Air Source Heat Pumps

stevehead

yes i know it's milder - 11C outside as i write this - but the ASHP's haven't been on at all today and I'm bloody sweltering. It's 24C / 56RH in the living room.
Bit of sunshine through the windows, followed by roast & a pie in the oven this evening has upped the indoor temp. No open windows so most of it's still in here!

richardc1983

Mad ain't it really when previously as I and many others probably o when were cooking we just open the windows to clear the steam or heat. What a wonderful piece of equipment they are.

mech_2

stevehead wrote: »

Hi Mech,
I see how you concluded this - I could duct the outflowing air over the ASHP compressor unit and grab the lost heat back. Neat. The outdoor ASHP unit is only about 4m from my HRV Exhaust.
I won't do it though. The ASHP outdoor unit moves a massive amount of air through it, whereas the HRV just gently exhales. As the HRV exhaust is only ~3C higher than ambient, I feel the benefit would be negligible.

I'm really only talking from a design standpoint. An integrated whole-house solution could be more efficient (and use less electricity for fans to boot). Not just because that 10-20% of heat from ventilation is no longer wasted (though if you have insulated and draught-proofed to the maximum, that air will be a major source of heat loss) but also because it will improve heat pump COP.

All ventilation is 'at odds' with all forms of heat. Just that all ventilation is 100% loss except MHRV, so it's the best of a bad job. Now if I didn't actually have to live in my house......

I've got a brand new - as in opened this month - gas generator 4 miles away. My leccy comes from the most efficient gas turbines in the UK just down the road. Add that to my ASHP's COP of 5.45 and 175% is on the low side.

No these figures are just misleading. You certainly don't have a heat pump with a seasonal COP that high. I have yet to see a field trial where an ASHP gets more than 3.8 and that's with a pipe buried in the ground as a kind of earth-source pre-warming air duct (apparently not uncommon in Germany) boosting the performance by 15%. The vast majority don't even get to 3.3, even ignoring domestic hot water (which a gas boiler does supply, let's not forget). Hell even some of the GSHPs don't get as much as 4 in trials.

Let's run through the maximum realistic figures... the CCGT power station has a headline figure of 59% efficiency. In the real world that's not accurate for the purposes of calculating the generating efficiency as it uses the lower heating value (LHV) of the gas. Real efficiency (using HHV) will be 90% of that... 53%. That's when it's at optimum load. And minus grid losses. Grid losses are usually between 7% and 8% for the UK as a whole. Though domestic customers (low voltage) incur greater losses than high voltage industrial consumers - usually more than 9%. Because you are close to the power station let's say it is only 7% though. 53-7=46% then multiplied by a generous seasonal COP of 3.4 and assuming your balanced-flue gas water heater is 75% efficient and your hot water demand is 20% of your whole heat demand (a pretty low figure if the house is insulated) that makes an overall PER of 140%. Not bad, but it is a maximum. And nowhere near 175%.

And in the real world that gas power station is connected to the National Grid just like everyone else. Gas isn't generally considered baseload: it's ramped up and down to meet demand. Centrica's gas power stations run at a loading capacity of less than 60%. Most of that will be during the day on weekdays with a peak just after everyone gets home from work and cooks their tea between 5 and 7 pm. Then for the rest of the evening the gas turbine will be spun down until early the next morning. What percentage of your electricity usage actually comes from your local gas power station is anyone's guess. I'm pretty sure it won't be near 100% though.

For everyone else the sums go like this: kg of CO2 per kWh for mains electricity = 0.591. Therefore an electrical ASHP with COP of 3 (incl. DHW) = 0.197 kg CO2 / kWh. Gas = 0.206. Therefore condensing boiler = 0.229. And this means an ASHP (IF it can supply hot water as well and still manage an overall COP of 3) will typically produce 14% less CO2 than a SEDBUK A boiler.

http://www.bre.co.uk/filelibrary/SAP/2009/STP09-CO201_Revised_emission_factors.pdf

Of course, if you have an air-to-air ASHP you still need to get hot water some other way. If you use an electrical immersion heater, this brings the figure vs gas down, but the overall COP even if the heating generally runs at a COP of 3.4 will go below 3. Whatever you do for hot water in this scenario, there's not much chance of it being cheaper than a mains gas SEDBUK A system boiler, even if it shaves a few percent off your carbon footprint.

