Janne Dragsted

SIDE-BY-SIDE TESTS OF DIFFERENTLY DESIGNED EVACUATED TUBULAR COLLECTORS

Six differently designed evacuated tubular collectors, ETCs, utilizing solar radiation from all directions, have been investigated experimentally. The evacuated tubular solar collectors investigated include one SLL all-glass ETC from Tshinghua Solar Co., four heat pipe ETCs from Sunda Technolgoy Co. and one all-glass ETC with heat pipe from Exoheat AB.

Advantages using inlet stratification devices in solar domestic hot water storage tanks

DTU Orbit (11/01/2019) Advantages using inlet stratification devices in solar domestic hot water storage tanks The thermal performance of a domestic hot water system is strongly affected by whether the storage tank is stratified or not. Thermal stratification can be built up in a solar storage tank if the heated water from the solar collectors enters the tank through an inlet stratifier.Measured thermal performances of two solar domestic hot water systems are presented. One system is a traditional high flow system with a heat exchanger spiral in the tank. The other system is a low flow system with an external heat exchanger and a newly developed inlet stratifier from EyeCular Technologies ApS installed in the tank. The two systems are otherwise identical which makes it possible to compare the thermal performance and the thermal stratification built up in each tank.Based on a measuring period of 140 days in the period from April 26, 2016 to September 25, 2016, the investigation shows, that the system with the stratification device has a higher thermal performance compared to the system with the heat exchanger spiral inside the tank.The relative performance (defined as the ratio between the net utilized solar energy of the low flow system and the net utilized solar energy of the high flow system), is a function of the solar fraction. The lower the solar fraction is, the higher the relative performance will be. Weekly relative performances up to about 1.10 are measured. That is, weekly extra thermal performances of up to 10% arc measured for the system with the inlet stratifier.

Behavior of a Solar Collector Loop During Stagnation

Behavior of a solar collector loop during stagnation A mathematical model simulating the emptying behavior of a pressurized solar collector loop with solar collectors with a good emptying behavior is developed and validated with measured data. The calculated results are in good agreement with the measured results. The developed simulation model is therefore suitable to determine the behavior of a solar collector loop during stagnation. A volume ratio R, which is the ratio of the volume of the vapour in the upper pipes of the solar collector loop during stagnation and the fluid content of solar collectors, is introduced to determine the mass of the collector fluid pushed into the expansion vessel during stagnation, Min. A correlation function for the mass Min and the volume ratio R for solar collector loops is obtained. The function can be used to determine a suitable size of expansion vessels for solar collector loops.

Validation of simulation models for differently designed heat-pipe evacuated tubular collectors

Differently designed heat-pipe evacuated tubular collectors have been investigated theoretically and experimentally. The theoretical work has included development of two TRNSYS (1) simulation models for heat-pipe evacuated tubular collectors utilizing solar radiation from all directions. One model is developed for heat-pipe evacuated tubular collectors with flat fins and one model is developed for heat-pipe evacuated tubular collectors with curved fins. The models are characterized by detailed calculations of the heat transfer processes in the fins, by detailed shadow modeling and by fins with selective coating on both sides. The input to the models is thus not a simple collector efficiency expression but the actual collector geometry. In this study, the TRNSYS models are validated with measurements for four differently designed heat-pipe evacuated tubular collectors. The collectors are produced by the Chinese collector manufacturer SUNDA Technology Co. The collectors have either flat or curved fins inside the evacuated tubes, the tubes have different diameters and the fins have selective coating on both sides. The collectors’ thermal performances have been measured side-by-side under the same operation conditions in an outdoor test facility.

MEASURED REFLECTION FROM SNOW

This paper presents an investigation of reflection from snow. Based on measurements of the reflected radiation and global radiation from a test facility in Sisimiut, Greenland, the effective albedo for the ground is determined. The influence of snow on the effective albedo is presented, and a high contribution from the specular reflection is seen. The effective albedo from snow-covered ground is compared with the effective albedo on a summer day without snow. The effective albedo is much higher for the snow-covered ground than the effective albedo for the ground without snow. In the summer the reflection happens as diffuse reflection and as backward reflection, because of the uneven terrain. There is no specular reflection.