First Off-Grid Solar Freezer (for Dummies)
"One note on efficiency. Standard front door refrigerators and freezers"
Another important consideration is auto-defrost system that is used to prevent frost buildup in the freezer. Ideally you want a freezer that doesn't have this option or you can turn off auto-defrost. I also noticed that Auto-defrost causes freezer burn as food thaws on the surface causing surface ice crystals to form.
A little off topic since probably none of the 12v freezers have autodefrost.
Re "freeze burns" as techguy noted. Our neighbor served great peach pie from peaches they grew. When asked she said the peaches were from a good season 5 or 6 yrs. ago. I was shocked & commented such; she said the key was manual defrost[& she made sure there was enough juice to cover the peaches]. They tasted fresh. Re defrosting she said 1x/yr tops; my experience has been 2 to 3 yrs. with getting into the freezer 6-10x/mo.
I would like to request people with solar freezer systems show us exactly what they have done. Please be specific and clear so that someone UNFAMILIAR with solar freezers can say "I understand. I could do that too."
Show us your wiring diagram.
What products exactly did you use? (Links and pictures)
Is it your system working well? Or would you do it differently based on what you know now? If you would do it differently now, please show us exactly what you would do.
Help guide us newbies who are trying to set up a system for the first time.
Techguy I believe 12v systems with adequate wire size & joints [robust i'd say also re heat & fire hazard] are almost always going to be more efficient than dc to ac systems. The whole point about adequate wire size is not to have heat loss, hence efficiency loss. Since batteries are going wear out they are the 'weak' part of any offgrid system; that along with the numerous points of possible failure in the system. All things being equal a DC to AC system of comparable size will need more batteries, than a DC system; & there are generally less points of possible failure.
AC system has a major expense & single point of failure, the inverter[& loss of efficiency]. However of course, having an inverter does allow serious flexibility to use tools. Your point about using 24/48 volt where possible [especially for longer cable runs, and larger motors] is very true.
Sandpuppy if expanding is a future possibility, the one thing i would do different from the beginning is get a higher capacity Controller that would allow for expansion[& flexibility]. Morningstar has a 45 & 60 amp[not the MPPT, $150-200] for a cabin-like sized offgrid system. [I'd provide links but not getting the paste function to work]. Also re a grid to battery charger that desulfatIes as well as charging[some] I am impressed with is Battery Minder.
More about this from our historic forums: https://www.peakprosperity.com/forum/grid-refrigeration/19539?page=1#comments
[quote=alfrede] Techguy I believe 12v systems with adequate wire size & joints [robust i'd say also re heat & fire hazard] are almost always going to be more efficient than dc to ac systems. The whole point about adequate wire size is not to have heat loss, hence efficiency loss. Since batteries are going wear out they are the 'weak' part of any offgrid system; that along with the numerous points of possible failure in the system. All things being equal a DC to AC system of comparable size will need more batteries, than a DC system; & there are generally less points of possible failure.
AC system has a major expense & single point of failure, the inverter[& loss of efficiency]. However of course, having an inverter does allow serious flexibility to use tools. Your point about using 24/48 volt where possible [especially for longer cable runs, and larger motors] is very true.[/quote]
DC to AC inverters are pretty efficient as long as the DC input voltage isn't low. A 48VDC to 120VAC inverter will be about 90% efficient and probably about 85% efficient for 220VAC. Higher efficiencies can be achived using even higher DC input voltages. a 144VDC inverter will be about 98% efficient at 120VAC and about 95% at 120VAC. Most of th conversion loss are caused by the voltage difference between the input and output voltages. The closer your input voltage is to your output voltage the more efficient the inverter will be.
In addition, Battery life can be significantly improved by including a supercap bank in parallel to the battery bank because the SuperCap will handle current surges (ie motor startup) reducing the stress on the battery. Companies prorduct SuperCap banks designed for use with UPS/inverters.
The problem as I see it is that we live in AC world, so getting DC appliciances and spart parts is going to be a challenge. In my opinion flexible it worth its more than the efficiency differences between DC and AC systems.
Thanks Sandpuppy… lots of in-depth discussion here of the various factors one should consider in system design, including an understanding of the relative "lossyness" of the various transitions as we go from DC to AC, in-and-out of batteries, and from low voltage to high. Everything after the cell is lossy.. the game is to minimize these losses without spending a fortune on the solar equivalent of (audio) Monster cables and such.
