24hr generator vs inverter/charger efficiency?

Re: 24hr generator vs inverter/charger efficiency?

In Africa some years ago, they started a Hybrid power program for villages.

Most of the time, the village would run from a large battery bank+AC inverter. Then in the evenings, everyone home, lights, homework, cooking, etc., they would fire up the diesel genset and power the village and recharge the battery bank. I am not sure, but I think they saved about 50% on fuel vs genset running 24x7 without the battery bank+inverters.

We can hand wave all we want--But I would suggest building a relatively simple model of your electrical loads/pattern of usage, and then cost out the system to run it.

For the moment, fuel is almost 1/2 the cost it was less than 1 year ago, so generators will have a big help right now. And batteries/electronics are pretty much at a peak, and I don't see a big dip in battery prices at the moment (cost of energy is a fundamental driver of any manufactured product--Perhaps folks have seen some weakening of prices?).

And, ignore capital/maintenance costs at your own peril. New battery bank every 3-5 years (or 7-10 for more expensive batteries), new electronics every 10+ years, etc. are huge drivers of costs (buying a "new" solar power system every 6-10 years).

Gensets--How many hours do you expect to get? 6,000 hours on a small Honda eu2000i is possible. There are 8,766 Hours in a year--Looking at almost two "cheap" Honda gensets per year for 24x7 operation. Plus fuel costs.

How much for fuel:

• 400 Watts * 9 hours * 1/1.1 gallons per tank = 3,273 kWH per gallon
• 8,742 hours per year * 1/9 hours * 1.1 gallons of fuel = 1,069 gallons per year
• 365.24 days per year * 0.4 kWatt genset * 24 hours per day = 3,506 kWH per year (at 400 watt average load)
• 1,069 gallons per year * $4.00 per gallon = $4,276 fuel costs per year
• 1,069 gallons per year * $2.20 per gallon = $2,352 fuel costs per year (cheap gas)
• $4,276 fuel costs per year / 3,506 kWH per year = $1.22 per kWH
• $2,352 fuel costs per year / 3,506 kWH per year = $0.67 per kWH

Add in 2x $1,000 per eu2000i and $1 per oil change once per 24 hours

• ($4,276 fuel costs per year + $2,000 genset + $365 worth of oil) / 3,506 kWH per year = $1.89 per kWH
• ($2,352 fuel costs per year + $2,000 genset + $365 worth of oil) / 3,506 kWH per year = $1.35 per kWH (cheap fuel)

Cost of fuel, while significant, is still only a fraction in the diver of overall costs (82% increase in fuel costs gives "only" a 40% cost increase in generated power.

That is pretty much in the middle of the range of $1-$2+ per kWH that a solar power system seems to cost (every time I have added everything up and divided by power used). Yes, you can get less than $1.00 per kWH for solar power--A few people here have done that--But I would plan for higher cost for budgeting/planning.

Generator costs per year go down if the generator is not used (i.e., use 1/2 the time, about 1/2 the yearly costs).

With solar, works best with ~9 months of the year usage or more... If you use the system 1/2 of the year--The batteries and electronics are still aging and will probably not last that much longer not being used--So, the $/kWH costs actually go up with less usage for solar...

Having a genset running 24x7 is not ideal (noise, smoke, refueling, oil changes, etc.)... But the high initial capital and maintenance costs of solar are not either...

And why I always push knowing your loads and conservation first--No matter your choice of system power--Conservation (insulation, buying very efficient appliances, turning stuff off when not in use, etc.) is still your best investment. Anything else to generate power is probably going to cost a lot more.

-Bill

Re: 24hr generator vs inverter/charger efficiency?

For diesels, the conventional wisdom is that you should load them around 40% to 60% of rated load minimum... Or they can wet stack/glaze cylinder walls/build up carbon.

One poster here said that modern diesels (high tech, designed for reduced emissions, etc.) should not wet stack even at lighter loads.

From the olden days, it was a quick and dirty tactic (and probably very dangerous) to fill some 55 gallon drums with salt water and run electrodes in the tank--Basically boil water to keep the genset at 60% rated load.

Figure out what your minimum rated load (and maximum) for your genset should be (sometimes it is nothing more than a guess and use 40% as a minimum load for some calculations and see what pops out).

Diesel cycle is much more thermally efficient vs a normally aspirated gasoline/petrol engine. Gasoline engine fixed RPM gensets typically have their minimum fuel flow very need 50% of rated output capacity. For diesels, their fuel consumption continues to drop below 50% of rated load (very fuel efficient)--But the potential of running below 40% continuous may raise the maintenance/reliably issues... I don't know.

Do I like did above... Figure out your power needs and time frames they are needed. If your loads are very low outside dinner/evening time--A small battery+inverter system + genset for dinner time running/charging (plus solar, if you wish) can be a very viable system.

It really depends on your loads and usage history... Running a diesel at 50-80% fixed load (say irrigation)--A solar + battery bank is going to struggle to match the costs of the pure diesel solution (of course, price of fuel is a wild card). Solar is very expensive in up front capital costs, plus long term costs for battery/electronics replacement.

If your loads are 100 watts for 12 hours per day, 400 Watts for 8 hours per day, and 2,000+ watts for 4 hours per day--Then a mixed battery+inverter+diesel is probably quite viable.

Basically 400 watts from an inverter+battery+solar (if you wish), and the 2kWatts+ from genset.

Doing the paper designs/modeling can teach us a lot. Use the various rules of thumbs to get "close" to an optimum solution--Then look at the results and figure out if there are any things to change/adjust once you have the basic system sizing/information. All before you layout your first $$$.

-Bill