I do believe one of the problems with phase angle control is it can induce harmonics back into the electricity supply.
Ah... well that seems to prove the case.
In discussions with senior engineering managers at NGED, they have cited harmonics from heat-pump installations as one of the two significant factors which contribute to losses.
Whereas phase-imbalance only creates losses at the local sub-station, harmonics propagate through the network and are detected at the higher-voltage transformers.
I've just plotted this from voltage measurements on a 33kV line (Bulk Supply Point transformer output):
The graph shows the voltage imbalance (plus or minus) as a percentage over a 20hour period. The area beneath the red line shows the amount of energy which gets lost as heat.
So that's a negative outcome from the way we control motor speed on heat-pumps. ☹️
I think that we may be referring to different things when we talk about harmonics. To me harmonics are frequencies above 50hz, often caused by inductive and capacitive effect in circuits creating oscillation.
I believe that your plot more likely shows the voltage imbalance due to heat pumps switching on and off, rather than any harmonics. The base load of our home is normally in the 100W to 200W range, but when our small (2.6kW) A2A heat pump starts up, this can quickly jump to 1000W. If you have one or two larger heat pumps switching on and off, in an otherwise heat pump devoid area, I suspect the load variations could be quite dramatic. Unfortunately your plot does not detail the time of day. Maybe when there are more heat pumps on the system, the overall loading will be higher, and provided that all the heat pumps do not switch on and off at the same time, then voltage imbalance may be reduced.
Modern electronic devices are required to contain line filters to prevent harmonics being fed back into the electricity supply system, both within the home and external.
I believe that your plot more likely shows the voltage imbalance due to heat pumps switching on and off, rather than any harmonics.
The raw data was supplied to me by the most senior engineering contact I have in NGED.
He was responding to a point I'd made that losses were confined to the local substations. I was wrong of course.
He described how/why harmonics at frequencies above 50Hz were becoming an increasingly serious problem, not least of all because they pass through the transformers.
just putting something in here that relates back to the discussion several pages back (sorry folks, been busy on other things) with some fascinating input from all regarding @cathoderay 's data.
I've got daily gas kwh monitoring going back more than a year. I stopped using gas for DHW at that time so known to be only CH. I've only had a heat pump only 2 months and only doing CH also for the majority of the time.
Bottom line : my data , albeit only for a short period, shows me that my heat energy output into heating the house with the heatpump is considerably higher than that with gas. I think its about 2x (ish - not enough data to be more accurate yet). I compare a) gas meter usage * 80% for modcon boiler = heat energy gas CH vs b) DT * flow rate integrated over time from the modbus of the heat pump = heat energy heatpump
with gas, we only heated the rooms we were using. setback for a room not in use at the time was 15 or in one case 12. overnight setback was 10C in most of the house - basically off. some rooms would be at 12 in the morning and we'd leave them that way as those rooms not needed in our morning routine. when our schedule required us to use a room, we'd turn up the TRV (with an automated system) 30-60 mins before needing the room, and it would "feel" acceptable by the time we were in it. The air temp might not have been at 21, and the walls might still have been cold, but the big warm airflow from the radiator would make it "feel" ok. 18C was our temperature in most circulation rooms (hall, kitchen, landing etc) - we'd wear jumpers. 21C was only for 1 room, the tv snug.
with the heatpump, the approach used above simply doesn't work. the heatpump craps out. cycling, insufficient emitter volume, rooms won't warm up quickly. we all know it.
so I've set my heatpump system as per best practice. It works best with all the downstairs emitters on , doors open, and a constant temperature of 20C in all downstairs rooms. which we all know. that makes for a higher "comfort factor" of living - there is no "this rooms a bit cold" any more, because it isn't, and we're mostly not wearing jumpers, or we're down from 2 thick ones to one thin one. I don't know my SCOP yet - more work to do on the monitoring, just installing a proper SDM120 input electric meter rather than the CT clamp which I know has been overreading - but I'm confident I'm in the 3's.
In summary, my words to a person considering installing of a heat pump into their large and/or lossy house that they used to run a bit cool / partially heat with a fossil source to save money, are:
We run the house warmer on simple numbers basis, and also warm in all rooms rather than just soe. we have to do this for the heatpump to work properly. you will have to as well. you'll get a more comfortable house that way. but because of this:
overall heat energy going INTO the house over the year could be 2x what it was with the fossil fuel. (caveats on the 2x, more data needed)
A typical heat pump has a 3x input to output efficiency. So you've consumed 2/3 of the INPUT energy that you did before (SCOP 3x assumed for simplicity)
But that energy costs 3x more per unit that in used to (assuming 3:1 electricity to gas as market rate) . so your actual run cost could be (3x2)/3 = 2x what it was before ☹️
in my case that doesn't bother me as I have the ability to get my electricity at below market price - I have substantial PV, and I have batteries for storage importing on TOU cheap rate. I wanted to do this anyway for green reasons. BUT I know that is a somewhat different position to typical joe public
For the time being, to help overcome the possibility of too frequent cycling during milder weather, the trusted room thermostat should be utilised to control the starting and stopping of the heat pump.
