Solar Assisted Refrigerated Delivery Truck

Re: Solar Assisted Refrigerated Delivery Truck

BB: thank you for the long and detailed response. Much food for thought there. I was hoping for such erudite responses when I posted my original article. I signed a bill of sale for the new truck and am about to sink most of my remaining cash into the Mark 2 truck,

Firstly, my sausages are very fine indeed, and I also carry a few other producer's product. Lots of great artisanal and farm product out there that can't get to market.
I am mostly using the truck for otr use, have been going to Atlanta and New Orleans regularly for about 6 months now. I'm still going through the math, thanks for laying it out so fine.
We actually plan to add a 24 volt alternator to the setup providing my mechanic can fit it in, but we are looking at an alternator designed for bulldozers, a lot cheaper than anything with 'marine' in the name. We plan to use it as a backup in case solar and battery give out and plug-in is not available.

The economics of mobile solar are quite a bit different to land-based solar. This is because gas and diesel are (still) so expensive compared to mains AC. Assuming we take 2 or 3 units of AC at night, it costs less than 50 cents (electricity is cheap here) to run the reefer per day, compared to about 10 gallons of gas for a conventional engine-powered reefer for otr, where the internal combustion power source has to run 24 hours. I assume that the economics of running an alternator would be similar. I estimate I have saved about $1700 over the last 6 months from using the combination of solar and mains AC power, and it would have been more at last year's gas prices. This is esentially because internal combustion engines have an efficiency of less than 20%, the wasted energy disappearing as excess heat in the engine and as friction in the rest of the machinery. It seems to me that the payback period for mobile solar is potentially a lot less than land-based installations.

I am very conservative with my temperatures. I run my freezers in the 0 to 5 degree range. For complicated reasons to do with Boyle's law, that my refrigeration guy has tried to explain many times, refrigeration equipment actually works harder the higher the temperature. Running at zero requires less energy than 31 degrees. The drive-in freezer at the warehouse runs at minus 3 to minus 10 F, so when we load up the new product is colder than the freezers, and this helps. We also try to minimize opening and closing the freezers. And I never park in the shade if possible.

One of my big worries is a van breakdown. I would have to find a mechanic who would plug the van in while working around. And there is a fullrange of similar headaches.

If I expect a hard day, rainy with little light, I usually max out the freezers overnight and get them down to minus 10 or so, and then I can get to noon before the freezers take any power. Still, I have had solar give up the ghost in mid-afternoon on really bad days. The upside of this is that it takes up to 6 hours for the zero-degree freezers to get up to where I get worried and I can get 400 miles to a plug-in in that time. The upside to not much solar is that I don't need much cooling either.

At the same time, in mid-winter, I make sure that I am at a place where I can plug in by 8 PM or so. Sort of a cinderella thing.

As I did not put my current unit into service until September, I have limited experience of summer, and, as I am reluctant to risk temperature-deviating my stock, I have been plugging it in at night. The system was new at that time, but one night I let the system run off battery power until 7 AM, when it conked out. I was waking up every couple of hours to check it and I was totally knackered the next day, so I have not repeated the test. In mid September I saw the solar panels still generating past 8 PM. It is one of our obectives to have the system run all night on the solar power generated during the day. When I am out of town I stay in hotels where I know I am allowed to plug in at night, or RV parks where they always have power available.

For the new build, I plan on using 3 inches of closed cell binary polyurethane foam for insulation, with an r-value over 20. This is about 3 times the insulation in our current commercial freezers, so if I do a good job of sealing the door, I'd expect the load to last upwards of 12 hours without any power, during winter.

Before we built 'Man the Van' mark 1, everyone who we talked to in the reefer business was dubious. Factors in our favor were when we needed most cooling, there was more sun to provide the energy. Also, the thermodynamics dudes neglected what we call the 'parasol effect'. The solar panels on the roof absorb or reflect about 20% of the sun's energy, so the system has less work to do, and this is also the hardest part of the work for a freezer compressor.

The bad news is that we don't think we can freeze any more than half the space under the solar panels, in the new truck. We'd like to be able to freeze the whole vehicle, but there are numerous applications (e.g. food distributors who have a combination of ambient products like dish detergent and frozen items), where the whole cargo space is not frozen. And our version 3 will be based on a pickup truck, in the style of a slide-in camper top, where there will be extra panel space above. We have efficiencies of about 50% over our mark 1 in the current development, so we might be able to the whole cargo area frozen eventually.

The major obstacle to widespread adoption of solar for mobile refrigeration are more commercial than technical. Businesses are reluctant to risk possibly millions of dollars of frozen food on an unproven technology. Our aim is to prove that the system is actually more reliable than conventional reefers. As we have three redundant power sources (solar, AC and alternator backup) compared to a single source in a conventional reefer we think that we will be able to prove this actuarily when we have enough data. Making it past the equinox was a major deal for us, when we get to the summer solstice we will be able to complete the data picture.

You might say we could get the same effect by having more batteries, and, we agree, that would probably be cheaper for hardware on the front end. But if we charged up on mains power, we're using fossil fuel in coal-burning power stations, and solar is renewable and free. We believe the investment in free solar, leveraged by the excess cost of energy from portable gasoline or diesel, will pay off short term.

The not-directly-economic consequences are also considerable in my business. Basically, solar is cool. I have pulled up at a restaurant, within a few minutes the entire kitchen staff were having the van tour, and the chef was ordering sausages before he even tasted them. I have tripled my business with the help of 'Man the Van' which is the main reason why I need to build a bigger van so soon. Doubtless, if solar powered reefers were more common, the 'wow' effect would diminish, but this generation of chefs, especially the farm-to-table guys actually like the idea that their ingredients have had renewable inputs, and this includes the logistics of getting it from the farm into the kitchen.

We are in an industry that appreciates green stuff and understands sustainability and the husbanding of resources, and they think about their carbon foodprint.

By the way, conventional reefers are expensive to maintain, requiring specialized factory service every 1000 to 3000 hours. As you know, solar panels last 25 years without service, all you'd have to do with a solar reefer is replace the batteries every few years. Our current prototype, based on a used Ford Econoline, cost about 10K to convert, about a third the cost of a conventional reefer conversion. Cheaper to build, maintain and run. I am a businessman, I respond to bottom line considerations, and that is why I am investing in mark 2 less than 6 months after mark 1. I want more of the same. Even though it has been frustrating and even scary at times My customers call me the mad sausage scientist.

Finally, next time you roll on the slab, look at all those 18-wheeler reefers in the slow lane. Each of them uses up to 40 gallons of diesel per day keeping the load frozen. Wouldn't it be cool if, in 20 years or so, they were all running on a roof-full of solar panels? We think it's possible and we plan to market our mark 3 version to micro food distributors.

Bill, thank you for all the info. I am digesting the math and it is very helpful. I had forgotten about the inductive surge from a DC motor being much less than AC. One of our freezers was losing a lot to excessive firing up. And apologies for the wrong units, I should know better than that.

By the way, I love your forum. I found the definitive answer to one of our most vexatious problems, a succession of busted AC chargers, and there is a wealth of other info in there. A wonderful resource, you are doing good work curating it.

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Here's a pic of Man the Van mark 1, parked at a hotel in Atlanta on one of its early road trips. Anyone know of another commercial, road-going solar powered reefer? I am claiming to be the first and no-one has been able to contradict me. Man the Van 1 has complete 20,000 miles OTR and not temperature-deviated a single sausage. Mark 2 will be better.