Swipe

I received my electricity bill today. I'm very impressed. I used just £49 for the period 23rd July to 19th Oct. That includes 3 weeks or so of using at least one of my ASHPs most evenings for 4 hours.

It's currently 12C outside. My living room air source heat pump is maintaining an indoor temp of 19C and drawing just around 300w. I'm also adding an extra 170mm of loft insulation this weekend and getting a new sitting room patio door in a couple of weeks. Combined with my ASHPs and multifuel stove (lots of free wood), I'm looking forward to 70% lower electricity bills this year compared to my previous house (heated via storage heaters) last winter.

stevehead

Nice, well put post Mech.

Regarding the stated & seasonal COP.
This comes down to 2 things
Firstly location. I'm sure the Germans, Swedes and Scots struggle to obtain an impressively high COP given the cold winter climate. I can't help where I live - it's in the South West which is very very mild. So mild, that it approaches the conditions under which the manufacturers COP is stated. (5.45 @ 7C ambient)

Here's the temp chart for Plymouth. Note that the average 'low' will be recorded just before dawn when my ASHP's aren't in use.

Average pre-midnight lows during Dec/Jan/Feb are 5/6C, therefore only a very small COP drop.

Which brings me to the 2nd and unreconcilable thing - do you accept Mitsubishi's figures or not?
Would it be fair to accept their claim that the Ecodan COP is 3.7 @ 7C, and 3.1 at 2C? If so, then why reject the same company's claim for a COP of 5.45 for a different unit? Or at least 'seasonally adjust' them both by the same factor.

Is it not the case that an air-to-water ASHP will always have a lower COP than an air-to-air ASHP because it has to heat water to 65C? Air heaters surely have a lower compressor temp?

As for DHW, I use an instant gas heater. It's rugged, old and non condensing. My understanding is that modern condenser boilers don't have the incoming cold water supply passing through the condensing unit, only the closed loop heating circuit. This means that DHW is always obtained in non condensing mode. There's nothing to be gained by me using a SEDBUK A boiler at all.

stevehead

Swipe wrote: »

My living room air source heat pump is maintaining an indoor temp of 19C and drawing just around 300w.

What you got Swipe?

Swipe

Two air to air daikin 3.5KW split system inverters

stevehead

Swipe wrote: »

Two air to air daikin 3.5KW split system inverters

Ditto and going into my 2nd winter. Looking forward to my bills too

paceinternet

Which brings me to the 2nd and unreconcilable thing - do you accept Mitsubishi's figures or not?
Would it be fair to accept their claim that the Ecodan COP is 3.7 @ 7C, and 3.1 at 2C? If so, then why reject the same company's claim for a COP of 5.45 for a different unit? Or at least 'seasonally adjust' them both by the same factor.

Is it not the case that an air-to-water ASHP will always have a lower COP than an air-to-air ASHP because it has to heat water to 65C? Air heaters surely have a lower compressor temp?

Mitsubishi do not readily quote outputs and COP's over a range of air and water temperatures, so it is difficult to know how they perform.
One brochure quotes:
A2/W35 = in 30, out 35 - COP 3.4
and A7/W in 30, out 35 - COP 3.9

Another shows a graph:
Water average flow temperature 45
Air -15 = COP 1.9
Air 2 = COP 3.1
Air 7 = COP 3.7
Air 15 = COP 4.8

So, Mitsubishi have similar COP's to other brands.
And you can see that the higher the water temperature, the lower the COP as with all other brands.
The lower the air temperature, the lower the COP
To get the higher levels of efficiency from any brand, you need to use low water temperatures, which means underfloor heating, large radiator surface areas, or fan convectors in a similar way to air to air heat pumps.

Cardew

paceinternet wrote: »

To get the higher levels of efficiency from any brand, you need to use low water temperatures, which means underfloor heating, large radiator surface areas, or fan convectors in a similar way to air to air heat pumps.

'No such thing as a free lunch'