Laboratory of Solar Energy Conversion – STET

Laboratory of Solar Energy Conversion

Experimental study of a parabolic trough linear CPVT system

Figure 1: CPVT system.

The new prototype of a parabolic trough linear concentrating photovoltaic-thermal (CPVT) system operating on the roof terrace of Department of Ingegneria Industriale is shown in Fig. 1 and Fig. 2. Four parabolic trough mirrors concentrate the solar radiation onto a linear receiver, where a photovoltaic-thermal module is placed. The aperture area of the present system is 6.86 m2 and the geometrical concentration ratio is nearly 130. The device has a modular arrangement and more modules could be added by increasing the number of mirrors and the length of the receiver. The system moves about two-axes (azimuthal and zenithal motions), to have the beam radiation normal to the plane. The motion is governed by a solar algorithm when approaching the sun and by a solar sensor when achieving the best receiver alignment.

The photovoltaic-thermal module is the test section. A secondary optics device has been designed for reducing optical losses. The module is equipped with triple junction solar cells soldered on a ceramic substrate, which in turn is in thermal contact with an active cooling system including an aluminium heat exchanger and a closed loop for pumping water as the coolant. The heat exchanger is applied to the back side. Each PV package has a nominal efficiency of around 35 %, as reported by the manufacturer.

Figure 2: CPVT prototype and Photovoltaic-thermal module

A scheme of the hydraulic loop built up at the Dipartimento di Ingegneria Industriale to test the solar concentrator is reported in Fig. 3. The water coming from the active cooling system of the photovoltaic cells enters the storage 1 and then passes through a plate heat exchanger that acts as a heat sink. The water enters the storage 2, which contains four electrical heaters for the temperature regulation. According to the operating mass flow rate, it is possible to set the electric power to obtain a desired and constant temperature of the liquid at the inlet of the test section. From storage 2 the water is pumped through a mass flow meter before entering the heat exchanger of the module. The inlet and outlet water temperatures in the test section and the ambient air temperature are measured.

The electrical terminals of the module are connected to a rheostat and a power analyzer that measures the current of the circuit, the voltage across the resistive load and the electrical power supplied by the PV cells. During the test runs, the sliding contact of the rheostat is set in order to make the PV module work close to the maximum power point.

Figure 3: Schematic view of the experimental test rig.

Experimental evaluation of solar thermal collectors

The new laboratory for experimental measurements regarding solar energy conversion installed on the roof terrace of DFT includes a test facility for measuring the performances of thermal collectors. This apparatus has been realized also with purposes of student.s laboratory.

An accurate knowledge of the performances of the solar collector is very important for a proper design of a solar thermal system. In fact the diffusion of solar installations is in relationship with the payback time with respect to more conventional systems. Therefore, the right choice of the collector for each application is a key aspect. The experimental apparatus for thermal collectors has been built according to the standard EN 12975-2 (Thermal solar systems and components. Solar collectors. Test methods). The goal is to measure the efficiency and the pressure drops along flat-plate and evacuated solar collectors.

Three collectors have been installed in the apparatus: two standard glazed flat-plate collectors, connected in parallel, and a direct flow-through evacuated tube collector. For all the collectors there is the possibility to modify the tilt angle to the horizontal plane. A mixture of water and propylene glycol is used as this fluid ensures no problems during the winter season. The circuit is divided in two lines: the first one for the plate collectors and the second one for the evacuated collector. Two pumps are used to move the liquid. Before entering the collectors, the fluid temperature is controlled in a storage, where four electrical heaters are located. Each heater has a power of 5 kW. A controller, commanded by a temperature sensor inserted in the storage, acts on the electrical heaters to ensure an accurate control of the liquid temperature at the inlet of the collectors. The liquid temperature at the inlet and at the outlet of the collectors i measured, both for plate and evacuated collectors. The fluid, coming from the collectors, enters a first storage and then it goes to a plate heat exchanger which works as a heat sink. In the plate heat exchanger, the heat flux provided by solar radiation i taken away by a secondary fluid, which is also a mixture of water and propylene glycol Finally, the heat flux is taken away in a second plate heat exchanger by the ground water of the Dipartimento.

