Posted by: @derek-m

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@jamespa

Thank you for the vote of confidence, but as I feel certain that you realise by now, it is still a 'job in progress'. Having discussed various aspects with you over the past weeks has led me to realise that there are further improvements that can be made.

You are indeed correct that the cells I10 to AG10 are used to select the probable operating range of the heat pump (min, mid, nom and max).

Without having empirical data from a working system, it is impossible to calculate what energy loss may occur during cycling, particularly since the frequency of cycling will probably vary dependent upon how the control system has been configured. If anyone has any actual data then please send it forward.

I added the DT calculation only recently, and you are correct that I should also consider a fixed flow system.

The energy balance calculation was again only added recently, and will hopefully lead to the calculation of thermal mass and actual changes in indoor temperature under different operating conditions.

My understanding of the internal workings of a heat are as follows, but if anyone knows better then please clarify.

The heat pump controller sends the required LWT setting to the compressor controller. The controller runs the compressor to raise the temperature and pressure of the refrigerant gas. The hot gas flows into the condenser, where it comes into contact with the cooler plates and hence starts to condense. Because a phase change is taking place, thermal energy is transferred through the plates to the central heating water. The speed of the compressor will therefore be varied to try to maintain the LWT at the desired setting.

The refrigerant gas, in liquid form, then flows through a pressure reduction valve into the evaporator. As the pressure falls, a phase change occurs once more, this time from a liquid to a gas. In doing so it absorbs thermal energy from the outside air.

To produce more thermal energy output from the heat pump, requires the water flow rate and/or DT to increase. To transfer more thermal energy from the refrigerant gas to the water therefore requires a higher gas flow and/or temperature, both of which require the compressor to work harder.

As the refrigerant gas flow increases, this may limit the overall quantity of thermal energy that can be absorbed in the evaporator, and hence the compressor needs to work harder to make up the loss.

I hope you find this useful.

Derek,

John Cantor reckons 4-5% in his video if I recall.  

I had a go at quantifying it last year and came to a similar conclusion

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/heat-pump-truth-or-myth2-cycling-uses-a-lot-of-power-and-ruins-your-cop#post-15322

@cathoderay

Physical testing should be carried out whenever possible, both for peace of mind and also to help validate the accuracy of any models.

There can be several benefits from modelling, since it should be possible to simulate the response of a system under various possible operating conditions. It would be nice to know if your newly installed heating system is likely to cope with the depths of Winter, before the depths of Winter arrives.

It may also be possible to try 'what-if' operations without trying to do so on the actual system. Comparisons is also difficult with an actual system since I suspect it could be very difficult achieving the same operating conditions. Possible efficiency improvements can also be investigated without touching the system.

A possible further benefit may be to simulate operation outside the normal accepted limits, without breaking your heat pump in the process.

Modelling a heat pump system can be quite complex, since there are many parameters that may need to be taken into account, many of which are variable and can even interact with other variables.

To ensure a model is as accurate as possible requires using data from verifiable sources and try not to incorporate assumptions without justifying the accuracy of such assumptions. As the computer world saying goes 'garbage in - garbage out'.

Posted by: @derek-m

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The heat pump controller sends the required LWT setting to the compressor controller. The controller runs the compressor to raise the temperature and pressure of the refrigerant gas. The hot gas flows into the condenser, where it comes into contact with the cooler plates and hence starts to condense. Because a phase change is taking place, thermal energy is transferred through the plates to the central heating water. The speed of the compressor will therefore be varied to try to maintain the LWT at the desired setting.

The refrigerant gas, in liquid form, then flows through a pressure reduction valve into the evaporator. As the pressure falls, a phase change occurs once more, this time from a liquid to a gas. In doing so it absorbs thermal energy from the outside air.

To produce more thermal energy output from the heat pump, requires the water flow rate and/or DT to increase. To transfer more thermal energy from the refrigerant gas to the water therefore requires a higher gas flow and/or temperature, both of which require the compressor to work harder.

