First Off-Grid Solar Freezer (for Dummies)

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First Off-Grid Solar Freezer (for Dummies)

 

“Building Your First Solar-Powered Off-Grid Freezer (for Dummies).”

 

Jeffery Yago, is an electrical engineer who writes for the website, Back Woods Home.   In one article, he talks about how to set up a freezer in a remote back woods cabin without grid electric power.  He does a nice job breaking it down to the level where us non-engineers can understand the issues.  I’ll summarize my synopsis here and link to a very useful handout from Grape Solar (see age 4) aimed at rank beginners who need to compute their power requirements.  (Hat tip to TallestManOnEarth and my neighbor, Gerry.) 

Danfoss is an OEM manufacturer that has made a highly efficient, very dependable compressor used by many companies to run their small refrigerator / freezer products marketed for RVs, boats and back-woods cabins.  It runs on 12 Volts DC and uses a brushless motor making it ultra-dependable.  It is ideal to run off a bank of deep cycle batteries.

 

Freezers build around this compressor are expensive ($1,000 +) but very energy efficient and durable.  The 12 Volt DC requirement makes it possible to run off batteries without requiring the equipment to convert DC battery power to AC (an “inverter”).  So the expense of the freezer is partially offset by a much less-expensive, more-efficient electrical solar system.

I’ll focus on one model, this 8 cubic foot chest freezer: the SunDanzer CDF225.  It is just under 4’ long x 3’ tall, and has about double the capacity of the 4 cubic foot freezer found in most American kitchens.  Cost is about $1,400.  (I know. I know. Ouch!!)

 

So we have decided to put this freezer in our hypothetical remote cabin and power it from batteries.  The batteries will be recharged by solar panels.  What kind of system will be needed to support this freezer?

 

Refresher:  A couple of basic electrical terms.

 

Amps.  Electricity can be envisioned as a river of electrons flowing through a wire.  Amps describes how many electrons per second are going through the wire. Amps measures the flow of current

 

Volts.  How much pressure the electrons are under as they flow through the wire.  When a battery “pushes” electrons out into a wire we want to know two things: how many electrons per second (Amps), and at what pressure (Volts).

 

Amp-hours.  Describes how many electrons a battery can store

 

 

How much “juice” does this freezer require?

 

Yago includes a table that gives the power requirement for several models during summer and winter.  Warmer days make the compressor run more.  For this model:

 

Summer:  40 amp-hours/day.

Winter:  36 amp-hours/ day.

(~3 amps of current per hour)

                                   

An amp-hour is a measure of charge capacity--how much electricity a battery can store.  Knowing that our freezer needs 40 amp-hours/day lets us calculate how many batteries we need.

 

 

Choosing our batteries

 

Lets assume that we have occasional rainy or cloudy days where little sunlight falls on our solar panels.  Lets choose battery capacity large enough to run the freezer for 4 cloudy days.

 

40 amp-hours/day x 4 days = 160 amp-hours.

 

Unfortunately, deep cycle lead acid batteries are damaged if discharged past 50%.  So to have 160 amp-hours of stored electricity we will need to buy 320 amp-hours (or more) of battery capacity.

 

We must choose batteries designed for deep repeated discharging.  Car batteries will give out after only a few months if used this way.  Golf cart batteries work the best.  6V Trojan T-105 golf cart batteries, $148 each.   (Thanks again Tall.)

 

Each of these 6 Volt batteries has 225 amp-hours storage capacity.   2 of them hooked together in series, will give the needed 12 Volts DC current, but still only 225 amp-hours—not quite enough stored electricity to run our freezer for 4 cloudy days.  To get 320 amp-hours capacity, we’ll have to buy 4 batteries.  Here is how to hook them up.

 

 

Diagram: one golf cart battery

 

Diagram:  Two 6-Volt batteries hooked together in this way (in series) behave just like a single 12 volt battery which is what our freezer needs.  But they have only 225 amp-hour capacity—not quite enough.

 

Diagram:  Four golf cart batteries can store more than enough juice (450 amp-hours) to power our freezer for 4 days, at 12 Volts, without discharging below 50% capacity.

 

 

How many solar panels do we need to charge the batteries?

