Two Types of Calorimeters for Assessing Fresnel Lens Performance in Concentrating Solar Systems Available to Purchase

Concentrating solar energy systems can use either refractive or reflective approaches to achieve the desired concentration ratio. However this is done, there is always a question about what the flux might actually be incident on the target of interest after the concentration process. Assessing the losses due to the concentration process is quite important in understanding the overall performance of the solar concentrating system. An issue that impacts this measurement is the type of system being evaluated, as the total flux at the focal point could be quite large. We have been working with concentrating PV units that utilize acrylic Fresnel lenses to achieve the necessary concentration on a single multi-junction cell. The magnitude of the losses associated with these types of lenses was desired. We developed two calorimeters for the purpose of evaluating the optical efficiency of Fresnel lenses utilized in various point focus concentrating systems. The first calorimeter developed utilizes a transient technique whereby a time-measured pulse of the beam is directed to a mass of material in a cavity form. The material has a high conductivity, so the lumped mass approximation can be used for the analysis of the energy absorbed if the temperature rise of the material is measured. The other calorimeter developed was a steady-state type that employs a technique commonly known as flow calorimetry. In this approach, the concentrated flux is beamed into the core of the calorimeter which is cooled to some steady-state value of temperature with a liquid (water near the ambient temperature was used in our tests). Knowing the liquid flow rate and temperature rise of the fluid allows the total heat input to be assessed. This paper discusses the development, testing, and comparison of the two calorimeters. Results are given for the evaluation of several types of commercial acrylic Fresnel lenses having different characteristics such as groove density and focal length.