Monday, 25 February 2013

Is it a fair COP?

Just looking at the spec for our eco cute in search of hard data on the temperature performance of the heat pump, it reveals very little. 

The spec is for two models, the CHP-H4619AT, recommended for "Region III" and the CHP-H4619ATK, recommended for Region I. The regions corresponds to the Next Generation Energy Efficiency standards, where Region I is Hokkaido, the coldest part of Japan, and Region III is Miyagi, Yamagata, Tochigi and Niigata in the lower North East of Japan, and the warmer parts of Nagano, which is central but mostly high in the mountains. 

Then it shows annual hot water efficiency of 3.1, which apparently is the ratio of the heat coming out in hot water to the electricity going in. So for a kWh of electricity, you get 3.1 kWh of heat. The note next to this says it is the situation using low energy mode, under certain conditions referred to in another note. The other note says that this is the average performance between Tokyo and Osaka, which are neither in Region I nor III but in Region IV.

Further down the spec, the mid-range rating for the heat pump COP is given as 4.5, which has no notes attached, but there are several notes to the numbers around it, which show the mid-range output heat and mid-range power consumption. "Mid-range" means an ambient temperature of 16 degrees (12 degrees wet bulb) and mains water temperature of 17 degrees with an output water temperature of 65 degrees. The electric power consumption is 1.33 kW and the hot water output is 6 kW; a ratio of 4.51. It also gives a summer figure with ambient temperature 25 degrees (21 degrees wet bulb) and mains water temperature of 24 degrees. The power consumption is then 0.97 kW and hot water output is 4.5 kW; a slightly higher ratio of 4.63). For the winter it gives ambient temperature 7 degrees (wet bulb 6 degrees) and mains water temperature 9 degrees with hot water at 90 degrees. The electric power in the winter is then 1.99 kW producing 6 kW of hot water; a significantly lower ratio of 3.0.

Two things leap out of this data when you actually look at the small print. The mid-range performance is suspiciously close to the summer performance. The winter performance is much worse. Since hot water use is going to be less in the summer, and more in the winter when the heating is on, the performance at the middle of the actual range is going to be between what it calls the mid-range performance and the winter performance. Also, the ambient temperature for the winter performance is about 5 or 10 degrees warmer than the actual ambient temperature we get in the winter. That's in Region III, let alone in Hokkaido in Region I.

It does give another note that under cold conditions, the performance will be lower. 

So there you go. 

The COP is 4.5...
Or 3.1.

Or some other number...