R-pod Owners Forum Homepage

This site is free to use.
Donations benefit a non-profit Girls Softball organization

Forum Home Forum Home > R-pod Discussion Forums > Podmods, Maintenance, Tips and Tricks
  New Posts New Posts RSS Feed: Costs of Alternatives for Running Fridge
  FAQ FAQ  Forum Search   Calendar   Register Register  Login Login

Topic ClosedCosts of Alternatives for Running Fridge
 Post Reply Post Reply Page  123 4>
Author
Message
  Share Topic Share Topic  Topic Search Topic Search  Topic Options Topic Options
offgrid View Drop Down
Senior Member
Senior Member


Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290 		Direct Link To This Post Topic: Costs of Alternatives for Running Fridge
    Posted: 07 Sep 2018 at 6:59am
I was enjoying the debate on what energy source to use to run our fridges. So, nerd that I am, I thought I'd add another dimension to it. This table gives an estimate of the daily fridge run costs under various scenarios. I am NOT getting into the debate about the safety of driving with propane or not. This is strictly about costs. 

Ranked from cheapest to most expensive. Solar is "free" if you have extra because you've already made the investment. So if you're boondocking and have a sunny day with excess solar, consider switching your fridge over to 12V for awhile. Grid electricity is "free" as well if you've already made the decision to pay for hookups for other reasons.  Even if you're paying for it its still the cheapest after using your excess solar. This was based on US average electricity costs, it gets really close to propane though in high electricity cost states.  

What's interesting to me is that propane is cheaper than 12V from the TV even while driving. That's because the first law of thermodynamics says there ain't no free lunch, the extra load on your alternator running your fridge while driving will cause you to burn more gas.

Then come the various genny based electric operations, avoid those for sure. And the worst is to run the fridge on 12V from your TV, an astounding $7 bucks plus per day because of all the wasted fuel idling. 


mode source demand unit assume high setting kWh/d kWh/gal thermal efficiency comments electrical efficiency comments cost per unit unit cost per day Notes
12V solar 130 watts assume high setting 3.1 N/A N/A when excess is avaialble 90% battery round trip $0.00  N/A  $0.00 switch to 12V if you have extra solar
120V grid 135 watts assume high setting 3.2 N/A N/A 99% conductors $0.12  kWh  $0.39 zero incremental if you're paying a fixed fee
Propane 20lb propane cylinder 349 grams/d assume high setting 4.9 26.8 100% included N/A N/A $4.68  gallon  $0.85
12V Gas TV alternator while towing  130 watts assume high setting 3.1 33.7 22% incremental  80% alternator and conductors $2.75  gallon  $1.45
120V inverter generator on gasoline 135 watts assume high setting 3.2 33.7 11% EU2200i drawing 450W 100% included $2.75  gallon  $2.40
12V inverter generator on gasoline 130 watts assume high setting 3.1 33.7 11% EU2200i drawing 450W 80% charger and battery $2.75  gallon  $2.89
120V inverter generator on propane 135 watts assume high setting 3.2 26.8 11% EU2200i drawing 450W 100% included $4.68  gallon  $5.14
12V inverter generator on propane 130 watts assume high setting 3.1 26.8 11% EU2200i drawing 450W 80% charger and battery $4.68  gallon  $6.18
12V Gas TV alternator idling 130 watts assume high setting 3.1 33.7 4.3% assume .3 gph idling drawing 450W 80% alternator and conductors $2.75  gallon  $7.47


1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
Back to Top
Leo B View Drop Down
podders Helping podders - pHp
podders Helping podders - pHp
Avatar

Joined: 13 Jan 2012
Location: Lyndonville, VT
Online Status: Offline
Posts: 4508 		Direct Link To This Post Posted: 07 Sep 2018 at 7:22am
Really interesting! Nice work!
Leo & Melissa Bachand
2017 Ford F150
2021 Vista Cruiser 19 csk
Previously owned
2015 Rpod 179
2010 Rpod 171
Back to Top
furpod View Drop Down
Moderator Group - pHp
Moderator Group - pHp
Avatar

Joined: 25 Jul 2011
Location: Central KY
Online Status: Offline
Posts: 6128 		Direct Link To This Post Posted: 07 Sep 2018 at 7:27am
great write up and comparison.. remember to "run the fridge on solar", there is going to be some cost in a large enough storage solution to get you through the night at least, and a solar array big enough to top it off even on not so sunny days...

