The use of laser doppler velocimetry (LDV) to improve district heating measurement, metering and operation

Measurements from large district heating (DH) flow meters are used as a basis for controlling the supply of district heat and as a basis for billing heat consumption and it is important that they measure correctly. For meters used for control purposes this is important with regard to energy consumption, whilst for meters used for settling bills it is important for the parties involved that billing is based on reliable measurements. During the course and influence of time, DH meters can become inaccurate and in the worst case deviate so that the registered flow rate and energy consumption is not the same as the correct values. In Denmark and throughout the DH sector generally, it is a difficult task to remove large district heating meters for calibration. Calibration with hot water is not possible in Denmark or abroad and no suitable method for on-site calibration has been available. Laser Doppler Velocimetry (LDV) measures the speed of tiny particles in a flowing fluid (i.e. water), by using two laser beams that cross each. An interference pattern is formed at the intersection of the two beams and as moving particles pass through this pattern they reflect light which is detected and transformed to a speed. A flow profile can be determined by taking measurements across the diameter of the pipe and the volume flow can be calculated. Modern LDV systems are compact and robust and fibre optics allow free movement of the laser probe, making the technology suitable for in-situ check and calibration of large district heating flow meters. This application has been illustrated and developed during the project. Results indicate that LDV measurements can be made with an accuracy of approximately +-0.5% for flow velocities over 1.5 m/s and ±1% for lower flow velocities. This is a vast improvement compared with the legal measurement demands for installed district heating meters, which stipulate 2 x +-3% = +-6% for nominal flow rates and ±10% for minimum flow rates. It is believed that further improvements can be made to the robustness of LDV measurements and their accuracy through increased practical experience and using improved measurement strategies. Improved equipment and methods of data analysis will also contribute to better measurements. There is scope for further work with regard to measurements close to the pipe walls, where reflections and uncertainty about determining the measurement starting point can make measurements difficult. To conclude, it has been demonstrated that flow in large district heating pipes can be measured using LDV. The results of the project indicate strongly that the method is relevant for future check or control of flow meters in district heating systems, with the advantages this gives with regard to better control of district heating distribution and a fair method of billing heat consumption. Similarly the use of LDV will also be suitable for checking large cold water meters as well as being a useful tool for improving the accuracy of meter calibration in general