Time Independent Production

Power Efficiency Guide

Ultimate Guide to Power Efficiency

Get Instant Access

Improved base-load system design is demonstrated by four exploring journeys. The executable tools "SystemTL.exe," "DesalTL.exe," and "NovelsysTL.exe" are used. Automated local optimization in the three tools assumes a single dissipation price for {cdi} and 1 for {Kzl). The first journey deals with gas turbine power systems and is generated by the tool "SystemTL.exe." The second deals with seawater distillation systems and is generated by the tool "DesalTL.exe." The third deals with testing new ideas applied to coal-fired power plants and is generated by the tool "NovelsysTL.exe." The fourth deals with gaining insight in the improvement of fuel cells and is generated by the same tool "NovelsysTL.exe." The highlights of the systems and their results are presented. Running the executable tools and referring to detailed flow diagrams obtain the detailed results.

7.1.1 The gas turbine power systems

"SystemTL.exe" was used to optimize the design of five gas turbine configurations, in the range 50-100 MW nominal power output, each operating under the same boundary conditions. These are the simple gas turbine, the gas turbine systems with steam generated at 1, 2, and 3 boiler pressures all of maximum firing temperature 1600°F (870°C) and a 2-pressure, blade-cooled turbine of maximum firing temperature 2200°F (1200°C). The search for optimum was both automated and manual. In this application, the automated search proved to be more effective than the manual. The program displays the results in detail by stating properties of each stream, performance of each process, distributions of exergy destructions {£)}, characterizing surfaces {A}, and costs. Figure 7.1 is a summary of the investigation on a cost-efficiency plane and Figure 7.2 shows an outline of the flow diagrams of the five systems. The flow diagrams in more detail are presented in Section 8.3. The fuel price cF is assumed .01 $/kWh higher heating value. The {ca} set of Appendix 9.3.1 is assumed. The unit power production cost is the break-even cost and the efficiency is the conventional first law efficiency along with the corresponding second law efficiency that assumes the exergy of the finally leaving streams is wasted. The 2-pressure blade-cooled configuration (Case 4), shows the most cost-effective improvement. For the first four cases, the saving of fuel cost per unit product by raising efficiency was not eaten up by increases in the cost of

Product Cost cents/kWh

I fuel i devices

I fuel i devices

lies

gn:

automated

manual

search

search

1

simple gas turbine |

D

2

IP combined cycle #

3

2PC.C.

4

2P C.C. cooled blades '

5

3PC.C.

-H-

Efficiency

0.35

0.435

0.45 0.489

0.525

0.544 0.571

1i 02

First law efficiency (using higher heating value of fuel) Second law efficiency (wasting leaving streams)

Figure 7.1 Comparing five gas turbine design concepts on a cost-efficiency plane.

devices, after which a point of diminishing returns is approached. For the 3-pressure system, Case 5, the raising of efficiency became cost-ineffective.

7.1.2 The seawater distillation systems

Six systems of the 27 systems of "DesalTL.exe" were considered. All of them are 1860 m3/h (10 mgd) receiving seawater at 1 atm, 27°C and 0.045 salt by mass and rejecting brine at 0.065 salt-content. The multi-stage flash unit operates in the temperature range 100-38°C and the vapor compression unit below 60°C. The six systems were selected in order of increasing complexity. The first is the simplest. In this system 80% of the fuel exergy is destructed before reaching the MSF unit and 90% of the destruction occurs in four units: the combustor, boiler, throttle valve and the recovery stages. There is no way to improve the first three losses. The destruction reduced in a unit, moves to rCh

. Low firing T simple gas turbine

2 Low firing T Simple Combined Cycle

2 Low firing T Simple Combined Cycle cm jhp.lp stm turbines

3 Low firing T

4 High firing T

2-Blr Pressure Combined Cycle frehtr I

[hp,mp,lp stm turbines |

5 Low firing T 3-Blr Pressures Combined Cycle ip sh,blr

up ecom

Trz.

Figure 7.2 The gas turbine power configurations analyzed.

another. The next three are low capital cost improvement and the last two are high capital cost improvement. The first three import their power needs, the fourth produces its power needs only. The fifth co-generates power and water and the sixth produces power to produce water. Each system has a reference design and an improved one by optimization.

Table 7.1 and Figure 7.3 compare the six distillation systems. One reverse osmosis system is included for comparison with distillation. The production cost of

Table 7.1 The highlights of the journey results.

Case

System

Break-Even Water Cost

Fuel & Power

Input Energy

Capital

Efficiency+

($/ton)

(kWh/ton)

Cost ($/ton)

Cost ($/ton)

Wideall Wactual

Reference

Improved

Ref.

Impr.

Ref.

Impr.

Ref.

