what's the difference?

i would like to know is there any difference in designing a PV system for
Backup(4-6hrs) and complete off grid(24hrs)

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: what's the difference?

    First--It is all about the loads... A backup system only has to run long enough until power is restored (afternoon outages in many countries/regions) or until an alternative power source is up and running (such as a computer UPS--Run for 30 minutes maximum until the backup genset is started).

    Completely off grid--The power is supplied by batteries when needed (evening at a cabin) at a relatively slow discharge rate (say two days of discharge at C/20 rate from the battery bank at 5 hours per night, to 50% discharge, then recharge the battery bank via available power--solar, backup genset, etc.).

    The components (in theory) are the same between both types of systems--But in practice, the actual choices are different.

    A UPS system that can discharge in 30 minutes needs a battery designed to do this. For lead acid, that is usually an AGM or GEL type battery. And it does not matter too much how quickly the battery is recharged. AGM can take very fast recharging, where GEL usually take C/20 recharge. Also, for a true UPS system, it is not unusual to replace the entire battery bank ofter a major outage or for the batteries to only last a few discharge cycles before replacement (UPS system batteries are designed to "float" for 1-2 years, or upwards of 25+ years for phone company systems, and if discharged by more than 15-25% or so, be replaced).

    Also, sometimes, other tradeoffs are made. UPS systems, many times, use MSW (modified square wave) AC inverters for AC Backup power. MSW can cause many power supplies to overheat, but the 15 minutes it takes to start the backup genset or safely shutdown the computer, means that the excess heating that can occur with MSW powered devices is not usually a big issue.

    The converse of designing an Off grid system... The battery bank has to recharge quickly (the amount of power removed is typically fully replaced in the ~3-5 hours of solar power available the next day. That means usually we are looking at C/10-C/8 rate of charge--Or even higher (such as when using a backup genset during bad weather--high charge rate to reduce genset run time and keep the genset heavily loaded for best fuel efficiency). Deep cycle batteries designed for off grid power use will typically take higher rates of recharging, and obviously need to withstand many more deep cycles without failing when compared to batteries designed to "float".

    In summary, the two systems have pretty much the same components, but the actual components chosen are different between short term/random usage of a UPS system vs the day in/day out needs for an off grid system.

    There are lots of compromises made when designing any power system...And we use a lot of rules of thumbs to design the basic system (give us your power needs, we run through some math to size the components, then you go through the available hardware that meets your needs and bank account's capabilities). The rules of thumbs are used because they take into account of various load types (i.e., continuous low current loads of lights, laptop computers, tv, etc. vs those of things with lots of surges/short term use--Well pump, refrigerator, microwave oven, etc.).

    We can talk about the various general design requirements--Or if you give us the basic information about loads, we can work together to design a system for your needs.

    In general, I prefer to design a system for your needs. It is much less frustrating for you and us. Once you have been through one design cycle--The pieces fit together in your mind (the basic education part), and then we can get into the details of tradeoffs and what different systems may require depending on different/larger/smaller loads.

    Make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: what's the difference?

    Nice, long answer Bill.

    I would have said "time". :D

    For any given load the Watt hours increase with time: Watts over hours.
    If you run a 200 Watt (average) load for 6 hours that's 1200 Watt hours. If you run it for 24 hours it's 4800 Watt hours.
    So the battery bank increases in size and thus so does the recharging capacity needed. The back-up power system may be easily recharged from the grid when the grid comes back on. The off-grid system needs solar (and/or other sources) because there is never any grid.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: what's the difference?

    i'll simplify it even more for you.
    the 24hr system will need 3x to 4x the amount of pv and battery capacity than one designed for only 6hrs. i do lean closer to 3x because most often loads are reduced when one sleeps.