Heat pump ?? in DHW production 3

[kikonas68]

Hello.
We say that heat pumps function with a ΔΤ of 5°.

Let's say we want to heat fresh water entering our storage tank at 15°C (as it comes from the water main) to 54°C at a flow rate of 0,8m³/h (our secondary circuit).
The primary circuit should have a ΔΤ of 5°. Roughly, by energy balance we obtain primary flow equal to 6,24m³/h.

We size accordingly the plate heat exchanger and both of circulators and then we install them and things work as expected.

Now, it happens sometimes that fresh water may enter our tank at a higher temperature, e.g. 25°C or maybe even 30°C (due e.g. to a hotter month or due to a preheat from some heat recovery or combination of both etc.). The desired temperature of our DHW is still 54°C so secondary ΔΤ is lower than before. Considering also the same secondary flow rate, the heat power needed now is less than before.
So for the same initial heat exchanger and considering the primary same flow rate with primary hot temperature still of 55°C the return temperature will be not 50°C(ΔΤ of 5°C) but higher (since less heat power is demanded by secondary) so primary ΔΤ is messed up.

1)We use to say then that heat pump will cut off too quickly, turn on and off frequently etc.
If it is so how will it ever be possible to heat a significant volume of water? Is there any risk that pretty much all of our tank water volume will stay at 25°C?
(In practice somehow it works)

2)Theoretically if we want to keep our primary conditions constant(temperatures and flow rate), when secondary ΔΤ drops we could raise accordingly the secondary flow rate-through a variable speed circulator- in order to absorb the same heat power(and not less) by having the secondary circulator always targetting the primary outlet temperature at 50°C.
Is it something that can work?
Is there any point in doing it?

 

[SAK9]

There is a limitation on the number of times the heat pump compressor can start in an hour usually not more than 6-7 times in an hour. Therefore you need a buffer storage tank that will smoothen the fluctuating field flow requirement. This buffer tank sizing will also take into account seasonal variation of feed water temperature. In fact the tank sizing is to be done for peak summer conditions of feed water temperature 25C-30C in your case.

 

[MintJulep]

Theoretically if we want to keep our primary conditions constant(temperatures and flow rate)

There's really no reason to do this.

when secondary ΔΤ drops we could raise accordingly the secondary flow rate-through a variable speed circulator- in order to absorb the same heat power

No. You can't do this. You're process power requirement is determined by the secondary ΔT.

(54-15)/(54-30) = 0.61. If the water supply temp increases to 30, then you only need 61% of the power that you do when the water supply is 15.

In practice, whatever seasonal factors that affect the supply water temperature also increase the condensing temperature of the heat pump, so it will be working lower down on it's performance curve, partially offsetting the reduced power demand.

On the evaporator side, the seasonal factors probably also make it easier for the heat pump to extract heat from... what? Is it an air-source heat pump?

You could reduce the primary flow rate, allowing the primary ΔT to float to whatever it needs to be to balance. But doing this will move the heat pump out of its happy zone of efficiency. Likewise, you could slow the evaporator fan.

There is no form of turn-down control in the heat pump? You can't vary the compressor speed? Hot-gas bypass?

 

[kikonas68]

Thanks for both answers. Will look into it, I'm seeing a lot of new questions on the way
It's a geothermal heat pump (evaporator exchanges with vertical closed loop).
The thing is that constant flow rate at the primary has been put as a non-negotiable prerequisite.
Regarding regulations at the refrigerant cycle that you propose (of any type), I hear it as something normally very logical and possible (and to be honest was counting on it as a last resort), but such possibility was never mentionned (nor any space for further discussion) although manytimes respective concerns were expressed, therefore we ended up that whatever can be done will be at the secondary.

 

[kikonas68]

Maybe the solution could be just a three way mixing valve electronically controled(such as those used for adjusting at will the distribution temperature of DHW) maintaining 50οC at the outlet?

This way heat pump always sees the same flow rate and same ΔΤ.

 

[LittleInch]

A quick look shows that the 5C delta on the heat pump is primarily due to the choice of refrigerant.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kikonas68]

Maybe the solution could be just a three way mixing valve electronically controled(such as those used for adjusting at will the distribution temperature of DHW) maintaining 50οC at the outlet?

This way heat pump always sees the same flow rate and same ΔΤ.

Absurd, cannot happen.