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ScienceDirect Energy Procedia 105 (2017) 343 – 349

The 8th International Conference on Applied Energy – ICAE2016

Experimental Investigation On A Solar/Hydrogen-Based Microgrid M. A. Ancona1, M. Bianchi1, L. Branchini1*, A. De Pascale2, F. Melino2, A. Peretto1 1

Alma Mater Studiorum - Università di Bologna, DIN – viale del Risorgimento, 2, 40136 Bologna, ITALY 2 Alma Mater Studiorum - Università di Bologna, CIRI-EA – via Angherà, 22, 47900 Rimini, ITALY

Abstract In this paper an experimental investigation on the performance of an integrated microgrid, installed at the laboratory of the University of Bologna, is presented. The integrated microgrid is made up of two photovoltaic solar panels, two batteries as electricity storage device, a hydrogen generator and an electronic load emulator. The direct current generated by the solar modules charges the batteries by means of a load regulator unit. The power electronics, including an inverter and a DC converter, provide the user with 12 V DC and 230 V AC to feed the electronic load and hydrogen generator, respectively. The aim of this experimental activity is to investigate batteries charging/discharging process and, as a consequence, to quantify batteries roundtrip efficiency value. Experimental trend of voltage, current and power input/output from batteries are observed during charging/discharging operation. Results of the experimental activity show that during the charging process, the voltage from the solar array tapers according to the batteries conditions and recharging needs, thus increasing the charge acceptance of the battery. Once batteries are about to reach full charged condition, the controller starts to hold constant voltage and reduce the charging current to avoid overcharging. Taking advantage of experimental activity results, average value of batteries roundtrip efficiency is determined. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: microgrid; experimental setup; solar, hydrogen; storage; battery.

1. Introduction Distributed generation (DG) is significantly growing in the last decade. Improvements in small size generation technologies and storage systems make DG settled as active networks, working together with conventional power grids. In addition, issues of exhaustible natural resources, fluctuating fossil fuel prices and uncertainty of electricity supply push governments to behave positive toward the development of integrated microgrids, which are supposed to play a fundamental role in the next future [1, 2]. Preliminary investigations on abovementioned context have been carried out by Authors in [3-5] dealing with optimal size and performance analysis of grid-independent hybrid solutions for residential application. In this context, a new laboratory has been designed and set-up by the Energy and the Environment Interdepartmental Centre for Industrial Research - CIRI-EA of the University of Bologna at Ravenna Technopole. The “microgrid and storage” laboratory testing facilities are within the scope of the High Technology network of Emilia-Romagna. The carried out research activities, in the field of renewable energy exploitation and electricity storage, are, at the present, aimed at characterizing the overall solar-hydrogen generation chain efficiency on the basis of system experimental behavior.

* Corresponding author. Tel.: +39-0512093320; fax: +39-0512093315. E-mail address: [email protected].

1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.324

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First step, presented in this study, of a planned comprehensive research activity, is the investigation of batteries charging/discharging process and the estimation of roundtrip average efficiency value. 2. Microgrid description The experimental test bench, presented in Fig. 1, consists of an integrated microgrid connecting a renewable power source with an energy storage device, to supply loads. More in detail, the microgrid accommodates the following components: two solar modules, a power management cabinet including a solar charge regulator unit, two block batteries, a DC/DC converter and a DC/AC inverter, a hydrogen generator, three metal hydride storage canisters and, finally, a DC electronic load emulator. The solar arrays, the power management cabinet and the hydrogen generator are from Heliocentris mobile unit for solar hydrogen production [6]. The integrated microgrid is intended to maximize the hydrogen generation starting from a renewable source through the use of batteries, working as electricity storage device, compensating for solar over/under-production. In Table 1 main technical data of integrated microgrid installed components are summarized. Within the next future the migrogrid will be equipped with a solar emulator and an electronic AC load (dotted components in Fig. 1). Fig. 1 shows the schematics of the electric microgrid where measuring sensors are installed. In order to collect data of microgrid operation the test bench is endowed with sensors for current (IR), voltage (ER), temperature (TR), solar radiation (RR) , mass flow rate (QR) and water quality (LR). Table 1: technical data of microgrid main components [6]

System Max. input current, photovoltaics System voltage, photovoltaics Max output current 12 V DC Max. continuous output 230 V AC Momentary peak load Output voltage frequency

Batteries (each) 30 A 24 V DC 2A 700 W 1050 W ( 10 sec) 230 V, 50/60 Hz, true sinus

Solar Modules (each) Type System voltage MPP output Efficiency Short circuit current MPP voltage

Polycrystalline 24 V DC 220 W 15% @ AM 1.5 8.62 A 27.54 V

Type System voltage Capacity

Solar lead-acid battery 12 V DC 55 Ah

Hydrogen generator Type Production capacity Hydrogen purity Input voltage Max. consumption

Proton Exchange Membrane 30sl/h @10.7 bar 6.0 (99.999%) 120 VAC /50- 60 Hz 300 VA

