...Continued
The diagram is a crude illustration of how the various technologies compare in installation and running costs. The heat pump starts off at a minimum of £500 per year due to the initial cost and that is after the grant. It’s over a £1,000 a year without a grant. The AAHP is standalone, £800 over 10 years. The gas boiler starts off at £4,000 over 15 years but probably 20 as it’s very lightly loaded. It’s only really going to be used on the coldest of days or if the electricity prices are high for some reason. With TRVs (that you can’t use with a heat pump) the gas boiler will only be heating the other rooms in the house as required to balance the heating. You are only asking the AAHP and the gas boiler to do what they do best so the combination is saving both money and gas at the same time. The immersion heater is saving gas on the lowest priced days but it’s capable of storing a certain amount of extra heat (as a battery) for later use so may save more than is initially estimated.
Some final ramblings
Two or three AAHP distributed around the house is roughly the maximum heat load for the house. Total cost about £2,400. Each one is only 800W so it will easily come off the ring final circuits (used to be called ring mains). No major re plumbing. I’m now beginning to wonder why we need to have a central heating system at all and what would be needed to get through the 4-7pm peak. There is still is the need for water heating but there are solutions for that as well. If you are not paying for a gas boiler then there is the heat pump water heater instead. That is too far for where I’m going but the AAHP has been ordered.
Jumping ship
I changed my energy supplier because Ovo wasn’t offering a tariff that meant I could reduce the cost of charging the car but it has turned into far, far more than that. Yes, being on the wholesale day-ahead market isn’t going to be what most people would want to do. There is also a lot of active management of usage for the car and immersion heater at the moment but I have now joined a group at Octopus to explore the use of IoT technology that can read Agile prices and they can switch on and off when compared to a threshold and other criteria.
For the immersion heater the controller would have ‘when agile < gas price / boiler efficiency’.
For the AAHP, it would be on a time switch and work perfectly well but its controller would have ‘when agile < gas price x COP / boiler efficiency’ control. COP and boiler efficiency are both constants so that effectively just translates to ‘when agile < 24.5 then Switch On else Off’. To be more green you can set the threshold higher but it’s probably still best to keep electrical equipment off during the peak.
The car is more difficult and the ‘when’ condition is a lot more complicated and is related to ‘price’, ‘battery level’ and ‘future use’ and it may not be possible to automate that satisfactorily. The combination of Future use / Battery Level would have to override price and some benefit could be obtained from not only knowing the price tomorrow morning but the price for the day after that in that delaying charging could take advantage of lower future prices. On the other hand it may not result in a significant decrease in cost.
Side note: Heat Pumps and all that new-fangled nonsense - What is the issue with heat pump used with central heating systems?
There is nothing fundamentally wrong with heat pumps. In fact it’s the only way we can go to improve energy efficiency and have a greener future. The issue is the way that it’s being done. The AAHPs work very well. They have been around for a long time and they have a COP of around 3-4 and higher for most of the temperature ranges you are going to see. They work well for two reasons. They only have two heat exchangers. The outside air to cold refrigerant and then the hot refrigerant to the inside air. They work with air as a medium which has a low viscosity and the heat exchange is assisted by fans and the air channels are wide and very short.
In contrast, central heating heat pump systems have three heat exchangers. The first one is the same as above. The next one is hot refrigerant to water. Water has a high specific heat which is very useful but has a much higher viscosity and pumping losses than air and it’s very difficult to push it through the all the thin, bendy pipework that constitutes an average house central heating system. In fact, this is such an issue that although the water loop goes outside to the heat pump you can’t add antifreeze (normally glycol) to it as it further increases the viscosity to the point where it won’t work effectively. The Heat Geek surveyor had charts with him to show that. The third and worst heat exchangers are the room radiators. These typically don’t have a fan so the heat transfer rate is very poor and it ultimately reduces the COP of the heat pump unless you effectively double the surface area of the radiators. The weakness of a heat pump coupled to a conventional central heating system is therefore the viscosity of water in long, thin pipes and the poor performance of room radiators. You can overcome the last issue by changing to underfloor heating with floor insulation. That increases the surface area, decreases the water temperature necessary and decreases the resistance to the water due to parallel paths. Retrofitting the average house is very expensive and uneconomic and not possible with all houses. New houses should be built with insulated floors and underfloor heating. As far as I can see it’s seen as a desirable feature for new build houses but not a mandatory requirement even at this late stage.
The surprising physics of heat engines
In physics the COP (coefficient of performance) is a part of Carnot’s Theorem on heat engines and there is a theoretical upper limit to the COP for a heat pump given the input and output temperatures. From the graph below for an input temperature of 0C and an output of 40C (slightly lower than that required for a central heating system) the upper limit is about 8. This means spending £16.5k to get a COP of only 3 or 3.5 from a central heating heat pump is rubbish. It tells us there are limitations in the heat exchanger (HE) design (they need to be bigger) particularly the last HE, the radiator, which is particularly poor. Changing to underfloor heating increases the surface area and reduces the water temperature but water flow rates are still an issue.
