[ASK] Designing utility interactive system with battery backup

V3120
V3120 Registered Users Posts: 5
Hello, my name is Dede. I'm new here. I designing a utility interactive system w/ battery back up using PV array. I need some help regarding the design. I'm planning to use Honda Soltech CIGS thin film solar module dan Xantrex GT90E inverter. I have some trouble sizing the battery and charge controller.

1. The Xantrex GT90E inverter DC input range from 300 V - 650 V, does it means that my battery banks have to supply min 300 V DC ??

2. From my calculation, my battery banks must have approx 1900 Ah capacity, does anyone have any suggestion what kind of battery I should use and how to connect the battery banks ??

Thanks.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: [ASK] Designing utility interactive system with battery backup

    I am not at all familiar with the large Xanterx GT inverter product lines--So I my be completely wrong...

    As I understand the GT90E (or is it now the GT100E)--These are pure Grid Tied inverters that would only accept Solar PV panels and no battery bank.

    Are you looking for an Off-Grid inverter (one that can supply power if the utility power has failed--for emergency or off-grid use)--Or are you looking for a Grid Tied / Utility Interactive inverter which is designed to take power from a solar array and output the energy to the utility grid (spin the meter backwards and do not use any storage batteries)?

    They are really two different type of products. There are some Hybrid Inverter products that can do both Grid Tied and Off Grid functions--But the GT family is not one of those...

    Do you have a link to the product you are trying to use?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • V3120
    V3120 Registered Users Posts: 5
    Re: [ASK] Designing utility interactive system with battery backup

    http://www.xantrex.com/web/id/2713/docserve.aspx

    here is the link for the product I'm planning to use. Actually I'm looking for an off grid inverter than can use the utility power when the PV array can't supply the power. In my country, the utility provider do not support net metering. Do you have any suggestion for the inverter ??
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: [ASK] Designing utility interactive system with battery backup

    Are you in Indonesia?

    And are you trying to have Backup power (storms/utility problems), trying to save money, or trying to "go green"?

    For example, there is the Schneider/Xantrex XW hybrid inverter system. It does both Grid Tied and Off Grid emergency backup power (and Off Grid power too).

    It will output up to 6,000 watts (12,000 watts surge) of power at 120 or ~240 VAC (the 50Hz version may be a bit different specifications--if there is one yet). I am not sure, it may even support three phase (a possibility--I just do not know).

    I understand your issues of wanting to go with solar PV power--However, there is a huge cost when adding battery backup to a solar system.

    In the US--We pay (using very rough numbers) around $0.10-$0.20+ per kWHr for our utility power. For a Hybrid system, it will cost around $0.45 per kWH for a hybrid/power shifting system. And for pure off-grid--Your effective power rate is around $1-$2+ per kWHr (before any tax credits or local green rebates). Pure Grid Tied systems (like I think you are looking at) can actually be competitive with utility power ($$/kWH)--depending on local costs and any government tax credits.

    At this point, there are some very large battery systems that are being sold to utilities to address wind/load leveling/etc. issues.... Even those seem to cost at least $0.10 or more per kWHr. And these systems may use lead acid, lithium or even molten metals/salt batteries (some pretty "cutting edge" type stuff). Even then, they are still looking at 10-20 year life for their battery banks.

    In the end, at least from the back of the envelope calculations I have done, it is very difficult to justify Solar PV + Batteries + Inverters to time shift power when Net Metering is not available.

    In general, that money is (most of the time) better spent on conservation measures for the home/office (lighting, insulation, energy efficient appliances, high efficiency heat pumps for heating and cooling, shading/double pane windows, shifting heating loads like hot water and space heating to alternative fuels or heat pumps or A/C desuperheaters etc.).

    If you are still interested in using Solar PV systems... You may want to check if you can get a partnership going with the local utility (using local environmental groups, government, universities, etc.) to see if you can get your laws changed, or at least set your system up as a demonstration for Solar PV Grid Tied power (good press)...

