In an earlier article, published in two parts, Passive Solar Adaptations in Existing Houses, we examined a hypothetical modest bungalow with passive solar potential. We assumed it was located at the 40th parallel, in Columbus, Ohio, one of the ten best cities in the US, I understand. By the time we hit our stride in the second part of the article, we had assumed the house had a typical basement, and pressed on towards making it virtually self-heating in winter with a passive solar strategy.
We saw in Chart 5 of the article, that under average January conditions of occupancy, temperature, wind and amount of sun, that the house would actually have a slight net heat gain over the twenty four hour period. Of course our renovation specifications were beyond typical building code standards, which represent only the legal minimum in construction quality.
Our passive solar strategy included:
· modest window areas on north, east, and west walls, with low-emissivity sealed double glazing
· extensive windows on the south wall, with heat gain low-emissivity sealed double glazing
· stringent air-tightness, with energy-recovery ventilation
· insulation standards 50% or more above building code
· manually controlled insulating night blinds in lieu of conventional window treatments
· innovative phase change material heat storage and retrieval system
We predict this strategy would allow us to collect, store, and release as needed, enough heat that conventional heating (involving the combustion of fossil-fuels, normally) would be largely eliminated over the heating season. Compared to a typical level of design and construction, a fair but perhaps conservative estimate would be 80% of heating load eliminated.
What would be the expected seasonal heat load of a typical conventional bungalow with basement?
We'll stick pretty closely to the code R-values for Ontario that I have at hand. Chances are the 2003 International Energy Conservation Code, still apparently the current document for design of heating, cooling and ventilation of single residences in Columbus, is not too far off the 2006 Ontario standard.
Besides heat storage, a very important distinction between a conventional house and a passive solar house, of course, is the amount of window area, and its arrangement on the exterior. The Columbus Code appears to require a minimum 5% glazing to floor area ratio for bedrooms and living rooms. A typical ratio might be from 10 to 15% overall, let's use 13% or 130 square feet of window. The perimeter of our 25 by 40 foot bungalow is 130 feet. Let's also arrange the windows as a fixed proportion (one eighth) of the wall area. North and south walls each have 40 square feet of window, east and west walls each have 25, for a total of 130 square feet. We'll still use the same quality of windows as we did in Chart 5.
Here are the results:
CHART 6: AVERAGE JANUARY PERFORMANCE OF CONVENTIONAL HOUSE
HEAT LOSS/SOLAR GAIN CALCULATION, IMPERIAL VALUES | LOW-E GLAZING | ||||||||
40 N LATITUDE, COLOMBUS, OHIO | |||||||||
JANUARY AVERAGE TEMPERATURE, F | 32.1 | ||||||||
AVERAGE DAY INSOLATION | CONVENTIONAL HOUSE | INDOOR TEMPERATURE F | 69.6 | ||||||
DAYTIME CALCULATION | 12 | HOUR | DELTA T | 37.5 | SLAB DELTA T | 15 | |||
COMPONENT | AREA | R-VALUE | BTU/HOUR | DAYTIME TOTAL BTU | |||||
WALLS | 910 | 20 | -1706 | -20475 | |||||
ROOF | 1000 | 40 | -938 | -11250 | |||||
N/E/W WINDOWS/DR | 90 | 3.3 | -1023 | -12273 | |||||
SLAB | 1000 | 10 | -1500 | -18000 | |||||
FOUNDATI0N | 1040 | 10 | -1560 | -18720 | |||||
SOLAR WINDOWS DAY | 40 | 3.3 | -455 | -5455 | |||||
SUBTOTAL | -86172 | ||||||||
GAIN | FRAME | SOLAR GAIN | |||||||
DBL GL | ALLOW | COEFF LOW-E | |||||||
NORTH WINDOWS | 40 | 120 | 0.8 | 0.92 | 3533 | ||||
EAST/WEST WINDOWS | 30 | 240 | 0.8 | 0.92 | 5299 | ||||
SOLAR WINDOW GAIN | 40 | 603 | 0.8 | 0.92 | 17752 | ||||
