Development and Test Results of a Calorimetric Technique for Solar Thermal Testing Loops, Enabling Mass Flow and Cp Measurements Independent from Fluid Properties of the HTF Used

Abstract

Thermal heat transfer fluids (HTF) used in solar collectors (e.g. synthetic oils) are known to age and degrade [1]. This degradation is impossible to control, affecting the fluid heat transfer capacity and thus the ability of measuring the performance of an HTF heating device (e.g. a solar collector) based on known specific heat values.

Collector testing is also crucially dependent on an accurate measurement of HTF mass flow rate. Such measurement relies on flow meters suitable for the accuracy, operating temperature and flow range requirements of the testing procedures, often an expensive and demanding component in particular when no-intrusive measurements are to be done ia a close circuit.

For power measurement purposes, as those performed in solar collector testing procedures, a direct measurement of the product between specific heat and mass flow rather than a separate measurement of both quantities is suitable. A calorimetric technique delivering this direct measurement is thus a suitable strategy to overcome such difficulties with acceptable (and even higher) measurement accuracy.

Solutions of this kind have already been proposed [2,3]. In this paper we revisit and improve the solution presented in [2] and demonstrate its usefulness in a solar collector testing loop, for temperatures up to 200 °C. A calorimeter prototype was thoroughly tested and calibrated with water as HTF (enabling accurate independent measurement of specific heat and mass flow rates values). Calorimeter calibration results where then used in the testing with thermal oil whose specific heat values were previously known from manufacturer and independent laboratory measurements.

A comparison of Cp measured by the calorimeter with the value given by the HTF manufacturer is used to test the calorimeter capacities. The agreement achieved was very good. It is noted that the technique can be easily implemented in any high temperature loops, large or small.