14 Nov 2020

Are heat pumps the way forward?

There is a lot of talk about heat pumps. Some tout them as the future of home heating, a surefire way of combatting our carbon intensive heat habit. There's no doubt it's a big problem, as this diagram clearly shows, but are heat pumps an easy fix?



First things first. What exactly is a heat pump? The clue is in the name. Heat pump. Unlike every other form of heating we have ever used, a heat pump doesn't actually produce heat, it shifts it. It takes the heat out of a large body of stuff (be it the air around us, the ground underneath us or some body of water nearby), and shifts this heat into a much smaller body of stuff, either air or water inside the house. 

That is the magic of the heat pump. 

 

Another way of looking at this is through the medium of colour. Imagine you had a large square drawn on a sheet of white paper and that this large square is filled evenly with very pale pink colour. It took just three drops of red inkwash to get that pale pink colour. Next to the large square is a small empty square, about a third of the size. If we were to somehow suck the red inkwash out of the large square and pour it into the small square, the three drops of red inkwash would be enough to turn the small square bright red, whilst the large square would now appear to be empty. We haven’t made any new red inkwash, we have simply moved it from one square to another, whilst increasing its concentration. The pale pink colour in the large square is gradually replenished by more red ink wash (incoming solar radiation): in this way, heat pumps are (mostly) a renewable technology.

 

Heat pumps are not new. They have been around almost as long as electricity has been wired into peoples’ homes. And heat pumps don’t just make small spaces hotter: the heat can flow the other way, in order to make spaces colder. Think of the domestic refrigerator. Every home you have ever lived in already has a heat pump working away in the background, at the back of the fridge. The same technology underlies the world of air conditioning as well. It’s a huge worldwide industry. 

 

But what concerns us here is the home heating aspect of it. It’s a technology that has energy-efficiency burned into it and, used well, it can achieve enormous energy savings with the potential to make the switch to electric heat a lot more palatable.

 

The key point of interest to engineers here is to measure the level of energy efficiency. This is conventionally expressed as the ratio of the amount of heat being pumped into a house set against the amount of electric energy required to run the heat pump. 

 

This ratio (heat in:power used) is known as the Coefficient of Performance or COP, and when you start getting versed in the world of heat pumps you will hear the term COP (or sometimes SCOP, where the S stands for Seasonal) used a lot. The higher the COP the better; it means the heat pump is more efficient and this makes it both cheap and green to run. 

 

The downsides for heat pumps is that they are relatively expensive to install and that they are electrically powered. Whilst electricity is now less carbon intensive than gas, and appears to be on course to become nearly carbon-free over the coming decades, domestic supply is currently three to four times the cost of gas.


If you want British consumers to switch over from gas boilers to heat pumps, you really need the heat pump to be at least three to four times more efficient in order to cancel out the pain of higher fuel bills. So if you manage to get a COP of 3 or 4 (which is technically quite possible), then your new electricity fuel bills will be comparable with your old gas bills. Get your COP higher than 4 (unlikely, but it has been known) and you will have the most thrifty heating system imaginable. Not only greener, but cheaper to run.


So why not just buy a heat pump with a very high COP? This is where it starts to get complicated. Unlike a gas boiler, which simply responds to a command to “Burn, baby, burn”, the actual performance of a heat pump is down to what you demand from it. Ask it to lift the internal water temperature by 30°C, and it will turn in a five-star performance with a COP way over 3. However, ask it to lift the water temperature by 60°C, and it will be a very different story. The same heat pump will be performing with a much lower COP and the cost of your home heating will appear to be astronomical. In fact, for every 1°C temperature increase you ask from your heat pump, the COP falls by 3%. The lab tests used to advertise the relative performance of heat pumps are all conducted at low temperature lifts.

 

So how much work will you require from your heat pump? 30°C lift (success) or 60°C lift (failure)?

 

There are some big variables to consider here.  Let’s look at the two most important.

