Back-up systems for convenience stores

JMincheyJMinchey Registered Users Posts: 1
I have been asked to compile a study on providing a solar back-up power for a chain of convenience stores. The system will be installed to keep the fuel pups, lighting, POS components (cash registers, modems, computers,) My estimate this will be around 25% of the stores energy usage due to studies that show refrigeration and food service equipment account for around 75% of energy usage. Most fuel dispensing pumps are 750 watts, we have 5 of those present on large stores, then 2 registers plus modems, lighting, etc...I am experienced on small commercial and residential but this is new to me...any input is appreciated.

Comments

  • EstragonEstragon Registered Users Posts: 3,039 ✭✭✭✭✭
    To size the system, you need the following inputs:
    1. Daily power use in watt-hours. For example, a pump might use 750w on a 50% duty cycle 24hrs/day, so this load would be 750*.5*24=9000wh. Total daily use for each load for a wh/day total.
    2. Peak power required. For example, a pump might draw 4000 watts briefly as it starts. If it's possible for 5 pumps to start at once, peak for these loads would be 4000*5=20000 watts.
    3. Length of time the system needs to operate with no charging sources.

    Ideally, actual loads should be measured or sampled.

    With this info, the battery bank and supporting gear can be sized.

    Solar may not be the most cost-effective primary charging source. Presumably an outage is more likely during adverse weather exents, which may mean little sun and potential damage to panels. If the objective is to keep operational for an extended time (more than a day or so), it may make sense to use a generator capable of bulk charging batteries, and have a smaller solar array provide lower current absorb and float charging to the extent solar power is available.

    It may also make sense to consider a system that provides power for self-consumption to partially offset utility costs as well as emergency battery charging.

    Anyway, just a few thoughts to start the conversation.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • bill von novakbill von novak Solar Expert Posts: 814 ✭✭✭✭
    JMinchey said:
    I have been asked to compile a study on providing a solar back-up power for a chain of convenience stores. The system will be installed to keep the fuel pups, lighting, POS components (cash registers, modems, computers,) My estimate this will be around 25% of the stores energy usage due to studies that show refrigeration and food service equipment account for around 75% of energy usage. Most fuel dispensing pumps are 750 watts, we have 5 of those present on large stores, then 2 registers plus modems, lighting, etc...I am experienced on small commercial and residential but this is new to me...any input is appreciated.
    1) Best bet is a generator, period.  They have a long track record, are well understood, can use easily-storable fuel (like propane) and are always available.

    2) If no service interruption is the goal, then a UPS+generator is the way to go, with a manual switchover.

    3) If he really wants solar to make a statement or something, install a small standard grid tie system and just tell everyone it's part of the backup.  If you want the option to use solar during an outage, then keep string voltages below 250/300 volts to allow for the option of using a Classic 250 or Flexmax 100 to charge the batteries in the UPS.
  • BB.BB. Super Moderators, Administrators Posts: 28,184 admin
    You really need to know the total energy usage by day, over the whole year... Solar power is expensive and you don't want to over design a system (costly) or under design (waste of money if it does not do the job).

    Watts is a rate (like miles per hour)... Watt*Hours is an amount (like miles driven). So if a 750 Watt pump runs 6 hours per day that is (750W*6h=) 4,500 WH per day (or 4.5 kWH per day).

    Electricity is billed in kWH--Typically between $0.10 to $0.40 per kWH (depends on what state the systems will be installed and their local energy polices).

    More or less, I suggest that a full time off grid system will cost around $1.00 to $2.00+ per kWH--More or less (capital costs and installation costs, replacing batteries every 5-7 years, fixing electronics--charge controllers & inverters every 5+ years, replacement every 10+ years), around 5-10x their current cost for power. Most businesses are pretty cost sensitive--And that will be a huge issue for them. And they "pre-pay" those system costs for 10 years up front (unless you have an MBA that can get green subsidies).

    Other issues--In decades past, service stations (with fuel pumps) had more stringent electrical requirements--Not sure how that will affect the hardware you need to install (explosion proof hardware?).

    Most solar power systems need a genset for backup power (when weather goes bad for a few days in a row). And solar panels are not the most storm resistant of items (sea of thin tempered glass panels on roofs).

