How much battery storage does a solar PV system need?
euanmearns.com
euanmearns.com
You need to spec your system so at the very least it generates enough power on the worst days of the year to cover your needs, and surplus power on good days. You also should have a second means of power generation, wind works nicely as it's uncommon to have a day that isn't either windy or sunny.
I've used 100% solar for the past 2 years and there has not been a single day that I needed a second source of power.
Also my system is quite modest. Just 500W of panels and about 600Wh of lithium ion battery (lifepo4) energy storage.
The 500W of panels is plenty. Most days I charge my batteries fully before noon.
The 600Wh of batteries is enough to get me through 3 days of heavy cloud weather ( you get about 10% of your maximum generation on a heavily clouded day ).
This system is for two people using our computers and 3g wireless for the entire day. LED lights at night which are bright and warm, water pump, food processor, stereo, iPad, hella gadgets, raspberry pi backup server, etc etc
If I changed something, I'd double the batteries that I use.. going to 24V. You get more of the rated power of the solar panel when using a 24V system. I am considering getting an electric bike & storing extra power there.
The total cost of the system was about $800.
In the summer I can generate about 2 kWh / sunny day -- I only need about 400Wh / day though. You do the math for the cost per kWh. It's damn cheap.
Exactly. It's a total nonsense proposition to offset seasonal swing using battery power. Running a PV system is about conservation first and foremost and some planning second. Your battery is for the day/night cycle and overcast days, not for seasonal variations. Maybe he should run a PV system for a while before writing articles like these.
From the article "Even with these rough numbers it’s hard to see how anyone living any distance from the Equator is going to be able to justify the cost of installing enough batteries to go off-grid with a domestic rooftop solar PV installation."
If you go off grid on purely solar your backup is not a battery, your backup is a genset.
It's a nonsense proposition leading to nonsense conclusions based on nonsense numbers. Very tiring.
With that said, from reading some articles about off-grid living, it's not outwith the bounds of possibility that people are looking at pure solar (or pure renewable anyway) off-grid living, for priniple-based reasons , rather than a more pragmatic rationale.
Back in the real world you wouldn't design your system that way. Instead, you would look at your worst case scenario - mid-winter, and calculate how many solar panels you would need to capture a day's worth of energy in one day. This would also tell you how large your battery would need to be. A prudent engineer would probably introduce a factor of 2 or so both in the size of the panel installation and the size of the battery to allow for a sequence of unusually dark days, but the multiples being talked about in the article are just ridiculous.
Do you mean 600Wh?
The fridge would probably double our power requirements. Add in a stove and we may need 3x our requirements.
The total bill would still be pretty cheap, but I may be running out of roof space on a camper this size.
The whole winter/summer storage thing is nonsense, nobody sizes a PV system like that.
But if you're going to think like that grid power is also impractical anywhere, since it too could fail. Power is so important that no single source should be relied on anywhere. In inner cities of course your only chance of getting power is the local electricity company hookup, but if that should fail for any extended period of time such a city would rapidly become a place where you could no longer live, especially at latitudes where it tends to get very cold in the winter.
So, nobody (in their right mind, at least) will do a 'pure' photovoltaic system. There will always be some kind of backup-power mechanism, either the grid, a wind-turbine, a battery to tide you over, a pelton wheel fed from a stream, grid power or any combination of the above.
In reality, what most people (today) who want to live off grid end up doing is hybrid-systems. Natural Gas/Propane for heating, cooking, cooling, and Solar for electrical use. (computer, lights)
But then I have seriously big batteries, and am careful with my electricity consumption, especially in winter.
FWIW, I'm living in a cold climate with high rainfall.
So, to stick with HN's tradition of making negative comments about articles: This author is totally clueless, that's not at all how you size a PV system (and the battery to go with it). Battery power in a solar system is not used to offset seasonal variation but the day/night cycle and a possible overcast day (or two).
