emergency power system for cooling insulin

[spectraThug]

I'm brand new to the forum so be easy in the event that I'm posting in the wrong area etc or otherwise doing something stupid. My son is type 1 insulin dependent diabetic. I'm wanting to put a PV backup system in place to power some kind of emergency cooling system i.e. a small refrigerator to hold the insulin and enough room to place a large block of aluminum or copper in the cooling unit to act as a heat/cold sink to buffer any short term sunshine outage. The medicine is very sensitive to even moderate temperature fluctuations. We have roughly a two year rotating supply to keep cool. Keep in mind this is to keep my kid alive, not sell power back to the grid. Reliability is very important so redundancy in certain key areas will be expected.

So off grid is probably the way this project is headed. I have 4 250 Watt Grape GS-S-250-Fab5 panels in my hands with the ability to get more if needed. They seem to be 30volt panels; so am I to assume that this would be a 24 volt DC based system with an inverter capable of the wattage I need? I'm just wondering what any of you would recommend for a charge controller/ inverter and batteries. I realize the next question will be "what is the energy requirements for the refrigeration unit you plan to use". Well, I don't really know for sure. With the way things are dumbed down these days, it's hard to get numbers from any of the retail sites on the web. But let's just say for argument 200-300 watt hours per hour @120volts (does that even make sense).

I live in northern AZ, so we get plenty of sunshine. If someone could sketch out a rough system based on these needs I would be in your debt. Thanks.

[Sunking]

You do not need solar to do what you want. You need to have a dedicated refrigerator with a modified UPS which is real easy to put together. First thing you have to know is how much power the fridge uses each day, and the size of the compressor in amps from there it is real easy. So let's say the fridge uses 500 watt hours per day, and uses 100 watts..

All you would need is a battery with up to 10 day reserve capacity in this example 5 Kwh, so at 12 volts = 400 AH, a 25 amp battery charger, and a small 200 watt to run the fridge. Power goes out and you have 5 days reserve capacity no sweat, and up to 7 or 8 days if needed. Commercial power is far more reliable than solar.

[spectraThug]

thanks sunking.

Huh! I hadn't even considered that. Would it be more efficient to use 24v DC over 12v to power an inverter?

[Sunking]

Huh! I hadn't even considered that. Would it be more efficient to use 24v DC over 12v to power an inverter?

Higher voltage is always more efficient. With that said a lot depends on how much power the fridge uses in watts. Do not confuse watts which is power with watt hours which is energy. They are two different things. If the fridge uses low power you can use 12 volts or you can use up to 48 volts.

Whatever you do, DO NOT use parallel batteries. So here is your home work and it is critical homework. You need to determine how many watt hours the fridge uses in a day, and how much power in watts the compressor uses. It would be wise if you went and bought a meter called a Kill-A-Watt meter. Run it for 7 days to get good data input. It will read the watt hours and the wattage. Once you have that info it is real easy.

[Sunking]

Huh! I hadn't even considered that. Would it be more efficient to use 24v DC over 12v to power an inverter?

Higher voltage is always more efficient. With that said a lot depends on how much power the fridge uses in watts. Do not confuse watts which is power with watt hours which is energy. They are two different things. If the fridge uses low power you can use 12 volts or you can use up to 48 volts.

Whatever you do, DO NOT use parallel batteries. So here is your home work and it is critical homework. You need to determine how many watt hours the fridge uses in a day, and how much power in watts the compressor uses. It would be wise if you went and bought a meter called a Kill-A-Watt meter. Run it for 7 days to get good data input. You run it for 7 days and divide the total watt hours used by 7 and you got your daily wh usage. It will read the watt hours and the wattage. Once you have that info it is real easy.

[spectraThug]

I'm on it like a bum on a 40.

[peakbagger]

Sundanzer has has a battery free solar refrigerator that doesnt use batteries if you are in a sunny climate, you just hook panels directly up the unit. It is lined with "ice packs" that freeze belween the freezing point of water so if the sun stops the unit keeps cold for three days.

http://www.sundanzer.com/images/pdf_BattFreeSpecs.pdf

http://sundanzer.com/solar-power-refrigerators/vaccine-unit/

[bonaire]

The cooler should be in the basement (if available) or perhaps even dug into the floor of the basement (ie. a "root cellar style") to allow surrounding land to act as a heat-sink. Along with the electrical refrigeration on top of that.

