Using excess solar to heat house

[robby]

I just didn't see a point in responding anymore. My head isn't as hard as it used to be. I now try to avoid banging it against walls. lol

I felt the same way in that thread about using LiFePo4 for storage. People want to hear what they want to hear and anything that goes contrary to that requires that the messenger be shot.

 

[Aussie_Pete]

You clearly don't understand how solar irradiance changes during the year.

Solar Irradiance Calculator

www.solarelectricityhandbook.com

You didnt specifically say but I chose Glasgow as a "central" point for the sake of demonstration.
On a "perfect" winter day. Your only get to get 7% of the energy out of the PV array as you would in the summer at "your" location.

There is no such thing as "excess" PV power in the winter to heat something.

Perhaps if you lived in Sunny Spain or southwest US (Arizona) and the system was sized to run a MASSIVE AC unit in the summer, then you might have extra power in the winter.... but the cost to put such an oversize system in for you.... Who am I to tell you how to waste your money....

“I clearly don’t understand” ....... very helpful opening statement. Maybe this isn’t the helpful forum I thought it was supposed to be. For me every day is a school day and solar is today’s topic and I’m trying to learn.

I’m about 30 miles from Glasgow, so using your information I’d like to extrapolate.

My system is 5.2kwp and has open southerly aspect that is completely unshaded at an angle of around 40 degrees. I expected a bell curve in terms of daily generation, with the obvious morning lows, midday highs and afternoon lows, and a similar bell curve for monthly generation across the year with the least in winter, more in autumn and spring and most in summer. Using your figure let’s say I get 7% on a winters day at my peak time (midday). If you are right I’d produce max around 360w. This would gradually increase to max generation mid summer at peak time (again midday), which could be up to 5kwp and as the summer rolls on it will decrease again dropping down to the low point. I won’t get into a debate about the actuals until I have som actual data to demonstrate it, however given my consumption is relatively low, for a number of months - say 9 of them, my hourly generation will exceed my hourly consumption. If this excess is sufficient (say 2kw in a given hour) and my battery is fully charged, and my hot water is filled heated, I will have energy to burn.

In Scotland it can be very bright with direct sunlight, but not too warm. If I can use that excess energy to provide household heat and avoid burning oil it’s got to be good for me and good for the planet.

I’ll come back to you in a year and either top my hat to your knowledge and experience or maybe, just maybe, I’ll be able to say I did it. On,y time will tell.

 

[noenegdod]

Yes and no. If you’ve got solar & you’ve got excess you want to find a way to use it.

I keep seeing threads about using excess solar production for heat. A panel is ~15-20% efficient correct? Would it not be far more productive to instead have a dedicated solar thermal panel instead? You could then even use a mini split, powered by your excess solar production to move the heat from the thermal panel into the conditioned space and see a 3 or 4:1 return on your excess solar production and increase the efficiency of the thermal panel.

Which is precisely what that does, but with 3-4 times the effect.

 

[robby]

“I clearly don’t understand” ....... very helpful opening statement. Maybe this isn’t the helpful forum I thought it was supposed to be. For me every day is a school day and solar is today’s topic and I’m trying to learn.

I’m about 30 miles from Glasgow, so using your information I’d like to extrapolate.

My system is 5.2kwp and has open southerly aspect that is completely unshaded at an angle of around 40 degrees. I expected a bell curve in terms of daily generation, with the obvious morning lows, midday highs and afternoon lows, and a similar bell curve for monthly generation across the year with the least in winter, more in autumn and spring and most in summer. Using your figure let’s say I get 7% on a winters day at my peak time (midday). If you are right I’d produce max around 360w. This would gradually increase to max generation mid summer at peak time (again midday), which could be up to 5kwp and as the summer rolls on it will decrease again dropping down to the low point. I won’t get into a debate about the actuals until I have som actual data to demonstrate it, however given my consumption is relatively low, for a number of months - say 9 of them, my hourly generation will exceed my hourly consumption. If this excess is sufficient (say 2kw in a given hour) and my battery is fully charged, and my hot water is filled heated, I will have energy to burn.

In Scotland it can be very bright with direct sunlight, but not too warm. If I can use that excess energy to provide household heat and avoid burning oil it’s got to be good for me and good for the planet.

I’ll come back to you in a year and either top my hat to your knowledge and experience or maybe, just maybe, I’ll be able to say I did it. On,y time will tell.

