Solar assisted Ultra Supercritical steam power plants with Carbon Capture and Storage

This paper focuses on the evaluation of the potential benefits arising from the integration of concentrating solar systems into coal-based Ultra Supercritical (USC) power plants with Carbon Capture and Storage (CCS). In particular, different solutions for the solar field, based on direct steam generation (DSG) with parabolic trough and linear Fresnel collectors, and for using the steam in the Rankine cycle were analyzed and compared. The comparative analysis was carried out with reference to a USC plant based on a double reheated Rankine cycle with four steam turbines and nine regenerative steam extractions. The USC plant was also integrated with a post-combustion CO2 removal process (USC-CCS) based on chemical absorption with an aqueous solution of MEA. The CO2 removal section requires a large amount of steam to be extracted from the low pressure turbine in order to satisfy the reboiler thermal power requirement, reducing performance of USC plant (about 10.5 percentage points of efficiency reduction for a CO2 removal of 90%). The performance of the reference USC-CCS power plant were evaluated by means of simulation models specifically developed through Aspen-Plus and Gate-Cycle software platforms. In order to offset the efficiency penalty introduced by CO2 removal, the USC-CCS plant was integrated with a concentrating solar field with direct steam generation (DSG) based on parabolic trough and linear Fresnel collectors. The solar field is based on several lines of collectors connected in parallel to achieve the required steam mass flow and therefore the required thermal power output. Owing to the difficulties related to the storage of steam, the presence of a thermal energy storage was not considered. Performance of the solar field were evaluated on a yearly basis by means of a specifically developed simulation model. In particular, performance of collectors were evaluated as a function of solar radiation and solar position, for given values of the main geometrical and technical characteristics of collectors. The study was carried out by using a data set for a typical meteorological year for the site of Cagliari, in Sardinia (Italy). Direct solar energy is available only for a limited number of hours in the year, due to cloudiness and nights; moreover, during most of the solar field operating hours the DNI is below its design value. Therefore, the design conditions of the USC-CCS plant considered here refer to the absence of steam production by the solar field. As a consequence during periods of solar energy availability, steam production from the solar field increases mass flow rate of the steam turbines leading to the off-design operation mode of the USC plant, with a corresponding efficiency penalty. For this reason an in-depth analysis of USC performance due to solar integration was carried out to assess in particular the influence of sliding pressure on steam turbine performance. A preliminary cost analysis was also performed. Overall, integration with concentrating solar collectors allows to reduce USC efficiency penalization due to CO2 capture, assuring an efficiency increase up to 1-2 percentage points, depending on solar field size. On the other hand, solar fields require a very large land availability (about 1-1.5 Km2 for 300 MW of thermal power, depending on collectors technology). The results of the comparative performance assessment demonstrate that owing to their better optical efficiency, the use of parabolic troughs gives better performance than linear Fresnel collectors, even if the latter give the higher energy production per m2 of occupied land.