Figure 4 – Solar collectors installed on the roof terrace of DFT

The global solar radiation on the same plane of the collectors is measured by a pyranometer. Other two pyranometers are installed on the horizontal plane: the first one for measuring the global solar radiation; the second one is provided with a shield against the direct radiation and measures only the diffuse component. This way, experimental measurements of total, direct and diffuse solar radiation can be obtained. An anemometer measures the air speed and a temperature sensor measures the surrounding air temperature in agreement with the standard indications.

The heat flux gained by the fluid flowing through the collectors is obtained by measuring the inlet and the outlet temperature and the mass flow rate. A Coriolis effect and a magnetic type flow meter are used to measure the liquid flow rate. The magnetic flow meter measures a volumetric flow rate, so the density of the mixture water-glycol must be known. For this purpose, there is the possibility to connect the two flow meters in series to check the values of the mass flow rate obtained using the magnetic flow meter. Then the collector efficiency is obtained as the ratio of the useful heat flux extracted and the solar radiation intercepted by the collector area. Several experimental measurements can be taken by modifying the fluid temperature at the inlet of the collectors: in this way the whole experimental efficiency curve can be built in agreement with the standards. Three experimental curves of efficiency can be obtained using the gross, aperture or absorber area.

The pressure drops along the collectors are measured with a differential pressure transducer.

Figure 5 – Rig for tests on solar collectors installed on the roof terrace of DFT

Figure 6 – Schematic view of the test rig for solar thermal collectors

Measurement and modelling of solar radiation

The measuring system is composed of a pyranometer for the measurement of the global solar irradiance on the horizontal plane, a shaded pyranometer for the measurement of the diffuse irradiance on the horizontal plane and a pyrheliometer, mounted on a sun tracker, for the measurement of the DNI (direct normal irradiance). Other pyranometers are installed on tilted and oriented planes to get data of interest for solar energy systems. The measuring system also includes an anemometer for the measurement of the wind speed and a probe for the measurement of the ambient air temperature. The experimental data are used for the study and evaluation of prediction models. For instance, the accurate knowledge of DNI is fundamental for the design and development of solar concentrating devices.

Figure 7. Pyranometers and pyrheliometer for the measurement of the global horizontal irradiance, diffuse horizontal irradiance and DNI (direct normal irradiance).

Experimental evaluation of photovoltaic systems and components

The photovoltaic apparatus consists of 5 modules connected in series to get a total peak power of 1025 W. The apparatus reproduces a typical grid connected installation. The standard efficiency can not be measured, because it needs some conditions that can be obtained only in an indoor laboratory with a solar radiation simulator. So the aim is to check how a typical grid connected installation works and to measure its instantaneous efficiency.

Figure 8 – Photovoltaic modules installed on the roof terrace of DFT

The modules are made of a thin single-crystalline silicon wafer surrounded by ultra-thin amorphous silicon layers. The nominal efficiency of the module is 16.4 %, while for the cell is 18.2%. The tilt angle to the horizontal plane can be changed for measurement purposes. The photovoltaic system is connected to the electrical grid of the University.

The DC power produced is converted in AC power by an inverter and finally it is put in the electrical grid. A data logger allows to acquire data of solar radiation on the same plane of the modules, instantaneous electrical power, voltage, current, total and daily energy produced, CO2 saved, modules and air temperature.

A pyranometer installed on the same plane of panels measures the global solar radiation as for thermal collectors. The instantaneous efficiency is measured as the ratio of the electrical power and solar radiation intercepted by the panels.

Figure 9 – The electronic display at the entrance to the department

Close to this photovoltaic system, another experimental apparatus is going to be setup: its purpose is to measure the characteristic curve of a photovoltaic module, which means current versus voltage, before the DC current is transformed into alternate current. The measurement of the solar radiation is made by means of Kipp&Zonen pyranometers.