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How can I store my solar generated electricity to use at night?

Posted by Cathy Debenham on 18 December 2014 at 10:24 am

Q: How can solar power produced in the summer during the day be stored for use at night or in the winter? Converted to hydrogen or stored in batteries? 

A: There are a number of options that you can take to use more of your solar generated power. Some are (relatively) cheap and cheerful. Others will be serious investments. 

First, the free option. By making a few changes to how you use electricity you can ensure that you maximise your usage while the solar panels are generating. Things that use lots of electricity include the vacuum cleaner, washing machine and dishwasher. Always aim to use them while the sun's out. If you're out at work during the day, you could get timer plugs so that they start up when there's most sun (well not the Hoover, unless you've got one of those clever robot cleaners!).

You could also invest in a monitor that both tells you how much electricity you are using in the house and what you are generating. The one I use displays it in both digits and coloured lights, so I know that when it's shining green we are exporting, and it's a good time to use electricity. When it's red, we're using more than usual (and it might be possible to turn something off).

Next up price wise are switching devices which divert excess electricity generated by your solar panels to your immersion to heat the water in your hot water cylinder. They monitor the amount of electricity generated as well as the amount that your home is using. When there is a surplus this energy is diverted to your immersion instead of being exported to the grid. You can read more about this here

Electricity can also be stored in battery systems ranging from the very basic outlined in this blog to more sophisticated systems that are outlined in this blog by one of our expert installers.

You can read an expert assessment of Nedap's Power Router and the Victron Power Hub here. A stand-alone solution from Germany is the SunBat.

Inverter manufacturers are now combining storage systems with their inverters. Some are also offering whole house management systems with the inverter. These are interesting options if you haven't yet installed, but very expensive if you have a functioning inverter already. One example was launched this year from SMA & other manufacturers are doing similar things.

However, prices are still high, and unless you're very keen to be self sufficient, self-storage is not cost-effective for most people. Even if it were, not everyone thinks it's a good idea.The Centre for Alternative Technology (CAT) says "the cost of battery storage generally outweighs any savings from storing your own electricity". It also points out that there are environmental reasons not to do it too. Hazardous substances are used in batteries such as lead and acid. Also, if excess renewable energy is fed into the grid then it can replace electricity from fossil fuel power stations, meaning a power station somewhere burns less fossil fuel.

CAT also argues against the use of immersion switches for similar reasons: "In fossil fuel power stations two or three usinits of heat energy from coal or gas are required to produce a unit of electricity. So be feeding a unit of renewable electricity into the grid you replace two or three units of fossil fuel primary energy, whereas by heating water you only replace one unit of heat energy from gas."

Photo: Rob Baxter

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Comments

12 comments - read them below or add one

Gary

GaryComment left on: 11 January 2015 at 10:44 am

Sorry not to come back sooner.

Re MuppetKeeper's initial comment, my admittedly simplified maths below is pretty much how things pan out in reality for me.  Over the summer I generally used about 6kWh of gas per day to heat water, now I use none for between 5 and 6 months.  The replacement energy equates broadly to 2 units of solar enecticity.  In winter when the heating is on the clculation is much more complex but suffice to say, my gas usage is down by about 12% on its lowest ever figure since 2009 when I started logging use.

One part of the calculation/complexity I did not mention is that using gas to heat water in the summer gives rise to heat losses between boiler and tank - in my house about 15-20m round trip of mainly uninsulated pipes under the upstairs floors.  This is the "lowest grade" heat you can ever produce because at 20-25 degrees outside temp in the summer you do not want the hot water system venting any unlagged heat into the house and as I cannot get to my pipes to lag them without ripping up every floor that is an impossibility with gas hot water (for me).

My water is hot by lunchtime on an average day and is kept hot all day.  My presumption would be that as it cools a little and is re-heated throughout the day, convection will ensure more of the tank is heated but either way, I can guarantee that fossil fuel is not being used to heat my water for at least 5 months a year and, importantly, that energy is not being wasted in achiving that hot water by leaking heat from the hot water circuit as mentioned above. 

