experimental analisys of a cogeneration system

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______________________________1 Engenheiro Mecnico NEST/IEM UNIVERSIDADE FEDERAL DE ITAJUB2 Engenheiro Mecnico NEST/IEM UNIVERSIDADE FEDERAL DE ITAJUB3 Prof. Dr., Engenharia Mecnica NEST/IEM UNIVERSIDADE FEDERAL DE ITAJUB4 Prof. Dr., Engenharia Mecnica NEST/IEM UNIVERSIDADE FEDERAL DE ITAJUB

IBP2976_10EXPERIMENTAL ANALISYS OF A COGENERATION SYSTEM

COMPOSED BY A GAS MICROTURBINE AND A LiBr-H2O

ABSORPTION CHILLER

Bruno R. Cantarutti1, Alexandre A. Salioni da Silva2, Osvaldo J. Venturini3,Electo E. Silva Lora4

-

Copyright 2010, Instituto Brasileiro de Petrleo, Gs e Biocombustveis - IBPEste Trabalho Tcnico foi preparado para apresentao na Rio Oil & Gas Expo and Conference 2010, realizada no perodo de 13a 16 de setembro de 2010, no Rio de Janeiro. Este Trabalho Tcnico foi selecionado para apresentao pelo Comit Tcnico doevento, seguindo as informaes contidas na sinopse submetida pelo(s) autor(es). O contedo do Trabalho Tcnico, comoapresentado, no foi revisado pelo IBP. Os organizadores no iro traduzir ou corrigir os textos recebidos. O material conforme,apresentado, no necessariamente reflete as opinies do Instituto Brasileiro de Petrleo, Gs e Biocombustveis, seus Associados eRepresentantes. de conhecimento e aprovao do(s) autor(es) que este Trabalho Tcnico seja publicado nos Anais da Rio Oil &Gas Expo and Conference 2010.

Resumo

A utilizao de sistemas de cogerao pode ajudar a descentralizar e aumentar a confiabilidade do sistema detransmisso de energia eltrica brasileiro, diversificando as fontes energticas e aumentando a capacidade de gerao

de eletricidade a partir de capital privado. Os sistemas de cogerao constitudos de microturbina a gs e chillers deabsoro esto se tornando atrativos no s pela economia de energia, devido possibilidade de recuperao de calordos gases de exausto, mas tambm pelo aumento da utilizao de gs natural e gs liquefeito de petrleo (GLP) no

pas pelos crescentes investimentos neste setor, e tambm, pelas questes ambientais. O objetivo principal deste art igo apresentar uma anlise experimental de um sistema de cogerao, composto por uma microturbina a gs Capstone de30 kW, um recuperador de calor e um chiller de absoro de 10 TR (35 kW) de simples efeito com H 2O-LiBr. Osresultados experimentais sero usados para demonstrar detalhes de desempenho do sistema e caractersticas deoperao para diferentes condies.

Abstract

The use of cogeneration systems can help to decentralize and increase the reliability of the electricity generation andtransmission systems, diversifying the energy sources and increasing the installed capacity using private capital.Cogeneration systems using micro turbines and absorption refrigeration chillers are becoming more attractive due the

possibility of primary energy savings, as a result of the energy recovery from the micro turbine exhaust gases, theincrease in the natural gas and the liquefied petroleum gas (LPG) utilization in the Brazilian energy market, asconsequence of the increasing investments in this sector, and also, due the environmental concern. The main goal ofthis paper is to present an experimental evaluation of a cogeneration system, comprised of a 30 kW Capstonemicroturbine, a heat recovery and a 10 TR (35 kW) single effect LiBr-H 2O absorption chiller. The results obtained byexperimental tests are used to demonstrate system performance details and operation characteristics at differentconditions.

1. Introduction

The search for new energy sources and new ways to supply the growing electricity demand, motivated by the

fast industrial and social world development in this new century, has been the focus of many scientific communities.This is an attempt to develop new systems capable of meeting the continuous improvement on energy utilization,combining compatible costs to market demand, reliability and sustainable development in energy production, not onlyregionally but also globally. This is also a global scenario that is also valid to the Brazilian case.


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Rio Oil & Gas Expo and Conference 2010

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The difficulty in replacing fossil fuels by others energy sources, that can full fill the growth of the energydemand, has led to a new search for the development of systems to convert energy more efficiently and safely in orderto compete with the resources already available at an affordable price and not impacting the world economy. Evenmore, the International Institute of Refrigeration (IIR) estimated that 15% of the total electricity produced in the worldis destined only to the refrigeration and air conditioning systems (IIR, 1992).

