Estimating the Size of the Renewable Energy Generators in an Isolated Solar-Biodiesel Microgrid with Lead-Acid Battery Storage

Estimating the Size of the Renewable Energy Generators in an

Isolated Solar-Biodiesel Microgrid with Lead-Acid Battery

Storage

GRAMA Alin, PATARAU Toma, LAZAR Eniko, PETREUS Dorin

Technical University of Cluj Napoca, Romania,

Department of Applied Electronics, Faculty of Electronics Telecommunications and Information Technology, Str. Memorandumului nr. 28, 400114 Cluj Napoca, Romania, E-Mail1: alin.grama@ael.utcluj.ro

Abstract – Climate change, fossil fuel decline, expensive power grid extensions focused the attention of scientist in developing electrical power systems that use as primary resources renewable energy generators. Romania has a high renewable energy potential and presents interest in developing renewable energy microgrids using: solar energy, wind energy, biomass Hydro, etc. The paper presents a method of estimating the size of the renewable energy generators in an isolated solar-biodiesel microgrid with lead-acid battery storage. The mathematical model is first presented and then an algorithm is developed to give an estimation of the size of the microgrid. The microgrid is installed in the region of Oradea, Romania. The results are validated through comparison with existing sizing software programs like: PV*Sol and PVSyst.

Keywords: renewable energy; photovoltaic; biodiesel; intelligent microgrid.

I. INTRODUCTION

Global decline of fossil fuels, climate change, high costs of power grid extensions, sudden increase in electrical energy consumption, unstable and increasing oil prices have focused the attention of scientists and policy makers on developing renewable energy microgrids to supply electrical energy to consumers. An efficient utilization of renewable energies can have great impact on both stimulation of the economy and reducing pollution [1]. Despite the increased benefits of using renewable energy resources technical and economical constrains limit their full potential [2].

Romania has a very high renewable energy potential having the 13th All Renewable Index (ARI) in the world [3]. The main renewable energy support schemes used around the world are: tradable green certificates, feed-in tariffs, tenders for contracts, investment subsidies, and different fiscal instruments. According to the annual report of the Regulatory Authority for Energy (ANRE) Romania promotes renewable energy investments using the tradable green certificates system [4]. Although

Romania has a very high renewable energy potential only 23,4% is shared by the final consumers [5], this factor being strongly influenced by fluctuations in Romanian economy. This amount of shared renewable energy ranked Romania the second place in EU in 2006-2010 [1]. According to the ANRE [4] the electrical energy generation divided based on the type of resources used is represented in Fig.1.

Fig. 1. Electrical energy generation

The figure demonstrated that there is interest in developing renewable energy microgrids that use as primary resources biomass and solar energy. The present paper develops a method of estimating the size of the renewable energy generators of an isolated microgrid based on a solar generator, a biodiesel generator and VRLA (valve regulated lead-acid) batteries as storage. An algorithm is also developed to give the estimation of the size of the microgrid. The results are then compared for validation with existing sizing software programs like: PV*Sol and PVSyst. This two software programs are intended to be used only to design photovoltaic system and cannot be used to integrate other types of renewable energy generators hence the need of developing an algorithm and software program that can integrate also the biodiesel generator.

is that it can work islanded and can be used in remote areas to supply the load [7]. The microgrid studied in this paper will be installed in the region of Oradea Romania and will be designed to supply a vegetable greenhouse. Thus the meteorological data used are from Oradea and the loads of the isolated microgrid are the loads necessary to operate correctly a vegetable greenhouse. The loads include: fans – 600 W, special lamps – 2800 W, window opening systems – 600 W, electric drives – 100 W, lighting – 60 W, and water pumps – 600 W. The total maximum power consumed by the considered loads is 4.86 kW.

Fig. 2 presents the microgrid under study. It is composed of master generator and two slave generators. The master generator is formed of a battery pack and a battery inverter. The slave generators are the solar system composed of PV panels and a solar inverter with MPPT and the biogas generator including a biodiesel generator as prime mover that drives an induction machine. All the components of the microgrid are controlled by a management system that reads the power generation and demand and computes the power references of each generator.

Fig. 2 Microgrid

The rest of the paper is organized as follows: section II presents the estimation of the size of the renewable generators and storage procedure, section III presents the algorithm, results and discussions, and section IV concludes the paper.

II. SIZE ESTIMATION PROCEDURE

A. Solar array sizing

The sizing estimation of the solar panel array is based on the average load consumption and also on solar irradiation data.

The model of the solar panel can be represented by the following relationship:

t m g

PV

N

A

G

P

=

η

⋅

⋅

⋅

(1)

where

ηg

is the instantaneous efficiency of the

solar cell,

G

t

solar incident radiation,

A

m

the surface

area of one cell, and

N

the number of panels [8]. In

standard test conditions the output power of the

solar array is:

STC m g Array

PV N A G

P _ =η ⋅ ⋅ ⋅ (2)

Multiplying (1) with time and substituting (2) in 1 the ideal energy yield of a solar array can be expressed as:

tilt STC

array pv ideal Array

PV H

G P

E = _ ⋅

_

_ (3)

where Ppv_array is the output power of the solar array measured at GSTC (solar iradiation in standard measuring conditions (1kWh/m2)), and Htilt (kWh/m2/day) is the medium daily solar iradiation specific to tilt angle.

