Solar-bicycle project. Charge controller and inverter questions

Miguifly · April 2019

Hello there!

I run a community garden in San Jose and we are installing an off the grid solar system to power a few appliances: 2 Led lights (20W) + phone charger (6W) + immersion heater (120W)

We will start there and eventually expand the system to generate enough power for a small 12V van-type fridge. Someone suggested to keep the system 12V to avoid any possible safety issues.

Our max energy use per day would be 200W, although this is not likely to happen often, or at all. The garden is not open everyday and the appliances would not necessarily be used every time the garden is open. This would change once we incorporate the fridge, but that is a future concern.

The fun part of the project is that we are going to build a bicycle blender (only mechanical) and add a second chain to the rear that will tie to an alternator to provide extra energy to the battery.

What I am wondering is what type of charge controller would we need? I imagine we would need 2 inputs to draw power from the alternator and the solar panel.

In case this helps, we were thinking of getting 2x250W solar panels and 2x1200W deep cycle batteries (12V). And just to make sure, since it's a 12V system, there is no need for an inverter right?

Any input or ideas would be appreciated!

Thank you :)

mike95490 · April 2019

What's the duty cycle of the immersion heater ?

> add a second chain to the rear that will tie to an alternator to provide extra energy to the battery.

No need for a charge controller - unless folks from the gym are lining up. Maybe the Hulk could over charge the batteries, Human power is a novelty. The extra 50-70 watts that will be going into the large batteries, will hardly be noticed.

What's the Ah capacity of the batteries and the voltage of each one? Generally it's better to use 2 six volt batteries in series, and not 12v in parallel.

BB. · April 2019

Welcome to the forum M,

To be clear--You need to calculate both the load in Watts (rate of energy usage, like miles per hour). And total energy usage in a day in Watt*Hours (an amount, like miles driven).

For example, you are using 200 Watts in a day is like saying you are driving 25 miles per hour in a day... We need to figure out your actual energy usage per day (Watts*Hours per day).

In general, off grid solar energy is very expensive (like 5-10x the cost of your home's utility bill--Well this is California, so closer to 2.5 to 5x our home energy costs). And if you are planning on things like a refrigerator, immersion water heater (tea and coffee?), etc... You can end up with a "tragedy of the commons" situation. A nice (not cheap) solar power system that nobody takes care of (or the first person got tired of taking care of), and people over drawing the amount of energy the system can produce in a day (leave refrigerator open, run the coffee maker all day long, forget to turn off at hight, etc.).

For example... Say you have 2x 10 Watt LED flood lights that run 10 hours per night, a cooler that runs 400 WH per day, and the immersion heater runs 2 hours per day (120 Watts)

2 x 10 Watt LED * 10 hour per night = 200 WH per day

400 Watt*Hour cooler

2 x 120 Watt heater = 240 WH per day

=============================

840 WH per day

That is still a "relatively small" off grid solar power system. Let's say we design for 1,000 WH per day. Just a very quick rule of thumb design for a reliable system would be:

1,000 WH per day * 1/12 volt battery bank * 2 days storage * 1/0.50 maximum discharge = 333 AH @ 12 volt battery bank

Say you choose 4x 6 volt @ 200 AH batteries (golf cart--cheap and fairly rugged). 2x 6 volt in series, by 2x parallel strings = 12 volts @ 400 AH.

To charge this, we have two calculations. 5% to 13% rate of charge typical. For a full time off grid system, highly suggest 10%+ rate of charge:

400 AH * 14.5 volts charging * 1/0.77 solar panel+controller derarings * 0.10 rate of charge = 753 Watt array nominal
400 AH * 14.5 volts charging * 1/0.77 solar panel+controller derarings * 0.13 rate of charge = 979 Watt array "cost effective" maximum

And based on hours of sun per day by season:

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

San Jose, fixed array, tilted to 53 Degrees from Vertical (best year round harvest)--Choose worst case December at 3.67 hours of sun per day:

1,000 WH per day * 1/0.61 DC off grid system eff * 1/3.67 hours of sun (Dec) = 447 Watt array December "breakeven" size

So, the 753 Watt 10% rate of charge array is over your December harvest needs--In general, batteries are "relatively expensive and fragile" (easy to "murder") and solar panels are cheap and reliable. Typical golf cart batteries would (if well maintained) last around 3-5 years. Your charger (electronics) ~10+ years. Solar panels 20+ years (assume they don't get "rocked").

You can wire in a DC multi-port USB charger for people to charge their cell phones:

https://www.amazon.com/Anker-5-Port-Charger-PowerDrive-iPhone/dp/B00OHE8AOI

Lighting--I would highly suggest a motion controller (or possibly timer activated if only for evening gardening). It would save a lot of energy:

https://www.amazon.com/GLW-Waterproof-Daylight-Security-Equivalent/dp/B00W4ZMIP4

Note: Links above are suggestions for starting your search. I use Anker chargers and I am pretty happy with them--Have not used this model.

