Setting up an off grid system with no battery

diallodjeri

Hello folks,
I would like to know if it's a good idea to set up an off-grid system with no battery, just panels, and inverter. The only load on the system will be a heavy duty commercial grade freezer. As long as I can harness enough energy during the day, I should be fine at night.

mcgivor

A couple of bad sun days would be a bit of a downer, actual sun hours are less than light hours, meaning say 5 hours production, to maintain the temperature for 19 hours, maintaining temperature to safe levels for food, an assumption, would be a challenge. So I'm going with bad idea. 

Estragon

It would also have to be pretty heavily overpanelled to handle the starting load. Even in full sun, starting a pump draws the current from batteries initially on my system. In theory, there should be enough pv capacity to start, but it doesn't work that way. Maybe something related to the way mppt works.

Dave Angelini

A battery based inverter needs a battery, right? Ever see one without and how unstable it is?

Offgrid people, if you are brave disconnect your battery and see what happens, only if you feel lucky. 

Even the Outback Skybox (not out yet) needs the grid to work without a battery.

If the OP is talking about a grid tie inverter then it is a different question. He said offgrid.  He is not clear on this to me.

BB.

As always--The answer is "it depends". Yes it is possible. There are DC powered refrigeration compressors that take a wide range of DC input voltages (12-24 volt Vmp solar panels) and will start/run correctly when the sun is up.

http://commercialrefrigeration.danfoss.com/products/compressors/dc-compressors-for-mobile-cooling/

In theory, order the correct compressor, call a refrigeration technician, have them cut out the old pump and install the new pump, and connect your solar array.

In practice, may not be so easy. First question--Will you be putting in "warm foodstuffs" during the day, when the sun is up and compressor is running? If not, warm food will heat the rest of the refrigerator/freezer above safe temperatures.

Next, is the "box" designed for optimum off grid cooling needs? Is it well insulated? In general, a vertical fridge/freezer, every time you open the door, it dumps out the cold air. A chest freezer type design can be much more efficient (in fact, a lot of off grid folks use chest freezers with a new thermostat--And they only use ~1/3-1/4 of the cooling energy of a typical energy star refrigerator).

A method used to "store cooling energy" is to line the freezer with salt water or similar, or a heavy metal plate attached to the evaporator.

And there is the issue of temperature cycling... More or less, for long term frozen storage, you want to keep the freezer below 0F... Even 5F is "warm enough" to shorten the storage life of food.

http://forum.solar-electric.com/discussion/302/question-best-freezer-temperature

Another issue with DC/Solar powered freezer conversion is the question of thermostat--12-24 VDC power through the freezer's AC thermostat contacts will typically burn out or weld the AC designed contacts (DC power is much more difficult to make/break, and 120 VAC is 1/10th the current as 12 VDC).

The DC refrigeration compressor from Danfoss is basically a multi-phase motor (2 or 3 phase) and a VFD--Variable Frequency Drive.

For larger compressors, it may be possible (and even practical) to get a Solar Input VFD (these are becoming popular with off grid water pumping/wells) and to drive a two phase motor (typical compressor has a second winding with a starting compressor) or a three phase AC compressor.

So--The question really is "what do you need a full off grid solar/battery-less commercial freezer for"? A big help is to measure the loads (starting current and kWatt*Hours per day).

-Bill

jonr

Most refrigerators and freezers don't have enough excess compressor capacity to provide enough cooling for 24 hours over a ~6 hour period.  Ie, battery-less needs significant compressor over sizing.   The amount of thermal storage needed and temperature stability are also issues.

I'd stick with the standard - batteries.  You can optimize a little bit by turning it off (a simple timer) for the morning hours (just before solar power is available).

Dave Angelini

He did say an Inverter Bill in an offgrid system. Your point on DC refrigeration and holding plate systems has fond memories from our boat.
We could stay below zero for 3 days in the tropics. I saw a land based system that was built out under a palm/thatch roof that was made to go for 10 days. It was 10 days until parts could get to the island and there was a bar to run

BB.

I would suggest a real VFD configured for solar vs some sort of a.c. off grid inverter. MW&S does or did sell a magic box to run a standard off grid inverter from a solar panel (I think a voltage controlled relay that cut DC power below 11 volts DC -- Resets the low voltage cutoff for the inverter).

Dave, what sized/type cold plate for 3-10 days of storage?

-Bill

Fixed auto correct typo. VFD

Dave Angelini

Ever see the really large stainless serving trays they use for big catering affairs? I seem to remember they were 36" X 48" and about 10 of them with a eutectic brine solution.
The reefer / freezer was a top door about 15 feet long and pretty deep. (needed a stepstool).
The freezer had doors and a fan that leak controlled the reefer temp. The insulation was over R60, way over!
Each of the plates was dual plumbed copper for a DC loop and and engine drive loop.
Nice little island bar off the pacific coast of Panama. An expat cruiser ran it circa 1995.
I remember he said it took 2 days to defrost it every year.

Estragon

Damn, now you've got me thinking

Dave Angelini

Look at Technautics, if they are still there, they pioneered small boat holding plate systems. We had one for 10 years and it was solid.
Are you not up in Canada? Seems kind of a mismatch as they really are for the tropics or deserts.

Estragon

Yup, up in Canada, but at a seasonal cabin.  Lots of insulation makes the most difference, but the notion of freezing brine when the sun shines has some appeal.  Kind of the opposite of heating water as an opportunity load.