Residential Fuel cells for off grid

Re: Residential Fuel cells for off grid

In general, a true fuel cell (noble metal + hydrogen + oxygen) is a pretty expensive and inefficient process (as I recall, fuel cells <-> hydrogen are less than 50% efficient--before you spent the energy compressing the H2 for storage).

Regular Lead Acid storage batteries are roughly 80-90% efficient and much less expensive for up-front costs.

17 kWhrs per day is a pretty heavy load for an off-grid system (in my humble opinion)... A good point to aim for is 100 kWhrs per month (~3.3 kWHrs per day)...

So, the first question is do you have access to grid power and does the local utility support net metering?

If yes, then either a pure grid tied system (solar panels - GT inverter) is the most efficient use of money.

If you do not have grid power, or you have grid power + net metering and want to plan for prolong outages (storms and such), then a Hybrid type solar+charge controller+battery+GT/Off-Grid Hybrid Inverter can be a nice compromise. You have the advantages of a GT system (efficient and using the "grid" as your storage--typically up to one year of net metering) and you have a smaller battery bank for emergency power and your backup genset.

Otherwise, pure off-grid with solar is certainly do-able--but it will not be cheap... Money wise, for the same amount of "power"--You will need to spend (very roughly) 4x the amount of money for off grid power (more solar panels, charge controllers, battery banks, cabling, breakers/fuses, heavy wire, battery replacement every 4-10+ years, etc.).

Very roughly, the costs per kWhr for electric power (assuming 20 year system life):

• $0.10-$0.30 per kWhr for Grid/Utility power
• $0.15-$0.35 per kWhr for Grid Tied system (+utility power)
• $0.45-$0.75 per kWhr for Hybrid System (GT+Off-Grid emergency)
• $1-$2+ per kWhr for Off Grid power

To size a solar PV system for your needs... The PV Watts website does a very nice job.

For a Grid Tied system, use 0.77=Derating Factor (default). For an Off Grid system, use 0.52=Derating Factor (battery and extra inverter losses).

For example, a 10kW solar PV GT system for New Orleans LA:

"Station Identification"
"City:","New_Orleans"
"State:","Louisiana"
"Lat (deg N):", 29.98
"Long (deg W):", 90.25
"Elev (m): ", 3
"PV System Specifications"
"DC Rating:"," 5.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 30.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.1 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.49, 398, 32.24
2, 4.84, 499, 40.42
3, 5.09, 566, 45.85
4, 5.60, 585, 47.38
5, 5.75, 608, 49.25
6, 5.64, 562, 45.52
7, 5.51, 571, 46.25
8, 5.35, 558, 45.20
9, 5.18, 524, 42.44
10, 5.39, 586, 47.47
11, 4.50, 489, 39.61
12, 3.86, 438, 35.48
"Year", 5.02, 6386, 517.27

For 17 kWhrs per day, that is 510 kWhrs per month. A 5 kW GT system will pretty much match your planned loads (assuming utility supports some version of 1 year net metering).

A 5kW GT system will cost around $6,000 to $10,000 per kW to install--so you are looking at around a $30,000-$40,000 system price (less 30% Federal tax credits and other rebates). Please note: I am neither an installer or in the solar RE business--I am just giving rough rules of thumbs to size/cost your system (level set expectations). Always do your own estimates for your particular needs/location.

A similar Off-Grid system would need to be ~7.4 kWatts or larger (because of less efficiency and you can only store ~3 days of useful power vs the 1 year "storage" of net metering with Grid Tied Utility connected systems):

"Station Identification"
"City:","New_Orleans"
"State:","Louisiana"
"Lat (deg N):", 29.98
"Long (deg W):", 90.25
"Elev (m): ", 3
"PV System Specifications"
"DC Rating:"," 7.4 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 3.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 30.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.1 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.49, 385, 31.19
2, 4.84, 489, 39.61
3, 5.09, 555, 44.95
4, 5.60, 573, 46.41
5, 5.75, 594, 48.11
6, 5.64, 547, 44.31
7, 5.51, 556, 45.04
8, 5.35, 544, 44.06
9, 5.18, 511, 41.39
10, 5.39, 575, 46.58
11, 4.50, 477, 38.64
12, 3.86, 425, 34.43
"Year", 5.02, 6232, 504.79

And a full Off Grid system would cost probably 2x per kW to install (call it $12,000-$20,000) or $88,000 to $148,000 to install (retail). Also, many rebates and credits are not available for Off-Grid systems. Also, this does not include the costs of fuel and genset installation / operation.

In the end, anything is possible. But, in general, extreme conservation through insulation and energy use is almost always a better investment of time and money vs installing any type of solar RE system.

Solar thermal (for hot water / heating) is always a good option to explore. Much less costly to install (although ongoing maintenance can be a pain--lots of plumbing/tanks/pumps to go wrong). If you are a Do-It-Yourself type person--you can even build collectors and install them yourself.

