[Greenbuilding] DHW COP

Paul Eldridge paul.eldridge at ns.sympatico.ca
Mon Oct 15 20:14:54 CDT 2012


Nick Pine wrote:

 >Paul Eldridge <paul.eldridge at ns.sympatico.ca> writes:

 >> I installed our new Nyle Geyser heat pump water heater earlier today
 >> (http://www.nyle.com/water-heating/geyser-r/)

 >Hmm... "the GEYSER® is an add-on unit that can be added to any existing
 >storage tank water heater. The GEYSER® will save you an average of 
50%-65%
 >over the operating costs of an electric tank water heater... for every 
unit
 >of electricity used, the GEYSER® produces over two units of heat for 
making
 >hot water."

 >> I'm pleased to report that the results are even better than I had
 >> anticipated.  It took 1.64 kWh to bring 115 litres of water from 
16.7?C to
 >> 49?C... All in all, not too shabby given that it would have taken 
4.3 kWh
 >> to do the same job using electric resistance.

 >115Lx1000g/L(49-16.7)4.186J/g-K = 15,548,897 Joules, ie 15,548,897/3600 =
 >4.319 kWh, with a COP of 4.319/1.64 = 2.63. I guess that saves
 >100(1-1.64/4.319) = 62%. Nice...

One thing to bear in mind: the first 6°C temperature rise was 
accomplished in 20 minutes and required just 0.16 kWh of electricity, 
with a maximum draw of 492-watts.   However, getting from 43°C to 49°C 
took 38 minutes and 0.41 kWh with a peak draw of 672-watts, so the heat 
pump is obviously working harder and longer to process those last six 
degrees than the first six, as one would expect.  In normal day-to-day 
operation, the start temperature of the tank is unlikely to be 16.7°C, 
as true here and, with that, I expect the actual COP to be closer to 2.2 
or 2.3.

 >> I've plugged ours into a simple mechanical timer that has a twenty-four
 >> push-pull pin wheel, so it's not scheduled to come back on until 
06h00, at
 >> which point it's free to run for up to two hours before the timer kills
 >> the power again; if need be, we'll add a second cycle for the 
evening, but
 >> I expect that once per day will suffice.

 >Last time I checked, a 4-watt mechanical timer would use more daily 
energy
 >than than the heat loss from a well-insulated electric resistance (vs 
heat
 >pump) tank running 24/7.

It appears that our Noma timer uses less than one watt because it's 
plugged into a power monitor and the display shows "0" watts (see: 
http://i362.photobucket.com/albums/oo69/HereinHalifax/Img_1920.jpg). The 
display reads "1" watt when the Nyle is operating in standby mode, so 
presumably it has the ability to read down to this level. Thus, until 
evidence is provided to the contrary, I'll assume that this particular 
timer uses less than 0.025 kWh over a 24-hour period.

The sole purpose of the timer is to prevent the heat pump from short 
cycling, an issue touched on by Richard in his post.  Our SuperStor 
Ultra is smaller than the minimum size recommended by Nyle, so by 
limiting its operation to just one run per day we can avoid any excess 
wear and tear as well as the energy losses inherent in frequent on-off 
cycling.  We'd also prefer to restrict its operation to off-peak hours, 
even though we're not on TOU rates (consider this my gift to Nova Scotia 
Power).  With occasional exceptions, all of our DHW demand occurs during 
the early morning hours, so letting the temperature of the tank drift 
downward through the course of the day presents no real hardship.

 >I estimated a 1.6 COP for a dehumidifier with a measuring cup and a
 >Kill-A-Watt meter. What's the COP of a fridge or freezer? We might 
cool 50 1
 >liter water bottles from 70 to 10 F with a freezer and measure the
 >electrical energy consumption. If that's promising,  we might heat DHW by
 >running a little outdoor air through the freezer.

I guess that's one approach, but I'm more inclined to let our new Nyle 
do the job for us since it doesn't involve any intervention on my part 
and we require the hot water it provides anyway.

Cheers,
Paul





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