 

How much current does one 100 Watt solar panel produce?  Solar panels are marketed by their peak power output in ideal conditions.  Since solar panels all pump out electrons at a “pressure” of about 16 Volts, the current produced by one 100 Watt panel, during one hour of full sunlight, is given by:

 

100 Watts / 16 volts = 6 amps of current

 

During a sunny summer day, a single panel makes 6 amps/hour x 6 hours of sunlight / day for a total of 36 amps of current per day.  Recall though that our freezer needs 40 amp-hours / day.  So one 100 Watt panel is not quite enough. We will need 2 panels--a 200 Watt system.

 

The addition of a second solar panel will allow putting more amp-hours of charge into the battery than will be consumed by the freezer that day. Having extra amp-hours going into the battery on a sunny day will carry us through those days without sun.

 

 

Hooking it all together using a charge controller.

 

 

 

 

This system uses the Morningstar-SS-10L-12V-SunSaver-Charge-Controller available at Amazon for $56.   The places to hook the wires to the solar panels, batteries and freezer are all labeled.

 

 

 

 

TLC for batteries

Batteries are damaged if overcharged.  Since our solar panels don’t “know” when the batteries are full, we must use a “charge controller” to shut of current flow when they are full.

Likewise, batteries useful life is reduced when they are discharged too deeply.  For golf cart batteries, consider a 50% discharge the limit.

However, like horses pulling a heavy cart, the strain put on each horse depends on how many horses are sharing the load.  Adding extra batteries in the system makes them all last longer as it does not force them to discharge so deeply.  In a prolonged grid-down situation, replacement batteries will probably not be available.

And if the grid is available, using the grid (rather than the battery) to run the freezer, similarly prolongs battery life.  (This is why Yago includes the grid connected 12 Volt battery charger in the wiring diagram above.)

 

 

Solar Kits from Costco

 

Grape Solar is a reputable company that makes small off-grid solar kits.  The 200W kit, enough for our freezer, is available at Costco for $650.

 

This kit comes with an inverter (used to convert DC to AC current) something that is NOT needed for this refrigerator.  But it conveniently includes connecting cables.  Each panel has an aluminum mounting frame on it.  The user will need to build a mounting rack.   (Face the panels to the south, and tilt them up according to your latitude—38 degrees where I am in Virginia.)

 

 

For those of a more DIY bent who would prefer to not use a kit, individual solar panels can be purchased at Costco for ~$140 each and the charge controller (12V-10Amp) from Amazon for $56.   Connecters, wires and the batteries would need to be purchased separately.

 

 

Cost

Off-grid freezers are not cheap. 

For the 8 cubic foot SunDanzer CDF225 discussed above:

$1,400 for the freezer

$   600 for 4 batteries

$   650 for a solar kit with 2 panels and some lumber to mount the panels.

$ 2,600 total

 

The next smaller sized freezer, the 5 cubic foot SunDanzer 165 is less expensive and requires a less expensive system to support it.

$ 950 for the freezer,

$ 300 for 2 batteries,

$  140 for 1 solar panel (or this 100 Watt kit)

$     56 for the Morningstar 12V-10Amp charge controller

$     20     for some wiring and connectors

$1,466

 

 

 

The Ice House at Monticello

This cost makes me reflect on a recent trip to Monticello, the plantation home of Thomas Jefferson.  Three centuries ago he had an ice-house, a mammoth brick and mortar cylindrical structure dug deep into the ground under a shady side of his house.  During the winter, slaves would go down to the river and cut surface ice with hand saws and carry it back on horse drawn carts.  If the ice-house could be filled by March, they would have refrigeration until late August. 

The ice-house allowed the household to preserve game greatly improving the nutritional quality of their diet.  Nutrition and sanitation are the twin pillars of our ability to resist infectious diseases and to remain productively healthy.

Compared to Monticello, a “mere” $2,600 for a solar freezer doesn’t sound quite so bad.

 

james_knight_chaucer's picture
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I wonder if there isn't a way

I wonder if there isn't a way to do without the batteries and charge controllers and use the 'coldness' of the freezer for the storage? I guess you might need to beef up the insulation.

