A possibly hare-brained scheme

ggunn
ggunn Solar Expert Posts: 1,973 ✭✭✭
in Wind Power Generation #1
We have been approached by a potential customer who wants to put in 75kW of PV and 75kW of wind power to connect to the grid via a PPA. The utility has stated that they want no more than 75kW to flow from his installation at any one time, so he wants us to design a system which will take all his wind power and then however much from his PV necessary to make up the difference, or vice versa.

Please hold your comments about the importance of evaluating the wind resource before proceeding, which is indeed a valid concern, but not relevant to my question. I have never heard of a system that will govern the output of a combined PV and wind installation to a predetermined power level, or even one of them singly. You?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: A possibly hare-brained scheme

    the turbines would have to be connected at all times or they will free spin and self-destruct. that means they will disconnect the pvs when wind is producing. wind doesn't produce well, along with high maintenance, and solar is limited by the amount of sun falling upon the pvs. imo they won't like wind, but i will say try a single turbine, (say no more than 20kw) and go with the rest as pvs and let the combined total of both not exceed 75kw to satisfy the ppa requirements.

    later on if wind fails, does not produce as expected, or just winds up being too expensive to maintain then later one can add more pvs to it while scrapping the turbine. if the turbine does acceptably then you can add more turbines later and figure out a way of switching the pvs out of circuit then, but i doubt wind will do as well or as economicably.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #3
    Re: A possibly hare-brained scheme
    niel wrote: »
    the turbines would have to be connected at all times or they will free spin and self-destruct. that means they will disconnect the pvs when wind is producing. wind doesn't produce well, along with high maintenance, and solar is limited by the amount of sun falling upon the pvs. imo they won't like wind, but i will say try a single turbine, (say no more than 20kw) and go with the rest as pvs and let the combined total of both not exceed 75kw to satisfy the ppa requirements.

    later on if wind fails, does not produce as expected, or just winds up being too expensive to maintain then later one can add more pvs to it while scrapping the turbine. if the turbine does acceptably then you can add more turbines later and figure out a way of switching the pvs out of circuit then, but i doubt wind will do as well or as economicably.
    I certainly share your doubts about the efficacy of wind in this scenario, especially where he is (southeast Texas nowhere near the coast), and I realize that it would be much better to throttle back the PV than the wind generator since PV doesn't mind an open circuit. The problem is how to make the output of the PV respond to the monitoring of the combined system and regulate itself so the the total will not exceed 75kW. I could, of course, size the system so it won't exceed 75 kW in any case, but that's not what the guy wants.

    The design problem is to limit the PV output current to a moving target established from monitoring the output of the combined system. I don't know of any way to do that. Sunny Boys can ramp down output in response to frequency munging by a Sunny Island in a microgrid, but this is a grid tied scenario so that's not an option.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: A possibly hare-brained scheme

    with current equipment hardware i don't know if that is possible to achieve as you are talking of a high power ac current limiting device and that does not discriminate between which shall take presidence, as it would limit in general and not just gate the turbine output. imo this is not worth pursuing as it would no doubt be extremely expensive to pull that off, if it's possible at all to do it.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #5
    Re: A possibly hare-brained scheme

    Your issue will be there is no UL1741 approved solution to have gridtie in the manor your asking about. Building a home grown solution won't cut it.

    The only way for this to work is using a buffer and the inverters sell from the buffer. For example, you would have 12 XW-6048's that could sell 72kw to the grid from battery's and wind and solar feed the battery's.

    You would need about 7000ah of 48V for this to work right, that alone will set you back 75k easy and expect to replace the bank every 5-8 years from wear and tear.

    You will also need probably a 100+kw dump load for the wind to prevent over-speed as the grid cannot be relied on for safety of the turbine for the load ( or an approve furling method on the prop



    ggunn wrote: »
    I certainly share your doubts about the efficacy of wind in this scenario, especially where he is (southeast Texas nowhere near the coast), and I realize that it would be much better to throttle back the PV than the wind generator since PV doesn't mind an open circuit. The problem is how to make the output of the PV respond to the monitoring of the combined system and regulate itself so the the total will not exceed 75kW. I could, of course, size the system so it won't exceed 75 kW in any case, but that's not what the guy wants.

    The design problem is to limit the PV output current to a moving target established from monitoring the output of the combined system. I don't know of any way to do that. Sunny Boys can ramp down output in response to frequency munging by a Sunny Island in a microgrid, but this is a grid tied scenario so that's not an option.
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  • arkieoscar
    arkieoscar Solar Expert Posts: 101 ✭✭
    #6
    Re: A possibly hare-brained scheme

    Since the voltage is fixed, couldn't you use CT's to monitor current and add or subtract PV inverters as needed? Since the utility would have to approve, you would probably need an engineering firm to design the system with some fail-safe features.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #7
    Re: A possibly hare-brained scheme
    arkieoscar wrote: »
    Since the voltage is fixed, couldn't you use CT's to monitor current and add or subtract PV inverters as needed? Since the utility would have to approve, you would probably need an engineering firm to design the system with some fail-safe features.
    That's the path of thought I have been going down sitting here staring off into space - some sort of relay assembly to take strings or inverters off-line in response to rising power on the output. Then there's the control program and the device drivers. Anybody want to write some software?
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