BRE heatpump calculator

[Beelbeebub]

Not sure if this has been posted before

 

https://tools.bregroup.com/heatpumpefficiency/index.jsp

 

Appears BRE have developed an efficency predictor tool based on more typical UK conditions than the SCOP calculation that manufacturers use in their literature. 

 

Their calculation also includes system loads like pumps and backup heating as well as hot water loads. 

 

 

One curiosity and I have to dig into the details) is that it appears the seasonal efficency (their version of SCOP) improves with undersized units using direct electric backup heaters for the few occasions needed. Their explanation is the reduction in cycling for the rest of the year more than compensates for the few days of inefficient direct heater use. 

[JamesPa]

11 minutes ago, Beelbeebub said:

Not sure if this has been posted before

 

https://tools.bregroup.com/heatpumpefficiency/index.jsp

 

Appears BRE have developed an efficency predictor tool based on more typical UK conditions than the SCOP calculation that manufacturers use in their literature. 

 

Their calculation also includes system loads like pumps and backup heating as well as hot water loads. 

 

 

One curiosity and I have to dig into the details) is that it appears the seasonal efficency (their version of SCOP) improves with undersized units using direct electric backup heaters for the few occasions needed. Their explanation is the reduction in cycling for the rest of the year more than compensates for the few days of inefficient direct heater use. 

I had spotted this and wondered if it was an artefact of the calculation, but the explanation you give is entirely plausible.  The difficulty I suspect is knowing with any certainty the cycling inefficiency.  Some calculations can be done based on flow temp but that won't account for compressor start up losses. 

 

There is a 'degradation factor' which manufactures are expected to declare as part of the spec, but if not declared there is.a default value (0.9) and most specs I have found use this.  Interestingly Daikin declare 1, which means no degradation, which may explain why their units perform so well in the bre results even though many of their models in any given range are just software limited versions of a higher output model (so might be expected to perform poorly at low outputs).  What I don't know is whether this degradation factor of 1 is real and if so how Daikin manage to do better than others.

 

Please do post what you find further.  After the religious issue of unnecessarily throwing out working dhw systems, whole house sizing  is probably the next most important issue to fix in the current heat pump retrofit deployment model, and proof that undersizing is beneficial would be extremely valuable.

[Beelbeebub]

24 minutes ago, JamesPa said:

What I don't know is whether this degradation factor of 1 is real and if so how Daikin manage to do better than others.

Maybe their swing compressor design has lower blown past losses on startup? 

[Beelbeebub]

It seems common knowledge that cycling is bad for efficency and longevity. 

 

But why?

 

What are the mechanisms of loss?

 

What are the accelerated wear mechanisms?

 

I get on the older shaded pole single speed motors there were contractors, capacitors and centrifugal switches that might have a finite number of operations. Surge current wouod also cause issues. 

 

But modern units use inverters with soft start and effectively infinite switching (certainly in the context of start stop) 

 

So what are the problems with mild cycling? 

 

 

[JamesPa]

1 hour ago, Beelbeebub said:

It seems common knowledge that cycling is bad for efficency and longevity. 

 

But why?

 

What are the mechanisms of loss?

 

What are the accelerated wear mechanisms?

 

I get on the older shaded pole single speed motors there were contractors, capacitors and centrifugal switches that might have a finite number of operations. Surge current wouod also cause issues. 

 

But modern units use inverters with soft start and effectively infinite switching (certainly in the context of start stop) 

 

So what are the problems with mild cycling? 

 

 

I can't answer any of the above, and suspect some of it may be outdated religion.

 

However there is one concrete thermodynamic argument namely that cycling inevitably means that the flow temp must be higher than if the pump were running continuously in order to deliver the same energy on average from emitters to house.  This compromises efficiency.  I can't quantity by how much however, because I don't have a model to estimate the rise in flow temp required.  It's probably not too difficult to work out if one put one's mind to it, which I may do in an idle moment.

Edited August 11, 2023 by JamesPa

[JohnMo]

Mild cycling is fine, with run time 10 mins or over. Its all to do with the amount of energy expended to heat things up, like compressor, heat exchanger and piping, all of which are not useful heat and get lost to the outside, between cycles. Think of opening a tap, the water will heat the piping and displace a load of cold water before you get hot water on your hands. Do the same 30 mins later it's almost a repeated process. In that case heat is lost to house, so who cares. The heat pump does the same but a big chunk of heat is lost to outside.

 

Any machine has micro wear during a start or stop cycle, that doesn't occur during steady state running, this happens on bearings and any surface that doesn't normally touch during running, due things like hydrodynamic lift that support a shaft from a bearing surface...

 

Equipment life is normally rated in number of cycles not running hours.

[JohnMo]

17 minutes ago, JamesPa said:

 

 

However there is one concrete thermodynamic argument namely that cycling inevitably means that the flow temp must be higher than if the pump were running continuously in order to deliver the same energy on average.  This compromises efficiency.  I can't quantity by how much however, because I don't have a model to estimate the rise in flow temp required.  It's probably not too difficult to work out if one put one's mind to it, which I may do in an idle moment.

