Why my heat pump performed worse this winter

Over the first winter my heat pump (space heating only) gave me an average COP (efficiency) of 3.1 which was higher than I had been told to expect. I was moderately pleased. This winter it was only 2.9 which was a great disappointment.

The difference is due to the weather. The mean COP for one year is not necessarily a guide to the next. The mean temperature is part of it but colder days require proportionately more heat, which biases the average COP. So I have done my best to work out what the average COP should be over time. I was surprised at the year to year variation.

Air source heat pumps heat the outside air even when heating your home

Standing in front of a heat pump when it is working hard you will feel an arctic blast of cold air. So you would think that if every house has one the outside air will get colder. After all, we know that cooling buildings with air conditioning (A/C) adds to the urban heat island effect. Since cooling our homes makes the outside air warm, you would think that heating our homes would make the outside air colder. In fact, using an air source heat pump to heat our home heats the outside air as well as our home. Read on to find out what happens to the air outside when we use air source or ground source heat pumps in heating or cooling mode, compared with a gas boiler for heating.

1 Simplified heating scenario.

Why do heat pumps get less efficient in warm weather?

This post has been modified 5/April/23 after discussion on twitter with @AR_Clark convinced me that the reason for the continuing decline in efficiency in warm weather is due to increasing use of the immersion heater for hot water heating. This is hopefully less common now, as heat pumps can provide all the hot water necessary except for sterilisation cycles, and sometimes even then.

We know that air source heat pumps (ASHP) are less efficient in cold weather. However this chart shows decreasing efficiency in warm weather too. The chart comes from an article recently published in Energy and Buildings [1]. It was generated using field data from UK heat pumps installed in 2012. These are quite old installations and performance has improved quite a bit since then. However, this data is still being used to predict national electricity demand in the future. In the chart each dot represents the average COP (efficiency) of all the ASHPs included in the analysis over one day. Efficiency is low in cold weather, and increases rapidly but only up to about 8°C. After that things go downhill rapidly. This is less critical than it looks as the heat demand also decreases rapidly - performance in cold weather is much more important - but it is still surprising and not a good sign. What is going on?

 

Figure from [1]

The article has an equivalent chart for ground source heating pumps (GSHP). At low temperatures, the GSHP efficiency remains high because the ground is still warm even though the air is cool. However in warmer weather the efficiency declines, much the same as with the ASHPs.

Avoiding the rated power fallacy

How much energy does an oven use? Today I have seen examples both on the BBC Food Programme and on the Uswitch website [1] where this question was answered by multiplying rated power by hours of use. This is totally wrong because ovens have thermostats. They turn the power off when they are up to heat. The power of the oven is mainly used during the initial heating phase. After that it turns down the heating and/or cycles on and off. 

Screenshot from Uswitch [1] As well as falling for the rated power fallacy Uswitch has confused watts (power) and watt-hours (energy).

How long can we rely on cheap rate electricity?

A number of my friends are considering or have already installed a Tepeo zero emissions boiler (ZEB) with storage as a low carbon heating solution for their home. This is less disruptive than installing a heat pump but it is only 100% efficient compared to 300% or so for a heat pump. It relies on cheap electricity to keep energy bills at a sensible level – either economy 7 overnight, or similar, or relying on prices fluctuating during the day with a tariff based on wholesale prices, such as Octopus Agile. I shall call all of these tariffs ‘cheap time’, for brevity. 

Cheap time tariffs could become cheaper – or more expensive

Historically, overnight demand is lower than during the day, and hence prices are lower because there is no need to run the expensive peaking plant. This has enabled tariffs such as Economy 7 to work well for homes with storage heating. Hitherto this has been only a small fraction of heating demand: I estimate just 2.2% of domestic heating demand in England [1]. However, there is going to be lots more demand for cheap time power due to switching to low carbon transport and the need for electricity storage to take account of intermittent renewables. Cheap-time tariffs could become cheaper, because renewable power is cheap, or more expensive, because there is more demand for it. On balance I fear the latter is more likely.

Greenhouse Gas Removal – and checking it works

According to the Climate Change Committee (CCC) balanced pathway to net zero, we should be capturing and storing 58 MtCO2e/year by 2050 [1]. This is called greenhouse gas removals (GGR) or sometimes carbon dioxide removals (CDR). It is hard to see how we can get to net zero without some GGR, but there is very little of this happening right now. There are several strategies likely to be deployed. Unfortunately, the ones which are most likely to be permanent tend to be most expensive as well, while with the cheap ones it is harder to measure how well they are working.

In the CCC balanced pathway, 90% of GGR is some form of BECCS – bioenergy with carbon capture and storage. That generally means burning biomass for energy and capturing CO2 from the flue gas. Most of the rest is direct air capture (DAC) which means getting carbon dioxide from the air rather than flue gases. The captured gas must then be stored typically in rock formations or depleted oil and gas wells.

 

Figure from [1] CCC sixth carbon budget

Keeping warm in cold conditions - field studies of personal heating devices

My last post discussed how much we need to heat our homes to keep them healthy. The short answer was, about 8°C warmer than outside. This could mean temperatures as low as 12°C or even less. So how do we keep ourselves comfortable? Wearable devices, warm cushions, portable electric heaters? This post reviews some field studies described in academic literature. It seems to be a hot topic in China.