water scource heat pump

[regen]

Has anybody any experience of water scource heat pumps like the Dimplex wi 9me or wi 14 me.  They talk about a COP of 5 with water in at 10c and out at 35C.  Stainless construction enabling use of untreated water and a usage of between 0.5 and 1.0 lps.  Seems ideal for my situation - spring water with min available flow of 1.5 lps and a 10 meter head.  So whats the catch?

What is the effect on the COP of say water in at 7C and out at 50C? Will this technology be accepted for RHI (if it ever gets off the ground?)

Regen

[Ted]

I know of a couple of water source pumps installed and running without any issues, but can't comment of the exact COP they are getting.

Water source HPs are included in the RHI, but of course only MCS accredited products will be eligible (unless you are looking at over 45 kW).

[dhaslam]

A lot depends on the water source.  It has to be a large volume of water  and deep enough to  have warmer water on the bottom.   If you have that  situation  it is ideal.

[BruceB]

Running a water source as an open loop system can mean you are forever having to clean filters and you can get a biological slime build up in the pipes.  Clean water from deep underground is probably OK.  I tried with lake water continually fed by underground springs, but it was not a success and I changed to a closed loop system in the lake.

[desperate]

Regen, hi

With all heat pumps the COP is low if the temp difference is high and vice-versa, with a temp difference of 43 your COP will be well below the 5 that the maker quotes.

I don't know about the RHI implications. Probably best to justify your install on its own merit, then if you do get some form of payment it's a bonus.

Desp

[regen]

HI All

Many thanks for your replies. 

Ted- Only looking at a small system (10 to 20kw range) so will have to be MCS aproved and i expect choice will be very limited as can only consider an open loop system as do not have a large body of standing water.

Bruce B- these are real concerns as was hoping to use overflow from existing well supplemented by seeps from below the well.  At 0.5 to 1 lps i dont think that water borne debris will be a problem but not sure about slime other than that the drinking water supply pipework has not shown any sign of slime build up oin last 10 years.

Desp- Dont suppose you have any idea where I can find a formula or tables that show the relationship between COP and water temp difference.

Regen

[Bodidly]

Hi Regen

For working out the COP I think there is a good download on the John Cantor website. I do not know if it is completely  accurate but I found it very handy.

Beau

[regen]

Thanks Beau this is exactly what I was looking for - I think! Will take a little time to digest it all.

Regen

[desperate]

Hi Regen, the physics behind a heat pump is quite simple,

basically the electrical efficiency is as follows



Eff==     T2
         T2-T1

where T2 is your inside/hot water temp and T1 is the outside/source temp, all temps being expressed in Degrees Kelvin or absolute temp.

So you can see that the  closer the source temp and the inside temp the greater the efficiency.

A working example then would be,  say you want an inside temp of 20C or 293K and your source is 5C or 278K you get



Eff==    293           == 293    == about 20 or 5%   in other words 1 unit of energy input will supply 20 units of usefull heat.
        293-278              15   


Now this is where the simplicity breaks down, the above formula takes no account of the losses in the system, and more importantly how fast you want that energy delivered to your house, as MacKay says in his book, "these limits could only be achieved by a system pumping heat infinitely slowly"
You will never reach anything like a cop of 20 and deliver a usefull amount of heat,  but you can use that formula to calculate the impact different internal and external temps would have on your COP.

That link posted up by Beau looks really usefull, I'm off to have a butchers myself now.

See ya
Desp

             

[Countrypaul]

Desp,

I'm curious as the formula you give suggests that the larger the temperature difference the higher the COP could be - which appears to contradict what the normal view is. Is this difference just between the theoretical COP as from the formula and the practical implementation being "so poor"?

Paul

[desperate]

Paul,

You've got me stumped here, if we lower the T1 term by 15 to 263, we end up with 298  = about 10
                                                                                                                 30  

similarly if we raised the T2 term by 15  we get   313         =313  = again about 10
                                                                313-278       35

implying a lower COP,  have I missed something?

I agree a larger temp difference lowers the COP

ATB

Desp

[billi]

Running a water source as an open loop system can mean you are forever having to clean filters and you can get a biological slime build up in the pipes.  Clean water from deep underground is probably OK.  I tried with lake water continually fed by underground springs, but it was not a success and I changed to a closed loop system in the lake.

sorry can you  explain ?

does water/water  heatpump mean a direct flow  of water ?  I doubt this and say there is a  heatexchanger  involved !


and surely  heatpumps will be more efficient  , when they  are able to keep their source of intake energy defrosted 

[regen]

Hi Billi,

I am only interested in a heat pump which takes water in at about 10 c direct from an underground source and discharges the same water at arround 7c the heat extracted can then be fed directly into the house at around 35c into underfloor system giving a COP of around 5. I believe that the dimplex wi 9 me is such a machine and it uses about 1 cu m per hour (0.5 lps) of single pass water and uses around 1.5kwh of electricty.  i suppose there is some form of heat exchanger built into the machine. Also what i dont yet know is how much temp vriation there will be with this source- for the past 3 weeks it as remained at 10c which is encouraging.

Taking water directly from an underground source should mean no filter cleaning as i have used this supply for drinking for last 10 years.  Slime   buildup is however a possibility but not sure how it would impact the machine efficiency.

Regen

[Countrypaul]

Desp,

You're entirely right, when I read it now I can't work out how I got the COP to increase by making the temp difference larger.
Put it down to being a Saturday night and ignoremy post.

Paul

Paul,

You've got me stumped here, if we lower the T1 term by 15 to 263, we end up with 298  = about 10
                                                                                                                 30  

similarly if we raised the T2 term by 15  we get   313         =313  = again about 10
                                                                313-278       35

implying a lower COP,  have I missed something?

I agree a larger temp difference lowers the COP

ATB

Desp

[dhaslam]

Hi Billi,

I am only interested in a heat pump which takes water in at about 10 c direct from an underground source and discharges the same water at arround 7c the heat extracted can then be fed directly into the house at around 35c into underfloor system giving a COP of around 5. I believe that the dimplex wi 9 me is such a machine and it uses about 1 cu m per hour (0.5 lps) of single pass water and uses around 1.5kwh of electricty.  i suppose there is some form of heat exchanger built into the machine. Also what i dont yet know is how much temp vriation there will be with this source- for the past 3 weeks it as remained at 10c which is encouraging.

Taking water directly from an underground source should mean no filter cleaning as i have used this supply for drinking for last 10 years.  Slime   buildup is however a possibility but not sure how it would impact the machine efficiency.

http://en.wikipedia.org/wiki/Direct_exchange_geothermal_heat_pump

There are DX heatpumps that transfer  heat directly from a source i.e. copper pipes  that are part of the heat pump circuit  go into the ground or water.   It  obviously  cannot be used in a well  if the water is used for drinking and the  refrigerant is toxic  but that is what is normally known as a direct system.