Its not so much frequency that causes the loss in efficiency, its the fact that, to deliver the same energy from the emitters, the heat pump must operate at a higher temperature if operating intermittently than is required if operating continuously. So Carnot inevitably gets you - its unavoidable so far as I can see and is dependent, to first order, on mark-space ratio.
Actually there may be an argument for faster on-off switching rather than slower (same mark-space, higher frequency) because then the temperature variations will be less and so the increase in flow temperature required to deliver any given amount of heat also less. In the limit a heat source that modulates on and off once per minute (say) with a 50% mark-space ratio is indistinguishable, as far as the rest of the system is concerned, with one operating at half the power, so the required elevation in flow temp versus continuous operation would be negligible. Obviously, at this point, wear and tear and start up 'costs' take over' but I need to think on this one a bit more!
I think that you may have taken my comment out of context, since I was referring more to short cycling rather than just general cycling.
I think that it may be useful if we not only consider what may be causing cycling, but also how it may be reduced in the most efficient manner.
As far as I am aware, the two main causes of cycling are an external input from a thermostat, switching the heat pump on and off, or the actual LWT straying from the calculated value by more than specified limits.
Possibly a good analogy for a heat pump system would be a water tank, being supplied from a pump, via a control valve, that can vary the flow rate from say 3 lpm to 10 lpm. The flow rate represents the thermal energy, and the water tank represents the heat pump system. The level of water in the tank represents the LWT which can be varied by the outside temperature. The size of the tank represents to volume of the heating system.
The discharge from the heat pump tank supplies a much larger tank, which represents the thermal mass of the home, the discharge from the home tank is also by a control valve which is varied by the outside temperature, from 0 lpm to 11 lpm.
Let's imagine that the system is operating with an outside temperature of 5C, a discharge flow rate from the home tank of 7.5 lpm, a flow rate of 7.5 lpm from the heat pump tank with a water level of 40%, and a supply flow rate of 7.5 lpm. All is in balance and the pump is happy, or at least as happy as pumps can get. 😊
Throughout the day the outside temperature has been increasing until it eventually reaches 15C. The discharge flow rate from the home tank has been reducing until it is now 2.5 lpm, but its level has remained reasonably constant at 21%, and the required level in the heat pump tank has been lowered to the present setting of 30%. The supply control valve has been reducing the inlet flow to achieve the calculated heat pump tank level, but has now hit the lowest flow rate of 3 lpm.
The system is now out of balance, since there is 3 lpm flowing into the heat pump tank, but only 2.5 lpm flowing out. The level in the tank therefore starts to increase, at a rate dependent upon the volume of the heat pump tank. Although the rising heat pump tank level does slightly increase the discharge flow rate to the home tank, it is not sufficient to balance the system.
DON'T PANIC. 🙃 Built into the control system are some safety features, such that if the heat pump tank level exceeds the level setting by 5%, the pump is switched off. So when the level in the heat pump tank reaches 35%, the pump is stopped.
When the heat pump tank level falls to 25%, the pump is restarted, but the level is now lower than the calculated 30%, so the supply flow rate is initially increased to say 5 lpm, and the level once more starts to rise, but more rapidly, since the pump is having to work harder. The system is therefore cycling, since it cannot do otherwise.
It is now possible to consider the effect that changes to the system may have.
What may happen if the calculated heat pump tank level is changed with reference to the outside temperature? (Changing the WC curve). If the calculated heat pump tank level is lowered (reduced LWT), the flow rate from the heat pump tank to the home tank would reduce, which in turn should lower the supply flow rate. But if the supply flow rate has reached its minimum setting, the net effect will be to stop the pump sooner, with the heat pump tank level falling lower before the pump restarts. This may extend the period between the pump starting and stopping, but I doubt that it would be by a great deal. It may also reduce pump running time and energy consumption slightly.
Raising the heat pump tank level, may again have little effect, and could slightly increase pump running time and energy consumption.
Adding a volumiser to the system would be the same as increasing the size of the heat pump tank, which would therefore take long to fill and empty, which should reduce the frequency of the pump stopping and starting. Along with reduced level settings, this could probably lead to better efficiency.