Looking richmc's comment, the energy conversion might be better with convection heaters but the heat is also transient at a time when the room is being heated by higher daytime temps anyway.  Storage heaters, ineffecient as they may be, would enable that heat to be released in the evenings when the house is occupied by the whole family and any heated gained during the day has been lost once the sun goes down.  Sometimes it's about practicality, not perfection, is, I guess, my thinking.

Finally, I think the discussion about whether it is irresponsible to use self-generated electricity for "low grade" heat is probably one of the most complex in the entire sector.  In a perfect world where I could guarantee that 100% of my generation would be used and used for high-grade purposes once it leaves my home and, crucially, where I could guarantee that an equivalent amount of energy would not be pumped into the grid from fossil fuel by the energy companies then the argument might be correct.  But as the energy generators have to build significant redundancy into what they produce to guarantee that our lights do not go out then I doubt very much that self-use of solar PV compared to export and local use has much, if any, impact on the amount of fuel burnt for mainstream electricity generation.  If that is true, then it is totally responsible to use as much as possible to begin reducing overall demand for energy, which  should ultimately impact on the levels of excess power that the main producers need to generate. And none of this even begins to consider grid losses between generator and me. Also, as MuppetKeeper suggests, if the majority of people locally are using high-energy devices (lights, fridges, old CRT TVs etc and leaving lights on and chargers plugged in) then the calculations become even more complex. 

My view is that if I can guarantee that I am not using fossil fuels to do things in my home then I can sleep easy... I'll now await the torrent of views from both ends of the "marmite" spectrum on this issue to tell me why I am either 100% right or 100% wrong.

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paul53

paul53Comment left on: 28 December 2014 at 8:42 pm

at the moment  my meter  goes  back wards but is on the  list to be  changed so i  am  looking  for  a  similar  heater i have  a tubular heater which  might be  of  some  use but its  trying  to  get something that  looks good and  has  build in  stat .

  

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MuppetKeeper

MuppetKeeperComment left on: 28 December 2014 at 6:08 pm

@ed1066 - Your comment really doesn't add any value does it, it's just a rant.

You have no idea how much of my generation is exported, whether it's the government guessed 50% or more, or less.

As for dumping energy into "low grade heat", I'm not in a position to know where my exported electicity goes, but my guess is its my neighbours, both of whom use incandescent bulbs throughout their homes. So I guess me using the electricity is probably better use than that particular form of low grade heat.

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omnikshan

omnikshanComment left on: 25 December 2014 at 2:26 am

The solar power system become more and more popular,every want to store the remaining electricity,but the cost is not cheap, I have heard a way to store the electricity, the specific process is more complicated, so we still await advances in technology it.

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paul53

paul53Comment left on: 24 December 2014 at 4:32 pm

the  most  important thing is  to get all houses in the  uk to have  solar panels  fitted. if  using  3  kw  a day to heat  a  tank of  water to make  it  more  financially  viable to  achieve this, its  a good  thing and  not  immoral at  all  better than  another subsidised  nuclear power station

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Cathy Debenham

Cathy DebenhamComment left on: 22 December 2014 at 10:02 am

A question posed by DavidW:

I have a 2.9kW PV system and would like to make better use of the generated electricity.  My preference would be to heat water but with a combi boiler installed and no hot water tank the installation required would be very expensive.  An alternative I am considering is fitting a storage radiator to use excess electricity.  The calculations I have carried out show powering a small radiator should be possible.   Does anyone have experience of such an installation and the type of controller I could use?

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ed1066

ed1066Comment left on: 21 December 2014 at 12:00 am

I'm sorry but in nearly all cases dumping the precious government-subsidised electricity as low-grade heat into a hot water tank is immoral.  You should be ashamed of yourselves recommending this.

