24VDC Solar and System Design

[Kugel8x57]

Hello Everyone. I'm not a complete newbie to DC systems, but new to "Off-Grid" type systems.

Let me start off with some basic comments:

1) I have a family member that is T1D (Type-1 Diabetic), and their insulin must be stored in cool box (ice-chest or refrigerator).
2) We live in an area regularly threatened by hurricanes,tornadoes and thunderstorms, which can, and have, caused long power outages.
3) Also, we live outside of town and have a well; so, when the power is out, we can only have intermittent water by running a generator.
4) We get plenty of sunshine, but not a lot of usable wind - too many tall trees around us.

Things I plan to do, to help mitigate power outage problems:

1) I plan to reopen an unused existing shallow well, install a 500 gallon water tank/tower and a 150 Gallon/Day Charcoal Water Filtration System.
2) Build two small 24vdc systems: one to power a 12vdc/24vdc refrigerator; and, one to power a 24vdc-5A submersible well pump for the water tower.
3) Build two solar panel systems; one each to charge the above refrigerator/well systems.
4) Build a 24VDC Generator system using a PMG or PMA and small gas or diesel engine. This will allow me to run the well pump for extended periods,
if necessary, and be able to charge both battery banks relatively quickly, during inclement weather.

Things that I think I have learned from Mr. Will Prowse and others in my personal research, so far:

1) 24vdc systems are a good compromise for higher voltage, lower current, safety, higher efficiency and cost.
2) Although the initial cost is higher, 24vdc appliances and equipment are more efficient that using 120vac and an inverter... probably around 20% better overall.
3) Although initial system cost in higher, mainly due to the extra battery cost, the lower system current helps to offset that by higher efficiency
lower wiring cost and lower operating temperatures, leading to longer life.
4) Budget constraints will probably put me into an AGM battery system, using two 12vdc batteries, wired in series to give me 24vdc.
5) I will probably spend the extra money on MPPT charge controllers.

Things I know that I need help with:

1) How to properly charge each system with the DC generator, without damaging the batteries or other system components.
2) Proper sizing of each system in Amp-Hours.
3) I'm sure there are many other things that will come to mind, as I get deeper into this.

Any feedback and/or direction would be greatly appreciated.

Best Regards and God Bless!

Kugel

 

[snoobler]

You're spending substantially more for greater efficiency and less flexibility. You could give up that 15-20% efficiency, spend the same, or maybe a little less and have additional features. A dorm fridge is dirt cheap. A 900W generator is cheap. An inverter capable of directly charging the battery bank from the generator isn't cheap, but it's not horribly expensive.

24V is good, but given that it doesn't sound like you're going to have a lot of demand, 12V might be better/easier.

1) Given that you're building a custom 24V generator, you should be able to control the charge voltage.
2) Depends on the power consumption of the devices used.
3) Probably way more than you think.

I would start with link #5 in my signature for the available solar in your area. Then use link #1 to compile a list of things you want to power and for how long they need to run per day. Once you have the power demand nailed down, you can properly size your system.

 

[John Frum]

2) Although the initial cost is higher, 24vdc appliances and equipment are more efficient that using 120vac and an inverter... probably around 20% better overall.

If you have to distribute the power any distance higher voltage really helps.
AC also helps in power distribution because you can use less beefy switching gear because of its ability to quench arcs.
This calculator will let you explore the benefits of 120/240VAC vs 24VDC.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

Of course there is ~15% overhead to convert dc to ac.
Remember the amps go down as the voltage goes up for the same power.

 

[Kugel8x57]

Thanks for the replies, guys. Still working on exactly what refrigerator I will purchase; however, I've pretty much settled on the pump.
- It is 24vdc-5amps = 120 watts Max.
- It will be indirectly controlled by a float switch, through a relay, that will turn it on and off.
- It will pump 3.2 gallons/minute.
- It will come on when the tank is at 350 gallons and go off when the tank is full, at 500 gallons. Usage should be only 150 gallons/day average.
- 150 gallons / 3.2 gallons/min ~ 47 mins/60 ~ 0.783 hrs x 120 watts = 94 watt-hrs / day.
- Also... 0.783 hrs x 5 Amps ~ 4 Amp-hrs.

I had not gone through all of the math for the pumping system, until just now. It is going to be a lot smaller than I thought! )

I'll do the same for the fridge, after I decide on what to purchase.

Best Regards and God Bless!

Kugel

 

[MisterSandals]

2) Build two small 24vdc systems: one to power a 12vdc/24vdc refrigerator; and, one to power a 24vdc-5A submersible well pump for the water tower.

This seems odd that you'd build 2 systems. Is this for redundancy?
I would consider building one hearty system with replacement(s) for questionable/critical items.

 

[Kugel8x57]

MisterSandals,

The shallow well that will be reused with the filtration system is about 75 yds away from the house, where the refrigerator will be. Line losses and wire costs would be a bit high! And considering how small that I think they shall both be, I believe that the costs for the two systems will be entirely reasonable.

Also, for Mr. Snoobler...

One of the main reasons that I want to use a 24VDC is that my experience with DC is mostly in Controls - PLCs, etc. - and I may, eventually, be incorporating a small PLC for programmable system control. These control systems are 99% 24VDC with many readily available and reasonably priced industrial components.

Many Thanks for your replies and God Bless!

Kugel

 

[Craig]

I use a mix of both 24v and AC. AC is for small fridge,small freezer,instant water heater(fan only), and power tools. DC is water pumps and all LED lighting and computer equipment(mainly raspberry Pi's. Routers and security cameras). This seemed to be the cheapest way to go with minimal inverter losses.

 

[Kugel8x57]

I use a mix of both 24v and AC. AC is for small fridge,small freezer,instant water heater(fan only), and power tools. DC is water pumps and all LED lighting and computer equipment(mainly raspberry Pi's. Routers and security cameras). This seemed to be the cheapest way to go with minimal inverter losses.

Thanks Craig!

Yes... as I have thought more about it, I'm sure that I will be using a mixture of both 24VDC and 1120/240VAC. I just have to look at what makes the most sense for my situation.

Thanks again and God bless!

Kugel