This would be interesting to know about the ground cover.  Whether the sun hitting the ground or not.
Where are you in the country? Do you get allot of snow.

We know that our winter heat recovery is up-welling heat from deep earth to surface.

Ground temperature at geo loop depths is determined by the average air temperature and not the heat conduction rate from miles down. So to maximize heating performance, you want to absorb lots of heat through the soil surface in the summer and minimize heat loss to the air in the winter.

Soil moisture has a big effect - so wet in the summer would help; and/or black plastic. Any ground cover that allows air to circulate freely isn't going to insulate. But it may block the sun from hitting the soil (a negative). Snow would help in Winter.

"you want to absorb lots of heat through the soil surface in the summer and minimize heat loss to the air in the winter."

Is there a reason we don't want the soil to absorb heat in the winter?

Personnally I'm not a supporter of the "solar banking" as absorbed heat moves away from loops if they are warmer than surrounding soil.

If one looks at horizontal loops for what the really are (in the context of heating), a giant solar collecter, then things such as sunshine and moisture are good, making insulation and other ground cover less than ideal.

Back to basics. In the absence of a geothermal loop field, my ground at 6 foot depth will still get colder in winter. This seems to indicate more heat loss than solar gain.

At the other site, I posted a graphic I found from an agricultural study which showed soil temps for various ground cover conditions: prairie, lawn, crop, forest, bare earth, asphalt, concrete. That graph showed all vegetative surfaces performing about the same. Peak of 23 C in mid-july. Bare earth was 5 degrees higher and asphalt or concrete about 10 degrees warmer.(Study only covered April-October)

There is clearly some impact of groundcover. Probably best left to students with time, research dollars and lab space to quantify. Going into year 2 on my loop field, I will hedge my bets and since i need to get rid of the leaves somewhere, I'll pile them over my loops. May provide insulation. Definitely a darker shade than existing, likely reduced albedo.

In response to Robinnc, I am in Southern Maryland.  Occasional snow, but not usually persistent cover.

Best,
Trent

So it would still get colder. But by extracting heat from it, you still gather more from sunshine by viryue of a higher delta T

There are some Web Energy Logger users who have installed temp sensors at various soil depths (i.e. 2', 5', 10', 20', etc.) as part of their monitoring associated with their horizontal loop fields.  Perhaps their charts would be informative to this subject.

To find the WEL units that have this information is somewhat tedious.  You'd have to go to the listing of all WEL users, first look at their description, and then look at those WEL units that look from the description that they may have soil information.

I think this is an interesting subject.  Unfortunately I'm not a thermal dynamics expert by any means, and, I don't have any temp sensors buried out in my borehole field.  If I come across any WEL sites that have ground temp info, I'll post it here.

Best regards,

Bill

"Bare earth was 5 degrees higher"

Your data supports my position. If loops make surrounding soil colder and bare earth is warmer, the Delta T will be more significant speeding the solar harvest.
Your course of action self admittedly keeps soil colder.

"Back to basics." Yes heat moves to cold, sunshine warms bare soil more........

> This seems to indicate more heat loss than solar gain.

If the air temp is lower than the surface soil temp, there will be a net heat loss and you want insulation (snow, leaves, dry soil, etc) to reduce it. This is common in fall and winter and is demonstrated by cases where snow cover reduces frost depths and mulch helps plants survive cold damage.

If the air temp is higher than the surface soil temp, then there will be a net heat gain and you don't want insulation in the way. This is common in spring and summer.

Solar gain can tip the scale a little bit towards the second case. But not enough when it's 20F outside and the soil is 40F.

The temperature of the soil measured at the loop depth doesn't change the above. But it will have a very small effect on surface soil temps.