Computing stuff tied to the physical world

What if the sun doesn’t shine?

In Musings on May 22, 2013 at 00:01

Welcome to the weekly What-If series, also available via the Café wiki.

Slightly different question this time – not so much about investigating, but about coming up with some ideas. Because, now that solar energy is being collected here at JeeLabs and winter is over, there’s a fairly obvious pattern appearing:

Screen Shot 2013-05-14 at 12.47.42

Surplus solar energy during the day, but none in the evenings and at night for cooking + lighting (it looks like the heater is still kicking in at the end of the day, BTW).

This particular example shows that the amount of surplus energy would be more or less what’s needed in the evening – if only there were a way to store this energy for 6 hours…

Looking at some counters over that same period, I can see that the amount of energy is about 2.5 kWh. The challenge is to store this amount of energy locally. Some thoughts:

  • A 12 V lead-acid battery could be used, with 2.5 kWh corresponding to some 208 Ah.
  • But that’s a lower bound: let’s assume 90% conversion efficiency in both directions, i.e. 81% for charge + discharge (i.e. 19% losses) – we’ll now need a 257 Ah battery.
  • But the lifetime of lead-acid batteries is only good if you don’t discharge them too far. So-called deep cycle batteries are designed specifically for cases like these, where the charge/discharge is going to happen day in day out. To use them optimally, you shouldn’t discharge them over 50%, so we’ll need a battery twice as large: 514 Ah.

Let’s see… three of these 12V 230 Ah units could easily do the job:

Screen Shot 2013-05-14 at 13.14.23

Note that the cost of the batteries alone will be €2,000 and their total weight 200 kg!

There’s an interesting article about the energy shortage after the Fukushima disaster, including a good diagram about a somewhat similar issue (lowering evening peak use):


Although driven by a much harsher reality in that article, I wouldn’t be surprised to see new “one-day storage” solutions come out of all this, usable in the rest of the world as well.

For winter-time, I suppose one could heat up a large water tank, and then re-use it for heating in the evening. Except, ehm, that there’s a lot less surplus energy in winter.

Are there any other viable “semi off-grid” options out there? A flywheel in the basement?

PS. New milestone reached yesterday: total solar production so far has caught up with the consumption here at JeeLabs during that same period (since end October, that is).

  1. I got a 4kW solar system installed about a month ago and have wondered about storing the excess rather than selling it back to the grid at a reduced price. Not sure how feasible it all is, but I know that some UPS manufacturers for datacenters have been looking at flywheels for a while now.

    I did also consider getting solar thermal to heat up a water tank, but that cost was a bit too high. There are also hot water boilers that heat the water from a gas supply, but have a sterling engine to capture the waste head and generate up to 1kW from it.

    For now I’m just monitoring the system with emoncms and a bunch of jeenodes. :)

    • Anyone interested in using not losing power may want to look here –

      and here –

      Its a system like EMMA but a fraction of the cost. I’ve also just started a discussion on the open energy site about battery storage its been around in Europe particuly Germany for the last two years. Battery life is predicted as 20 years with Li-ion devices.

    • Thanks for the links – I’m intrigued by this, but I don’t quite understand yet. You divert energy to heat a reservoir of water whenever production exceeds all the other consumption. What is it used for? And what are the economics for the UK? In the Netherlands, feeding energy back (currently!) still pays as much as drawing it out, so (for now!) using the grid as a battery is still economical (up to a 1000 kWh/year surplus, I think).

  2. Lack of storage (at an acceptable price) is why I don’t go down the solar path. Where I live the feed in tariff was dropped a couple of years ago. So you all you can possibly do is cancel out your day time consumption.

  3. Trojan Battery Co. ( used to publish a “total lifetime capacity” spec on their batteries, i.e., the total number of kWh it could be expected to store and return over its lifetime. It was generally equivalent to 300-700 times the rated “100%” cycle kWh, depending on the type and grade of the battery. (so, for example, you could get 1000 cycles at 80% depth, or 2000 cycles at 40% depth, etc.)

    It was a useful metric that made it shockingly plain the total expected lifetime battery cost. It’s much cheaper to store the energy in the form of a pile of coal (or diesel fuel) than in a pile of lead: you use up more cost of battery per kWh than you would by burning fuel.

