Air Conditioning and Solar and the Mojave Desert

Hardware Hub
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Ok so I am in a bit of a bind here. Kind of a “more than one way to skin a cat” thing and I am trying to figure out a way forward.

Situation:

I am building a Starlink connected off-grid solar powered security and monitoring station. I have a 20’ container to work with. It is located near the Mojave desert where daytime summer temps can be extremely hot.

Conundrum:

Keeping this thing cool. So air conditioning is a must but I am trying to figure out the best way to go about it.

Current plan:

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This is where I am at now. Problem is that the size of the solar array and the batteries are quickly ballooning to support the 24v DC A/C system. The container will have shade provided by the solar array on the roof and some sunshades set up on the exterior. It will also be insulated. According to this random calculator I found on the internet, I should only need 4500-5500 BTU to cool the entire container provided it is properly insulated. Even less if I constructed an insulated wall inside the container to reduce the sqft necessary to cool. So although the A/C unit can draw up to 1200w I don’t expect it to be necessary.

Now where the indecision comes in. I chose the 1200w A/C system because it was 24v DC and I expect it to be quite efficient running directly off the battery bank. However, Tripp-lite makes these rack cooling A/C units in 2k BTU and 7k BTU. The attractive qualities about these are that they are less BTU so not as “over spec”, easier to install, and they have a network card for remote control since no one will be onsite to monitor anything on a regular basis, potentially less battery and solar capacity required? The con’s, 120v AC power so I would need a properly sized inverter and would suffer more inefficiencies due to voltage conversion.

The equipment that will be inside is about 400w so lets say 500w for safety. That means to offset the equipment I only need about 1700 BTU. so 1700+4500=6200 BTU of cooling necessary to cool the entire container. Seems like that 7k BTU Tripp-lite unit is looking pretty good? Now what if we partition the container down to say 10ft? Even less. Now what if we just used the 2k BTU unit and mounted it directly on the rack and insulated the rack itself?

I hope my ramblings made sense here. I would really like some advice from anyone that has experience with either building solar systems or air conditioning or both.

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Oh dear god that’s a horrible, no good, very bad, completely useless calculator. I think I need to go lie down. Please at least start with a “data center” air conditioning calculator.

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you could bury it a little, but please get someone to reinforce it with welding some gussets in the corners! Something I was thinking about is running a swamp cooler to ‘pre cool’ the air before you passed it through the AC, given an adequate space to drop out excess moisture, but not so big as to heat up again, would lessen the impact on your A/C module. insofar as voltage, I’d say go with a simple 12v system, and even though you’d have to spend some major $$$$ on a high amp capable pure sine inverter, you’d still end up saving money because you halved your battery and panel costs. Speaking of batteries, don’t cheap out on back up power. Have at least a day or tow in reserve power. Also it would be convenient if it was 12v as you could in theory, simply drive to location, and connect your car system into the circuit. Just my thoughts.

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Please recommend one if you have a better idea of what to look for. I’ll search for a datacenter ac calculator.

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No heavy equipment or swamp cooler. It is a remote site that is unmanned so nothing that requires regular maintenance.

Would like to stay with 24v or 48v to keep high efficiencies and reduce losses.

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When you say “insulated” and “shaded” you mean like it’s basically a fridge in the desert where the external part of the container gets no sun at all? … or something else?

btw, basic thermodynamics say your AC will be less efficient in taking the heat out, the warmer it is outside. Look at detailed specs for the units in data sheets, not just the “achieves 3.5 COP at 23°C outside” numbers. Usually the test reports where units get certified will be publicly available and they’ll have more data.

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This struck me as funny. While not pertinent where its located, near the Mojave is about like saying near Czechoslovakia in land area.

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south of Vegas just on the California side along the I-15.

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Uncountable cars have overheated running AC out there.

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Yes lol

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Be aware solar panels have wildly fluctuating output voltages.
For maximum efficiency from the solar installation, you also need some sort of system to do MPPT.

Given the heatload of the sun baking your container, I would not worry about an inefficient inverter.

