Is load shifting always a financial loser?

danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
After due deliberation, and just deciding that it is righeous to dip into the thisCouldBeAFunProject account (not just the makesFinancialSense account), I have contracted for an AC coupled system. Per prior discussions on this forum
(http://forum.solar-electric.com/showthread.php?16354-Grid-interactive-what-benefits-make-it-worthwhile/page5&highlight=load+shifting),
the operative reasons are to provide a most-of-house UPS, and support during extended outages, for a set of critical loads.

Yet maybe load-shifting (time of use) is not a totally silly idea; an idea that battery degradation makes untenable.

Let's start with a comment from the inestimable BB
If you time shift (use battery power during the day), you not only have the cost of battery charging (typical AC to DC charger is around 80% efficient, battery ~80-90% efficient, and re-invertering around 85-90+% efficient = ~54% worst case efficiency to power shift) and you are cycling the batteries, which have a limited cycle life too (around 500-2,000 cycles, depending on how deep you are cycling, type of battery, how expensive of battery, etc.)...
Or, if charger/inverter/storage efficiency is 90% (AGM battery and Magnum MS-PAE inverter), 73%.

Now, my utility (PEPCO in suburban MD) is not great but not terrible. As a rough guess, I am assuming over the
next 10 years, transient and long-term outages will eat about one hundred 50% DOD charge cycles (or equivalent, per http://currentgeneration.co.nz/site/current/files/Partial%20AC-coupling%20in%20Minigrids.pdf).

Well, that's a lot less than the 500 or so 50% DOD cycles for a typical AGM battery. And if pure aging (even if the battery is always maintained at float)
causes the battery to just die in 10 years, that suggests 400 cycles are just wasted.

IOW: assuming that battery usage for UPS/outages is relatively low (albeit real nice when it is needed)....
under a limited number of load-shifting discharge cycles (say, 50 or so per year), the battery degradation losses
might be minimal. And if efficiency is ~70%, and assuming a base rate of $0.15/kwh, then a time of use rate over $0.21 would justify time-shifting.

Now, "limiting" this number of load-shift cycles might be untenable (at least with the current state of the 'smart' grid). And if there are a lot of
load-shift cycles, so that battery depreciation is effected more by cycling than by age, I expect the numbers would get a lot worse
(don't feel like doing the math right now).

Still, if done properly ... perhaps time shifting is worth considering.

Or am I missing something?

I.e.; I assume a more precise equation, that integreats over degradation rates and expected lifespans under x y z conditions, would yield results of the same order of magnitude.
Perhaps that is false.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: Is load shifting always a financial loser?

    I am not in the business--But I would estimate (see, I am not inestimable :p) maybe around 5-8 years of float/some cycling service for a good quality AGM. And given that AGMs cost something like 2x as much as a similar grade flooded cell--they do make the trade-offs a bit more muddled. Good forklift batteries will last 15 to 20+ years--But more maintenance/distilled water costs.

    It is a bit of a catch 22... Batteries that are designed for 20 year float service (I think I remember reading that is discharging to 85% state of charge or only drawing 15% off the top in "normal" duty) will not do that well in deep cycle. And deep cycle batteries do not do that well in float.

    If you are putting in a system purely for time shifting--Then it will be difficult to justify.

    If you are putting in a solar PV system for afternoon power outages and possible longer term power hits, and you can do Grid Tied / Net Metering back into the grid with your utility--It is not too bad (maybe $0.45 to $0.75 per kWH for hardware+batteries+maintenance + whatever your off-peak power costs for recharging).

    The basic equation I use is (I use 20 years--Because 25-35 year panel life just is too much for a "reasonable" expectation for most of us to live in one place):

    $/kWH = (cost of installation+labor+replacement batteries over 20 years+replace inverter/chargers after 10 years + 20 years * shifted kWH*1/system efficiency (if using grid/generator for power) + Interest+property taxes)

    You can do a similar equation for generator + fuel + oil + maintenance costs...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Is load shifting always a financial loser?
    danielh wrote: »
    Now, "limiting" this number of load-shift cycles might be untenable (at least with the current state of the 'smart' grid). And if there are a lot of
    load-shift cycles, so that battery depreciation is effected more by cycling than by age, I expect the numbers would get a lot worse
    (don't feel like doing the math right now).

