Solar or Wind? Off-grid system opinions.

Re: Solar or Wind? Off-grid system opinions.

In general, it is very difficult to design an off-grid system without knowing how much power is needed.

But, my two cents, design the system to use grid power when available (power your loads, recharge your battery bank), look at conservation (it is cheaper to conserve power than to generate your own), and then look solar PV panels and backup diesel/gasoline gensets. It will be sort of like a giant UPS system.

Usually, the only way wind will make any sense is if you can put up a 60 foot / 20 meter minimum tall tower and the trees in the area are "flagging" from prevailing winds.

And even then, wind turbines are relatively fragile animals. Exposed to weather (and salt spray in your case)--they may last months to a few years.

-Bill

Re: Solar or Wind? Off-grid system opinions.

I think I understand what you mean with "Grid Tied" --- Connecting to the utility power as one of your power sources.

We also use Grid Tie Inverters connected to solar panels to generate electricity. It is a great way to produce power--but may not be legal in many countries (you are back-feeding electricity to the utility company). Very popular--but does not provide any backup power (no battery for energy storage).

Once you know their loads--We can recommend some equipment for you to look at...

Obviously, this solar RE stuff is expensive--so they should minimize their loads as much as possible to keep the system small and cost effective (usually no microwaves, coffee pots, big desk top computers, etc.).

-Bill

Re: Solar or Wind? Off-grid system opinions.

2 MW?

2 MW of solar panels, peak power, 2,000 kWHrs per month of power???

In any case, a reasonably priced off-grid system (that I would aim for) is around 100 kWHr per month (3.3 kWH per day).

Bigger Off-Grid systems become very large outlays in money.

-Bill

Re: Solar or Wind? Off-grid system opinions.

The ones I have read about that sound interesting are "hybrid diesel" installations in Africa.

Basically, you have a battery bank that can manage loads for 16 hours per day (use your numbers) to supply night/morning light loads... Then during evening (cooking, cleaning, homework, TV, etc.) time, the diesel is fired up and both providing power for the heavy loads and recharging the battery bank.

The idea being that you want to only run the diesel when you can pretty much guarantee 50-60% power loading (evening loads + battery charging) and let the light loads to be handled by the battery bank+inverter.

Compared to a 24x7 diesel generator power station, you can save quite a bit on fuel costs (remembering that the charger+batter bank+inverter are around 50% efficient--so you don't want to handle heavy kWH loads with just the battery bank because it will use almost twice the fuel as running the loads from the genset directly).

Depending on the size of the village load (peak power)--the "small" (relatively speaking) Xantrex/Schneider XW 6kW hybrid inverter can probably be setup to automate such a system.

But, you are probably looking at something bigger.

[PDF] White Paper: Understand Hybrid Generator - Battery Systems
[PDF] A Study for Optimizing the Management Strategies of a Hybrid ...
Solar hybrid systems for rural electrification, Solar Power for Africa

And, once you have the generator, battery bank, inverter, and controllers--You should be able to add Solar/Wind/etc. as funding and needs permit.

-Bill

Re: Solar or Wind? Off-grid system opinions.

Bullethead and I have been talking by PM a bit too...

Yes, it is 2MWH per month--or 2,000 kWH per month power usage... A largish system, but can be doable.

But, in this case, an inverter/charger + battery bank will probably be the better answer (basically a large UPS made from separate components).

At this point, just need to know how long the outages are (longest needed to ride through) and how much power is needed to be supplied during this time (peak watts--like motor starting loads; and average Watt*Hours used during this time).

That way, we can size both the inverter (peak power needs) and battery bank (total energy used) during the power failures.

A good way to measure the loads is with a Kill-a-Watt meter (US/North American 120 VAC Version) and UK/International 230 VAC version.

-Bill

Re: Solar or Wind? Off-grid system opinions.

Transients can be a killer for an off-grid system.

For example, my very nice Energy Star Rated refrigerator uses around 120 watts +/- steady state... However, it has ~500 watts of defrost heaters and probably a 4-5x current spike for motor starting current...

So, for a "simple" frost free (no ice maker) that uses ~400 kWHR per Year, you would want a 1,200-1,500 watt minimum inverter to power that load (and you can put a few CFL lights + a small radio on there too).

Transient loads are difficult to measure--some of the more expensive Fluke (and other brands) can measure RMS (root mean square--the proper way to measure "arbitrary" wave form voltage, current, and power) and have "peak hold" capabilities.

Motors can easily take 4-5x current surge over rated current for starting.

Some inverters/battery systems can handle surges better than others. Asking the place where you will purchase your inverters/supplies may be able to offer guidance.

Also, in the end, you may be better off installing one system and measuring its performance, then procure the next 10 based on those results.

Also, you need to look at what loads need to be powered when the utility is down during those 8 hours... No microwaves, tea pots, etc. would be a very good idea--but you can design the system to power them.

So, some round numbers. Assume energy efficient refrigerator + a few lights and radio for a 1,500 watt inverter.

Assume 300 kWHrs per month usage or 10 kWH per day, of which 1/2 will be used during the 8 hour outage.

Assume 24 volt battery/inverter/charger (keep wiring sizes down). 5,000 Watt*Hour "UPS" function:

• 5,000 WH * 1/24 vdc batt * 1/0.80 inv eff * 1/0.50 max batt discharge = 521 AH @ 24 volts Minimum Battery Bank
• 521 AH * 3x longer outage = 1,563 AH "maximum" battery bank

The maximum battery bank would be the largest (numbers are very rough) I would go--Any larger and it is additional expenses that would be hard to justify (larger charger needed, larger battery bank, more losses, but you would have a longer battery bank life and 24 blackout protection before needed a generator/solar panels).

Now, if we assume that the refrigerator a a few lights are the only loads... Assume ~1.2 kWH per day for fridge and 3x18W-CFL*8H=432WH for lighting:

• 1,200 WH * 8 hours/24hour no power + 432WH lighting = 832 WH per 8 hour failure:
  • 832 WH * 1/24 vdc batt * 1/0.80 inv eff * 1/0.50 max batt discharge =87 AH @ 24 volts Minimum Battery Bank
  • 87AH * 3 = 261 AH @ 24 volts "Maximum" Battery Bank

So, you can see, playing with loads and assumptions can result in wildly varying system sizing...

By the way, since keeping food/medicine/etc. cold is a heavy electrical load--Look at Chest Freezer to Refrigerator Conversions--They can cut the daily cooling loads down to 1/4 of that of a normal refrigerator (and be cheaper too).

-Bill

Re: Solar or Wind? Off-grid system opinions.

Yep, Tungsten resistance goes up with temperature... A cold filament has much lower resistance (normal and halogen bulbs both):

Incandescent lamps are nearly pure resistive loads with a power factor of 1. This means the actual power consumed (in watts) and the apparent power (in volt-amperes) are equal. The actual resistance of the filament is temperature-dependent. The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms).[30][58] Since incandescent lamps are resistive loads, simple triac dimmers can be used to control brightness. Electrical contacts may carry a "T" rating symbol indicating that they are designed to control circuits with the high inrush current characteristic of tungsten lamps. For a 100-watt, 120 volt general-service lamp, the current stabilizes in about 0.10 seconds, and the lamp reaches 90% of its full brightness after about 0.13 seconds.[30]

-Bill