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Old 10-20-14, 02:28 PM   #1
buffalobillpatrick
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Default HP COP vs NG boiler $ Balance point

I want my Heating System controller (Arduino) to be able to automatically switch between HP and NG boiler, based on $ calculations.

Code must calculate the HP COP vs. Natural Gas $ Balance Point.

Code will use near real-time $ inputs for NG & Electricity (pots adjusted monthly), to calculate the $ balance point, calculate HP COP real-time,
then select HP or NG boiler.

Coefficient of performance - Wikipedia, the free encyclopedia

COP = Q / W
where
Q is the heat supplied to the reservoir.
W is the work consumed by the heat pump.

note: W is in Kwh & must include pumps.

In USA units:
Q = BTU/h = F x DT x 500 x SH
where
F is Gallons/min
DT is Delta Temperature *F between Load Out & Load In
SH is Specific Heat, water = 1, propylene glycol 30% water mix = .935

NG Mcf = 1,000 ft3
Therm = 100,000 BTU = 29.3Kwh
$Therms = $Mcf / 10.25

My energy prices vary greatly.
Over the past several winters my average delivered NG $8.00/Mcf = $.78/Therm
& delivered Electricity rate about $.16 / Kwh


Easy to use online Heat Cost calculator:
Electric Heat vs. Gas/Oil Heat Cost Calculator/Comparator

My Cost of heat (per 100,000 BTU)
$.16 / Kwh Electricity = $4.69
$.78 / Therm with 85% Efficiency NG boiler = $0.92

Convert to Balance point: 4.69 / .92 = 5.0978 COP

Balance Point Formula:
BP = ( $ per Kwh / (( $ per Mcf / 10.25) x (1 / Boiler Efficiency %))) x 29.3

COP of about 5.1 seems too high????

Please feel free to give corrections.............


Last edited by buffalobillpatrick; 10-20-14 at 03:31 PM..
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Old 10-20-14, 04:24 PM   #2
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COP 5 sounds unachievable?
What type of heat pump are you using?

I was told of a system like this by a Worcester Bosch sales rep last year, not sure if it's something they are experimenting with or if they have a product available.

Steve
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Old 10-20-14, 04:25 PM   #3
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http://www.climatemaster.com/downloads/RP881.pdf
p19 graph for model TMW036 W/W

To get a COP of 5.2
Interpolating between lines
ELWT 70*F needs ESWT 30*F
ELWT 80*F needs ESWT 40*F
ELWT 90*F needs ESWT 50*F

This appears to be 40* delta between ESWT & ELWT

I expect my ELWT to run from 70-85*F

So I would "possibly" need ESWT from 30*F to about 45*F

These ESWT are posible due to sourcing heat from my Solar heated 1,000G tank.

My HP is DIY W/W 2T 240vac Copeland scroll with 4T "snake" coax HX's for Evaporator & Condensor.

Last edited by buffalobillpatrick; 10-20-14 at 08:22 PM..
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Old 10-20-14, 07:51 PM   #4
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How cold do you expect the glycol to ever get?
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Old 10-20-14, 08:14 PM   #5
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Not sure, possibly below freezing in the Load 100' buried & insulated loop between HP in shop & house, if HP is not used for some days.

Another option to glycol would be to have a small pump circulate 24/7 in heating season?
I would prefer this to elemonate the glycol and to elemonate a flat plate HX in house.
This would also elemonate the higher viscosity pumping penality with glycol.

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Old 10-20-14, 08:23 PM   #6
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Not sure I would depend on a pumped liquid to not freeze. Despite the bane of having a HX, it is better than a split pipe.
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Old 10-20-14, 08:28 PM   #7
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I could pipe the connection to the load tank in the house to pick up a little heat.
This would waste some heat but elemonate the glycol.
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Old 10-21-14, 12:25 AM   #8
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That seems a risky strategy to me. What if you have a prolonged power outage? What if you have a pump failure (which is bound to happen when you are away, or can't get a replacement quickly)? To me the risk would outweigh the benefit.
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Old 10-21-14, 10:51 AM   #9
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I used to live in Denver and routinely saw lots of frozen car radiators all winter long. Nearly all the coolants inside were some shade of green. Subzero temperature and water just don't live together for long.I wouldn't have to think twice about this one, especially since you are in the mountains.
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Old 10-21-14, 12:30 PM   #10
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Thanks guys,
I can buy a flat plate HX on Ebay 5"x12" 30 plate, 120Kbtu for $150
I already have a 5gal unopened propylene glycol made for heating systems.

I could sleep better doing the 30% propylene glycol in everything outside house.
Not only is it 100' from house to HP in shop, it is also 75' from shop to Solar panels/tank.

Enthalpy of fusion - Wikipedia, the free encyclopedia

Also this opens the door to the possibility of using "Latent Heat of Fusion"
(enthalpy of fusion) by allowing ice to form around the HP source HX tubes in the 1,000g Solar tank.

This would only occur after a long period without Sun, and supports the bottom end of ESWT at 0*C increasing the Solar tanks heat capicity by about 2x

http://punktwissen.at/pdf/punktwisse...System-LEO.pdf

Laramie, Wyoming - Thermal Battery Systems | Thermal Battery Systems

I don't know if system would ever need to use 0*C ESWT as this would require my calculated economic "Balance Point" from post #1 to drop below a COP of about 5.0

BTW, has anyone checked my math?
OR
the thread idea presented?


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