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re: solar power efficiency pv vs heat collector
21 jun 1997
mosl roland wrote:
>there is a difference between thermal power and electric power.
nice to hear that :-) and between sunspaces and photovoltaics...
>you can put the electric power from the photovoltaic into a heat pump.
i've heard this before, and it always sounds stupid. seems expensive,
for one thing. i wonder...
>under certain conditions:
>
>low temperature and cloudy, the combination pv + heat pump
>is far superior a thermal collector in producing heat.
let's see... a 1 m^2 15% pv with an 90% efficient inverter and a heat pump
with a cop of 3 in 30% sun would produce about 300wx0.15x0.9x3 = 121 watts
of heat (a little more if the inverter is inside the house.) a square meter of
30 c metric r0.2 single-glazed sunspace with outdoor temp t would gather about
300x0.9 watts and lose about (30-t)1m^2/r0.2 watts, to equal the pv/heatpump
output when 300x0.9-(30-t)1m^2/r0.2 = 121 or t = 0.2 c. with r0.4 double
glazing, the breakeven point is t = -27.6 c. how often does that happen?
>if you need heat and cooling the same time, 1 part electric power
>can produce about 3..4 part heat and 2..3 part cold.
i wonder how many houses need heating and cooling at the same time...
>one heavy design failure in solar architecture is:
>create the solar collectors...
agreed. at today's oil and gas prices, using "solar collectors" and pv/heat
pumps whose only function is gathering $10/m^2-year worth of energy seems to
make very little economic sense, compared to less-expensive sunspaces which
also add floorspace to houses.
>...as big as needed for late autum. the big solar collector makes to much
>power in the summer.
a sunspace "solar collector" might have an overhang or a layer of shadecloth
to cover it in the summer and make it a more usable space...
>let's have a big season storage.
store summer heat for winter? not many houses need that, outside of
polar regions, where the sun so frequently shines in the winter.
>that's always the best way to make it very very expensive.
why make it very very expensive? :-) and if we really need seasonal storage,
how much would it cost to build a 5 meter cube full of water drums, sitting
outside on the ground, with a flat rubber roof, surrounded by strawbales?
>pv heat pump combination avoids this expensive storage problem.
or maybe a small insulated box full of water to store heat for a cloudy week.
>let's sell the power to the grid, when we have to much in the summer time.
i like the idea of selling/banking power back to the grid in the summer, but
most us electric companies buy power back for a small fraction of the selling
price, over a year. maybe the same price, over a month, until the customer
begins to produce more power than he/she consumes... but suppose we could sell
electricity for 10 cents per kilowatt-hour in the summer (assuming we didn't
use it for cooling--no free air conditioning here), and buy it back for
10 cents per kilowatt hour in winter. (can you do that in austria?)
and suppose my pennsylvania house required 2 kw for heating in december, ie
2kwh/degree day (c), when 3.1 kwh/m^2-day of sun falls on a 60 degree south
wall, and the average 24h temperature is 2.1 c, and the average daytime high
is 6.3 c. a single-glazed sunspace might gain 3.1x0.9 = 2.8 kwh/m^2-day and
lose 6h(30-6.3)1m^2/r0.2 = 0.7 kwh over a 6 hour winter day, for a net gain
of 2.1 kwh/m^2-day, so heating the house might require 24hx2kw/2.1kwh/m^2-day
= 22.8 m^2 of sunspace glazing, eg a 6 m wide x 3 m deep x 4 m tall 2-story
lean-to sunspace, something like half of a standard arched commercial plastic
film sunspace, costing about $10 per square meter of floorspace, eg $200.
my house has masonry walls with insulation outside, so it can store heat for
a few cloudy days in a row. a wood house might have a sauna full of containers
of water to store heat for a few cloudy days...
alternatively, we might use a heat pump with an output capacity of about 2 kw,
or more, if the house had little thermal storage. suppose we use 2 window air
conditioners with a total output heat capacity of 3 kw (10,000 btu/h) to heat
13 c (55 f) moist basement or crawlspace air up to 30 c (86 f.) say they cost
$200 each. cheap, at 13 cents/peak output watt or 40 cents/peak input watt.
winter heating in phila requires about 2700dd(c)x2kwh = 5400 kwh of heat or
5400/3 = 1800 kwh/yr or 5 kwh/day of electrical energy, on an 365 day basis.
phila has a 365 day average of 4.6 kwh/m^2-day of sun. with 15% pvs, this
requires 5/(0.15x4.6) = 7.2 m^2 of pvs with 1080 peak watts, right? something
like $4k for pvs, ignoring the cost of mounting, etc, at $4/peak watt. then
we need a 1 kw grid-tie inverter, another $2k?, and a cooperative electric
utility. a total of about $6500, or more, vs an add-on sunspace costing
something like $200, which adds 18 m^2 of floorspace to a house...
(you might say "my pvs can also provide all the electricity needed by my home."
but these pvs just heat the house. there is no electricity left over. adding
more pvs would fix that, and raise the price of the system. and an efficient
house may not need much electricity. it may be cheaper to buy than make it.)
all this is oversimplified, but it seems to me that sunspaces serve more than
one purpose, as well as being less expensive, simpler, and more reliable. they
seem clearly superior for house heating, unless there are other constraints.
nick
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