The input temperature

The warmer the heat source, the less work the heat pump has to do, and therefore the more efficiently it runs. Not all heat pumps are the same. Air-source heat pumps (ASHP), as their name suggests, draw their heat from the surrounding air and they therefore have to deal with a very wide temperature range. In summer, they can be immensely efficient but homes don’t need space heating in summer. In mid-winter, when the air temperature falls below zero, they are starting with one arm tied behind their back and their efficiency (COP) falls substantially. In contrast, ground source heat pumps (GSHP) draw their heat from below ground where temperatures are much more even throughout the year. It follows that, over the course of a year, GSHPs are more efficient than ASHPs. The downside is that GSHPs are much more complicated and expensive to install.

The output temperature


The two key factors affecting your heating demand are space heating and domestic hot water. Space heating is conventionally delivered via radiators, or occasionally underfloor heating. The water flow temperature in your radiators can be as high as 70°C, which is enough to put a huge strain on any heat pump. The solution may lie in fitting bigger radiators capable of working at lower temperatures. Underfloor heating is designed to work at around 40°C  and therefore makes a good partner for heat pumps, but unfortunately very few homes in the UK have underfloor heating systems. 

 

Domestic hot water is another kettle of fish. Most UK homes, at least most small ones, now use a combination boiler (aka a combi) to supply their heating needs. A combi heats hot water instantly with a quick blast of gas. Heat pumps are unable to do this: they work by supplying a steady trickle of heat, so hot water for the tap or shower has to be sourced from a storage tank such as a cylinder.  

 

This is a problem for many homeowners in that combi-heated homes don’t have any hot water storage, nor do many of them even have space for hot water storage. If you are very desperate, you can probably find a place somewhere in your home for a hot water tank, but it will involve lots of plumbing and probably carpentry as well, to create an airing cupboard. 

 

If you are building a new home, these issues won’t be a problem. Hot water tanks, underfloor heating, good insulation, efficient glazing: all these things are easy to build in. But if we wish to make our existing housing stock all electric, we have some major issues to address here if we want to switch over to heat pumps. We have developed a housing stock that is based largely on high-temperature, gas-fired instant heating. Weaning us off this will not be easy.


Good installers know all this and will be able to advise you as to whether your home is heat pump ready or even heat pump compatible. Cowboys will just sell you a heat pump and let you suffer the consequences - cold homes with high fuel bills.

Hybrids

If you don’t have a heat pump ready home, and you are not inclined to undertake the upgrades required to do this, there are hybrid options available. Typically this might be keeping your gas boiler and using it for domestic hot water and for very cold days in winter when a heat pump will struggle, but using a heat pump for the main bulk of your space heating requirements. As space heating typically makes up something like 80% of your homes overall heating bill, such a solution will make quite a difference to your carbon footprint, but it’s unlikely to be any cheaper to run. Hybrid solutions can also be combined with solar thermal or PV which will further reduce your carbon footprint. But you won’t be on course for a fully electric home whilst you still have a fossil fuel boiler as a back up.


If you want to do that, you can always fit an electric boiler. They even make electric combis, but they are underpowered when compared to their gas-fired cousins. And of course, they will cost three to four times as much to run. Not really a very enticing option.


 

Policy

The government is indicating that it wants us to switch to heat pumps for all new homes within a few years. Technically this should be no problem, as new homes can be designed to be heat-pump ready. The problem resides in what we do with the existing housing stock, something like 25 million homes heated by gas or oil which won't take easily to a heat pump without a fair bit of adaptation. This is what needs to be the focus of policy over the coming years. The switch over to heat pumps isn't going to happen without a few carrots (incentives) and sticks (taxes on fossil fuels), and how these issues are approached by our government will be a measure of how serious they are about de-carbonised home heating. 


The only document they have published so far on this topic, the Future Homes Standard, is a very tepid dish indeed, which will have very little impact on the rate of heat pump deployment in the retrofit sector.

 

 



 

 

 

 

 

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