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.
Highlights
In order to facilitate the transition towards a cleaner energy context, the European Union (EU) has updated its energy policy by fixing for 2030: (i) a binding renewable energy target of at least 32% and (ii) an energy efficiency target of at least 32.5%, with a possible upward revision in 2023 [1].With reference to the building sector, it accounts for about 40% of the final energy consumption and 36% of CO2 emissions in Europe
In previous works [14,15] some of the authors of the present paper have investigated the performance of a microsolar organic Rankine cycle plant based on linear Fresnel reflectors (LFR) solar field and designed for residential applications
Solar field, (ii) a 3.8 ton latent heat thermal energy storage system equipped with reversible heat pipes, and (iii) an organic Rankine cycle unit designed for a power production of 2 kWe/18 kWth, as extensively discussed in [19]
Summary
With reference to the building sector, it accounts for about 40% of the final energy consumption and 36% of CO2 emissions in Europe. For this reason several EU Directives, such as the Directive. 2018/2001 and the Directive 2018/2002 [2], have set the specifications for high-energy performance buildings and for the adoption of energy efficiency measures that will be transposed by 30 June 2021 by the Member States. Among the different technologies to efficiently convert and supply energy into buildings, combined heat and power (CHP) plants in combination with district heating (DH) networks have already proved their appreciable benefits as confirmed by various EU research projects since. Van der Heijde et al [5] presented the mathematical derivation and software implementation in Modelica of a thermohydraulic plug-flow model of thermal networks
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