Powering Commercial Refrigeration Unit With Solar 1

[bucknast]

Hey there,

Working on a project where we need to cool a shipping container down to 50 deg F and hold it there for long periods of time. Container will be outdoors under a sunshade where solar panels will be mounted. The goal is to power the refrigeration unit with a solar system with energy storage.

Looking down two design paths. Option 1) buy a refrigerated shipping container and design PV system to work with the reefer. Option 2) buy an insulated shipping container and design refrigeration and PV systems together.

Right now I'm struggling to size a solar system. Refrigeration systems are very power hungry. Even if they have a low duty cycle, when they are on, they will still draw lots of current. A battery storage system's discharge capacity will need to keep up with the current draw during operation, even if the batteries are not drained that much.

For instance, a Tesla powerwall 2 can output 5 kW continuous, and a reefer compressor will draw 7-8 kW during operation.

Soooo I'm thinking that the container refrigerators would require a larger than needed solar system just to accommodate these spikes. While a smaller refrigeration unit could run off a smaller solar system and still achieve the design requirements.

Looking for guidance. Does this make sense? What am I missing here?

Thanks.

 

[FreddyNurk]

There's a number of ways to approach this, but there's no getting around the reality of sizing the equipment, and what that might mean for your plant (and cost).

Its possible to trade off the size of the battery system against the solar system to allow for extra capacity for compressor starting, but you'll still need to size the system to both run the refrigeration unit during the day, plus allow for charging the batteries in order to maintain capacity for when the sun isn't shining.

For a flat load of 8kW for 24 hours, you're likely to need more batteries than you might be expecting (the Powerwall 2 is only 14kWh), same with the required surface area to provide the solar panels. If you need to cater for overcast conditions then your storage needs will increase too.

You'll likely need to oversize the inverter somewhere in order to deal with the high starting current for the refrigeration plant too.

EDMS Australia

 

[LittleInch]

If by shipping container you mean a 40 ft ISO container then you're going to need a lot of solar panels and batteries.

Most commercial systems available off the shelf are a balance between size and cooling capacity / cost. You probably need to double or triple the insulation to do everything you can to get the heat loss down to something reasonable. Then you can reduce the power requirement of your cooling unit.

Can you bury the container and get access from the top?

Also look at what is in the container and how much air you have. A large metal block can act as a good heat sink / cold storage device which can smooth out any temperature loads.

The cooling load will vary during the day so you're kind of luck y that if the Sun is really hot then you'll be generating lots of electricity and can run the cooling system more frequently / harder.

Battery systems are not easy to design from scratch. If I was you I would look at commercially available UPS systems which already have all the electronics sorted.

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[IRstuff]

You also need to consider the steady state separately from the cool-down. You can insulate all you want, but at some point in time, you're going to put actual stuff in there, and unless they've already been cooled-down, any system you size for the steady state will take forever to cool down the load to its desired temperature.

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[itsmoked]

I've had to deal with this type stuff several times. IR is correct. It is all about what the exact use of this refrigerated space is going to be.

I had to build temperature controls for refrigerated rail cars. We did many studies of the dynamics involved. This was Fruit Growers Express (FGE) and Pacific Fruit Express (PFE). These were insulated cars which held up to about 120,000lbs of whatever. They were all electrical refrigeration systems powered by on-board 4 cylinder diesel engines mated to generators. They had two speeds, 60Hz and 40Hz, this for high and low cooling. We did extensive full trip logging for many many loads because our product was a self calibrating temp controller/logger/energy saver.

What we quickly learned was that all that horsepower could not shift the load's temperature one iota! It could only keep it at its delivered temperature. Our discoveries caused an industry shift. It had always been the rail company's responsibility to maintain some temperature designated by the customer until our studies. The shipping companies were getting shellacked because customers would deliver product that was at, say, 36F but state they wanted the load kept at 34F. One degree can reduce shelf life a day and when lettuce is moving from Watsonville, CA to New York City an extra day or two is critical.

Anyway customers would embed temp loggers and then make claims because the load they supplied at 36F never even made it down to 34F on the entire transit. It simply stayed at 36F, which also showed a delivered load at 38F stayed at 38F, 33F at 33F. After us the rail shipping industry changed its acceptance policy to a signed-by-the-customer agreement that the load would be maintained at the delivered temperature or any temperature higher if allowed. Then the shipping company would embed its own loggers to confirm the delivered-for-shipping-temperature.

Where am I going with this? Just what IR was saying. If you want your container to "maintain temperature" then the task is considerably easier. Actually in, your case, achievable. If you're trying to "chill" anything you are in for a very rough, very expensive ride, using solar. You will likely need three to four times the power to provide chilling service.

You haven't stated if you are aiming for refrigerated space or frozen space. Long term or short term.

Instead of the dealing with all the multiple inefficiencies of charging batteries you might consider the tried and true method of chilled plate eutectic thermal storage. This is used all over the world as a delivery truck system. The fleet of delivery trucks all have cool plates in them. They remain plugged in all night then are unplugged for the day's deliveries. This completely avoids all the problems of remote on-the-move refrigeration.

The chiller plates are boxes usually about 6 inches thick filled with a tailored mixture of some solution that absorbs a huge amount of heat before changing state from solid to liquid. This 'box' is installed on the front wall of the box truck. Already chilled goods are then run into the box for the day's deliveries and the truck is dispatched. The plate absorbs a huge amount of heat keeping the box ably chilled for the day's deliveries. The system essentially keeps the air temp in the box at the plate temp.

In your case I'd get a standard container and then custom build the insulation system making the box leak the absolute least amount of heat possible. Then I'd put in the appropriate chill plate. I'd then use a custom compressor that would be fed from the solar to store every erg of 'cold' deliverable by the sun during the day. This might be a brushless DC motor run directly from the panels.

This will NOT work if you actually have to chill product.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

Great post there and completely agree. I've Mbeen doing a lot of work on the opposite end i.e. keeping stuff hot in an insulated system and the key issue is how long you want to heat it up from ambient. At 7 days it uses 4 times the power just to keep it hot. You need to go to something like 20 days or more before it gets close to the steady state heat loss.

Lots to think about. ...



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.