Having now had a heat pump working over a whole winter, It was time to see if in fact a heat pump was more expensive to run than a gas combi boiler. I had made many improvements to the house in addition that would reduce costs, so I did the comparison twice. Once as a straight like for like cost difference assuming the same factors as last year, then again including the cost savings through the extra solar panels and tariff prices I was able to obtain. Column J shows the standard comparison assuming things were the same as last year. It was interesting to note that the heat pump was slightly more expensive to run than a gas boiler between October and December. This was most likely due to this being the coldest dullest months when its COP and solar output would be lowest. However this effect was dwarfed by the savings in the other months, leading to a total saving of £178 over the winter period. This agreed with the measurement of the heat pumps SCOP of around 3.8 during the period which would make it cheaper than gas given the price differential between the two fuels.
Column K shows the comparison after all the changes I had made. Having extra solar panels probably didnt help that much on the dullest winter days, but getting an EV allowed access to much better overnight tariffs, which allowed the charging of the battery, hot water cylinder and general washing duties at a much reduced rate. The total saving taking all this into account was £363 over the winter period. The difference of £185 between the two analyses, shows that you dont need a lot of solar panels to make a heat pump cheaper to run than gas in the winter. The downside for me was that the small saving also didnt justify the extra expense that the new panels and battery had cost me. For a reasonable payback period, I was going to have to pin my hopes on a substantial income from electricty export during the summer months.
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After having additional solar panels installed for a year, I was now in a position to compare how the import of electricity compared to the export. The graph below, compiled using OpenEnergyMonitor, illustrates this. As expected, electricity import was low in the summer months, but picked up substantially during the winter months. Electricity export showed an opposite trend and at the end of the year I had imported 3503 kWh and exported 3325 kWh. An additional consideration was that we had purchased an EV in December, and this had consumed 769 kWh. Pleasingly, excluding the car, the house had exported more electricty than it had imported over the whole year.
Having now experienced a wide range of outside temperatures, I could begin to assess how my heat pump installation was performing. It was possible to create a table of Outside Air Temperature (OAT) v Coefficient of Performance (COP) and estimate how much 14 hours of heating would cost. Using my standard Octopus tariff rates, I could see that my crossover point between gas being cheaper than the heat pump was at an OAT of 8C. Colder than that gas would be cheaper, warmer and the heat pump was cheaper. This was useful to know, but I also knew that my SCOP would improve as I did more of the house renovations, which would allow me to run at a lower flow temperature. An additional variable was the cost of the electricity. For comparison I had used the standard rate on my tariff, but in fact I was making use of an excellent night time rate of 7.5p where possible and this was lowering my unit cost overall. Lowering my average cost to 25p per unit changed the calculation markedly The crossover point had now moved to an OAT of 5C. To calculate which would have been cheaper to run all winter, all that would be needed was the average temperatures of each heating day. Above 5C I was winning and below 5C I would be losing. A graph of average daily temperature, showed that the majority a values were well above this level. Comparing bills between this and last year, the heat pump was coming out on top.
After three months of winter heating, it seemed like a good point to assess how the new heating and hot water system was performing compared to the old gas combi boiler. Because of the changes in solar panels and tariffs a direct comparison in terms of monthly cost might be difficult, so initially a comparison in terms of kWh hours would seem more useful. The amount of gas used in 2022 was used to provide heating and hot water, with a small amount used for cooking on a gas hob. The gas hob was replaced with an induction hob and I wasnt monitoring how much electricity was being used by that, so could not account for it. However the effect would be small.
Ignoring that difference, the number of kWh used between the two systems was profound, with the new system using between 28%-53% of the gas figure. The wide variation in the size of the reduction was a little confusing, but I think this is down to several factors between the two years, including:
I know from my monitoring system , that my heat pump is running at a COP of between 4.6 and 3.6 during the winter so far. The lowest COP occuring during a cold snap where the temperature dropped to about 5 degrees C. I fully expected that the heat pump would be more expensive to run in very cold weather, than gas heating. However I also expected that in the shoulder seasons and summer this effect would reverse. Removing gas from my property, meant that I no longer had to pay a standing charge for it and getting an Electric Vehicle enabled me to access much cheaper off peak tariffs and these factors have helped with reducing my electricity bill. In addition my extra solar panels would also reduce bills and provide an income in the summer. So far comparing my total energy bill with 2022 (adjusting for inflation), I had saved £109.31 for the period Oct-Dec. A modest difference and not enough on its own to justify the costs of the improvements made. However I expect this to improve as the the newer EV tariff starts to kick in and solar export starts to increase. With the new Electric Vehicle bought and the old cars sold, we are now officially a carbon free household. :-)
Taking the car on its first long distance drive this weekend required a bit of planning with respect to locating charging facilities close by, but turned out to be easily achieved and not a big issue. Unfortunately one thing no one told us was that the charging cable is locked in the car until the doors are opened! Who knew! :-) Getting a charging point installed in the house was the next step and would allow me to change to tariff to Octopus Intelligent Go. We finally decided that the time was right to purchase an Electric Vehicle. Second hand prices are very low at the moment and the availability of charging points is getting better. We settled on ordering a 2020 BMW i3.
