[time-nuts] Practical considerations making a lab standard with an LTE lite

Jim Lux jimlux at earthlink.net
Sun Nov 23 19:45:50 EST 2014


On 11/23/14, 11:15 AM, Alex Pummer wrote:
> by us  in central California, we get 1kW/h square meter average around
> the year, the south even more, el Cajon will have today +29C° in the
> afternoon  as of 23 of November 2014

I suspect more like the insolation peaks at 1kW/square meter or a bit 
more, the average over a day is somewhat less. At JPL we have a weather 
station on line that displays this and I don't recall seeing 
significantly more than 1000 W/m2.


The nominal average 1.362 kW/sq meter at solar max is at the top of the 
atmosphere, and is normal to the incidence.

The surface insolation at the equator when the sun is directly overhead 
  is about 1.04 kW/sq meter.

I think you'd get pretty close to that at solar noon in the Summer in 
Southern California, which is 32-34 degrees latitude, so at the 
solstice, the zenith angle is 10 degrees, and cos(10) is pretty close to 1.

You do pick up some additional insolation from diffuse and scattered 
radiation from clouds or haze, but I'm not sure that makes up for the 
attenuation due to the same haze.

Some time ago, I calculated that in Los Angeles (34 degrees latitude), a 
horizontal flat plate gets about 8-9 kWh/m2/day in summer and about 1-2 
kWh/m2/day in winter..   Tilting the collector would help a lot in the 
winter (Zenith angle is 56 degrees instead of 10), but there's no making 
up for the short days.


Getting back to the time-nuts aspects, there are some charts around that 
show the temperature variation as a function of depth, latitude, soil 
and season.  I know that for DSN, they went through all kinds of 
gyrations to calculate (and measure) this for the optical fiber timing 
links between the antennas and the masers.  For small dissipated power 
(I doubt your oscillator is going to be putting kilowatts into the soil) 
you don't have to go very deep (single digit meters) before the diurnal 
variation is down in the 0.1 degree or smaller.  Annual variations are 
bigger.

http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm
has a bunch of charts for some unknown latitude (probably mid Atlantic 
states, since the data is from Virginia Tech).  They appear to use well 
water temperatures as the measurement technique.

A bit more googling found a paper by one G. Florides that refers to the 
Kasuda formula.. (the link is hard to cut and paste.. I'm sure if you 
google "Florides soil temperature" you'll find it)

and gives this reference
Kasuda, T., and Archenbach, P.R. "Earth Temperature and Thermal 
Diffusivity at Selected Stations in the United States", ASHRAE 
Transactions, Vol. 71, Part 1, 1965.






>> Horizontal ground heat means that you are harvesting sunshine
>> accumulated in the top one meter of the soil.  Much of the energy
>> is harvested from freezing the water around the pipe thus pulling
>> out the relatively high melting energy of water.
>>



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