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You have not given a budget so I’ll just assume you have a reasonable one

First of all, being a security site and off-grid at that either you’ll want a generator in the mix or you will need way more solar and battery to guarantee it does not go down. Being near Mojave, while guaranteeing few days of cloudy weather, will not do anything for your power consumption at night
Second, go with lithium phosphate batteries at 48v, they are more efficient, electrical installation requires more reasonably sized wires and you will find more offerings

For the air conditioner, if your BTU estimate is 7000 BTU, go for a 15000 BTU unit, you do not want the ac to always work at max, and if you underestimated it you’ll end up with cooked stuff
As for ac vs DC, have a look at this:

It is not a definitive guide, but will give you context and a real use case of ac vs DC cooling

You did not list your PV setup details, I’d suggest you check online what is the expected solar irradiation at you site, but some napkin math for you:

Assumption:

  • You get a somewhat efficient AirCon solution that will provide the amount of cooling for an average 300W at night, 600W during the day, so 10KwH per day for cooling
  • your other hardware consumption will be constant at 350W, so 8,5 KwH per day
  • total system consumption per day: 18.5Kwh
  • You are completely offline, your batteries should be able to guarantee 3-4 days of the daily output, or you need a generator

If you want to play with actual numbers, this site is cool:

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These estimates are for a 11Kwh Solar array, with a 13Kwh Inverter and 20KwH of lifePo batteries (15.5KUSD + taxes) at signature solar:

That system would cover your needs for the entire year as consumption during the day would be covered by the solar array, that would still be able to charge the batteries for the night, assuming it never rains for more than one day. If that happens, then you need double or triple the batteries and double the solar power to cope for winter production, and you’ll be wasting heaps of production during summer … that is why usually it makes more economic sense (if still costly) to setup a backup generator, to kick in when solar production is not enough, but that requires maintenance, either a gas line or a fuel tank and I am nit sure how viable that is in a completely unsupervised setup …

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Replying to myself,
a possible solar+generato setup that could live in a remote location:
the previous kit but a different inverter:

that would provide solar input, battery BMS, MPTT, 120/240VAC inverter and has a dry contact relay to start a generator when batteries go below a certain pre-set level

and a Natural Gas/Propane generator that can be started by said inverter:
https://www.generac.com/all-products/generators/home-backup-generators/ecogen/15kw-7163-wifi-enabled
and for which an appropriate sized gas tank can be installed without having to fiddle with diesel tanks and fuel pumps …

OIP

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I’ll talk to the client about a genset. The solar array and battery banks are getting very expensive and very big very quickly to run the air con.

This is the solar controller I was looking at using:

It has a dry contact for controlling a genset and voltage control for the PV array as well as BMS for the batteries.

Also using their inverter. They have a 2500w 48v one that I’ll probably swap out for.

48v rackable battery banks with UL rating and fire suppression. Chainable BMS that should be able to integrate with the solar controller.

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Yep, unfortunately using solar power for cooling/heating stuff is still an inefficient proposition, both from an economical and energy point

The equivalent (lower rating on AC output) for supporting a bigger solar array is 1500USD and you still need an inverter on top of that …

Anyway the projected energy balance with a 1.7Kwh solar array is going to be:

Screenshot 2023-05-25 at 15.22.38

That means you will never be able to even charge the batteries, because you will be using all the solar power to run the loads, and when solar declines, everything will shut down.
You are missing 10 KwH of solar production per day on average to maintain the estimated load , and my estimate is lower than yours …

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They look like a very good option for the US market, I like this guy’s style of explaining things

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@MadMatt Thank you! Your responses have been extremely helpful.

I talked with the client this morning and they are very adverse to anything that requires maintenance and would really prefer a fully solar solution so that is the direction I will head for the time being.

I’ll have to get a new quote together with a much larger capacity and they will have to deal with the solar panel mounting and such.

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When sizing A/C units how much should I oversized the unit? What is the duty cycle for the average A/C system?

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How exotic are you willing to go?

Because passive cooling paints are something you can make. Now this is passive, which means it won’t do squat for heat generating components but that Mojave sun well that heat could be improved a little bit.

It is not a commercial solution but there are plenty of tutorials online.

If you look up how to make infrared cooling paint you would have an interesting addition to your cooling system.

From what I understand it is a bit of work but you are the one building a solar Star link facility in the desert , you can probably call the shots if you think it’s not worth it.

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Good insulation can *GREATLY affect temps Wendell’s garrage stays 70 winter/summer

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