Aerodynamic and thermal characteristics of novel modular air-cooled condenser

To significantly contribute to global energy needs, the deployment of Concentrated Solar Power (CSP) plants in suitable worldwide locations, requires the development of more efficient dry-cooling solutions. A novel modular design, air-cooled condenser is proposed in this work. The condenser concept provides potential to increase plant efficiency whilst also reducing the capital cost of the condenser. A thermodynamic model was developed to predict the influence of the condenser concept on the output power of a 50MW CSP plant. The model showed the capability of the concept to constantly optimise plant efficiency for any condenser size or ambient temperature. While this would require tight fan-speed control, it offers a significant improvement on the current state-of-the-art. The air-side heat transfer is the dominant thermal resistance in an air-cooled condenser. Therefore the model relies on accurate airside heat transfer input data. The primary focus of this work was on the air-side design of the condenser and understanding the mechanisms leading to enhanced heat transfer.

Air-side heat transfer and aerodynamic measurements were performed on various circular finned tube heat exchangers in a wind tunnel facility to provide a benchmark in performance under standard flow conditions. The heat transfer measurements confirmed that a global reduction in heat transfer occurs as the number of tube rows decreases. For each design, the aerodynamic and heat transfer measurements were thoroughly compared to existing correlations. In the cases where good agreement was found, the appropriate correlations were recommended for application in the thermodynamic model. In the alternative cases, new correlations, or expansions to existing correlations, were proposed. The effect of grid generated free stream turbulence on the heat transfer characteristics of a single row heat exchanger was also investigated. For turbulence intensities of 8.3% and 24%, mean enhancements in Nusselt number of 6.4% and 11% were measured. Flow visualisation measurements demonstrated the turbulent flow structures responsible for the heat transfer enhancement.

The performance of three condenser designs were also investigated in a full-scale test facility. Despite the inherently different inlet flow fields associated with forced and induced draft fan configurations, heat transfer measurements on a six row, and a four row, circular finned tube design showed negligible change in heat transfer. This work validated the use of a number of correlations for forced draft flow fields. Measurements on an elongated plate-finned tube condenser design were also performed. These measurements validated the use of the method of correlating developing, and fully developed, flow asymptotes in fan generated flow fields. A number of suggestions were made with regard to the appropriate blending parameters which should be applied for the most accurate prediction of flow and heat transfer characteristics. These measurements also showed that for a fan-to-heat exchanger area ratio of 0.317, the minimum fan-to-heat exchanger spacing which should be considered is .44dfan.

Due to the lower capital cost associated with shallow circular finned tube designs, a techno-economic analysis demonstrated the potential of such a design to enhance the performance of the CSP plant over a fixed lifetime. On this basis, an investigation was performed to determine the heat transfer characteristics of a single row, and a two row, circular finned tube bundle under forced draft conditions. The study showed that under the appropriate conditions, the air-side heat transfer of a single row heat exchanger could be increased by up to 30% and a two row heat exchanger by 12%. The effects of the fan-to-heat exchanger spacing and the fan-to-heat exchanger area ratio were also investigated. For the lowest fan-to-heat-exchanger area ratio, σf-he =0.11, a strong dependency on the fan-to-heat exchanger spacing was found. Flow field measurements confirmed that this was due to the increase in the uniformity of the flow distribution. As the area ratio increased, less dependence was found on the fan-to-heat exchanger spacing. The study showed for fan-to-heat exchanger area ratios greater than σf-he = 0.22, negligible deviations in heat transfer occurred. New heat transfer correlations for a single row, and a two row, circular finned tube heat exchanger under forced draft flow conditions were proposed.

The forced draft correlations were integrated with the thermodynamic model and it was demonstrated that the heat transfer enhancements under forced draft conditions corresponded to an increase in plant efficiency. Overall, the techno-economic analysis showed that the two row circular finned tube condenser design provides the lowest LCOE (Levelised Cost Of Electricity) for a given capital expenditure. The four row design and the rectangular plate finned design had an LCOE approximately 0.6% higher for the same capital expenditure.