The daily average output power produced by the solar array must be equal to the daily average power consumed by the load. Considering losses introduced by the solar inverter, connections and PV array, and (3) the power required from the solar panels can be expressed as:

STC tilt LOSS PV fire invertor

biodisel L array

pv G

H E

E

P ⋅

⋅ ⋅

⋅ − =

_ _

η η

η (4)

where EL (kWh/day) is the average maximum power consumption, η_invertor is the solar inverter efficiency, η_fire is the efficiency determined by the connection wires,

Ebiodiesel is the energy generated by the biodiesel generator, and KLOSS is a loss factor determined by the solar aray. η_{PV_LOSS} is the loss factor determined by the solar panels. It accounts for dust particles fdepuneri, production tolerance

f

_prod, and the efficiency due to temperature drift [9].

prod depuneri temp LOSS

PV_ = f ⋅f ⋅f

η (5)

The efficiency due to temperature drift can be expressed as [9]:

[

( )

]

1 panou celleff STC

temp T T

f = −γ ⋅ − (6) where

γ

_panouis the temperature drift factor, TSTC is the tempreture in standard test condition usualy 25o C and

Tcelleff is the medium solar array tempreture and is a function of ambient temptreture Taday and can be considered:

C T

T o

aday

celleff = +25 (7)

B. Sizing the number of solar panels

The number of solar panels depends on the minimum and maximum input voltage at the solar inverter, Vmin_inv,

Vmax_inv, the MPPT voltage of the solar panel, VMPPT , and the minimum power determined with (4). The minimum number of solar panels connected in series is:

MPPT inv ms

V V

N ₌ min ₍₈₎

The minimum number of strings of solar panels connected in parallel is:

mod _ P N P N ms array pv mp ⋅

= (9)

The solar inverter size is chosen usually with a 25% safety margin [9]:

array pv

invsolar P

P =1.25⋅ _{_} (10)

C. Sizing the battery capacity

The sizing of the battery capacity is based on the number of days of autonomy Nd,depth of discharge rate

DODmax,DC bus voltage VDC, the load, and the efficiency of the battery pack, ηout:

out DC L d x V DOD E N C η ⋅ ⋅ ⋅ = max (11)

The number of batteries connected in series depends on the battery voltage, Vbat, and the DC bus voltage usually 12V, 24V, 48V, 96V:

bat DC bs

V V

N = (12)

The number of batteries connected in parallel is:

bat x bp

C C

N = (13)

The total number of batteries considering (12) and (13) is:

bs bp

tot N N

N = ⋅ (14)

The battery inverter is also chosen with a 25% safety margin [9]:

25 . 1 ⋅ = _S invbat P

P (15)

D. Sizing the connection wires

The connection wires between the solar inverter and panels accounts for the length of the wires lcab1, and the short circuit curent of the panels Isc. The crossection of the cable is determined as folows:

1 1 1 25 . 1 D SC cab V I l

A = ρ⋅ ⋅ ⋅ (16)

The crossection of the wires witch connect the batteries to the battery inverter is:

2 2 2 25 . 1 D DC invertor invbat cab V V P l A ⋅ ¸ ¹ · ¨ © § ⋅ ⋅ ⋅ = η ρ (17)

where VD2 is the voltage drop allowed on the wires. The load conductor’s crossection accounts for the maximum power of the loads:

3 3 3 25 . 1 D AC invbat cab V V P l A ⋅ ¸ ¹ · ¨ © § ⋅ ⋅ = ρ (18)

III. RESULTS AND DISCUTIONS

A software program with a user interface was developed to calculate the size of the microgrid. The flowchart of the program is presented in Fig. 3.

Fig. 3 Flowchart of the program

The load is divided in two parts: it is considered that the fans, computers, electric drives lighting, and the window opening system will be supplied by the solar system. The water pumps and the special lamps which mainly operate during nighttime will be supplied by the biodiesel generator.

The calculated values, determined using the proposed method, are presented in the following table:

Table I. Calculated size of the generators

Load EL 7.92kWh

PV output power PPV_array 4.075kW

Solar inverter power Pinv_solar 5kW

Battery inverter power Pinv_bat 6kW

Number of solar panels Npanels 20

Number of batteries Nbat 16

Crossection of panel wires A1 1.4 mm2

Crossection of battery wires A2 35 mm2

Crossection of load wires A3 10 mm2

The interface of the development program is represented in the Fig. 4. The values calculate using the proposed program are validated through comparison with two different dedicated software programs: PV*Sol and PVSyst. PV*Sol and PVSyst are solar system development tools which can only be used to determine the number of solar panels and the number of batteries at a specific location and a specific load. In this regard the load was divided as presented above and it can be observed that the value of the battery capacity and the value of the solar output power match the ones calculated by the algorithm. Also PV*Sol gives a battery capacity close to the one estimated.

Fig. 4 User interface of the propose estimation program

Fig. 5 Calculation using PVSyst

Fig. 6 Number of bateries calculation using PV*SOL

IV. CONCLUSIONS

The paper presented a method of estimation of the size of the renewable generators used in an isolated solar-biodiesel microgrid instaled in the region of Oradea, Romania. A program was developed that alows automatic calculation of the estimated size of the generators. It integrets also a biodiesel generator compared two other dedicated programs. The method was validated through comperison of the proposed algorithm with PV*Sol and PVSyst and it can be observed that the results are close toghether.

ACKNOLEDGMENT

This paper was suported by the Post-Doctoral Programme POSDRU/159/1.5/S/137516, project co-funded from European Social Fund through the Human Resources Sectorial Operational Program 2007-2013. This paper is supported through the programme "Parteneriate in domenii prioritare – PN II", by MEN – UEFISCDI, project no. 53/01.07.2014.

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