As the refrigerant gas flow increases, this may limit the overall quantity of thermal energy that can be absorbed in the evaporator, and hence the compressor needs to work harder to make up the loss.

So far as I know that's about it.

One thing I have found is that sometimes it helps to start by thinking about the emitters, or even the house, rather than the heat pump.  The emitters lose heat according to the difference between the average water temperature and the room, and that determines the deltaT between flow and return.  The heat pump responds to this by a) making up the energy in the water to achieve the desired flow temp and b) adjusting the flow rate to equal the desired flow-return delta T.  However (b) is optional, some systems do adjust flow rate and some dont.  If the pump isn't integral its possible that the heat pump is capable of adjusting the flow rate but the installer doesn't bother to fit the pwm cable.

1/10 I used a MCS accredited installer, (when I eventually found one) 10 years on, still not got the system promised, reported to RECC who passed to Napit, had an inspection, installer to thn put faults, bad design, incorrect installation etc right, now UFH and thermostats still not right, now Napit says they only cover the GSHP unit, as installer not signed up for the whole installation, back to MCS awaiting confirmations, even then back to RECC for the items missed in the contract/installation, even then is the plumbing as that not covered by any of them, wish I had got a wood burner and saved 10 years of my life and £30,000

Posted by: @cathoderay

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put them in hot water at a known temp. Take a pack of ten DS18B20 sensors, pair them off as closely as possible, and you have the making of a good enough temperature monitoring set up. 

I did this calibration method with the first few, but they were pretty close to right so I got lazy...depends on how keen you are!

@derek-m (and @jamespa) - I agree models definitely have their uses when used sensibly, but I fear these days far too many people believe models are the be all and end all of everything - and as a result are in awe of them - and as we have seen only too clearly models can get us into a terrible mess, for example, it can be clearly argued that the whole recent pandemic and pandemic response was driven entirely by models that precipitated a sequence of events that even at the time I said was the lunatics running the asylum and now increasingly others are rather late in the day realising that perhaps turning a pandemic into a panicdemic isn't such a good idea after all. The antidote to models on steroids is real world data. By mid 2020 we had enough real world data to see what was going on, and should have reacted to the real world data, not hysterical forecasts from dubious models. There was also the perennial problem of politicians not understanding science, leading to them allowing themselves to be bamboozled by a cabal of hysterical public heath doctors for whom normally the biggest event of the decade is the food poisoning outbreak (n=2, possibly 3 if we include patient DZ who felt a bit queer for a few hours on day 3 of the outbreak) from the local kebab take-away, who suddenly found they had seats at the top table, which they had no plans to leave before they absolutely had to.

I hardly need to point out that AI is a form of modelling.

In medicine, we often distinguish between efficacy, what something can achieve in ideal conditions eg a drug in a clinical trial, which is a highly abnormal setting, and effectiveness, which is how well the same drug does in the real world, where patients get back to being real patients and behave like real patients eg forgetting to take meds, experimenting with oral meds as suppositories (and quite possibly vice versa), feeding the meds to the ducks in the park or to the dog because she was poorly too etc etc. This is where I think our observational studies of our heating systems come in useful. By definition, because they are observational studies, nothing is controlled in the formal sense, but we are observing real systems in real houses with real people in them, and that has to count for something. Furthermore, although the external variables (eg weather) are changing all the time, most of the time they happen within certain limits, and there are periods of, for example, similar weather - an Indian summer, a cold snap etc. I think if the right data is collected then we should end up with good enough data, not perfect, but good enough, for example, to make an educated guess as to whether continuous or on/off running uses more energy overall. As I have said many times, better a good enough answer to the right question, than a perfect answer to the wrong question. 

@cathoderay

I don't disagree with most that you state.

To me a model should be created using real World data and then tested against a working system. Once proven to be reasonably accurate, it can then be used to see the effect of say installing larger heat emitters or fitting additional insulation.

