Louise Jivan Shah

Characteristics of Vertical Mantle Heat Exchangers for Solar Water Heaters

The flow structure in vertical mantle heat exchangers was investigated using a full-scale tank designed to facilitate flow visualisation. The flow structure and velocities in the mantle were measured using a particle Image Velocimetry (PIV) system. A CFD simulation model of vertical mantle heat exchangers was also developed for detailed evaluation of the heat flux distribution over the mantle surface. Both the experimental and simulation results indicate that distribution of the flow around the mantle gap is governed by buoyancy driven recirculation in the mantle. The operation of the mantle was evaluated for both high and low temperature input flows.

Correlation of experimental and theoretical heat transfer in mantle tanks used in low flow SDHW systems

Experimental and theoretical investigations of vertical mantle tanks for solar domestic hot water systems have been carried out. Differently designed mantle tanks have been evaluated in a laboratory test facility and a transient three-dimensional CFD-model of one of the mantle tanks is developed in the CFD-program CFX 4.1. The model is validated against the experimental tests and good agreement between measured and calculated results are achieved. The results from the CFD-calculations are used to illustrate the thermal behaviour and the fluid dynamics in the mantle and in the inner tank. The CFD-calculations are used to carry out a detailed analysis of the heat transfer from the solar collector fluid to the wall of the inner tank. The analysis has resulted in a local Nusselt-Rayleigh correlation for the heat transfer between the solar collector fluid and the wall of the inner tank.

Heat transfer correlations for vertical mantle heat exchangers

This paper investigates heat transfer in vertical mantle heat exchangers for application in low flow solar domestic hot water systems. Two new heat transfer correlations for vertical mantle heat exchangers with top entry port and bottom exit ports are developed. The correlations are based on computational fluid dynamic modelling of whole vertical mantle tanks. The correlations are combined with a heat storage model in a simulation program that predicts the yearly thermal performance of low flow solar domestic hot water systems based on mantle tanks. The model predictions of energy gains and temperatures are compared with outdoor measurements and the model is found to give reliable results.

Thermal performance of a large low flow solar heating system with a highly thermally stratified tank

In year 2000 a 336 m 2 solar domestic hot water system was built in Sundparken, Elsinore, Denmark. The solar heating system is a low flow system with a 10000 1 hot-water tank. Due to the orientation of the buildings half of the solar collectors are facing east. half of the solar collectors are facing west. The collector tilt is 15° from horizontal for all collectors. Both the east-facing and the west-facing collectors have their own solar collector loop, circulation pump, external heat exchanger and control system. The external heat exchangers are used to transfer the heat from the solar collector fluid to the domestic water The domestic water is pumped from the bottom of the hot-water tank to the heat exchanger and back to the hot-water tank through stratification inlet pipes. The return flow from the DHW circulation pipe also enters the tank through stratification inlet pipes. The tank design ensures an excellent thermal stratification in the tank. The solar heating system was installed in May 2000. The thermal performance of the solar heating system has been measured in the first two years of operation. Compared to other large Danish solar domestic hot water systems the system is performing well in spite of the fact that the solar collectors are far from being orientated optimally. The utilization of the solar radiation on the collectors is higher, 46% in the second year of operation, than for any other system earlier investigated in Denmark, 16%-34%. The reason for the good thermal performance and for the excellent utilization of the solar radiation is the high hot-water consumption and the good system design making use of external heat exchangers and stratification inlet pipes.

Thermal performance of Danish solar combi systems in practice and in theory

An overview of measured thermal performances of Danish solar combi systems in practice is given. The thermal performance varies greatly from system to system. Measured and calculated thermal performances of different solar combi systems are compared and the main reasons for the different thermal performances are given. Further, a parametric study on two solar combi system types is performed. Based on the investigation it can be concluded that the thermal performance first of all is influenced by the space heating consumption during the summer period and that the systems in practice perform as theoretically expected.

Modeling Shadows on Evacuated Tubular Collectors With Cylindrical Absorbers

A new TRNSYS collector model for evacuated tubular collectors with tubular absorbers is developed. Traditional flat plate collector performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbor tube. An all glass tubular collector with tubular absorbers with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility. Performance calculations with the new model are compared with measured results and a good degree of similarity between the measured and calculated results is found. Further, it is illustrated how the model can be used for geometrical parameter studies both for constant collector mean fluid temperatures and for varying temperature conditions in a solar heating plant. These investigations are performed for two climates: Copenhagen (Denmark) and Uummannaq (Greenland).