• Energy Research

Pressure losses on the condensing-side of an air-cooled condenser (ACC) have the potential to inhibit condenser performance and, ultimately, curtail plant efficiency. However, little information is available on the magnitude and effect of these losses in an ACC under typical Rankine cycle operating conditions. This article seeks to improve current understanding on steam-side pressure losses in ACCs by presenting an experimental study on the losses in a full-scale ACC circular tube bundle. Saturated steam at low pressure was condensed by a cross flow of cooling air, provided by a bank of axial fans. Full condensation occurred in all measurements, which were carried-out over a steam pressure and temperature range of approximately 0.05e0.14 bar absolute and 33e55 � C, respectively. These test parameters ensured that measurement program test conditions were representative of those expected in an operational thermoelectric power plant. Experimental mass fluxes, per individual tube, varied from 0.7 to 2 kg/m 2 s during testing. The pressure drop characteristics were, therefore, analysed over a vapour Reynolds numbers range of 1890e5150 and liquid Reynolds number range of 25e95. Results indicate that the measured pressure drop through the tube bundle was relatively small, in the range of 130e250 Pa. As shown in this article, the reason for this was due to momentum recovery as the steam condenses to form liquid condensate. This phenomenon offsets the frictional losses, which are shown to be comparable in magnitude to momentum recovery in a condensing flow. However, this may not always be the case. Therefore, since the frictional component is traditionally the most problematic to predict, a range of liquidegas two-phase frictional pressure drop predictive models were reviewed, and are presented herein. Comparisons between these models and the experimental data show that the most applicable model was found to be that of Lockhart & Martinelli. This demonstrated reasonable accuracy of ±18%. © 2015 Elsevier Ltd. All rights reserved.

Reliable and efficient cooling solutions for portable electronic devices are now at the forefront of research due to consumer demand for manufacturers to downscale their existing technologies. The power required for these technologies now has to be dissipated over smaller areas resulting in elevated heat fluxes. The most popular choice among engineers in terms of cooling solutions is to integrate a fan with a heat sink and for portable electronic devices this involves the use of a low profile solution. In this paper an experimental investigation on the thermal performance of a finned and finless heat sink integrated with an axial fan, for the purpose of cooling a microchip, is presented. The objective is to characterise the performance of each heat sink in terms of thermal resistance and to develop an understanding of the flow structures in such systems. One of the smallest commercially available fans is used in conjunction with each heat sink giving a total footprint area of 465m2 and profile height of 5mm. Thermal resistances are measured over a range of fan speeds and detailed velocity measurements were taken of the flow within the heat sinks using Particle Image Velocimetry (PIV). The thermal analysis results indicate that the thermal resistance of the system is of order 30 deg C/W for both heat sinks. However, the finless heat sink resulted in slightly lower values over a range of intermediate fan speeds. Hence, indicating that the maximum heat transfer density, for a range of fan speeds, can be achieved with a finless heat sink. The results also define the limiting heat fluxes that can be dissipated in low profile miniature applications.

Abstract Economic and environmental restrictions have resulted in an increase in the installation of air-cooled condensers (ACCs) in thermoelectric power plants located in arid regions. The traditional A-frame design is installed most frequently, despite an array of empirical evidence that shows it to suffer from significant inefficiencies. As a result, there is scope for improvement in condenser design and this paper presents one such approach – a novel modular air-cooled condenser (MACC). It is suggested that the unique ability of the MACC to continually vary fan speed could result in efficiency gains over a plant operating with existing state-of-the-art fixed speed ACCs. To determine the impact of installing the MACC on plant output, the steam-side characteristics were established through a series of experimental measurements taken on a full-scale prototype. The experimental arrangement and measurement technique ensured that conditions representative of an operational ACC were maintained throughout. The steam-side characteristics are quantified in terms of temperature, pressure and thermal resistance. Predicted values of these quantities are also presented, calculated from established theory. Both the measurements and predictions were used in a thermodynamic analysis to determine the performance of a 50 MW power plant. Results show that, for a given steam flow rate, increasing fan speed leads to a reduction in condenser pressure which ultimately, results in increased plant output. This occurs up until a certain point, at which further increases in output are offset by larger fan power consumption rates. Thus, an optimum operating point is shown to exist. The results from the thermodynamic analysis demonstrate discrepancies between the plant output evaluated from the measurements and that predicted from theory. In some cases, a difference as large as 1.5% was observed, equating to a 0.8 MW over-prediction by the theory.