I would submit that the electrons used while towing are nearly as free as the ones used while on solar.. the truck is going to make those, whether you use them to run the fridge or not. Modern alternators don't have near as much parasitic drag as old ones, or, if you are as old as me.. generators..

Also, here at least propane is 2.21 a gallon for refills. Don't ever exchange if you don't need a recertified bottle.

also.. as a side side note, never use those 1 pound cylinders of propane unless you have to carry them in a backpack or something.. They work out to being about $80 for the same amount of propane in a 20 pound cylinder, which can be refilled for about $12... in 2 refills you will pay for any and all hoses and connectors possibly needed to run on the Pod's onboard low pressure, or from a stand alone 20 pouinder at high pressure.
Back to Top
offgrid View Drop Down
Senior Member
Senior Member


Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290 		Direct Link To This Post Posted: 07 Sep 2018 at 8:59am
I wouldn't suggest adding to your solar and battery system just to run your fridge, at least not from a cost standpoint. With solar and batteries you are still paying for your electricity, just doing it up front, I'm certain propane would be cheaper than that, especially considering battery replacement costs over time..

Especially if you're getting your propane that cheap, I just got my big residential tank filled and it was over $3 a gallon. 

I beg to differ on the "free alternator electrons theory". That would be a violation of conservation of energy, and unless your TV runs on nuclear power that isn't going to happen. The alt has to put out an additional 11 amps which will require additional torque, which will load the TV internal combustion engine which will burn more gasoline....

ICE's are about 18-25% efficient (more basic thermodynamics). I assumed 22%, and 20% alt and wiring losses. That's just the way it is...which is why if you're heating something electricity is not the best way to go compared to directly burning the fuel.

If you really want to run your fridge on electricity cost (and weight) effectively, convert to a 12V Danfoss compressor based system. Those work great and have been around for over 35 years now (I tested and installed my first ones for NASA in solar powered medical fridges in Africa in the early '80's. That was an interesting trip.

It's what most of the blue water cruisers use who either don't want to or can't carry enough propane. The reason this is cost effective is they are heat pumps not absorption coolers and so have a coefficient of performance around 2. That partially makes up for the 20-ish percent engine efficiency, and also would reduce your solar and battery requirements by about a factor of about 2. 
1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
Back to Top
lostagain View Drop Down
Senior Member
Senior Member
Avatar

Joined: 06 Sep 2016
Location: Quaker Hill, CT
Online Status: Offline
Posts: 2587 		Direct Link To This Post Posted: 07 Sep 2018 at 2:29pm
offgrid, what a great analysis.  It really is informative and helps understand the real costs.

There is one question I have, the price of propane seems a little high - at least in rural northern NV.  Seems like last time I bought it the price was about $2.25 a gallon or something around there.  That puts the cost on par with shore power more or less.

And, a comment:  You left off my wind turbine idea.  I know it's new and novel, but, but, 

And, why doesn't someone invent a cold fusion generator or a perpetual motion machine that defies the first law of thermodynamics.  After all laws are there to be broken.  
Never leave footprints behind.
Fred & Maria Kearney
Sonoma 167RB
Our Pod 172
2019 Ford F-150 4x4 2.7 EcoBoost
Back to Top
TheBum View Drop Down
Senior Member
Senior Member


Joined: 26 Feb 2016
Location: Texas
Online Status: Offline
Posts: 1407 		Direct Link To This Post Posted: 07 Sep 2018 at 3:30pm
Propane was $3.99 a gallon the last time I filled up in the Dallas area (last Saturday). Nevada is probably not a good barometer because they are a sparsely populated state that gets a lot of income from gambling and hotel taxes. I doubt that they have the high fuel taxes that some other states have.
Alan
2022 R-Pod 196 "RaptoRPod"
2022 Ram 1500 Lone Star 4x4
Three cats
Back to Top
mcarter View Drop Down
podders Helping podders - pHp
podders Helping podders - pHp
Avatar

