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re: solar options
28 jan 2003
bill kreamer  wrote:

>hi all,

hi bill,

> principal, for highest efficiency, you would collect solar energy
>at the lowest temperature that is practical for the intended use; so why
>concentrate, to heat living space?

1. we take a hit on the reflector, but a smaller receiver can have less loss
at a higher temp: a square foot of flat plate with one layer of r1 glazing
with 90% solar transmission might collect 0.9x250 = 225 btu/h in full sun
and lose (80-30)1ft^2/r1 = 50 from 80 f air on a 30 f day, for 175 btu of
net gain. with 3 suns and 130 f water and a 90% reflector and r1 90% target
glazing, 3x0.9x0.9x250 = 608 btu enters and 100 leaves, for a 508 btu gain.

2. higher temps can make heat storage easier.

3. water can make heat transportation easier.

4. water can make heat distribution easier, with less noise and drafts
and smaller "ducts" with more insulation and less pump vs blower power.

5. higher temps can heat water for showers.

6. mirrors are cheaper than collectors.

>what about storage?  storage is rarely a paying proposition.

how about 12" diameter greenhouse poly duct (38 cents/ft) over 2"x4"
welded-wire fencing (10 cents/ft^2 in 48" rolls) screwed under basement
rafters on 2' centers, in a return air path to a large air heater or
sunspace? with 2.8" of water inside, we can store 75 btu per square foot
of floorspace with a 10 f daily temp swing and a 39 cent investment. 

>unless it is segmented, so that isolated storage regions are addressed
>in a last-in first-out sequence, storage is generally ineffective for
>space heating.

stratification only seems useful with low-flow collectors and backup fuel
costs. a 100%-solar-heated house might collect lots of low-temp heat on
an average day and store separate higher-temp water for cloudy days.

>this is non-trivial controls territory.

simple dampers and thermostats come to mind.

>the materials expense, design time, controls, space, installation, and
>maintenance can be avoided in an air system by venting excess heated air.
>the cost effective options in liquid systems are fewer.

a nearby house has about 3k/year of heating and water heating and electric
bills. it's 2 years old, well-insulated and airtight, about 3,000 ft^2, all
1-story, with a 40'x18' south-facing garage roof with a 9/12 pitch. we talked
about a trickle collector up there, but it's far from the furnace and water
heater, the roof is new, and the owner wants to avoid complex systems and
their maintenance. he also wants a 3-year payback :-) 

part of the south wall is about 27'x9' tall, with few windows. it might
provide heat and help heat water year-round, with a 24' wide x 8' tall
x 6" thick passive air heater with a $100 2-watt motorized damper at one
corner and $200 worth of fin-tube pipe near the top for water heating.

the air heater might have $300 worth of redwood 1x6 sides and $100 of black
aluminum window screen inside to act as a transpired mesh absorber. room air
would slide past the basement ceiling and enter the heater via a hole at one
corner and return to the house via the upper damper when the sun is shining
and the house needs heat (using a $20 thermostat.)

the heater could stagnate from june through september, heating the fin-tube
with the damper closed. the glazing could be a single layer of dynaglas clear
corrugated polycarbonate in 4 $80 51"x12' sheets with corrugations running
horizontally. a few years ago, i put up an unvented wall-warmer like this
over a dark stone wall on the art building at ursinus college. it's held up
well. it even survived art critics :-)

the house is now heated with propane, with a 500 gallon tank in the ground. 
the non-condensing new yorker boiler has 4 zones, 3 baseboards and a 300 ft^2
hydronic floor, with an indirect-fired amtrol 41 gallon domestic water heater.
the house has a fairly open plan. the owner is reluctant to add more baseboard

the fin-tube might be attached to a pipe loop thru another $200 water heater
in the basement that acts as a preheater for the present water heater, with
a $100 differential thermostat to turn on a small $100 circulating pump when
the wall space is warmer than the preheater. it would also turn on the pump
briefly when the fin-tube water is colder than 40 f in order to keep it from

here is a 2002 history of electrical use at 7.9 cents/kwh and actual heating
degree days and propane deliveries in gallons at 92k btu and $1.09/gal (in
"fixed font," one of the outlook express view options): 

     jan   feb   mar   apr   may   jun   jul   aug   sep   oct   nov   dec

kwh 1177   823   785   835   848  1000  1455  1632  1190   937   785   987
hdd  944   670   611   385   189     0     0     0     0   125   491   848
pro  310     0   285     0   405     0     0     0     0   400     0   345

we might exclude the may and october propane deliveries and figure that 940
gallons of propane heated the house during this 4263 degree-day season, ie
940x92kx0.85 = 73.5 million btu at 85% efficiency. we might also exclude 50k
btu/day of water heating for a family of 3, ie another 12 million btu from
october through may. omitting outdoor lighting, indoor electrical use added
about 20.7 million btu of house heating, which makes the house conductance
approximately (73.5m-12m+20.7m)/(24hx4263dd) = 804 btu/h-f. 

winter 2002 was warmer than average. nrel says phila's 30-year average winter
has 4954 heating degree days and 1101 cooling dd, with these long-term average
monthly (f) temps and south wall sun (btu/ft^2-day):

     jan   feb   mar   apr   may   jun   jul   aug   sep   oct   nov   dec

temp  30    33    42    52    63    72    77    76    68    56    46    36
sun 1000  1080  1070   950   830   790   820   940  1070  1150   990   900
x0.9 900   972   963   855   747   711   738   846   963  1035   891   810
loss 300   282   266   196   136    64    24    32    84   168   204   264
net  600   690   697   659   611   647   714   814   879   867   687   546
x192 115k  132k  134k  127k  117k  124k  137k  156k  169k  166k  132k  105k
need 599k  485k  360k  152k    0k    0k    0k    0k    0k   91k  268k  491k
%ht   19    27    37    84   100   100   100   100   100   100    49    21
prod 115k  132k  134k  127k   50k   50k   50k   50k   50k  141k  132k  105k
hdd 1073   896   701   378   123     5     0     0    32   283   558   905
cdd    0     0     0     0    58   209   363   326   128    17     0     0

on an average january day, 1000 btu/ft^2 of sun falls on the south wall. the
glazing passes 900 btu/ft^2. with 80 f air inside, it loses 6h(80f-30f)/r1
= 300, for a net gain of 600. at 800 kwh/mo (91k btu/d) of electrical use,
the house needs 24hx1073ddx804/30d-91k = 599k btu of heat. the wall warmer
might supply 100x115k/599k = 19% of that. 

it might provide 50k btu/day of water heating from may through october, for
a total yearly energy production of about 30dx1136k = 34 million btu,
equivalent to 34m/(92kx0.85) = 435 gallons of propane worth $474. materials
might cost about $1,500. adding say, 6 days of plumbing and carpentry labor
at $50/h might make the total $3,900, with a 12.2% tax-free return. 


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