Diminishing fossil fuel reserves and a growing collective environmental awareness has led to the development of alternative methods of power generation such as Concentrated Solar Power (CSP). Although almost all existing CSP plants currently use water-cooled condensers, limited water supplies in the designated desert regions for such power plants, the high costs associated with providing cooling water and environmental considerations will all restrict the future use of water-cooled condensers. Air-cooled condensers (ACCs) are therefore proposed, despite evidence to suggest that they suffer from significant inefficiencies [1]. It has been suggested that a modular design, addressed in this paper, could offer solutions to issues with current ACC technologies. To fully characterise the modular ACC design it is necessary to quantify the steam-side characteristics. A series of tests were performed under vacuum conditions representative of an operational condenser. The condenser vacuum was measured for a series of incremental fan rotational speeds, to determine both the qualitative and quantitative relationship between fan speed and condenser pressure. Results indicate that for a given steam mass flow rate, the condenser pressure decreases with increasing fan rotational speed. Furthermore, the choice of vacuum pump, used to displace air leakages, was shown to have a significant influence on the steam-side response. Larger displacement-capacity vacuum pumps permit lower condenser pressures. The steam condensation pressure drop through the condenser tubes was also measured. Results for the measured pressure drop revealed a large level of momentum recovery, which is not uncommon in steam condensation processes. Experimental frictional pressure drops were determined and these compared favourably with certain two-phase frictional pressure drop correlations. In particular, the Lockhart & Martinelli correlation was found to be most capable of predicting the frictional pressure drop trends encountered during testing. The large level of agreement between the measurements and predictions provide confidence in future use of the Lockhart & Martinelli correlation to predict frictional pressure losses.

Abstract The receiver in a concentrated solar power (CSP) tower system accounts for a considerable proportion of plant capital costs, and its role in converting radiant solar energy into thermal energy affects the cost of generated electricity. It is imperative to utilize a receiver design that has a high thermal efficiency, excellent mechanical integrity, minimal pressure drop, and low cost in order to maximize the potential of the CSP system. In the present work, thermal, mechanical, and hydraulic models are presented for a liquid tubular billboard receiver in a representative CSP plant. A liquid sodium heat transfer fluid as well as a number of receiver configurations of heat transfer area, tube diameter, and tube material have been analysed. The thermal analysis determines tube surface temperatures for an incident heat flux, thereby allowing for the calculation of thermal losses and efficiency. The mechanical analysis is carried out to establish creep deformation and fatigue damage that the receiver may undergo through a life service. The hydraulic analysis is concerned with calculating the required pumping power for each configuration. Results show that thermal efficiency increases for a decreasing heat transfer area, however reducing receiver area comes at the penalty of increasing tube surface temperatures and thermal stresses. The selection of tube diameter is critical, with small diameters yielding the greatest thermal efficiency and mechanical life, however the increased pressure drop reduces the overall plant efficiency due to a necessary increase in pumping power. The optimum receiver configuration is established by finding an appropriate trade-off between thermal performance, service life, pressure drop, and material costs, by using the levelized cost of electricity (LCOE) as the objective function. The analysis highlights necessary trade-offs required to optimise the design of a solar receiver.

Abstract Theoretical modelling techniques are used to compare the thermohydraulic performance and thermal storage characteristics of molten salt, liquid sodium, and lead-bismuth in a CSP solar receiver concept. For molten salt, the performance of a number of heat transfer augmentation techniques are also studied. Sodium and lead-bismuth both yield excellent receiver thermal efficiency (max ∼92%), when compared to molten salt (max ∼90%), due to high thermal conductivity values that lead to large heat transfer coefficients. A high pressure drop penalty for lead-bismuth largely offsets its thermal performance gain over molten salt, however sodium retains its advantage as a receiver working fluid with a low pumping parasitic. The implementation of heat transfer enhancement techniques can significantly improve the performance of a molten salt receiver when compared to smooth tube designs. The low specific heat capacity and high unit cost of lead-bismuth is prohibitive towards its use as a storage medium in storage-integrated plant designs, resulting in very high LCOE values. Sodium is the most economically feasible fluid for systems with low storage (

