Solar math - Redux

[Native_NM]

Sometimes the simple problems cost the most.  I have two quotes, from two reputable firms.  I also did my own calcs.  Here is the design spec:

Single, dedicated draw of 0.5 amps AC continuous.  No downtime permitted.  100% off-grid.  No generator allowed.  Mission critical.  No wet-cell batteries.  No modified sine wave inverter.

Take a guess - what do you think it costs?  It might surprise you.

[firefox]

My guess is ~$500
Bruce

[Native_NM]

Remember - 60 watts continuous.


Higher. 

[MountainDon]

Questions to enable better calculating, guessing   :
How many days of autonomy?
What maximum degree/percentage of discharge for the batteries?
NM sun hours?  working average 5?
Is there a prize?   


Note to wannabe winners... watch the specs...
No flooded wet-cell batteries means something like AGM, Li-Ion, Sodium-sulfur....    Raises cost some

[Native_NM]

Good questions. 

50% max discharge and 3 days autonomy.

Working hours is 5.2 per the tables based on 6 sun hours. 

Panels set at winter angle year-round at solar noon.

[Native_NM]

I can probably come up with a prize.

But you have to show your work.  I'll post mine at the end.

[MountainDon]

one more question;   minimum temperature where batteries would be stored?

[Native_NM]

5 degrees Celsius.  Batteries stored indoors in unheated space. 

[MountainDon]

41 F

[Squirl]

60 watts x 24 hours a day = 1440 watt hours.  I usually factor 25% extra for everything.  1800 watt hours per day.

5 sun hours per day = 500 watts of panels.  $700
$250 - suresine inverter
20  amp 24 volt pwm charge controller = $100
$300 - breakers/boxes
$300 mount

6 kwhs of battery autonomy, 9 in cold weather with 75% DOD,  24 volts at 375 amps. = $2400

I would guess about $4300 in materials.

The batteries are the killer.  Why no flooded?

[Native_NM]

The off-gas can potentially interfere with remote sensing equipment.  Even a vented box was not acceptable. 

The tray for the agm's I've used in the past were over $1000.

Heck, I once used a Honda portable generator and had to install two layers of visqueen and an oxygen and CO monitors for outdoor use. 

The gas cans had to go in an approved fire safe but the safety officer used to drive around with a 5 gallon jerry can of gas in the truck bed.  I quit wondering a long time ago.

[Native_NM]

I was going to comment earlier but didn't have time.  Your estimate is higher than the two commercial quotes, but not by much.  They didn't include wiring and mount.    I think you win the prize; I'll have to figure out what that is. 

I estimated about half as much.  I had 440 watts of panels, but fell well short on the battery capacity.  I'm not a solar engineer, but feel comfortable with small projects.  I went higher DOD and didn't have the capacity.  The engineer said I would probably be fine about 340 to 350 days out of the year in northern NM.  Remote sensing is complicated with equipment not designed specifically for DC operation.   They have a newer version of my equipment that runs on 24v DC and is more efficient.  It is very expensive and adds not much in the way of functionality.  Another vendor has equipment that uses flash memory and even less power.  And finally, some vendors are using solar and capacitor/battery combinations.  They are also very expensive.

These exercises are fun for those who are contemplating solar.  Too many people think they can plug in a 100 watt panel and power a 100 watt bulb. For real off grid apps, it is cheaper to focus on conservation.  Small power drains over time translate into big dollars - at least solar dollars. 

If I take an educated guess, capacitors are the next big thing in RE storage. 

Thanks for playing along.  This forum is amazing.

[flyingvan]

I vote for flywheels over capacitors.  Kinetic batteries are awesome, as long as they don't fly apart. http://en.wikipedia.org/wiki/Flywheel_energy_storage

[Native_NM]

While not directly related to the math, there is other interesting solar news.  California and several other states have been floating "solar taxes".  Spain has already opened Pandora's box.

http://www.bbc.co.uk/news/business-24272061

In California, they have floated the tax based on either Kw of installed panel or a flat tax.  They are discussing the tax for off-grid installations, including vacation homes, but I think they are primarily concerned with grid-tie systems.  The other option in CA was an annual inspection fee by a power company engineer.  For your safety ya know.  That would be a zoning law which would make it harder to dodge.