Did you run the system through PVWatts?
I just used your numbers of 5.2KWp and Glasgow along with a clear path south at 40 deg plus I lessened some of the wire losses etc and this is what it came up with. I have found this software to be very accurate.



Right now in February you would be producing 203/28days = 7.25KWh per day That is just barely enough power to charge your 7.2KWh battery much less have spare power. In December that would be 2.9 KWh per day. Your battery will never be fully charged. You can double the Amount of Panels but that still leaves you in a hole during Winter but does give you plenty of power in the summer.

 

[labeeman]

If you have excess power you can use an electric space heater to heat up your house in the winter much cheaper than what you are looking at.

 

[Aussie_Pete]

Did you run the system through PVWatts?
I just used your numbers of 5.2KWp and Glasgow along with a clear path south at 40 deg plus I lessened some of the wire losses etc and this is what it came up with. I have found this software to be very accurate.

View attachment 85489

Right now in February you would be producing 203/28days = 7.25KWh per day That is just barely enough power to charge your 7.2KWh battery much less have spare power. In December that would be 2.9 KWh per day. Your battery will never be fully charged. You can double the Amount of Panels but that still leaves you in a hole during Winter but does give you plenty of power in the summer.

I used something similar https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP. It’s numbers come out higher see below) but your principle is sound. However in off summer months I’m expecting to have to charge the battery overnight, especially if I’m am using the solar PV for other means

 

[Symbioquine]

@Aussie_Pete have you tried selecting the "Off Grid" tab in that PVGIS tool you linked? It can estimate how much of the time your batteries will be empty/full...

e.g.

 

[OffGridInTheCity]

The fundamentals of solar....more in summer, less in winter....go against any economic benefits to purchase special equipment for the winter excess. You will find that your panels won't be able to keep up with normal demands during cold periods.

Absolutely and I have hard numbers to prove it. I converted to all electric (heat-pump) for hot-water and heat for my 2600sq ft house in mild (Zone 4) climate.

It takes up to 1400kwh/month to heat the house in dead winter and a 1000kwh to cool in mid-summer using a 4-ton, max efficiency Lennox system. Its the 1400kwh in winter that is the killer as my 13kwh PV array (that's 45 panels / large area) typically makes 500kwh in December and this December was only 250kwh. 1367kwh heat - 250kwh PV = 1,117kwh deficit! I can reduce this by living at 60F/16C in a bad month - but still.

The hot water (Rheems 50gal) heat-pump ranges from 138kwh/month to 200kwh/month in a cooler area under the house with 200ft of water recirculation for 'instant hot water' at the taps losses.

First - I highly recommend hi-efficiency heat-pump technologies as an electrically driven heat solution.

Second - My goals have been changing and I've reached the point where I'm willing to sacrifice excess summer PV for more winter capability. Its definitely not good for ROI, but its viable from a prepper/survival standpoint. The big problem is just 'space for panels'! Since I'm in the city, my space is limited.

The first column is hot-water, the second column is heat/cool kwh by month for 2021

 

[Zwy]

The fundamentals of solar....more in summer, less in winter....go against any economic benefits to purchase special equipment for the winter excess. You will find that your panels won't be able to keep up with normal demands during cold periods.

Yes and No. I have found that having variable slope thru the year will mitigate this problem. Running various scenarios thru a calculator will show that by changing slope thru the seasons will help supply the PV needed. I used the calculator here https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP

and ran several different slopes and looked the calculations for PV production. My mount is variable slope https://www.mtsolar.us/top-of-pole-mount/ and I chose it based upon this reason. It wasn't cheap, it cost way more than what the panels cost. But I also have trees in my yard that I would prefer stayed upright instead of firewood, so the area for a PV array is somewhat limited.

Winter solstice I will have around a 60 degree slope, summer solstice is 35 degrees. Equinox will be halfway between.

Run some numbers, it might surprise you. I've seen tilt mechanisms on the cheap,

certainly one can add a cheap screw jack to an adjustable mount where the return would be short term.

 

[Denis]

Hello all from a solar & battery newbie,

I'm currently getting a 5.2kwp solar PV system and linked 7.2kwh battery on my house in Scotland. I've been researching for about 6 months and the system is due to be installed this month.

