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Forum Index : Other Stuff : Spa Heating Plan "B"
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| Phil23 Guru  Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
As anyone my gather by my first posts, a recently acquired Spa Tub is my current centre of interest. Solar heating is working, but here in Northern NSW we have already had a few cooler overcast days where the solar has simply had no output. The Tub does have a 1kW heating element & raise the temp 1°C in about 2 hours. Current overnight temperature losses mean I'd need at least 4 hours run time to get back to the 34°C temp we like. Not looking good for maintaining all year round temp. Not without giving the power bill a good nudge. Recently pulled an old window rattler air conditioner out of a rental & figured I see how it worked as a heat pump. The unit was rated at 1000 Watts input with a 1500 Watt cooling capacity. So ripped the cover off, unscrewed the condenser & immersed it in a 50 litre tub of water to see what sort of heating efficiency I could get. The Victim. 
Condenser in a tub, 12v transfer pump to circulate the water. Measured temperature rise & power consumed for calculations. 
The Results:- 
Not that bad, about 140% efficient, but a lot of effort required to make a decent heating vessel & plumb it all in. Idea went on hold. Spoke to a local plumber & air con guy, & he mentioned he had a few old heat pump hot water systems lying around I could have if interested. So yesterdays exercise was to retrieve & look at one of these. The unit he gave me was a Rheem with the heat pump rated at an input of 1300 Watts. It had a dead circulation pump & a blown logic board. And lots of sensors.... Removed the control board & found I could easily power the fan & compressor directly so nothing high tech required there. Victim Number 2.... 
Connected it to the same circulation pump & ran the water from the tub through the unit & took some measurements again. 
The Results of Round 2.... 
Really impressed with these figures. Over 400% efficiency. Interesting to notice the way the power consumption increases with water temperature. As mentioned the unit is rated at 1300 Watts (at 56°C), but at 36°C input it is only consuming 800 Watts as the compressor doesn't work as hard at lower temps. A real benefit for my intended purpose as that's about as hot as the spa water would ever be. Cheers Phil. |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hmmm interesting. I plugged your numbers into this process heating I assumed there is no heat loss and come up with a number for the spa of 1720 litres based on the old element heating time so raising the temp 4 degrees with the heat pump would be 8kw of heat energy 3.2 hours = 2.6 kwh electricity raising it 11 degrees, 22kw of heat 9 hours = 7.2 kwh electricity or 7.6 Kg of wood in a rocket heater  I guessed at 10.4 MJ of heat in the water for every kg of wood, that is still going to take a couple of hours.
that's 2.6 kwh per kg for wood my old waste oil burner knocks out about 12 Kwh per litre but catching all that with a heat exchanger that wont melt is a challenge. it is surprising just what it takes to heat water. Im off to boil the kettle for a cuppa! I'm confused, no wait... maybe I'm not... |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
it should increase with the difference in temperature between the evaporator and the water, cold day and it will work harder again. I have got a new burner on the drawing boards, smaller, better insulation to reflect the heat into the fire chamber for a hotter cleaner burn. fan forced with a repeat cycle timer to pulse the fuel pump. I will see how it goes, I haven't posted any before because they are either a bit finicky on the tuning or a complete monster on the heat output. Hopefully the new simplified design will iron most of the wrinkles out. I'm confused, no wait... maybe I'm not... |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
Google's not being my friend today... Trying to find a simple calculation that will give me an estimate of heat loss from evaporation. Seem to be hitting a lot of brick walls. Surely there's an equation out there that can give an Approximation, using 4 variables. Doesn't need to be spot on, 10 or 20% error for other variables not taken into consideration. Eg. Air temp = 20°C Water Temp = 35°C Relative Humidity = 60% Surface area of water = 3.5²m Thermal Loss = ???? Watts. Thanks Phil |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
probably googling the wrong thing. its called phase change or phase transition or latent heat ice to water is 334kj/kg water to steam is 2265kj/kg expressed as latent heat of fusion (Lf) or vaporisation (Lv) there is no time component just a transfer of mass about all you can do is estimate the water loss and multiply with the constant. if you need it better than that there are calculations for the enthalpy of vaporisation BUT it involves calculating the skin effect of a liquid on 3 atom water molecule and vapour pressure deficit (VPD) a measure of the drying power of the air. This is one of the numbers I use for trying to see how difficult it is for plants to evapotranspirate. yeah, I know, to much information, sorry! I'm confused, no wait... maybe I'm not... |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
Been thinking about my heating Strategy. Using solar to gain a few degrees during the day, then the heat pump for a few hours to get that last few degrees for the evening. Think I might be better off running the heat pump earlier in the day when the ambient temp is higher, say 22-24°C, as opposed to running it in the late afternoon when ambient is quickly falling to around 18°C after sun down. Does that sound more feasible? Cheers. |
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