Solar Thermal Hot Water Heating May Be a Time of the Past

A reader wants to know if the results he’s achieved with his newly designed solar thermal hot water heater are compelling.  He used his panel solar absorber area of 16.25 sf to heat 50 gallons water 55.2 degrees F in 5 hours.

Here’s what I get, using q = m * C * deltaT and energy = power * time:

The energy absorbed by your system:  50 gallons = 187,200g * 4.18J/g * 30.55 degrees C = 2,288,362 J

The theoretically available energy is 16.25 square feet = ~1500 watts * 5 hours * 3600 J/watt hour = 27,000,000 J

Thus your efficiency is ~9%, which isn’t good.  I’m betting that the tank isn’t well insulated, and you’re losing a great deal of the heat you collect.

In any case, I’m not sanguine about the whole solar thermal hot water enterprise, since the cost of PV has fallen to the point that it represents a better way to convert the sun’s energy into hot water.  PV will also last longer and require less maintenance.

This is an interesting consequence of advancements in cleantech, albeit a sad one, i.e., they have the potential to make other aspects of cleantech obsolete, just as they do fossil fuels.