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hi 'i've been using a solar

hi

'i've been using a solar freezer for about 5 years. i use the GMAT marine batteries from cabella's and they last about 3 years. cost of the batteries equals out to what one would pay for the same amount of electricity...so someone has this figured out. the batteries right now are the weak link and it's almost like the ptb are keeping it that way.

i've been thinking about experimenting with the idea of letting the sun (via the sundanzer)cool the freezer at night and if well insulated it should carry thru the night til it gets another boost from the sun. the heat from the condenser still has to be vented away from the freezer.once the food is frozen that mass should keep things cool.  also things like freezing food first in a reg freezer or putting warm foods in only during a nice sunny day.

my entire house works on this principle, of being so well insulated, that i can run the ac or furnace just twice in a 24 hour period.

so yes i think some other options are very possible, and i can verify sandpuppies costs analysis.

remember if you live where it snows, put your panels where you can scrape them off regularly

 

fh

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correction let the sundanzer

correction let the sundanzer cool during the day

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Thanks for sharing your research

Wow, thanks for sharing your research on this topic.  This is on my list and I have briefly looked into it before.  This info takes the edge off; I have not found a straight path for this exact issue of keeping a freezer going off grid.  Thank you

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Directly Connecting Solar Panel to Freezer

James_knight_chauser and ferralhen,  Good questions.  I took this issue to my friend Gerry who is a engineer, and he pointed me to some OEM spec sheets that mostly went over my head.

But I did get this much:

1.  Yes, it is possible to connect a solar panel directly to several of the Danfoss (now Secop) compressors as they are designed to work over a range of voltages from 12 V  to 40 V.  Theoretically, this could avoid the need for charge controllers and batteries.

2.  And, he agreed that one way to keep a direct solar freezer cold during hours without sunlight is called "thermal banking."  Basically, you put jugs of water in the freezer and freeze them solid when the sun is up.  Then during the night, the melting of the ice absorbs heat and keeps the freezer cool. Here is a graph of this happening.  An engineer could compute the exact number of jugs of water needed to keep a specific freezer cool for a specific number of hours.....

 .

But there are two problems with thermal banking:  1.  Space given to water jugs is space not available to store food.  And these freezers are little to start with.  2.  Once the ice completes its melting, the cooler warms.  And if the next couple of days are cloudy, your food thaws too, .... then spoils.....    It doesn't seem to me that thermal banking is dependable enough for food storage that is essential to survival.

3.  It takes a big surge of current for a fraction of a second to start a compressor.   A direct solar setup would have to have enough solar panels to supply this PEAK STARTING current.  

Graph of starting current:  Left axis is amps of current and bottom is time in milliseconds.

4.  Every time a cloud passes over, solar panels stop producing current.  The compressor would stop, then need to restart again, over and over, wearing it out prematurely.  

A battery bank would supply A) the starting current surge,  B) a continuously current even when sunlight was intermittent, and C) enough current to keep the freezer running through 4 days without sunlight.

An important distinction in Yago's setup above is to think of the freezer as battery driven.  And, the batteries as charged by solar panels.

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amorphous solar panels work

amorphous solar panels work when it's cloudy and that is why i have a combination of those and polycrystalline panels. i'm thinking about ac from these compressors, for just one room to sleep in.

ultimately, i keep thinking supply chain disruption and where am i vulnerable.

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Great report

Thanks for putting this together Sand_Puppy.  I have the panels and related components, but I'm waiting until we get our new roof on before installing them and buying the batteries.  I will be taking this through an DC/AC converter which will reduce overall efficiency, but allow me to use a standard AC freezer and also use the system for the occasional charge of other household electronics.  Another way to reduce the load of the batteries is to add extra insulation to the freezer or keep it in a root cellar or basement.  I have considered burying mine below grade, but have not looked into the technical challenges of that yet.

One note on efficiency.  Standard front door refrigerators and freezers loose a lot of cold air every time the door is opened because cold air sinks and with a front door freezer there is nothing to hold it in.  It spills out each time the door is opened forcing the compressor to go to work.  Top door freezers are much better in this regard because the cold air settles into the freezer itself rather than spilling out.  One simple way to avoid thermal loss with a top door chest freezer is to open the door very slowly which limits the vortex that can draw cold air up with the door and allow it to spill out.  Crack the door slightly and wait a second for the pressure to equalize then slowly draw it open.  Also, know what you want before you open it so that you don't keep it open longer than necessary. It is a small gain, but in a energy-constrained future the small gains will make all the difference.  As my Dad used to say to me when I was a kid, "Close that door, you're letting the bought air out."