Abstract The nature in which a solar receiver in a concentrated solar power plant interacts with an accompanying heliostat field plays a significant role in plant performance and economics. An appropriate heat flux distribution should help deliver maximum receiver thermal performance, while minimising mechanical damage – thereby maximising power production and reducing costs. The current work presents an investigation into the thermal performance and mechanical reliability of a sodium-cooled solar receiver operating under heat flux profiles generated by a novel heliostat aiming strategy. A modification of the HFLCAL model is used to generate heat flux profiles for individual heliostats in a representative plant, and simulated annealing optimisation techniques are used to produce a novel heliostat aiming strategy. The importance of giving consideration to receiver limitations under non-uniform thermal boundary conditions in the development of a heliostat aiming strategy is demonstrated in this study, with mathematical optical, thermal, and mechanical models used to complete the analysis. An investigation has been conducted for a point-in-time resulting in maximum thermal loading conditions, with theoretical modelling techniques used to calculate receiver tube temperatures for aiming strategy yielded heat flux profiles, thereby allowing for the determination of heat losses and mechanical reliability through creep-fatigue damage. Results show that the simulated annealing algorithm can significantly improve heat flux homogeneity on the receiver, potentially reducing peak heat flux to less than 10 % that of a single aiming point strategy, given an appropriate spillage allowance and aiming point grid size. A satisfactory configuration of spillage allowance and aiming grid size exists so as to supply maximum power to the receiver, while uniformly distributing the incident heat flux in order to meet mechanical reliability requirements. Based on the receiver design and conditions simulated in the analysis, a grid constructed of more than 81 aiming points (receiver area coverage of 32.7 % ), and an additional spillage allowance of 10 % allows the receiver to deliver maximum power output while retaining mechanical durability through a 30 year plant life cycle.

Abstract This paper presents the essentials of low temperature thermal storage (LTTS), a novel technique whereby thermal energy storage is employed to achieve sub-ambient condensation in air-cooled Rankine cycle power plants. It summarises work which was undertaken to explore the potential and the range of application of LTTS. The technology is most effective at geographical locations with large average daily temperature ranges, and high summertime temperatures. Hourly normal temperature data was sourced for five potential deployment sites, which provided a representative sample of different climate types. A steam turbine, a condenser, an air-cooled heat exchanger, and a chilled water thermal energy storage tank formed the LTTS configuration – a techno-economic model of which was developed to simulate system behaviour. The size of the air-cooled heat exchanger, the fan speed of the air-cooled heat exchanger, and the hours of charge, discharge, and bypass of the thermal energy storage tank were all modelled as variables to determine the effects of component sizes and operating patterns. LTTS performance was benchmarked by comparison with a direct air-cooled condenser model. Results presented in this paper include daily plant output, annual power output, and payback period. This study shows that LTTS can deliver all the advantages of dry-cooling, without suffering the usual performance degradations. The inherent flexibility of LTTS allows for configurations to be customised to exploit the prevailing site climate, and capitalise on the local power demand pattern. There are also clear indications that, in suitable climatic settings, LTTS outperforms traditional air-cooled thermal power plants by offering up to 10% additional generating capacity. Coupled to this are payback periods as short as 2.5 years, ensuring LTTS can be considered a viable alternative to current air-cooling strategies.

AbstractThis paper aims at developing a novel air-cooled condenser for concentrated solar power plants. The condenser offers two significant advantages over the existing state of the art. Firstly, it can be installed in a modular format where pre-assembled condenser modules reduce installation costs. Secondly, instead of using large, fixed speed fans, smaller speed controlled fans are incorporated into the individual modules. This facility allows the operating point of the condenser to change in order to maximise plant efficiency at all times. Two condenser designs were proposed and full-scale prototype modules were manufactured for experimental testing. Both designs comprise a bank of circular finned tubes, coupled to an array of axial fans. The air-side thermal and aerodynamic characteristics were measured using a purpose built steady-state test facility. The measurements were correlated and compared well with existing correlations in the relevant literature, thus validating their use in thermodynamic models to predict power plant performance. A plant performance analysis was conducted combing the experimental measurements with output data from a 50MW concentrated solar power plant. This analysis firstly verified that the plant efficiency can be continually maximised by altering the condenser fan speed. Furthermore, under certain conditions plant output can be increased by over 10% by varying fan speed by 10% of the full speed. The study also provides a comparison of the two designs in terms of their effect on the plant output power. With respect to the thermodynamic efficiency and cost-efficiency, the four row condenser substantially outperforms the initial six row design.