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re: reality vrs modelling
18 nov 2000
richard yeo wrote:
>after a weekend playing with some figures and spreadsheets, i have come
>up with a couple of questions relating to the practicality of my 'solar
>shed' (tm nick pine , i think) modelling.
perhaps you are thinking of a solar closet (tm), vs a sunspace or a shed.
>being up here in iceland, i get 0.5 mj/day in midwinter, that equates to
>around 140 wh or 475 btu.
per square foot of south-facing wall per day? not much. a frozen pond
in front of a sunspace might add 30%. what's the average air temp?
>...how efficient are real build sunspaces in terms of (ein from the sun) /
>(eout in the air to the store?)
we measured 55% for one solar closet (sunspaces have no thermal store,
in my book), which was in our theoretical ball park. this depends on
the distribution of solar intensity (i figure an average amount arrives
over 6 hours on an average winter day), outdoor and collection temps,
glazing transmittance and r-value, undesirable thermal mass, etc.
>i can calculate the expected eff. assuming the air temp is the same as
>the heatstore (this has the covert assumption that i have 100% eff.
>air/water heat transfer, which is of course impossible!).
you can come close with lots of insulation and airflow and water container
surface and a small amount of glazing and a small average thermal load
(only using the closet to supply heat for the house on cloudy days.)
>3. how practical is it to build r-30 walls ? in theory 150-200mm of
>polystyrene would give me r-30, but this is ignoring the thermal bridges
>produced by the structural members etc.
i like large prefab precut sips with no thermal bridges and few air leaks.
osb-beadboard-osb sandwiches with glue. premier (www.pbspanel.com) 6" sips
are rated r25, and oak ridge measured r21.7 for a wall, vs r13.7 for 2x6s
2' on centers with r19 fiberglass batts. their 10" r40 sips cost $3.90/ft^2
plus shipping.
nick
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