Numerical Investigation of Pipelines Modeling in Small-Scale Concentrated Solar Combined Heat and Power Plants

Abstract

In this paper four different detailed models of pipelines are proposed and compared to assess the thermal losses in small-scale concentrated solar combined heat and power plants. Indeed, previous numerical analyses carried out by some of the authors have revealed the high impact of pipelines on the performance of these plants because of their thermal inertia. Hence, in this work the proposed models are firstly compared to each other for varying temperature increase and mass flow rate. Such comparison shows that the one-dimensional (1D) longitudinal model is in good agreement with the results of the more detailed two-dimensional (2D) model at any temperature gradient for heat transfer fluid velocities higher than 0.1 m/s whilst the lumped model agrees only at velocities higher than 1 m/s. Then, the 1D longitudinal model is implemented in a quasi-steady-state Simulink model of an innovative microscale concentrated solar combined heat and power plant and its performances evaluated. Compared to the results obtained using the Simscape library model of the tube, the performances of the plant show appreciable discrepancies during the winter season. Indeed, whenever the longitudinal thermal gradient of the fluid inside the pipeline is high (as at part-load conditions in winter season), the lumped model becomes inaccurate with more than 20% of deviation of the thermal losses and 30% of the organic Rankine cycle (ORC) electric energy output with respect to the 1D longitudinal model. Therefore, the analysis proves that an hybrid model able to switch from a 1D longitudinal model to a zero-dimensional (0D) model with delay based on the fluid flow rate is recommended to obtain results accurate enough whilst limiting the computational efforts.

Fuzzy Logic Energy Management Strategy of a Multiple Latent Heat Thermal Storage in a Small-Scale Concentrated Solar Power Plant

“…Indeed, the TES thermal losses are decreased due to the fact that less modules are involved in charging/discharging phases in cases of limited energy being available (thus, less mass of the storage medium), while the pipelines thermal losses are bigger because the average oil temperature is higher (with less modules involved, the charging phase is slower and the oil temperature tends to increase). Therefore, also the wasted energy through pipelines is crucial for the whole plant performance, and a good estimation of it, using accurate models, is paramount, as shown in reference [29].…”

“…In particular, the LFR and ORC subsystems are quasi-steady state models, whilst the thermal storage is a purely dynamic model. In order to take into account the thermal losses of the tubes connecting the different subsystems and the influence of their thermal inertia, a 1D longitudinal model of the pipelines has also been included, as further explored in reference [29]. The different subroutines were validated with available performance data from the manufacturers, as better discussed in reference [16].…”

Prediction of the time constant of small-scale concentrated solar CHP plants

“…Moreover, the code vectorization in the longitudinal direction allows drastically reducing the computational efforts required by the code. More details of the pipelines model can be found in [15]. The same numerical scheme has been applied to make the Forristal model of the receiver tubes dynamic.…”

Development of a Fuzzy Logic Controller for Small-Scale Solar Organic Rankine Cycle Cogeneration Plants

“…The pipelines connecting all the different subsystems were modeled by means of a one-dimensional longitudinal model, as discussed in detail in [26]. In summary, the simplified advection Equation ( 4) was solved for each tube:…”