Numerical evaluation of the performance of an indirect heating integrated collector storage solar water heating system

Due to the impact of energy usage on the environment and the increase in the price of fossil fuel, people must be encouraged to use renewable energy sources such as solar energy, wind power, hydraulic energy, geothermal energy and biomass energy. The indirect heating integrated collector storage solar water heating system is one of the compact systems for domestic water heating. It incorporates a solar energy collection component and a hot water storage component into one unit. The indirect heating type is characterized by service water passing through a serpentine tube (a heat exchanger) that is immersed in the stored fluid. The objectives of this study were to investigate ways to reduce heat losses from the system and enhance heat gained by the service water with the aim of reducing both the initial and the running costs.

The continuity, momentum and energy equations were solved in a steady state condition, using ANSYS 13.0-FLUENT software and using the pressure-based type solver. The results for particular system using the realizable k-є and standard k-ω turbulence models were compared to available experimental results to determine the appropriateness of the turbulence model choice. The percentage error for the numerical simulation of k-є model was higher than for the k-ω model. The error varied between zero (no errors) and 15 per cent for k-є, and zero to 8.5 per cent for k-ω model. The radiation heat transfer was also included by using a surface-to-surface radiation model.

To minimise the heat loss from the system, a parametric study was conducted in a system of double glass covers. The air gap spacing between the absorber and the lower glass cover (L1) and the gap between the upper and lower glass covers (L2) for the system were varied within the range of 15-50 mm to investigate which combination of gap sizes (L1, L2) would result in minimum total heat losses, i.e. including radiation and convection losses. Three-dimensional CFD models for the absorber, the double glass covers and the air in between (i.e. the storage and service water were not included) were developed. The results showed that when the gap size was small, the heat loss through the gap was mainly due to conduction, while as the gap size increased, the velocity of the air in the gap increased and this increased the convection contribution to the heat loss. The optimum lower gap spacing was found in the range of 15 and 20 mm, while the optimum upper gap was found in the range of 30 and 35 mm.

To enhance the heat gained by the service water, important parameters of the heat exchanger were investigated. These parameters are tube length, shape, positioning and the cross sectional area of the pipe. The tube length was 16.2 m for the double row heat exchanger and it was varied to 8.1 and 10.8 m for the single row heat exchanger. Circular and elliptical tubes were also examined. The mass flow rate was chosen as 500 and 650 L/h. The outlet service water temperature was used as a measure of the performance, since it is a measure of the energy acquired from the solar radiation. Three-dimensional CFD models were developed and validated using the experimental results of Gertzos, Pnevmatikakis and Caouris (2008). A standard k-ω turbulence model was used in the optimization of the heat exchanger because it gave good agreement with the experimental results.

The results showed an increase in the outlet temperature of the system, and a significant reduction in the initial and operating costs of the system. The outlet temperature of the elliptical tube system was higher than the circular tube of similar length and cross-sectional area. The single row heat exchanger (HX) with 10.8 m length and elliptical cross sectional area gave a high service water outlet temperature of 57.9o C with low pumping power. The outlet temperature of the system with tube length of 10.8 m (single row heat exchanger) was higher than those of 16.2 m (double row heat exchanger). These resulted in an increase in the thermal performance and a significant reduction in both the initial and operating costs of the system.

The study was conducted in steady state condition assuming that the circulating water mass flow rate was 900 L/h, the storage water temperature was constant at 60oC and for two service water inlet temperatures’; 15 and 20 o C.