POWER QUALITY EXPERIMENTAL ANALYSIS OF A MICRO-COMBINED 1 COOLING HEAT AND POWER SYSTEM FOR THE CONNECTION TO A 2

SMART GRID 3 4

N. Badeaa,*, I. Paraschiva ,M. Oancaa and G.V. Badeab 5 aDunarea de Jos University of Galati, Domneasca,800008, Galati, Romania, 6

b University of Amsterdam , Faculty of Law, Spui 21, 1012 WX Amsterdam, 7 Netherlands 8

*corresponding author: nicolae.badea@ugal.ro 9 10

ABSTRACT

The paper analyzes the compatibility of a micro-

Combined Cooling Heat and Power (mCCHP)

system and low voltage grid in terms of power

quality. The analysed mCCHP system is

implemented on Lower Danube University

campus and includes a micro-Combined Heat

and Power unit (mCHP unit) with Stirling engine

(9 kWe and 26 kWt), a conditioning system with

thermal activated chiller (15 kWt), a photovoltaic

system (2kW), solar thermal panels (14 kWt)

and an additional pellet burner (30 kWt).

Experimental tests are performed to show the

problems which arise from the operation of the

system in two situations: when only a part of the

electricity produced is injected in the grid (most

of it is consumed in residence) and when all

electricity generated is injected in the grid. The

experimental results can help define the

technical conditions to be met by mCCHP and

smart metering for their integration in Smart

Grids at low voltage. This analysis focuses

primarily on issues of the current approach of

the power quality and compatibility with a new

Energy Efficiency Directive (EED). In this

legislative proposal (EED), the meter operators

ensure that, the meter account for electricity

produced on the final customer's premises and

exported to the grid is monitored.

Keywords: microgeneration, power quality, renewable energy, Smart Grids

INTRODUCTION

The residential households and commercial

buildings sectors together are responsible for

over 50% of Europes electricity consumption [1]. The current electricity distribution system, in

many countries, treats home and working

environments as consisting of isolated and

passive individual units that receive electricity

only from the Distribution System Operator

(DSO). The increasing consumption of

household consumers is having a substantial

impact on the environment. For the reduction of

the negative impact the energy sector has on

the environment, in December 2008 the EU

legislator agreed on a Climate and Energy

Package that sets ambitious targets for the EU.

The RES Directive [2] has the purpose of

creating a common framework for the promotion of energy from renewable sources. The goal is to contribute to EUs objectives using renewable energy sources (RES). To

achieve its objectives, the Directive sets national

targets for the use of RES and encourages all

actors from the electricity market to participate in

the production of energy through RES. The

Directive acknowledges the positive impacts that

RES have on local communities and consumers

and supports the use of decentralised

renewable energy technologies.

The CHP Directive [3] has the purpose of

creating a general framework for the promotion

and development of high efficiency cogeneration of heat and power based on useful

heat demand and primary energy savings in the

internal energy market. The goal is to exploit the potential that CHP technologies have in

order to save primary energy, reduce

greenhouse gas emissions and help the security

of supply. The Directive applies to cogeneration

implying therefore that it addresses all interested

subjects from the electricity chain. Meant to be a

general framework for cogeneration, the CHP

Directive creates a legal basis for consumers to

get actively involved on the energy market.

However, it leaves it to the Member States to

enact measures that could stimulate consumers

to actively participate on the market. Therefore,

the Directive creates the basis for an open field

to be developed.

The previously discussed Directives

acknowledge the important role consumers play

in the energy sector and aim at consumer welfare in the energy sector to provide an additional impetus for consumer empowerment [4]. The consumers are the last link of the

energy chain being the end beneficiaries of

energy. The development of conversion systems

that use renewable energy sources combined

with EU and national legislative incentives [5]

have determined household consumers to install

individual microgeneration systems (solar

thermal PV, wind turbines or combined heat and

power) diminishing in this way the electricity

consumption from the grid. The extraordinary

potential offered by decentralised energy

sources for the production of energy on

household consumer premises can be exploited

and such energy can be exported back to the

grid if a legal framework would be set up to

connect this category of consumers to low

voltage grids. Unfortunately, it is not the case in

Romania. This leads to a severe limitation in

achieving energy efficiency and sustainability,

as it ignores the potential delivered by homes,

offices, and commercial buildings that are seen

as intelligent networked collaborations.

