New to the whole solar thing, need help sizing a system for a school bus conversion

dvd545

Hi everyone, 
Been pouring over the forums and learning as much as possible for the last month or so. Im building a school bus conversion and planning on traveling the country. Trying to design a system thats going to be good in multiple different locations across the USA (mostly mid-west coast) maybe some time in Canada living throughout the winter times and chasing snow storms. I will have a vehicle towed behind so I wont need to park the bus in the middle of the storm (and cloud covered skies) but if possible would like to design this system to be able to survive in the winter.  

Im hoping someone will be able to check over my numbers and let me know where I'm off and give me some recommendations.

So first off my power requirements are about 4.3kwh/day
I believe I need  about 360AH @ 12v per day in batteries so doubled for 50%DOC would be 720AH bank. 
I also read that after a certain threshold of panels It makes more sense to move to a 24v system.
Im aiming for a 360AH bank @24v using 8 6v 220ah golf cart batteries. wired 4 in series with 2 strings of parallel. 

For panels I found some new panels on craigslist someone is selling. They are Hanwha 340w. Has anyone heard of these/would they work out? 
Im planning on purchasing either 4 or 5 panels. There are a few things im worried about and havent found the answer to. Hoping someone on here will be able to help.  Based on calculations below I may need to purchase one more. But first the winter power requirements. 

Ive run a solar output calculator for locations such as glacier national park, whistler, and seattle. Im trying to figure out approximately how much solar energy I will be able to produce during the winter months and if the (4-5) panels will be enough. I have calculated that with a bare bones energy use (fridge, diesel heater, lights, water pump) I will only need a minimum of 1.7kwh. I plan on mounting the panels to be able to tilt. 
 Im not 100% if I am entering the data correctly but heres what I plugged in to pvwatts.nrel.gov solar calculator.
Location: seattle
Panel size 1.7KW
module type: standard
Array Type: roof mount
System losses: 14.08 (default)
Tilt: 20deg
azimuth: 180
Obviously this is a perfect scenario but according to this im getting 58kw in Dec and 64kw in Jan. Divide that by 31 for a per day amount and I get 1.87 KW in Dec, and 2.06KW in Jan. These loads are in AC so whatever I was using directly DC current there would be less loss. Correct me if Im wrong but this would be enough for the bare min electrical requirements. Im planning on having a small generator on board to cover my bases but I would like to plan on using mostly solar throughout the rest of the year. I figured if I plan on the lowest common denominator I should be good the rest of the year.

I have also found a few other calculations that it seems you guys use on here to help newer inexperienced members (like myself) out but im not sure if these numbers are correct. 

For charge controllers Im looking at these numbers
1360 watts of panels (4 panels) * .77 for panel losses *  1/29v charging = 36A min for controller
1700 (5 panels) * .77 * 1/29v = 45A CC
2040 (6) panels * .77 * 1/29v = 54A CC
Would the bolded numbers be suitable for sizing a charge controller? I read somewhere else you should factor in an additional percentage for extreme scenarios. 

Panel sizing for battery charging. This is calculated by a 5-13% charging amount. Again please correct me if Im wrong at all. First time haha
430AH battery * 29V charging * .05 charging rate * 1/.77 system derating = 810W
430AH battery * 29V charging * .13 charging rate * 1/.77 system derating = 2105W
A little confused what these numbers mean. Does this mean I need at least 2,105W in panels to fully charge my 430AH batteries?
How does that affect the amount of sun I will be receiving in the winter time. 

So back to panel sizing and charge controller sizing. What would you recommend? Am I anywhere near the correct numbers here?

Sorry for such a long post. Really appreciate the answers. Believe it or not a few months ago I wasnt sure what a KWH, or AH was. 
Thanks,
David

706jim

I live in Canada. If last fall and this winter are any indication, you had better be looking at a generator if you need 4.3 Kwh/day. Sun has been low and sun hours dismal here.

Estragon

What seems to happen in a place like Seattle or Vancouver in winter is there will be a run of a week or two of gloom, a few nice days, then another system rolls in and it's back to gloom.  In those conditions, you end up using the genny a lot, and upping the pv can be limited by available space in a mobile application.  You can still get some charge if the cloud isn't really thick, so charging in bulk for an hour or two in the morning, and taking what solar gives through the day can work.

GC batteries would be a good choice.  Reasonably cheap, and available all over if replacement needed.

In CC sizing, I'd use nominal (24v).  In bulk, battery voltage will be closer to nominal than Vabsorb, so eg 1700w x .77 / 24 = ~55A (vs 45a using 29v).  In this application, I'd probably look for fanless (assuming it will live somewhere fan noise might be an issue).