A did see a video the other day where the guy was recording flow temps and CoP. In theory a flow temp of 30 would have been good, but the reality was 35 gave him a better CoP over the period, due to how the heat pump cycling occured. Difference in CoP was 4.0 compared to about 4.5.

 

So not always that straight forward.

[SimonD]

7 minutes ago, JohnMo said:

due to how the heat pump cycling occured

 

Somewhere I have a paper on this, looking at cycling/running time intervals and determining the overallimpact on efficiency. It was certainly not a binary answer. My initial search can't find the paper so I don't know whether I actually saved it, unfortunately.

[Beelbeebub]

I get the analogy with a hot water pipe and a tap at the end (partly because my one kitchen tap is that tap!😁). 

 

I'm not sure the heating of the various components is a major issue, especially as they are typically insulated. But also because the thermal mass of the compressor, pipes etc is relatively small vs the load (house). 

 

The unused water in the pipes outside the thermal envelope is a bigger issue. I can imagine a pump se way away from the house with big bore pipes might suffer more than one right next to the house. 

 

Things like the pipe runs, insulation of components and thermal mass of the house will be important, and will impact the cycle time below which inefficiency becomes intolerable. 

 

One aspect will be the thermal mass of the house. It may be that running a little hotter and transferring a little bit more heat than the losses would require is a better strategy than just turning off. If you "overheat" the house by 0.3C will anyone notice?  This may be an area where pure "open loop" weather compensation loses and a slightly more pragmatic algorithm focused on maximum efficency at stating above a certain temp (even if that means being a little bit more above than you would normally consider) might win. 

[JamesPa]

2 hours ago, JohnMo said:

A did see a video the other day where the guy was recording flow temps and CoP. In theory a flow temp of 30 would have been good, but the reality was 35 gave him a better CoP over the period, due to how the heat pump cycling occured. Difference in CoP was 4.0 compared to about 4.5.

 

So not always that straight forward.

Fair enough.  At this level all sorts of 'second order' effects intervene.  I doubt anyone has a comprehensive model so I guess the algorithm to optimise is, first minimise the flow temp then, if there is cycling, try tweaking it!

 

Or alternatively do the first step, have a pint, have another pint!  There comes a point where things are good enough!

Edited August 11, 2023 by JamesPa

[DanDee]

1 hour ago, JohnMo said:

A did see a video the other day where the guy was recording flow temps and CoP. In theory a flow temp of 30 would have been good, but the reality was 35 gave him a better CoP over the period, due to how the heat pump cycling occured. Difference in CoP was 4.0 compared to about 4.5.

 

So not always that straight forward.

The difference wasn't due to how cycling occurred, but due to how the heat pump manages its refrigeration cycle/pressure/expansion valve when it hits the steady state(target flow temp), in a particular range of flow temp from 29-30C to 34-35C.

 

yet when the flow temp is rising from 20 to 35 at the start, that issue doesn't occur.

 

 

Sometimes at the same flow temp during the same heating cycle there's no issue initially and then the issue starts.

 

It happens even at 35-37C(rare occurrence).

 

This shows how the issue temporarily occurs until the system finds its happy medium at the bottom range output of the HP.

 

[Archer]

This video by John Cantor has some interesting analysis of Heat Pump cycling and how it isn't always a bad thing. I'm going to post a related dilemma in another thread so as not to hijack this one - would be grateful for any thoughts from people though

 

 

[DanDee]

31 minutes ago, Archer said:

This video by John Cantor has some interesting analysis of Heat Pump cycling and how it isn't always a bad thing. I'm going to post a related dilemma in another thread so as not to hijack this one - would be grateful for any thoughts from people though.

This is an example of how John's 6KW Ecodan, instead of running smoothly at minimum it chases its own tail in between compressor speed and expansion valve, continuously adjusting its opening until the system overshoots the target temp and starts cycling. With the cycle COP being lower due to the system pressure adjustment, and not being able to set itself to a fixed condition(or more constant).

 

 

 

[SteamyTea]

1 hour ago, DanDee said:

This is an example of how John's 6KW Ecodan, instead of running smoothly at minimum it chases its own tail in between compressor speed and expansion valve, continuously adjusting its opening until the system overshoots the target temp and starts cycling. With the cycle COP being lower due to the system pressure adjustment, and not being able to set itself to a fixed condition(or more constant).

Not watched the video (watching the mist come in over Mount's Bay instead), but are those symptoms a sign of a poorly designed system, rather than just one component?

[DanDee]

20 minutes ago, SteamyTea said:

Not watched the video (watching the mist come in over Mount's Bay instead), but are those symptoms a sign of a poorly designed system, rather than just one component?

I need to re watch the video(I seen in the past, not today), I think the video was done with examples from other systems vs the one I mentioned(his current one).

 

The system in my analysis runs very well when it comes to delivering medium to high output, very rare does modulate efficiently as expected for minimum output, but often starts to cycle due to the flow temp rising as a result of the system increasing the compressor speed at the time when the condensing temperature/pressure is on the rise anyway due to how the system manages the opening of the expansion valve(closing and opening at certain levels within whatever the parameters set in the system).