Re: Hydro or Wind Power

AJH <sylva@xxxxxxxxxxxxx> writes
>On Fri, 25 Nov 2005 06:38:46 +0000, Oz <Oz@xxxxxxxxxxxxxxxxxxxx>
>wrote:
>
>>AJH <sylva@xxxxxxxxxxxxx> writes
>>
>>>Apparently it doesn't matter as ice conducts heat better than wet
>>>soil.
>>
>>Maybe, but ice conducts vastly less well than liquid water.
>
>Not so, ice conduct about 4 times better than water at freezing.

Probably true, but its level of convection is near zero.

>>I did post the theoretical results here earlier in the thread.
>
>OK, missed that, I take it we agree that heat pumps COP are generally
>quoted for modest temperature differences rather than the current case
>of a delta T of about 20C?

Hmm, I forget. Ahh, here it is:


====================
Its an efficiency thing. I forget the exact equation, oh heck I can look
it up. For 100% efficiency

(Heat supplied) = (work done) x (hotside temp)/(temp difference)

Noting that temp is degrees kelvin here.
So if you want a room temp of say 27C then hotside temp is 300k at 100%
efficiency you get:

degC deltaT gain
20 7 43
15 12 25
10 17 18
5 22 14
0 27 11
-5 32 9
-10 37 8
-15 42 7
-20 47 6
-25 52 6

The advantage of the system is that all energy losses (pretty much) end
up as hotside heat. The disadvanatge is that fridge units are not very
efficient and in particular get less and less efficient as the coldside
gets colder. They are also not at all efficient at low temp drops. So if
you look at an invented system with 1kW input power and 300K (27C)
hotside you are likely to get someting like:

pump Tot
degC deltaT gain eff% kW kW
20 7 43 15 6.4 7.4
15 12 25 20 5.0 6.0
10 17 18 25 4.4 5.4
5 22 14 30 4.1 5.1
0 27 11 25 2.8 3.8
-5 32 9 20 1.9 2.9
-10 37 8 15 1.2 2.2
-15 42 7 12 0.9 1.9
-20 47 6 10 0.6 1.6
-25 52 6 8 0.5 1.5

Which is a bit of a bummer because when the coldside is really cold, you
get very little gained heat for all your complex equipment.

BUT if you can have a stream as your coldside, which never drops below
0C then you have a nice source of very cheap heating giving you four
time the electrical input energy (and with care it could be even more).
If it freezes, though, you are stuffed.

Using a local source also doesn't work too well. A pool 10mx3mx4m has
volume 120m^3 or 120,000kg. Latent ht of fusion of water is (from
memory) 540calories/gm or um 2.2kJ/g so you get about 250x10^6kJ. Using
2.5kW day and night for heating gives you 100megaseconds or 1200 days.

Hmm, ok, that works fine....
[Unless I've slipped a decimal or two or three]

NB You have to unfreeze the ice during summer.....
================

So, probably works fine up to about 30C delta-T.
I would probably design to a 200% 'efficiency' and expect much shorter
running times when differentials are less unless a reliable source of
liquid water was available. Groundwater in the UK is usually about 12C.

--
Oz
This post is worth absolutely nothing and is probably fallacious.

Use oz@xxxxxxxxxxxxxxxxxxxx [ozacoohdb@xxxxxxxxxxxxx functions].
BTOPENWORLD address has ceased. DEMON address has ceased.

.

Relevant Pages

  • Re: Digital Camera as Sky Meter -- Ongoing Report
    ... > BUTTTTT...iffff you have ice with ANY significant amount of liquid water in ... The ice can be ... ambient temperature in the freezer, ... it does remain there for some time as it absorbs the heat of fusion... ...
    (sci.astro.amateur)
  • Re: De-humidifiers
    ... >> The efficiency drops dramatically with temperature - the performance ... > its efficiency by the amount of electricity it uses to do it. ... defrosting themselves at low temperatures because they rely on heat from the ... exchanger accumulates an enormous block of ice. ...
    (uk.rec.models.engineering)
  • Will the Ice Caps Melt?
    ... Will the Ice Caps Melt? ... degrees Centigrade in temperature, Al Gore's claim that ocean levels ... The scientific data necessary to calculate the amount of heat ...
    (alt.politics.bush)
  • Re: Venus terraforming?
    ... A massive outflow of heat will be taking place over the entire ... You need to get the temperature down to the CO2 freezing point ... Water ice very effectively insulates upward flow of heat - but does not ... The atmosphere of Venus has weight of 90 atmospheres. ...
    (rec.arts.sf.science)
  • Re: On-demand hot water heater for domestic and space heating...
    ... From a *heat transfer* pov, ... transfer efficiency, would have to be substantially larger in dimension than ... The only device I claim to be 100 % efficient is the fan coil. ...
    (alt.home.repair)