Then the first line listing the assumptions made starts from the average household but a PV system designer that is designing an actual system rather than a theoretical one will start with throwing everything out that you can miss, the household will be anything but 'average'. Electric heat, AC and a whole raft of other big consumers get thrown right out before you even start to think about what your consumption will be. No romantic array of incandescent spots for you, and no baseboard heat. Everything you can save you won't have to generate and store and that's your first gain. This will cut the consumption in half or even better and then you can start wondering about how much PV you need, if you're going to have wind or water to augment that, what size you genset will have to be (in case all of the above aren't available for a longer period) and what kind of storage battery you need. (And of course what size inverter(s) to convert your DC back to AC tied into the distribution panel of your house, you really don't want to run your household on DC)
A good rule of thumb for battery storage for a system that is off-grid and totally self-reliant is to have 5 days worth of consumption leading to 33% or 20% of battery discharge depending on how nice you want to be to your batteries. That's a whopping large battery, most likely larger than what your friendly car company can sell you in one chunk so you'll need multiple units ganged together via some kind of bus system.
When you go off-grid your life can depend on the working of the system, if you're snowed in and you can't get fuel, the sun lets you down and there is no wind then you fire up your trusty genset (preferably a diesel, you might be running it for a week straight in an emergency) to put some much needed charge into your batteries. Of course you anticipated this and you have two weeks worth of fuel stored.
Also, in very cold weather battery capacity tends to be reduced.
So, in short design your house and your life around your energy budget and even then you'll find you need a rather very large battery.
But people must change & figure out how to do it. (hint: 50 cent per watt solar and cheaper.. you just may be able to heat with solar. storing energy as heat... you don't need a battery. You need concrete.)
Evolve or die.
BTW, lithium batteries do not off gas hydrogen (like lead acid batteries do). Therefore, you just keep your batteries inside with you. They'll work just as efficiently and any of their 5% inefficiency will become heat.
I think you are misconstruing the author's objective here - they were simply trying to do the analysis of what it would take to live 24x7x365 on solar + batteries as a replacement for grid electricity. They weren't trying to provide practical advice.
Of course we'd need to get away from the huge mismatch between feed-in tariffs and consumption tariffs.
Ideally the difference would be a small spread (like in a currency exchange rate) and a nominal daily connection fee.
In which case a 10KW Tesla powerwall is a pretty handy size.
Maybe first we could nationalize the whole outfit and run it on a cost-plus basis. There is way too much profit being made from being a monopoly grid operator.
That probably has more to do with why solar is such a good deal than the green energy subsidies do.
The latter may simply be a matter of geography, but even then unless you live in a cabin in the woods then I believe e.g. in Australia some utilities are creating solar/battery/genset microgrids for remote communities, rather than try to maintain long power lines from the main grid.
And of course, nobody uses storage like that. Off-grid system must combine solar and wind, wind typically gets stronger in winter, plus about 24 hour worth of energy consumption in storage (a couple Tesla systems), and finally a diesel generator for a few percent of the total generation for the unlucky days when sun doesn't shine and the wind doesn't blow.
This results in a crazy large system due to our appalling wasteful house designs (which in turn are due to historically cheap electricity).
A better approach is to decide how much you can spend on solar, then design a house which will function within that limitation.
To succeeded you will need to change house designs, select different type and size of appliances, and most importantly, make some personal sacrifices.
If you can live within these limits, then yes, you will enjoy having Solar. If not, you would be better to continue with your existing expensive lifestyle.
Speaking only regarding residential energy: In North America, other than Hawaii as of a couple weeks ago, solar generation banks credit for electricity exported to the grid. In effect, a 100% efficient, and free, battery. In Austrlia, exported power receives credit only equal to the offset generation -- less than 1/3 of the total price of electricity. In those cases, there is an economic evaluation.
Otherwise, batteries are for reliability. And apparently there are more residential backup generators in the US than there are residential rooftop solar.
So in that situation you try to cut your consumption when you generate little power or you take the hit by paying for grid power.
The larger chunk of your bill in that case will be the termination fees, the electricity bill typically has two components, one where you pay a fixed fee for the connection whether you use it or not and another you pay per used KWh. The first component can handily outstrip the second making those very expensive KWs, especially when you're close to running of your own power most of the time.
Still, if you think of it as a replacement for both your genset and your very large battery it may make economic sense to do this anyway. It all depends on the location and reliability of the grid. Where I lived our PV system was many times more reliable than the local grid so we ended up ditching the hookup but that's a very exceptional situation.
For winter/summer cycle, if you're south of 40° N (and north of 40° S) just oversize PVs. Otherwise, you need a second power source - grid, wind or geothermal.
Not to mention that having a second source is really good if you ever plan for maintenance or emergency repairs.
