Radiative Cooling in Hot Humid Climates

Aubrey Jaffer

Abstract

Passive radiative cooling of buildings has been an underachieving concept for decades. The few deployments have generally been in dry climates with low solar angles.
The greatest need for cooling is in the tropics. The high humidity endemic to many of these regions severely limits the passive cooling available per radiative area. To wrest temperature relief from humid climates, not just nocturnal cooling but solar irradiance, both direct and indirect, must be addressed.
This investigation explores the extent to which thermal radiation can be used to cool buildings in the tropics. It concludes that inexpensive materials could be fabricated into roof panels providing passive cooling day and night in tropical locations with an unobstructed view of sky.

2006 Full Text (PDF)

March 2009

I have discovered reflector configurations which are significantly more efficient for radiative cooling than the corrugated reflectors. Read about these in Optics for Passive Radiative Cooling.

ThinkCycle: Open Collaborative Design

Project Status December 2007

In the year since cool.pdf ("Full Text" above) was last updated, I have obtained better data for most parts of the simulation model:

Dr. William D. Vacca of SOFIA-USRA NASA Ames Research Center has been kind enough to run ATRAN for both wet and dry equatorial atmospheres. With this data, my radiative-transfer calculations no longer need to smooth or interpolate atmospheric transparency, which was a weakness of my earlier computations.
Evaluating radiative-transfer at over two million uninterpolated samples of transparency data would take hours, much longer than with 14200 interpolated samples. Dan Stanger suggests a Monte-Carlo approach. Preliminary tests indicate that sub-sampling by a factor of 20 will keep the calculations within 1% of the exhaustive case.
It turns out that there is a TMY2 (typical meteorological year) for a tropical area: Guam (Pacific Islands). Its full year can be used instead of hacking Miami's summer data.
Using programs that extract measurements from digitized graphs, I developed better spectral refractive-index data for polyethylene in FreeSnell.
I fit quadratic functions to the graphs of convection given in the 1997 ASHRAE Handbook-Fundamentals. It is bizarre that such an important formula isn't widely available; but the ones I found on the web weren't suitable.

In order to properly evaluate this cooling method, a typical tropical dwelling should be simulated through Guam's typical meteorological year with various roofs, radiative and conventional. The ASHRAE model for indoor comfort has questionable applicability to primitive housing in the tropics. Instead, I will calculate the net cooling for an unoccupied dwelling; that number divided by 150.W gives the number of people who can occupy the dwelling comfortably at that time. Because the roof panels condense water, the division (of personal heat) between sensible heat and latent heat is unimportant.

With the convection formulas and radiative models developed, simulating the dwelling is straightforward, except for the floor. Tasks to complete are:

Create model (network of thermal masses and thermal conductances) for a tropical dwelling; also determine an air-exchange rate for the dwelling.
Find or create a simple thermal model of a dirt floor.
Simulate said model (with conventional roof) on existing software in order to validate operation of the new simulator.
Write thermal simulator driven from TMY2 data and radiative-transfer calculations.
Run simulations with conventional roof, radiative-cooling panel roof, reflective metal roof, and white painted roof.
It was suggested that warehouses might also benefit from radiative-cooling panel roofs; create model for a warehouse and run.

On the practical side, Luciano Cardoso has designed a clever system of folded metal with tabs for the (internal) side reflectors.

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