Thermo-Economic Optimization of a Tower-Trough Combination Thermal Solar Power Plant

Current energetic context involving skyrocketing oil prices, population growing rates without precedents and increasing concerns about nuclear energy and climate change issues requires rapid, economically feasible and reliable progress in the field of renewable energies. We have the responsibility today of building the energetic scheme of tomorrow. Amongst renewable energies, thermal solar energy has already proven both reliable and competitive with adequate feed-in tariffs and offers some key advantages such as the possibility to store the energy as well as the good controllability of the transformed energy that translates into a great connectivity to the grid. The goal of this project is to perform a thermo-economic analysis and optimization of a solar thermal power plant combining two existing technologies, namely a tower and a parabolic trough power plant. To do so, accurate and reliable modelling of both power plants has been necessary. For the energyintegration, software developed at LENI (EPFL) has been used such as Osmose or Energy Technologies [2]. For the multi-objective optimization MOO [3] software, a LENI tool as well, has been used. Trough–tower combination power plants, when properly integrated, present higher energy efficiencies than those of either the tower or the trough power plants individually. Large power plants (around 80 MW) can be thus imagined (as large as current trough power plants) yet featuring an energy efficiency (17%) even higher than current state-of-the art tower power plants and similar levelised energy costs (0.08 USD/kWh) to typical values of trough power plants. These results represent another step forward for concentrating solar power, technology that is certainly called to play a major role in tomorrow’s energetic scheme.

Thermo-economic optimisation of a tower/trough combination solar power plant

Jorge López Moreno

Today a crude oil barrel has five times the prize it had fifteen years ago, moreover, we have increased our global energy consumption by almost 40 percent since then and population growing rates are higher than ever before. In addition, recent tragic events in Fukushima nuclear power plant, Japan, earlier this year, have increased concerns even more about energetic schemes based on nuclear energy. This challenging energetic context requires rapid, economically feasible and reliable progress in the field of renewable energies. We have the great responsibility today of building the energetic scheme of tomorrow. Amongst renewable energies, thermal solar energy has already proven both reliable and competitive with adequate feed-in tariffs and offers some key advantages such as the possibility to store the energy as well as the good controllability of the transformed energy, which entails a great connectivity to the grid. The goal of this project is to perform a thermo-economic analysis and optimization of a solar thermal power plant combining both a tower and parabolic trough power plants and to find out whether or not the combination of these two (with different temperature levels) is advantageous from an energetic or an economic point of view. This idea is fairly innovative and no previous studies have been found on this particular research line. A new, detailed model has been developed for the trough power plant not only for the thermal side but also for the hydraulic and economical side. This model has been then combined with an existing model for the tower power plant. Afterwards, a performance analysis and a multi-objective optimization of the combination have been carried out. Energetic integration, modelling of the power cycles and multi objective optimization all have been performed using LENI tools to analyze and optimize energetic systems such as Energy Technologies and MOO. Trough – tower combination solar power plants present several advantages both from an energetic and economic perspective. Combination power plants feature important efficiency gains compared to either the tower or the trough power plant individually. This efficiency gain comes from the sake of combining both types of power plants, allowing better cycle efficiencies. Efficiency of these combined trough- tower power plants can reach 18.7% (solar to electricity) for certain combinations, with a levelised energy cost of 15 UScts/kWh. Other interesting combinations have been pointed out, for instance, an 80 MW combined power plant, with an efficiency of 16.9% and a levelised energy cost of 8.8 UScts/kWh which is fairly impressive compared to the current solar thermal power plants. Many different interesting combinations can be imagined, each one offering particular advantages. This project confirms that trough – tower combination, when properly integrated, result into more efficient, with a lower levelised energy cost with respect to tower schemes and larger power plants than either tower or trough schemes separately. This new possibility definitely contributes to make solar thermal power an even more promising and interesting technology.

2011

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

Preliminary Study of a Solar Trough/Tower Power Plant

Jorge López Moreno

The possibility of combining both the advantages of a solar tower thermal power plant and of a solar parabolic trough power plant could be interesting from an energetic point of view as well from an economic point of view. The preliminary study of this combination feasibility is the goal of this project. The study of the project viability consist first of all on the modeling of the trough system, the energy balance of the parabolic troughs will be established at first. Secondly, and choosing a perfect solar day simulating a day when the solar radiation is rather stable, with no cloudy periods along the day, the power available from the trough plant can be calculated. Once we have estimated the power that we can introduce into the cycle of the tower plant, the main issue consists on how and where it is better to use this energy in the cycle. Several possibilities seem to be interesting a priori, however a more detailed study of each one is to be done in order to get to the most economic and efficient layout. The work related to the tower thermal power plant will be done taking as starting point the master thesis of James Spelling, who did an incredible work in this field and whose work will be a valuable help to continue the research on the proposed concept in this project of combining two plants, tower and trough. The main advantage of the tower design compared to the parabolic trough design is the higher temperature. As it will be explained further, thermal energy at higher temperature can be converted more efficiently, the exergy efficiency is better. In terms of storage, the high temperature results in a cheaper and a more efficient storage system. Nevertheless, the set up of the field of heliostats is more complex than the set up of the parabolic trough lines even though the area for a trough plant must be previously flattened. The lower temperature level that entails an inconvenient itself for a parabolic trough power plant may become an advantage when combining both types of concentrating solar power plants. If the results are positive enough that it seems interesting to combine both types of power plants, another step forward will be done towards the use of the sun energy as a renewable source thus making it a more interesting choice for power generation. Sun potential has already shown as an important technology not only at the present time but also in the future. Commercial solar plants have already achieved levelized energy costs around 12-15 UScts/kWh, and the potential for cost reduction is expected to follow this trend even further, to costs as low as 5 UScts/kWh. In addition the taxes imposed for the non-green power should make even more interesting the use of concentrating solar power plants. The requirements of high beam normal radiation lead to a limited amount of suitable places where the implantation of this technology is possible. However many places exist on Earth where the conditions are perfectly adapted, we talk about regions such as United States (the country with more carbon dioxide emissions in the world), some regions in Mexico, North Africa, India, Australia, northeastern Brazil and some locations around the Mediterranean, especially in the south of Spain. The challenge is therefore set for a change in our energy production scheme.

2010

Thermo-economic optimisation of a tower/trough combination solar power plant

Jorge López Moreno

2011

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

2009

Preliminary Study of a Solar Trough/Tower Power Plant

Jorge López Moreno

2010