Other good options to look at. Using Propane for heating/hot water/cooking. Look for the most efficient heat pump systems you can find (systems that use the earth or local water ways/lakes for heat sinks can greatly increase efficiency). Also, if you need/use lots of A/C--A desuperheater (use waste heat from A/C) to heat your hot water for "free". You are in a warm climate, so you can even look at a heat pump to heat your hot water (save 1/2 of your electric costs or more to heat hot water--a nice option if you don't have natural gas or other "cheap" fuel source.

Anyway--that is my 2 cents. There is also a thread I started where lots of the above issues are further discussed and/or links provided to other areas with more details (solar hot water, conservation, etc.). It needs to be edited and cleaned up--but it is a good beginning.

-Bill

PS: If you have local conditions that help provide energy (local wood lot, manure for methane production, wet gas well, etc.)--those can be added to your help meet your total energy needs.

We are here to help.

Re: Residential Fuel cells for off grid

Here is a good Government website about hydrogen fuel...

From one of the PDF presentations some very surprising results. Basically, the equipment is still expensive and can output hydrogen gas at the equivalent of $2-$3 per kg (~equivalent to 1 gallon of gasoline) on $0.03 per kWhr or so (... For a home sized solar PV system, your power costs will be ~10-100x larger.

Note that electrolysis/fuel cells are still very expensive and have limited "surge" capabilities (I would guess none). For reasonable day to day use, they need storage batteries to buffer their input/output throughput for best cost/performance (i.e., you would need a <1kW fuel cell system, on average, but need batteries to take a ~7 kW pv array input and output a 6kW surge inverter output for your loads.

It appears that hydrogen powered piston engines (and I assume gas turbines too) are a better means to output power to the home/grid than a fuel cell from the above presentation at this time.

You can do further research--but I just don't see fuel cells in the near term future. There are other developments out there than may make hydrogen possible (direct conversion of solar to hydrogen through chemical/biological means)--but that is far in the future too.

I would really think about the issues of using too small of battery bank for a pure off-grid system. If you have variable sun (i.e., marine layer vs high desert), that means that you have almost no capability to store power for use more than 1 day in advance. That will either force you to use much more fuel for your genset and/or reduce power usage. However, there may be load characteristics that minimize the issue--such as perhaps in cloudy weather you don't need the A/C.

Using PV Watts for 4.3 kW of solar with 0.52 derating factor (off grid) for New Orleans:

"Station Identification"
"City:","New_Orleans"
"State:","Louisiana"
"Lat (deg N):", 29.98
"Long (deg W):", 90.25
"Elev (m): ", 3
"PV System Specifications"
"DC Rating:"," 4.3 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 2.2 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 30.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.1 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.49, 223, 18.06
2, 4.84, 284, 23.00
3, 5.09, 322, 26.08
4, 5.60, 333, 26.97
5, 5.75, 345, 27.95
6, 5.64, 318, 25.76
7, 5.51, 323, 26.16
8, 5.35, 316, 25.60
9, 5.18, 297, 24.06
10, 5.39, 334, 27.05
11, 4.50, 277, 22.44
12, 3.86, 247, 20.01
"Year", 5.02, 3621, 293.30

Say that you want to be on PV only for 9 months of the year, that would mean that February with 284 kWH per month would be your minimum power (without genset use).

• 284 kWhrs per month / 30 days = 9.5 kWhrs per day average (February)
• 12 kWhrs * 30 days = 360 kWhrs per month

At your planned array + usage levels, you will have to use the genet every few days throughout the year to keep up with your proposed power usage.

Personally, if this was my off-grid system and I was watching my dollars--I would probably go with a battery bank sized for 5% rate of charge based on my 12 kWHr daily load and minimum array size (say 9 month calculation above).

Then if you needed to add more solar arrays, you could add up to ~2.6x the amount of solar panels later and still be sized nicely for your existing battery bank.

It is hard to cost effectively add capacity to an existing battery bank. Batteries have an aging cycle that tends to cause newer batteries to age quicker when added to an existing bank. If you have a 5 year old bank and add new batteries to it--the new batteries will tend to cycle more current (because they are newer) than the old batteries and age quickly. All of the batteries will still start failing out at the 8 year normal replacement date (using made up numbers here as an example--battery maintenance and actual cycling, temperature, etc. all can have dramatic effects on battery life).

But batteries, as you have read here, are not ideal devices--and can be a a real problem to maintain for long life. Another reason that you want to use the minimum amount of batteries needed for your home. And why conservation is really a good place to invest (keep your PV electric system as small as practicable). If you don't use a kWhr, you never have to pay to generate it in the first place.

-Bill

Re: Residential Fuel cells for off grid

BleuSoleil wrote: »

Perhaps this would not be a huge problem?...Any idea what kind of runtime for each top off session? Could that be calculated using HOMER??(i do not have proficiency using that software)

I am sorry--I don't know HOMER.

To "model" your system--I would suggest that you pick rule-of-thumb setup first (fewer variables, and there are reason rules of thumbs are used--they give quick answers and size the :"optimum" system based on a variety of reasons--Once you have looked at the "optimum/rule of thumb system"--you can always make some changes based on your local needs).