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Comments on 12V systems

years ago I investigate using low voltage DC applicances for off-grid or grid down scenerio. I've reached the conclusion that switching from AC to low voltage DC is not the best option

1. Low voltage DC loses a lot of energy in cabling. To power a 120W device at 12V requires 10 Amps of power. With at AC, 120W requires about 1 AMP. Thus for applicances that need a lot of power, very heavy  gauge cabling is needed to be installed. The cost of the heavy cabling is expensive. Even with the heavy gauge cabling there will still be a large loss because the much higher current will result in higher resistance losses. Power loss =  I^2R. Power loss increases at square of the current load. 10 times the current and the power loss is 100 times.

2. Low voltage DC system can be prone to fire if the cable heats up or begins to arc. AC power switches off 120 times per second which provides a sufficient time to quench an arc.

3. In a long term grid down, it make become impossible to source replacement parts locally. Few People will have Low voltage DC appliciances. However there will be excessive amount of AC applicances that can probably obtained for free in abandon homes.

4. An off grid AC power system is more flexible since you can power just about every electrical device since they are all designed to use AC Power. Its much more difficult to find DC appliances that manufactures always charge a premium compared with DC systems. In addition to a refriderator perhaps you want to power up a laptop/desktop PC, well pump, clothes washer, cordless drill battery rechargers, gas stove/oven (electric controlled oven thermostat), window fans, LED lighting, etc. While it may be possible to source all these devices that except DC power, its fair easier to source them with AC power inputs.

If you choose to go with a DC system, I would recommend looking at using 36VDC or 48VDC systems instead of 12VDC since it would cut the amount of current needed by 1/3 or 1/4, which would reduce cabling costs and improve overall system efficiency. Danfoss also sells 24VDC and 48VDC compressors:

http://www.danfoss.com/Products/Categories/Photos/ra/Compressors/Direct-...

One last comment is that its possible that the PV panel may not recieve sufficient light to keep the batteries charged. Perhaps the region experices extended overcast for many days which causes a deep cycle drain, or complete depletes the battery, or snow coverage obscures the PV panels. One option is to incorporate a portable inverter generator with an electric start that can automatically start and recharge the battery as well supply power to run refrigerator. The electric start inverter generator can be started using a microcontroller (ie Arduino)  or a SBC such as a raspberry pi. I believe some charge controllers or AC inverters also offer auto-generator start to recharge the batteries if the PV panel cannot provide sufficient power if your not the DIY electronics type. 

 

 

 

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Direct PV refriguration

"1. Yes, it is possible to connect a solar panel directly to several of the Danfoss (now Secop) compressors as they are designed to work over a range of voltages from 12 V to 40 V.  "

I don't think so,and here is why.

1. The PV panel may not always provide sufficient voltage. During partly cloudy days or when the sun angle is large relative to the PV panel, the output voltage can be significantlly lower. for instance, a 12V Panel may produce only 8 or 9 volts. This will either cause the motor to stall or put excessive stress on the motor as the windings draw more current and saturate the windings. Motors (DC or AC) are subject to saturation which cause input power to be dissipated at heat instead of mechanical work. This is a bit complex to explain. In general practice, a motor should not be operated at low at a voltage below its rated input. Operating a motor below its operating voltage will sigificantly reduce its operating lifespan.

2. DC/AC Motors under load will pull a current surge when starting up. The PV panels may not be able to source sufficient power to start up the motor. Especially if the Panel aren't providing 100% of the thier output because of the sun angle.If the motor fails to start, its possible that the current flowing will saturate the windings and begin overheating them, damaging the motor.

3. The Batteries also serve to provide power at night and during overcast. I am certain that a direct tied PV refrigurator system will have exteremely poor temperature regulation resulting in frequent food spoilage, especially in periods of overcast which exceeds thermal banking measures.

 

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Autodefrost

"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. 

 

 

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manual defrost

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.

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Show me

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.

Thanks

 

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12 volt[DC] efficiency

 

      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.

 

 

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earlier thread on this

More about this from our historic forums: http://www.peakprosperity.com/forum/grid-refrigeration/19539?page=1#comments

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alfrede wrote: Techguy I
alfrede wrote:

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.

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.

 

 

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Excellent thread

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.

 

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idea

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.

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Supercaps used to avoid high DC starting motor currents

Hi Jim

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.