Having a PHE, buffer tank or LLH in the system would be similar to fitting a flow restriction between the heat pump tank and the home tank. This would mean that it would be necessary to have a higher level in the heat pump tank, to achieve the required flow rate to the home tank, otherwise the level in the home tank would start to fall (colder rooms).
In milder weather conditions it may be possible to reduce the frequency of cycling by using the home tank level as a controlling element for the pump. If the system can be operated such that with minimum flow rates the home tank level (indoor temperature) is allowed to rise by 1% (1C) or so, until the pump is switched off by the home tank level sensor (thermostat). With the pump stopped, the level in the heat pump tank will fall at a faster rate than the home tank, but will not restart the pump when the heat pump tank level reaches the minimum setting. The pump will not restart until the level in the home tank has fallen by say 1%, which will take much longer because of the larger size of the home tank.
The difficult setting is now the heat pump tank level, since if it is too low, the pump may be switched off and back on before the home tank has reached the switch off level. That is, the heat pump cycles whilst raising the indoor temperature to the thermostat setting. This is where trial and error will play a big part.
@iancalderbank This is an interesting facet of heat pumps not much discussed. It's going to vary by person and property and probably also time of year. To be honest I don't know what the 'average joe' does with their heating these days, if the people I used to work with are anything to go by it's whack the heating up to 24 form September to mid may and ignore the cost and (for me at any rate) the discomfort. I don't have a clue if this is 'normal'.
The energy companies have this info (on a per customer basis for those with smart meters), it would be good to see some of it.
For some it will be irrelevant, for others very relevant I suspect.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@jamespa yes definitely by person and property. I read cathoderays write-up and the subsequent 6 pages of discussion and thought that might well be at least part of the issue. For me - not cost conscious so much as gas burn conscious - we did try to use only what we needed. So someone very cost conscious might well be doing the same thing for cost reasons. I'm sure you've read the same stories in the press that I have about those less fortunate who can't afford to heat.
I've read a lot of discussion about "if you only part heated a few rooms then your heat loss was still the same it was a false economy", but not true for me - we basically ran our house with less heat on fossils. average room temp over the whole house in the upper-mid teens at best.
time of year - classic example today. very cold day for mid may today, about 11C ambient here. I turned the heating completely off last sunday when it was mid 20's ambient. I've turned it back on today. The ASHP has put 20kwh into the house so far, for an input of about 5kwh. I did this because I knew that it would cost me nothing, the PV has completely covered it, and why not, its nice to have a warm house and not feel guilty.
But when on gas, I'd have just put another layer on today and left the boiler switched off so the heat used would've been Zero. There'd have been plenty of times like this on the milder winter days when you'd just "put up with" 16-17C and put a layer or two on...
I believe that your plot more likely shows the voltage imbalance due to heat pumps switching on and off, rather than any harmonics.
The raw data was supplied to me by the most senior engineering contact I have in NGED.
He was responding to a point I'd made that losses were confined to the local substations. I was wrong of course.
He described how/why harmonics at frequencies above 50Hz were becoming an increasingly serious problem, not least of all because they pass through the transformers.
Unfortunately your plot does not detail the time of day.
The x-axis starts at 15:00
The point at which there were fewest losses due to harmonics was 07:40
I would be interested to know how he ties the harmonics to heat pumps, rather than EV chargers or battery chargers or solar PV inverters? Is he anti heat pumps by any chance?
Is this phenomenon throughout the country or localised in certain areas?
@derek-m Thanks for the analogy but I'm not sure how it changes the fundamentals. If the HP is forced to cycle because it can't maintain a steady state output at the (low) level of the heat loss from the house, then during the on periods the flow temperature will need to be higher than would be the case if it were on continuously, for any given total energy delivered. Thus it will be operating at a lower COP than would be the case we're it to be operating continuously, because because cop reduces with increased flow temp.
The period of the on-off will depend on various system parameters most particularly volume, permitted hysyerisis of flow temp and the difference between the min hp output and the demand, but the mark-space (on-off) ratio will be the ratio of required output to minimum output, irrespective of system volume etc.
All of course 'to first order', ie based on the fundamentals only, and assuming that the target temperature is reached.
Are you arguing that this is not the case or am I missing some other point?
@derek-m don't go there. A2A would have not been an easy piece of engineering in my house and no point discussing... in any case, in my case its good to have a more comfortable house. mrs likes it lots 😉 . and I have improved my run costs by putting in other things (PV, batteries) which are what make the major difference.
I'm trying to think from the point of view of joe public who won't have those.
