Can you use a refrigerator system for a heat pump?

[rjsimmon]

Hi!

I'm building a 1/5th scale size Eco house and my part of the project is the heat pump.

I was wondering, if I took an entire refrigeration system (the compressor, heat exchangers ect) off a working frige, could I use the system as a heat pump?

Hope you can help.

[The Viking]

The short answer is: Yes, you can.

But (there is always a but);

The heat exchangers (evaporator and condensor) will be noisy (compared to something that been designed for the specific task).
The control system will have to be altered.
And the system performance will not be on par with a properly designed heat-pump.

[rjsimmon]

Cheers for the quick response!

Ahh as I thought. Just needed another opinion from someone who knows a bit more than me to ensure what I thought was correct.

[Peter_1]

Yes you can, but these components were chosen and adapted for a fridge application.
The fact given that you will build an ECO house says also that this solution isn't per definition the best system to heat the house because it uses the principle of a heat pump.
All the components must fit to each other for your specific problem.
But most components can be use.
Were you thinking on a household fridge? In that case, the answer is no.

[US Iceman]

I'm going to agree with Peter.

In fact, if you are building a proof-of-concept home (or school project) for showing the basic principles you might be better using the components from a window air conditioner.

This would allow the concept to be more closely matched with similar operating conditions and equipment.

Using the equipment from a household fridge might present some operational problems that could lead to varying perceptions on the usefullness of the concept.

[rjsimmon]

Hi.

The 1/5th scale eco house is my final year University project and the heat pump was one of three devices aimed to heat the house. The others are a solar water heater and some natural ventilation from a trombe wall but they're my colleagues concerns. I think the project is the first of its kind in the UK and it is hoped that it will gather popularity for a Uni competition in future years.

My main focus is on the design of the system heat exchangers, which have been designed with R-134a and I have obtained the surface area/volume etc from my calculations.

I was thinking of a household fridge to make things easy. BUT, I have the option to get my heat exchangers made and the other bits bought in.

I'm starting to think making my own system will be the better idea.

So, can I use a household refrigerator compressor and thermostatic valve for my heat pump (that I've already got)? Or should I buy specific ones in? Can you recommend any compressors?

Thank you for your interest and responses so far, really appreiciated!

If any technical information is needed to help me, it can be provided.

Cheers, Rob.

[The Viking]

Sorry,
I had missed the part of your message that said "1/5th scale" when I post my first message.

So, one question (or maybe 5!)

-Do you have to prove performance or just function?
-Does it has to last or is it just going to be used once?
-How are you proposing to introduce the heat in to the building?
-How are you proposing to collect the heat in to the system?
-What capacity are you looking for?

[US Iceman]

Can you recommend any compressors?

What I would recommend is the compressor and other components be based on the heat duty required for the 1/5th scale requirement, at the specific operating conditions that will be encountered.

Granted , the opportunity for finding parts from a household fridge may be greater however, the operating conditions (for your heat pump) may be totally different.

You want to have the equipment rated for it's exact set of operating conditions. Forcing some equipment that is readily available may allow you to get started quickly, but you may be faced with some operational details that do not measure up.

If you had to calculate the heat transfer surface and heating/cooling duty, then you would also want to ensure the equipment operates as designed, not as you HOPE it might.

If you are using an air/air heat pump you might do something different than you would if you used a ground source heat pump.

You really have not provided sufficient details to say either way, so my opinion is to always start with the basics.

What do you expect to do?
&
How are you going to do it?

For cooling you need a certain kW at a set of operating conditions (or range of conditions), and the same for heating.

The system has to work in these conditions and should be designed accordingly....

[rjsimmon]

-Do you have to prove performance or just function?
-Does it has to last or is it just going to be used once?
-How are you proposing to introduce the heat in to the building?
-How are you proposing to collect the heat in to the system?
-What capacity are you looking for?

I just have to prove function, although the performance will be monitored over a year, it doesn't matter if the COP's arn't as high as I have predicted.

It will have to last a year if possible, only in heating mode.

I prepose to introduce heat into the building via the use of an air-air source heat pump.

I plan to collect heat for the heat pump by gathering low temperature heat from the outside air to evaporator heat exchanger (out side). The cold HE will have a fan to assist in convection heat transfere.

The capacity is tiny. I have to replace 100 Watts of heat lost by the house per hour. It's so small due to the extreme insultaion and size etc.

Hope this answers your questions.

[rjsimmon]

What do you expect to do?
&
How are you going to do it?

What are you expacted to do?

My response to The Viking may answer some of your questions.

I expect to produce a Air-Air source heat pump. It is only needed for heating.

I need to replace a minimum of 100 Watts per hour of heat into the house.

