Contribution to the Technique of Compressed Air Energy Storage

Development of intermittent solar and wind energies and the demand peak increase has made energy storage very important in energy policy. Thus, energy storage will play a key role in enabling the world to develop a low-carbon electricity system. An interesting approach to do so can be storing energy in form of the compressed air. Recently Compressed Air Energy Storage system (CAES) has attracted attentions as a promising technology for energy storage. In the general frame of CAES and after presenting liquid piston, the LEI Laboratory of EPFL has introduced the concept of finned piston. The main goal is to achieve energy storage by means of compressed air thanks to high isothermal efficiency compression/expansion processes. As mentioned, compressed air with a liquid piston and with a performance of nearly 65% has been realized already as a promising solution for the cost-effective small/medium electricity storage applications. As a post-development, and to improve the problems related to the complexity of a liquid piston, a so called "dry finned piston" system was proposed. The effective work for this new system at LEI was be to realize an analytical and a finite element model for such a system. The key development of this task is to solve the problem of non-isothermal behavior of the dry piston systems utilizing a directly integrated exchanger, inside of the compression / expansion chambers. In this regard, theoretical modeling and simulation has been started with a classic reciprocating piston based on principles of mass and energy conservation. The model consists of different subsystems like driver mechanism, cylinder head, valves, heat transfer, etc. This will serve as a basis for developing the more complicated finned piston compressor. Besides, the Finite Element Method (FEM) has been used to account for the detailed local thermal characterization in 3D geometry. Next, analytical modeling has been extended to finned piston. The heat and mass transfer has been accounted for using thermoelectric and pneumatic-electric analogy. Also FEM modeling has been carried out for the finned piston. The analytical model was verified using experimentation. In order to propose an experimental validation, a test bench has been developed for both of the pistons. The experimental results confirms a higher exergetic efficiency for the finned piston due to its close to isothermal behavior thanks to increased heat transfer surface. The last chapter is dedicated to a parametric study to investigate the effect of change of the design parameters on system performance. This thesis open doors toward more investigation to improve the design. The effective tool here is the comprehensive model developed in an innovative way.

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. However, all currently existing solar thermal power plants have been based around the use of Rankine, or steam turbine, cycles which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009

Analysis and Development of a New Compressor Device Based on The New Finned Piston

Mahbod Heidari, Alfred Rufer

In the general frame of Compressed Air Energy Storage system (CAES), the LEI Laboratory of EPFL has introduced the concept of dry finned piston. The main goal is to achieve energy storage by means of compressed air thanks to high isothermal efficiency compression/expansion processes. For achieving this goal, a new compression and expansion machine has been defined, using a new piston-cylinder assembly consisting of a series of concentric annuli’s that fit together during compression/expansion. This will increase the heat transfer surface, allowing the heat to be removed from the system during compression and to be absorbed by the system during expansion. This feature together with low speed movement makes the process close to isothermal. First the concept of annular compression chambers is explained, with the goal of increasing the heat exchange surface. Then the new compression system with imbricated annulis is described. A test bench has been developed in order to propose an experimental validation of such a compression process concept. It is also of a great interest to compare the dry finned piston to the classic pistons available in the market. The experimental results show a higher energetic efficiency for the finned piston due to its close to isothermal behavior thanks to increased heat transfer surface.

2013

Electrical hubs: An effective way to integrate non-dispatchable renewable energy sources with minimum impact to the grid

Dasaraden Mauree, Vahid Moussavi Nik, Amarasinghage Tharindu Dasun Perera, Jean-Louis Scartezzini

A paradigm change in energy system design tools, energy market, and energy policy is required to attain the target levels in renewable energy integration and in minimizing pollutant emissions in power generation. Integrating non-dispatchable renewable energy sources such as solar and wind energy is vital in this context. Distributed generation has been identified as a promising method to integrate Solar PV (SPV) and wind energy into grid in recent literature. Distributed generation using grid-tied electrical hubs, which consist of Internal Combustion Generator (ICG), non-dispatchable energy sources (i.e., wind turbines and SPV panels) and energy storage for providing the electricity demand in Sri Lanka is considered in this study. A novel dispatch strategy is introduced to address the limitations in the existing methods in optimizing grid-integrated electrical hubs considering real time pricing of the electricity grid and curtailments in grid integration. Multi-objective optimization is conducted for the system design considering grid integration level and Levelized Energy Cost (LEC) as objective functions to evaluate the potential of electrical hubs to integrate SPV and wind energy. The sensitivity of grid curtailments, energy market, price of wind turbines and SPV panels on Pareto front is evaluated subsequently. Results from the Pareto analysis demonstrate the potential of electrical hubs to cover more than 60% of the annual electricity demand from SPV and wind energy considering stringent grid curtailments. Such a share from SPV and wind energy is quite significant when compared to direct grid integration of non-dispatchable renewable energy technologies.

Elsevier Sci Ltd2017

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

2009