Nevertheless, new discoveries in the oil sector (oil and gas) including the Brazilian context, has become this

last resource attractive in power generation of large and small scale, competitively priced with other existing forms ofgeneration, such as: hydro and nuclear, among others.

Cogeneration is a well-known technology for energy conservation in industry and in commercial buildings.Internal combustion engines and combustion turbines are widely recognized as important power generationtechnologies for combined cooling, heating and power (CCHP) applications. Nowadays, especially for large-scale,there are many applications considering the use of cogeneration such as, district energy systems (10 MW applications)and industrial cogenerations (5100 MW applications). Furthermore, cogeneration system is not only applied in large-scale systems but also in small ones.

Many small cogeneration technologies have been developed based on different prime movers such as internalcombustion engine, stirling engine, gas turbine and fuel cell. Small scale gas engine have many advantages, such as itsavailability in small sizes, fast start-up and shutdown capability, good part load operation, high electrical efficiencyand the ability and flexibility to run with different working fuels, such as LPG and natural gas (Kong et al., 2005).

However, limited research has been conducted for energy conservation in small scale buildings. Thus, thechoice of cogeneration system depends on many factors such as the choice of prime mover, the demand for power andthermal energy, and the technology development (Sun and Xie, 2010).

Normally, absorption chillers are considered for utilization in cogeneration, because they can be driven byseveral thermal alternatives such as combustion exhaust gases with an energetic potential availability, steam, hotwater, or even direct fired with natural gases or oil (Kong et al., 2005). Considering the possibility of using absorptionrefrigeration systems in cogeneration plants, added to the fact of its low power consumption, the study related to thesesystems has become increasingly attractive.

In additional, to the advantageous possibility of use waste heat discharged from industrial processes, theabsorption chillers also have a low consumption of electricity, what can reduce the electricity demand for coolingsystems, beyond a better utilization of the primary energy source for electricity generation. Moreover, these systemsare more environmental friendly, because the working fluids used (a binary solution) are not harmful and do notcontribute with the ozone depletion, such as the CFCs and HCFCs largely used by compression refrigeration systems(Balghouthi et al., 2006).

The Kyoto Protocol sets targets to be met by 2012 for the participating countries, which aim to reduce

emissions of greenhouse gases to 95% of the indices in 1990. This corresponds to the need to reduce, in 2006, levelsby about 30%, which impacts strongly on the energy issue, considering that it accounts for 86.6% of these emissions(Dantas, 2009).

Besides providing advantages to reduce the electricity consumption and the possibility of using waste heat,these systems have noise and vibration levels reduced when compared to vapor compression system (Dorgan et al.,1996). Among the working fluids used, such as water-ammonia (NH3-H2O) and lithium bromide-water (H2O-LiBr),the most used in absorption systems (Li and Sumathy, 2000), do not affect the ozone layer.

However, these systems have disadvantages, such as its low performance, the higher installation cost andcrystallization possibilities, in the case of use H2O-LiBr solution. However, when it has a residual heat source or lowcost fuel is available, the absorption refrigeration systems can be advantageous compared to vapor compression.

2. Description of the Cogeneration Plant

The small capacity cogeneration plant, shown in Figure 1, which is installed in NEST/IEM laboratories of theFederal University of Itajub (UNIFEI) was designed to produce chilled water recoveries of energy from exhaust gasesof a microturbine (Capstone, model 330), with a net power of 30 kW at ISO conditions (15 C/ 101.3 kPa/ 60% RH).

The recovery of energy from the microturbine exhaust gases is accomplished through a heat exchanger(Enedi, Model ITC-1) capable to produce about 8 m/h of water at 90 C. The hot water produced is used as an energysource of the single effect absorption chiller of H2O-LiBr, which design capacity is 10 TR (35.2 kW). This coolingsystem is capable to produce 5.5 m/h of cold water at 7 C when operating at nominal conditions.

The Figure 1 presents the microturbine (1), the heat exchanger (2) and the absorption chiller (3)compounding the small cogeneration plant.


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The cold water produced in the chiller is, then, directed to a reservoir where electric heaters are installed. Thecontrol of the power of these resistors allows its variation from zero to full power (35 kW). This control mechanismallows the simulation of a thermal load, for example, it's able to simulate the load profile of an air conditioning systemfor thermal comfort.

1

2

3

1

2

3

1

2

3

Figure 1 Cogeneration system installed at NEST/UNIFEI.

