Solar fantasy — One more time. It cannot work.
Solar, wind and battery storage.
There is a problem with energy payback, and this cannot be unlinked from cost. You cannot run an electrical grid where your life cycle energy cost of manufacture and installation may be near or even higher than the amount of energy the system can generate. Let me outline the problem using the example of the 3 day battery standalone system being sold now by Solar City. This is sometimes called the “off-grid” problem.
Solar City has a system they sell. It stores 3 days of power in batteries, because there are low solar energy periods where you can’t produce enough power. (Winter, cloudy times, etc.) That system charges those batteries in 7 hours. There is an average of 11 hours of usable sunlight in an average day. (We will flatten this calculation to averages for our purposes, although the real numbers make it worse than this. Peak demand is actually critical for energy supplies, but we will ignore this to make another fundamental point.)
If it takes 7 hours to store 3 days worth of power while still operating the home, then at most, the average power consumption of the house for 1 day must be 7hr/3days = 2.33hr/day of energy storage. This means there is 11hr — 2.33 hr = 8.66 hr per day of excess PV electrical generation. So, 8.66hr/11hr = 78% of the PV energy thrown away unused from that system. That means that 1–0.78 = 0.22 (or 22%) of the PV energy potential is actually used on a normal basis. This is the level of oversupply required to run our home mini-grid that can run on storage for 3 days.
A best manufacturing efficiency solar panel takes ~3 years to generate the energy required to make it. But that’s an optimal installation that assumes 100% of potential PV energy is collected. In practice, it’s somewhat less, but let’s assume really excellent over 50 years, and it averages 90%. So 3yr/0.90 = 3.33 years to give back the energy to make it. Then, we have to divide 3.33yr by 0.22 (our 22% in the previous paragraph) to get the time required for the system to give back in usable energy what was required to make the PV solar panels. 3.33yr/0.22 = 15.15 years. It will take a bit over 15 years in an excellently operated installation to give back the energy that was required to make the solar energy power generation system.
A solar-on-grid installation (which collects and uses all of the available PV energy) can pay back financially in 5 years taking into account tax incentives, depreciation, etc. Doesn’t happen for everybody, but let’s accept that. In our above system, the PV part of it would pay back in 5yr/0.22 = 22.7 years. (Most PV on grid systems are sized for 20 years.) So here we see that energy cost payback time of 15.15 years is ≈ 68% of the financial cost payback time.
However, we aren’t done because we have to add in the battery. Those batteries Solar City is selling have 10 year warranties. They should have ≈12 year life spans. But financially, those batteries take 38 years to pay off their financial cost. Usually, cost is a fair stand-in for energy required to produce. We will assume that financial payback has the same ratio of energy payback time that solar cells do. So, we will assume 68% x 38yr = 25.9 years.
So, if we assume an absolute best case 50 year life span for our solar power system and no major PV degradation (which is plausibly defensible based on Swiss long-term numbers for wafer solar — -though those long-term numbers are not for thin-film which has a claimed life span of 20 years), how many battery sets will we need to replace in that time? Assuming it’s 12 years per battery set, we have to pay for the initial battery system plus 3 more battery sets at year 12, 24, 36, and 48. If the batteries are 25% of the initial system cost, and 50% is PV and 25% is installation, then your total 50 year system cost will be: Battery cost x 4 + PV cost x 1 + 3 x battery installation cost. Let’s assume battery installation labor etc is 3% of the initial price. Add it up in percentages of the initial system and you have:
(25% batteries x 4 batteries) + 25% installation + 50% PV’s + (3% replacement x 3) = 184%. The total financial system cost is 1.84 times the initial cost. (I leave as an exercise for the reader to figure out the total system cost for a 20 year PV system.)
But, energy payback on batteries is worse than this. In theory, 92% of the energy that goes into the battery should come out. In practice, because load doesn’t fit the optimum discharge of a battery, it’s observed to be ≈80%. I haven’t put that figure in, assuming that it’s already contained in the above estimates of charging time. However, it quite well may not be.
We have figured out that our energy payback period on the PV’s for this system is 15.5 years and our energy payback period on the batteries is 25.9 years.
So, 4 battery sets x 25.9 years/battery set = 103 years. Add the 15.5 years for the PV’s to pay back their energy to that, and subtract the amount of PV energy net produced by the PVs in 50 years.
103 years for batteries + 15.5 years for PVs — (50 years — 15.5 years) = 84 years to pay back the energy for the 50 year life-cycle. Even if we eliminate 25.9 years for one battery set and accept some degraded performance for a couple of years, we are still stuck with an energy payback time of 58.1 years.
So, it is probable that the 3-day energy storage system would not give back in 50 years the energy required to make and maintain it. In fact, it should incur a net energy debt of 69% of the energy that it produced over 50 years, or in the light degraded performance scenario, a debt of 16% for that 50 year period. Below is a graph of solar energy production during the day versus load. If we could add day vs. night it would be more interesting. The more excess power this system produces, the worse the storage and excess energy production becomes.
The more batteries that you put on this kind of system, the worse the problem is, and there is incentive to try to do that if possible. Remember that a 3-day battery system is not enough to cover all the lows. In the above graph, the three day battery storage is obviously not going to tide that system over for 6 months of less energy than needed. Such a system will require energy rationing, and have blackout periods. There is also the problem that batteries do not operate well if fully discharged. EV owners learn early that if you have a 220 mile battery, it is really a 150 mile range, because the charging gets slower as the battery charge capacity goes up, so it will rarely see a full charge, and you don’t discharge fully, but leave 35 miles or so of charge. So, in the real world, is a 3-day battery system really three days? That depends, but likely not.
What this tells us is that in the real world, batteries are only useful if they have a virtually unlimited cycle life span. So if performance degrades by low single digit percentage over 50,000 deep cycles, that’s perhaps good enough. Best batteries today have 300–500 cycles of charge-discharge before rapid degradation sets in. Discharge cycle limits also tells us that in the real world any solar or wind energy system that overwhelms the capacity of the grid to accept its energy will have similar problems even if there are huge batteries. And this tells us what would have to happen for a solar or wind power generator on the grid is that the energy system operator must lock in revenue for all the “excess power” in order to make it pay off. And that is exactly what we see in energy delivery contracts. Wind farms in the UK have been paid more to not deliver electricity than they are to deliver it — -granted without batteries, but the point of this discussion is that even with batteries, huge batteries, they would need the same deal!
Our 3 day house system is a small, tractable exercise that is works the same as a regional grid does to supply energy. The overall numbers work the same, they are just much larger. Look back at the graph above. To make efficient use of the energy, batteries would need to hold the energy required to operate cities and states for 4 to 6 months of supply. Those costs become enormous.
Inevitably, industrial and consumer use of the cheap-rate “excess power” would grow, which could incentivize more solar and wind power installation. But batteries would not have the same incentive, so the battery system would, inevitably shrink as a fraction of the power mix. Probably it would shrink in absolute terms as well, simply not being replaced, because the public and private purse would not be willing to pay for replacements at scale.
Which all comes back around to dispatchable power once more. Solar and wind can be part of the mix, but over 10% they start to have problems, and over 25%, those problems become severe. Those problems get more severe as you go north for solar, because the summer peak is so much higher than the winter low.
Pumped storage? The global supply of hydroelectricity is 2.54% of the total. Where are these pumped storage reservoir facilities going to be sited? Where will the water come from?
