Most wells, springs, or ponds produce water with temperatures ranging from 40 to 60 degrees Fahrenheit (500 to 520 degrees Rankine). A refrigerant such as Ammonia, Propane, or a "Freon" that can maintain a pressure of 30 PSIG across an expansion engine is capable of producing shaft power. Despite it's flammability hazard, Propane (Refrigerant 290) is the least toxic, non-corrosive, environmentally friendly, lubricant compatible, chemical of the group.
Propane Temperature-Pressure: Degrees F Degrees R PSIG -30 430 7 -20 440 12 -10 450 18 0 460 25 10 470 34 20 480 42 30 490 53 40 500 65 50 510 78 60 520 93 70 530 110 80 540 128 90 550 150 100 560 177 The Carnot Efficiency (T,hot - T,cold)/T,hot times the expander-generator efficiency determines the wall-socket power available. IOW. using propane with well water at 50 F (78 PSIG at 510 R ) rejecting heat into a condenser at 20 F (42 PSIG at 480 R) with a pressure differential of 36 PSIG, gives a Carnot efficiency of (510 - 480)/510 = 5.9%. An expander-generator efficiency of 60% would give a wall socket efficiency of 0.6 * 5.9% = 3.5% Well pump lift-head and piping loss at 55% efficiency gives a rule-of-thumb equation for determining horsepower(hp), head (hd), and gallons per minute (gpm): hp = gpm*hd/2178 or, hd = hp*2178/gpm For example a 0.25 horsepower pump delivering 12 gpm (100 LBs/minute) for a 10 degree temperature drop for heat extraction (1,000 BTU/minute) or 60,000 BTU/Hr available geothermal heat can lift and tolerate a piping head loss of 0.25*2178/12 = 45 feet. At 3.5% wall socket power output 60,000 BTU/hr, 2,100 BTU/hr or 0.62 Kilowatts will be available at the wall socket and a 0.25 horsepower (0.187 kilowatt well pump) can run off of it, leaving about 435 watts left over for other use. Fred