Before you start picking equipment--Does this look anything like you are expecting? The bicycle charging--Wire it up if you want--But I would not really worry about even placing a dump controller on it--I doubt that people would really use it that much (roughly 100-200 Watts maximum from alternator if an athlete was peddling for 20-60 minutes at a time).

The charge controller would be roughly (you probably will need an MPPT type charge controller--more expensive--But you can use cheaper >200 Watt solar panels):

979 Watt array (13%) * 0.77 panel+controller deratings * 1/14.5 volts charging = 52 Amp minimum MPPT type solar charge controller rated output @ 12 volts

Your thoughts?

-Bill

Miguifly · June 2019

https://forum.solar-electric.com/discussion/comment/396774#Comment_396774

Miguifly · June 2019

Thanks Mike and Bill.

Sorry I never replied. You gave me a lot to work with! I ended up simplifying the project a bit, incorporating new considerations from what students and coworkers thought was appropriate.

-> I came up with the following system. According to what I have learned, they should match, but I need a second opinion and have no one to ask! Hopefully someone can take a look and let me know if it makes sense.

[Note: according to my research, for this size of a system, 24V is more desirable than 12V. The main 2 reasons are that the charge controller will be half the amperage, and the copper wiring a lot thinner. This will save a significant amount of money. It is also more more efficient overall. Therefore, the components listed below make a 24V system, but it could be easily adjusted to 12V.]

4 Panels: 240W each (30.4V, 7.9A). Total, connected in series: 121.6V, 7.9A

We already got these because we found a great deal on craigslist

Assuming that our panels will perform at 50% of their rating on an average day (conservative estimate from the little research I have done) 960W x 0.5 = 480W

On average, we get 5.9 hours of sun in San Jose (more in summer, less in winter). Therefore, on an average day, the panels should be able to provide 480W x 5.9h = 2838Wh.

Battery (Lithium ion, deep cycle): 2x 12V, 100Ah = 24V, 100Ah

Alternatively, they have a 12V 200Ah battery if we go for a 12V system. In any case 2.4kW

Charge Controller: 40A. Max PV open circuit voltage: 100V/150V/200V,
Inverter 24V, 1500W or 2000W
Fuses 200A
Fuse Block
Circuit Breaker 50A
Converter 24V to 12V for 12V appliances (96% efficiency)
Bus Bar 100A-250A
4 Gauge copper wire
Branch Connectors (panel to panel)
Panels to Charge controller cables 10 AWG, 30 ft.

Thank you!

Tecnodave · June 2019

It would be better to not have all the panels in series as you propose. At 121 volts in to 24 volt system you would have a 4.5- 1 voltage down conversion ratio and with a 12 volt system it would be more like 9-1 voltage conversation ratio. The higher the conversation ratio, the less efficient the system and the controller will run hotter, thus wasting power as heat. Much better to have two strings of two panels each. For a 24 volt system for a conversion ratio of 2.7-1, if you end up using 12 volts, all the panels in parallel would be a 2.6-1 conversion ratio and will produce more power and be easier on the controller. Heat is the arch enemy of electronics,, the hotter you get it, the less effeciency and less life.

That said I do not recommend a 12 volt system as it does not leave room to grow your system. A 30 amp controller at 12 volts is 360-420 watts max but at 24 volts the same controller is capable of 720-840 watts. (Calculeted at charge voltage of 14 and 28 vdc.)

Estragon · June 2019

In this case, I agree with @Tecnodave that 2 strings of 2 may be a better setup. The CC buck conversion efficiency is offset by voltage drop to some degree. With a longer wire run (eg >100-150') from panels to CC, the higher string voltage might be better, but at 30', voltage drop isn't as big an issue.

Doing 12v with 4 in parallel would mean adding a combiner box and string breakers or fuses, but 2 strings would be okay with 'Y' connectors.

I'd use heavier than 4awg from battery to bussbars and inverter - as big as will physically fit on the inverter. 4awg is good for CC to busses though. I'd also consider a breaker for the battery circuit as they're handier as disconnects than fuses.

Miguifly · June 2019

Thanks for the input @Estragon and @Tecnodave!

Main take-aways: The components match in terms of volts and amps. Sticking to 24V is a good idea. I will switch to 2 strings of 2 panels and use thicker wire from battery to busbars and inverters. I will look into getting a circuit breaker for the battery circuit too.

As for the heat situation, this is for a community garden, and it can get pretty hot in the summer. I was thinking of putting the elctronics under a shaded table and inside a truck box to mitigate the heat. Is there any other good way to keep them chilly that I am missing?

Thanks again 😀

Estragon · June 2019

Although not specified, note that you will need an mppt type charge controller to use those panels effectively in series.

As well as shading, one possibility for heat mitigation might be to make a small simple below ground vault. The thermal mass of the surrounding earth keeps temps closer to average, avoiding more extreme daytime highs. With lithium batteries, there should be very little self-heating during charging, as there is no absorb cycle to speak of. Some in extreme heat have used chilled water baths etc, but those sorts of measures may be overkill in your application.

Miguifly · June 2019

Great thank you! The below ground vault is a great idea, and very easy to do at our location

Solar-bicycle project. Charge controller and inverter questions

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