So, for a 12 kWH per day load on a 48 volt battery bank and 85% efficient inverter:

• 12,000 Watt*Hours * 1/0.85 * 1/48 volt * 3 days * 1/0.50 discharge = 1,765 Amp*Hours (48 volt bank)

Now, for a genset, I would suggest that you look at ~5% 13% rate of charge for your battery bank (bigger generators tend to be setup for autostart/off grid use and slower 1,800 RPM type units). Also, assume that the battery charger is 80% efficient and we will aim at a 50% load on the genset (reasonably fuel efficient at 50% and greater loads.

• 1,765 AH * 0.13 rate of charge * 58 volts charging * 1/0.80 eff * 1/0.50 gen load = 33,000 Watt generator maximum
• 1,765 AH * 0.05 rate of charge * 58 volts charging * 1/0.80 eff * 1/0.50 gen load = 13,000 Watt generator minimum

So, roughly, we are looking at (very roughly) a 13,000-33,000 watt genset (to power a 5,000 watt to 13,000 watt battery charger)

Now, you have a 12kWhr per day load--and lets assume worse case--no sun for 24 hours.

• 12,000 Watt*Hours * 1/5,000 watt batter charger * 1/0,80 eff battery * 1/0.85 eff inverter = 3.5 hours (small charger per day)
• 12,000 Watt*Hours * 1/13,000 watt batter charger * 1/0,80 eff battery * 1/0.85 eff inverter = 1.4 hours (large charger per day)

A lead acid storage bank can take full charging current from 0% to approximately 80-90% state of charge. To get that last 10-20% charging takes time (a few more hours). Typically, it is recommended that you do your generator based charging in the morning and let the solar array finish off the charging the rest of the morning/afternoon (lower current charging to top off the battery > 90% state of charge is when the genset is less fuel efficient).

If your difference in average power usage between 12 kWhrs used and 9.5 kWHrs per day (4.3 kW array in an average February) is 2.5 kWhrs per day deficit--It would take 12/2.5=4.8 days to drop your battery capacity by "1 day's worth of use" (1/6th of bank capacity or 16.7%).

So--guessing on 2-8 hours of generator use every 5 days (depending on weather and genset/battery charger capacity).

Can you elaborate on this? I do not want to have to build a barn as a battery box

Well--it gets back to the 5% to 13% rate of charge (based on 20 Hour battery capacity rating) rule of thumb.

Pretty much, you need 5% charging current to stir the cells up and equalize. And as the battery bank ages--you could get a upwards of a 1% per day self discharge (near bank end of life)... So, too small of main charger (solar panels or generator) can cause undercharging (if the bank is not in use--you can use smaller array to do a maintenance charge and use a genset if you need to equalize or if cycling the bank with loads when needed).

The 13% maximum rate of charge is normally recommended because above 13% rate of charge, the battery bank can begin to overheat for a standard flooded cell battery bank.

AGM's (a type of sealed lead acid battery) can take lower or higher charging currents.

But 5% is a good minimum for a useful solar PV based system. A smaller array would take forever to recharge the bank (possibly leading to under charging with normal loads-or excessive generator use).

Agree. The problem is the hot humid climate and the need for a combination of constantly running dehumidification and or AC(looking at the Sanyo Split). The Sanyo AC would draw 300w when in use and the dehumidifier 264w(an Ebac triton). The rest of our lifestyle needs would be hyper-conservation.

Sort of the Catch 22--You need power build your home and to figure out how much power you use. But you can't design the "optimum" solar PV / Genset system until you have built your home and lived in it for awhile.

I guess I need to knuckle down and learn HOMER or some way of predicting run times for the ventilation systems. We are building a 1200 sq foot passive solar ranch home with super insulation and Energy10 modeling shows our working design will have no need for mechanical systems of heating.

It sounds like you have taken the conservation theme of the forum here to heart--Whether it is because you are "green" or "cheap" -- the advantages are real for both. :D

The problem is that getting to within a factor of 2 estimate of your actual power usage is probably a pretty good guess. Using your existing residence as a yard stick of your energy measurements and as a lab (change out to efficient appliances, stop using the dishwasher, turning off unused electronics, switching to laptop computers from desktops, etc.) can help you better gauge your estimated power usage.

The Kill-A-Watt meter is great for measuring plug-in appliances (especially those that cycle such as refrigerators, freezers, or have variable usage like computers and many home appliances like waster/dryer/small A/C).

For larger/fixed loads--a T.E.D. type device for measuring whole home loads may make sense too (the simple versions are not that expensive).

Also, the battery will be the "heart" of your off-grid system. Study about them in these two FAQ's:

Deep Cycle Battery FAQ
www.batteryfaq.org

In the end--you may end up building your first system with less expensive batteries (4-8 year life) and use them as Training Batteries. And, after you get some runtime on your system--you can decide where to go next without feeling too bad about spending too much money on the first system.

-Bill