ECOS: Computational Simulation of Cogeneration Plants for Energy Efficiency Strategy

ECOS: Computational Simulation of Cogeneration Plants for Energy Efficiency Strategy Ricardo Antonio do Espirito Santo Gomesa , Ednildo Andrade Torresb a

b

USP – Universidade de São Paulo, São Paulo, Brasil UFBa – Universidade Federal da Bahia, Salvador, Brasil

Abstract: The intense penetration process for evaluating new projects and retrofit second energy efficiency standards and sustainability parameters have attracted interest in the computer programs development to simulate accurately the buildings overall efficiency. Besides its power and thermal own generation is possible to employ the cogeneration technology as a strategy for energy efficiency in the certification process. This paper shows the computer program results was developed to interact with the Energy Plus in the commercial buildings analysis that have cogeneration plants in its main energy efficiency strategy. The program was developed in Software Engineering Equation Solver - EES. The computational model is characterized by its flexibility since the cogeneration plants geometric factors are not determined. To become eligible for LEED certification, for example, the cogeneration plants must meet the minimum annual energy efficiency required in the standards. The intention to qualify the cogeneration plants through its annual energy efficiency is to encourage efficient operational systems avoiding peak demand. The results simulated are for systems in this study may in accordance with the operational reality of any existing plant or project.

Keywords: Cogeneration, Simulation, Energy Plus, EES, LEED

1. Introduction The strong penetration of procedures aimed at the evaluation of new projects and retrofit front of energy efficiency standards and sustainability criteria have attracted interest in the development of computational tools to assess reliably the overall efficiency of buildings. Besides the search for thermal cycling more efficient and less environmental impact associated with the evolution of gas turbines and the growing availability of natural gas, has been a major factor behind the boom in combined cycles and systems of co-generation. Given the great diversity of commercially available machines, the introduction of cycles of this nature requires a review of a technical and economic point of Design and off design. The co-generation power takes advantage of the fact that the activity, whether industrial or commercial, because you need heat to produce a low-cost electromechanical energy. Given the current trend in the tertiary sector buildings, such as large institutions, hotels, hospitals and shopping centers, look for ways to achieve self-

sufficiency in power, the subject cogeneration has received considerable attention. The aim of this study is to show the results of a computer program designed to interact with the Energy Plus in the analysis of commercial buildings that have cogeneration in its main strategy for energy efficiency. The program was developed in Software Engineering Equantion Solver EES. The cooling loads for the building were calculated by the Energy Plus project the days of the city of Sao Paulo. The conditions of peak used to scale the capacity of the central chilled water were obtained from three-day project set out in Appendix G of ASHRAE Standard 90.1-2007 with the following probability: 99.6%, at temperatures of project heating and 1% for temperatures cooling design dry bulb and wet bulb. The algorithm also provides an analysis energy-economic system of cogeneration, incorporating aspects regarding the configuration of the steam generator. This way you can show the direct influence of parameters such as Pinch Point and Point Approach, establishing an analysis that links important variables inherent in the system boundaries. The program developed at the same time

makes the thermodynamic study, the economic analysis and evaluates the performance of the system of cogeneration for 8760 hours in accordance with input data provided. Established the point of the project is to study possible scenarios, to estimate reliability, the system performance under new operating parameters. With this modeling is to be a powerful tool in the design and operation of systems for cogeneration and combined cycle plants.

2. Building description and systems This study analyzed a building of 25 floors generic type. In defining that geometry sought to represent the zoning in accordance with the guidelines of ASHRAE Standard 90.1-2007 for the design of HVAC systems. Each floor of the building was divided into 4 zones conditioned and 5 core areas. Figure 1 shows a schematic diagram of the thermal areas considered for the pavement.

Core

Zone 2

Zone 4

Zone 3

Tabela 1: Brief descriptions of the building and HVAC systems. Items Description General information Location São Paulo, SP Buildin type and Office building, 25 stories stories above ground Floor áreas Total floor =23105m2 Air-conditioned =16264 m2 Height Floor-to-floor = 3.4 m Fenestration área WWR=50% Operation hours Design values of internal heat gains Occupancy 7 m2/person (conditioned area density 25 m2 person (core) Lighting density 12 W/m2 (conditioned area 5 W/m2 (core) Plug equipments 21.5 W/m2 (conditioned area load 0 W/m2 (core) Space design 25ºC temperature Infiltration 0.3 ACH Sizing runs SAO_PAULO/CONGONHAS - BRA TMY HVAC system and plant information HVAC systems VAV No-reheat and Fan-coil Chiller types Absorption Double Effect, Indirect-Fired Cooling nominal 1.2 COP (kW/kW) Fans and pumps Variable speed drive (VSD) for the central fan and chilled water loop pumps Cooling Tower Axial fan cooling tower with two-speed fans

Zone 1

type.

