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re: solar heat pump
31 jul 2001
oo wrote:
>i have a customer who wants to install a geo-thermal system for his seven
>figures home... i think i've convinced the guy to go radiant infloor
>instead and spend the money on a solar system instead...
"radiant infloor" is a method of heat delivery
that might work with any heat source, no?
>...evacuated tubes of course operating at ~110-120f.
of course. gee, where would we buy something like that? :-)
>flat panel collectors can be operated at near 100% efficiency if the
>collector temp is slightly lower than ambient.
maybe more than 100%.
>so what about using the flat panels as the "solar" evaporator
>of a heat pump system. yup, full of r-22 refrigerant and everything!
sounds expensive and complicated and energy-inefficient, altho heat
storage is easier. an uninsulated 70 f tank in a 70 f room, with a
water loop and a water cooler heat pump in each room?
>the big problem with flat plate collectors in cold / windy / low sun areas
>is that their efficiency is crap. look at the specs...
let's see. seattle in december... 40.5 f outdoors, with 420 btu/ft^2 per
day on a south wall... a collector with an r1/90% cover and 120 f water
gains 378 btu and loses about 6h(120-40)/r1 = 480. yup. not good.
but over 60% of the btus arrive in the form of beam sun. nrel says
a horizontal e-w axis concentrator (non-tracking, in december) can collect
an average of 254 btu/ft^2-day over about 1 hour. we might heat a 180 f
water trough with a 4:1 concentrator with 90% reflection and collect
4x254x0.9x0.9 = 822 btu/ft^2 of trough and lose 1h(180-40)/r1 = 140,
for a net gain of 682, and store the heat in a large indoor insulated
tank and use it to heat the hydronic floor. how big a tank? how often
does this beam sun arrive? time for a tmy2 simulation? we might start up
the heat pump if the tank temp ever reached 75 f, and cool it to 40 f.
an r4 south window with 50% solar transmission gains 210 btu/ft^2-day
and loses 24h(70-40)/r4 = 720. not very helpful. very few windows and
lots of insulation and electrical energy use would be helpful.
>the heat pump then... "steps this heat energy up" to 110-130f at a cop
>of 5 or so...effectively cutting down power consumption by five times!!
bad. we might say this lowers the cop of a water-based system with
a small pump by a factor of 200, and adds a lot to the first cost.
>same hot water tank with three heat exchangers etc etc......
no heat exchangers.
nick
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