Electronic DC Load emulator Input Voltage range Input power range Input current range

0-360 V 0-300 W 0-30 A

Solar energy is converted into electrical energy with the solar photovoltaic modules (PV) connected in parallel and located on movable racks with an adjustable angle so that they can be positioned facing various directions. Each of the solar modules delivers up to 220 W as maximum power output at standard condition (AM 1.5). A sensor kit is connected to the PV array recording ambient (TR1) and module temperature (TR2) and solar radiation (RR). The direct current generated by the solar modules charges the batteries by means of the solar charge regulator unit: in fact, charging a battery through a PV module without a regulation device is to be avoided because it can damage battery itself and shortening its life cycle. In practice, a DC/DC converter is necessary to regulate and provide suitable charging voltage/current according to the battery specifications. Several type of charge controller units can be used; devices can be divided into two main categories: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) controllers. While units belonging to first category work regulating the current and voltage from PV panels depending on battery state of charge (SOC); controllers of the second group perform battery charging keeping PV panels close to maximum power point condition for given incident solar radiation and weather conditions. The controller unit currently installed at the laboratory is a PR 3030 using the PWM method [7, 8]. A PWM controller is less expensive compared to a MPPT and represents a good trade-off choice for small systems. Moreover, performance advantage of MPPT controllers is significant, in particular, when the solar cell temperature is low (below 45 °C), or very high (above 75 °C), or when irradiance is very low [9]. In detail, once charge takes place, the current is controlled via PWM shunting of the module input. Depending on the batteries SOC, the regulator adjusts charging rates to allow charging operation closer to the batteries maximum capacity as well as monitors batteries temperature to prevent overheating. The two block batteries, Banner Stand by Bull Block 55Ah/12V, work in series to store the renewable energy coming from PV arrays. The DC/DC converter, Meanwell SD-25B-12, feeds the electronic load converting 24 DC voltage from batteries down to 12 V DC. The DC/AC Inverter feeding the Hydrogen Generator (HG) is a Meanwell TS-700-224B generating 230 V AC voltage for the HG unit. The Hydrogen Generator, HG 30, enables the

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production of high-purity hydrogen taking advantage of Proton Exchange Membrane (PEM) technology, in combination with an innovative gas dehydration system. Generated hydrogen is stored into three metal hydride canisters with a capacity equal to 760 sl each. The ARRAY 3711A is a direct current programmable electronic load emulator. It basically works as a resistance, fed with a constant direct voltage, dissipating the input electrical energy into heat. Grid

Solar Simulator

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Fig. 1. microgrid schematic layout with installed measuring sensors.

3. Results And Discussion Several experimental activities have been carried out in order to investigate batteries charging/discharging process and its effects on the overall microgrid performance. Batteries charging/discharging process required approximately 6 and half hours; the time step of data acquisition system is equal to 5 s. For brevity, results of a single test are highlighted in Fig. 2 a)-f) showing the experimental trend of several variables versus time: a) solar radiation (RR), b) PV power output and batteries input/output power, c) PV efficiency, d) batteries input/output DC voltage, e) batteries input/output DC currents, f) batteries SOC. Figures 2 b)-d) also included estimated trend of PV maximum power output (PPV), maximum power point efficiency (ηPV), maximum power point voltage (VMPP) and maximum power point current (IMPP), as function of recorded solar radiation (RR). Results show that during the charging process (from zero to about 2600 s) the solar charge regulator unit regulates the voltage and current coming from the solar panels going to the batteries as function of batteries SOC, in order to maximize charging process (i.e. maximum amount of energy to the battery in the shortest time). Thus, voltage and current input to batteries differs from optimal PV I and V values function of solar radiation: the solar panels, therefore, are far from MPP condition.


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Fig. 2: experimental trend of a) solar radiation, b) batteries power input/output and PV maximum power, c) PV recorded efficiency and PV maximum power point efficiency, d) Batteries DC voltage and PV maximum power point voltage, e) Batteries DC currents and PV maximum power point current; f) batteries SOC. side note: (1) in charging mode, power to the batteries coincides with PV production minus auxiliaries consumption; (2) maximum PV power production has been estimated considering the auxiliaries consumption

