978-1-4244-8409-6/10/$26.00 ©2010 IEEE 336

Design Criteria for mCCHP Electric System

Viorel Stanciu , Mihaela Chefneux , Maricica Pesteri and Alexandra Valentina Toma Electrical Research Institute - ICPE-SA, Bucharest, ROMANIA, e-mail: viorel.stanciu@icpe.ro

Abstract — In the frame of the European Research Project RO 0054, entitled: “Integrated micro CCHP-Stirling Engine based on renewable energy sources, for the isolated residential consumers from South-East region of Romania”, there are analized the main performances required by the mCCHP electric system. The authors of the paper propose a method that leads to the selection of the optimum solution depending of the specific requirements of the residential house. The general method presented in the paper is applicable for such kind of residential homes and leads to a design algorithm that will be used for the project.

Index Terms — Design methodology, Battery storage plants, Inverters, Power generation

I. INTRODUCTION

In the frame of the European Research Project RO 0054, entitled: “Integrated micro CCHP-Stirling Engine based on renewable energy sources, for the isolated residential consumers from South-East region of Romania”, there are approached some aspects concerning the main performances required by the electric system. Dimensioning the electric system for a residential isolated house is an iterative complex process, with many parameters that must be evaluated

The electric system can be regarded as a compact block which has input and output parameters, as shown in fig.1.

Mechanical

energy

Solarenergy

CONVERSION ELECTRICITY

Fig.1. mCCHP electric system

The functional block that represents the electric system converts energy and transfers it to consumers. That requires, as an ideal target, high efficiency transfer and low costs, conditions that in most cases are in opposition.

In the case of hybrid cogeneration, the main input parameters of the system are power and speed for mechanical energy and the available capture surface for solar energy

The output size is supply voltage- electricity- for the consumers, with parameters corresponding to local requirements.

The electrical system for a mCCHP installation must be done in accordance with mCCHP models which must satisfy some local energy requirements.

The main criteria for the mCCHP electrical system performances that have to be considered are referring to :

A. Technical parameters: - input/output voltage - operating power - load curve

B. Electrical system structure: - electric generators - electric converters - electrical energy storage assembly

C. System component commercial availability : - components existing on the market - possibilities of replacement/service

D. Operating autonomy (without external energy sources) - the distribution of energy demand on time periods

- while electricity production characteristic

II. ELECTRICAL SYSTEM DESIGN

Although an installation type mCCHP is an unitary system, the design approach is not only for the whole assembly, but also for module components. The conceptual model of the installation is modular and that is why the electrical system design algorithm , as well as it’s subassemblies components, can be addressed separately, with it’s own criteria.

The designer of the whole system may have other criteria – e.g. costs – and as a consequence the subassemblies design can be regarded as an iterative operation, the final solution being established after the simulation of the global system.

To exemplify the method we proposed for the electric system design, we’ll present in this paper how we intend to solve the tasks required by the project.

The conceptual model of the electrical system that we will analyze has the following structure:

- electric generators - photovoltaic panels - charge regulators - network of direct current - accumulators - inverters

This structure can be seen in figure 2: The Stirling engines have variable speed depending of

the quantity of the generated heat. Electric generator coupled to this Stirling engine will have variable voltage depending of the speed. Also, the frequency will be variable depending of the speed. D.C. supply networks are usually 12, 24 and 48 V. For an efficient dimensioning of the conductors, we chose 48 V d.c.

For the solar panels will be carried out a serie-parallel connection, so the output voltage will result 48 V.

337

Change controller

Input voltage= 65-150V

Maximun charge current=45A

Change controller

Input voltage= 300-600V

Maximun charge current=90A

StirligEngine

Generator600V/3kW

+Rectifier1200V

10A

StirligEngine

Generator600V/3kW

+Rectifier1200V

10A

48Vdc

3kW220V50Hz

Accumulators48V

800Ah

48Vdc

3kW220V50Hz

48Vdc

3kW220V50Hz

Inverter 1

Inverter 2

Inverter 3

sync

ro 12

0sy

ncro

120

Photovoltaic panel70-90V1500W

Network ofdirect continuu

Three-phaseconsumers

Installedpower9kW

Instantaneouspower9kW

Fig. 2. Structure of electrical system

So, for the project RO 0054, in order to carry out the

experimental model, the power sources will have the following parameters:

A. Electric generator: - Power: 6 kW

- Voltage: variable - Frequency: variable B. Photovoltaic panels system

- Power: 1.5 kW - Voltage: 48 V

From the market study for mCCHP were chosen, in order to satisfy the power requirements of 6 kW, two Stirling engines, each one having a 3 kw generator, that will operate in parallel. This generators have output rated voltage between 300 and 600 V. They have integrated a rectifier bridge, so as to obtain a maximum output voltage of 600 V.