Having an EV opens up a new range of electricity tariffs available from Octopus and crunching the numbers shows some quite surprising results. Currently on Octopus Cosy and Fixed Octopus Export, I had already picked up on the fact that the cheap rate electricity price and the export price were getting very close (16p v 15p). This meant that there wasnt a great deal of financial difference whether I heated my hot water tank with excess solar via my Eddi PV diverter or did it during the cheap rate periods. One of the new tariffs I could now access with an EV is Intelligent Octopus Go. This tariff is still compatible with Octopus Fixed Export, so I could still get 15p per kWh exporting my excess solar. The idea behind this new tariff is that you hand over the control of charging your EV overnight to Octopus. You just tell them how much you want the car charged and when it needs to be done by. As a reward for this you only pay 7.5p per kWh. This price is a game changer! If I switched to this tariff, then I would obviously get very cheap electricity for my car, but it also meant that I would not get the second cheap rate period in the early afternoon that I currently have when I recharge my 5kWh house battery. Therfore if I assumed that I fully recharged my battery twice on the Cosy tariff, this would currently cost (5 x 2 x 18.13p) £1.81. If I switched to Intelligent Go this would now cost (5 x 7.5 + 5 x 30.52) £1.90. No big change. However heating my domestic hot water now becomes much more attractive during the cheap overnight period, rather than using diverted solar which could be exported instead. If I assume I use 4 kWh of hot water a day, this currently costs (4 x 15p) 60p per day in lost export. If I switched to heating it at night, it would cost (4 x 7.5p) 30p per day. This equates to about £12 per month in savings! As well as saving, I am also helping the grid stay clean by providing my green energy to them in the day when it is needed and using green energy from the grid at night when it isnt needed. So the question is, what is the point of my £500+ solar diverter now? I suppose the pricing of electricity could change in the future and it might make more sense to use my own solar again, but that seems unlikely to me. Perhaps I should look to sell it on to someone who doesnt have an EV? After using the Octopus Cosy tariff for a month, I could now see how charging my 5kWh battery twice a day at cheap rate, was effecting my unit cost.
Over the month, I used 2.1% of my electricity at peak rate (4-7pm, £0.42), 17.4% at mid rate (£0.26) and 80.5% at low rate (£0.16). This gave me an average unit cost of £0.18 per unit. It seems that even a small battery can have a significant effect on electricity cost and this was in addition to the extra savings I was making by utilising more of my solar panel output. Now that the heating season has started, I am now able to make some comparisons between last year when I was using a gas combi-boiler and this year when I was using my heat pump.
Last year between the 26/09/22 and the 25/10/22, I used 570 kWh of gas to provide my space heating and hot water. During the same period this year, I used 197 kWh of electricity. The 2022 figure is probably an underestimate because I had recently fitted my wood burning stove last year and I know we where using it in October, whereas we hadnt used it at all this year. I also know I could reduce the 2023 figure further by not heating my water with an immersion heater, but by buying a heat exchanger and using the heat pump instead. Adding up how much electricity I used to heat my water and calculating how much I would get if I exported it instead came to about £14 for October. If I could get a COP of 2.5 for water heating, then using the immersion had acutually cost me just over £8. This should be relatively consistant over the year, so using the immersion instead of the heat pump costs about £100 a year. This would be about a 4 year payback term on buying the heat exchanger. Looking at my bills, last October cost £61.49 as I was on a good fixed tariff, at todays prices that would be £87.41, this October my bill was £38.80. So a saving of £48.61 but this included all the improvements I had made including the extra solar panels and battery. A modest saving so far compared to the costs of the improvements, so it will be interesting to how how this changes over the year and what period would be needed to recoup the investment I had made. As the colder nights started to draw in I decided that it was time to do some tinkering with my heat pump settings to try and get the maximum efficiency. Heat pumps work most efficiently at lower flow temperatures, so the trick was to use the lowest flow temperature I could whilst still heating the house sufficiently. Obviously as outdoor temperatures get lower, the house will lose heat more quickly and the flow temperature will need to be increased to keep up. The way heat pumps automate this is called weather compensation. All that this involves is entering two set points in the the software for outside temperature and leaving flow temperature from the heat pump. Initally mine was set up so that an outside temperature of 15C, the flow temperature was 38C and at an outdoor temperature of -2C my flow temperature would be 45C. For all outdoor temperatures in between the software would calulate a flow temperature in between my set points. I felt that I could probably reduce the 38C flow to perhaps 35C to get better efficiency and so over a few days tried doing just that and studying the results. What I found was that as I dropped the flow temperature, the heat pump started to turn itself off periodically, as shown in the graph below: This behaviour is called cycling and is generally thought that it is best to be avoided if possible, as it puts extra wear on the compressor and might lead to premature failure of it. The question my mind was, why had it started doing this at lower flow temperatures.
After some research and help on forums, I came to the conclusion that the problem was that the heat pump could not get rid of enough heat into the house, fast enough and so was turning off until needed. This initally seemed counter-intuitive because I was using a lower flow temperature and so trying to put less heat into the house! The answer was that because my flow temperature was lower, the difference in temperature between it and the house was lower and so less heat was being lost by the radiators! The only way I could avoid this was either to use bigger or more radiators, or increase my flow temperature again. As my extension was planned for next year, this would add extra radiators to the circuit, so in the mean time, I decided to raise the flow temperature again and also change one of my set points. This had the desired effect, and the cycling stopped. My new set points were 38C @17C and 45C @ -2C. When the very cold weather came I would have to see if 45C flow was enough. After a couple of days on my new tariff and charging schedule, the smart meter was reporting quite high daily costs. It looked like I was being charged the standard daily rate for my imports so a check was in order to see if anything untoward was happening! Fortunately my OpenEnergy Monitor was showing just what I expected, with imports occurring during the 04:00-07:00 and 13:00-16:00 cheap windows and virtually nothing in the 16:00-19:00 peak price window.
The smart meter also appeared incapable of factoring in any export that was going on, so it would be interesting to see what the bill looked like at the end of the month. |
AuthorEx Radiographer, Information Analyst, Teacher and Self-builder. Now retired Archives
December 2023
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