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"Can you link to some that may be out there" (starting caps)

The Supco brand seems most popular (SUPCO HS6 (SPP6) Hard Start Relay Capacitor) and the bigger  (most cost) the better.  These are for the AC motor at 120 volts or higher.  If you have all 12 volt or 24 volt system: I would use a stereo power capacitor (Walmart sells a giant one rated at more than 24 volts) to put in parallel with the power lead to the low voltage DC motor.  BTW there are starter circuits for pumps that do a similar job of helping water  pumps alleviate high pulse starting and can get a water pump to operate off solar panel directly without a battery so if you want to run a pump directly off a solar panel and the starting juice isnt there, you have that option too.  futurlec.com sells a kit  for the latter.  best wishes

Mots's picture
Mots
Status: Silver Member (Offline)
Joined: Jun 18 2012
Posts: 198
Super starting caps: correction

Correction: the super starting cap is AC so the leads are NOT + and - (no polarity).

MoonShadow's picture
MoonShadow
Status: Member (Offline)
Joined: Jul 3 2014
Posts: 3
Direct PV refrigeration, without batteries.

Yes, it can be done.  Much cheaper than with a dedicated battery storage bank, as well.  The SunDanzer DDR165 is designed to run directly from a solar panel with a 120 watt minimum rating, and uses "holding plate" style ice packs built into the walls for storage of "cold" energy.  It's a 5.8 cubic foot cavity, and it can be set to be a true freezer (if you have the sunshine to support that, which ups the solar requirements hugely) or a refrigerator, but not both at the same time.  I believe it's priced around $1600 before shipping.

 

sand_puppy's picture
sand_puppy
Status: Diamond Member (Offline)
Joined: Apr 13 2011
Posts: 2041
My Conversion Experience (DC to AC)

My understanding of "the best way" to set up a small, grid-down, type of system for my home to run a few essential appliances such as a freezer, is evolving with this discussion and ongoing reading.  

I think that I'm coming around to the view that Mots just articulated above:

1.  Lots and lots of solar panels

2.  An inverter to make DC current into AC

3.  Using "normal" AC household appliances and electric shop tools because they are 1)  ubiquitous and common, 2)  much much less expensive than their DC counter-parts, and 3)  I already have many of them (such as a freezer, table saw, LED lights and a washing machine).

The advantages of keeping the electricity DC (the way it comes fresh out of a PV panel) seem to have diminished with the falling price of panels.   The losses that result from the DC --> AC conversion fade in significance when a few more cheap panels are added on.

One poster here, about a year ago, indicated that the had set up an entire wood shop in a shed in his yard with solar panels directly powering ONE outlet in his shop that permitted him to use one electric tool at a time during daylight.  His wood shop was entirely off-grid.  His secret was Lots of Panels, an Inverter, and being comfortable restricting his woodworking hobby to daylight hours.

----------------------------------------

A request from those of you who have done this successfully:  Beginners need specific information.

1.  Show us which panels you are using?  Where did you buy them, which size, what connectors.

2.  Show us how you mounted them and what connection hardware you used? 

3.  Which charge controller and inverter are you using?  Specific model?  Where did you buy it.

4.  Which batteries? (Please give a link.)  Do you house them?

5.  Which super capacitor?  Do you need one for each motor and water pump?  Which model, how much money are you spending?

6.  How big is your system?  What appliances do you run off it?

7.  Can you link to a wiring diagram?

Thank you for taking the time to show us what you have done with enough detail so that we could retrace your process.

 

sand_puppy's picture
sand_puppy
Status: Diamond Member (Offline)
Joined: Apr 13 2011
Posts: 2041
Mots: Are these the panels?

The company website for Sun Electric in Miami:  http://sunelec.com/

Which are you using.  It sounds like they would like to you buy a pallet at a time, but I can't find the number of panels on the pallet or the price for a pallet.

1.  http://sunelec.com/datasheet-library/download/sun/SUN-85-95_ce8tra8los.pdf

or these,

2.  http://sunelec.com/datasheet-library/download/sun/SUN-SF-SERIES-THIN.pdf

or these?

3.  http://sunelec.com/datasheet-library/download/sonali/Sonali_Solar_240-SS_230-240-250_Series.pdf

This third link mentions "high efficiency in diffuse light condition"s and "amorphous" and "thin film."