This is not the only solution for the project- any other combination that leads to the required parameters is permitted.

The main steps of the algorithm that leads to the choice of the energy source are shown in fig.3.

NO

YES

YES

YES

YES

NO

NO

NO

PowerVoltage

FrequencyCost

GeneratorChoice

Choosing generatorblock withP=n*Pu

Number ofiterations >3P= Pc

U=Unf=fn

ConvertorChoice

C= Ci

Generator(+Convertor)

chosen

Fig. 3. Algorithm for selecting the energy supply

III. DESIGN OF ENERGY STORAGE ASSEMBLY

We do not intend to present the entire design of the

electric system, but only to give an example of how the problem was solved for each module and component. That is why we present the specific criteria for the energy storage system design.

According to the conceptual model chosen, the energy storage needs a network of batteries.

The design of this module- as a part of the electric system- is an independent step, for which we must consider some technical and economical parameters. Also, we must take into account that the batteries are the last sustainable element of an electric system and usually they have to be replaced the first.

Especially, for the energy storage system, a main design criteria is the price, and that is why for the RO 0054 project, we proposed to choose BSB lead batteries with ventilation valve of 200 Ah. The price cost/lifetime report is optimal. The price for these batteries is 350 Euro/pc. and they have a lifetime of eight years. These batteries have the terminal voltage of 12 V.

To get the 48 V d.c., we need four such batteries. That will lead to 48 V and 200 Ah. As we need 800 Ah, we will connect four such groups in parallel.

The battery network is shown in fig. 4:

338

DC +-

C20=800A/h

Udc=48V

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

DC +-

Fig. 4. Network batteries conection

IV. ELECTRIC SYSTEM PROPOSED FOR THE

RO 0054 PROJECT Starting from the conceptual model of the the mCCHP

electric system, we must establish the power conversion parameters, that means the power supply, energy storage and electricity for residential consumers parameters.

As shown in fig.5, there are four electric power lines with the following main parametrs:

- D.c. line with Ucc=35-45V, which has to bear a power of 1.5 kW, line resulting from the photovoltaic panels - D.c. line with Ucc=350-600V, which has to bear a power of 6 kW, line resulting from use of several electric generators operating in parallel - D.c. line of 48 V, line necessary for the energy storage system, which has to bear a power of 7.5 kW, - A.c. line with Uef=220V and f=50Hz, line necessary for electrical consumers, which has to bear a power of 7.5 kW, As shown in the figure, appears the necessity for

electric conversion between different voltage lines. The three devices (noted as 1,2,3) must carry out the power conversion between lines, transferring electrical energy from input to output, with high efficiency and reduced losses.

Depending on the parameters of the electric power input and output, will be designed and chosen the devices.

Battery network with40Vdc800Ah

Photovoltaic networkwith Pv=1,5kW

D.C.

Volt

age

Udc=

35-4

5V

A.C.

Volt

age

Uef=

220V

D.C.

Volt

age

Udc=

350-

600V

D.C.

Volt

age

Udc=

48V

Elec

tric ge

nera

tors w

ith P

t=6k

W Electric consumers7,5kW peak power

220Vac voltage50Hz frequency

Load profile8

4

6 12 18 24 hour

kW

1

2

3

Fig. 5. Electric system

V. A METHOD OF DESIGN OPTIMIZATION

APPLIED FOR THE BATTERIES As always in such kind of complex system design , the

solution is not unique. Choosing one or another way depends of the requirements of the beneficiary and of the ability of the designer.

The problem is very complex and the optimum is very difficult to establish, but we will present a method that we consider that can lead to very good results.

Of course, in the first stages of design we intend to apply it for only a module, and that is why we will present the steps done for choosing the batteries.

The method, known as “value analysis method”, reflects user’s opinion, with his personal preferences, the ability of the designer consisting in establishing the main criteria and applying the results that have been obtained. Because the method is reflecting the beneficiary options, it may be considered as an optimization method.

