• Energy Research

In the quest for more efficient solar thermal systems, accurately determining the thermal emittance of low-emissive materials is crucial in determining the power losses. This paper describes the calorimetric method designed to precisely measure the thermal emittance of Selective Solar Absorbers (SSAs) to be used in High Vacuum Flat Plate Collectors (HVFPCs). The method’s capability is demonstrated through the successful correction of thermal emittance values for copper samples of varying sizes, including dimensions down to 49 cm2. Results highlight the method’s potential to significantly reduce measurement errors associated with small-size and/or low-emittance samples, providing a path forward to improve the design and efficiency of SSAs. This research marks a significant step in advancing solar thermal technology by enabling emittance measurements with a precision better than 0.003, which is essential for the development of high-performance solar thermal absorbers. The method has also been applied to correct the thermal emittance value of SSA measured in previous measurement campaigns, and it allows a better estimation of the SSA efficiency conversion curve.

Abstract In this paper, design charts for the evaluation of thermal parameters for natural convection with air in a channel–chimney system are proposed. In the thermal analysis of natural convection in channel–chimney systems, the variables that play an important role are: the ohmic heat flux, maximum wall temperatures and geometrical parameters such as the height of the heated channel, the channel spacing and the height and spacing of unheated extensions. A simple numerical procedure to obtain the thermal design charts, a thermal optimization of the system and an uncertainty analysis due to the thermophysical properties are presented. Results are carried out for symmetrically and asymmetrically heated channels with walls at uniform heat flux and a simple estimation procedure is proposed to evaluate the error in the relevant geometrical and thermal parameters due to a different value of the reference temperature. The estimated error, however, is less than the uncertainty of the experimental data employed. Some simple examples are given to show the use of the charts. The proposed results are obtained from experimental data in the following dimensionless parameter ranges: 5.0⩽Lh/b⩽20; 1.5⩽L/Lh⩽4; 1⩽B/b⩽4; 102⩽Ra⩽106.

Multilayer absorber coatings based on a Cr2O3/Cr/Cr2O3 tri-layer structure with SiO2 anti-reflective layer have been optimized to work at mid temperatures for Evacuated Flat Plate Collectors, maximizing the efficiency without solar concentration.

Industrial heat and cooling applications are an essential fraction of the overall energy demand, mainly produced by fossil fuels. Solar thermal energy production can satisfy such a need by adopting the High Vacuum Flat Plate Collectors (HVFPCs) and increasing their efficiency. The absorptance and emittance of Selective Solar Absorbers (SSAs) determine the thermal efficiency of HVFPCs. Being the absorptance already maximized, the thermal emittance of the absorber should be minimized to increase further the operating temperature of the collector and its efficiency. This research aims to reduce the thermal emittance of commercially available Selective Solar Absorber by depositing a thin silver film on the aluminium substrate. So, in this work, the thermal stability of a silver coating has been investigated, and a diffusion barrier layer has been adopted to stabilize the coating performance up to 360 degrees C. The low-emissive layer of Ag and a diffusion barrier of CrOx guarantees a decrease of 11% in thermal emittance at 200 degrees C of commercially available SSA deposited on aluminium. Further emittance reduction can be obtained by depositing a thin Ag film on both sides of the aluminium substrate before the SSA deposition, proving to be a promising way to enhance the efficiency of HVFPCs.

From January 1, 2011, in all UE countries the combined production of electric (or mechanical) and thermal energy (also called Combined Heat and Power, CHP, or cogeneration) is recognized as a high efficiency technology only when it is able to ensure a minimum value of energy saving with respect to the separate production of the same energy flows. The Directive 8/2004/EC, and a few successive Decisions of the European Commission, introduced a methodology to establish whether any cogeneration plant, existing or new, can be acknowledged as a high-efficiency CHP plant, and can therefore be supported from the UE member states. In the paper, such methodology, based on the evaluation of a standard Primary Energy Saving (PES) index, is briefly described, and then a metrological analysis is presented, in order to evaluate the uncertainties affecting the field evaluation of such index. Three numerical examples are also presented and discussed, referred to natural gas plants, showing that the evaluation of the PES index can be quite critical, especially for values close to the minimum limit fixed by the Directive, and in particular for small and medium scale CHP units, mainly due to the low accuracy that usually affect, in such cases, the measures of the fuel input.

The growing need for renewable energy sources has highlighted the importance of technologies that can bridge the generation-demand gap in the energy system. Thermal Energy Storage (TES) can help to smooth out peaks in energy demand, thereby reducing waste resulting from excess capacity during offpeak periods. Due to their widespread use, significant effort has been dedicated to optimizing the control logic of TES components to maximize the harvested-to-stored energy ratio. The aim of this study is to compare MIX number and dimensionless exergy, two parameters commonly used to identify the storage’s ability to generate and maintain optimal temperature stratification. The investigation is conducted by comparing the parameter response to the thermo-fluid dynamic variations inside the store under several inlet temperatures and flow rates, using a two-dimensional CFD model validated on experimental data collected from an operating commercial stratified tank. The sensitivity of the two performance indicators in detecting stratification losses that are due to different charging conditions is compared, to better understand the applicability of the latter as control parameters during operational phases. Results show that the derivatives of such indicators are less sensible to the tank starting conditions and can be more robust indicators during the charging phase.

Abstract Reducing thermal losses in solar thermal devices is fundamental for enhancing conversion efficiencies, particularly at high operating temperatures. In this work, we consider the benefits of adding an InfraRed (IR) mirror coating to the inner surface of the glass encapsulating a High Vacuum insulated Flat Plate solar thermal Panel (HVFP). The IR mirror helps recover the radiation emitted by the absorber by sending it back to the absorber itself. This mechanism, known as cold-side external photon recycling, allows a reduction of radiative losses and, consequently, an improvement of the panel efficiency. The performance of the structure presented in this manuscript is studied via a thermal model. A detailed discussion on the increasing efficiency is presented, and results are presented by taking into account different parameters, like the mirror transparency, reflectivity and reflection bandwidth, as well as different operating temperatures of the panel. Finally, the annual energy gain associated with the IR mirror is analyzed in the case of three different cities, using historical data, showing that improvement higher than 50% can be obtained at operating temperatures above 300 °C.

High Vacuum Flat Plate Collectors (HVFPCs) are the only type of flat plate thermal collectors capable of producing thermal energy for middle-temperature applications (up to 200 °C). As the trend in research plans is to develop new Selective Solar Absorbers to extend the range of HVFPC application up to 250 °C, it is necessary to correctly evaluate the collector efficiency up to such temperatures to predict the energy production accurately. We propose an efficiency model for these collectors based on the selective absorber optical properties. The proposed efficiency model explicitly includes the radiative heat exchange with the ambient, which is the main source of thermal losses for evacuated collectors at high temperatures. It also decouples the radiative losses that depend on the optical properties of the absorber adopted from the other thermal losses due to HVFPC architecture. The model has been validated by applying it to MT-Power HVFPC manufactured by TVP-Solar. The dissipative losses other than thermal radiation were found to be mostly conductive with a linear coefficient k = 0.258 W/m2K. The efficiency model has been also used to predict the energy production of HVFPCs equipped with new, optimized Selective Solar Absorbers developed in recent years. Considering the 2019 meteorological data in Cairo and an operating temperature of 250 °C, the annual energy production of an HVFPC equipped with an optimized absorber is estimated to be 638 kWh/m2.