In figuring out pv layout, beware of any potential shading on the roof, as even small stuff like vents and antenna can be a problem.

mcgivor

Welcome to the forum David

Your proposed consumption figure is way too high for a mobile application, you need to trim that by 50% minimum, even then it would be a challenge, especially during winter months. The real estate simply doesn't exist on a bus to provide that amount of energy, there is little point in working on a calculation without a conclusion, economy is the best option. Perhaps it's better to discuss your loads, and how to reduce them.

dvd545

mcgivor said:

Welcome to the forum David

Your proposed consumption figure is way too high for a mobile application, you need to trim that by 50% minimum, even then it would be a challenge, especially during winter months. The real estate simply doesn't exist on a bus to provide that amount of energy, there is little point in working on a calculation without a conclusion, economy is the best option. Perhaps it's better to discuss your loads, and how to reduce them.

Are you only referring to the winter? I plan on having a genny to keep the batteries topped off. 
Regarding size restrictions, the panels are about 6'x 3'. I have plenty of room on the roof for more panels if necessary but I will be running out of money for the supporting CC/batteries before  roof space. As I said in my original post I need only about 50% of that as a bare minimum. About 1.7kw, are you suggesting I should aim to cut that down to 50% of that? 
I could consider installing up to 6 panels but I dont know how that would affect what controller size etc. 
I plan on moving from place to place so finding hookups/running the generator during an intense storm then moving again would not be at all out of the question. 

How would I calculate the expected solar output during winter months? 

dvd545

Estragon said:

What seems to happen in a place like Seattle or Vancouver in winter is there will be a run of a week or two of gloom, a few nice days, then another system rolls in and it's back to gloom.  In those conditions, you end up using the genny a lot, and upping the pv can be limited by available space in a mobile application.  You can still get some charge if the cloud isn't really thick, so charging in bulk for an hour or two in the morning, and taking what solar gives through the day can work.

GC batteries would be a good choice.  Reasonably cheap, and available all over if replacement needed.

In CC sizing, I'd use nominal (24v).  In bulk, battery voltage will be closer to nominal than Vabsorb, so eg 1700w x .77 / 24 = ~55A (vs 45a using 29v).  In this application, I'd probably look for fanless (assuming it will live somewhere fan noise might be an issue).

In figuring out pv layout, beware of any potential shading on the roof, as even small stuff like vents and antenna can be a problem.

Thanks for your input!
How noisy are the fan units? Are they constantly on or only during times of high load.  I plan on housing all the electric inside a utility area that will be insulated in spray foam. 


swapping in 24v for charging would my calculations be correct otherwise? 
panel requirements for charging my battery bank (assuming this is how its calculated)?
430AH battery * 24V charging * .05 charging rate * 1/.77 system derating = 670W
430AH battery * 24V charging * .13 charging rate * 1/.77 system derating = 1742W 

BB.

I use 29 volts for charging

Bill

BB.

More or less, 500 to 1,000 WH (1 kWH) will run a small laptop, some LED lighting, cell phone charger, and RV water pump...

Add a full size Energy Star rate refrigerator, at 1-2 kWH per day, and you are pretty much at needed a 3.3 kWH per day system. And it can support a washing machine a couple times a week, LED TV, and such. About as near to "normal" electrical life (albeit with lots of conservation and careful usage of appliances).

You suggest a 20 degree tilt... In reality, it really depends on where and when you are... And the farther north you go, the more you want panel tilt (vs flat which works pretty well down in the southern USA and further south/closer to the equator).

This solar calculator gives the basic answers and is quick/easy to figure hours of sun based on where you are and angle of panel/seasons.

http://solarelectricityhandbook.com/solar-irradiance.html

Say you want to go to Calgary Alberta Canada in Winter:

Calgary
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
1.03	1.90	3.21	4.67	5.49	5.83
Jul	Aug	Sep	Oct	Nov	Dec
6.15	5.04	3.58	2.27	1.23	0.88

And the same thing 24 degrees from vertical (76 degrees from horizontal):

Calgary
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 24° angle:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
1.94	2.99	3.97	4.58	4.39	4.37
Jul	Aug	Sep	Oct	Nov	Dec
4.75	4.56	4.08	3.35	2.22	1.84

You get alost another hour of sun November through February... 1.7 kWatt array, with AC inverter usage:

• 1,700 Watt array * 0.52 average DC off grid system eff * 0.88 hours (Dec Flat) = 778 Watt*Hours per avg Dec day
• 1,700 Watt array * 0.52 average DC off grid system eff * 1.84 hours (Dec 24 degrees from Vert) = 1,627 WH per avg Dec day

More or less, these are average energy harvests... Some days more, some days less, and get a streatch of bad weather, and you may harvest 5% or less of your average day (near zero harvest in dark/stormy weather).

When designing a system, generally you have a "base load" that you don't want to go below (without using a genset)--Say a refrigerator and some LED lighting, laptop computer for work... The minimum "base load" would be somewhere around 50% to 65% of your predicted output... On clear/sunny days, then you do your optional loads (washing clothes, TV, etc.).

Designing your RV Bus for minimum power is critical for a good solar experience... There are some things that just take a large amount of energy--And they seem that they should not.