Fig 1. Thermal Zone of the building. Table 1 summarizes the parameters of construction, load data and the characteristics of air-conditioning of the building under study

3. Interface between Plus and ESS

Energy

The construction proposal was initially simulated in Energy Plus for lifting the thermal and electrical demands in accordance with the peak conditions set out in Appendix G of ASHRAE Standard 90.1-2007 and the file's climate Sao Paulo.The values demands were simulated for three typical days: a day of summer, a winter day and a day of transition process.The days of summer and winter day were used to establish respectively the maximum and minimum cooling loads. The days of transition was used for the establishment of conditions moderate load. These demands are exported to the computer program developed in the Engineering Equantion Solver - EES to simulate the system of cogeneration in terms of design and off

design. To evaluate the system of cogeneration operation in the whole year were used multiplication factors for the demands of summer, winter and transition as the tables below:

Table 2. Duration of the seasons. Southern Hemisphere Spring Equinox Summer Solstice Autumn Equinox Winter Solstice

Date 22/09 21/12 20/03 21/06

Duration 89.80 days 88.19 days 92.66 days 93.65 days

ambient conditions, these values are available in catalogs for a reference state, called ISO. With the use of computational befitting, you can size the turbine setting the local installation conditions. The temperature of the gas turbine inlet is determined from the energy balance in the combustion chamber. The fuel can be burned as natural gas or a gas from a gasifier. The main parameters of the gas turbine are shown in Tables 1 and 3.

6. HRSG modeling

Table 3. Multiplication factors Typical Days Summer

Multiplication factors 88.19 days

Transition

182.46 days

Winter

93.65 days

4. Configuration studied In this analysis we simulated the configuration shown in Figure 1, according to its compact nature, widely proposed for systems of cogeneration of the tertiary sector. The prime mover used corresponds to a gas turbine consists of an axial compressor, a combustion chamber and the turbine itself. In general, the turbine and compressor are mounted on a common axis so that the work required for compressing the air is obtained from the work of expansion of gases in the turbine. For the generation of electricity, the gas turbine is coupled to an electric generator. The ideal thermodynamic cycle for the modeling of plants consisting of gas turbines is known as the air standard cycle Joule, also called the Brayton cycle. The irreversibility associated with a turbomachine real, and the losses are considered through the isentropic efficiencies and coefficients of pressure loss.

In the modeling of recovery boilers, the product of the global coefficient of heat transfer area for heat exchange is determined for each heating surface at the point of the project. In this product are applied correction factors that take into account variations in the flow of gases in the steam and the thermodynamic properties and transport of gas, to assess conditions outside the project (Ganapathy, 1991). These properties are an exclusive function of temperature. The program allows to evaluate the auxiliary firing point out the project through the burning of fuel in the recovery boiler. This is possible without the injection of air "new", since the gases exhausted from gas turbine contains a significant amount of oxygen in a range from 14% to 16% by volume.

G

G Gas Turbine

EC

5. Modeling cycle gas turbine The main parameters that describe a gas turbine is the compression ratio and temperature of gases exiting the combustion chamber. Because of the variation constant

EV

Fig 2: Configuration: evaporator

economizer

and

The main parameters of the recovery boiler are shown in table 2 and 4. Table 4. Parameters related to gas turbine at the point of the project. Compression Ratio

12

--

Fuel Flow

329,4

kg/h

Airflow

24000

kg/h

Isentropic efficiency of compressor Isentropic efficiency of turbine Efficiency of electric generator Loss of pressure in the combustion chamber Loss of pressure in the turbine Local conditions

89

%

89

%

98

%

3

%

4

%

Ambient pressure

101

kPa

Ambient

30

ºC

Level

640

m

Relative

76

%

Table 5. Parameters related to the recovery boiler at the point of the project. Temperature of water supply Steam Pressure

110

ºC

735,5

kPa

Purges

1

%

Heat loss

2

%

Pinch point

11

ºC

Approach point

11

ºC

consumed in the building, produced by the plant co-generation, are treated as free energy. The fuel burned in the mill is charged based on actual market rates. If a surplus of electricity is produced by the plant, the power of extra fuel consumed to generate this surplus is treated as load process. In addition to the system to become eligible under the certification process for the minimum efficiency should be greater than or equal to 60%. This annual efficiency is a function of the system operation and should be evaluated through the simulation process for the total number of hours of operation of the plant.

8. Economic modeling The economic analysis applied to the computer program compares the investment costs, production costs of steam and electricity, to those practiced by the market for a decision. The methodology provides a possibility to share costs between the electricity and steam products. The economic viability of systems of cogeneration analyzed, is the cost of electricity production to cover the costs associated with conventional telephone systems. The computational code has as main results of economic analysis, the "pay back" simple, "pay back" discounted annual gross revenues and annual net revenue. Tables 5 show the input data needed for economic analysis.