I have based the design of the heat exchangers on a Mass Flow Rate of 0.05 kg/s and R-134a.

How are you going to do it?

I am going to do it by designing an Air-Air heat pump system.

The heat exchangers have already been designed, (evaporater and condensor).

The evaporater pressure and temps are -10 degrees C and 0.02006 MPa. (Is this too cold for and outside heat exchanger? I'm worried about ice forming and fouling)

The Condensor pressure and temps are 50 degrees C and 1.318 Mpa.

The problem I have is that I'm not sure how to select a compressor.

I'm also not sure about getting the system charged with refrigerant. Do I need a safety certificate or something?

Hope this helps.

[The Viking]

100w total heating capacity?? !

In which case you are looking for a compressor with around 60W cooling capacity!

No, you will not find one of those. (that's a light-bulb)

Let's think about this.
80 m2 /5 =16m2 so I assume this is roughly the size of your project?
You say it's an eco project so I assume there isn't much heat transfer.....
Fresh air through natural ventilation?
Peak load in the mornings?

If your figures checks out (if you email me the drawings, I will run the design through Hevacomp if you want, just for comparison) a heat-pump will not be your best choice, purely due to the low duty.

My choice would be solar panels and a radiator or two.
You could also install one of the caravan-battery charging solar panels, a battery and a 12 volt electric heater.

But, I guess the spec of your project states that it has to be a heat pump?




BTW, -10 isn't unrealistic here in UK, what the heatpump need to do is to defrost the outdoor coil with regular intervals. 50 Degrees inside is on the low side for a normal heatpump but as I assume the house to be well insulated it might be sufficient.

[taz24]

The problem I have is that I'm not sure how to select a compressor.

I'm also not sure about getting the system charged with refrigerant. Do I need a safety certificate or something?

Hope this helps.

The Norseman is correct. 100 watts is a very small amount. Have you thought of combining it to heat water as well. You could then increase the duty of the comp.

As for the refrigerant side of it yes there are restrictions on who can charge a system. The system would need to be proved safe for the pressure and gas tight. You need a freindly fridge man to do the last bit.

As for the comp you can get 37watt comps so at a COP of 3 to 1 you will easily add 100watts of heat.

I can give you as much info as you want.
I have a special intrest in heat pumps.

Cheers taz.

[rjsimmon]

Yeah I know, the figure is tiny!
But that is the calculated heat loss from the building. If anything, it may be a bit on the small side.

If I ignor the calculations and put the capacity up to 500W, is there a compressor for that duty?

Any idea what the smallest compressor you can get is?

I definatley know now it is not possible to scale a heat pump system easily, lol.

How did you work out the compressor would need a 60W cooling capcity?

Sorry about all the questions. I'm really miffed about the compressor part.

The house is about 9.6 m2 floor plan. You're correct in saying there's not much heat transfer. The house is super insulated.

Ventilation will be controlled by some sort of controll system. That's one of the other guys projects. The estimated air circulation is 0.5 changes an hour. Peak load is in the mornings.

Thanks for the offer of running the design through Hevacomp (I take it, it's some kind of analysis programme? I've never come across it before.), I'll see if I can get my hands on the drawings. I'll have to chase the architects who designed the building structure. What format do you need it in?

The spec of my project does say it needs to be a heat pump. I feared from the start the house may get too hot. Especially, to add to the heat pump we've got a photovoltaic panel for charging batteries (also emitting heat) to power the house and a solar water heater for underfloor heating!

It's no big problem if the house is too hot, as long as my part works. It's the first try of many my Uni's going to do. Things are going to go wrong!

If you say 50 degrees C is on the low side, I'll take it up to 60 degrees. It was really guess work on the temperatures.

Thanks for you feed back and interest so far!

[taz24]

It's no big problem if the house is too hot, as long as my part works. It's the first try of many my Uni's going to do. Things are going to go wrong!

Thanks for you feed back and interest so far!

As I said earlier there are comps on the market under 40w in size.
Heat pump out put is RE (refrigeration effect) + WD (comp power) so if you have a COP of 3 and you have a comp that is 40 watts then your heating capabilities are 40w x 3 = 120w.
COP of 3 is poor so you could get as much as 4 to 1 or 5 to 1. 5 to 1 = 40w x 5 = 200w.

taz.

[rjsimmon]

As for the comp you can get 37watt comps so at a COP of 3 to 1 you will easily add 100watts of heat.

I can give you as much info as you want.
I have a special intrest in heat pumps.

Cheers taz.

The water heating has been thought about. My associte is taking that on, combining it with the underfloor heat from the solar panel, that luckily for him a very generous company has custom built for him free of charge!

