INTRODUCTION

• Correct sizing of geothermal drilling requires exact knowledge of thermal parameters of the ground, most importantly the average thermal conductivity (λ)

• This parameter can be accurately determined by an experimental method called Thermal Response Test (TRT). This involves inserting a rigorously constant amount of heat in the borehole for 72 hours, and monitoring the flows of temperature every 20 seconds

BENEFITS OF A THERMAL RESPONSE TEST

• Overestimating the value of thermal conductivity λ leads to under-sizing the ground heat exchanger, thus making the heat pump consume more electricity than required (the COP, coefficient of performance, decreases) or even stop working

• Underestimating the value of thermal conductivity should also be avoided, because it leads to a larger scale geothermal drilling than needed, and hence to an unnecessarily high initial investment

6_TRT_diagrama

For example, if the requirements are:

- 60 kW heating 1800 h/year
- 30 kW cooling 800 h/year

Let’s assume that estimating λ=2.3 W/mK leads to 10 boreholes, each 100 m depth.
At an electricity cost of 130 euros/MWh, we get the following results:

ADDITIONAL ANNUAL COSTS DUE TO OVERESTIMATING λ

(if actual λ is less than 2.3 W/Mk)

Actual λ

SPF

Additional annual costs

Cumulated additional costs for 15 years

2,3

3,8

0 €

0 €

2,1

3,3

570 €

8.550 €

1,9

2,9

1.150 €

17.250 €

1,7

2,6

1.700 €

25.500 €

SPF = seasonal performance factor

EXAMPLE

Click on the pdf icon to see a sample report from a TRT we performed recently (for 3 boreholes)

pdf-geoterm