Having as background EUs 2020 Strategy in which energy efficiency is one of the key

priorities and after setting out the intent to use

Smart Grids and smart metering as means to

accelerate the reach of such ends, the

Commission put forward a proposal for a new

Energy Efficiency Directive [6], because the

current legal framework on energy efficiency

(the amended ESD and CHP Directives in

particular) is not fully covering the potential for

energy savings. In order to achieve the next-

generation energy efficiency and sustainability,

a novel Smart Grid ICT architecture based on

smart cogeneration-trigeneration systems

interacting with Smart Grids is needed.

This architecture enables the aggregation of

houses as intelligent networked collaborations,

instead of seeing them as isolated passive units

in the energy grid (empowering consumers).

MICRO-COMBINED COOLING HEAT AND POWER SYSTEM ANALYSIS

A very important application of using energy

supplying produced from renewable resources is

the so-called green house. The autonomous

energetic systems that are based on renewable

resources represent a field recently approached

in the literature, in connection to the concept of

intelligent building. They generally use CCHP

(combined cooling, heat and power) systems.

They provide the necessary electricity and heat,

thus ensuring the energy independence from

centralized energy systems. Such an

autonomous power system using renewable

energy for a residential building is located in the

campus of Dunarea de Jos University of Galati. The system is designed to meet the

thermal energy needs (heat in winter, cold in

summer and domestic hot water) and electricity

needs, with the two components: the operating

of the household equipments and the own

consumption of the supplying system with

thermal and electrical energy [7].

Figure 1 presents with continuous line, the

overall structure of the micro CCHP system. It

consists of two subsystems: electrical energy

supplying subsystem and thermal energy

system.. For power supply two sources are

provided, i.e.: a Stirling engine micro Combined Heat and Power (it also provides

modulating thermal energy 9-26 kWt and power

2-9kWe) and photovoltaic systems (2kW).

In case of insufficient solar irradiation a micro

CHP unit is used. The maximal return

temperature of the Solo V161 from Cleanergy is

60C .

Figure 1: Micro-trigeneration system

Solar thermal

collectors

Cold

water

Photovoltaic

panels

Chiller

Battery Electricity

Hot

water

CHP

unit

Pellet

boiler

Heat

Cold

Inverter Controller

Smart

Grid kWh

kWh

The electrical energy provided by the Stirling

engine and the photovoltaic systems is stored in

a battery of 48V/ 1000Ah. It aims to provide the

necessary electricity in the peak loads.

Considering the thermal energy outside the

Stirling engine, which delivers a maximum

power of 26 kWt , there are two more energy

sources: a pellet boiler that provides modulating

thermal energy in range 19-49 kWt and a solar

collector system that can deliver a maximum 14

kWt. The thermal energy produced by the three

sources is stored in a thermal accumulator

(thermal storage 2000l), in which the

temperature of the thermal agent varies

between 70 80 Celsius degrees. The heating system is supplied from this accumulator in the

winter with thermal agent the domestic hot water

and the thermal source for the cooling system in

the summer are also provided. It can be

mentioned that the cooling is done using an

adsorption system that needs a thermal energy

of 42 kWt with 15 kW cooling output. The

thermal cooling system based on adsorbtion

technology consists of three hydraulic circuits:

driving heat, cold distribution and heat rejection.

The basic performance data for adsorbtion

chiller type ACS 15 , SorTechh AG product are

presented in Table 1.