Table 6 - Data used in economic analysis. Investment in Gas Turbine

600000

US$

Investment in Recovery Boiler

102000

US$

Investment in Conventional Boiler

60000

US$

7. Cogeneration LEED

Cost of fuel burned in Gas Turbine

0,011

US$/kWh

The methodology for simulation and validation of plant co-generation is based on procedures in Appendix G of ASHRAE Standard 90.1-2007 and the understanding of the LEED ® for the combined heat and power. In buildings that have co-generation in its main strategy of efficiency gains associated with the plant can only be considered when dealing with plant On-Site. In this case the CHP is located within the building. The electricity and thermal energy

Cost of fuel burned in the boiler

0,013

US$/kWh

Purchase Price of Electricity

0,07

US$/kWh

Selling Price of Electricity

0,05

US$/kWh

Interest Rate

12

%

Load Factor

90

%

Cost of Civil Works and interconnections-[System of Co-generation] Costs of Civil Works and interconnections[Conventional System] Electric Energy Demand Life System of Cogeneration

35

%

30

%

7000

kW

20

anos

9. Results The results are presented through the screens working computer program. Among the most important are the temperature-entropy diagrams for the system gas turbine electric power produced, the income associated with the system, the temperature profile of the recovery boiler, and power flows for each heating surface, and the results of economic analysis cited above. Figure 3 shows the curves of thermal and electrical demand of the building proposed for the typical days of summer (a), winter (b) and transition (c).

(c) Fig. 3. Electric and cooling demand Table 7 shows the thermal and electrical efficiency of the gas turbine, the efficiency of the recovery boiler according to ASME and overall plant efficiency in design conditions.

Table 7. Efficiencies of plant co-generation Thermal Turbine Gas Electrical Turbine Gas Thermal HRSG Overall

(a)

(b)

29.2 (%) 28.6 (%) 56.8 (%) 85.3 (%)

Figure 4 shows the temperature profile of the recovery boiler supplied by the computer program.

Fig. 4. Temperature-heat flux diagram

10. Conclusions Table 8 shows the performance of the recovery boiler rated by the computational code. Table 8. HRSG Performance Temperature (ºC) Power Flow Gas Water/Steam (kW) (kg/h) Surf. In Out In Out Evap 507 178 167 156 1890 3229 Eco 178 121 156 75 312 3261 Pinch 11 (ºC)

Approach 11 (ºC)

Table 9 shows the production costs of electricity and steam to the system of cogeneration considered in addition to the annual net revenue, gross and times of return on investment. Table.9. Economic Performance Production costs of electricity

0.05475 US$/kWh

Cost of steam

0.01571 US$/kWh

Gross Annual Income

177992 US$/year

Annual Net Revenue

146234 US$/year

Simple Pay back

6.0 anos

Discounted Pay back

11.0 anos

Table 10 shows the consumption and the annual cost of energy efficiency and annual plant co-generation. This parameter is required to verify that the system is eligible under the certification process.

Table.10. Annual Performance Plan for cogeneration Annual Consumption Annual Cost Annual Efficiencie

3558625.8 kWh 774176.2 R$/year 65.7%

This study aims to show how a computational model can be used to study the economic feasibility of cogeneration systems of gas turbine / heat recovery boiler at the point of project outside of the project. The main advantage of the computer program is the assessment for the heat recovery boiler. The influence of the Pinch Point and Point Approach in the flow of steam generated was a determinant in establishing the characteristics of the system cogeneration.

References [1] GANAPATHY, V.- Waste Heat Recovery Deskbook. Fairmont Press, Atlanta, GA, 1991.

[2] ALTAFINI, C.R. – Estudo Computacional dos Ciclos Combinados Gás/Vapor na Co-geração de Calor e Potência. Universidade de Caxias do Sul, 2003. [3] SILVEIRA,J.L; WALTER, A. C. S. e LUENGO, C.A.- Co-geração para Hospitais Estudos de Casos com Turbina a Gás. V Encontro Nacional de Ciências Térmicas, São Paulo, 1994. [4] ANTUNES, J.S.; SILVEIRA, J.L.; BALESTIERI, J.P. – Modelagem de Sistemas de Co-geração Utilizando Turbinas a Gás. VII ENCIT, Rio de Janeiro, 1998 [5] GYARMATHY, G. and ORTMAN, P. – The Off Design of Single and Dual Pressure Steam Cycles in CC Plants. ASME COGENTURBO,1991. [6] ANDREYEVICH, S. T. ; PINTO, J.H.F. – Shopping Centers: Motores ou Turbinas a Gás? Eletricidade Moderna, agosto, 1996. [7] OLIVEIRA,P. C.; NASCIMENTO,M.A.R.; NOGUEIRA.L.A. Análise Termodinâmica e Econômica de Ciclos com Biomassa Gaseificada em Sistemas de Co-geração para a Industria Madeireira. VII ENCIT, Rio de Janeiro , 1998. [8] BIASI, V. (Publisher), “For Project Planning, Design, Erection and Operation”, Gas Turbine World 2000 – 2001 Handbook, volume21, Pequot publication, USA, 2001.