Solar Powered Refrigerator

[jamesk]

Hello again,

I have a university project to complete and i am finding it very difficult to get anywhere, feel as though it should have been a seperate degree:

My project: 'Performance investigation and optimisation of a Solar Powered Refrigerator'.
I keep going in circles, first the compressor, then the refrigeration space and now the evaporator. Very hard to pick something to improve. Where would you start? any tips or insider views,, i feel i have to read everything to get anywhere but i don't have unlimited time and really need some direction to moving in, before i get somewhere. Is there really that much scope for improving the evaporator, i.e. can it be easily changed in its design to improve the Coefficient of Performance COP of the ref. cycl? I am focusing on using the vapour comp ref. cycle at the moment as i have learnt what i like to call 'a lot' about them.
Thank you for all of your knowledge

'every journey starts with first step' hahaha 'every circle starts with a point'

[hookster]

Hi interesting project but as you say time would be needed to redesign every part of the system and trying to get all the efficiency savings out there to work in a single application. For me I would concentrate on an absorption refrigeration system utilising solar collector for driving energy and PV for battery back up. The cycle is simple and you can then concentrate on optimising. Or try the Einstein refrigerator cycle.

[Tesla]

Good advice from hookster using a solar absorption system, you could even use a bank of solar heat tubes (made with steel not copper of course).

[jamesk]

Thanks Hookster could be a break through, will look into the absorption cycle in more detail starting now. And thank you as well Tesla, i didn't know you were into refrigeration, or being alive, will keep the heat tubes in mind but it's more about the refrigeration rather than where the solar power is used (i know, what's the point of having 'Solar' in my project, i don't know)

[Gary]

I would go with a DC compressor run off batteries charged by solar. Or maybe a solar powered liquid line booster pump.

[hookster]

I like the thinking Gary of a solar powered Liquid pump, personally I have never been a fan of booster pumps and they always felt like selling "snake oil". The idea that the energy to run one is harvested seems to make more sense. I guess install costs, running costs and energy saving will have to be considered.

[install monkey]

morning gary- boss has had me having a go at reinventing the wheel- used a 3.5kw wall mount unit-using 12v batteries via a 2000w inverter to create 240v to power the inverter circuit, doing this we were drawing 2 amp on the 240v circuit (approx 40% duty-maximum current 5.1amp on 240v mains)- but 12v off the battery its dragging 120amp, used a 4x4 battery rated for 700amp load, 25 amp continuous- it ran the compressor for about 10 mins, he also wants to charge the batterys via a dynamo coupled off the cond fan- thats the problem,also a changeover panel to revert back to mainswhen the battery was low- currently the project has stopped
I would go with a DC compressor run off batteries charged by solar. Or maybe a solar powered liquid line booster pump.

[hookster]

Hi Install, you should suggest to your boss that inverter is going to waste energy. I think Gary was talking of a DC driven motor. A better bet would be pulsed DC or digital motor.

[Gary]

Surely there must be all sorts of equipment designed to run on DC... and a solar panel outputs DC.

Currently the emphasis seems to be on converting the power to AC. Where is the advantage to doing so?

[Gary]

morning gary- boss has had me having a go at reinventing the wheel- used a 3.5kw wall mount unit-using 12v batteries via a 2000w inverter to create 240v to power the inverter circuit, doing this we were drawing 2 amp on the 240v circuit (approx 40% duty-maximum current 5.1amp on 240v mains)- but 12v off the battery its dragging 120amp, used a 4x4 battery rated for 700amp load, 25 amp continuous- it ran the compressor for about 10 mins, he also wants to charge the batterys via a dynamo coupled off the cond fan- thats the problem,also a changeover panel to revert back to mainswhen the battery was low- currently the project has stopped

What I had in mind was running the equipment directly off the solar panel output, with solar charged batteries providing backup... and mains charging the batteries only when absolutely necessary.

[MikeHolm]

Check out the "sunfrost" fridge which uses a Danfoss DC comp (I believe), if you want to just use batteries and PV. I would look ad ADSORPTION to run a small chiller for the fridge if you wan to use thermal to drive the process. Don't think you can get freezer temps out of it though.

[jamesk]

Cheers Mike
I don't have much knowledge on ABsorption or ADsorption systems but it seems from the little i have read that they do not perform with a comparable COP to vapour comp. systems.

One paper I have found states (not the best paper, it has no date on it!):

"Barriers to uptake of the technology
The main barriers to uptake of adsorption refrigeration technology:
• In their current state of development systems are bulky and of higher cost compared to competing vapour absorption systems"

And arn't vapour-comp. systems even less 'bulky' and costly than ABsorption ones?

This is one of my reasons for questioning why people advise i use these systems.
Also my project is aimed at refrigerators the size of the top-opening ones you would find icecream in, in supermarkets. hoepfully canada has the same for this case.
Thanks,
James

[hookster]

It is true that absorption systems have a low COP, 0.3 or there about. The benefit is the long term operation, few moving parts, environmental impact and low maintenance costs. There is in reality no perfect "green energy" or even perfect technology ie. screw versus reciprocating you will always have to consider best option for the operating conditions. The size system you are basing on would allow you to achieve -18 and your heat input would probably be in the region of 100-200 watts. A wish full thing is if you can improve on the bubble pump efficiency!!

[Gary]

I have an absorption refrigerator in my camper that runs off propane or 12VDC. In fact I can run pretty much everything in the camper off 12VDC except the air conditioner, which uses 120VAC.

[jamesk]

thnx hookster,,

I have added some research on absorption systems to my research and if i don't use them to 'theoretically' optimise i will include them in comparing vap-comp power consumption/life span etc, so they can be an alternative.. but i think for now i will stick with vap-comp cycles as that's what i know best at the moment, ahve not much time left and can't do loads more research

[jamesk]

Can anyone tell me where I can find a data sheet of a real life vapour-compression-cycle, i.e. the properties at various stages of the cyc. which allow the whole system to be analysed in detail:
What temp/pressure/state refrigerant is in after compression, condensation? Data about the reaction in the control valve and same for evap. ???? Prehaps someone has an instruction document in mind they can guide me to that gives many properties for the whole cycle?

I think if I have a real example of a system I can at least do some calculations and it will help a lot in being able to say what could be changed for the system in order for it to consume less energy (just egsample). My theory might not be very accurate when it comes to real life but it might be something, better than what I have now anyway.

I understand a bit about P-H diagrams and how to read them, prehaps if I knew the real life conditions of a vapour comp. ref. cycle I could apply the P-H diagram and analyse the cyc. this way.