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Wind, solar, storage and back-up system designer - Page 9

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Posted by Jim Wilkins on July 8, 2019, 6:05 pm

On Monday, 8 July 2019 04:21:21 UTC+1, ads  wrote:

Thank you for your on-topic questions Ads.

There's the off-grid table for 4KWh maximum daily usage and 8% solar  
capacity factor.
http://scottish.scienceontheweb.net/Wind%20power%20storage%20back-up%20calculator.htm?usage=4&units=kw&spc 0&scf=8#off


The way I see it, your backup system generator should be capable of  
meeting the maximum instantaneous essential power demand, such as the  
fridge starting while the microwave or other short-duration, high  
demand cooking appliance is on. This assumes non-essential loads like  
computers are off while cooking, and your batteries are depleted and  
can't contribute.

If the generator you have isn't powerful enough for both the fridge  
needs a power monitor to show when it's running, and will turn back on  
immediately after being unplugged while cooking, so you know to wait  
while the pressure drops. I set a wind-up timer for 5 minutes when  
switching from grid to inverter power or back, unless the fridge is  
off. I need a visible monitor like a Kill-A-Watt because I can't hear  
the fridge running over the inverter fan.

Unless you run the generator overnight the battery bank should support  
at least the fridge and heating system, I assume from dusk to dawn at  
the minimum, or however long it's likely to be unattended..

If those conditions are satisfied the alternate energy source can be  
any size you're willing to allocate space and pay for, and the  
generator can make up the difference.

A grid-powered DC supply draws minimal wattage when diode-OR'd with a  
solar system set to a higher float voltage, unless the solar output is  
insufficient. The ones I've built smoothly share the power, with grid  
demand falling as solar output increases. This is a DC UPS with the  
two diodes handling the switching and sharing. The higher voltage  
supply's diode conducts while the lower voltage supply's diode blocks.  
The AC-powered supply is set to the lower end of the float voltage  
range, the solar controller maybe 0.1V or 0.2V higher but below the  
level of significant gassing.

When I use a Radio Shack 13.8V, 19A DC switching supply as the  
AC-powered source its input wattage drops to zero on a KAW when solar  
takes over. I had to experimentally confirm that it's output current  
wouldn't burn out the diode if the grid powering it came back up while  
the battery was discharged to 10V.

Posted by Scottish Scientist on July 8, 2019, 9:23 pm
On Monday, 8 July 2019 19:04:45 UTC+1, Jim Wilkins  wrote:


My designer's "Off-grid daily usage focus table" doesn't specify peak power
 output because small systems can have peak power loads which are much and  
unpredictably higher than the average power.

So I leave it to all you experienced home power experts to decide how you a
re going to manage your peak power arrangements.

What my designer offers is that providing the off-grid system's energy usag
e keeps within the limits of the maximum daily energy usage which your conf
iguration specifies and providing the specified backup power starts up auto
matically when needed -

My modelling suggests that would be to power up the specified back-up power
 whenever the energy store drops to 75% of maximum capacity.

- then the off-grid system ought to be able to meet the daily usage require
ments, without normally needing any additional back-up power and with the b
attery never getting flat.

Exceptionally, when energy demand exceeds the designed for performance that
 the configuration specifies, when the energy store empties (flat battery,  
empty pumped hydro reservoir etc) then the specified system cannot be expec
ted to meet demand and then some other external power source is required to
 import power from to meet the demand.

In the case of an off-grid system that may mean starting up a legacy fossil
-fuel generator, kept on stand-by to keep the lights and everything else on
 in emergencies.

When designing for a 100% renewable energy system the designer tables speci
fy a capacity of routine back-up power that should ideally be supplied usin
g renewable energy.

So that could be a generator that routinely burns renewable energy bio-fuel
 - bio-ethanol or bio-diesel for example.

So for the off-grid scenario, there could be one smaller routine back-up ge
nerator burning bio-fuels that all being well completes your 100% renewable
 energy system.

Then you can complete your system for all eventualities and worst-case scen
arios, with a larger emergency back-up generator on stand-by to burn fossil

The off-grid table doesn't specify the emergency generator requirement for  
the same reason that it doesn't specify peak power demand. That's for you t
o decide.

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