Rwrek's picture
Rwrek
Status: Member (Offline)
Joined: Feb 9 2012
Posts: 19
Insulating Freezers

I am running a small ac freezer with a solar system(inverters, charge controller, etc) in a very hostile environment (Texas heat) and the insulation question came up.

I learned that many freezers use their outside metal surface to dissipate heat (unlike many older ones that had coils on the back to aid cooling). This means trying to insulate a freezer may actually be preventing it from cooling properly.

Research this subject for further details.

I have had to settle for making sure space was clear around the freezer and "brute force" battery power and charging capacity was available. Even then I have to wind up supplementing the charging of the batteries for cloud cover lasting too long.  Not ideal for grid - down possibilities.

If you find ideas that work, let me know.

Great info from Sand Puppy though.

 

 

 

sand_puppy's picture
sand_puppy
Status: Diamond Member (Offline)
Joined: Apr 13 2011
Posts: 2041
Going in on a Pallet of Solar Panels?

I am contemplating buying a pallet of high quality solar panels and would like to know if any pp'ers on the east coast would like to go in with me.  A pallet is 30 of these BIG 275 Watt panels.  I would like to keep 8 - 10 myself.  Would anyone else like to participate in this bulk purchase with me?  Please PM me if interested or want to talk.

I live in Charlottesville, Virginia and would be willing to drive a couple hundred miles (to West Virginia, Washington DC, Maryland, North Carolina) to distribute them to others.  If people lived beyond this radius, we might be able to figure out a two person hand off.

Here is the website and page for this panel

http://www.wholesalesolar.com/products.folder/module-folder/SolarWorld/sw-275.html

And, here is the product specification sheet for this 275 Watt panel.

They are 66" x 38" and have a metal frame.  Weight is 47#.  "24 Volts" DC.  Rated at 275 Watts each (Much bigger than the 100 Watt panels discussed in my freezer post above.)  Cost will be $275 (for a 275 Watt panel) plus shipping to my house (which is yet to be determined).  This is $1 / Watt!!

They are Mono-crystaline (the higher quality design), made in the USA (not China), and seem to be from a very high quality and reputable company according to the reviews I have found.

----

Professional quality mounting of PV panels is now about as expensive as the panels themselves.  To save money, I am personally intending to mount them myself, on the ground, with frames that I make inexpensively.  Should the grid go down, I'll get very busy building mounting racks quickly!  Here is a picture of someone's low tech mounting arrangement.

And a much more expensive arrangement:

Smaller ground mounts:

I have been won over to the approach of purchasing "too many" panels so that I can tolerate imperfect positioning, intermittent shade, overcast weather, and all the other inefficiencies from a system that is not laid out too perfection (which I cannot do at my house).  I don't have a perfect south-facing roof and the large shade trees (which are most welcomed in the Virginia summer) leave no section of our home or yard sunny all day.  I'll need to cluster panels in 3 different groups spread out through our yard in order to have PV power all day.

Since I am prepping, I'll keep several in my basement in reserve for the day that they are suddenly needed.   I will use PV power to pump from a rainwater collection tank to my garden.  

Possibly a neighbor may want to run their freezer also.  You know, one of those people who were sure that "nothing like this could every happen...." 

 

james_knight_chaucer's picture
james_knight_chaucer
Status: Silver Member (Offline)
Joined: Feb 21 2009
Posts: 160
This is my rainwater

This is my rainwater collection system Sand_puppy. It doesn't need a pump as it works on gravity. You can just about see the yellow timer below which waters my hydrangeas for 5 minutes every evening.

johnln's picture
johnln
Status: Member (Offline)
Joined: Aug 9 2014
Posts: 1
Solar powered freezer

This is my first entry onto a forum. I started with a new medium chest freezer using a Danfoss BD50 compressor. The compressor is running off a solar system array of 1.5 KW feeding into a 700-amp hour 24-volt battery bank. The charge controller between the solar panels and the batteries is an Outback MX60. I have built the freezer into the house and am not using the built in condenser. This saves floor space, allows for an external condenser to heat water and dissipate waste heat outside the house and triple the insulation around the freezer. To minimize cost, voltage drop and copper cable I have used a ring circuit. This is commonly used in the UK. The BD50 compressor is designed for R134a gas. This is a synthetic gas and has been linked to cancer. I charged the freezer/compressor with a mix of isobutane and propane and changed compressor synthetic oil to pure mineral baby oil. The 24-volt power system also powers all of the lighting in the house using 12-volt strip LEDs and DC-to-DC converters. This system powers all other low voltage devices including a modem, desk top computer, phone chargers, anything else that had a wall plug transformer and even charges an electric bike.

