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Solar vs heatpumps in sunny Ireland ? - Page 3

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Posted by iain-3 on April 30, 2005, 12:08 am
TDI> Using such a system with ground water temperatures of around 50F
TDI> will give you a system that will typically provide a COP of 3.5 to
TDI> 4 depending on the system. COP is the number you were referring to
TDI> - a COP of 4 will give you four watts of heat out for one watt of
TDI> electricity in.

Here in California, four watts of heat from natural gas costs about
the same as one watt of electricity.  What are the relative rates
in Ireland?

Remember that to make that one watt of electricity, they have to burn
fuel to make multiple watts of heat, then extract electricity from
that heat.  The process is governed by the same heat-engine equations
that govern your GSHP.  Inefficiency in the various stages --
thermal to mechanical, mechanical to electrical, transmission,
electrical to mechanical, mechanical to thermal again -- usually
overcome the leverage you have of pumping heat up from 4C to 32C (+10%)
versus dropping heat in a turbine from 870 C to 100 C (-67%).

My point is just that the physics make it inevitable that electricity
will cost a lot more than fuel, and thus make heat pumps a marginal
idea.  They work great in places like Florida where the amount of
heat needed per year is small, and the heat pump is needed anyway for
refrigeration (A/C) for most of the year.

I'm using unglazed plastic rooftop panels for heating the pool in my
current house, and glazed metal rooftop panels for heating the house
in the next one.  The new system does not break even economically.
I'm doing it because (a) it decouples the value of the house from the
cost of fuel, and (b) I'd rather pay money to American labor to
assemble stuff than pay slightly less money to folks in the mideast
with a scary religious agenda.

Posted by daestrom on May 1, 2005, 2:56 pm

Your physics is quite right.  But another point is the relative cost of heat
in different forms of fuel.  While it may take four watt-hours of heat to
make one watt-hour of electricity, if the fuel used to generate that it is
cheaper than the fuel you might use for direct heating, you can still come
out ahead.  If the electric is from coal, nuclear, hydro or wind, the
per-watt-hour cost of electric can make heat pumps still attractive.

For example, if residential natural gas costs $.80 per therm (100 000 BTU),
and you have a 90% efficient furnace, you pay $.889 for 100 000 BTU
delivered.  If you have a heat pump operating with COP of 3.75, you would
need about 7.8 kWhr of electricity to deliver the same 100 000 BTU. (1 therm
is about 29.3 kWhr).  If your electric is only $.08 / kWhr, then the heat
pump wins the day.


Posted by iain-3 on May 2, 2005, 1:23 am
 Daestrom> Your physics is quite right.  But another point is the
Daestrom> cost of heat in different forms of fuel.

Daestrom> While it may take four watt-hours of heat to make one
watt-hour of
Daestrom> electricity, if the fuel used to generate that it is cheaper
Daestrom> the fuel you might use for direct heating, you can still come
Daestrom> ahead.

But here's the thing: the OP is building a house.  These things have a
lifetime of about a century (or 30 years here in the Bay Area, after
they get knocked down in a "remodel").  I wouldn't want to bet on the
relative cost of fuels in 30 years, because it's so far out, and there
so much regulation that can change in that time.

That said, though, it's a lot more likely that the electric utility
figure out a way to use the cheapest fuel around, than your future
homeowner.  Score a point for the heat pump.

I can't find it right now, but I've seen a report on various solar
heaters which listed, among other things, the effective COP: in this
the heat delivered divided by electrical energy used to run the pumps.
The tests were done in Florida, and as you can imagine, the solar
creamed heat pumps.  IIRC, some systems had COPs in the mid-60s!  It's
worth noting, though, that most did not.

I would imagine that the challenge in Ireland is to collect heat when
cold outside.  The efficiency curve for Heliodyne Gobi glazed panels
         x = (inlet_temp - outside_temp)/(radiation*0.317)
efficiency = 0.737 - 0.804*x

excuse the units: inlet and outside temp in degrees F, radiation in
watts/m^2, efficiency tells how many of those watts end up in the water
coming out of the panel.  To make this useful to the OP:

                  outside  tank      radiation      eff
California, Jan:   50 F    120 F    900 watts/m^2   0.540
Ireland, Jan, est: 35 F    120 F    800 watts/m^2   0.467

The latitude there is 54 N as opposed to 37 N here, so the collectors
will have to be at more of an angle.  My simulations indicate that the
angle isn't really that critical, but it's probably more critical up
there.  At that latitude, it's probably better to think of the panels
being wall mounted rather than being roof mounted.

I spoke to the owners of a house nearby which was heated with glazed
solar collectors.  The system looked pretty beat up.  In 30 years, it
had the controller go once, both sensors failed once, and it froze
three times.  So, a failure every five years, half of which were real
money to fix, but that half could have been avoided with anitfreeze.
I spoke with a local solar contractor who, summarizing what he's seen
for 20 years, suggested that pumps eventually fail too.

It seems like a solar system is more maintenance than a furnace, but if
the OP is building the house himself I'd imagine the maintenance
wouldn't be too difficult, mostly a matter of getting the parts.

Posted by daestrom on May 2, 2005, 9:55 pm

Agreed.  And the utility can invest in energy sources to generate cheap
electricity that the typical home owner cannot.  The fuels usable for
heating is a much shorter list (oil, gas, coal, wood, biomass, solar) than
for electric generation (add on hydro, wind, nuclear, to name a few).  I
think the heat pump leaves more options open.  But the lifetime maintenance
may be higher.

Agreed.  Althought they have a 'mild' climate thanks to the Gulf-Stream,
they are really quite a was north compared to CONUS.

Of course, heat pumps have maintenance issues too.  And if the OP is
intending to 'retire' there, they may get tired of maintenance that they
have to do themselves.  If it wasn't a '30-year' home, then I'd bring up
resale values as well ;-)


Posted by News on May 4, 2005, 3:49 pm

Ireland is the same latitude as England. Panels on roofs are the best
option.  Parts of Ireland never get below freezing on many years.  A
temperate climate.

Electrical items in many cases have no annual servicing which shouls be
added to any equation. Only service on breakdowns.

A heavily to superinulated home that only requires a very small heat input,
may be more economical running the small heating system by electricity, even
thought it cost 4 times as much to run than gas.  Once the lack of an annual
service is costed in it, it may be quite cheap to run.  Also electrical
equipment is generally cheaper to install than gas or oil equipment.

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