[Greenbuilding] Direct gain again???

[nick_pine at verizon.net]

Sacie Lambertson <sacie.lambertson at gmail.com> writes:

> This is very interesting, a prototype for a quickly-built highly energy 
> efficient 'pop-up' house.  I'm a bit skeptical to say the least.
>
> http://dornob.com/this-beautiful-passive-house-can-be-built-in-just-four-days/?s=pop+up+house

Highly energy efficient, with no night insulation for all that glass?

>The home has an airtight thermal envelope and, in sunny climates, requires 
>no heating system at all.

It doesn't look like it has much thermal mass either.

Clarke Olsen <colsen at fairpoint.net> writes:

> A dark roof will not absorb any meaningful heat from the brief, shallow 
> sun of our discontent.

Got numbers?

"Lynelle A. Hamilton" <lynelle at kos.net> writes:

> I am looking to tap the collective expertise of the group for house #3. I 
> am aiming for R-50 or so in the walls...

How about R30, with dense-packed cellulose and a layer of foamboard?

> Location is south Lake Simcoe... exposed southern face for sunspace.

http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data3.cfm/region=4_north_and_central_america_wmo_region_4/country=3_canada/cname=CANADA 
says an average December day in Toronto is -2.5 C (28 F), with a -0.2 C 
high, which makes the average daytime temp about 30 F. The 4 m deep soil 
temp is 6.7 C (44 F) in December. Toronto has 3956 annual heating 
degree-days (18.3°C baseline), ie 1.8x3956 = 7121 F HDD.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/International/pvwattsv1_intl.cgi 
says 1 PV watt can collect 1.08 kWh/year, and 1.93 kWh/m^2 (612 Btu/ft^2) of 
sun falls on a south wall in December, so 1 ft^2 of R2 low-mass sunspace or 
air heater glazing with 80% solar transmission and 80 F air inside could 
collect about 0.8x612-6h(80-30)1ft^2/R2 = 340 Btu/day.

A 40'x60'x8' house with 100 ft^2 of U0.25 windows and 1500 ft^2 of R30 wall 
and 2400ft^2 of R40 ceiling surfaces and no air leaks would have a 
25+1500/30+2400/40 = 135 Btu/h-F conductance. According to Professor Kelly 
Kissock at U Dayton, an A ft^2 slab on Rf foamboard with Rp vertical 
perimeter insulation down to the frost line would have an approximate G = 
A(0.1140/(4+Rf+Rp)+0.8768/(16+Rp)) Btu/h-F conductance from the room air 
temp to the 4 m deep soil temp. With A = 40'x60' and Rf = 30 and Rp = 30, G 
= 50 Btu/h-F. At 70 F, the house would lose 24h((70-28)135+(70-44)50) = 167K 
Btu/day. A frugal 600 kWh/mo of indoor electrical use could provide 68.2K 
Btu, leaving a need for 99K Btu from 99K/340 = 291 ft^2 of sunspace or air 
heater glazing, not counting window heat gain.

The house would need 5x99K = 495K Btu for 5 cloudy days in a row, eg 
495K/(163-80)/62.33 = 96 ft^3 of water cooling from 163 to 80 F in a 
4'x8'x3' tall plywood tank with a 10'x14' folded EPDM liner and R30 
insulation, which would lose 24h(163-70)136ft^2/R30 = 10088 Btu on an 
average day. With a 167K/24h/(70-28) = 166 Btu/h-F effective conductance and 
2400 ft^2 x 4" 20K Btu/F concrete slab, RC = 20K/167 = 120 hours. With a 
28+68.2K/24h/167 = 45 F balance point temp, the house would cool from 70 F 
at 3 PM to 45+(70-45)e^(-18h/120) = 67 at 9 AM.

OTOH, Fig 10 on page 14 of http://www.nrel.gov/docs/fy11osti/52175.pdf shows 
a heating COP of about 2 at -10 F and 3 at 30 and 4.5 at 60 for a $2100 air 
source Mitsubishi 
mini-split.http://www.amazon.com/19-20-Mitsubishi-Single-Zone-Split-System/dp/B0060OUI2A/ref=sr_1_10?s=home-garden&ie=UTF8&qid=1395490096&sr=1-10&keywords=mitsubishi+mini+split+heat+pump 
so with no indoor electrical use, it would need about 24hx7121HDDx167/3412/4 
= 2091 kWh/year, eg a 2091/1.08 = 1936 W PV array at a materials cost of 
about 1936($0.60 PV+0.40 inverter) = $1936, excluding rack hardware, before 
any rebates. The total materials cost of this system would be about $4K.

Nick