2.1. Experimental ProcedureThe tests were performed with the microturbine using liquefied petroleum gas (LPG). The power of themicroturbine was maintained constant at a fixed net power value. The exhaust microturbine gases heat the water in theheat recovery. The water is supplied at the temperature required to initiate the process of evaporation of the water inthe desorber of the absorption chiller.

When the chiller started to produce water at 7 C the process of applying heat load on the system, through theelectrical resistances, was initiated. For each microturbine operation condition, the power of the electrical resistancewas changed from zero to 35 kW, at increments of 5 kW.

In each of these load steps data were collected the operation, such as: temperature, flow rates and pressures ofthe points of entry and exit of microturbine exhaust gas, inlet and outlet water from the recovery boiler, chilled water,cooling water, etc, as can be seen in Figure 2.

2.2. Performance ParametersThe experimental investigation carried out focused on the behavior of cogeneration system operating at part

load conditions. Among the analyzed parameters, are included the turbine efficiency ( MT ), the absorption chiller

coefficient performance ( COP ), the efficiency heat recovery (HR ) and the cogeneration system efficiency ( cog ).

Figure 2 Small cogeneration plant schematic.


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To study the performance of the cogeneration system, the heat flows, the efficiences and other parameters areobtained by the follow equations:

..

6 , 7 6

. .

4 , 5 4

. .( )

. .( )

p wo

g p w

m c T T QCOP

Q m c T T

(1)

. .

, 2 3. .( )gases p gasHRQ m c T T

(2)

.

,

.

.

MT c

MT

fuel LPG

W

m LHV

(3)

. .

,

.

.

MT c ocog

fuel LPG

W Q

m LHV

(4)

where,.

oQ Cooling capacity (kW);.

gQ Heat supplied to the chiller desorber (kW);

.

HRQ Heat recovered in the heat exchanger (kW);

.

MTW Net power (kW);.

gasesm Exhaust gases flow rate -. .

air combm m (kg/s);.

airm Air mass flow rate (kg/s);.

fuelm Fuel mass flow rate (kg/s);

,p wc Water specific heat 4.187 (kJ/kg C);

,p gasc Gas specific heat 1.148 (kJ/kg C);

LPGLHV Liquefied petroleum gas lower heating value 46670 (kJ/kg).

The Equations 3 and 4 are the efficiency equations, MT is the microturbine efficiency and cog is the small

cogeneration plant efficiency, respectively.To accomplish the correction of temperature and atmospheric pressure the following equations were used:

1014atmP (5)

273.15

288.15ambT

(6)

where, is the coefficient of correction for the ambient temperature and is the coefficient of correction for the

atmospheric pressure. The atmospheric pressure atmP and ambT is the ambient temperature. Using these parameters the

air mass flow rate (.

airm ), the fuel mass flow rate and the power output from the microturbine (.

MTW ) are corrected bythe equations below, where the subscript c mean corrected:


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5

..

,.air

ai r c

mm

(7)

..

,.fuel

fuel c

mm

(8)

..

,.MT

MT c

WW

(9)

The results of the measurements analyzed were computed only under steady conditions. To separate thisresults at steady condition it was segregated an interval of 1-2 minutes and calculated the average and standarddeviation which were compared with the smallest range of the measuring instrument.

2.3. Measurement Instruments and Data AcquisitionThe main equipments used to measure the parameters of the cogeneration plant are:- Bourdon Manometers to measure water pressure in all circuits;- Thermoresistances Pt-100 to measure temperature in the chilled, cooling and hot water circuits;- Turbine type flowmeter to measure the volumetric flow of chilled, cooling and hot water;- Orifice plate flowmeter to measure the volumetric flow of air and fuel;- Pressure and temperature transducer.The water temperature controls is made through a three-way valve located in the cooling, chilled and hot

water circuits. The simulation of the thermal load was made by a set of seven water immersion tubular heaters, 5 kWeach one.

All measured quantities are converted into electric signals and compose a data acquisition system. Thesoftware, Elipse Scada, allows to visualize and to memorize all parameters necessary to analyze the behavior of thecogeneration system. The Figure 2 shows the screen of the Elipse Scada acquisition data.

Figure 3 Acquisition data software.


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The main parameters acquired by the systems are reported in the Table 1 below:

Table 1 Parameters acquired by the system in real time.

2T Exhaust gases temperature.

4m Hot water flow rate

3T Emissions temperature

.

6m Chilled water flow rate5T Inlet hot water temperature

.

8m Cooling water flow rate

4T Outlet hot water temperature.

airm Air flow rate

8T Inlet cooling water temperature.

combm Fuel flow rate

9T Outlet cooling water temperature.

