Direct gain in the 2013 Solar Decathlon?

Doug Kalmer writes:

>Do you really believe that when EVERY ONE of the 19 entries in this years Solar Decathlon uses Direct Gain passive solar just for the "visual appeal"?

I wonder what that means. There were 20 teams and 19 entries this year, including...

ASU/UNM, in which

>Phase-change material distributed throughout the house stores thermal energy and buffers temperature fluctuations by redistributing the energy as passive heating and cooling.

>The capillary radiant system in the ceiling, working with the phase-change materials in the floors, passively charges at night while the ceiling mechanics shift the peak load.

Las Vegas, in which

>Solar thermal collectors provide radiant floor and water heating.

Missouri, in which

>A mixed-mode residential HVAC system marries the automation system and the house HVAC system.

>A radiant heating system with tubes beneath the concrete floor circulates water to heat the interior space from the bottom up.

Norwich, in which

>A mini-split heat pump HVAC system with a single supply diffuser provides widely available, compact, and easily serviceable heating and cooling

Santa Clara, in which

>The radiant heating and cooling system embedded in the ceiling drywall uses radiant panels to heat the house with hot water or cool the house with cold water—ensuring a uniform environment.

>As part of the water heating and storage system, a solar thermal panel supplies heat to a tank containing organic phase-change material.

Sci-Arc/Caltech, in which

>The HVAC system uses state-of-the-art solar water heating technology with solar thermal evacuated tube collectors to maintain a comfortable interior temperature.

Stanford, in which

>A heat-recovery ventilator works with an efficient heating/cooling system, automated windows, phase-change materials, energy-efficient ceiling fans, and a tri-zone ductless mini-split system

Austria, in which

>A heat-recovery ventilator works with an efficient heating/cooling system, automated windows, phase-change materials, energy-efficient ceiling fans, and a tri-zone ductless mini-split system

DC, in which

>A unique under-floor heating and cooling distribution system supplies air at floor-level from ductwork connected to a central air handler

Ontario, in which

>An integrated mechanical system provides space heating, cooling, dehumidification, and domestic hot water through a single system.

Texas, in which

>A ductless radiant heating and cooling system circulates fully as a closed loop of radiant heat.

So Cal, in which

>A combination heat pump system provides heating, cooling, and domestic hot water, and

West Virginia, in which

>The climate-control system enables room-by-room temperature and lighting adjustments. Through smart HVAC technology, users can set different zoning preferences without disturbing the settings of other rooms.

Oodles of PVs, of course. Oddly enough, none of the entries touted simple direct gain solar heating with its refreshing sub-freezing indoor temperatures after 2 cloudy days in a row.

Nick

A direct gain rant

John Canivan writes:

 

>Dug What happens if you have a week without sun in December?

 

If it's a direct gain house with sun shining through windows onto a massy floor, you freeze or wave your hands and say it's really warm inside when it isn't, or wear sweaters and arctic army pants and sit in one small room with an electric space heater. Direct gain house owners and builders and designers often fool themselves and others.

 

What's the solar heating fraction of the Britton's house?

 

>Every day is sunny for George and Charlotte Britton of Lafayette Hill. The Britton's 2,900-square-foot house is blessed with energy bills 20 percent lower than one of comparable size... The design of the house incorporates "passive" solar principles. There are large double pane windows and sliding glass doors on the south side. Inside, tile floors and a Trombe wall absorb the sun's heat during the day and radiate it at night... A stone fireplace on the south wall of the living area provides additional heat during colder months. Britton said "We have a fire every day of the winter."

 

Direct gain (aka "direct loss") houses in cold cloudy places rarely have solar heat fractions greater than 50%, according to Passive Solar Institute (aka Sustainable Building Institute) Guidelines... 30% is more common, ie the sun only provides 30% of the house heating over an entire year, but indirect gain houses can have solar heating fractions of 90% or more in the month of January. 

 

An 8' 70 F direct gain cube with R20 walls and ceiling and floor and A ft^2 of R4 south windows with 50% solar transmission needs 24h(70-30) (A/4+(384-A)/20) Btu of heat on an average 30 F January day in Phila with 1000 Btu/ft^2 of sun on a south wall... 0.5x1000A = 24h(70-30)(A/4+(384-A)/20) makes A = 41 ft^2, with a total cube conductance G = 41/4 + (384-41)/20 = 27.4 Btu/h-F, right? (Or would you like to argue about that? :-)

 

With a 4" 533 Btu/F concrete floorslab, time constant RC = C/G = 19.5 hours. With no sun, 300-year-old high-school physics says the direct gain cube temp drops to 30+(70-30)e^(-24/19.5) = 41.7 F after 24 hours and 30+(70-30)e^(-48/19.5) = 33.4 after 48 hours, and so on. That's MISERABLE solar house heating  performance! (Would you like to argue about that? :-)

 

OTOH, an 8' 70 F indirect gain cube with R20 walls and ceiling and floor with a 384/30 = 19.2 Btu/h-F conductance needs (70-30)19.2 = 768 Btu/h or 18.4K Btu/day. A T (F) 64 ft^2 radiant ceiling heated by thermosyphoning sun-warmed air (as in the Barra system) with a 1.5x64 = 96 Btu/h-F slow-moving airfilm conductance can keep the cube 70 F if (T-70)96 = (70-30)320/20, ie T = 77 F. If the cube has 8'x8' of R2 air heater glazing with 80% solar transmission (eg 2 $42 4'x8' sheets of twinwall polycarbonate) over the R20 south wall,

 

0.8x8'x8'x1000 = 51.2K Btu/day

= 6h(T-30)64ft^2/R2 for the air heater during the day
+ 18h(70-30)64ft^2/R22 for the south wall at night
+ 24h(T-30)64ft^2/R20 for the ceiling all day
+ 24h(70-30)4x64ft^2/R20 for the other walls

 

makes T = 167 F.

 

We can keep the cube exactly 70 F for 5 cloudy days in a row with a slow ceiling fan and a room temp thermostat and 5x18.4K/(167-77)/62.33 = 16.4 ft^3 of water cooling from 167 to 77 F, with an approximate 1-2^-5 = 0.97 solar heating fraction. But that's an overestimate, since the ceiling can be 77 vs 167 F most of the time.

 

Nick