[Sunking]

The cooler should be in the basement (if available) or perhaps even dug into the floor of the basement (ie. a "root cellar style") to allow surrounding land to act as a heat-sink. Along with the electrical refrigeration on top of that.

He lives in AZ, there are no basements in AZ. YOu do not dig in AZ, you drill, jack-hammer or use dynamite.

[billvon]

I'm wanting to put a PV backup system in place to power some kind of emergency cooling system i.e. a small refrigerator to hold the insulin and enough room to place a large block of aluminum or copper in the cooling unit to act as a heat/cold sink to buffer any short term sunshine outage. The medicine is very sensitive to even moderate temperature fluctuations.

Try this; it will give you 7 days of backup without batteries. (And can be run on regular DC, you don't need solar if you don't want it.)

http://spinoff.nasa.gov/spinoff2003/er_1.html

[peakbagger]

Its good some else figured out the sundanzer option. I am trying the salt brine method for my chest freezer where I have several containers of water mixed to saturation with rock salt, this drives the freezing temp done to below freezing so it supplied more carry through power outages. I am not sure what Sundanzer uses but its the same methods

One thing to remember with most chest freezers is that the coils that get rid of the heat are frequently mounted under the sheet metal outer casing. If you try to insulate the cube while it is running you may cover over these coils and cause damage. Its okay to cover it when the power is off in an emergency but not something you want to leave in place.

[inetdog]

I'm brand new to the forum so be easy in the event that I'm posting in the wrong area etc or otherwise doing something stupid. My son is type 1 insulin dependent diabetic. I'm wanting to put a PV backup system in place to power some kind of emergency cooling system i.e. a small refrigerator to hold the insulin and enough room to place a large block of aluminum or copper in the cooling unit to act as a heat/cold sink to buffer any short term sunshine outage. The medicine is very sensitive to even moderate temperature fluctuations. We have roughly a two year rotating supply to keep cool.
I live in northern AZ, so we get plenty of sunshine. If someone could sketch out a rough system based on these needs I would be in your debt. Thanks.

Depending on the volume of a two year supply (I have no idea how to estimate that), and if you want a semi-commercial prefab unit with very high reliability and provision for solar DC input, you should look at vaccine refrigerators designed for third-world use. Their volume may not be enough for your needs, but they would provide a highly reliable storage for at least part of your stock.

I infer from your description that you are also interested in something that will keep going for months or more without grid power, and one of those would meet that need. They are extremely will insulated, so your results if have to open it multiple times per day may not be as good.

[inetdog]

This is by no means an endorsement, but you might want to take a look at another commercial option. You could use batteries with it to ride through cloudy days.

One concern I have is just how stable the temperature has to be. Will overcooling the refrigerator during the day to avoid getting too warm during the night also be a problem? Would you use your thermal mass to stabilize the temperature under those conditions?

Have you tried looking for a phase change material with a transition point at the temperature in the range that are trying to maintain? That will give you maximum heat capacity in a small weight and volume.

[Vern Faulkner]

Just interjecting my two cents: I've been impressed by the stats on the Steca line - the PF-166 states it will use only 70 watt/hours per day, keeping the interior at 3C in ambient temperatures of 20C outside; 165 whr/day to lower to 3 from 30, straight draw from 12 or 24 volts.

It's the item I am leaning towards purchasing for the new home.

[spectraThug]

This is by no means an endorsement, but you might want to take a look at another commercial option. You could use batteries with it to ride through cloudy days.

One concern I have is just how stable the temperature has to be. Will overcooling the refrigerator during the day to avoid getting too warm during the night also be a problem? Would you use your thermal mass to stabilize the temperature under those conditions?

Have you tried looking for a phase change material with a transition point at the temperature in the range that are trying to maintain? That will give you maximum heat capacity in a small weight and volume.

Thanks Inetdog,

I'm not sure what the threshold is for temp variations. I'm sure the pharmaceutical boys know the degradation curves on stuff like that. Less is better obviously. That's what the aluminum or copper heatsink idea was for; to moderate any temp spikes and "hold the cold" as long as possible. I was even planning on rolling my own digital thermostat. There's plenty of gadgets on eBay for that.

I've never looked into phase change technology; I'll make it a point to do so. Are we talking something like ammonium nitrate?