So, we've just discovered our new house has a biodigester and not a septic tank as was previously believed. However the air blower is not connected so its not bio digesting. I'm obviously going to recommision it (particularly as it discharges into the ditch ).
My question is - the air blower is 240v, 75W so similar to leaving an incandescent light bulb on and will be sited in a shed. It needs to run 24/7 365 days and I'm not keen on using mains power both due to cost (80p/day at present, soon to be £80/day ) and also I generally don't like leaving things running unattended.
I'm therefore considering using solar or wind , battery bank and inverter to run it. Does anyone have experience of anything similar or signpost to handy online calculator s?
We live in N Scotland so during the winter expect sub zero temperatures for extended periods (weeks), ~12" snow and only about 6 hours of daylight in midwinter. On the plus side we're on an exposed hillside.
I'm OK with topping up the battery bank with mains charger on occasion.
Some Yankee sites list a unit that uses solar with a grid tie so when the sun goes to bed the mains switches in to keep it running over night but obviously they are 110v / 12v not much listed on the UK sites at all
If your roughing it, Your doing it wrong
Lack of planning on your part doesn't make it an emergency on mine
Hi NurseAndy,
An interesting conundrum. You might need to dig deep into your savings.
Looking at back of the envelope numbers for doing it with Solar.... It's very much on par with my system running an 80W fridge freezer in slightly sunnier Cheshire.
Estimated energy need 75W
Fair sunlight available maybe 6 hours per day, so a 100W panel might push out 50W for 25% of the day, 25W for 25% of the day and 0 for 50% of the day. Average = 19% efficient
Ballpark figures but anything else is a bonus.
You will get some 30% inefficiency in your charge converter, so overall efficiency =19x(100-30) = 13.3%
So on average, to supply 75W, you'd need panel capacity of 75 x 100/13.3= 560W
So.... maybe 3x200W panels. I'd suggest buying them in pairs so you can run a 24V system, so 2x280W or 4x 150W
Now, you need Some batteries..... Let's say enough to run it 24H. Let's assume a 12V system
That's using current of 75/12=6.25A
Over 24Hrs, that's 6.25x24=150 Ampere Hours. You don't want to run batteries full to empty, so 2x or 3x 110Ah leisure batteries, or 4 60Ah Car batteries.
I don't know the nature of this blower, but I'd definitely research replacing any 240V fan with a 12V fan or multiple 12V fans, because if you are to generate 240V, that would require spending on an inverter, which itself would waste maybe another 25W.
All the above assumes standalone and off grid. It's roughly on par with my setup which drives an 80W fridge freezer. Where resilience was my primary concern.
Now, if you are prepared to supplement the energy from mains, then you could use relays and a simple timer as I have. That lets you get away with a wholly smaller solar system.
The timer switches on a regular car battery charger at night*, thus smoothing out the troughs in battery drain and extending the life of the batteries. Then use another timer or the dusk* detector of your charge controller so that the blower works off the mains from dusk till dawn. and from battery/solar during daylight. Effectively, in daylight hours the current goes straight from panel to blower with the batteries just smoothing the dips.
Likely Expense, shopping around and buying pre-owned.....
2x280W panels ~ £200-£500
PWM Charge controller £9 or MPPT Controller £100+
4x60Ah Car batteries (Cheap option) ~ £240 or 2 x 110Ah Leisure batteries ~£300
Inverter, if you opt for 240V ~£50 ( I'd suggest Pure Sine of 600W+)
Or spend the £50 on some 12V fans for the blower.
Battery monitor Recommended £20
Assorted cables £50
If you decide to use the timer and relay solution, maybe another £50
Now, all that sounds like expensive overkill, but with the use of the battery charger, you can build your system progressively, and pick up components as you go along, such as starting with one battery and one panel.
Once you start building your power station, you will get hooked. Soon you'll be running the street lights from it.
Now..... Wind power. No experience of it, but the principles and estimations are the same. You'll still need batteries and some sort of charge controller.
Graceful Degradation! Prepping's objective summed up in two words. Turning Disaster into Mild Inconvenience by the power of fore-thought
This may be stating the obvious but is there a lower power air pump that will do the job?
t'other half was chatting with an off grid solar expert at a show recently about his shed which has similar power needs. The expert conclusion was not so different to the advice already given here. Two extra points though.
1) To keep costs down see if your local garage has any old car batteries available. The take was that the nice leisure batteries soon drop to a similar level of performance as a used car battery so are only worth while in situations where space/weight is important.
2) 400W of panels is enough for most of this type of application but remember to sweep the snow off your solar panels or they won't generate anything.
Thank you all so much, incredibly useful info. Andy - passive overflow so only power for air pump required, thanks for Google search, I couldn't find much either.
Jenny - thank you very much for calculations.
Gilly- I have an old car battery (quite small) I can start with. Good tip about approaching mechanics.
I'm at work at the moment so can't check numbers but I have a (very) old solar panel off my boat from maybe 25 years ago, the old car battery AND a small inverter that plugs into car cigarette lighter. I'll cobble it all together at the weekend, I know it's not matched to each other and old knackered kit but it may be sufficient for proof of concept before spending a wedge. Quite excited about this little project
You ideally want a 12,v pump to save on the losses stepping up to 240v
Is yours a aeration block like a fish tank or the center air pumped water lifter and diffuser think sprinkler with a mushroom on top to spread the effluent over the dense plastic mesh ?
Good idea - 12v would be simpler and more efficient. Not sure that ones up to job though. The diffuser is an 8" diameter aeration block, I connected it up last weekend to the pump in a very temporary fashion that involved a garden hoe and my wife sitting on my legs as i leant in and there was significant agitation in the tank (from the diffuser, not me).
I can mount the pump directly over the tank but it would still need to pump the air down nearly 3m and its a 25mm pipe coming off the diffuser.
When I get home ill have a look at the rating plate on the pump to see what the l/min is.
A 12v pump is very tempting though and keeps everything simpler.
GillyBee wrote: ↑Tue Aug 30, 2022 12:00 pm
This may be stating the obvious but is there a lower power air pump that will do the job?
...
Two extra points though.
1) To keep costs down see if your local garage has any old car batteries available.
Seconded. Old scrap car batteries are a cross between 'Industrial Waste' and 'Valuable Scrap Metal'
If I recall, they have scrap value of up to about £5, so worth asking your friendly neighbourhood motor mechanic or specialist battery shop. Mine cost a fourpack of beer, and my man tested through the dozen or so available The 90 or so Ah battery might be good for 30Ah, but so long as it doesn't have a dead cell, it's perfectly serviceable.
Yorkshire Andy wrote: ↑Tue Aug 30, 2022 11:35 am
They are a electromagnetic driven diaphragm pump Jenny
I just love learning all this new Sh1+ and now I can add 'Linear Diaphragm Air Pumps' to my repertoire. I'd thought it might be a sort of fan blower.
A quick google search reveals 12V ones seem to exist, and that link from Yorkshire Andy looks just the job.
Andy..... No. I'm not ordering one from ebay.
If the original poster discovered that his was not pumping, then getting a new pump might already be in the plan.
Graceful Degradation! Prepping's objective summed up in two words. Turning Disaster into Mild Inconvenience by the power of fore-thought
). 
) and also I generally don't like leaving things running unattended.