If they were already running their house constantly at 21C with gas, and they change to an ASHP, keep the same 21C run temp, and get a SCOP of 3, and the import cost is 3x worse per unit, then the run cost is the same with the ASHP.
If they previously were being very frugal with the fossils (for whatever reason) and running anything similar to what I described for their house, then even if they get SCOP 3, their run cost goes UP. Because its not possible for them to run the ASHP in the frugal scenario that I described. (ok yes they could , but then the SCOP wouldn't be 3, so it doesn't help them).
thats what I'm trying to introduce to the debate. from the point of view of joe public not me.
with the heatpump, the approach used above simply doesn't work. the heatpump craps out
we run the house warmer on simple numbers basis, and also warm in all rooms rather than just so[m]e. [W]e have to do this for the heatpump to work properly
I think this is the nub of the whole thing. This is why I all but said at one point (deliberately provocatively, to provoke discussion) that heat pumps are designed to fail, with the flaw chain being: COP is king => low LWT => 24 hour running (and bigger rads) and its that final bit of the chain that doubles or trebles the amount of energy delivered to your house compared to timed yet still very capable fossil fuel boiler. With a standard set up you cannot avoid this because it is designed into heat pumps, you have to run them as Steady Eddy. If you try to make Eddy run faster, he complains bitterly.
Trawling through some old records of oil boiler maintenance, I noticed that in many years I made a note of the date when I put the old oil CH on auto (ie the timer) and in some year when I took it off auto, and it looks like it was always later going onto auto (sometimes late Oct, occasionally November) than the starting the heat pump heating (1st Oct last year), and in some years I took the oil boiler off auto earlier in the year than I turned off the heat pump heating this year (1st May). With the oil boiler, I could and did manually run it on particularly cold days either side of the main auto period, but those manual runs were infrequent. This is another consequence of the Steady Eddy nature of heat pumps, you can't just use them on an ad hoc basis on cold days. Just as you have to run them 24 hours a day, you also need to run them for more days in the year, to get reasonable comfort.
I think this problem (to get the same comfort levels), you are bound to have to put more energy into the house than you did with a fossil fuel boiler, but the heat pump's better efficiency will mean that the raw amount of energy going into the heat pump will be less, and you may well find you break even, as I did: three times more delivered heat, but at three times the efficiency cancel each other out, and you end having to put roughly the same amount of raw energy into the system. This effect is likely to be particularly the case in home previously heated with a timed fossil fuel boiler. If you always heated the home all the time with a fossil fuel boiler, you should see savings, but how many people heat their homes with a fossil fuel boiler?
I wonder is any research has been done on this, to see how many homes are affected, and by how much? My hunch is that it could well be over 50%, but that is only a guess.
I’m sure the Octopus approach of targeting standard modern houses with systems that through experience become designed correctly will have a big impact on the transition to renewable technologies. It doesn’t help the older housing stock, but maybe in the big picture that doesn’t matter and price pressures will ease and those houses stay on fossil fuel.
I'm sorry, I really am (so much so I nearly didn't post this, but it does need to be said) but this doesn't stack up, because most of our housing stock is old, and this proposal (leave the oldies on fossil fuels) will mean that perhaps two thirds of homes will be on that regime, and only one third on heat pumps. I couldn't find UK data, but here is year of build data for England (and I bet the other home nations are similar, perhaps even more heavily weighted to older housing):
Older housing stock needing properly designed retrofit solutions are very much part of the big picture; correspondingly, Octopus's tentacles will only reach perhaps one third of homes, and have a much smaller impact.
Midea 14kW (for now...) ASHP heating both building and DHW
Thus it will be operating at a lower COP than would be the case we're it to be operating continuously, because because cop reduces with increased flow temp. [emphasis added].
Yes, but is also running for less time, and the two may cancel out, partly orwholly. This is in its way partly another way of explaining my paradox: the heat pump runs at much better efficiency, but all the time, my old oil boiler ran at much worse efficiency, but only for part of the time. Surprisingly there seems to be precious little real data comparing overall energy use running a heat pump all the time with running it with setback and boost, perhaps precisely because we are all running them all the time ('because we have to', but in fact we don't have to) and setback and boost by and large is out of the reach of a factory gate heat pump (another design flaw?). Ecodans (and possibly Homely) apart, you can only do it with dedicated custom controls, not something your average Joe is likely to want to do. My hunch is that setback and boost will reduce overall consumption, because the home spends less time at design temp, and so its overall heat loss per period of time is reduced. But we need empirical evidence. Once we are into the next heating season, I may even have some data of my own.
Midea 14kW (for now...) ASHP heating both building and DHW
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