Solar Design Time

[JoshInReno]

Hello all. I figure it is time to consult the brain-trust that is this forum with a question: I am interested in a 12v system for things like a dome light, porch lights, a CO2 detector and a portable DVD player.

What I need is some help in choosing panel wattage and a battery. I have been compiling a list of energy requirements, and I think I have them nailed down pretty well:

2 LED porch lights: 1 amp each, maybe a few hours a day.
1 LED dome light: 1 amp, maybe a few hours a day.
DVD player: 3 amps @ 9.8v, maybe 2 hours a day.
12v CO2 detector: 0.6 amp, 24 hour draw.

So my current calculations indicate a 26.4 amp hour draw on the battery, per day. Let's call is an even 30 for simplicity sake (I don't see it going over 30, for almost any reason).

Now, I figure I can cut about 7 amp hours by shutting down the carbon monoxide detector via master switch during the day and turn it back on in the evening. So on second thought, lets call it 20 amp hours. That is, if I understand this correctly?

So assuming my calculations are correct, what kind (watts) of panel should I be looking for?

Thanks in advance.

[bdosborn]

Lets look at an 85 watt panel to start with, say this one from eBay:

UL Solar Linky

The peak current output is 4.84 amps . A conservative calculation for the daily amp-hr harvest of a flat panel is:

peak current*5 hours/day full sun*80% typical efficiency=daily amp-hrs

4.84*5*0.8~20 amp-hrs.

You can increase your harvest by 20% by tilting your panel. You can get 10-20% over that by moving the panel during the day to track the sun.

26 amp-hrs in a night is a lot, I think you're over estimating the LED draw. You should be able to get LEDs that draw a half to a quarter of what you've estimated.
Bruce

P.S. A group 24 deep cycle battery has around 85 amp-hr capacity. That should carry you through a couple of rainy days.

[JoshInReno]

Thank you very much for the info. Your suggestion of an 85 watt panel matches what I was thinking. I got confused after asking the same question on another site and recieved this answer:

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You don't say where you will be traveling and what seasons it will be done in... However, lets start where you live.

Using PV Watts, a 1,000 watts (1 kW) of solar panels (round number, smallest web site will accept). Elko, mounted flat to roof, 0.52 system efficiency, we get (note: this assuming you use an AC inverter for system efficiency):


Quote:
"Station Identification"
"City:","Elko"
"State:","Nevada"
"Lat (deg N):", 40.83
"Long (deg W):", 115.78
"Elev (m): ", 1547
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 0.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 9.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.12, 31, 3.01
2, 2.86, 39, 3.78
3, 3.97, 61, 5.92
4, 5.11, 75, 7.27
5, 6.51, 96, 9.31
6, 7.22, 100, 9.70
7, 7.16, 99, 9.60
8, 6.59, 92, 8.92
9, 5.40, 75, 7.27
10, 3.80, 54, 5.24
11, 2.28, 31, 3.01
12, 1.90, 26, 2.52
"Year", 4.59, 778, 75.47

Lets toss out the "bad" three months of the year, we get 2.86 hours of sun per day average for February.

Say your 30 AH @ 12 volts is the amount of DC power you need (no inverter losses), then your system efficiency is

~0.80 battery eff * 0.77 solar panel + charger eff = 0.62 derating
30 AH * 14.5 volts charging * 1/0.62 sys eff * 1/2.86 hours of sun = 245 watts of solar panels.
Say you want to store enough energy for 2 days of "no sun" and 50% maximum discharge (for longer battery life):

30 AH * 2 day * 1/0.50 max discharge = 120 AH minimum
To support charging of that battery bank with solar panels, the starting rule of thumb is 5% to 13% rate of charge:

120 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.05 = 113 Watts minimum
120 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.13 = 294 watts ~cost effective maximum
So--if you mount the panels flat to the roof, you should install roughly 245-300 watts of solar panels for ~9 months of mostly generator free camping with your defined loads.

Now, if you plan on camping in winter (and/or going farther north), you should consider tilting the panel up. For example, in your area tilting the panel to latitude +15 degrees to ~55 degrees gives you:


Quote:
"Station Identification"
"City:","Elko"
"State:","Nevada"
"Lat (deg N):", 40.83
"Long (deg W):", 115.78
"Elev (m): ", 1547
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 55.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 9.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.05, 66, 6.40
2, 4.47, 64, 6.21
3, 5.06, 79, 7.66
4, 5.04, 73, 7.08
5, 5.45, 79, 7.66
6, 5.51, 74, 7.18
7, 5.73, 77, 7.47
8, 6.23, 85, 8.24
9, 6.46, 90, 8.73
10, 5.86, 87, 8.44
11, 4.04, 60, 5.82
12, 3.93, 63, 6.11
"Year", 5.16, 897, 87.01

...almost 2x as much useful power in winter (with the added cost and hassles of tilting the array). This will allow you to either consume more power or build out a smaller array.


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Any thoughts on this? 300 watts seems REALLY high for my use.

I appreciate your plain english answers to questions here.

[bdosborn]

I don't think 245W is that far off for 30 amp-hrs a day minimum. He assumed you would be using the camper most of the year, so he based the amount of sunlight one of the worst months (February ) using a flat panel. He also was *very* conservative with the system losses you should expect. I haven't seen loses that bad on my system. Bottom line is that 30 amp-hrs would be the minimum harvest you could expect with a 245 watt flat panel in February, it would be at least 50% more in the middle of the summer.

We had an 85 watt panel on our teardrop and ran the kind of loads you've described (except for the CO2 monitor). Our battery was usually charged by noon with full sun. It worked so well I stopped tilting the panel. We have a CO2 detector in the Boxcar and its powered by AA batteries. It would help your energy usage a lot if you got one of those.

Bruce

[Rock]

To echo what Bruce said regarding the LED lights - a 1 watt LED (single or array) is a lot of light compared to an equivalent incandescent. 1 watt at 12 volts is about 0.1 amp draw.

For comparison I started buying LED units for the actual night lights in my house (only because the incandescents burn out too often.) They are 0.25 watts (I think) versus the traditional 5 watts for incandescent. So for a given light output you only need 1/20 the amp draw with LEDs.

In short an 85 watt panel is probably sufficient, especially if you consider carrying around a canary and getting rid of the CO2/CO detector all together.

Eric

[Ratkity]

I doubt you have a carbon dioxide detector. These are usually portable:
http://www.co2meter.com/

However, the CO detector can be battery operated:
http://www.myboatsgear.com/mbg/product.asp?prodID=332 or you can just plug in your current CO monitor in while you sleep.

Drop that electric requirement and your needs drop by half. Portable DVD players usually come with their own battery and just need recharging (I have one in my TD).

I'm running an 85W panel and running a fantastic fan, portable dvd player and a couple of 12v lights. I charge my phone too, btw. My battery is charged by mid morning.

Hugs,
Ratkity

[JoshInReno]

Thank you very much for the information, ladies and gentlemen.

Yes, it is a carbon DIOXIDE detector that I am thinking about - don't want to suffocate in a small enclosure. However, I am beginning to think of other options.

So lets cut my usage down to 12aH on the high end. At this point I am thinking a 50 watt panel may be about what I need. Something around 2 amps?

http://cgi.ebay.com/RAMSOND-50-W-WATT-5 ... 2a1179551f

[bdosborn]

Yes, it is a carbon DIOXIDE detector that I am thinking about - don't want to suffocate in a small enclosure. However, I am beginning to think of other options.

Just checked and I have a COx detector. One option for CO2 might be to open the window..

Bruce