Best way to add diversion loads

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Best way to add diversion loads

Postby HughF on Tue Feb 02, 2016 5:16 am

We have very frugal power requirements, on average between 1.5-2kwHr per day. In the summer, with 2.5kw of panels, we have plenty of time in float. What is the best way to use this 'spare' power? Should I add diversion loads on the AC side, switching them in with the AUX contacts on the MX-60, or is there a 'better' way? I'm undecided what to use for a dump load as yet - I think it will be a combination of air compressor (we have a shop and it would be nice to fill our HUGE air receiver with air so we can use hand tools instead of electric tools when we play in the shop when it's dark) and water heating.
HughF
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Re: Best way to add diversion loads

Postby SailorThor on Thu Apr 28, 2016 4:15 am

Good morning,

There are a couple of extensive threads about using every last watt from PV panels once critical loads are satisfied:
viewtopic.php?f=17&t=3413
and
viewtopic.php?f=29&t=2865

There are a couple of complications with either plan (compressor and water heating.) Compressed air storage is sort of abysmally inefficient, and water heating suffers standby losses. If the heat of compression can be captured and utilized, then compressed air storage regains some efficiency.

Battery storage is useful because the standby losses are smaller than the heat of compression losses with compressed air, and the heat loss of most water heating situations. Direct use of the PV output for any load you can time properly to match the PV output is the most efficient way, to avoid battery system losses.

I'd say go with whichever is going to provide the most utility to you. If you have your air system really well sealed (leaks are efficiency killers on compressed air systems) and allowing the PV to spin up the compressor can be done while you are out of the shop so you don't have to listen to the compressor run, that may be well worth the inefficiencies.

The challenges with an AC side load for diversion generally deal with matching the load to the excess PV -- it is tough to run a compressor at just the right speed to match the PV excess, since most compressors use bang bang controllers. If you could replace the compressor motor with a 3 phase motor, use a single phase to 3 phase variable frequency drive inverter, and run the speed input of the VFD from the "SSD" output of an Outback Charge controller (maybe with an RC filter to get the right voltage range), you could tailor the speed of the compressor to just match the excess output of the PV system. This may not keep the compressor lubed properly if it is a splash lubricated compressor at low speeds.

Modifying the speed of a DC motor is generally easier than an AC motor. Finding 48VDC motors is tougher than finding 3 phase motors. Adding a second smaller maintenance compressor may be the easiest solution, rather than hammering the inverters/PV/battery with a heavy load that gets switched on and off frequently due the heavy current draw of the compressor on marginal sun days. Avoiding relay cycling and oscillations in the system is important to reliability and longevity. The tiny HDX compressor at Home Depot and the Blue Hawk tiny compressor at Lowes both have DC motors in them, with a bridge rectifier diode and noise suppression capacitor. I verified that the HDX compressor from Home Depot will spin from a 48V battery bank (but the switch and pressure switch must not be used on DC.)

Pumped water storage can be more efficient than either compressed air or water heating, suffering nearly NO standby losses, and operating at any speed needed with a positive displacement pump and variable speed DC motor with simple controls. Switching half an array over to PV direct pump on float status could pressurize a large water tank, or pump to an elevated water tank quite efficiently. Or a mosfet driving a DC motor right from the battery bank with the SSR aux output feature. Some deadband and minimum speed considerations may come into play, to avoid heating the motor windings when the motor isn't spinning.

Have you built up a diversion load system since you posted in February? What did you choose? How well is it working for you?

Cheers,
Michael
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Re: Best way to add diversion loads

Postby HughF on Thu Apr 28, 2016 5:53 am

Thanks for your reply Michael. I haven't built a diversion load since February, no. We are running about 7 hours a day in float, which is nice, the PV carrying all the loads during the day (refrigerator, laptop, a few tools in the shop sometimes).

We plan to add another 100ah of batteries (another string in parallel) so we will have some spare capacity for cloudy days.

Still undecided about the diversion loads...
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Re: Best way to add diversion loads

Postby JRHill on Thu Apr 28, 2016 11:03 am

SailorThor wrote:Pumped water storage can be more efficient....


Yup, I agree. But what to do with the water is the next challenge. A neighbor has the wonderful asset of having a well that was dug into a spring which blew a column of water 6' into the air. When capped and redrilled, he has ~ 25 psi at some crazy volume that he uses to run a hydro generator. But It only gives him 300 watts with a pretty substantial flow. I love the idea and have a 3000 gallon storage tank up the hill, but that tank would be drained quickly compared to the fill time for 300 watts as its running. And the hydro generator was not at all inexpensive. So far, the water storage has more value for emergencies (drought season, fire, well failure, etc.). So I guess its back to what ever utility uses makes for the best advantage in the situation.

BTW, keep an eye in the newspaper for a cheap treadmill (Proform, Weslo and the like). Most run on 90VDC 1.5hp brushed motor via a PWM or SCR variable controller which can be re-purposed for many cool things. Not to mention the running belt, rollers, etc.

Best,
Jim
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Re: Best way to add diversion loads

Postby SailorThor on Sat Apr 30, 2016 1:56 am

What to do with water?

Drink it, take a bath, a shower, wash a car, or irrigate a garden. With elevated water storage, constant feed to a slow sand filter could purify water for you with little energy input. Fill an emergency wildfire fighting tank. Fill a pond, a pool, or run a water feature to add pleasant sounds to the yard. Water storage can also be heat storage or "cold storage." The pumped water might be returned to a well from whence it came in a geo-thermalmass system for cooling or heating. Add another pressure tank inside the thermal envelope of the house for greater temperature stability inside. Jet a new well, water trees, add an aquarium, start an aquaponics system...

Last resort, if you have a huge tank/elevated pond that you don't mind running dry is pumped hydro-power for electricity. But that isn't terribly practical on the small scale, as far as I can tell. You could overspeed an induction motor with a turbine pump running backwards to backfeed the output of an Outback VFX inverter, but I think finding other uses for the water would be more cost effective.

Run a waterjet cutting machine, fountain...

Any others?
Michael
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