Surface heat loss assessment at the Waiotapu Geothermal Field, Taupo Volcanic Zone, New Zealand

Assessments of surface heat loss from geothermal systems can provide information and insight into the size and dynamics of the underlying geothermal resource and provide constraints for numerical models. Historically, many different methods have been used to measure heat loss. However different techniques, and assumptions can result in a wide range of estimates with large errors. Surface heat loss is estimated in this paper using a variety of terrestrial based measurements and remote sensing data. Aerial thermal infrared data, calorimetry, temperature-depth profiles and chloride flux measurements were collected at the Waiotapu Geothermal Field, Taupo Volcanic Zone, New Zealand. Terrestrial measurements are integrated to determine a total heat loss for the area using a groundcover (vegetation) map determined from aerial photography.

A variety of methods for calculating heat loss are compared, with each technique providing insight into surface and subsurface heat transfer characteristics. Of note is the vast difference in the amount of heat lost from the transport of geothermal fluid to the surface compared to the amount of heat transferred through the ground compared to other New Zealand geothermal fields. Over 50% of the surface heat loss is emitted through evaporation, convection and conduction processes at the surface of water bodies, and an additional 40% is lost through directly discharging geothermal fluids into the Waiotapu Stream and tributaries. Less than 10% of heat is lost through heat transport from the ground.

Comparisons between heat loss determined from temperature-depth profiles and calorimetry vary drastically. We propose two potential causes for this. Firstly, a sinter deposit is present in the shallow subsurface which is acting as an insulator and preventing heat from reaching the surface, or secondly, the high air temperatures at the time of measurements suppressed the surface heat loss signal.

Aerial thermal infrared data are used to estimate the total heat loss from the ground and pool surfaces, while chloride flux of stream water is used to estimate the mass discharge into stream channels. A total heat loss from the Waiotapu Geothermal Field is estimated to be 456 MW which compares favourably to historically estimated heat loss values, ranging from 410 to 520 MW, using a variety of techniques.