Off the grid solar is unpractical the world over if you don't adapt your consumption.
I think the big problem people have with going PV, is that there isn't a good manual on how to go solar. & much of it is changing and improving as we speak.
A Potential Solution to the Problem of Storing Solar Energy – Don’t Store It.
http://euanmearns.com/a-potential-solution-to-the-problem-of...
As commenters here and on the blog suggest, this is a much more cost effective approach (although still prohibitively expensive in extreme latitudes).
Is there some context outside of these two posts that makes them reasonable things for someone to have spent so much time and effort on?
That is extraordinarily wasteful. In most of Australia the average is about 2300kW per quarter, and even that is probably a high estimate. But if 5000kW is low then I'm assuming the average has to be 6000kW, or 6500kW! That a HUGE amount of power!
What on earth are people in America using?!?
According to http://shrinkthatfootprint.com/average-household-electricity... the average (in MWh/a) is 11.7 for USA and 7.2 for Australia, both at the high end of first-world countries. It's 3.5 for Germany for instance.
I always wondered why we measure consumption in those units, instead of just using average power directly. 11.7 MWh/a = 1.33 kW.
"Households with 1-2 residents use about 1,400 kWh of electricity per quarter
Households with 3-4 residents use about 2,100 kWh of electricity per quarter, and
Households with 5 or more residents use at least 2,700 kWh of electricity per quarter."
http://www.ipart.nsw.gov.au/Home/For_Consumers/Compare_Energ...
Edit: I must be tired. My calculations are way off. Sorry about that.
In other news, name brand sneakers are also expensive and probably not really worth it.
http://www.forbes.com/sites/jeffmcmahon/2015/05/05/why-tesla...
A consumer to operate a small gas turbine and store their energy in the form of hydrocarbons. If this is to be sensible, then 43MJ/kg would have to be enough, because that's what they're going to get with kerosene/jet fuel.
Now let's consider lithium. The problem with metal-air batteries has mostly been poisoning by unwanted gases. Let's assume for a moment that a membrane material is developed that only allows high purity oxygen through, but suffers low throughput. This is no problem for our off-grid solution because we're talking about large storage relative to power, similar to some flow battery applications. Using some method of specific ion or biological style solution, such a membrane is not implausible.
Lithium-air weighs in at 40MJ/kg specific energy. How much does it cost? Given that a metal-air battery uses lithium oxide, the lithium carbonate and lithium hydroxide prices are not quite valid guides, but the price of $7/kg is obtainable for the carbonate of "battery grade." If the pure oxide can be the material used in manufacture, then possibly the $7 value is in the ballpark.
Jet-A is $0.45 per kilogram. 1.41 $/gal / 3.78541 l/gal / 0.820 kg/l
Higher efficiency must be allowed because even with combined cycle at high latitudes where it's cold, we're looking at 90% round-trip for a battery vs ~75% for our Brayton cycle + home heating.
There are external costs to a large tank of kerosene and a pile of lithium carbonate. One requires a gas turbine and the other requires a battery & significant manufacturing. Excepting those two things, we're left with a roughly 7:1 factor of cost for the raw material, so a seven year payoff it is.
Lithium might not be the cheapest rechargeable precursor. It might be great for EV's (high power output is a requirement) and maybe something else will show up as the cheapest energy storage without requiring off-grid people to rely on ARES gravity trains.
Perovskites (new paper just suggested that hot-carrier phonons can be used to raise theoretical efficiency to 66%) throw yet another wrinkle into solar. Both energy storage and generation could become quite a bit cheaper.
All hypothetical, yes, but my napkin does tell me not to rule out off-grid, not that I'm sympathetic to off-grid, anti-social Ayn Rand acolytes =D Whatever matters for off-grid matters for a ton of settings, such as places where there is no grid still. It's tough to say definitively that a straight-renewable energy source won't become cost-competitive even in grid areas of high latitude within the decade.
Source on Jet A specific heat: http://www.exxonmobil.com/AviationGlobal/Files/WorldJetFuelS...
Source on Lithum Air theoretical specific energy: https://en.wikipedia.org/wiki/Lithium%E2%80%93air_battery
Jet A price: http://www.indexmundi.com/commodities/?commodity=jet-fuel
Lithium Carbonate Price (ev grade): http://www.globalstrategicmetalsnl.com/_content/documents/40...