I've been looking at what technologies are out there to exploit excess solar once the batteries are charged. I'm aware of heating water using an Eddi or I-boost, charging an EV using a zappi, but was wondering if anyone had considered installing an electric flow boiler (like the one linked below) on their existing central heating system, driven by excess solar to reduce cost of central heating? It seems to be a relatively new technology, but when integrated with solar could significantly reduce gas or oil use for central heating.

For clarity, I’d be considering installing in addition to the existing oil or gas boiler, so that when there’s excess solar this would heat central heating and when there’s not solar the gas or oil boiler would run heating

Wow, is what I say after reading this thread, but I agree with you 100%. You know what you need and how much solar production you are getting, so no need for any reason not to do it.
I'm in the same boat, here in Canada Ontario, we get the best solar production starting in mid January, when it is the coldest time of the year, and snow season.
On sunny days I can make over 40kwh a day, I've just installed a boiler in my attached garage and I've installed the baseboard heaters in the basement of the house also.
On those sunny days I have a 12000 btu senville mini split, and three other heaters at different locations,and I still have power to spare.
One day I saw 10 300 watts coming in from the charge controllers. That's almost maximum watts input the controllers can see.
I found a heater in a yard sale over 20 years ago, which I wanted to add to my boiler system, check the following pictures for details.
I am waiting for the next year to install it, so I will be able to monitor the difference in gas consumption it will make.

Another thing to notice is i have over 10kw of panels, and 45kw of LFP.
I was happy to see someone else have the same interests, as I know this is doable.
It's not for everyone but everybody knows what they can do with their own system.
Thanks

Slimjim Electric Flow Boiler | @electricheatingcompany

The SlimJim is traditionally installed in properties such as apartments, domestic dwellings, holiday homes & light commercial properties with limited space

www.electric-heatingcompany.co.uk

 

[Swing]

Just go for a heat pump system (which can already be a standard AC heating unit).
So you use more of your own electricity. When the solar production is not there, you probably still save on your energy bill compared to gas and oil. Or they can run at same time.
And since UK is going big on wind energy it is good to choose for a proper electric heating appliance.
You might keep your burners for when the heat pump / AC is not strong enough.

 

[efficientPV]

I saw a youtube video of heating water with a single solar panel. He used the shadow of a pencil to keep it perpendicular to the sun all day east, west and tilt. I was absolutely amazed at just how flat the power curve was. I can imagine a newbie being misled thinking he will get about the same result by sticking it in one place

 

[robby]

Wow, is what I say after reading this thread, but I agree with you 100%. You know what you need and how much solar production you are getting, so no need for any reason not to do it.
I'm in the same boat, here in Canada Ontario, we get the best solar production starting in mid January, when it is the coldest time of the year, and snow season.

Okay you lost me right here.

On sunny days I can make over 40kwh a day, I've just installed a boiler in my attached garage and I've installed the baseboard heaters in the basement of the house also.

So your saying that your 10Kwp array of panels can produce 40KWh a day of power in January.

On those sunny days I have a 12000 btu senville mini split, and three other heaters at different locations,and I still have power to spare.
One day I saw 10 300 watts coming in from the charge controllers. That's almost maximum watts input the controllers can see.
I found a heater in a yard sale over 20 years ago, which I wanted to add to my boiler system, check the following pictures for details.
I am waiting for the next year to install it, so I will be able to monitor the difference in gas consumption it will make.

Another thing to notice is i have over 10kw of panels, and 45kw of LFP.
I was happy to see someone else have the same interests, as I know this is doable.
It's not for everyone but everybody knows what they can do with their own system.
Thanks

So what is the secret for making it doable? Are you saying it's tilting the panels for max solar radiation.

 

[Swing]

Apart from everybody having its own experience: I think it is important to understand the difference between the lowest point of solar energy and the seasonal energy required for heating.

We just had a couple of grey months in which I really needed other power than the sun to heat our home. This was also during lowest sun height in the midday. However, right now there is already much more solar power available while still heating is necessary.
So the heatpump is running on solar power during the day.

 

[OffGridInTheCity]

Apart from everybody having its own experience: I think it is important to understand the difference between the lowest point of solar energy and the seasonal energy required for heating.

We just had a couple of grey months in which I really needed other power than the sun to heat our home. This was also during lowest sun height in the midday. However, right now there is already much more solar power available while still heating is necessary.
So the heatpump is running on solar power during the day.

In addition, the heatpump is much more effecient with higher ambient temps. As I wrote above, I burned 1400kwh heating my house with a heat-pump BUT.... a lot of that was raising the temp from 68F/20C to 75F/24C at 6am - at the coldest ambient temp of 30F/-1.