My Uni has pressure testing facilities to test the pressure, one of the guys on the project is a pro brazer of small pipes like the ones I'll be using, so I should be ok there. All I need now is a friendly fridge man, amoungst other things, lol!

If you can get 37 Watt compressors thats great news! Where from? I'd love some more information on this. Although I now think the compressor may have to be a little bigger, I haven't added any safety factor yet.

Have you any information about selecting the right kind of thermostatic valve? Do you have to select a specific one for the task?

Thank you for your interest, you guys are really helping me out.

[The Viking]

Yes, Hevacomp is a heat - load/loss program.
What you need is the sizes of walls (and windows/doors in them), floors and roofs/ceilings.
It has the K values for all normal building materials but I guess this wouldn't be standard so we would need the K values for your walls/floors/roofs as well.
And not forgetting fresh air.

[taz24]

My Uni has pressure testing facilities to test the pressure, one of the guys on the project is a pro brazer of small pipes like the ones I'll be using, so I should be ok there. All I need now is a friendly fridge man, amoungst other things, lol!

.

You have to be careful when pressure testing because there is a set procedure to adere to. You can't just bang some gas in, you have to test it to a pressure that is compatible for the gas its on.

I,m sure if you keep in touch with this site you'll get a fridge man to do it for you.

taz.

[taz24]

If you can get 37 Watt compressors thats great news! Where from? I'd love some more information on this. Although I now think the compressor may have to be a little bigger, I haven't added any safety factor yet.

Thank you for your interest, you guys are really helping me out.

Danfoss comp between 20 and 40w
low temp.
upto 120w high temp.

http://www.danfoss.com/United_Kingdo...64f34a14647%7d

Cheers taz.

[rjsimmon]

Cheers Taz.

I'm in contact with some suppliers of Danfoss compressors at the moment.

So I take it once I get the heat exchangers made up, buy in the compressor and the thermostatic valve. All I have to do is assemble them and get the friendly fridge man to charge and tune the system?

The Viking:
I'm still trying to get my hands on the drawings. It will be interesting to see how different the values will be compared to the analysis that has already been done.

Cheers lads.

[The Viking]

So I take it once I get the heat exchangers made up, buy in the compressor and the thermostatic valve. All I have to do is assemble them and get the friendly fridge man to charge and tune the system?

Errr, Hmmm.....
Speak to your friendly fridge guy first, it could save you some hassle later.
You will also have to design a control system with defrost control and so on.

BTW, whereabouts are you located?
You might find someone on this board in the neighbourhood.

[rjsimmon]

OK, I'll try and find a friendly fridge guy then ask him/her.

Control system. Great, more work for me to do. Can you not buy them in? Isn't there some sort of control system fixed to the compressor?

I know one of the other guys on the project is designing a control system for the house, so that my heat pump comes on when it needs to, but I don't think it'll include a defrost cycle.

The defrost cycle will be the reversed cycle, right. So I'll need a reversing valve?

I'm located in Sheffield if anyone's from/near here.

Cheers, Rob.

[taz24]

Cheers Taz.

So I take it once I get the heat exchangers made up, buy in the compressor and the thermostatic valve. All I have to do is assemble them and get the friendly fridge man to charge and tune the system?


Cheers lads.

There are two ways to build somthing.

First way. Calculate, design, run simulation, redesign, recalculate, then build a prototype and finaly build the real thing.

Second way. Think about it, build it, run it, re build it, run it, so on and so on until you end up with the working thing.

I'm more of the second method but its not my heat pump . Both can prove to be fanstastic or desastorous.
I would involve a fridge man early on though. He may well save you a lot of wasted time.

Cheers taz.

[The MG Pony]

And for a heat pump don't forget to over size your evap to maintain a low delta between the pipe work and the air, lower delta less likely for icing up

[mashan]

is there possibillity to design air to air heat pump. did You think for water to air, or w to w. You can spent more monay. Of course, the invesments will be greater. but...

[rjsimmon]

Cheers for the feed back guys, you've given me plenty of stuff to think about and helped me out a lot!

The heat pump design is coming along nicely now, I've designed the heat exchangers now and added a safety factor: cheers, MG Pony.

The compressor has been selected. I've just got to order in the stuff and get it built.

Mashan, I'm afraid the project is only for an air - air source heat pump. Other designs were considered but although efficiencies are better, the eco-house needs to be transportable. No digging holes for me.

Cheers guys!

[nanda]

[Josip]

Hi, Nanda

welcome to RE, (we have one thread New to RE where you can introduce yourself much better )

confused,mad,cool,rolleyes,rolleyes

Please can you translate your post

Best regards, Josip