Solar Microhydro hybrid system

jbreezyjbreezy Registered Users Posts: 3
I'm wondering about how to maintain battery health during the year given our plans.

We have moved our farm to a new property that has good hydro 3/4s of the year.  We have done surveys and have been monitoring water flow for the past two years.  During the fall, winter and spring we will be able to produce peak output of 1900w at the controller.  During the summer we will often go to 0w on the hydro to maintain stream flow so we will also need solar.  

We have a kubota 10kw gen that we brought from our last offgrid house and is way bigger then our current setup needs.  Our biggest loads 30-40kwh a day will come during the late winter early spring when we are at peak hydro production .  My plan was to size the battery bank for the summer months when our loads are much lower 15-25kwh a day with help from the generator when needed.  

We are still building most of our infrastructure our so our loads won't meet the power being produce for a few years.  We currently have a temporary small 1.5kw offgrid system that powers our house we are still building.  We a have a 450 gallon hydronic system we can dump extra load into(likely expanding it to another 375 gallons in the next year) and our propagation greenhouse during the night will take as much extra heat as this system could make.  
Thanks for any advice.


  • BB.BB. Super Moderators, Administrators Posts: 31,425 admin

    I will take a shot at this... All of this is just SWAGs--So it may not be correct/applicable to your needs... But it is a start.

    First, roughly (nearest large city) will the system/home be located (for hours of sun per day by season).

    And going backwards through your post:

    Using solar electric panels for heating can be done--But if you need heat when the sun is not shining, then you are looking generating the heat from batteries or other methods... Solar Thermal is one of the projects that can be done pretty nicely with DIY panels/insulated storage tank/pumps, etc... But you do have to like plumbing and be really careful about pipe runs (not to trap air or hold water in winter/freezes), etc... Some nice links: Low Tech solar thermal panels (very sophisticated solar thermal system--Plus lots of information on what needed to be fixed) Low Tech DIY solar thermal shed

    As I have been going through the older FAQ posts--I have found that many of the products, especially for solar thermal systems, have "dissapeared" from the Web or become much less popular/common (such as heat pump water heaters--Sometimes issues of finding an A/C guy that can work on a heat pump water heater???). And these were posts from around 2008--Things change--And you probably want to be very careful before you take a large amount of cash and spend it on the latest system.

    Besides solar thermal... There are the mini-split heat pumps... The modern ones can be pretty efficient down below freezing (vs just resistance heating).

    I like to design "balanced" systems... Loads=>battery bank. Battery Bank=>Charger size (larger batteries, bigger charging systems). And Loads=>solar array by hours of sun per day (less sun, larger array). Also, like to size backup gensets to size loads/battery bank capacity too.

    Make starting assumptions using Flooded Cell Lead Acid batteries... You can always look at AGM, LiFePO4, or other chemistries--But to keep things simple, start with FLA.

    When sizing a battery bank, there are several conditions address... First is AC inverter capacity vs battery capacity. Assuming a typical AC inverter can surge 2x rated power (i.e., 2,000 Watt inverter can surge 4,000 Watts for seconds/minutes). For FLA batteries, the typical Minimum battery AH capacity per 1,000 Watt of AC inverter:
    • 100 AH @ 48 volt FLA battery => 1,000 Watt max AC inverter rating
    • 200 AH @ 24 volt FLA Battery => 1,000 Watt max AC inverter rating
    • 400 AH @ 12 volt FLA Battery => 1,000 Watt max AC inverter rating
    And, roughly, that is also the rough maximum of 1,000 Watts of solar array too (can do more solar--But need to watch charging current/battery bank temperature with remote battery temperature sensor for chargers--Batteries can overheat with too much charging current).

    And there is sizing the battery bank for loads... A good start is 2 days of storage and 50% max planned discharge... This is chosen for several reasons--One is hours of sun per day for FLA batteries--It becomes difficult to fully charge an FLA battery when discharged by >25% in one day with "average time" the sun is in the sky. You can choose 1 day of storage (can be typical for a weekend home/RV system). Or you can go with 3+ days of storage--But that gets expensive (more batteries, more solar panels, more space, etc.).

    Since you have a constant 1,900 Watt charging for 3 seasons--But you said "peak"? What is the average (Watts or Watt*Hours per day) harvest from hydro?
    • 40 kWH per day / 1.9 kWatt hydro = 21 Hours of hydro per day (is this right)?
    • What is your load profile like?
    • 40 kWH / 8 hours per day = 5 kWatt loads (1.9 kW from hydro and 3.1 kW from battery bank 8 hours a day?)
    • 40 kWH / 24 hours per day = 1.7 kWatt average load (vs 1.9 kWatt hydro output?)
    You can see what I am trying to figure out--Sizing battery bank vs loads vs available charging power in winter with hydro.

    For summer, similar questions--Need to better understand the load so as to design the battery bank and solar/genset charging. 24 hour per day loads, 8 hours a day working equipment, or what? Do loads have heavy starting current (pumps, machinery, compressors, fridge/freezers, etc.)?

    Why solar/hydro vs just running the genset? If your loads are fairly steady (8 hours a day, 24 hours per day, such as running fans/driers for silos/fish smoking, etc.)... A diesel genset running efficiently is probably going to cost you less than $1 per kWH for fuel (depending on what happens to fuel prices now...). Off grid solar+battery bank+hardware will cost you something like $1-$2+ per kWH (very rough numbers).

    Solar is great for smaller systems/quiet time operation... Say 3.3 kWH per day for a very efficient small home... Or a small solar system for overnight (lights, TV, cell phone charging, laptop computer, TV). And run the genset during the day for cooking / cleaning / washer / drier / shop / tools...

    Possibly start with genset and measure daily power usage--And better characterize your energy profile/needs? Perhaps segregate your loads for solar/hydro/battery vs shorter term/seasonal loads that go on genset....

    For solar, much of your costs are "up front" (panels, electronics, battery bank with 5-15 year life, installation/labor). Genset--Actual generator cost is not that expensive, and you only pay for fuel as you use it (not a lot of "sunk capital" with generators vs solar). Also, with solar, the battery bank can be damaged/ruined pretty easily (over discharging/under charging/poor maintenance/equipment failure/etc.). It is pretty common for first time off grid solar folks to "murder" their first battery bank or two until the operations are worked out.

    More questions than answers--Sorry.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.