Table 1: Thermal cooling system

Basic performances data

Statistical Evaluation

Cooling capacity , max 23kW

COPtherm max 0,65

Cooling capacity nominal 15kW

COPtherm,nominal 0.6

Nominal working conditions

Temperature in/out

Driving heat circuit 72/65oC

Heat rejection circuit 27/32oC

Chilled water circuit 18/15oC

Temperature range (out) 6-20 oC

To assure the cooling capacity during all

weather conditions, other thermal heat sources

like a CHP unit and a pellet boiler are used. A

cold-water storage tank (500l) with hydraulic

separation is used for air conditioning in summer

time. A fan coil system assumes the cold

distribution of the building. This system can be

defined as combined heating system and can be

used for heating in colder periods as well

SYSTEM OPERATION AND CONTROL STRATEGY

Control strategy for off-grid operation

Energy conversion system allows modern

systems to operate in the autonomous mode

type off-grid. In this way the mCCHP system

must ensure both electricity and thermal needs

for the household. Using Programmable Logic

Controller (PLC) to control the two subsystems,

two scalar variables attached to the storage

elements have been chosen, namely the water

temperature T in the thermal accumulator and

the voltage from the battery terminals that has to

be maintained between the minimum and the

maximum value [8]. The control architecture is

show in Figure 2.

Figure 2: Architecture control

For the electric subsystem, the intermittent

production of electricity from the photovoltaic

panels is stored in batteries from which, using

triphased inverters, is converted to alternative

voltage. The CHP unit ensures the battery load

its level drops below the minimum value. Electrical energy balance is:

ElEilEgridPVEcgB PPPPP

dt

dW

(1)

where:

dWB is the variation of the power in the battery

[kWh];

PEcg is the power produced by the cogeneration

unit [W];

PPV is the power produced by photovoltaic

panels [W];

PEgrid is the quantity of electricity consumed from

the grid [W];

PEil is the power required to cover the internal

load (pumps, fans) [W];

PEl is is the power for the electrical useful load in

the house [W];

mCHP

unit

Battery

The energy

demand of the building

Electricity

load

Electrical

Subsystem

PEcg

PEil

T

Electrical

Energy

Balance

PV

source

PLC

kWh

kWh

Smart

Grid

PT

source Thermal

tank

Thermal

Energy

Balance Pellet

burner

Electrical

Internal

Load

Thermal

Subsystem

Heating

load

Cooling

load

DHW

Ta

V

Va PPV

PPT

PTcg

PTad

PTto

PEBo

PEl

PEgrid

PTH

PTC

PDH

W

For the thermal subsystem, the intermittent

production of heat of the solar panels together

with the intermittent production of the CHP unit

are storage in the thermal tank. The additional

pellet boiler ensures the thermal load of the

buffer is the temperature drops below the

minimum value. The heating system is supplied

from this accumulator in the winter with thermal

agent; the domestic hot water and the thermal

source for the cooling system in the summer are

also provided. The control system in this case

maintains a temperature in the cold storage

buffer between 5 and 15oC.

Thermal energy balance is:

DHWTCTHTadPTTcg

T PPPPPPdt

dW

(2)

where:

dWT is heat variation of heat storage tank [kWh];

PTcg is the thermal power produced by the

cogeneration unit [kW];

PPT is the thermal power produced by solar

thermal panels [kW];

PTad is the thermal power produced by the pellet

burner [kW];

PTH is the useful thermal power consumed in the

house for heating [kW];

PTC is the thermal power consumed in the house

for cooling [kW];

PDHW is the thermal power consumed in the

house for hot water [kW];

Control strategy for the connection to a Smart Grid

The Smart Grid introduces the concept of ICT,

architecture and technology for smart houses as

they are situated and intelligently managed

within their broader environment.

This concept seriously considers smart homes

and buildings as prosumers (proactive customers) that negotiate and collaborate as an

intelligent network in close interaction with their

external environment [9]. The context is key

here: the smart home and building environment

includes a diverse number of units: neighbouring

local energy consumers (other smart

households), the local energy grid, associated

available power and service trading markets, as

well as local producers (environmentally friendly

energy resources such as solar and (micro)CHP

etc.)

In a Smart Grid, there is only the possibility of

exporting/importing electricity depending on the

tariff for production or consumption. The typical

consumption wave is represented in Figure 3.

