UC Berkeley

Distributed solar thermal combined heat and power is an area where there is potential for significant innovation to reduce the cost of solar energy. Two areas for innovation were explored: reduced thermal losses in a novel dual-chamber absorber tube and parametric study for optimal efficiency of small-scale (5-10 kW) radial inflow turbines.

Increasing the peak temperature in solar thermal systems typically increases the cycle thermal efficiency. However, the outer wall temperature of the absorber tube also increases causing greater thermal losses to the surroundings. A novel finned dual-chamber absorber tube design was presented as a possible solution. Fins attached to the outer wall could serve as a heat sink to send heat into the inner chamber. Computation modeling was validated by a reduced-scale experiment to the accuracy of the correlations used. From these models, it has been demonstrated that the heat loss to the environment from the outer wall of the absorber tube could be reduced by 10-60%. The best performance was found when water enters the outer chamber at the highest mass flow rate tested. This impressive improvement encourages further development of the dual chamber design.

One of the primary obstacles of scaling down solar thermal technology for distributed power generation is finding an appropriate expander design. The radial inflow turbine was considered. A parametric study of seventeen input parameters was completed. Of these parameters, the mass flow rate, r_2,hr_2,t (ratio of rotor exit hub radius to rotor exit tip radius) and r_2,tr_1a (ratio of rotor exit tip to rotor inlet radius) had the greatest influence on the thermal efficiency of the system. Decreasing the mass flow rate both increased the turbine efficiency and increased the pressure drop across the turbine both of which enhanced the thermal efficiency. The optimized value of r_2,hr_2,t and r_2,tr_1a depended on the type of fluid. Wet fluids had an optimal performance with low values of r_2,tr_1a but high values of r_2,hr_2,t. Dry fluids had an optimal thermal performance with low values of both r_2,hr_2,t and r_2,tr_1a . Of the fluids studied it appears that dry fluids with low molecular mass and high critical temperature yielded the greatest thermal efficiency. Such fluids typically have low viscosity, allowing there to be less frictional losses in the turbine leading to better thermal efficiencies. Unfortunately, these fluids also share the characteristic of being highly toxic. In order for the radial inflow turbine to scale with reasonable efficiency, a non-toxic dry fluid with low molecular mass and high critical temperature would need to be found.