• Energy Research

Abstract In arid areas, dry cooling technology is a preferable alternate of wet cooling mainly owing to the scarcity of abundant water supply. However, the supercritical CO2 power cycle still offers considerable thermal performance even at higher ambient temperature using dry cooling. The novelty of this work is the exhaustive designing of dry cooler for supercritical CO2 cycles (recompression and partial cooling) in concentrating solar power application. Prior to the design of tower, a preliminary analysis is conducted in achieving the optimum main compressor inlet temperature (33 °C-recompression and 40 °C-partial cooling) at which the cycle delivers the maximal efficiency. The comparison is performed at same higher and lower pressure and for the partial cooling, the intermediate pressure is optimized. At relatively higher compressor inlet temperatures (above 50 °C), the partial cooling achieves higher efficiency while at lower temperatures (30–49 °C), the recompression shows superior performance. An iterative nodal method is used for the air-cooled finned tube heat exchanger units that takes account of the dramatic variation in thermodynamic properties of CO2 with the bulk temperature. Kroger’s detailed methodology of designing dry cooler is adapted with the implementation of nodal approach for CO2 property variation. A dry cooling tower with 52.45 m height is essential for the recompression cycle, whereas the partial cooling requires two towers of the height of 35.4 m and 38.7 m. A thermal assessment is carried out on the dry cooler under various cycle fluid inlet temperatures and ambient temperatures. During hot and humid ambient conditions, lower compressor inlet temperatures (up to 53.1 °C) are obtained with the recompression cycle compared to partial cooling (up to 64.5 °C). In extreme climate condition of 50 °C air temperature, the recompression cycle provides superior thermal efficiency (46.5% against 45.5%). For future commercialization of dry cooled sCO2 power plant, the recompression cycle is preferred due to its superior performance and lower capital cost for cooling tower design and solar field. The work demonstrates the impact of dry cooling tower design strategy in the context of cycle thermal assessment under various working condition.

The thermal performance of a supercritical CO2 (sCO2) recompression cycle is expressively influenced by main compressor inlet temperature. Design of the cooling system is imperative since the compressor inlet temperature substantially influence the system performance. Due to nonlinear variation of both thermal and transport properties of the CO2 under critical condition, the cooling tower design and selection for the sCO2 cycle power plant is quite different from the power plants with steam cycle. The present work comprehensively investigates the effect of cooling system design on the optimal cycle performance under different operating condition. An iterative section method is applied while designing and optimizing the air-cooled heat exchanger bundles inside the tower. Prior to the design of natural draft dry cooling tower (NDDCT), an optimal operating condition is rectified at which the cycle efficiency is maximal. The tower performance is investigated by demonstrating unit height heat rejection and average heat rejection by each heat exchanger bundle. A detailed economic analysis of NDDCT is performed which takes account of capital cost, maintenance cost, annual cost, and specific investment cost. The thermo-economic assessment of the NDDCT is conducted by the influence of sCO2 inlet temperature inside the tower and variation of ambient air.

Abstract Supercritical carbon dioxide (sCO2) based Brayton cycle integrated with concentrated solar power applications is a promising technology to exploit solar energy for electricity production. To reduce the energy cost of this solar power plant, spray-assisted dry cooling technology is developed, which makes electricity more affordable for isolated and arid regions. However, pure dry cooling technology suffers from low efficiency under high ambient conditions and a spray cooling system has been proposed to address this problem. Due to the high cost and great complexity, experimental test of a designed spray cooling system on a natural draft dry cooling tower is never reported. Here a spray cooling system consisted of multiple nozzles was tested on a 20 m high experimental tower. This is, to our knowledge, the world’s first attempt to practice spray enhancement of NDDCT at full scale. It is found that the introduced spray cooling can effectively precool the inlet hot air and consequently reduce the circulating water exit temperature. The promising application of this new technology in solar power plants was firstly revealed by integrating the tower into a 1 MW concentrated solar thermal sCO2 Brayton cycle. As spraying water flowrate increases, cooling tower dissipates more waste heat, lowering the compressor inlet temperature and consequently improving the efficiency of thermal cycle. Power cycle simulations also show that cycle efficiency can be higher than 40.5% at the optimal circulating water flow rate, i.e., 4–5 kg/s, depending on the sCO2 flow rate.

Abstract Crosswind is a significant concern for natural draft dry cooling towers. The concern is more serious for shorter towers. Therefore, the crosswind influence is a significant threat to the use of natural draft dry cooling towers in concentrating solar thermal power plants, which are generally built at sizes smaller than conventional fossil-fired plants and employ relatively shorter towers. While some numerical studies and small lab-scale test reports exist, very few full scale experimental studies have been reported for conventional cooling towers and none for relatively short cooling towers suitable for renewable thermal power plants. To address this gap, a 20-m tall fully instrumented natural draft dry cooling tower was built by the University of Queensland. The tower was designed to serve a future 1-MWe concentrating solar thermal plant on the same site. Its performance was tested under different ambient temperatures and crosswind speeds. The detailed experimental data of the crosswind condition, air temperature distribution inside and outside of the cooling tower and the cooling performance are presented. The experimental data demonstrate the substantial yet complex impact of the crosswind on cooling tower performance. Significant non-uniformities in air and hot water temperature distributions and strong air vortices inside the tower were observed in high crosswind speeds. Unlike tall cooling towers used in large conventional plants, the cooling tower performance does not monotonously decrease with the increase of the crosswind speed. In fact, after the tower performance drops to its lowest level at a wind speed around 5 m/s, the trend is reversed and further increases in the crosswind speed help the tower performance. Analysis shows that this reversal occurs because the tower heat transfer mechanism changes. As crosswind rises above the critical speed, the airflow inside the cooling tower becomes increasingly controlled by the crosswind instead of the natural draft.