    My suggestion as to what will be both "green(er)" and more cost effective... Install a grid tied solar array (solar panels=>GT inverter=>main electrical panel + Utility Metering per local requirements). Very common in our area ("solar powered" gas stations). You have the utility and low cost of grid tied power--And you have the (frequently) lower cost of GT Solar Power (probably below $0.10 per kWH).

    Add a backup genset for this power (probably somewhere around $1.00 per kWH for genset+fuel costs--Where fuel cost is incurred only during actual power outages for a couple hours to a few days a year--Typically). Still get the "green energy" with GT solar, possibly save some money on power costs, and a completely separate backup genset for emergency power. A much more cost effective solution (in general).       

    Please note--GT Solar relies on local State Public Utility Commissions' rules that are presently in place. Initially, the subsidies paid for by rate payers were tilted towards the folks that installed GT solar (GT customer got to buy and sell power at retail prices, very low connection fees). Now, some states are approaching the 5%+ of their grid power being GT solar... And are reducing subsidies (i.e., monthly connection fee goes from $5 per month to $10 per month in my home's case--In a few locations is it approaching $100 per month, and when you sell power back to the utility it is at $0.05 per kWH or less). And some states like Hawaii & Nevada have stopped new GT solar (Hawaii) or completely removed subsidies (Nevada).

    Also, there are many smaller utilities (co-ops and such) that do not allow GT solar. Also--Solar power does not really work if there is any shading on the panels (power lines, nearby buildings/trees/deep valleys, etc. that block sun during parts of the day). Add marine layers, local climate, etc.... Each location needs to be assessed for regulatory and a "solar inventory" to determine if it is possible/practicable to install solar (GT or Hybrid GT+Off Grid solar).

    If you can get a daily (seasonal?) energy usage (kWH per day) and a sample location, we can go through a quick back of the envelope set of calculations to size a system and point at some hardware to get an estimate of costs.

    Please note, in general, there are lots of companies that install and service gensets... And a fair number of companies that install GT solar... Not a lot of companies that install and service Hybrid solar (GT+Off Grid or pure Off Grid solar) power systems. Also, you can run into issues with GT solar power--The utility has to have the distribution network inplace to accept your solar power generation (do an engineering study of the local system). For the most part, systems under 10 kWatt (10,000 watts of solar panels) are "small systems"... If you go over ~10 kWatts, you may end up in another nightmare (more regulatory hoops to jump through). And in some cases, comercial billing/rate plans actually increased power costs with GT solar:

    https://forum.solar-electric.com/discussion/comment/93113#Comment_93113

    GT Solar power systems are highly susceptible to local politics... In Nevada, one day there were lots of companies selling and installing GT solar power systems, then the next day, zero new installations. In Canada (Ottawa province as I recall)--They gave people permits to install GT solar farms, but then delayed or refused utility connections when the time came to "go live" due to political and engineering issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,137 ✭✭✭✭✭
    My input is have you considered going solar?  Just kidding!

    My real input is have you considered what happens in a utility outage in your area. In many cases the Police (for whatever reason) want people to either leave/stay off the streets in an evacuation (fire, earthquake, named windstorm or ?) or they want control of a situation by shutting down Utility power. Public safety with energized power lines down and they are coming to you?

    Do you really want your employees exposed to a situation where the local power is out and they are the ones to deal with a possible dangerous event?

    Maybe you can shed some light on why, where, and what for?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • oil pan 4oil pan 4 Solar Expert Posts: 759 ✭✭✭✭
    What kind of service does the store have?
    If it's 480v 3 phase power, don't bother.
    480v 3 phase USP systems are really, really expensive, like hundreds of thousands of dollars. Then you have batteries that will need to be replaced every few years.
    Get a generator and a transfer switch.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • BB.BB. Super Moderators, Administrators Posts: 28,184 admin
    I believe the original poster is out of Florida (possibly Alabama).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tr0ytr0y Solar Expert Posts: 86 ✭✭✭
    This is a much better place for a generator ,maybe roof mounted 10 to 15K watts and diesel preferred as the fuel stores better. The nice part is you will have fuel on-site, if something is bad maybe pull the fuel you will need into cans or a spare tank.  If you want to throw in a small solar system that maintains a UPS type system until the generator is up and producing power. Your refrigeration need is the killer here. Solar sounds more fun but my guess it wont be as effective. 
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