In contrast an AAHP has an input of 0C and an output of about 20C which means its COP could be as high as 15. This means that if you were to be able to realise this value you could heat a 4 bedroom, detached house with average insulation using a maximum of only 500W of electrical power. This is obviously nonsense because the relatively small size of AAHP means there are compromises on the two heat exchangers that reduce this value and also the type of refrigerant used but it has the potential to be much higher that a heat pump feeding a conventional central heating system.
The ethics of negative prices
(inset picture; My 6th April electricity usage: 23kWh of electricity; Cost = -£0.19)
On Speak EV this point came up in a discussion about Agile prices when they went negative (again). If Agile are paying you to use electricity then anything you can do to use electricity will result in an actual income. I’ve had one or two days where my usage was concentrated on half hour periods where the price was strongly negative and my net cost for the day was negative. Not a huge amount but on one day I actually got paid to charge the car, heat the house and do loads of higher temperature washing. If you are using a fan heater then you may get to the point where it’s so hot in the house you need to put it out in the garden and heat that up instead. You could say that this is unethical and it’s a moot point but you have to understand where negative prices came from in the first place.
Side Note: Day Ahead prices and when it goes negative
This is my understanding (in the simplest possible terms and with probable omissions and errors) on what happens and it starts with the law that says that the National Grid (and the through the utilities) has to meet the peak demand of all its customers all the of time. To do that the NG has to arrange sufficient generating capacity, though long term contracts agreed at a strike price. This is the Contract for Difference (CfD) mechanism. This means that whatever price the electricity actually gets sold at, at the moment of generation, the generator will get that price. If it’s sold for less than the strike price then the LCCC (Low Carbon Contracts Company) will make up the difference and if sold for more, then the LCCC are paid the difference. The LCCC was created to allow for renewables generation to enter the generating market because their output can’t be defined in advance. The utilities have to ensure they have enough energy to pass on to their customers to meet the full demand. To ensure they can meet the contracts (Fixed Price and SVT) with their customers they overbuy their energy needs on the futures market some time in advance based on expected number of customers, expected load, expected weather conditions etc. However, none of those guesses are going to be right on the day so they hedge their energy demands.
The Day Ahead Market: About 36 hours before the actual moment of generation (and near instantaneous consumption) the NG now has a good idea on demand and generating capacity on line at the time of generation. It’s at this point where the utilities (or at least their computers) note that they have overbought based on tomorrow’s actual conditions that are actually not average. The demand is lower, it’s warmer and the generators have sunnier or windier conditions than the average. They then push their unwanted demand back onto the market which according to the supply/price curve pushes the prices down. The demand/price curve is opposite; if the price goes down the demand goes up. So, the NG is stuck with the supply that was bought on the futures market by the utilities and it can’t do anything with that but allow it to be generated. What it can do is push the price down the see if it can drive the demand up to match the supply. There will always be extra offtakers if you push the price down. Me for one! The generators will willingly offer lower and lower prices to stay in the ‘generation game’ because their strike price will be met by a top up from the LCCC. This is until prices reach zero. The generators cannot go negative as this would create a ‘negative market’ which doesn’t have a bottom limit. They would also forfeit the CfD condition in being compensated by the LCCC if they do. However, if there is still too much generation capacity then the NG can create a negative market and the generators still get the LCCC top up. The lowest negative price in 2023 was -19p which equates to a generator paying out about £90 for each MWH generated but still being fully compensated back to their strike price. In the worst case the NG could pay a curtailment charge to the generators to scale back generation but this is very costly and it’s generally cheaper to stimulate higher demand with negative prices.
So, the LCCC could be out of pocket big time. They are a government owned company but they don’t have a budget as such. Their costs are not paid by taxes. Any loss that the LCCC makes (including running costs) is paid for by the electricity supplier obligation levy on the utilities. This is then paid for by the users as part of the SVT and Fixed Price contract price. For Agile pricing the price paid per unit is 2.1 x Wholesale Cost. After paying the Wholesale Price the remaining 1.1 is the profit element for Octopus plus the LCCC levy and their running costs.
The last part of the market occurs at the time of generation and is called the Balancing Mechanism where an NG computer issues sharp changes in the unit price to encourage or discourage power generation (possibly even very agile demand) to keep the frequency within limits. Adjustments are made to the generators via the CfD contracts.
Meanwhile, The Only Way Is Ethics
So, negative prices and people putting heaters out in their garden is perhaps more of an issue about shoehorning renewables into a CfD mechanism that it wasn’t designed to accommodate and less about the rights and wrongs of wasting electricity. Because of the law the cheapest option that the NG has to meet its contractual obligations with the generators is to set negative prices on occasions and quite low prices a lot of the time. The average, long term, all-day price for Agile is 15p.
And finally, the strange world where HMRC pay you to use electricity
I’m not even sure if this is true but it follows on from the maths. If you consume electricity and pay for it then you pay VAT at 5%. However, if electricity is negatively priced then the VAT is also negative. I.e. HMRC is paying you to consume electricity. Now, it’s only likely to be only be a penny or two at most in a day but the sheer madness of that is out of all proportion to the financial reward and how strange is that!