MTW Net power

6T Inlet chilled water temperature atmP Atmospheric pressure

7T Outlet chilled water temperature ambT Ambient pressure

2.4. Experimental UncertaintiesAll measurement instruments, such as, flowmeters and termoresistances, present a measurement error. This

kind of error, denominated direct measure error, is transmitted to other datas which depends on these parametersmeasured. The error propagation of the indirect measures is calculated through partial derivative of each parameters ofthe equation. An example is showed below (Equation 10), which represents the partial derivative to obtain the error ofthe coefficient of performance ( COP ).

2 2 22.2 2

6 7 6.7 66

2 2 22.2 2

4 5 4.5 44

. . .

. . .

COP COP COPm T T

T TmCOP

COP COP COPm T T

T Tm

(10)

3. Experimental Data and Results

The experimental results, obtained from the testes realized in the small cogeneration plant installed inlaboratories of the Federal University of Itajub, are presented in this section. As mentioned before, among all the datacollected, only the ones obtained when the system were operating at steady state were considered in this analysis.

The values of some parameters measured, such as, inlet cooling temperature, hot water temperature, outletchilled water temperature, flow mass rate of cooling, chilled and hot water and net power are showed in the Table 2.

Table 2 Data acquired

Absorption chiller Microturbine Heat recovery

5T 7T 8T.

4m .

6m .

8m .

,MT CW .

,air Cm .

,fuel Cm 2T 3T

C C C m/h m/h m/h W Nm/h Nm/h C C86.1 7.1 26.8 7.7 3.6 9.3 21807.3 1049.9 6.1 267.0 148.590.5 6.6 26.7 8.1 3.7 9.3 21862.1 1050.0 6.1 266.4 151.389.9 6.8 26.5 8.0 3.6 9.3 22207.3 1048.7 6.1 265.4 152.991.9 6.6 26.4 8.0 3.7 9.3 22401.2 1047.0 6.2 264.5 157.588.6 7.4 26.5 7.9 3.6 9.3 22661.1 1040.3 6.2 264.4 151.588.1 7.8 27.2 7.8 3.7 9.2 22673.8 1041.3 6.2 264.4 151.290.2 7.0 26.2 8.0 3.7 9.3 23137.6 1047.5 6.3 265.1 153.388.9 7.0 27.4 8.0 3.7 9.4 23580.6 1048.9 6.3 263.2 153.391.6 6.8 28.3 8.0 3.6 9.3 24140.2 1055.6 6.4 262.6 153.2


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The cooling capacity, desorber capacity, heat recovery capacity, the coefficient of performance, themicroturbine and cogeneration efficiency are based on the Table 2 data and presented in Table 3.

Table 3 Evaluated parameters

.

o

Q .

gQ

.

HRQ COP

MT

cog

kW kW kW - % %9.41 39.82 47.44 0.24 27.73 39.6910.59 39.77 46.10 0.27 27.77 41.2210.77 40.63 44.98 0.26 27.94 41.4811.31 40.14 42.76 0.28 28.08 42.2516.73 43.11 43.06 0.39 28.10 48.8515.94 42.73 43.25 0.37 28.04 47.7517.11 41.40 42.94 0.41 28.30 49.2216.25 41.74 42.21 0.32 28.66 48.4117.20 40.81 42.31 0.47 28.94 49.57

As can be seen in the Figure 4, with the increase of the microturbine output power, the cooling capacity andcoefficient of performance increase. During the operation of the microturbine at 24 kW, the absorption chiller reaches50 % of its nominal capacity, producing chilled water at 7 C.

The relative error on COP and.

MTW is approximately 8 % and 2 %, respectively. The evaluation presented byAsdrubali and Grignaffini (2005) shows a 9 % relative error.

0.0

5.0

10.0

15.0

20.0

25.0

21500 22000 22500 23000 23500 24000 24500

Microturbine net power (W)

CoolingCapacity(kW)

0.1

0.3

0.4

0.6

0.7

0.9

1.0

COP(-)

COP (-)Cooling capacity (kW)

Figure 4 Variation in the net power to cooling capacities and coefficient of performance.

Analyzing this Figure 4, it is possible to see that the microturbine power varies from 70 to 80 % of itsnominal capacity, while the absorption chiller cooling capacity raises, approximately, from 25 to 50 % its capacity.Even if the microturbine achieves its nominal capacity, the absorption chiller will not reach 10 TR. This occurs

because the nominal microturbine thermal load isnt able to demand the nominal chiller capacity.In the Figure 5, the exhaust gases flow rate in the heat recovery increases with the elevation of the

microturbine output. This increase allows a better heat exchanged use and increases the cooling chiller capacity.


