Efficiency and Environmental Comparisons Combined Heat & Power … · The enclosed information is...

Efficiency and Environmental Comparisons Combined Heat & Power (CHP) Separate Heat & Power (SHP)

Combined Heating & Cooling (CHC)

The enclosed information is provided to explain and illustrate some of the differences between Cogeneration and Trigeneration; Combined Heat & Power (CHP) and Separate Heat & Power (SHP); and to introduce Combined Heat & Cooling (CHC) as a (re)emerging technology for providing heat, power, and cooling in district energy systems. For these illustrations it is necessary to select a common fuel type for calculation and comparison of overall system efficiencies. Whereas natural gas is the fossil fuel of choice for many new base load grid power plants as well as distributed cogeneration plants it is the fuel selected for the comparisons herein. Although the figures for Stanford University energy loads and system comparisons are real, the comparisons of efficiency in the initial Examples are for simplicity based on balanced heat, power, and cooling loads. The Examples also represent overall average system performance and in reality heat, power, and cooling loads are rarely balanced and change regularly through the day and year. While these Examples can be used for indicative high level comparisons of efficiency and environmental performance of these technologies, specific calculations on much more discrete intervals (e.g. hourly loads) using actual equipment performance data will provide more accurate comparisons of any energy systems being compared. Nevertheless as the Examples indicate, for a wide range of heat & power loads in cogeneration settings; or heat, power, and cooling loads in trigeneration settings; the calculations suggest that the use of SHP with modest amounts of heat recovery and/or renewable power will quickly eclipse the overall efficiency and environmental performance of even the best 100% gas fired cogeneration systems. The comparative economics of the Example systems herein are dependent on site specific energy, capital, and O&M costs. At Stanford local economics favored SHP with Heat Recovery and Renewable Power, even with state mandates for 33% renewable power, due to the substantial amount of heat recovery possible. Substantial amounts of heat recovery and/or the use of ground source heat exchange may also be possible in many other district energy systems and yield surprising results when considering long term life cycle costs. For complimentary assistance in evaluating the potential for heat recovery in your system please contact: Joseph Stagner, P.E. Executive Director Department of Sustainability and Energy Management Stanford University 327 Bonair Siding Stanford, CA 94305-7255 650-721-1888 (work) [email protected] More information on SESI may be found at: http://sustainable.stanford.edu/sesi A copy of this document may be found at: http://sustainable.stanford.edu/sesi-technical

Introduction

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Definitions: CHP vs. SHP vs. CHC

Cogeneration: Heat & Power Combined Heat & Power

(CHP) Separate Heat & Power

(SHP)

POWER

HEAT

Steam Boiler or

Hot Water Generator

On-site or Off-site Power

Generation

Cogeneration Unit

POWER

HEAT

Trigeneration: Heat, Power, & Cooling

Steam Boiler or

Hot Water Generator


Generation

Combined Heat & Power (CHP)

Separate Heat & Power (SHP)

COOLING

Steam or Electric

Powered Chillers

Cogeneration Unit

Steam or Electric

Powered Chillers

POWER

HEAT

Trigeneration: Heat, Power, & Cooling


Generation


Combined Heat & Cooling (CHC)

COOLING

Steam or Electric

Powered Chillers

Cogeneration Unit

Heat Recovery

Chiller

Efficiency (Natural Gas fuel basis)

Gas In

Gas In

Gas In POWER

HEAT

Steam Boiler or

Hot Water Generator


Generation

Cogeneration Unit

POWER

HEAT

Steam Boiler or

Hot Water Generator


Generation

COOLING

Steam or Electric

Powered Chillers

Cogeneration Unit

Steam or Electric

Powered Chillers

Gas In

Gas In

Gas In

Cogeneration Efficiency = Heat + Power Total Gas In

Trigeneration Efficiency =

Heat + Power + Cooling

Total Gas In


Separate Heat & Power (SHP)

Note that for SHP the Total Gas In may be reduced by the use of Renewable power generation

Note that for SHP the Total Gas In may be reduced by the use of Renewable power generation

Comparative Efficiency (Example) CHP vs. SHP vs. CHC

Example Energy Diagrams

Combined Heat & Power (CHP aka Cogeneration) Separate Heat & Power (SHP) Combined Heat & Cooling (CHC aka SHP + heat recovery)

Natural gas selected as comparative base fuel All figures based on gas High Heating Value (HHV) Example figures based on balanced electricity, heating, and cooling loads

