Dynamic Model Development of Linear Fresnel Solar Field
Authors: Hakkarainen Elina , Tähtinen Matti , Mikkonen Hannu 
Published: October 2015

As a dispatchable clean energy source, concentrated solar power (CSP) can be one of the key technologies to overcome many problems related to fossil fuel consumption and electricity balancing problems. Solar is a variable location, time and weather dependent source of energy, which sets challenges to solar field operations. With proper dynamic simulation tools it is possible to study dynamics of CSP field under changing weather conditions, find optimum control strategies, and plan and predict the performance of the field.

CSP technology considered in this paper, linear Fresnel reflector (LFR), is a proven line focusing technology, having simpler design but suffering in optical performance compared to more mature parabolic trough (PT) technology. Apros dynamic simulation software is used to configure and simulate the solar field. Apros offers a possibility to dynamically simulate field behavior with varying collector configuration, field layout and control mode under varying irradiation conditions. The solar field applies recirculation (RC) as a control mode and direct steam generation (DSG) producing superheated steam. DSG sets challenges for the control scheme, which main objective is to maintain constant steam pressure and temperature at the solar field outlet under varying inlet water and energy conditions, while the steam mass flow can vary.

The design and formulation of an entire linear Fresnel solar field in Apros is presented, as well as the obtained control scheme. The field includes user defined amount of collector modules, control system and two modules describing solar irradiation on the field. As two-phase water/steam flow is used, an accurate 6-equation model is used in Apros. Irradiation on the solar field under clear sky conditions is calculated according to time, position and Linke turbidity factor. Overcast conditions can be created by the clear sky index. For LFR single-axis sun tracking system is applied.

In order to test the model functionality and to investigate the field behavior, thermal performance of the field was simulated at different dates at two different locations, and the results were compared. Similar field dimensions and control schemes were applied in each case, and simulations were done for full 24 hours in order to study the daily operations and ensure process stability. Control scheme functionality is evaluated based on the plant behavior in simulation cases having different operational conditions.

The proper operability of the configured LFR model is evaluated. Obtained performance results show differences between locations and variation depending on season and time. The importance of a proper control system is revealed. The results show that the dynamic model development of a solar field is necessary in order to simulate plant behavior under varying irradiation conditions and to further develop optimal field control schemes and field optimizing process. The future work in the development of the LFR model presented will focus on dynamic response behavior development under transient conditions and field start-up and shut down procedure development.