    You will, many times, get push back from your utility concerned about whether the GT Inverter System is safe. For the most part, they are safe for the utility and there are various national standards to support their use.

    You may also find that your local electric grid cannot support GT systems for various technical reasons (some systems cannot be "back fed" electric power without tripping safety circuits). It is rare (at least in the US)--but possible. Also, there are issues of maximum rating for power distribution--A large system may need money spent on infrastructure upgrades.

    The last issue is real--And that is the utility with Grid Tied solar inverters does lose money with a true Net Metered billing system (meter turns forward, you pay for power, meter turns backwards, you get a credit on your bill).

    In the US, there are several forms of metering... For residential, many times is just one meter that turns forwards or backwards--and at the end of the month, either money is paid by the customer or a credit goes "into the bank".

    Another form is use for commercial/businesses (at least in California)... Very roughly, about 1/2 the bill is based on how much power is consumed (pay the generator/fuel fees). The other 1/2 is the "reservation charges" -- Basically the 15 minute peak power used by a company in the last year. These types of billing plans can really mess-up a solar grid tied system rate of return on investment.

    The problem is that for some places, the reservation charges actually go up with solar power (the peak power output of solar is higher than the peak power used by the system)--Some school districts got hit by that in California a few years ago... Their power bill with GT solar was nearly equal (if not higher) with solar GT + increased reservation charges + loan payments than it was before they installed the systems (and they had huge government green credits too and still "lost money").

    You really need to look at your total needs and look at the risk/reward for any investments you choose to make.

    Solar PV sounds nice--but like any expensive project--you really need to be careful before spending the money.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: [ASK] Designing utility interactive system with battery backup

    the subject of being gt is moot as he said he is not allowed to be grid tied there. he is looking purely at off grid and the inverters he has shown are not for off grid. i also must ask the op to be sure of what the frequency and voltage of the ac appliances there that you plan on running for the inverter, be it off grid or a hybrid type used as off grid, as the voltage and frequency must match. the op will need batteries and at least one controller as the inverter will not run on just the pvs.
  • V3120
    V3120 Registered Users Posts: 5
    Re: [ASK] Designing utility interactive system with battery backup

    @Bill

    Yes, I'm in Indonesia.

    I'm trying to "go green", actually I'm an apprentice in a manufacturing industry, I got a project from my boss to do a feasibility study and cost study on PV system for the waste water treatment at the factory. So I am trying to design a PV system that can use the utility power when the PV array can't supply the power to the loads.

    I'm still looking information for the probability for net metering in my country, but the chance is slim.

    I know it's very expensive to use PV system, rather than using the utility power. But I still have to complete design whatever the result.

    I already read the XW hybrid inverter spec, but my PV system will produce about 60 kW, so the XW hybrid inverter isn't suitable. I'm looking for another hybrid inverter that can handle the power from my PV array. Do you have other solutions or suggestions ??

    Thanks

    @niel

    The loads mostly consist of water pumps using 3 phase 380 V motor.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: [ASK] Designing utility interactive system with battery backup

    I am going to guess that you may have better luck looking for manufacturers out of Germany. They do some interesting things over there.

    Perhaps some other folks here are more familiar with large battery backed 3 phase solar Hybrid power systems than I...

    Also, see if you can find anyone that has done large battery backed computer installations in your region--perhaps you might find some local engineering and sales talent that can work with you (better than me XX,000 miles away).

    Short of just starting with Google and looking for manufacturers--I don't have any better information.

    Sorry,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    Re: [ASK] Designing utility interactive system with battery backup

    I see no reason for battery based system at all. Pure grid-tie will work and is cheapest option. You just have to make sure your plant always consumes more power than available from solar. So no power will flow into the grid. You need to find a device that would monitor power load of your plant and turn on extra pumps if power demand falls too close to zero.