First, we must establish the main functions of the battery, than, using the statistical survey method that compares the functions that achieve the battery use value, we will come to a conclusions about the importance of every function.

We shall analyze the following battery functions: A – energy storage B – aesthetics C – autonomy D – reliability

The components of the battery are (fig.6): - plastic housing - positive plate - negative plate - separating element - electrolyte

339

Negativeplate

Pozitiveplate

Electrilyte

Fig.6. Battery components Step 1: Setting the level of importance of battery functions Determination of share value functions

TABLE 1 THE LEVEL OF IMPORTANCE OF BATTERY FUNCTIONS

Functions A B C D TOTAL

A 1 0 0 0

B 1 1 1 1

C 1 0 1 0

D 1 0 1 1

Function share 4 1 3 2 10

Important factor 0,4 0,1 0,3 0,2 1

Adapted share [%] 40 10 30 20 100

From the table, result these percentage value of share in

value of functions battery: XA = 40 %; XB = 10 %; XC = 30 %; XD = 20 %.

In fig.7 it shown the arrangement of battery functions compared with their value.

0

5

10

15

20

25

30

35

40

45

A B C D

[%]

Figure 7. The diagram of share battery functions Step 2: Determination of cost share of each function in

total cost of battery (Economic distribution of battery function)

Distribution costs of the battery functions was made in matrix functions – costs of table two.

TABLE 2 DISTRIBUTION COST OF THE BATTERY FUNCTIONS

Com

pone

nts o

f bat

tery

FUNCTIONS

Cos

t of

bat

tery

com

pone

nts

(Eur

o)

A B C D

1 Plastic housing 20 20 20

2 Positive plate 40 10 20 20 100

3 Negative plate 40 10 20 20 100

4 Separator 40 30 30 100 5 Electrolyte 10 10 10 30

Total cost (Euro) 130 40 100 80 350

Report 0,3714 0,1142 0,2857 0,2295 1 Cost functions

[%] 37,14 11,42 28,57 22,95 100

Percentage values of the functions involved in the

total cost of battery are: YA = 37,14 %; YB = 11,42 %; YC = 28,57 %; YD = 22,95 %.

37.14

11.42

28.57

22.95

0

5

10

15

20

25

30

35

40

A B C D

[%]

Figure 8. The diagram of distribution costs on functions

The next step of the study is to compare the share value

functions and their cost of use. This study must identify: - Functions that are too expensive in comparison with

the others - Functions with low contribution to battery value but

too expansive - Functions that are too expensive but difficult to

make The regression line from figure 9 shows the

disproportions in the costs/functions repartition and their contribution to the battery capacity.

Examining the graph from figure 9 we see that the functions B and D are placed above the regression line, which means that these functions are expensive compared to battery capacity, but, as we can see, these functions are not very far from regression line what means that this is not a major problem.

340

Regression line

40; 37,14

10; 11,42

30; 28,57

20; 22,95

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40 45

A

Figure 9. The graph of functions’ shares of batteries in value and cost

VI. CONCLUSION This paper was made in the project “Integrated micro

CCHP - Stirling Engine based on renewable energy sources for the isolated residential consumers from South-East region of Romania (m-CCHP-SE)” and presents a case study to exemplify approach of design algorithms that we propose for the electrical part of the system.

System optimization and design criteria are very important, even if the optimal solution is subjective, depending on the options of the beneficiary; we tried to quantify the functions of every component module of the system and we proposed a scientific method very easy to apply for separate parts and for the entire system.

ACKNOWLEDGMENT This work was funded by means of the EEA grant no. RO 0054 for the project entitled: “ Integrated micro CCHP-Stirling Engine based on renewable energy sources for the isolated residential consumers from South-East region of Romania”.

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[1] Course of economic efficiency, master year 1, Mircea Covrig

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[3] User’s and installer’s Manual Sine wave Inverter, Battery charger, Transfersystem HP-COMPACT - STUDER INNOTEC

[4] Ghid de alegere a sectiunii conductoarelor electrice – S.C. Iproeb S.A. Bistrita

[5] A Guide To Photovoltaic (PV) System Design and Instalation - Endecon Engineering

[6] Manuel d‘Installation et d‘utilisation – Steca Xtender [7] Panouri fotovoltaice POLICRISTALINE- fise

tehnica – S.C. ASON Trading S.R.L. [8] Baterii acide cu plumb sigilate, cu valva regulatoare

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