Impr.

Ref.

Impr.

1

Blr + msf

1.557

1.514

99.3

2.0

84.8

1.9

1.083

0.934

.474

.580

.0383

.0445

2

Blr + msf + 2s ejector

1.454

1.407

89.2

2.2

7.5

1.9

0.990

0.790

.464

.616

.0421

.0530

3

Blr + msf + tc-effect

1.519

1.454

96.5

2.1

79.3

1.8

1.058

0.875

.461

.582

.0393

.0477

4

Blr-msf + aux pwr

1.495

1.463

102.0

-

91.3

-

1.020

0.913

.477

.551

.0395

.0443

5

Blr + msf + pwr

1.001*

.954*

58.8

-

44.7

-

0.588

0.447

.413

.507

.0709

.0925

6

Blr + msf + vc-effect°

1.034

.958

53.2

-

34.4

-

0.532

0.344

.502

.615

.0759

.1173

7

One RO case

1.050

-

10

0.450

.600

.1345

*Cost allocation: a number of cost allocation methods has been reported. The fuel allocation method and the proportional method are frequently used. In the first, fuel is allocated by the ratio of produced powers. Water cost consists of the cost of allocated fuel plus the cost of the distillation plant unit. In the second method, three costs are computed by producing the products separately and combined. The benefit of cogeneration is allocated proportionally to each product. In the above table the first method is used. The second gives higher water costs (1.171, 1.119).

"Attractiveness of Case 6 is retained for a VC cost up to SlOOO/kW or $10000/ft2 blade surface. Efficiency measures around 20 on the gained output ratio scale.

+ Wideai = ideal separation work from sea water .045 salt content at 80°F= 1.345 kWh/ton.

Waauai = any work input + input fuel/3 (work that input fuel produces in a power plant 33% efficient).

Economy Creates Tobacco Cultivation

.01 .02 .03 .04 .05 .06 .07 .08 .09 .10 .11 .12 H-1-1-1-1-1-1-1-1-1-1-1-1-I-

Figure 7.3 The six cases compared on a cost-efficiency plane.

.01 .02 .03 .04 .05 .06 .07 .08 .09 .10 .11 .12 H-1-1-1-1-1-1-1-1-1-1-1-1-I-

Figure 7.3 The six cases compared on a cost-efficiency plane.

water by the co-generating system Case 5 requires an allocation assumption. Curves 5a and 5b bound the cost by reasonable assumptions. The penalty of running at 70% design power for 50% of time is indicated in Figure 7.3 (about 12% increase). System 6 by producing only water does not suffer this penalty. It has a good economic potential with specially designed steam compressors. The compressors in use at present are centrifugal that handle only 1/10 the unit capacity of the MSF and they should handle about the same capacity or even larger. A patent of a suitable steam compressor has been proposed (El-Sayed, 1997). Figure 7.4 shows an outline of the flow diagrams of the six distillation systems. Figure 7.5 presents a longer exploration journey. It compares 30 configurations on a cost-efficiency diagram. The flow diagrams of this journey are included in Section 8.3.

7.1.2.1 Production cost allocation: The allocation of production cost to power and water in a cogeneration system has been a topic of extensive discussion in desalination literature. Various methods have been proposed. None is free from debatable arguments. The result is a cost scatter of maybe up to 20%. Examples of proposed methods are:

• Assign the production cost in proportion to the cost of producing power and water separately. Various designs of different costs exist for the separate plants.

air prhtr to air ejctr

boiler v r-

hrottle I-

[MSF r jbrnhtr rej

[MSF r jbrnhtr

0 HLS

air prhtr to air ejctr stm turbine for pumps

|MSF r jbrn htr u

|MSF r jbrn htr

Kg boiler

thermocompression boiler

[suprhtr |

er u thermocompression

[suprhtr |

er u rej to

Figure 7.4 Analyzed seawater distillation design concepts.

• Divide the system devices into three sets. One set serves power, one set serves water and one set serves both as given by Equations 4.18 and 4.19. The selected sets can be as large as those who decide them.

• Divide the system devices into two sets, one for power and one for water. Assign fuel in proportion to the exergy destructions of the devices of each set. Sets of devices serving both power and water are ignored. This method, however, sets a suitable lower bound to the cost of water.

3.oa

2.75

1.50

1.25

0.7S

0.5a

0.25

Cost S/ton water product

Sola Distillation uster

3: Target lower cost solar stills

Envelope Cw = 5.12* T| Configurations ~ 30 Compatible boundary conditions Cf = $.01/kWh hhv Range n 3 to 60 L/kWh Cw 3.5 to 0.7 $/t

Lower capital -►cost cluster

Lower capital -►cost cluster

^Higher capital cost cluster

By Innovation

^Higher capital cost cluster

By Innovation

Efficiency Liters/kWh flld

0 10 20 30 40 50 60 70 80 90 100 110 120

Figure 7.5 Cost/efficiency trend of a longer seawater distillation journey.