    • This is the major problem: Storing energy in lead-acid batteries is not cost effective in grid connected systems! Not even in regions where the kWh price for electrical energy is high (like here in Germany, ~€0.25) . Taking the abovementioned “total lifetime cappacity” data of different lead acid batteries and manufacturers, you always arrive at a cost of between €0.30 and €0.50 for one stored kWh! LiFeYPO4 batteries are better by about a factor of two. If you believe in the manufacturers data about possible number of charge cycles (5000-7000 at 70% DOD) and lifetime in years (20).

  4. Here in India, with regular power cuts everyone who can afford a battery backup has one, usually to bridge a short gap. However, more and more people are opting for off grid solar setups. Typical setups for a household are between 1 and 2 kw of panels and about 400-600 ah at 48v of 2v lead acid cells, sealed. This lets people power general loads and with a bit of planning, a washing machine and even an air conditioner for several hours. We have been monitoring the devices with wattmon, and it has helped people feel confident about their energy backup. IMHO there are several issues that need to be addressed in pure off grid systems in general:

    • High cost of lead acid batteries
    • Short life (5-8 years) of batteries
    • proper monitoring and control making a user confident to use it
    • lifestyle changes to match the power from the sun

    All of these are important, and I do believe that off grid systems have there use especially in power deficient zones, but grid feed inverters and the concept of using the grid as a giant battery has a lot of merit. I believe that even with gross inefficiencies ultimately it may be a more green solution if a large portion of energy could be generated from renewables.

  5. Look up “liquid metal batteries”.. hopefully soon..

    There are lots of regulations here in the US about ‘Islanding’.. eg if the grid goes down, the solar has to be disconnected. I want the opposite. If the grid goes down, I want my house to keep on, and just disconnect from the grid.

    • I imagine that’s to protect the engineers.

      If the feed from the power station is broken, and they go out to repair it, the last thing they want is AC coming back up the other way from the house!

      Unless they are working on your property, you should just be able to pull the main isolator and keep your island powered, although they probably don’t trust end users to do that.

      Then again, 110v? Pah!

  6. I never did the math on it, and I expect it to be inefficient, but what about a hydro-power plant: Put a water bassin on the roof and pump up water from the pond behind the house with surplus energy. I could even make use of rain as additional potential energy in the cold, dark and wet season.

  7. Best solution is to avoid electrical power consumption if there is no sun. E.g. run the washing machine (=1kWh, similar for dish washers) at day times, control the fridge that it runs longer in the afternoon and accepts a higher temperature during night (= 0.5kWh). Also the hot water can be produced when the time is right.

    Also interesting is the standby load of all (small) devices at home (eg door bell standby at our home = 4W). In our house it is around 50Watts and should be further reduced.

    With these tricks a battery might still be necessary but at half the size…

    PS: doing the math, it comes out that a hydro-power plant needs several ~100m or the stored energy/l is too low.

    • +1 exactly and absolutely what I wanted to say. I think the smartest thing to do is to break down your 2.5kWh consumption and see how much of it you can reduce or shift to daytime. Definitely cheaper than solar batteries

  8. SevenW’s remark made me remember an article I read a while ago about storing energy (heat in that case) in a large volume of liquid for a longer period of time. The article is in Dutch, I didn’t find the English equivalent immediately: Not related to this brand in any way, the concept of storing heat in a large vessel of liquid/ice seemed very interesting. Here, the basis is a heat-pump, but since heat=energy=electricity if I’m correct (with losses here and there of course), couldn’t this idea be reused in our context, storing surplus generated electricity by heating/freezing a watertank, too?

  9. Nice acticle, but typo “Fukushima” and not “Fukoshima”. Interesting to note is that “Fukou” would mean disaster.

  10. Darren, using PV rather than traditional solar thermal for heating water is not completely mad:

    I’m planning an off-grid house (currently stuck in a bit of a legal snarl up on the plot purchase). For this I expect to use LiFePO₄s for the primary electricity storage. The main reason is that off-grid you tend to have very variable electrical input so some days you won’t recharge the batteries fully. Apart from cycles, the thing that kills lead acid batteries is being left in a partially charged state – they sulphate. This is not a problem with LiFePO₄ batteries.

