[Greenbuilding] hotter water

Tue Jul 10 06:11:23 CDT 2012

"Nick Pyner" <npyner at tig.com.au> wrote:

>> I suppose there are situations where climatic conditions are such that trying to get all-solar is really a hopeless cause...

Barrow, AK is hopeless, with 0 Btu/ft^2 of sun on the ground on an average -13.4 F January day.

larencorie at axilar.net wrote:

>... A good sizing for a Solar water heating system is that it supplies all the hot water during the easiest few months, and functions as a pre-heater the rest of the year.

Maybe not, if we build our own. Thom Styles and I bought a 12'-diameter x 30" deep Intex metal-frame pool http://www.amazon.com/gp/product/B000BVKO0E/ref=s9_simh_gw_p86_d0_g86_i1?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=0FXTG7053CFDZNAAHG2W&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846 and a 15' diameter clear solar pool cover for $160 from Amazon and an $80 1"x300' NSF black plastic pipe from Home Depot http://www.homedepot.com/Plumbing-Pipes-Fittings-Valves-Polyethylene-Pipe-Fittings-Pipe/h_d1/R-100124369/h_d2/ProductDisplay?catalogId=10053&langId=-1&productId=100124369&storeId=10051

We plan to coil the 13-gallon pressurized pipe around the inside perimeter of the pool near the top and suspend the cover from the frame and take it to a few summer festivals. 

Phila gets 1890 Btu/ft^2 of sun on the ground on an average 76.7 F July day, so the pool would gain about 0.8x113ft^2x1890 = 170.9K Btu/day at the Phoenixville Green Earth festival on August 4, where it could provide hot water for a benefit car wash... http://www.pxvgreen.org/2012-Green-Earth-Festival/index.html

With no sidewall insulation and a 113ft^2/R2+94ft^2/R1 = 151 Btu/h-F thermal conductance, 170.9K = 24h(T-76.7)151 makes T = 124 F, approximately. (Or less, with significant radiation loss.) With 3 wraps of a 12'x24' $46 solar pool cover cut in strips around the outside and no hot water use and a 113ft^2/R2+94ft^2/R6 = 72 Btu/h-F thermal conductance, 170.9K = 24h(T-76.7)72 makes T = 176 F, approximately. With C = 113x30/12x62.33 = 17608 Btu/F, it could make (176-110)C = 1.16 million Btu of hot water for car washes on the first day. The water temp would initially increase by about 170.9K/C = 10 F per day. At (110-81.4)x1.25gpmx10mx8.33 = 2978 Btu/wash, it could provide 390 10 minute 1.25 gpm car washes on the first day, with 8 car washers working continuously for 8 hours. At 110 F, it would lose about 24h(110-76.7)72 = 57.5K Btu/day, for a net gain of 113K Btu/day in July.

This year's PA Renewable Energy Fest http://www.paenergyfest.com/ has a DIY emphasis. The heater might provide hot showers for 200 campers on 9/23.

The winter version could have a 12'x12' Styrofoam cover under a 12'x12' piece of EPDM with a greenhouse containing a north reflector on top and a pump http://www.pexsupply.com/Grundfos-59896341-UPS15-58FC-3-Speed-Circulator-Pump-1-25-HP-115-volt-4701000-p that moves pool water over the shallow pond drainback cover during the day. The controller http://www.sun-pump.com/controller.htm could run the pump at night to protect the Styrofoam from overheating.

20 PI=4*ATN(1)
30 TA=31.8'average December Allentown temp (F)
40 TM=39.2'average daily max
50 TD=(TA+TM)/2'average daytime temp
60 LC=12'cover length (feet)
70 WC=12'cover width (feet)
80 RCU=2'upper cover US R-value (F-h-ft^2/Btu)
90 RCL=10'lower cover R-value
100 GCU=LC*WC/RCU'upper cover conductance (Btu/h-F)
110 SUN=3.01'Dec sun on 60 degree south wall (kWh/m^2-day)
120 LS=12'south wall slant height (feet)
130 SGAIN=.8*LC*LS*SUN*317'solar gain (Btu/day)
140 LDAY=6'solar collection day (hours)
150 DLOAD=41000!'SRCC OG-300 DHW load (Btu/day)
160 TW=TD+(SGAIN-DLOAD)/LDAY/GCU'average day cover temp (F)
170 DS=12'heat store diameter (feet)
180 HS=30/12'heat store height (feet)
190 TS=2'wall/floor thickness (inches)
200 RS=3'wall/floor R-value per inch
210 AP=PI*(DS/2)^2'pool surface area (ft^2)
220 CS=AP*HS*62.33'heat store capacitance (Btu/F)
230 GS=(AP+PI*DS*HS)/(RS*TS)'conductance
240 FOR CDAY=1 TO 10'cloudy days
250 TW=TW-DLOAD/CS'hot water loss (Btu/day)
260 TW=TW-(TW-TA)*GS/CS'conductance loss (Btu/day)
270 PRINT 500+CDAY;"'",TW
280 NEXT CDAY

01       192.396  F
02       189.7591
03       187.1275
04       184.501
05       181.8797
06       179.2636
07       176.6525
08       174.0466
09       171.4458
10       168.8520 

The cover could hold 6 240 watt PV modules that each make about 320 watts when water-cooled under 2 suns.

Nick
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