Fig 2 b), d) and e) show differences between batteries input variables and PV output variables function of RR. During the charging process the controller regulates PV voltage according to batteries needs at the time: DC voltage slowly increase with the increasing of SOC (see Fig 2 c) and e)) reaching about 29 V at the end of the charging process (i.e. SOC equal to 93%). An opposite trend, decreasing with the increase of SOC, is recorded for the DC current input to batteries/output from solar panels†. Once batteries are about to reach full charged condition, the regulator unit starts to hold constant voltage and to reduce the input current to prevent from overcharging. During the discharging process (from 93% to about 35% of SOC), batteries generated power (negative in Fig. 2 b)) is dissipated trough HG: output voltage and current appear slightly decreasing and increasing, respectively. Fig. 2 c) highlights the difference between recorder PV efficiency, ηPV, and estimated maximum power point efficiency function of RR, ηPV f(RR). As clearly visible in figure, PV recorded efficiency value decreases, with the increase of batteries SOC, down to about 4% at the end of the charging process. Maximum gap between efficiencies achieves up to 6 percentage points. Focusing on batteries performance, different tests have been carried out to determine the batteries roundtrip efficiency. Fig. 3 shows, for four experimental tests batteries roundtrip efficiency values calculated as ratio between total energy output and total energy input. Obtained values, in the range between 81 and 93 %, highlight that batteries †

except for transformation and auxiliaries consumption


Batteries round-trip efficincy [-]

efficiency can be subject to variations depending on initial and final SOC. Additional tests will be carried out in the next future in order to have a deep characterization of batteries return efficiency. Based on carried out experimental tests an average roundtrip efficiency value equal to 89 % has been obtained. 1 0.9

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Fig. 3: batteries roundtrip efficiency values for different experimental tests.

4. Conclusions The study presents an integrated solar/hydrogen based microgrid set up by the University of Bologna at Ravenna Technopole. The microgrid is made up of two photovoltaic solar panels, two batteries as electricity storage device, a hydrogen generator and an electronic load emulator. A comprehensive experimental activity will be carried out, in the next future, in order to characterize the efficiency of each microgrid components and of the overall solar-hydrogen generation chain. In this paper a preliminary investigation on batteries charging/discharging process is presented. Experimental behavior of batteries input variables shows that, when in regulation, the solar charge unit, operating according to pulse width modulation system, selects DC voltage output from solar panels according to batteries state of charge value. Resulting DC current is derived from solar module I-V characteristic curve. Therefore during batteries charging process, as confirmed by experimental results, solar panels operate far from maximum power point conditions. Once batteries are about to reach full charged condition, the regulator unit slowly reduces the charging current to avoid heating and gassing of the battery, yet the charging continues to return the maximum amount of energy to the battery in the shortest time. Based on several experimental tests, batteries average roundtrip efficiency value has been estimated equal to 0.89.

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References [1] S. Munchb, M. Prasada, State-level renewable electricity policies and reductions in carbon emissions, Energy Policy, Vol. 45, June 2012, pp. 237–242 , doi:10.1016/j.enpol.2012.02.024. [2] P. Tendayi Manditerezaa, R. Bansalb, Renewable distributed generation: The hidden challenges – A review from the protection perspective, Renewable and Sustainable Energy Reviews, Vol. 58, May 2016, pp. 1457–1465, doi:10.1016/j.rser.2015.12.276 [3] M. Bianchi, L. Branchini, A. De Pascale, F. Melino, A. Peretto, Preliminary Investigations on a Test Bench for Integrated Micro-CHP Energy Systems, Energy Procedia, Vol. 45, 2014, pp. 1275–1284, doi:10.1016/j.egypro.2014.01.133. [4] M. Bianchi, L. Branchini, C. Ferrari, F. Melino, Optimal sizing of grid-independent hybrid photovoltaic-battery power systems for household sector, Applied Energy, Vol. 136, 2014 , pp 805–816 doi:10.1016/j.apenergy.2014.07.058 [5] M. Bianchi, A. De Pascale, F. Melino, Performance Analysis of an Integrated CHP System with Thermal and Electric Energy Storage for Residential Application, Applied Energy, Vol. 112, 2013, pp. 928–938, doi: 10.1016/j.apenergy.2013.01.088 [6] Heliocentris, Solar Hydrogen Extension Mobile Unit for Solar Hydrogen Production, http://shecey.com/wpcontent/uploads/2015/09/Solar-Hydrogen-Extension_Brochure_EN_1106.pdf [7] http://www.steca.com/index.php?Steca-PR-10-30-en [8] Morningstar Corporation, Why PWM?, Preprint from the 14th NREL Photovoltaic Program Review, November 1996, http://www.morningstarcorp.com/wp-content/uploads/2014/02/8.-Why-PWM1.pdf [9] Victron energy, Which solar charge controller: PWM or MPPT? https://www.victronenergy.com/upload/documents/White-paper-Which-solar-charge-controller-PWM-or-MPPT.pdf

Nomenclature Acronyms AC Alternating Current DC Direct Current DG Distributed Generation ER Voltage sensor [V] HG Hydrogen Generator I current [A] IR Current sensor [A] MPP Maximum power point MPPT Maximum power point tracking LR Water quality sensor [%] P power [W] PEM Proton Exchange Membrane PV Photovoltaic PWM Pulse Width Modulation QR mass flow rate sensor [kg/s] TR Temperature sensor [°C] RR solar radiation [W/m2] V Voltage [V] SOC State Of Charge Symbols η efficiency [-] Subscripts open circuit OC


SC MPP

Short circuit Maximum Power Point

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