[Ko_deZ]

Originally Posted by AC_Hacker

So if I understand you, if you have continuous circulation, you have the same advantages as if you have large mass?

Yes. The water in the accumulator acts as the large mass.

Also, I notice that you seem to be thinking of one heat pump that performs multiple functions. Have you considered multiple small pumps each tuned to a specific task?

Yes, and it does not have a positive impact on overall COP to go for several small units. Explained below.

The reason I mention this is that I came across a comment on some blog, by someone with more experience than I, who flatly stated that 4.5 Tons was the upper limit before COP started dropping off. Since then, I looked at the COP figures for some of the better performing ASHPs, and noticed that the smaller units had higher COPs than larger capacity models from the same manufacturer. Also, larger multi-head units were worse than multiple smaller single-head units of the same capacity. Added to this, there is a lower likelihood of multiple small units all failing at the same time (if they all have power), than one large unit. So you have the promise of higher efficiency and higher reliability with many smaller units...

The reason for this is using too small heat exchangers. You will notice the heat exchangers are not as much larger as the difference in power. Also, for multi-head you have additional fans running with a COP of 1, reducing the system overall COP. Multiple heads have the advantage of heating i more places, giving a more even temperature and making you stress the system less, so in the real world, they give you a lower bill. If you have the same temperatures ( superheat, subcool and energy provided, drawn ), a larger system will usually give you some improvement in COP due to the fact that there is less energy lost in friction in a larger scale system (internal in compressor, water in hoses, waterpumps and so on and forth). The gained thermal energy is based on the physical properties of the coolant being used, so removing the compressor and other losses, and assuming equal operating conditions, the efficiency must be the same for all systems. The only difference in real world efficiency lies in the efficiency of the components, and larger components are usually more efficient.

The reason for cooling the hot side trough multiple heat exchangers is to utilize the energy more efficiently. A typical heat pump design will give a superheated gas temperature of maybe 55C (I am guessing, but that sort of makes sense at least). During the phase change from gas to liquid, most of the energy is removed, but the liquid still has some heightened temperature in it, and as a result, some energy. If you continue to cool that trough several heat exchangers, that would give you more energy from the system from the same input energy. As the hot tank approaches and maybe passes the phase shift temperature, the condensing and phase shift is moved downward towards to the second heat exchanger, providing the floor heating with more energy than the hot water. If a lot of hot water is used, the hot tank will be cooled down, and most of the phase change will happen there, putting most of the energy in the hot tank.

The last heat exchanger is really liquid to liquid, but as I mentioned, there is some energy to be gathered there too, as long as we don't cool it so that it freezes and blocks our hot side. That is unlikely as anything below 10C is pretty much unusable for anything but pushing up the temperature a little in your garage or something. 10C could hold a very cold basement at a higher than freezing temperature. Anyway, this energy is completely free. Gathering this will put a larger strain on your ground loop, but will not affect the compressor power usage at all. It is free power, by definition increasing the COP of your system. A simple example would be to run your tap water trough a cold accumulator tank, pushing the normal 4C water to maybe 10-15C. That is energy that will not be drawn from the other accumulator tanks, which calculates to about 12C * 4.2J/gK ~= 50kJ/litre. Not very much, but certainly something that will reduce the cost of having a shower.