• Assign an established cost for power production and charge the rest of production cost to water. There is no single cost to power production.

• Assign production cost in proportion to the exergy of the products. The exergy of water is negligible to that of power. Production cost goes to power.

There is a need for a standardized procedure that is simple and applies to any cogeneration configuration at any design point or operational state. The following is a proposed procedure:

1. Assign system devices to either power or water but not to both as far as capital cost is concerned.

2. Identify three main system characterizing exergies: Efueh Ethermah and Edesa/. The first is the fuel input chemical exergy. The second is the converted thermal exergy from the fuel chemical exergy. This is the exergy available to make products (power or/and water). For steam power systems, it is the exergy of prime steam state. For a gas turbine power system it is the firing state minus the compressor work (an inlet state to a free power turbine). The upper limit of Ethermai reflects the prevailing technological state of power generation. The third is the exergy of the stream feeding desalination. Its upper limit reflects the prevailing technological state of distillation

Fuel cost = Cf * Efuei Fuel cost assigned to water = Fuel cost * Edi,sa\IEthermai Fuel cost assigned to power = Fuel cost * {Ethermal - Edesai)/ElhermaI

7.1.2.2 Profitability objective function: The configuration of a cogeneration system decides the provisional water-to-power ratio desired. A steam power system configuration with backpressure steam turbine gives higher ratio than a configuration with extraction steam turbine or a gas turbine power system configuration. The expected marketplace prices for water and for power of the profitability objective function give the optimal ratio that maximizes profitability.

7.1.2.3 Power generation and power for separation: The above two journeys may indicate the following interesting observations:

• The range of second law efficiency in power generation is 20-55% while that for seawater desalting is only .04-. 13%, a case shared by many industrial processes. A room for future improvement of many industrial processes does exist. The appropriate technologies of industrial processes are yet to be discovered.

• The direction of lower unit product cost at higher efficiency appears to be toward more investment in devices to increase a system's product.

• Cost-efficiency diagrams seem to indicate the trend of envelopes that encompass lowest costs at highest efficiency for a prevailing technological state-of-art. The envelope can be extended to higher efficiencies by research and development and innovation. Figures 7.3 and 7.5 show such trends.

7.1.3 Higher efficiency coal-fired power plants

Two directions of raising the efficiency are examined along with an estimation of their cost-effectiveness. A conventional 50 MW pulverized coal power plant treating exhaust by precipitators and scrubbing is taken as a reference. The conventional plant has five feed heaters. Its prime steam is at 2400 psia, 750°F. Condensing temperature is 100°F. Adiabatic firing temperature is 4065°F (excess air ratio 0.1). Adiabatic efficiencies of steam turbines and pumps are .9 and .8 respectively. These parameters are kept constant for all solutions. Cost of fuel is assumed 0.003 $/kWh higher heating value.

The first direction tests the idea of raising the top cycle temperature to 1200-1400° F instead of the current 700-1000°F using high temperature material for the superheater and the reheater. The second direction tests the possibility of bringing down the adiabatic firing temperature to about 2600° F by radiation exchange with water-wallsboiler and exchanging heat with air as a working fluid of gas turbine in a high temperature exchanger (e.g. ceramic). Four alternative solutions assuming blade-cooled turbine are considered in this direction. Alternative 1 uses only a superheater and assumes a low-pressure ratio gas turbine open cycle with a regenerator. Alternative 2 is the same but uses a reheater. Alternative 3 uses a high-pressure ratio gas turbine open cycle without a regenerator. This exposes the high temperature heat exchanger to high pressure. Alternative 4 is the same but subjects the high temperature exchanger to low pressure by a closed air cycle of below atmospheric intake pressure. All high temperature surfaces are rated at double the cost of conventional per unit area.

The major results of the reference system and the five proposed solutions are compared in Table 7.2. Figure 7.6 is the flow diagrams of the reference system and the first direction solution. Figure 7.7 is the flow diagram of the four solutions of the second direction indicating the inactive devices in each case.

Economy Creates Tobacco Cultivation
Table 7.2. The cost effectiveness of raising the efficiency of coal fired plants.