  11. In time, perhaps flow batteries may be a solution:

    Sounds just about right: you buy a fuel cell that matches your power profile and enough electrolyte to store all the surplus energy you produce.

  12. I can improve the efficiency of your house, but it will require a complete rewire…

    We seem to be spending an awful lot of time and energy converting solar up and down voltages and from DC panels to AC mains to DC batteries and DC computers.

    How about a 12v and 5v DC ring main in the house? The fridge could be a challenge ;-)

    Then again, the size of copper required to carry the power (current goes up as voltage goes down) might be prohibitive ;-)

    I remember as a child in the 70s, we had a lot of power cuts at the time. My father was an electrical engineer, and he’d made up little 12v lights in jam jars with a 13A UK mains plug on the end… Sounds like a recipe for a bang, but when there was a power cut, he’d unplug the TV and fridge (the only mains powered devices deemed sensitive to 12v DC), pull the main breaker, then connect a 12v lead acid battery across the ring main feed.

    Then we could just take the little jar lights wherever we needed them. As a small child that generally meant “to bed”.

  13. I think the 12v ‘main’ is a good way to go for some domestic projects – this page has given me some very good ideas and I’m implementing quite a few here. The fridge etc is a challenge, but in reality, so long as it isn’t opened much, the contents will remain in good condition for several hours.

    Another source of ideas is the yachting world – there are a number of new build sailing yachts being developed with direct drive backup electric motors with some interesting electricity storage systems.

    • The fridge is a challenge? It draws a mere 150-200W, same for the freezer. Not a big problem at 12V (although already at the edge, taking transportation losses into account). But what about cooking plate, coffee machine, flatting iron, hair dryer, washing machine, dishwasher ……. (ok, most of this stuff is only for women :-)) At least in my household, the major part of the consumption is from loads drawing more than 500W. And there the 230VAC is clearly an advantage.

      Wherever you need 5V or 12V, this is easily done with a high efficiency switch mode power supply (>90% efficiency, sometimes even >95%). And if a load needs 43.5V (LED driver?), no problem, same or even higher efficiency than for conversion to 12V.

      Hmmm, I really don’t see the efficiency improvement by using a low voltage DC bus ….

  14. there are promising idea’s about converting excess elecrical energy into gas (Hydrogen by electrolysis, and further conversion of that H2 into methane). Storage is necessary, because when everybody offers energy in daytime, the price at that moment drops to zero, maybe even negative. and since you cannot store the stuff, you will be paid at the time you offer that energy to the grid. So you will be paid zip.

    btw, the reason your solar panel switches off when the grid goes down, is that normally you expect the solar electronics to generate 230 AC (ok 110 AC) IN PHASE WITH THE GRID. Therefore it needs to sense the 50/60 Hz frequency. otherwise your solar power would not contribute but substract…

    but the engineers are grateful too, I guess.

  15. While its an interesting idea to store the excess as methane I’m not sure you’d want methane storage in every home. If things go wrong then it could be a bit of a problem.

    For my solar system I’m currently looking at using the excess electricity to heat my hot water tank using the immersion heater. In time I will install a thermal store so that the stored heat can be used for both heating and water. While it will not provide all the power required in the winter time the thermal store technology means I could augment this with a solar thermal system or other heating system. So I will get paid for the energy I produce and can use it to save on the gas I would have normally used.

  16. It’s not storage, but I have a system which looks at the power being imported from the grid, and the power being generated by the solar array. When the power imported has been zero for 15 minutes, and the average power generated by the PV array exceeds 1kW over the same time, it switches on an ‘optional’ load. This is done using a HomeEasy remote switched mains socket (Klick An/Klick Uit in the Netherlands, I think). I can plug a suitable load into this socket which gets moved around appropriately. Generally either the washing machine or the dishwasher.

  17. Grid tie systems only run when the grid is up; if there is no grid power – you don’t have electricity either and that kind of defeats the idea of running on solar power. On the other hand storing excess energy in batteries means using off-grid type of system, which require that you disconnect from the grid..Maybe a hybrid solution would be best, but in any case PV solar power will never pay off unless it is heavily subsidised. I do have evacuated tubes solar heater on the roof, that will pay off for 3 years. This is about as much as I will go solar :)

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