Eric Walker <[email protected]> wrote:
> I wonder what the second set of calculations would look like with an > assumption of ε=1 -- since the COP was only ~2, perhaps it would get > uncomfortably close to 1 with full emissivity? > First of all, the COP wasn't ~2 it was ~2.6 +/- 0.5, with the assumption that the power supply consumed no power. As explained on p. 24, Eq. 37 is more reasonable: 816 W/283 W = COP Of 2.9 +/- 0.3. So that's ~3 not ~2. They measured the power consumed by the power supply during the null run calibration, so that is not a guess. As described on p. 25, the difference in COP is because the temperature was lower in the second test. In test 1, look at the difference in temperature with actual emissivity (around .95) versus emissivity set to 1. It is 512 deg C versus 497 deg C. Not a big difference. One thing I didn't feel too sure about was the contribution from > convection. It looked like a fairly complex calculation that depended upon > a number of factors, that needed to be looked up in a table in some > textbook and that would be easy to get wrong. > Why would it be easy to get that wrong? I have no trouble looking things up in tables. Way easier than doing arithmetic. > When I re-did some of the calculations for the December 2012 run without > the contribution from convection, the numbers were still impressive (I > think the COP was ~4). It might be interesting to obtain lower bound > calculations for both December 2012 and March 2013 with ε=1 and ignoring > all convection. > You can do that from the numbers in Table 8. With average emissivity, radiation is 460 W and convection is 282. Throw out convection completely (ignore it; pretend it did not happen) and you get: 460 W / 283 W = COP of 1.6 Saying there was no heat lost to convection goes beyond conservative. Every estimate in this paper is conservative. - Jed