For example, a common RV heating system (propane heat, 12 VDC fan forced air)--RVs tend to be poorly insulated... And if you want to run the heater overnight (assuming 50% duty cycle), it would consume (roughly):

• 8 amps * -0.50 duty cycle * 12 hours = 48 AH @ 12 volts
• 48 AH * 12 volts = 576 Watt*Hours to heat overnight

You can see, picking this as a typcial heater running your 1,700 Watt array on a typical December day, it would consume something like 2/3rds of your average daily harvest just to run the heater.

I highly suggest that you do a lot of back of the envelope calculations for loads and solar system configurations before you purchase any hardware.... The idea is to design a "balanced" system (daily energy needs drive battery bank design... Battery bank design, daily loads, location+hours of sun per day/season drives the array sizing.

-Bill

mike95490

The solar electronics (Charge controller & Inverter) WILL generate heat that has to be removed.  Sealed in a closet will cook them.

Estragon

To clarify, my suggestion to use 24v was for sizing the charge controller capacity, because in the early stages of charging, output voltage will likely be closer to nominal bank voltage.  At lower voltage, current is higher for a given wattage, so this results in a more conservative sizing (so the controller won't end up limiting harvest running full out in early bulk).

For array sizing, I'd use 29v so you size the array to be able to provide the design current in the later stage of bulk, when voltage is closer to Vabsorb.

Fan noise is a pretty personal thing.  Some find it really annoying, others find it more like white noise that can be sort of welcome.  Noise tends to get a bit worse as the fan(s) age.  With or without a fan, you wouldn't want them in a really small, tightly sealed locker.  Fanless use big heat sinks, and the heat has to be able to go somewhere.

dvd545

@BB.
Thanks! Im going to have to take a step back and consider what the best approach will be. Maybe consider relying on a generator more or an alternator vs beefing up a solar system. 

Do you have any suggestions as far as correctly calculating a fridge energy consumption based off the energy star rating? I havent bought a fridge yet but I know the energy star ratings vary wildly. 
I read somewhere you divide the energy usage by 3 for the amount of cycling it will do during the day. 

Estragon

For an (AC) fridge, I'd look for an "inverter" type.  Although not very common in North America, they have been used elsewhere for some time.  The main advantage to this type is low start-up power draw.  A fridge may take 150-200 watts running, but typical N.A. fridges take many times that to start, which means running a bigger AC inverter just to start.  Some large inverters can take almost as much as a fridge just being turned on, so not oversizing is important.

Cycling will depend a lot on usage and ambient conditions.  A design duty cycle of 1/3 would be typical, but in use could be 2x or 1/2 that.

There are DC alternatives which can have much lower consumption than AC, but tend to be more costly.

dvd545

Estragon said:

For an (AC) fridge, I'd look for an "inverter" type.  Although not very common in North America, they have been used elsewhere for some time.  The main advantage to this type is low start-up power draw.  A fridge may take 150-200 watts running, but typical N.A. fridges take many times that to start, which means running a bigger AC inverter just to start.  Some large inverters can take almost as much as a fridge just being turned on, so not oversizing is important.

Cycling will depend a lot on usage and ambient conditions.  A design duty cycle of 1/3 would be typical, but in use could be 2x or 1/2 that.

There are DC alternatives which can have much lower consumption than AC, but tend to be more costly.

I looked online for them. They are very difficult to find! Do you know of any? Looks like most of them are only available over seas. I also need one thats smaller than a full size. Somewhere close to 10cu. ft. 

BB.

Your best bet is to buy a Kill-a-Watt type energy meter (many brands out there these days) and measure the energy usage of each appliance and create a W*H per day value for each.

Energy Star numbers are pretty good for typical fridge/freezer usage in warmer climates. Very hot, you can use more. Very cold, you can use less (and for fridge+freezer combinations, many will not work correctly below 55F or so--The freezer may defrost). Making ice, putting warm food in fridge/freezer to cool (shopping, cooking, etc.) all drive up energy usage... If you pick the right unit, 1,000 WH per day. If you convert a chest freezer to chest refrigerator, you can get down towards 250 WH per day (chest freezers/fridges can be a pain to move baskets/goods around to get/store your goods).

For DC refrigerator/freezers, HomeDepot is listing some now (I know nothing about the product except that DC units tend to be very expensive and have manual defrost). Over the years, some DC fridges have some construction issues--doors that leak/don't fit over time, etc.:

https://www.homedepot.com/s/dc%20refrigerator?NCNI-5

Some folks save money with DC refrigerators (smaller solar power systems, no AC inverter, no compressor surge). Others find a standard AC fridge (cheap) plus larger solar power system (not cheap) is a better deal. Paper designs first before you buy anything is usually the best place to start and see what works best for you.

Refrigerators are the loads that generally push a "small" solar power system to "medium" size. If you are going to have propane, a propane powered fridge may be a better alternative (something like 1/2 to 1 lb of propane per day). Venting propane refrigerators properly is important.

Some folks pick up good deals for propane fridges from RV Wrecking Yards.

Inverter refrigerators (smaller types) seem to be available in Mexico...

-Bill