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0

10

20

30

40

50

60

21500 22000 22500 23000 23500 24000 24500

Microturbine net power (W)

Coolingcapacity(kW

0.28

0.29

0.30

0.31

0.32

0.33

0.34

Exhau

stgasesflowrate(kg/s

Exhaust gases flow rate (kg/s)Cooling capacity (kW)

Heat recovery boiler (kW)

Figure 5 Variation in the exhaust gases temperature to the net power and cooling capacities.

The Figure 6 shows the efficiency of the microturbine and the cogeneration system as a function of themicroturbine net power variation. With the increase of the net power due the elevation of the fuel flow rate in thecombustion chamber, the inlet microturbine temperature increases. Consequently, the microturbine efficiencyincreases and also the nominal load microturbine tend to achieve the maximum efficiency.

With the microturbine net power increase, it allows a greater heat exchange in the heat recovery whichprovides higher cooling capacity. Therefore, the increase of the cooling capacity and net power microturbine areproportionality bigger than the fuel mass flow rate elevation, what leads to a cogeneration efficiency increase.

0

20

40

60

80

21500 22000 22500 23000 23500 24000 24500

Microturbine net power (kW)

Efficiency(%)

Cogeneration system

Microturbine

Figure 6 Microturbine and cogeneration system efficiencies.


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The microturbine reaches efficiencies of 27-29 % operating between 21.5 to 24.0 kW. According to Hwang(2004) regenerative microturbine can reach high efficiencies of 27-30 %.

4. Conclusions

This paper presents an experimental analysis of a small scale cogeneration plant, composed of a gas

microturbine, a heat recovery boiler and a single stage absorption chiller. The results showed the microturbineoperating between, approximately, 21.5 and 24.0 kW. The chiller capacity increases when the electrical power rises, asexpected, reaching 55 % of its capacity when the microturbine was operating at 80 % of its nominal power.

For on-site power generation the microturbine didnt reach the nominal load due the operation conditionswere different from the ISO conditions (15 C/ 101.3 kPa/ 60% RH). As a suggestion for improvement the powergeneration can be used, for example, two microturbines working in parallel.

Performance tests showed that the entire cogeneration system performed more efficiently when themicroturbine produces more electrical power. The coefficient of performance also increases with the outputmicroturbine variation, reaching a value of 0.47 when the microturbine was producing 24 kW of electrical power.

5. Acknowledgements

The authors would like to express their acknowledgement to FAPEMIG, CNPq, Petrobrs and Cemig for thefinancial support provided for the present study.

6. References

ASDRUBALI, F., GRIGNAFFINI, S. Experimental evaluation of the performance of a H2O-LiBr absorptionrefrigerator under different service conditions. International Journal of Refrigeration, v.28, p.489-497, 2005.

BALGHOUTHI M., CHAHBANI M. H., GUIZANI A. Feasibility of a solar absorption air conditioning in Tunisia. Building and Envirolment v.43, p.1459-1470, 2006.

DANTAS, F. Sistema de climatizao do Salvador Norte Shopping incorpora tecnologias para eficincia energticae reduo de emisses. Climatizao & Refrigerao, n. 110, p. 52, 2009.

DORGAN, C. B.; STEVEN, P. L; DORGAN, C. E., Application Guide for Absorption Cooling/Refrigeration usingRecovered Heat, ASHRAE, 1995.HWANG, Y. Potencial energy benefits of integrated refrigeration systems with microturbine and absorption

chiller. International Journal of Refrigeration, v.27, p.816-829, 2004.IIR (INTERNATIONAL INSTITUTE OF REFRIGERATION), 1992, "Solar Energy for Refrigeration and

Air Conditioning: Commissions", IRR, Paris.KONG, X. Q., WANG, R.Z., WU, J. Y., HUANG, X. H., HUANGFU, Y., WU, D. W., XU, Y. X. Experimental

investigation of a micro-combined cooling, heating and power system driven by gas engine . International Journalof Refrigeration, v. 28, p. 977-987, 2005.

LI Z. F., SUMATHY K., Tecnology development in the solar absorption air-conditioning system. Renewable andSustainable Energy Reviews v. 4, p. 267-293, 2000.

SUN, Z. G., XIE, N. L. Experimental studying of a small combined cold and power system driven by a micro gasturbine. Applied Thermal Engineering, v.30, p. 1242-1246, 2010.

experimental analisys of a cogeneration system

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