 

This is a heavy and provocative statement, but I believe it with a passion.  Ed

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muymalestado

muymalestadoComment left on: 19 December 2014 at 8:53 pm

Possibly not dead-centre in the domestic market - yet - here is something in the line of renewable energy storage from (fast) locomotion experts Williams F1.

http://williamsf1.com/AdvancedEngineering/Case-Studies/DECC/

This was a more detailed set of web pages but now is a PDF outlining their very high mass magnetised flywheel which is shown inserted in a PV setting.

I have not seen any cost data, but one can imagine there will be several zeros in the price tag until mass-production and mass-utilisation is achieved.

The website pages gave power curves of quite large capabilities of instantaneous kW of extraction and cycled kW of re-energising and high daily duty-cycle (for evening-out wind generator power output).  Lifetime performance seemingly measures far better than various "normal" battery technologies.

The thought is neat.

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richmc

richmcComment left on: 19 December 2014 at 5:03 pm

I keep on banging on this one, divert to an immertion heater, then a convection heater, use all the generated power you can, feed in as little as possible. Storeage heaters are incredibly inefficent. not only that but a convection heater will only cost £20.

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MuppetKeeper

MuppetKeeperComment left on: 19 December 2014 at 4:00 pm

I feel the two or three units of fuel for one unit of electricity is a little simplistic for water heating in the home.

I too have a diverter, and the amount of electricity used to heat my water is less than the equivalent gas. There are probably two main reasons for this.

1. The gas boiler is not 100% efficient, at best, when it's condensing, it will be 90%, when the water gets warmer, I believe condensing stops, and it drops to 70 - 80%. Almost all of the electricity from the diverter goes to heat.

2. My immersion heater heats the top 50 - 60% of my water tank to about 65 degrees, as it is top mounted, thats all I need for myself and my wife.  My gas boiler heats the water from the bottom, so the whole tank needs to be heated, which is more water than I use. (My tank is triple insulated, but heat still loses)

In summary, electic heating of the water is a maximum of 3 units per day, where gas heating can be 8 to 10.  That aside, gas is a very versatile fuel, so you should only use it when that versitility is required.

Finally, the percentage of electricity being generated that is from renewables continues to grow, so those 2 to 3 units of fossil fuel are reducing all of the time.

 

 

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Cathy Debenham

Cathy DebenhamComment left on: 19 December 2014 at 2:01 pm

Thanks Gary. It's always interesting to hear how things work in practice. Afraid I can't help on the storage heater.

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Gary

GaryComment left on: 19 December 2014 at 1:28 pm

This is a topic I am particuarly interested in. 

I have an Immersun and am really pleased with it.  I turn my gas off when the heating goes off in spring and get all my hot water needs from the immersion heater, saving something like £70 per year on gas.  What is also interesting is that it takes about 2 units of electricity to heat our water each day but about 6 of gas, presumably because of the losses between boiler and tank (and back) which in our house must be a 18m round trip.  It has been in place for 18 months and has routed nearly 1megawatt to the immersion heater and secondary outlet (see below). "Yield" to date is about £120 and it cost about £300 to fit, so should pay for itself in 3 years or so.

The Immersun can also power a second (or with the mark II a third) resistive load so I am currently experimenting with an oil-filled radiator on that second output.  It works by switching the output to the second circuit once the hot water is hot and checking back to top up the hot water regularly after that.

The intention was to use this only in the spring and autumn to take the chill off the living room in the "in between" phase when it is chilly in the evening but you dont really need the heating on for the whole house.  This is the first winter of playing with this and today (sunny and nearly cloudless) in the middle of December, the water is hot and the radiator is on from about 1pm.

So far this has worked well.  The only downside is that the radiator (a 2kW oil-filled) reaches its full temerature and turns off for a while before the sun goes in etc.  A 3kW radiator would probably be a little better in this regard to maximise the heating potential.

What I would ideally like to find though is a good-looking and movable storage heater - something which is not an eyesore in the winter and which can be moved to the garage or whatever in the summer.  This would then charge up during the day and discharge its heat in the evening, rather than immediately which happens with the oil radiator.  Trouble is, I cannot find one, so if anyone has any ideas...

Gary

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