Joined: 07 Apr 2016
Location: Greenbrier, TN
Online Status: Offline
Posts: 3419 		Direct Link To This Post Posted: 07 Sep 2018 at 5:02pm
Honestly I have monitored gas mileage on many trips. The alternator load didn't effect mileage in anyway. The AC did as did road conditions. The alternator output does change, the drag on the belt didn't change, the output of the alternator did. Because of demand. I like the effort but not sure that increased alternator output has a measurable difference to gas mileage and the cost of fuel.
Mike Carter
2015 178
" I had the right to remain silent, I just didn't have the ability."
Back to Top
offgrid View Drop Down
Senior Member
Senior Member


Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290 		Direct Link To This Post Posted: 07 Sep 2018 at 5:29pm
Propane prices are really volatile (pun intended). And as furpod says swapping tanks at the gas station is the worst. But I admit to doing it often rather than trying to hunt down a place that will actually fill my existing tank. I’d rather be camping.. Here on the OBX it costs nearly the same to swap or get your tank filled because they can, it’s a tourist area and there aren’t many alternatives.

If you want to adjust the prices for the various fuels and for electricity for your area then you can just scale the numbers by the ratio between what I put in my spreadsheet and your actual costs. Or if you send me a PM I can send you the spreadsheet. I doubt things will change very much though in terms of ranking costs.

I’m thinking your wind turbine would be one of the ones sailors use? You’d have to figure out a quick sturdy mounting system, they need a lot stronger structure than solar. Might work great in the NV desert. Some Burners use them on the playa don’t they? Would be fun to do and a big attraction at your campsites 😜.

Wind is going to be like solar, you’re buying your energy up front. If you have extra, I’d use it instead of propane for the fridge and water heater. If not then run the propane.

You can go ahead and try to break the laws of thermodynamics. I’m getting too law abiding in my old age I guess. If you choose that path I also have a good deal for you on a bridge over the East River in NYC. Let’s talk.
1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
Back to Top
offgrid View Drop Down
Senior Member
Senior Member


Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290 		Direct Link To This Post Posted: 07 Sep 2018 at 6:09pm
For those who forgot or never learned the 3 laws of thermodynamics.

Snow’s corollary:

You cannot win (that is, you cannot get something for nothing, because matter and energy are conserved).

You cannot break even (you cannot return to the same energy state, because there is always an increase in disorder; entropy always increases).

You cannot get out of the game (because absolute zero is unattainable).

Or just remember the acronym TANF (there ain’t no free lunch).
1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
Back to Top
lostagain View Drop Down
Senior Member
Senior Member
Avatar

Joined: 06 Sep 2016
Location: Quaker Hill, CT
Online Status: Offline
Posts: 2587 		Direct Link To This Post Posted: 07 Sep 2018 at 6:20pm
TheBum, sorry you live in such a high tax state.  Here in Heaven/aka Nevada, I think the only tax you pay on propane is sales tax which varies county by county, but is always in the 7% range.  I didn't count the tax in my suggestion of a price.

offgrid, yes, I was thinking of marine wind turbines.  They are usually mounted on the stern rail, with a post to keep it up out of the way.  I'd be a bummer to back into those spinning blades.  The anticipated wind they have to withstand in normal use is often in in excess of 60 miles per hour or over 50 knots.  Not sure what the wind speed is that they feather the blades and cut off generating.  I think the trailer is in danger of tipping over before land wind speeds would become a problem for the turbine, assuming a solidly mounted post.  

i suppose, like the high efficiency boiler PSEG wanted to sell me in New Jersey, it all depends on how much of the cost you can recover before you die or otherwise stop using it.  Checking Amazon, they have some pretty interesting units (some regular propeller blades and some kinda U shaped) and they are pretty cool looking in the order of magnitude of $500, give or take a few hundred.  Who knows how well they'll really hold up and what their real service life would be.  

But the thing I like the best about the idea is how cool it would look on top of a Pod, centered on the forward roof, with struts connected to the aluminum frame on the side walls; like it's part of the propulsion system for the trailer.  Wings attached to the back sides would really top it off.

As for me, everyday in my old age I have less and less reason to be law abiding.  
Never leave footprints behind.
Fred & Maria Kearney
Sonoma 167RB
Our Pod 172
2019 Ford F-150 4x4 2.7 EcoBoost
Back to Top
 Post Reply Post Reply Page  123 4>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.64
Copyright ©2001-2009 Web Wiz