The likely benefits can then be weighed against the cost.

Posted by: @derek-m

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To me a model should be created using real World data and then tested against a working system.

I agree, and that is pretty much what we did in the old days, only we didn't call it a model, we called it fitting an equation, or whatever. The cyclical nature of the activity is also very clear in the way you have expressed it, start with real world data, analyse it, then test the analysis against the real world, adjust analysis, test again, and so on. That process is at the core of experimental science. It is also at the core of why there is no such thing as 'the science', because science is a process, and a process that constantly evolves, not an end result.   

Posted by: @cathoderay

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Posted by: @derek-m

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To me a model should be created using real World data and then tested against a working system.

I agree, and that is pretty much what we did in the old days, only we didn't call it a model, we called it fitting an equation, or whatever. The cyclical nature of the activity is also very clear in the way you have expressed it, start with real world data, analyse it, then test the analysis against the real world, adjust analysis, test again, and so on. That process is at the core of experimental science. It is also at the core of why there is no such thing as 'the science', because science is a process, and a process that constantly evolves, not an end result.   

Science advances through a combination of theory and experiment.  True also with engineering.  Fitting an equation (to experimental data) is only one kind of model, there are others such as models based on 'theoretical' equations (eg e=mc^2, f=Gm1m2/d^2, V=IR) and there are models which are hybrids.  Its horses for courses. 

Any model should of course be tested against the real world, but is still capable of informing us about real world scenarios that it may be difficult to reproduce, or about trends/relationships which are important but not easily observable, in the real world.

If experiments were the only way to advance understanding theoretical science wouldn't exist at all, and working out whether an airplane is likely to stay in the air in particular conditions would involve sacrificing people (or as a minimum airplanes) to perform the experiments.  Yet theoretical science does exist and we don't generally sacrifice aircraft to find out whether they will fly and decide the limits on their permissible conditions of operation.

Posted by: @jamespa

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If experiments were the only way to advance understanding theoretical science wouldn't exist at all, and working out whether an airplane is likely to stay in the air in particular conditions would involve sacrificing people (or as a minimum airplanes) to perform the experiments.  Yet theoretical science does exist and we don't generally sacrifice aircraft to find out whether they will fly and decide the limits on their permissible conditions of operation.

I did say '"That process [observe > analyse > observe] is at the core of experimental science. [emphasis added]". Perhaps I should have added a reference to theoretical science, but it wasn't really relevant to the point under discussion, which was the importance of grounding research in the real world, while theoretical science is rather the opposite, being at its core a creative process, imagining something that wasn't there before. In passing, I might note that I have always considered that is the reason why (theoretical) science and the arts have no need to be at loggerheads, because at their core they are both remarkably similar activities, in which human beings imagine something that describes the real world.

All this leads to the rather absurd conclusion that all art is also nothing more or less that the creation of models. Da Vinci's Mona Lisa isn't a painting, it is a model of what a real woman looked like. Oliver Twist isn't a novel, it is a model of the fight of good against evil in a metropolis, Vivaldi's Four Seasons isn't music, it is an audio model of what the four seasons sound like. For Shakespeare, as he liked it, it was 'All the world's a stage', but if he was writing today, he might just as well have written 'all the world's a model'.

The problem is, when everything is a model, then there are no models. It high time we ditched the overloaded and now effectively meaningless word, and used better more descriptive terms.  

@cathoderay

Would you be happy with simulator?

Posted by: @derek-m

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Would you be happy with simulator?

Yes, I think that is a good description. However, the OED has the following as the fourth meaning of the verb simulate:

"To imitate the conditions or behaviour of (a situation or process) by means of a model, esp. for the purpose of study or of training; spec. to produce a computer model of (a process)"

so we may end up with tautologies, but we should also perhaps take note of the first OED meaning for simulate:

"To assume falsely the appearance or signs of (anything); to feign, pretend, counterfeit, imitate; to profess or suggest (anything) falsely."

Caveat scriptor...