[Greenbuilding] Central vs window AC

Fri Jun 8 03:56:54 CDT 2012

> NREL says an average year in NYC has 1096 CDDs with a 65 F base... so a house with no internal heat gains or solar gain and (say) 400 Btu/h-F of thermal conductance needs 24h/dayx1096F/dayx400Btu/h-F = 10.52 million Btu (note the units) per year of cooling...

Oops. That should be 24h/dayx1096F-dayx400Btu/h-F = 10.52 million Btu. A degree-day is a temperature-time product.

> or 10.52M/26 = 405 kWh worth $72.84 at 0.18/kWh with a 26 SEER

http://www.greenbuildingadvisor.com/blogs/dept/musings/window-mounted-air-conditioners-save-energy?utm_source=email&utm_medium=eletter&utm_term=cooling-options&utm_content=20120530-wall-mounted-air-conditioner&utm_campaign=green-building-advisor-eletter says

>For typical residential air conditioners, EER equals about 0.875 SEER.

Comparing apples to apples, the 10 EER window AC would have a 10/0.875 = 11.4 SEER (maybe more, if the fan doesn't run all the time), so it would use 10.52M/11.4 = 923 kWh worth $166.10, with a simple payback of ($2100-$210)/($166.10-$72.84) = 20.3 years, compared to a $2100 SEER 26 split system, without the use of room occupancy sensors.

>How would this change for a house with 8 rooms each occupied 2 hours per day, with doors and lots of wall insulation between rooms and 30-minute occupancy sensors?

With no common walls or time delay, a 10-SEER window system would be like an inexpensive 80-SEER central AC system...

> NREL says an average July day is 76.8 F in NYC, with a 68.2 min and a 0.0123 humidity ratio, which is fairly comfortable. Smart night ventilation on comfortable nights can help.

How much, if a small box turns off the window AC and opens a 2-watt outdoor damper or runs a $69 2470 90 watt fan on comfortable nights, using the ASHRAE comfort standard?

http://www.amazon.com/Lasko-Electrically-Reversible-Window-2155A/dp/B00002N5Z9/ref=sr_1_1?s=appliances&ie=UTF8&qid=1339144279&sr=1-1

The box might look something like this: http://www.youtube.com/watch?v=wexdNx_StRc

My Quaker friend Thom Styles could build one. Any takers? This could work well in a massy brick house or brownstone.

Phila has 1101 CDD, with an average 67.2 F July min and a 0.0133 humidity ratio...

Nick

50 CLO = 1'clothing insulation (clo)
60 MET=1.1'metabolic rate (met)
70 WME=0'external work (met)
80 TA=19.6'air temp (C)
90 TR=19.6'mean radiant temp (C)
100 VEL=.1'air velocity
120 RH=86'relative humidity (%)
130 PA=0'water vapor pressure
140 DEF FNPS(T)=EXP(16.6536-4030.183/(TA+235))'sat vapor pressure, kPa
150 IF PA=0 THEN PA=RH*10*FNPS(TA)'water vapor pressure, Pa
160 ICL=.155*CLO'clothing resistance (m^2K/W)
170 M=MET*58.15'metabolic rate (W/m^2)
180 W=WME*58.15'external work in (W/m^2)
190 MW=M-W'internal heat production
200 IF ICL<.078 THEN FCL=1+1.29*ICL ELSE FCL=1.05+.645*ICL'clothing factor
210 HCF=12.1*SQR(VEL)'forced convection conductance
220 TAA=TA+273'air temp (K)
230 TRA=TR+273'mean radiant temp (K)
250 TCLA=TAA+(35.5-TA)/(3.5*(6.45*ICL+.1))'est clothing temp
260 P1=ICL*FCL:P2=P1*3.96:P3=P1*100:P4=P1*TAA'intermediate values
300 P5=308.7-.028*MW+P2*(TRA/100)^4
310 XN=TCLA/100
320 XF=XN
330 N=0'number of iterations
340 EPS=.00015'stop iteration when met
350 XF=(XF+XN)/2'natural convection conductance
360 HCN=2.38*ABS(100*XF-TAA)^.25
370 IF HCF>HCN THEN HC=HCF ELSE HC=HCN
380 XN=(P5+P4*HC-P2*XF^4)/(100+P3*HC)
390 N=N+1
400 IF N>150 GOTO 550
410 IF ABS(XN-XF)>EPS GOTO 350
420 TCL=100*XN-273'clothing surface temp (C)
440 HL1=.00305*(5733-6.99*MW-PA)'heat loss diff through skin
450 IF MW>58.15 THEN HL2=.42*(MW-58.15) ELSE HL2=0'heat loss by sweating
460 HL3=.000017*M*(5867-PA)'latent respiration heat loss
470 HL4=.0014*M*(34-TA)'dry respiration heat loss
480 HL5=3.96*FCL*(XN^4-(TRA/100)^4)'heat loss by radiation
490 HL6=FCL*HC*(TCL-TA)'heat loss by convection
510 TS=.303*EXP(-.036*M)+.028'thermal sensation transfer coefficient
520 PMV=TS*(MW-HL1-HL2-HL3-HL4-HL5-HL6)'predicted mean vote
530 PPD=100-95*EXP(-.03353*PMV^4-.2179*PMV^2)'predicted % dissatisfied
540 GOTO 580
550 PMV=99999!:PPD=100
580 PRINT TA,RH,CLO,PMV,PPD

19.6          86            1            -.4778556      9.769089
23.9          66            1             .4732535      9.676994
25.7          15            1             .5239881      10.74283
21.2          20            1            -.4779105      9.770202
23.6          67            .5           -.4747404      9.706658
26.8          56            .5            .5145492      10.53611
27.9          13            .5            .5003051      10.23146
24.7          16            .5           -.4883473      9.982468

Innova AirTech Instruments has an excellent comfort web site...
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