Good COP bad COP
A ground source heat pump (GSHP) is more efficient than an air source heat pump (ASHP) but is this relevant, or even true, for central Europe where air temperatures are warmer than in England for example? Taking the daily high and low average temperatures for Perugia in Central Italy and converting them directly into COP figures for a modern ASHP gives a chart showing the performance envelope. The straight black line shows an approximation of GSHP performance. Note how the GSHP starts really well and declines over the season as the ground energy gets depleted. Even so it performs strongly throughout and has the ASHP licked overall especially at night when the air temperature falls dramatically.
That’s the simple story but can the ASHP fight back in any way?
You’ll see that the big red line at the top shows that every day there is a time when the ASHP will always be running a better COP so the ASHP could be elevated with a total commitment or even a bias towards day time running. This assumes adequate power and bigger energy storage with tanks of some 2,000 litres. A house with high thermal mass and heated concrete floors will store energy too.
This regime pulls the average COP for the ASHP a bit higher than halfway between the day and night lines so over the season the ASHP is a match for the GSHP; but the battle isn’t over yet. Time to introduce the solar shed.
The solar shed in its simplest form is a greenhouse for the ASHP and apart from adding a little sunlight to the equation it allows 2 extra sources of warmer air to be added. The ASHP fan blows air out of the shed and replacement air can come from vents leading from the house air exchange (bathrooms and kitchen for example) and also from large diameter tubes buried in the ground. The latter is old technology and has been shown to raise the air temperature by around 5 degrees C. The green dotted line on the chart shows the night time COPS being elevated by 5 degrees and as you can see the COP envelope is an easy match for the GSHP and draws ahead in the late season.
Finally some ball park economics starting with a €10,000 ASHP and the equivalent €20,000 GSHP. Assigning a typical bill of €2,000 to run the GSHP a standard ASHP would cost €2,500, at the very worst, and thus take 20 years before the total cost overtakes the GSHP – already a no brainer for the ASHP before we even start on economy measures. Increasing the storage to a 2,000 litre twin tank system would add up to €3,000 and that would be recouped in 6 years, which is useful, but the air recovery and ground heating systems don’t look feasible on top of that although eco-warriors might like to go the whole hog.
The conclusion is that Air source is the way to go in Southern Europe and big tanks are worthwhile if the quotes look reasonable.
What about England?
If you live in England you might be wondering how relevant this graph is to you. Comparing monthly average temperature highs and lows for Perugia vs. Horsham gives these results for the 6 cold months only:
Nights are only 1 or 2 degrees colder in England except for Jan and Feb when Italy is 1 degree colder so on balance there is no real difference.
The average monthly lows for both zones are just above freezing – i.e. no trick technology required for air source heat pumps.
Days are always warmer in Italy, 1 – 3 degrees. The swing between night and day is a little more and so air source heat pumps make more sense particularly if biased towards daytime running with big tanks.
For the 3 coldest months England is only a degree or two colder during the day than Italy so in COP terms they are in the same ball park.
The bottom line is that an ASHP is just as valid wherever you are, the graph holds true and it makes sense to run during the warmer day and store in bigger tanks. Running an ASHP on economy 7 at night is also valid of course and I’ll make comparisons when I’ve updated my models.
If anyone is into the nitty gritty of COPs and stuff I have prepared a COP matrix for a modern ASHP in Excel. You can select an air temperature and the target temp such as floor, rads, fancoils and read off the COP. Just contact me if you’d like a copy.
If you are starting a project and need advice on integrating wood, solar, gas and an ASHP by using a heat bank just let me know. I’ll be in England full time after Summer 2015.