Heatpumps are most efficient around a minimum of 40F/5C ambient. So if I waited till 2pm when it was 45F/7 outside to raise up to 75F/24C, the power requirement would go down significantly "I think". But I don't know how much.

Does anyone have charts or info on the relative power needed for heat pumps based on temp differential over ambient temps?

 

[Swing]

That depends on the model. In general you can make some assumptions for a complete category.
Basically you are looking for a graph which features COP (efficiency) versus temperature.

For example, the average (not lowest quality) AC split system that can heat, gets a hard hit in its efficiency when the temperature goes below freezing. However, even for AC units there are Scandinavian / Nordic models, that keep up the efficiency in lower temps. They even continue working down to -30 degrees Celcius!

However, for air to water heatpumps, I think you can make some assumptions on the efficiency.
You can typically find numbers for -7 and +7 degrees Celcius.
But this categorie has a high dependency on output temperature of the water into the house, that makes it efficient or not.

In the end, a higher outside temp is always a reason for a heat pump to run more efficient, but it is not a big deal.
For me, it needs to start in the morning anyway, otherwise, it isn't very comfortable anyway. During the day the sun comes up and takes over.
Since we have a small house and a small AC installation for heating, we are talking about a few kWh before sun is shining. Which could be from the battery from yesterdays sun.

 

[shavermcspud]

I used something similar https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP. It’s numbers come out higher see below) but your principle is sound. However in off summer months I’m expecting to have to charge the battery overnight, especially if I’m am using the solar PV for other means

HI Pete, I'm in the UK but much further south, near Cambridge , technically I'm in a slightly better location than you for being further south for PV production, My findings on my very large 22kWp system with 32kW of battery storage and an iBoost water heater, array 22 degrees from south. As others have stated, winter months are pretty bad, low sun position in the sky, short sun days, all make up for not great production, I had one day in late December where I made 18Kwh which was my best for that month on record, that was just enough to run the house and 70% in to the battery pack with no water heating. The UK is already crap for sunshine as it is. In contrast as the days get longer, Sunday just gone (27th) I produced 64Kwh, charged my pack up, heated the water tank and charged the car a bit, but we're nearly in to March now, then today, raining and crap and produced 16Kwh not even enough to charge the batteries up to full. With a 5kWp system you would struggle to achieve what you think you will. Not trying to piss on your chips but this is a reality check.

 

[Denis]

Okay you lost me right here.

Around that time of the year, the sun is really bright and the sky is very clear and clean, and it seems that with the coldest days the rays are strong, please note I said it starts mid January to have better solar charging. So it climbs up eventually more and more every day when it's sunny,of course.

So your saying that your 10Kwp array of panels can produce 40KWh a day of power in January.

I've got a few pictures that shows that 15, and 16 days from today (so on February 12-13) I made 43 kw, and 42.5kw in those two days. A lot of days of 20-30 kw. Middle picture shows 10.4kw at 156 VDC max pv day 10, so yes it's doing better than in the summer months with all the haze in the air.

So what is the secret for making it doable? Are you saying it's tilting the panels for max solar radiation.

If you have enough panels and sun, power is there to be used, unless you can sell back to the utilities company. Not my case. Hope this clarifies any doubts, and I'm happy to share. Thanks

Edit: controllers are stacked, so CO1 AND CO2 HAVE TO BE ADDED on pictures taken, magnum data don't go over 25 kwh for each controller. PT-100

 

[sunshine]

Yes and No. I have found that having variable slope thru the year will mitigate this problem. Running various scenarios thru a calculator will show that by changing slope thru the seasons will help supply the PV needed.

A problem specific to the OP is the location, Scotland, where the average winter sun output is only 10% of the summer.
Even 10% would be useful for lights and other small loads but not for heating.

With a 5kWp system you would struggle to achieve what you think you will. Not trying to piss on your chips but this is a reality check.

This puts what I'm trying to say much better!
Load diversion is great....but not in your case.
Another point is to maximise any output the load must be varied to utililise the production. It is very well to say that over a day my panels sent x amount to the grid when the mppt has been able to vary the load for maximum production but try running a reasonable high fixed load like your central heating unit. It doesn't work.

 

[timselectric]

If op is already producing excess. I don't understand why so many people want to say that he can't.