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re: higher temps in winter than summer? (was: "radiant wall" to deliver heat)
2 may 2002
daestrom  wrote:

>if i understood nick correctly, he was discussing using the soil around
>a home as a heat sink in the summer for a home-cooling heat pump.

more like john hait's passive annual heat storage system than a heat pump
using compressors. op jim was thinking about "earth cooling tubes," which
might become "warming tubes" in the winter. mr. hait uses lots and lots
of earth under plastic film and foam insulation to store heat, and biases
the local ground temp up to room temp with solar gain from house windows
in a clever scheme using passive earth tubes in pairs, with concentric
heat exchangers and bidirectional airflow. 

>if you incorporate a large enough volume of soil, you may only heat
>the soil up to 75 by the time the cooling season is over. conversely,
>during the heating season, you would remove heat from the same volume
>of soil to heat the home.


>how many cubic feet of soil would the system need to encompass so that
>at 30 btu/f-ft^3, and only cooling the soil down to about 65 degrees
>by spring.

we have 4954 heating degree days in phila. a superinsulated house with
200 btu/h-f needs 24x4954x200 = 24 million btu for a winter, which might
come from 24m/30/10 = 79k ft^3 of earth cooling 10 f, eg a 43' cube
surrounded by perfect insulation...

>or i could also be totally confused ;-)

>i think the lower limit of 65 for the 'source' of the heat pump for the
>heating season, and the 75 limit for the 'sink' mode during the cooling
>season are assumed as limits for the heat pump's efficiency.

a desiccant (eg licl) "chemical heat pump" could have a very high cop,
compared to a conventional heat pump. the papers i mentioned seem to have
enough information to estimate the efficiency based on sink and source
temps. i suggested a large pond under or outside the house as a source
of water vapor for heating, but it might be more practical to trickle
groundwater through another packed tower (a plastic 55 gallon drum full
of rocks?) let some evaporate, and return the remainder outdoors slightly
cooler, especially where i live, over a 9' water table. 


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