Tank volume available for heat pump

[ld70]

Hello, I have a question regarding matters outside my area of expertise. It regards optimization of a 10 kW domestic ground source heat pump. The volume of the radiator system is not big enough causing short cycling. An optimum addition of 200 liters has been calculated, and of course a purpose-built or ready-made tank can do the job, with 2 connections, one in the top as inlet for heated water from the heat pump, one in the bottom as outlet to the radiators. However, in the old boiler-room where the heat pump is located, sits an unused old 500 liter buffer tank with several connection possibilities. A colleague of mine suggested we use the old tank connected top-bottom, but I think the volume is too big. He then suggested that we could move the outlet pipe to a connection halfway up the tank's side, and that this through thermoclining would halve the volume available for the heat pump. Unfortunately I am no expert in fluid mechanics, but my limited knowledge tells me he is wrong. I just can't see the mechanics behind 250 liters of water in the bottom half of the tank not being circulated at all. Why wouldn't the outlet "suck" water from all directions, including from the lower parts? Thankful for some input on the problem.

 

[imok2]

It seems to me that you could use the unused old 500 liter buffer tank to hold the excess 250 liters with inlet on top and outlet at the bottem and install a water makeup float valve at the 250 liter mark to maintain the proper amount of water in the system.

 

[ld70]

Thanks, but I believe float valves are used in tanks with an "open" surface, yes? The tank in question is part of a closed water circuit which is pressurized to 1.5 BAR, all air is vented off.

 

[walkes]

If the tank is well insulated such that the temperature variation in it is minimized, then the volume shouldn't be an issue. Your system is then similar to a compressed air system with a receiver, the larger tank will reduce the run timer per hour of the heat pump compressor in this case.

You could determine the final cycle time from:

V = [t * (Qheat source - q load)]/ 500*dT

V = buffer tank volume
t= desired cycle time
Qheat source = heat output rate
qload = rate of extraction from tank
dT = temperature rise of tank from the time when the heat source is turned off.

 

[chicopee]

Of course you could take the 500 liter tank and cut out a midsection and reweld the two ends to make it appx. 200 liters.

 

[ld70]

Thank you for all answers. However interesting, they have been a bit too practical! Maybe my English is not good enough to convey my intentions, but the original question was more theoretical, meaning:

I can't get my head around my colleagues opinion that the water in the bottom half of the tank won't circulate. Can someone explain this to me?

 

[berkshire]

I will try,
The key word here is thermocline.

When one body of water is colder than other the two masses separate with the warmer body on top.
Think of oil floating on water. The interface between the two masses is called the thermocline.
Since the cold water wants to move down and the warm water wants to move up there is no mixing.
True the outlet will want to suck water from all directions and as a result the thermocline may establish several centimeters below the outlet. but not enough to upset your friends calculations.

However if the buffer tank is well insulated there is nothing wrong with circulating all 500 litres, the only thing it will do, is to slow down your warm up time a little when the system is first started.
B.E.

 

[cry22]

Consider looking at it as deep see diving, the deeper you go, the higher the pressure, the colder the water, even though it is the same body of water.
You could say the same thing about water in a 500-liter tank, albeit, the difference is so infinitissimal.

 

[Compositepro]

If your problem is short cycling It means that your heat pump capacity is greater than the capacity of the radiators or the heat requirement of the house. So you are proposing to use heat storage to lengthen both the on and off periods of the heat pump. Domestic tank style water heaters are also used for heat storage so follow the same design principles. Cold water goes in the bottom and hot water goes out the top. Heat storage capacity depends on the mass of water in the tank and the range of allowable temperature fluctuation of the water leaving the tank. Mixing warm and cold water in the tank is wasted storage capacity.

When you hook-up the tank you want the heat pump to draw cold water from the bottom and return hot water to the top of the tank. You want the radiator loop to draw hot water from the top of the tank and return cold water to the bottom. This will maximize both storage capacity and responsiveness of your heating system. The thermostat controlling the heat pump should be on the bottom of your tank. If the house thermostat controls the heat pump then a storage tank will not fix short cycling.