One thing I question is the idea that, were you to run the freezer only during the daytime (i.e. powered by a system w/no batteries) that the contents would begin to thaw overnight. As well, it is not clear to me why you would need bottles of water to be frozen as thermal "ballast", when in fact the contents of the freezer.. the food that you have frozen, would essentially act as thermal ballast. When I put ice in a normal plastic walled cooler, like I would take to the beach… it's still ice overnight and into the next day if left closed. These 12V freezers are very well insulated and I am pretty sure that the amount of heat transfer during a normal overnight would be relatively small. I think, left in a closed freezer, the food would stay frozen for a matter of days if power were lost completely. If anyone can counter this from real life experience, I would appreciate knowing. Based on the same reasoning, my gut is that a system that had no batteries, i.e. were powered only during the day, would be fine as long as there was enough panel area to assure some minimal electron flow, even on cloudy days.
One question for Techguy; The idea of having supplementary super capacitors (don't assume everyone knows what a "cap" is) is completely new to me. Can you link to some of these that may be available in the market… are their any pre-configured systems out there for sale? The idea certainly makes sense – super caps to handle the motor start loads, etc., while the batteries handle the steady state.
Isn't it amazing that it takes so much research to set up one solar powered appliance. Most people who want a freezer go to the box store, bring one home plug it in and they're done. It's no wonder non-grid-tied systems are still fringe. I'm fairly technically inclined, and reading some of this makes my head spin.
Someone could start a nice little business just offering off-the-shelf solutions to some basic solar backups. e.g. – If you're in USDA zone 8 and between latitude 40-45 buy this kit that includes freezer, battery bank, panels and all other components. OR If you're in USDA zone 5 between latitude 35-40 buy this kit to provide solar backup for a 1/2 hp well pump.
The three most important systems I think people would want to backup are a well pump, a freezer and hot water (solar thermal). It would be pretty straight forward to develop regional solutions all three and offer them online cutting out most the technical work. With those three, a good outdoor cooking area a wood stove (with access to wood) and maybe a couple of solar powered LED lights, it's possible to maintain a almost first world standard of living even in an extended grid-down scenario…..at least until the batteries give up on you.
Sand_Puppy these 2 are excellent for your purposes, & since you linked Yago I figured you had seen his numerous archived articles; 2 for example on batteries & battery banks [lotsa detail like you provided-thanks].
Handybob[Handybob's Blog] has lived with RV scale solar, discovered the need for generally higher voltages[for the charging of standard golf cart batteries], how to care for batteries via a measuring device[strongly recommends Trimetric- reports getting 8 yrs w/golf cart batteries], & has retired from doing installations but will provide consultation via email or phone for DIY folk]. His site has some helpful pictures too. Good Luck!
You mentioned how does one use supercaps to help a compressor motor get over the large startup current pulse: Such electric motors already have a "starting capacitor" attached to their surface to help them start. Due to cost concerns, such supplied capacitor is small and cheap. When you buy a super starting capacitor, the capacitor comes with instructions for simply connecting the capacitor in parallel with the existing motor capacitor. (the motor has a starting capacitor with two leads, labelled + and – (red and black wires usually). The super sized cap has the same + connect to + and – connect to -. The instructions that come with the super starting capacitor explain all this with pictures.
By the way, my garden has a solar shed with a cheapo (ca. 150$) black and decker 8 cubic feet freezer in it that has been solar powered for about 14 months continuously now with zero problems. To save money, I just spent most money on panels (1800 watts about 1500$) and use a cheap 100$ charger/controller, 200$ 2000 watt inverter and four (100$ each) 12 volt batteries for a 48 volt system. I am kind of surprised at how well the thing works and local fisherman uses to make ice to take fishing most mornings for over a year now. Excellent freezing (garden produce goes right into the freezer, keeps taste and nutrients) and zero problems during the 14 months. ALso, I use the copious solar electric electricity in the garden to run my double wide electric rototiller (The green machine in my regular picture) and also I run electric grass cutters and sometimes electric coffee pot and griddle in the garden when people visit WHEN the sun is shining. By spending most money on the panels, there is less stress on the batteries as even during "dark" overcast days, you can get 10% of maximum power (in my case, ca. 180 watts). Panels and chargers/controllers/inverters are so cheap nowadays, doing this saves much money and you get a wonderful extra power source for regular use. I buy electronics direct from the factory and panels from sunelectric in Miami. Spend your money on ample panels and overcome the overcast /early morning etc limitations with greater power output to help battery life and get tons of power for gardening or other uses when the sun invites outdoor garden work.