# Parabolic Tube

• But this requires replacement of insides with a u- copper tube, which is a valiant effort. Tbis is because evacuated tubes have a closed heat exchanger mechanism - a closed copper tube with heat exchanger bulb.
• Unless one can find low cost u-tube heat exchangers, the evacuated system appears to be too complicated compared to plain black tubes.
• Does not appear highly relevant to saturated water storage.

## BOM

• \$25 per 6 foot evacuated Tube, no inner tube - water directly in glass? - [1] - affordable.

# Calculations

• Max T that a metal can get when in full sun in 90C - see nice explanation at [2] combined with Stefan Boltzmann constant - [3]
• So we see that the max temp in K varies as the 4th root of power density hitting the object. For example, to double the temp in K, need 16x solar concentration.
• 90C = 363K. Double to 726K - max 450C.
• At this point, conduction and convection play a good role.
• We need T to be 1.3x this
• We need 482K, or 1.33x this K
• 1.33^4 = 3.1 - so only 3x concentration is needed if there were no other losses
• Convective loss calculator - [4]