A Holistic Approach to Geothermal Power Plant Option Selection for the Te Ahi O Maui Geothermal Project

Abstract

There are a number of power plant cycles that can be employed to convert thermal energy in geothermal fluid into electricity. They include back pressure steam turbines, condensing steam turbines (single, and multiple flash), Organic Rankine Cycle (binary), Kalina Cycle (binary), and combined cycles. In addition the conversion cycles require rejection of low grade heat at the surface and various air and water cooling options can be employed for this duty. An optimum selection of the cycle depends on a number of considerations. The plant should be matched to the reservoir conditions including reservoir enthalpy and chemistry and the production well characteristics. The selection process needs to consider the project location and associated topography, land access, geotechnical conditions and transmission options. Environmental impacts, such as emissions and impact on the reservoir also need to be assessed. Traditionally project developers will select a power plant option in order to maximise their return on investment by maximising net power and plant availability whilst minimising project development schedule, capital and operational expenditure for the project. These aspects can be quantified and compared across prospective power plant cycles. Environmental and cultural considerations are sometimes considered as an afterthought or are more difficult to quantify. The Te Ahi O Maui project, in Kawerau New Zealand, aims to develop the resource beneath the lands of the Kawerau A8D Ahu Whenua Trust. As part of the Te Ahi O Maui geothermal project we have combined a conventional optional selection process with the Mauri Assessment Model. The Mauri model is a decision-support tool that has been developed to address contemporary practices in a manner inclusive of indigenous (New Zealand) Maori world views and values. It recognises the parallels between New Zealand legislation on sustainable development and Maori values of kaitiakitanga (guardianship and conservation). The four key pillars of the model are: wellbeing; economic; environmental; social and cultural. Under the model a geothermal development has to have a net positive impact across these pillars to be considered sustainable. The inclusion of the Mauri model provides a holistic option selection framework, and has allowed the cultural impacts of power plant cycles to be compared and assessed in a structured way alongside engineering and economic factors. It has particular application for future geothermal projects in New Zealand developed in conjunction with Maori partners. In addition the principles here can be readily applied globally to achieve sustainable geothermal projects with indigenous communities.