#

Efficiency Work/Fuel

Cost Devices

Fuel ($/h)

Prdctn

($/kWh)

Work

Relative Masses

Exhaust T

Surfaces

stmt

airt

pmp

stm

Air

cgas (F)

stmt (ft2)

gt

hx (1000ft2)

hxht

1

0.3867

304

388

692

0.0138

445

_

13

1.162

_

319

261

_

232

_

2

0.4263

261

353

613

0.0122

485

8

0.695

485

173

166

3

0.4457

663

337

1001

0.0200

332

176

10

0.932

1.058

400

210

192

272

254

4

0.4499

820

333

1154

0.0230

337

175

9

0.790

1.058

400

183

191

471

417

5

0.4661

612

322

933

0.0186

322

209

10

0.928

1.052

400

191

182

323

194

6

0.4683

753

320

1073

0.0214

327

207

10

0.941

1.101

400

194

188

329

368

1 = Conventional as reference.

2 = Raising prime steam temperature tol300°F (gas temperature leaving boiler is adjusted to 2700°F, first and second feed heater pressure adjusted to 600, 400 psia).

3 = Open air turbine cycle of pressure ratio = 5.4 with regenerator. Adiabatic efficiency of compressor= 0.88. Equivalent adiabatic efficiency of turbine = 0.92.

5 = Open air turbine cycle of pressure ratio = 13.6. Regenerator and reheater are removed. Pressure to the high-temperature heat exchanger = 200 psia.

6 = Closed air turbine cycle of pressure ratio= 13.6. Air admitted to compressor at 114°F and 6 psia. Pressure to the high temperature heat exchanger = 80 psia.

Economy Creates Tobacco Cultivation

Alternative 1: Reheater not active, low press-ratio open-air-cycle Alternative 2: Same as 1 but uses a reheater

Alternative 3: Reheater and regenerator not active, high press-ratio open-air-cycle Alternative 4: Same as 3 but uses a closed-air-cycle, sub-atmospheric press intake

Alternative 1: Reheater not active, low press-ratio open-air-cycle Alternative 2: Same as 1 but uses a reheater

Alternative 3: Reheater and regenerator not active, high press-ratio open-air-cycle Alternative 4: Same as 3 but uses a closed-air-cycle, sub-atmospheric press intake

Figure 7.7 High temperature heater driving air turbine cycle: four alternatives.

All details of the coal-fired plants are available when running "NovelsysTL.exe." Although automated optimization is included for these systems, its use is not important at this stage of analysis. Cost effectiveness of systems 2-6 in descending order is: 2, 5, 3, 6, 4. The order is the same if the high temperature surfaces are rated at the same price as conventional, but cost effectiveness is relatively improved.

7.1.4 A fuel cell system

Direct conversion to work from fuel without moving parts is a tremendous advantage of fuel cells. Ideally work equals the exergy of fuel. The exergy of a fuel differs a little from the fuel higher heating value. The difference depends on the entropy of formation.

No successful fuel cells exist for fuels other than hydrogen. Fossil fuels have to be processed to produce hydrogen. This, for now, creates a barrier against simple systems of high conversion efficiency. A fuel cell burning natural gas is considered to gain insight in the added complexity of using a commercially available fuel and the inefficiency factors of the added complexity. Figure 7.8 shows a 200 kW fuel cell developed by ONSI Corporation, South Windsor, Connecticut. Since the actual information on the system is proprietary information, logical assumptions and published information on fuel cells (Appleby 1987, Karl Kordesch and Gunter Sinader 1996, Minh and Takahashi 1995, Singhal and Dokiya 1999) are used to model and analyze the system.

Onsi Fuel Cell

| storage/treatment "|

Figure 7.8 "ONSI" 200 kWe low temperature fuel cell cogeneration system.

| storage/treatment "|

Figure 7.8 "ONSI" 200 kWe low temperature fuel cell cogeneration system.

The system has eight main heat exchange devices, a reformer, a combustor, a hydro-desulfurization unit and a shift converter beside the fuel cell and its inverter. The heat tapped by exchanger 19 is delivered as co-generated heat to improve overall system efficiency. An ethylene glycol loop helps recover as much as possible from the HzO needed for the reformer by exchanger 18. The analysis of this application example is limited to thermodynamic analysis. Costs and automated optimization are not considered. Because of the large number of devices involving heat exchange, a special attention is given to the T-Q diagrams along the full heat exchange path to reveal temperature crossing or pinch points. Exchanger 18 has an unavoidable pinch point. One or more decision parameter can be changed manually to trace their effect on the system. As an example, five parameters are selected to show their influence on efficiency and co-generated heat. The parameters are changed one at a time. The parameters are the extent of fuel cell reaction, the fuel cell efficiency (cell work/cell enthalpy of reaction), the extent of reformer reaction, the reformer excess steam ratio and temperature sub-cooling by exchanger 17. Table 7.3 gives the parameters and their results. The first three parameters have appreciable effect. The last two have negligible effect. When the fuel cell efficiency (second parameter) becomes as high as 90%, no heat becomes available for cogeneration.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment