Solar pond powered liquid desiccant evaporative cooling

Liquid desiccant cooling systems (LDCS) are energy efficient means of providing cooling, especially when powered by low-grade thermal sources. In this paper, the underlying principles of operation of desiccant cooling systems are examined, and the main components (dehumidifier, evaporative cooler and regenerator) of the LDCS are reviewed. The evaporative cooler can take the form of direct, indirect or semi-indirect. Relative to the direct type, the indirect type is generally less effective. Nonetheless, a certain variant of the indirect type - namely dew-point evaporative cooler - is found to be the most effective amongst all. The dehumidifier and the regenerator can be of the same type of equipment: packed tower and falling film are popular choices, especially when fitted with an internal heat exchanger. The energy requirement of the regenerator can be supplied from solar thermal collectors, of which a solar pond is an interesting option especially when a large scale or storage capability is desired

Chemical Looping Electricity Storage

Developing grid-scale energy storage technologies is the key element for broader deployment of renewable sources of energy. This is due to bench-mark technologies like pumped hydro and compressed air being geographically restricted, i.e. require large reservoirs to store air/water, new storage solutions must be found. Hydrogen and pumped thermal storage have emerged as options, but hydrogen suffers from low round-trip efficiency and pumped thermal has a relatively low capacity. In a pumped thermal electricity storage system, electricity is converted to heat using either an electrical heater or a heat pump. This heat is stored and converted back to electricity using a heat engine. In these storage schemes, heat is mostly stored as sensible heat which leads to a low storage capacity. In this regard, this Dissertation examines a simple cycle which makes use of a thermo-chemical store, with a view to achieving high storage capacity by using the chemical looping concept. To do so, a simplified model of a packed bed reactor is developed enabling faster analysis of different layouts and materials. The objective is to find layouts and material properties that are optimising the performance of the storage system including round-trip efficiency and capacity. Round-trip efficiency describes the proportion of electricity put into the storage system during charge returned to the user during discharge. Capacity specifies the size of the storage system and is generally proportional to the capital cost. Results show that a Chemical Looping Electricity Storage (CLES) system can achieve a high capacity, in the range of 250-350 kWh/m3, second only to hydrogen electricity storage systems. Its round-trip efficiency (40-55%) is potentially higher than that of the hydrogen electricity storage systems. By achieving a higher capacity than pumped thermal energy storage and higher round-trip efficiency than that of hydrogen systems, CLES has the potential to fill out the gap between these two grid-scale storage technologies. Thus, this system may play an important role in our future energy mix. In early schemes, a heat pump is employed to convert electricity to heat, but its operating temperature is limited and only those solid oxides capable of releasing oxygen at low temperatures (below 900 K) are feasible. Therefore, ways of using materials with a higher decomposition temperature, i.e. the commonly used materials in chemical looping systems, are investigated. Two methods are proposed: using a vacuum pump to reduce the charging pressure or an electrical heater to increase the charging temperature. Results show using a vacuum pump to be infeasible whereas a simplified charging cycle only comprising of an electrical heater and a recuperator is deemed optimal. This system capacity can be as high as 600-800 kWh/m3 with round-trip efficiency in the range of 40-55%. Finally, a detailed model of a packed bed reactor is developed to study the performance of the CLES for one cycle of charge and discharge. This model of the packed bed is better equipped to capture the transient response of the system to changes in the operating parameters. Previous findings showed that a combination of manganese and copper oxide may have high potential. This stage led to three important outcomes and helped redirecting the future work. First, it showed that the initial simple model of the system is able to capture the dynamic nature of the system to an acceptable degree and therefore it can be used for further investigation of other materials. Second, it showed that a mixed oxide performance can be explained by its constituents. Copper manganese oxide capacity was higher than that of the manganese oxide and lower than copper oxide. Similarly, its efficiency was higher than that of the copper oxide and lower than manganese oxide. This is important because it helps directing the future search for potential materials best suited for electricity storage. Third, the practicality of the concept is ensured by studying the system with more accurate material properties, e.g. reaction kinetics, and a transient model of the reactor

Comparative analysis of power augmentation in air bottoming cycles

The air bottoming cycle (ABC) is a proposed plant configuration in which the steam turbine bottoming cycle in conventional combined power plants is replaced by another gas turbine cycle. Nevertheless, ABC's relatively low efficiency reduces the likelihood of having an ABC power plant in the near future. In this research work, steam injection in the topping cycle combustion chamber and supplementary firing are recommended to improve ABC's performance. Three different configurations of ABC, including simple ABC, ABC with steam injection, and ABC with supplementary firing, are investigated. A thermo-economic analysis is performed to study the effects of the proposed power augmentation approaches thermodynamically and economically. Moreover, optimisation is carried out with the objective of minimising the total cost of the plant for different configurations. Furthermore, a multi-objective optimisation is performed and the results are presented to further understand the trade-off between higher efficiency and lower operating cost

Thermo-economic optimization of air bottoming cycles

In this work a thermo-economic optimization analysis is performed on two air bottoming cycle (ABC) configurations with and without intercooler in the bottoming cycle. Thermo-economic optimization modeling is developed and the effect of the mass flow rate ratio of bottoming cycle air mass flow rate with respect to the topping cycle air mass flow rate is examined in terms of both ABC plant efficiency and total operation cost

Limits of performance of chemical looping air separation in packed bed coupled with electricity production

Oxyfuel combustion, as a carbon capture method, requires oxygen to be separated from nitrogen. Currently, cryogenic air separation is used for this purpose. An alternative is to use chemical looping, where an oxygen carrier is cycled between reducing and oxidising conditions. In this paper, the feasibility of using packed bed reactors in chemical looping air separation is studied. By introducing a compressor and turbine, oxidisers can be operated at an elevated pressure and the proposed scheme can be viewed as a power-station in its own right, in addition to its more recognised application of producing oxygen for a downstream oxy-fuel combustion. A single packed bed reactor is not able to meet the 0.30-0.35 oxygen molar fraction needed for an oxyfuel combustor. Therefore, multiple beds must be used in series or heat must be added radially along the bed length to increase the oxygen molar fraction in the bed.EPSR

Theoretical and experimental study for shortening laser pulse width by pinhole plasma shutter

In this article, a theoretical model is presented to calculate the laser clipped pulse temporal width by the pinhole plasma shutter, and then the model results are compared with the experimental results of CO2 laser clipped pulses by aluminum and copper pinhole plasma shutters. In this model, it is assumed that the laser clipped pulse width is approximately equal to the sum of the plasma formation time and the plasma propagation time in order to reach from pinhole edges to the pinhole center. Furthermore, we assume that the plasma formation time is approximately equal to the time for the surface temperature of pinhole metal plate to reach the boiling point by absorbing the laser pulse energy. Heat conduction equation is used to calculate the time of plasma formation, and Taylor-Sedov’s model is used to calculate the plasma propagation time to reach the pinhole center. By these assumptions, a relationship has been established between the laser clipped pulse width on the one hand, and thermo-dynamical and optical parameters of plasma shutter and the involved laser optical parameters on the other. Results of this model are in good agreement with experimental results