them and how many to use per element? My elements are rated at 2600W each.

I don't know if it would make any difference, but maybe pump your sand battery down to a vacuum? Your experimenting anyway.

 

[Turnermjt]

thanks all for your help - i started heating 210ltr oil drums filled with sand - using the excess solar from our panels - all worked well in august/september, but i found that i just dont have enough excess solar this time of year to make it work as i thought.

So plan B was heat a larger volume on sand june/july/aug/sept and extract it oct/nov/Dec.

The reason for the 3kw load, is that what it takes for me to heat my greenhouse during these winter months approx.

Currently i use air to air heat pumps, but i am just trying to find away of capturing energy in summer and using it in winter.

Any body got any other thoughts on insulation?? that sounds my biggest worry now - how much bigger does the sand box have to be to use the sand as the insulator??

don't believe i can go down the water root, as i cant store enough heat in it for use winter - i believe.

thoughts??

 

[sunshine_eggo]

Hey, I used to live in Chandler! Worked for a certain semiconductor company in your neck of the woods. Back in 2000.

Interesting that that recommends 57deg as your optimum year-round. At 33 deg latitude I would have thought 35-40 would be the best for year-round.

I have a contact in Arizona - his year-round collection numbers are staggering. We get it pretty sunny most of the year with an average of over 2100 bright sunshine hours, but Chandler gets around 3500. Not sure your USGS is calculating them the same as we are, though. ou sure don't get twice as much UV as we do - we have a massive ozone hole and you can burn badly in 20 minutes.

57° from vertical, not horizontal. (90-57 = ) 33° from horizontal.

 

[MattiFin]

thanks all for your help - i started heating 210ltr oil drums filled with sand - using the excess solar from our panels - all worked well in august/september, but i found that i just dont have enough excess solar this time of year to make it work as i thought.

So plan B was heat a larger volume on sand june/july/aug/sept and extract it oct/nov/Dec.

The reason for the 3kw load, is that what it takes for me to heat my greenhouse during these winter months approx.

Currently i use air to air heat pumps, but i am just trying to find away of capturing energy in summer and using it in winter.

Any body got any other thoughts on insulation?? that sounds my biggest worry now - how much bigger does the sand box have to be to use the sand as the insulator??

don't believe i can go down the water root, as i cant store enough heat in it for use winter - i believe.

thoughts??

Your target is about 3 to 6 decades too small to work with sand insulation over 3-month perioid.
Put it in other way 1000 to 1000000 times too small to work. 100x100x100m cube of sand with 10m outer layer of sand working as insulation would surely work but it sounds kind of big.

Sand Battery’s Efficiency Explained – Polar Night Energy’s ‘Sand Battery’ Has Efficiency Up To 95 Per Cent — Polar Night Energy

On what basis can we make the claim 'Efficiency up to 95%'? Here’s our Lead Scientist’s take on the efficiency of our energy storage system. Resistive heating of sand is essentially 100% efficient, but the efficiency is inevitably lowered by heat loss through the boundaries of the system. However,

polarnightenergy.fi

95% efficient for 1 or 2 week cycle on 1GWh =1000MWh scale and they supposedly would also use other insulation than sand. Scale up to 3 month storage and I'd look start with 10000MWh minimum feasible. Use sand as insulation and I'd start with 100000MWh for 3 month target at reasonable efficiency.

Waste of time to do the actual maths as nobody is going to have 100000MWh sand battery in domestic use.
Might explain why we don't store heat in sand for winter months.

Water would be totally doable if you dont care about any sort of ROI. Built wisely under the house it might? be even economically feasible in some cases. Even low temperature sand or concrete would be doable but you need ALOT of it.
For example 150m2 house foundation with 1m of sand under the foundation, insulated with about 3x typical insulation thickness ie something like 300-600mm PIR foam would probably get you close. Lot easier than 1 cubic kilometer of hot sand.