Experimental and Computational Analysis of Small-Size Solar Receiver for Industrial and Residential Application

Use of solar thermal energy in residential and industrial applications has to be sustained to reduce the concentration of greenhouse gas in atmosphere due to the exploitation of fossil fuels in producing energy. In this context, the renewable energies play an important role. The energy request in industrial and residential sector involves a noticeable fraction (more than 50%) of the total requested supply for human activities. Concentrating collectors could be the right technology to produce heat at medium temperature (between 85 and 250°C) to provide thermal energy to users with high consumption rates and low-temperature heat demand like domestic hot water and space heating in addition to the industrial process heat applications. Thus, in this study UF-RT01 receiver (University of Florence Receiver Tube 01) of small size parabolic trough collector called m-PTC were investigated experimentally by indoor and outdoor tests and computationally by 3D heat transfer FEM model. The m-PTC suitable to be integrated in the roof of industrial environments where the space for installation of solar collectors is in general limited and the heat demand temperature is below than 200°C. The UF-RT01 receiver has a specific design, being formed by two coaxial tubes so that the fluid inlet and output are at the same side. It was properly developed to scale the PTC technology toward smaller size (chord length from 6-8 meters to around 0.5 m): the purpose is the installation in urban context and the application in industrial process. The outer absorber tube is made of steel and has a diameter of 10 mm (1 mm thickness) for a length of 1860 mm; the smaller coaxial tube is made of steel and has an internal diameter of 6 mm (0.5 mm thickness). Furthermore, a selective coating has been selected to reduce the emission in infrared range and increase the energy absorption in solar spectral range. Inside, a vacuum level is fixed at 10-4 mbar to reduce the heat losses to the radiative ones. In order to study the thermal losses of the receiver, two different indoor test stand have been realized. The thermal loss measurement is set up under indoor test without Sun irradiance, imposing a controlled internal heating. This process is based on the Joule effect, feeding electric heaters with current to obtain a steady state condition at different reference temperatures. In preliminary test stand by removing the inner coaxial steel tube, two nickel-chrome wire heaters are inserted along the length of absorber tube. An additional external heater is also placed before the Kovar part to meet the adiabatic condition and minimizing the temperature gradient. The UF-RT01 has been analyzed experimentally and performances are evaluated as a function of different operating temperatures, reaching up to 180°C. A maximum value for heat loss amounts at about 24 W when ΔT is 161°C (receiver average temperature of 180°C). In order to obtain more uniformity of temperature along the absorber tube the second test set up has been developed for thermal loss measurement and instead of nickel-chrome wire heater, an industrial cartridge heater made of resistance wire (NiCr20/80) as a core covered with stainless steel 304 as a sleeve (sheath) has been used. Three different tube from same type (UF-RT01) have been tested in the range of interest and the procedure was repeated for about 150 cases. In comparison to preliminary test stand, results showed more uniformity in temperature distribution along the tube. A maximum value of 17.89 W is found when ΔT is 163°C (receiver average temperature of 190°C). In order to achieve production assurance and have more clear vision about the results due to the different results obtained from test on RT03 in comparison to the RT01 and RT02 with higher thermal loss, new tests have been conducted on additional tubes. Similar setup and test procedure have been conducted in order to evaluate the uniformity of temperature along the tube and estimate the heating supplier parameters in additional tubes. Seven different tube from same type have been tested and labeled as RT04-RT10. Results from tests on RT01 and RT02 are in accordance with new results obtained from heat loss test on RT04-RT10. Therefore, the different results related to the RT03 are to be expected as a result of variation in production quality by manufacturer of receiver tube. The Finite Element Method (FEM) has been used in order to predict the thermal performance and analyze the relevant physical characteristics of the receiver tube (specially the value of emissivity at higher temperature). Heat transfer model using FEM simulation method has been realized with Comsol Multiphysics software. An adaptive mesh refinement (AMR) with different mesh configurations has been conducted in order to increase storage and computational savings. By using a parametric sweep to vary the maximum element size, the model solved using meshes with different mesh density in order to study how it affects the solution. The heat transfer model is able to precisely predict the heat losses at low temperature of the absorber tube with constant value of emissivity reported by manufacturer. The estimation of emissivity at the higher temperature obtained by solving the model with various emissivity values for each test at specific input power until the average temperature inside the absorber tube obtained by simulation were in agreement with experimental value. The obtained emissivity function has been used in model in order to solve the model for various input power values and the results showed that the model and emissivity function are able to predict the thermal loss with high accuracy. In order to perform the out-door test according the designed and assembled test rig platform at first phase has been slightly modified to reduce the heat losses and reach stable inlet temperature . The reliability of implemented test bench and output power and efficiency of a novel small size parabolic trough collector have been evaluated by preliminary test. For this purpose an out-door tests at ambient temperature on the designed small size PTC test rig is carried out during clear sky day. Furthermore, the peak optical efficiency test has been conducted based on introduced requirements at quasi-steady condition. The general point of the outdoor efficiency test is extracting the efficiency curve of the collector for normal incidence based on the efficiency curve coefficients. 24 tests have been done under various inlet temperature and irradiance under clear sky condition and the exemplary performance measurement data for present research stems from 153 experimental points. The preliminary out-door experimental test on the collectors showed that the test rig meets the initial design expectations in order to control the system in stable condition. The peak optical efficiency test has been conducted at quasi-steady condition and the average peak optical efficiency of the collector is 61.8% with total absolute error of 1.4%. With regard to the peak optical efficiency and for assuring that experimental results from the outdoor testing are valid, a cross check with the efficiency curve of the collector by weighted least squares (WLS) fitting shows almost similar values. The obtained value for peak optical efficiency from efficiency curve is 62.1%. Efficiency measurement of solar collector have been conducted from inlet temperature of 28 °C up to 123°C for various DNI values. A Maximum of 63.1% for thermal efficiency is found when the inlet temperature is 28.41°C and a minimum of 54.6% corresponds at 122.90°C. The total standard absolute uncertainty of thermal efficiency for test at inlet temperature of 28.41°C and 122.90°C are 0.7% and 0.8%, respectively. The efficiency curve of the collector by WLS fitting were also obtained from outdoor test results.