| Title | Stored heat calculations – Time for review |
|---|---|
| Authors | B. White |
| Year | 2024 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Stored heat calculations, power density calculations, geothermal development |
| Abstract | This paper reviews some preliminary methods of assessing power potential of geothermal fields (particularly stored heat and power density), while (re)introducing an additional stored mass calculation. Equating stored heat estimates with two different reference temperatures (e.g. USGS's original ambient temperature versus a 180°C abandonment temperature) shows that recovery factor is a function of reference temperature and reservoir temperature (Ra = Rb x (Tres - Tb)/(Tres -Ta)). This is not always recognised, such that differing recovery factors from different reference temperatures have been mixed in calculation schemes and ascribed to formation type or effects of reservoir temperature. Alternatively, reference temperatures have been changed without adjustment of recovery factors from the original reference temperature. The paper brings the various methodologies into a single power density graph to show how they may relate particularly to accessible depth of reservoir, porosity and temperature. The family of curves suggests that Wilmarth’s “Main Sequence” and “Hot Arc” lines may require re-examination. The convergence of various quick capacity assessments about real world data either speaks to the usefulness of these methods, or to the dominance of the stored heat paradigm in ultimate sizing of geothermal developments. Nearly twenty years ago, the USGS revised its stored heat calculation methodology which included stochastic modelling and led to a recalibration of recovery factor for condenser reference temperature using a limited database of US operating geothermal power stations. In the last twenty years many stations have been added globally such that a significant database of real-world operating plant could be created for recalibration of the recovery factors in stored heat calculations. If the associated numerical reservoir simulations for these plants are also considered in the recalibration, then a clear view on sustainable capacity for each field would be available for this recalibration purpose. The paper calls for a review of stored heat methodology and especially recovery factor, ideally using a single reference temperature as a means of calibrating effects of formation type and reservoir temperature as examples. It then goes on to question who could lead such an initiative. |