The mCCHP system allows both an increased

electricity production using the CHP unit

compared to the electricity consumption and

temporary storage possibility.

Figure 3: Electric load graph for 24 hours [10]

Depending on the electricity tariff in the Smart

Grid, the system can be formatted to various

operating systems. It aims at a better local

match between demand and supply, at

customer acceptance of management strategies

and at a more effective reaction to near-real

time changes at the electricity market level (e.g.

due to fluctuations in large-scale solar energy

production) and grid operations (e.g. for

congestion management and reserve capacity

operations).

In order to realise the electricity management

strategy, the consumer has at his disposal to

two control ways: the battery-loading period and

the load source (PV, CHP unit and Smart Grid),

depending on the electricity tariff. The electricity

export is controlled by the CHP units operating modes.

Depending on the requirements, the mCHP unit

may be operated using different types of

operating modes. Time-variant operation modes

may be implemented by means of an energy

management system, which selects the

optimum operating mode for the specific

requirements.

Heat-driven operation mode. The

control variable for the operation of the

CHP unit in this operation mode is

always the heat demand. The

architecture control from Figure 2 is

modified in the sense that the value

commanding the functioning of the CHP

unit is the temperature in the tank. In

this case, the loading of the battery, if

the battery voltage drops under the

minimum imposed value, is done from

the grid. The CHP unit with a Stirling

motor produces a constant power flow

to the grid. The power level is equalised

by the pressure of the working gas

(helium).

Electricity-driven operation mode.

For this operation mode, the electricity

demand is the control variable of the

power output from the CHP unit.

Principally there are two operation

modes possible: o When the system works connected with

the grid. The CHP unit supplies the consumers and charges batteries until it reaches its maximum electrical output. The architecture control from Figure 2 is modified in the sense that the value commanding the functioning of the CHP unit is the temperature in the tank. The power is supplied to the grid only if the battery voltage reaches its maximum value. In this case the electricity exported to the grid is variable and intermittent (see Figure 3).

o When the system works independently. The CHP unit (very often in combination with a battery system) has to cover the consumers demand on its own. The value for commanding and controlling the CHP unit is the battery voltage.

The mCCHP system is supported by an

additional boiler in order to cover the heat

requirements. The thermal energy, which is

produced simultaneously by the CHP unit,

should be used wisely so that the temperature

from the heat storage tanks is not released in

the environment.

It is also feasible to apply combined operating

modes: - Heat driven with peak-electricity

function; - Maximum electricity and/or heat

demand; - Minimum electricity and/or heat

demand.

POWER QUALITY EXPERIMENTAL ANALYSIS

Since the energy is given by the power-time

product, the primary quality indicators of the

electricity refer to:

The quality of the product (power

quality) such as frequency, load tension

amplitude and its sinusoidal form,

temporary overloads and tension dip;

The quality of the electricity from the

service supplier defined by the length of

the interruptions;

Taking into account the power quality aspects

from the supplier and user perspectives, the

power quality indicators can been classified in

two categories: primary indicators, that relate to

the quality of the product electric energy and the electric power supply service, and

secondary indicators, that relate in particular to

the disturbances caused by nonlinear receivers.

The implementation of these indicators allows

the delimitation between the attributions of the

supplier and the users for maintaining the

disturbances in the established limits.

The indicators are classified as follows:

frequency deviations, slow variations of the

supplied voltage, voltage surges, voltage dips,

short and long-term power supply interruptions,

voltage fluctuations, harmonic distortion, and

voltage unbalance. The limits of indicators are

presented in Table 2.

Table 2: EN 50160 Compliance Limits

Supply voltage characteristic

Statistical Evaluation

Compliance limit

Power frequency 95% of the time in 1 week 100% of the time in 1 week

50 Hz 1% 50 Hz + 4% to -6%

Supply voltage variations

95% of the time in 1 week

Uc 10%

Rapid voltage changes (and Flicker)

95% of the time in 1 week

Plt 1

Supply voltage dips

1 year None given1

Short interruptions 1 year None given2

Long interruptions 1 year None given3

Temporary over voltages

1 year None given

Supply voltage unbalance

95% of the time in 1 week

The importance of the harmonic distortion

problem is justified and based on the effects

caused in the supplier and user power networks.