Abstract The dry cooling unit is integrated with a 25 MW recompression super-critical CO2 (sCO2) cycle for solar energy application. Based on the optimal operating condition, a natural draft dry cooling tower (NDDCT) is modelled for an ambient temperature of 20 °C and sCO2 inlet condition of 67 °C and 7.96 MPa. The Kroger’s detailed working principle of NDDCT is adapted in the simulation processes. The sCO2 property variation with the change of bulk temperature is considered while modelling the heat exchanger unit inside the tower. The draft equation of the tower is solved by including various air-flow resistances at different parts of the tower. The heat input to the cycle is assumed to be supplied by the solar field with sufficient capacity of thermal storage. The seasonal effect on the performance of a dry-cooled sCO2 recompression cycle is performed in Alice Spring, NT, Australia by using the daily meteorological data. The daily net power generation fluctuates in the range of 2.4% to 22.4% of the design value for the consecutive days. The year-round mean net power generation is 24.66 MW which corresponds to 50.9% cycle efficiency. The weekly and monthly seasonal variation of the plant performance in summer, autumn, winter, and spring is performed based on the mean maximum and minimum temperature data. The mean net power produced in summer and winter is 22.9 MW and 26.4 MW respectively. The fluctuation of heat input to the cycle, heat rejected by the cycle and air mass flow rate in NDDCT are demonstrated. Seven performance indicators (Thermal efficiency, exergetic performance criteria, exergy efficiency, maximum available work, ecological coefficient of performance, cooling efficiency, and ecological function criteria) are determined to evaluate the plant performance for the period of 1941 to 2018 using the mean temperature data.

Abstract The aim of this study is to investigate the off-design performance of an air-cooled supercritical carbon dioxide recompression Brayton cycle for concentrating solar thermal power generation. Off-design component models were developed in a system modelling framework. The components were designed for 25 MWe net power generation at ambient temperature 30 °C, with design point cycle thermal efficiency of 46.2%. The off-design performance was investigated for a range of heat source temperatures, ambient temperatures and cycle mass flow rates. Key elements of the off-design control scheme used are independent compressor shaft speeds, fixed low side pressure (assuming inventory control), and fixed turbine speed (for synchronous operation). The cycle can maintain nominal net power generation at 50 °C ambient temperature with increased cycle mass flow rate and turbine inlet temperature. At design point turbine inlet temperature and mass flow rate, net power generation decreases by approximately 10% for each 10 °C increase above the design point ambient temperature. The high design point ambient temperature limits the beneficial effect low ambient temperature. The effect of decreasing the design point ambient temperature was investigated. While this allows higher peak cycle efficiency, it also leads to much greater deterioration of cycle efficiency with increasing ambient temperature.

Abstract Concentrating solar thermal power system can provide low carbon, renewable energy resources in countries or regions with strong solar irradiation. For this kind of power plant which is likely to be located in the arid area, natural draft dry cooling tower is a promising choice. To develop the experimental studies on small cooling tower, a 20 m high natural draft dry cooling tower with fully instrumented measurement system was established by the Queensland Geothermal Energy Centre of Excellence. The performance of this cooling tower was measured with the constant heat input of 600 kW and 840 kW and with ambient temperature ranging from 20 °C to 32 °C. The cooling tower numerical model was refined and validated with the experimental data. The model of 1 MW concentrating solar thermal supercritical CO 2 power cycle was developed and integrated with the cooling tower model. The influences of changing ambient temperature and the performance of the cooling tower on efficiency of the power system were simulated. The differences of the mechanism of the ambient temperature effect on Rankine cycle and supercritical CO 2 Brayton cycle were analysed and discussed.

Abstract The purpose of this paper is to model an Enhanced Geothermal System (EGS) power plant using an Organic Rankine Cycle (ORC) cooled by a Natural Draft Dry Cooling Tower (NDDCT) and to investigate the influence of the variation of performance of the NDDCT due to changing ambient temperature on cycle performance. The ORC used in this work is the supercritical butene recuperated ORC. The EGS heat source conditions used are those found at the Habanero 1 MW pilot plant in South Australia, with geothermal brine inlet temperature of 220 °C, minimum brine temperature of 80 °C, and brine mass flow rate of 35 kg/s. A one dimensional NDDCT model was developed and integrated into the cycle model, enabling a novel method of coupled analysis of ORC and NDDCT interdependence, which allows analysis of plant performance for varying ambient temperature. The analysis finds that annual average W net is 2.82 MWe, the typical daily range of W net is 0.62 MWe (±11%), the typical change in W net , mean for consecutive days is 0.07 MWe (3%), and the largest is 0.5 MWe (20%). The maximum range at any given time throughout the year, based on historical temperature data extremes is ±31%, but the typical expected range ±10%.