I had no prior experience with refrigeration. All the systems have been functioning perfectly for 4 years. Almost all learning was through the Web.

Danfoss has a controller for running the BD50 and BD35 compressors directly from solar panels. Look up Secop solar panel powered BD compressors. 

Boomer41's picture
Boomer41
Status: Silver Member (Offline)
Joined: Nov 30 2008
Posts: 137
A more general approach.

May I humbly submit that a solar powered freezer is just a sub-set of the larger problem of keeping a household functioning off grid. If you need a solar powered freezer you probably also need a solar powered TV, microwave, lighting and communications. I believe it is simpler (and possibly cheaper in the long run) to make a battery-powered, whole-house power system which provides 120 VAC, just like the utility company, and can run whatever you plug into it. No special DC motors required, just off the shelf appliances.

This can be achieved with a nice, big fork-truck style battery and an inverter to convert battery power to 120 VAC. All appliances are then plug and play.

The battery can be charged from solar panels, wind power, the grid (when it is on) or a small generator when it isn't. In all cases, bigger is better, so the highest capacity battery and most solar panels you can afford is the way to go.

Nothing about this system precludes the efficiency improvements of extra insulation or an external condenser on the freezer, it just makes the whole endeavor much easier.

See my posts at http://www.peakprosperity.com/blog/portable-solar-generators-hybrid-emer... for more details.

 

TechGuy's picture
TechGuy
Status: Gold Member (Offline)
Joined: Oct 13 2008
Posts: 453
Putting them at ground level

Putting them at ground level instead of the roof is a much better in my opinion. In the event that you need to take them inside, ie a hail storm, hurracane, etc. its far easier to move them. I would sugguest some sort or easy remove fastners to the frame so you can remove them quickly and with out aggravation. That said you need to mount them to a reboust frame so that can't blow away in a windstorm, as well as two legged predictors that may spot your panels from a distance.

FWIW: I would consider mounting them on a fulcrum frame (ie like a teeter totter) so that you can set up a sun tracker that would track the sun as it rises and sets in the day so you can maximize output for more of the day. The pole could be actulated using a motor(s) attached to a screw drive(s). Its probably not possible to get a full 180 degree travel, but even a modest 60 degress of travel would do a lot. That said you probably need some basic metal working skills and welding to make it right.

Another consideration is thermal panels for Domestic hot water as perhaps using extra hot water to extend temperatures in greenhouses.

TechGuy's picture
TechGuy
Status: Gold Member (Offline)
Joined: Oct 13 2008
Posts: 453
"You mentioned how does one

"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."

Starting Caps are using in AC motors improve starting torque. However these have tiny capacitors and are not useful when the input voltage is low or the power source is unable to supply sufficient current. An AC motor cap will have ~70uf to ~200uf (or 7x 10^ -6 Farads) The are intended to increase the draw of power from the power source because motors have large inductances can have high impedances preventing sufficient current to start up the motor.A SuperCap will work more like a battery and has a charge storage measured in Farads, or about a half million times more than an AC motor cap. 

I was presuming the person asking about avoiding batteries was because batteries don't last very long when they have a large number of discharge cycles. A typical lead-acid battery probably will last 300 to 700 cycles, depending on the depth of discharge. Often oversized battery banks are used to limit the depth of discharge so they can last a longer number of cycles but this gets expensive and large banks costs lots of $$$. A SuperCap has an extremely larger number of cycles (100K to 1M). so would last essentially forever, but they don't have any where near the capacity of batteries. A 10F SuperCap bank would probably only be able to run the motor for a few seconds where as a battery can run a motor for 30 minutes or longer. The idea is to use the Supercaps to supply sufficient startup current to get the motor running when the solar cells can't supply enough power for startup. (ie low angle, shadows, partial overcast, etc).

Alternativatly Super caps can also be used with Battery banks to extend the life of the batteries. The Super-Cap will reduce the stresses on the batteries by providing the power for sudden surges when a device is turned on (ie compressor motor). Super caps have a much lower internal resistance the bulk of the inital current will be drawn down from the supercaps before drawing from the batteries. 

 

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