Because the majority of the nonlinear receivers

that exist in the house are single connected -

phase power network, the voltage unbalance

phenomenon is very important.

The delivered currents are unbalanced (Figure

5) and non-sinusoidal (Figure 6) since the

consumers residences are nonlinear (light sources and electronic equipments) and

frequency converter starters are used to reduce

the start current of the pumps driven by

asynchronous motors (deformed consumers).

Figure 5: Phasors diagram of currents

Figure 6: Currents waveform

The harmonic content of the load currents

defined by relation 3 is high. The experimental

measurements (Figure 7) indicate values

between 10% and 40%, exceeding the admitted

values.

]][i[A

]n][i[A

Aharm

n

harm

thd1

50

2

2 (3)

where i: phase (0, 1, 2); n: rang (250)

Figure 7: Current harmonics

Power quality experimental analysis for CHP unit

The CHP unit must satisfy the EN 50160

standard and is limited to low and medium

voltage distribution systems. Specifically, the

following characteristics of the supply voltage

must be fulfilled: frequency, magnitude,

waveform and symmetry of the three-phase

voltages.

The experimental results relating to the voltages

generated by the CHP unit indicate a sinusoidal

symmetrical triphased voltage system (Figure 8)

with an 0,4% unbalance (Figure 11), within the

maximum admitted limits (max 2%).

Figure 8: Voltage waveform

The voltage frequency (Figure 9) is maintained

within the normal limits (50Hz +/-1%), with a

frequency deviation of 0, 24%.

Figure 9: Frequency

0

50

100 Athd1 Athd2 Athd3

49.5

49.7

49.9

50.1

50.3

50.5Frequency

The unsymmetrical and nonsinusoidal

consumption of the residence (Figure 6) is

compensated from the triphased inverters that

pass in rectifier mode loading the batteries. The

voltages produced by the asynchronous

generator become sinusoidal (Figure 10) and

symmetrical.

Figure 10: CHP unit currents waveform

The phase diagram of the voltages and

generator currents is presented in Figure 11.

Figure 11: Voltage and currents phasors

diagram

CONCLUSIONS AND RECOMMENDATIONS

The power quality experimental analysis of a

micro Combined Cooling Heat and Power

system with Stirling engine and renewable

energy sources for the connection to a Smart

Grid leads to the following conclusion:

The power quality indicators (voltage,

frequency, magnitude, waveform and symmetry

of the three-phase voltages) from the

experimental results fulfil the EN 50160

compliance limits.

The energy management of the CCHP system

is entirely up to the prosumer which, depending on its own seasonal energetic needs,

sets the operation mode of the microCCHP

system using a Programmable Logic Controller.

Depending on the chosen energy management,

the microCCHP system can also satisfy the EN

50160 requirements relating to short and long

interruptions.

The Heat-driven operation mode is used mostly

during the colder period of the year when the

prosumer wishes to satisfy the heat demand of the residence. The control variable for the

operation of the CHP unit is the tank

temperature. In order to fulfil the admitted value

for interruptions the CHP unit should be

connected directly to the Smart Grid, fully

exporting the electricity to the grid. The

electricity consumption of the residence will be

ensured from the battery and the loading of the

battery is done from the PV and/or intermittent

connection to the grid.

The Electricity-driven operation mode is used

mostly during spring-summer-autumn when the

prosumer wishes to satisfy the electricity demand of the residence. The control variable

for the operation of the CHP unit is the tank

temperature because the CHP unit should be

used wisely so that the heat is not rejected in

the environment. In this situation the CHP unit

charges the battery through triphased inverters.

The off-grid operation mode avoids the

overcharging of the battery by installing an

output triphased resistive load (with a protective

function) that is connected only if the battery

voltage exceeds the maximum value (56V in

real case). If instead of this resistive load, the

system is connected to the Smart Grid, the

number of interruptions would exceed the

admitted values and the currents supplied to the

Smart Grid would be nonsinusoidal and

unbalanced.

