Bennny
Member
Hi folks
I'd like to calculate the energy consumption of a Sanden 315L Heat Pump HWS, based on:
1) Ambients of 0, 5, 10, 15 and 20 degrees
2) Average daily hot water consumption of 200L
Does anyone know how to do it? I have a chart showing the COP at different ambients, not don't know how to work out actual power consumption.
Many thanks!
arttt
Member
good study of the sanden here showing COP v temp.
http://neea.org/docs/default-source/lab-test-reports/laboratory-assessment-of-sanden-gau-heat-pump-water-heater-lab.pdf?sfvrsn=6
The accepted answer here shows you how to do the calculation - just change to use the numbers you have. ie volume of water, heat rise from cold water to hot.
https://sustainability.stackexchange.com/questions/1199/how-many-kilowatt-hours-of-electricity-does-it-take-to-heat-180-litres-of-water
The COP is just an efficiency factor so if COP is 3 divide the required energy by 3.
There is also heat loss from the tank. If you know the R value for the tank insulation the heat loss calculation is along the same lines as what you have been doing in the other thread for your house.
Bennny
Member
You are a wealth of information, thanks Art 
Using information from those sources, I calculated the following:
Sanden 315 tall
Ambient Temp 3
HW Temp 60
Power cost 0.27
Daily use (L) 180
COP 3.5
Capacity (L) 315
Height 2.09
Diameter 0.57
Area (sqm) 4.25
R Value 2
U Value 0.5
Loss (kw) 0.12
Use (kw) 11.92
Total (kw) 12.05
Actual usage (kw) 3.44
Daily cost $0.93
Bennny
Member
Compared to gas instantaneous below, the Sanden is more expensive (assuming you are relying on grid rather than solar).
I couldn't find any data on the Rinnai for different flow rates, assumed gas consumption is linear with flow, calculated flow and consumption based on max flow to then work back for time.
Rinnai Infinity 26 Enviro
Flow rate 26
Daily usage (L) 180
Run time (mins) 6.92
Mj/hr 172
$/mj 0.0157
Daily cost $0.31
arttt
Member
I havent checked your maths but a few comments
If using electric and have excess solar the per kWh cost is the difference between the feed in tarrif and the purchase price. Off peak is also an option.
Dont forget that if you can go off gas fully you save the substantial connection cost. Gas is also predicted to keep rising in price. With electricity you have the potential with batteries later on as they get cheaper to reduce the pain of price hikes.
If you are aiming for a net zero house then you need to put a price on offsetting the carbon from each option.
If your heating load is small then there is potential to combine heating and hot water from the Sanden. It voids the warranty and you would have to be sure of the heating load calculations but worth considering as it could save some money.
The Sanden when running puts out 4kW of heat you only need 3 hours of that for the hot water and it has another 21 hours in the day to do nothing or.... heat the house. One day i might do this on my place if i decide i need a bit of heating.
Lastly, Bennny can you look up the difference between kW and kWh. You keep using them interchangeably but they are different. You seem to understand the difference and get the math right just not write the Units correctly.
Benny
Member
I can see how you're trying to calculate the energy (gas) needed to heat 180L/day with the Rinnai Bennny but 1 issue you might have with that is that at the full flow rate (26L/min) the Rinnai won't be heating the water to 60C.
From your specified run time of 6.9mins and 172MJ/hr that means the unit would use 19.8MJ which is 5.5kWh. A ball park figure for 100% efficiency is 1kWh will give you 20L of water at 60C (from 20C) so that would be 110L at 60C - not 180.
Also you gas price is extremely cheap! Is that all you pay ?
The HP calculations show 3.44kWh at 27c = 93c.
The gas shows 5.5kWh at (.0157 * 3.6) = 5.6c/kWh = 31c. So thats the main reason for the gas being cheaper than HP even with its 3.5 COP, your gas only costs 1/5th what electricity costs.
Bennny
Member
Thanks Art, I hadn't considered the extra bonus for not having gas connected (an extra $335pa).
I'm looking at an additional heat pump from these guys to drive Hydronic Radiators, I'd heard bad things about pushing the Sanden too hard? http://www.hydronicheatingvictoria.com.au/radiators.php
Does anyone know what sort of lifespan is typical from the Sanden? Factoring in servicing/depreciation costs into my model.
Bennny
Member
Benny said:
110L at 60C - not 180.
I see, cheers. Even at that the gas still looks (on face value) the better option. Still think I'll go for the HP, supplied by PV's.
Benny said:
Also you gas price is extremely cheap! Is that all you pay ?
Yeah mate, jut pulled out my last bill again. It's definitely $0.0157/mj plus GST, then I get 18% discount by paying on time. That's through Energy Australia.