    If utility still refuses to allow grid-tie inverter to be connected, you could power additional pump motors from Variable Frequency Drives (VFDs) with their DC current bus fed directly from solar panels. Theoretically this will work if suitable motor control algorithms are used. But it should be easier to just convice your utility or government to allow grid-tie PV.
  • V3120
    V3120 Registered Users Posts: 5
    Re: [ASK] Designing utility interactive system with battery backup

    Ok then, let's say that my utility company provide net metering. Do I still need battery banks to stabilize the output flow to the grid ??
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: [ASK] Designing utility interactive system with battery backup

    No--Not at all.

    Basically Solar Panels to Grid Tie (Utility Interactive) Inverter to your Utility Feed Panel.

    The utility AC grid (AC Generators, Off-Grid Inverters too) does look a lot like a giant AC battery bank. The grid is very low impedance (resistance). Or, in electrical terms, is a Voltage Source.

    A Grid Tied inverter, on the other hand, is very high resistance source (or Current Source). Battery Chargers are also (to a degree) current sources too (the battery voltage is set by the voltage, not by the battery charger--unless it is very large and overwhelms the battery current capacity).

    Basically, the GT inverter simply follows the AC Voltage Wave Form and "injects" current into the AC Mains (it will raise the local voltage a bit due to "voltage drop/increase" because of the addition of current into your wiring).

    From your "systems" point of view--this is a parallel energy source into your electrical system.

    Generally, for commercial systems, say you have a main panel--You have 200 amp service coming in, and you have 200 amps from your PV system. So, you need to have a 400 Amp rated panel (because of the two power sources).

    So, in that panel, you have 200 amps from the utility, 200 amps from the PV system, and you have your loads... Assuming your loads are between 0 and 200 amps, the power for those loads will be supplied by the sum of your PV and Utility power... If your load is less than than the power supplied by the PV system, then the excess energy (current) will be pushed back out your 200 circuit breaker back to the utility grid.

    In general, at least for "home sized" systems--A Grid Tied system costs about 1/4 the price (per kWHour) of a similarly capable Off-Grid (Battery Backed) system.

    You have the extra losses of charging the battery bank (extra losses of 20% for flooded cell, 15% for the extra battery to inverter), and the costs of replacement batteries every 5-15 years (depending on battery type, cycling frequency and depth, etc.).

    For an end to end efficiency--you can assume around 77% of solar panel to AC output (solar panels STC ratings tend to be higher than real life usable power except in very cold locations/seasons). For Off Grid Systems (where you are charging during the day and discharging at night)--Your Solar Panel to AC output is around 52% efficiency.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: [ASK] Designing utility interactive system with battery backup

    batteries come into play only if the grid shuts down for any reason and you would want to still power things in the grid's absence. straight gt systems (no batteries) lose all power when the grid goes down even if you have pvs.
  • V3120
    V3120 Registered Users Posts: 5
    Re: [ASK] Designing utility interactive system with battery backup

    Hmm.. What if I have a back up generator in case the utility power down ?? I should use the hybrid inverter then ??
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: [ASK] Designing utility interactive system with battery backup

    Generally, you would use a UPS (Uninterruptable Power Supply--Basically a battery charger to battery to off-grid inverter--Many have an AC bypass switch that allows the load to run directly from the AC line power and only switch to the inverter in the event of a power failure) to power your computer equipment and a genset to power the UPS + Large Pumps when the power fails.

    The genset will take a few tens of seconds (more or less) to start and get up to power before it will power your loads--So the UPS powers just a the critical equipment during the power failure.

    You might be able to run a GT inverter to supply power to the genset--but that depends on the generator being accurate enough frequency (XXHz +/0 0.1%).

    Also, you would need to work out with the generator and load management folks the GT inverter operation... Gensets generally do not like to be "back fed" power at all--And damage may result.

    A Hybrid (or off-grid system with AC charger and a bypass switch) is really just a "piece part" version of a UPS.

    I assume that this will be a large installation--Once you understand the basics, you will need a professional engineering/power engineering firm to do that design and installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset