Matching Grout Thermal Conductivity to Formation Thermal Conductivity

Frequently, GeoPro is asked about the importance of trying to match the thermal conductivity of a grouting material to the formation thermal conductivity value. To answer this question, we’ve created a hypothetical example where we examine the impact of grout thermal conductivity with respect to design bore length reductions in four different locations with different formation thermal properties.

After reading this document, hopefully it can be seen that the grout thermal conductivity value should be selected based on the results of an honest economic analysis. We’ve created a tool which allows our customers to perform such an analysis. Refer to our Bore Cost Comparison Calculator to perform these calculations.

To begin with, let’s assume that a hypothetical building with a 100 ton (1,200,000 Btu/hr) cooling and a 100 ton (1,200,000 Btu/hr) heating load exists where the annual full load operating hours are estimated to be 1,040 hours per year for cooling and 870 hours per year for heating. Then let’s assume that this same building is designed in four separate locations with the same annual weather conditions but the ground thermal properties are all different.

For the four separate locations, let’s also assume that the following thermal conditions exist:

Site Location	Deep Earth Temperature	Formation Thermal Conductivity (k)	Formation Thermal Diffusivity (α)
1()	58°	0.66 Btu/hr ft °F	0.43 ft2/day
2()	58°	0.94 Btu/hr ft °F	0.58 ft2/day
3()	58°	1.20 Btu/hr ft °F	0.74 ft2/day
4()	58°	2.77 Btu/hr ft °F	1.20 ft2/day

For all four locations, we designed a well field with 100 bores on a 10 x 10 grid, separated 20ft on center. In these designs, we assumed a 5.5 inch diameter bore using a 1.25 inch HDPE u-bend assembly and also assumed a “B/C” bore configuration. We then calculated the bore lengths per ton for every grout thermal conductivity value listed in GeoPro’s Thermal Grout literature plus the value of 0.38 Btu/hr ft °F for a 20% solids, standard bentonite-based grouting material (value obtained from the IGSHPA Grouting for Vertical Geothermal Heat Pump Systems, Engineering Design and Field Procedures Manual, page 2-3) and graphed the calculated lengths as seen above.

It is important to understand that all the above lengths represent that exact same thermal performance. In all cases, the heat pumps will operate identically and will see the same design entering water temperatures for summer (Cooling EWT) and winter (Heating EWT). Therefore, system operation (from the heat pump point-of-view) will be identical for all designs.

When we compare the calculated lengths for a grout with a thermal conductivity value of 0.38 (no silica sand additive) to that of 0.88 (200 pounds of silica sand additive to 50 pounds of bentonite) for all four locations, we will see the following:

Site Location	Calculated Length for 0.38 Grout	Calculated Length for 0.88 Grout	Bore Reduction
1()	292 ft;	239 ft	53 ft
2()	258 ft	205 ft	53 ft
3()	237 ft	184 ft	53 ft
4()	179 ft	126 ft	53 ft

If we do the same thing and compare the calculated lengths for a grout with a thermal conductivity value of 0.88 (200 pounds of silica sand additive to 50 pounds of bentonite) to that of 1.20 (400 pounds of silica sand additive to 50 pounds of bentonite) for all four locations, we will see the following:

Site Location	Calculated Length for 0.88 Grout	Calculated Length for 1.20 Grout	Bore Reduction
1()	239 ft;	228 ft	11 ft
2()	205 ft	194 ft	11 ft
3()	184 ft	173 ft	11 ft
4()	126 ft	115 ft	11 ft

From this analysis, we see that the effect of calculated bore reduction due to grout thermal conductivity is independent of the formation thermal properties. In that the formation thermal properties are driving the overall design length (lower thermal values deliver longer bore lengths versus higher thermal values deliver shorter bore lengths), grout thermal conductivity values have the potential to adjust the actual design length within a fixed range for any formation.

Bore reductions can vary from project to project, especially in the presence of unbalanced ground loads, so it is imperative that the designer calculate the actual bore reductions and not assume these values are a standard.

You can also see that there is a diminished return when increasing the thermal conductivity value of the grout beyond a certain point. Therefore, it is critical that the design professional look at the economic impact of the grouting material and select the grout that makes the most economic sense (select the specific value that reduces the initial installation cost the most).

If a design using a 0.38 thermal conductivity grout can honestly be installed for less money than a comparable, properly designed system using a 0.88 value, then the 0.38 design should be selected over the 0.88 design. However, that is normally not the case.

Download a copy of this example:
Matching Grout TC to Formation TC ( 297kB PDF )

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