DAYTIME SOLAR GAIN | 26584 | ||||||||
NET DAYTIME GAIN | -59588 | ||||||||
NIGHTTIME CALCULATION | 12 | HOUR | DELTA T | 37.5 | SLAB DELTA T | 15 | |||
COMPONENT | AREA | R-VALUE | BTU/HOUR | NIGHTTIME TOTAL BTU | |||||
WALLS | 910 | 20 | -1706 | -20475 | |||||
ROOF | 1000 | 40 | -938 | -11250 | |||||
N/E/W WINDOWS/DR | 90 | 3.3 | -1023 | -12273 | |||||
SLAB | 1000 | 10 | -1500 | -18000 | |||||
FOUNDATION | 1040 | 10 | -1560 | -18720 | |||||
SOLAR WINDOW NIGHT | 40 | 3.3 | -455 | -5455 | |||||
SUBTOTAL | -86172 | ||||||||
WHOLE BUILDING NET 24 HOURS CONVECTIVE + SOLAR GAIN/LOSS | -145760 | ||||||||
VOLUME | ACH | H-CAP AIR | |||||||
INFILTRATION LOSSES/DAY | 8000 | 0.5 | 0.018 | -64800 | |||||
VENTILATION LOSSES/DAY | INCLUDED | ||||||||
KWH | BTU/KWH | ||||||||
DEGRADED ELECT'Y CONTRIB'N | 12 | 0.9 | 3412 | 36850 | |||||
OCCUPANCY CONTRIBUT'N/DAY | 3 | 12 | 400 | 14400 | |||||
WHOLE BUILDING NET 24 HOURS GAIN/LOSS ALL SOURCES | -159311 | ||||||||
It's only responsible to talk about the EROEI values of an energy technology, i.e. energy return on energy investment. Let's talk in annual figures. Appendix 5 of The Passive Solar Energy Book has the annual degree-day figure for Columbus, Ohio, as 5277. The glossary defines degree-days for heating with reference to a base temperature of 65°F. Chart 6 represents (37.5-(69.6-65)) i.e. 32.9 degree-days. Recall that we chose a day of average sun, and windows are evenly distributed. As average as can be.
Accordingly, we estimate our realistic annual heating load for this conventional bungalow to be about
(160186 x 5277/32.9)BTU's. In MBTU's (million BTU's) the annual heating load is 25.6 MBTU. 80% of this figure is 20.5 MBTU. This number will be our annual Energy Return in calculating EROEI.
Now, to calculate the denominator of the ratio, the Energy Invested, we'll compare our strategically improved bungalow with the conventional base case, and apply embodied energy values to the quantities of additional materials needed for the strategy. What I quickly discovered in setting up this spreadsheet was that insulations derived from petrochemicals, such as expanded polystyrene, have a very high embodied energy content compared to other insulations. Semi-rigid rock wool batt insulation, however, is waterproof, so I'm suggesting it is more appropriate than polystyrene on top of the basement slab, obviously with proper detailing of moisture retarder, vapour barrier, and floor structure.
The value for the PCM is a best guess based on the value of gypsum plaster as being the most similar material on the list I used (google embodied energy coefficients - alphabetical). Better data is welcome.
CHART 7: EMBODIED ENERGY OF PASSIVE SOLAR ADAPTATION MATERIALS (ADDITIONAL ABOVE CONVENTIONAL CONSTRUCTION) | ||||||||||||||
AREA | DEPTH | VOL | VOL | MJ/ | MJ/ | LIFE | ANNUAL | |||||||
MATERIAL | SF | FT | CF | CM | LBS | KG | CM | KG | MJOULE | KBTU | YEARS | KBTU | ||
CELLULOSE | 1000 | 0.42 | 417 | 11.8 | 112 | 1321 | 1253 | 100 | 12.5 | |||||
POLYURETHANE | 740 | 0.125 | 93 | 2.6 | 2340 | 6129 | 5810 | 100 | 58.1 | |||||
ROCK WOOL | 2040 | 0.25 | 510 | 14.4 | 139 | 2007 | 1903 | 100 | 19.0 | |||||
GLASS | 170 | 0.021 | 3.5 | 0.10 | 37550 | 3766 | 3569 | 50 | 71.4 | |||||
CaCL2.6H2O PCM | 4500 | 2041 | 4.5 | 9185 | 8706 | 50 | 174.1 | |||||||
TOTALS | 21241 | 335.2 | ||||||||||||
It's just a small matter to move the decimal over for MBTU's, and our annualized embodied energy input for the passive solar adaptation materials is 0.34 MBTU. So 20.5/0.34=60.3, and Eureka! our EROEI is 60! This is the kind of number we want in a post-peak-oil future. If you do a bit more math, you'll see that PCM is about 7 times better than a concrete Trombe wall in terms of EROEI. PCM is the way to go.
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