    Still, if done properly ... perhaps time shifting is worth considering.

    You also have complete freedom to decide how much you cycle the batts every day, so you could optimise this to find the sweetspot between battery life vs. energy payback.

    I think you've covered all the ground, but perhaps the only variable you haven't considered is the AF (Awesomeness Factor). Some solutions like a straight grid tied solar system have a clearer financial payback but have a relatively low AF of around 1.5 (Using IEC values). An AC coupled battery backup system tends to be more complicated to justify on pure financial grounds but has a higher AF of around 1.8. Throw in load shifting and your AF goes to around 2.3 at least ;)
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Is load shifting always a financial loser?

    BTW, SMA have announced a 5kW grid tied inverter with built in Lithium Ion battery, specifically for load shifting. Not available yet as far as I know.
    http://en.sma-sunny.com/2012/08/07/smart-home-with-solar-electricity/
  • Eric LEric L Solar Expert Posts: 262 ✭✭
    Re: Is load shifting always a financial loser?
    You also have complete freedom to decide how much you cycle the batts every day, so you could optimise this to find the sweetspot between battery life vs. energy payback.

    I agree; if you have a shallow battery discharge to, say, 90% SOC the battery life is not going to be nearly as significantly as affected, and as you suggest you might find it would make sense to do so relative to letting them just wear out from old age. Depending on the bank size and your usage, that 10% of battery power could amount to several hours/day of peak-rate use.

    I know you said this project was contracted, but if you can, try to get a sense of local battery distributor pricing, to make sure your contractor is getting the best prices for you. There seem to be a lot of regional variations in battery prices, I think in part depending on how close you are to a major port. But I found that there were also significant (think 40-50%) local differences in battery prices among distributors, especially once the haggling started. By the way, I additionally found AGMs were only about 40% more than FLA batteries locally, not two times more. Getting a good price on your battery bank now will make a massive difference to the financial viability of the system; likely more so than the load-shifting will.
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Is load shifting always a financial loser?

    Hi estimable Bill!
    BB. wrote: »
    I am not in the business--But I would estimate (see, I am not inestimable :p) maybe around 5-8 years of float/some cycling service for a good quality AGM. And given that AGMs cost something like 2x as much as a similar grade flooded cell--they do make the trade-offs a bit more muddled. Good forklift batteries will last 15 to 20+ years--But more maintenance/distilled water costs.

    It is a bit of a catch 22... Batteries that are designed for 20 year float service (I think I remember reading that is discharging to 85% state of charge or only drawing 15% off the top in "normal" duty) will not do that well in deep cycle. And deep cycle batteries do not do that well in float.
    I should clarify that the system will primarily be grid-tied; the battery stuff is an addon (not used 99% the time).
    For a variety of reasons, AGM is way I am going: less outgassing (they will be in a basement near ng boilers), more robust
    to higher discharge rates, etc.

    Also it was my understanading that AGM's were better for imy kind of install: where they sit dong nothing most of the time (in float), while FLA's were better in cases where there was a lot of cycling?
    If you are putting in a system purely for time shifting--Then it will be difficult to justify.
    Nope, due to battery degradation I had decided against any time shifting... until I realized that I might be able to SOME time shifting without effecting battery degradation.

    Actually, thinking some more: let say 500 cycles at 50% (or 2500 at 10%). To make the math easy, say 10kwh of capacity (200AH @48v, rounded up)
    So that is 5kwh per cycle x 500 (or .1 x 2500)=2500kwh. So unless the time of use differential is huge, a well planned time-of-use strategy would net just few hundred $$. Still, I might as well investigate the possiblities (if only for AF reasons)



    The basic equation I use is (I use 20 years--Because 25-35 year panel life just is too much for a "reasonable" expectation for most of us to live in one place):

    $/kWH = (cost of installation+labor+replacement batteries over 20 years+replace inverter/chargers after 10 years + 20 years * shifted kWH*1/system efficiency (if using grid/generator for power) + Interest+property taxes)

    You can do a similar equation for generator + fuel + oil + maintenance costs...