Comparing the experimental results for this

operation mode regarding the currents absorbed

by the domestic consumption (Figure 6) and the

currents produced by the CHP unit (Figure 10)

when the battery is in charging mode, the result

is a symmetrisation of the currents due to the

functioning of the inverters in rectifier mode. In

order for the system to fulfil the admitted limits

for the interruptions, the connection to the Smart

Grid should use a different triphased system

with inverters connected in the DC bus bar. In

this way, the number of interruptions in

supplying the grid is limited. The influences of

the consumers currents harmonics over the Smart Grid are limited as well due to the

protection of the grid through the DC link.

It must be pointed out that the entire analysis

has been conducted on the existing

experimental system from the Dunarea de Jos

University Campus in Galati, Romania.

The power quality analysis for the connection of

prosumers to a Smart Grid has been done according to the existing legal framework (both EU law and national law).

From a legal perspective, even with the newly

adopted Energy Efficiency Directive [6], there

are still gaps and uncertainties that have to be

further addressed. For instance, there is no

regulation on the Technical Compliance Limits

between prosumers and Distribution System Operators (DSO). Moreover, a clear contractual

framework should be established between

prosumers and DSO taking into account all the relevant factors influencing their relationship for

instance the characteristics of the electricity

produced by prosumers (intermittent). To sum up, the concept of Smart Grids is

feasible taking into account the results of the

analysis in this paper. However, the legal

framework should be consumer friendly, leading

to an accelerated use and development of

microgeneration systems and continuous

deployment of Smart Grids.

REFERENCES

[1] European Commission - EUROSTAT,

Electricity production, consumption and

market. Final energy consumption, Sept.

2012. Web. Dec. 2012.

[2] Directive 2009/28/EC of the European

Parliament and of the Council of 23 April

2009 on the promotion of the use of energy

from renewable sources and amending and

subsequently repealing Directives

2001/77/EC and 2003/30/EC (2009), OJ L

140, pp. 1662.

[3] Directive 2004/8/EC of the European

Parliament and of the Council of 11 February

2004 on the promotion of cogeneration

based on a useful heat demand in the

internal energy market and amending

Directive 92/42/EEC (2004) OJ L 52, pp. 5060.

[4] European Commission, Commission Staff

Working Paper, An energy policy for

consumers, SEC (2010) 1407 Final, 2010.

[5] Romanian Law 220/2008 on establishing the

promotion system of energy production from

renewable energy sources, republished

2010 Published in the Official Gazette, Part

I no. 577 of 13 August 2010.

[6] DIRECTIVE 2012/27/EU of the European

Parliament and of the Council of 25 October

2012 on energy efficiency, amending

Directives 2009/125/EC and 2010/30/EU and

repealing Directives 2004/8/EC and

2006/32/EC (2012) OJ L315.

[7] N. Badea, E. Ceanga, S. Caraman, M.

Barbu, Numerical Simulation of the

Conceptual Model for mCCHP-Stirling

Engine based on renewable energy sources

Proceedings 9th WSEAS International

Conference on System Science and

Simulation in Engineering (ICOSSSE 2010),

Iwate Prefectural University, Japan; 2010,

ISSN 1792-507X; ISBN 978-960-474-230-1.

[8] S. Caraman, M. Barbu, V. Minzu, N. Badea,

E. Ceang, Modelling and Control of an Autonomous Energetic System Obtained

Through Trigeneration Proceedings of the

14th International Conference on System

Theory and Control; Sinaia, Romania ISSN

2068-0465 and Buletinul Univ. Gh.Asachi Iasi

Tomul LVI(LX) fasc.4 -2010.

[9] K. Kok, et al., Smart Houses for a Smart

Grid, CIRED-20th International Conference

on Electricity Distribution Prague, 8-11 June

2009.

[10] Conrad Gahler., Micro-CHP with Stirling Engine-Activities at Siemens building

Technologies- International Conference

SEEC09, Trento, Italy, 2009..uilding T

ABSTRACTintroductionReferences