Measuring Techniques for in Situ Measurements of Thermodynamic Properties of Geothermal Water

The energy content of geothermal water flow is determined by the chemical and physical water properties and the special flow conditions on site, like mass flux, temperature and pressure. For dimensioning geothermal power plants and/or district heat systems, energy content of the brine is the most important parameter. Flow rate, temperature and pressure measurements are common but determination of thermodynamic water properties at power plant operating conditions is often uncertain. High amounts of dissolved minerals and gases influence the water properties significantly. Sample taking of geothermal water and measuring of the physical properties in the laboratory tends to lead to big errors if temperature and pressure level cannot be kept constant. Gasification and/or precipitation of minerals might occur which change the fluid properties substantially. Against this background a measuring technique was developed and installed which allows for identifying the main thermodynamic properties of the brine on site in a bypass line of the pipe system. The mobile configuration consists of a flow calorimeter for isobaric heat capacity determination, a capillary viscometer for kinematic viscosity ascertainment, and a density measuring device. All components are our own developments and are tested for water and aqueous salt solutions. The accuracy of heat capacity measuring technique amounts to less than 1 per cent based on literature data, the uncertainty of the kinematic viscosity measurements is within a range of 1-2 per cent. All measuring units operate in a temperature range of 0-170°C and pressures up to 30 bar. In addition to our own developments a commercial measuring technique for thermal conductivity determination is appended. Heat capacity, kinematic and dynamic viscosity, density and thermal conductivity of geothermal fluids can now be determined at geothermal sites as either single point measurement under stationary operating conditions or as multi-point measurement at different temperatures and/or pressures. Detailed knowledge of these fluid properties will serve for a better dimensioning of power plant components, e.g. heat exchangers.