    -Bill

    That may be the biggest unknown in the whole decision: if we move in 6 to 8 years, does the house sale price reflect these wonderful pv panels/ batteries. I decided to just not think about that.
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Is load shifting always a financial loser?
    Eric L wrote: »
    I agree; if you have a shallow battery discharge to, say, 90% SOC the battery life is not going to be nearly as significantly as affected, and as you suggest you might find it would make sense to do so relative to letting them just wear out from old age. Depending on the bank size and your usage, that 10% of battery power could amount to several hours/day of peak-rate use.

    I gotta ask this: as noted in the paper from .NZ that I linked to in my first post: given the cycles/DOD charts show that the total power output (say. kwh output over working life of battery) is farily constant over different DODS'... why should aiming for 10% DOD buy you that much. In fact, if it means you can get by with a smaller battery pack: why not permit > 50%?

    IOW: the important metric of degradation is total power produced by the battery, and not so much whether it is done in big chunks (50% DOD or even greater) or small ones (10% DOD or so).
    I know you said this project was contracted, but if you can, try to get a sense of local battery distributor pricing, to make sure your contractor is getting the best prices for you. There seem to be a lot of regional variations in battery prices, I think in part depending on how close you are to a major port. But I found that there were also significant (think 40-50%) local differences in battery prices among distributors, especially once the haggling started. By the way, I additionally found AGMs were only about 40% more than FLA batteries locally, not two times more. Getting a good price on your battery bank now will make a massive difference to the financial viability of the system; likely more so than the load-shifting will.

    At this point, it is too late. He claimed to be getting a good price via his social networks. Given the package deal nature of the whole install, and that it has taken 5 weeks to research/price/decide on whether or not to add ac coupling to the basic grid-tied system, I think it is best NOT to micro negotiate this point.
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: Is load shifting always a financial loser?

    For Lead Acid, it appears that if you get 2 times the battery bank, it will last 2.x times longer. So--The costs are similar. But you do have longer between change-outs.

    On the other hand, if you have an "oops" where somebody left some loads on when away from the home--The bigger bank can be killed just as easily and still cost 2x to replace.

    For portable use (RV's, etc. that have space/weight limitations) or seasonal cabin use where the batteries are only used on weekends/few weeks a year--Then why not discharge to 50% or even 20% state of charge--The batteries will probably age out before they wear out. A good sized array (and/or backup genset) to quickly recharge will still take good care of the battery banks.

    One issue with "balancing" the battery bank to load/charging capacity--You don't want to discharge them too quickly (faster than C/8 or so), flooded cell batteries are less efficient. Similar with surge current--if you have motors/well pumps/etc., then you need a minimum battery bank size to support surge current (C/2.5 or so).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Is load shifting always a financial loser?
    stephendv wrote: »
    You also have complete freedom to decide how much you cycle the batts every day, so you could optimise this to find the sweetspot between battery life vs. energy payback.
    Does the magnum ms-pae series have good features for doing such "time shifting". Or is more stuff (control hardware/software) needed?

    I think you've covered all the ground, but perhaps the only variable you haven't considered is the AF (Awesomeness Factor). Some solutions like a straight grid tied solar system have a clearer financial payback but have a relatively low AF of around 1.5 (Using IEC values). An AC coupled battery backup system tends to be more complicated to justify on pure financial grounds but has a higher AF of around 1.8. Throw in load shifting and your AF goes to around 2.3 at least ;)
    KA ZING! You have outed me!


    I might have put the AC coupling bonus at greater than 0.3, since when one explains how "one can not use PV power during outages", everyone expresses astonishment. With AC coupling, one can turn that astonishment into a but-we--beat-the-system vicarious accomplishment. Or is that already factored in?

    That said, where can I find a complete list of IEC values :p
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: Is load shifting always a financial loser?

    The magnum is not authorized to sell back to the grid. It is good for ac coupling as long as you don,t sell to the grid with it. I have a magnum that I am in the process of ac coupling but won,t have it finished for a while. :Dsolarvic:D
Sign In or Register to comment.