[Greenbuilding] DeltaCool and direct gain solar heating

[David Delaney]

John

Thanks for bringing the DeltaCool phase change material 
(pcm) product to my attention. (http://tinyurl.com/2ue2sx) 
This extraordinary material freezes at 22C and melts at 24C, 
with an enthalpy of fusion of 158 kJ/kg.

In a solar house the best application of DeltaCool would 
seem to be in a layer above, and in good thermal contact 
with, a thermally thin ceiling.

By simultaneously providing substantial thermal mass and  
clamping the temperature of the ceiling just above room 
temperature, this pcm product offers the possibility of 
improving the performance of direct gain solar heating by 
reducing the air temperature required to absorb solar heat 
into the thermal mass of the house -- allowing a larger 
thermal mass to be charged easily.

DeltaCool is packaged in bags as documented at http://tinyurl.com/2ue2sx. 
Each bag is 30 cm x 15 cm, about 12" x 6". The picture suggests 
they are about 5 cm, 2" thick, so you could get two of them 
in each square foot.  Estimated  total volume per bag of 
0.30m x 0.15m x 0.05m =  2.25 liter => 2.25 x 1.5 kg/liter = 
3.4 kg/bag, providing 6.8 kg, 15 lbm per ft^2, 73 kg/m^2.  
Since the melt enthalpy is 158 kJ/kg, this would provide a 
storage capacity at essentially constant temperature just 
above room temperature of 1074 kJ/ft^2, 1019 Btu/ft^2 or 11.6 MJ/m^2.  
The same storage capacity as a 6 inch thick layer of water 
over the whole ceiling swinging 32F and often being 
at an inconvenient temperature.

Assume a thin steel or aluminum panel ceiling having essentially no 
thermal resistance compared to the R 0.6, RSI 0.106 air film 
adjacent to the lower surface of the ceiling panel.  The 
DeltaCool bags are laid directly on the upper surface of 
the thin metallic ceiling panel. The temperature of the lower 
surface of the ceiling panel will be clamped by the DeltaCool at 
approximately 23C. A very hot layer of air can lie adjacent 
to such a ceiling and above the much cooler air of the room 
below without overheating the room. Essentially the only 
heating of the rest of the room by the system composed of 
the hot air layer and the ceiling will be by (quite moderate) 
radiation from the 23C ceiling. (Well, there are also the tops
of the walls, but we could clad these with steel and DeltaCool too,
if necessary.)  Assume the presence of a hot 
air layer 28C above the 20C temperature of the rest of the 
room, 25C above the 23C temperature of the pcm (air layer 
temperature of 48C, 118F).  Energy will flow into the  pcm 
at a rate of 25C/RSI 0.106 = 236 W/m^2, 75 Btu/ft^2.hr.

A good solar absorber arrangement to exploit this ceiling 
would consist of vertical steel or aluminum venetian blinds behind 
south windows. One side of each vane of the blind would be 
painted a dark color for converting sunlight to heat, the 
other side would be as shiny as possible to inhibit thermal 
radiation into the room.  Heated air would rise in a sheet 
on either side of the hot metallic vanes to feed the hot air 
layer at the ceiling. The vanes could be rotated to face the 
shiny side out when the DeltaCool ceiling mass approaches 
full charge.

David Delaney, Ottawa