…but conclusions hold for other typical district energy system load balances too

Respective line losses for delivery of gas and electricity from production sources to final point of use are excluded but may be included for your site if you know the centroid of the respective production sources serving you

Example Calculations- Perfectly Balanced Cogeneration Plant

Cogen unit Power out: 35,849 btu

(=10.50 kwh)

CHP Efficiency

Prime Mover (Cogen) Efficiency = [Useful H + P Work] / Gas In Trigeneration Efficiency = [Net useful H + P + C ] / Gas In

Example: Cogeneration work (usable CHP unit output) = 31,000 + 35,849 = 66,849 btu Trigeneration work (usable overall system output) = 31,000 + 31,000 + 31,000 = 93,000 btu If total gas in to CHP = 95,650 btu: • Prime Mover efficiency =

66,849/95,650 = 70%

• Trigeneration Efficiency = 93,000/95,650 = 97%

.55 kw/ton *2.58 ton hr = 1.42 kwh to chiller &

CT

31,000 btu waste heat

to chiller

35,846 btu to atmosphere (incl.

machine heat)

1.42 kwh

9.08 kwh

31,000 btu to hot water

POWER to Buildings 31,000 btu (=9.08 kwh)

HEAT to Buildings 31,000 btu

COOLING to Buildings 31,000 btu

(= 2.58 ton-hr) (70% prime mover

cogeneration unit; maximum efficiency using high

efficiency electric chillers and no steam chillers)

Example Calculations- Separate Heat & Power + Heat Recovery (without renewable power)

9,300 btu (30%)

waste heat to HRC

1.32 kw/ton * .77 ton-hr = 1.02 kwh to

heat recovery chiller

9.08 kwh

.99 kwh

SHP + HR Efficiency

Prime Mover (Grid Power Plant) Efficiency = Power Out / Gas In Trigeneration Efficiency = [Net useful H + P + C ] / Gas In

Example: Prime Mover work = 11.09 kwh * 3,413 btu/kwh = 37,850 btu Trigeneration work (usable overall system output) = 31,000 + 31,000 + 31,000 = 93,000 btu If gas to grid plant = 74,216 btu • Prime Mover efficiency =

37,850/74,216 = 51%

If total gas to system = 74,216 + 21,434 = 95,650 btu • Trigeneration Efficiency =

93,000/95,650 = 97%

.55 kw/ton *1.81 ton hr = .99 kwh to chiller & ct

25,079 btu to atmosphere

(incl. machine heat)

21,700 btu (70%)

waste heat to chiller

11.09 kwh = 37,850 btu @ 51% efficiency =

74,216 btu gas in to grid power plant

18,219 btu @85% efficiency = 21,434 btu gas in

to hot water generator

1.02 kwh




(= 2.58 ton-hr)

12,781 btu hot water (incl.

machine heat)

18,219 btu hot water

(Combined Heat & Cooling Plant with 30% Heat Recovery and 51%

Grid Gas Power Plant)

Example Calculations- Relative Trigeneration Efficiency (without renewable power)

Cogen unit Power out: 35,849 btu

(=10.50 kwh)

CHP Efficiency

Prime Mover (Cogen) Efficiency = [Useful H + P Work] / Gas In Trigeneration Efficiency = [Net useful H + P + C ] / Gas In

Example: Cogeneration work (usable CHP unit output) = 31,000 + 35,849 = 66,849 btu Trigeneration work (usable overall system output) = 31,000 + 31,000 + 31,000 = 93,000 btu If total gas in to CHP = 95,650 btu: • Prime Mover efficiency =

66,849/95,650 = 70%

• Trigeneration Efficiency = 93,000/95,650 = 97%

.55 kw/ton *2.58 ton hr = 1.42 kwh to chiller &

CT

31,000 btu waste heat

to chiller

35,846 btu to atmosphere (incl.

machine heat)

1.42 kwh

9.08 kwh

31,000 btu to hot water




(= 2.58 ton-hr)


machine heat)


9,300 btu (30%)

waste heat to HRC



9.08 kwh

SHP + HR Efficiency



37,850/74,216 = 51%


93,000/95,650 = 97% 11.09 kwh = 37,850 btu @ 51% efficiency =


18,219 btu @85% efficiency = 21,434 btu gas in





21,700 btu (70%)


.99 kwh

1.02 kwh

9,300 btu (30%)

waste heat to HRC



9.08 kwh

.99 kwh

SHP + HR Efficiency



37,850/74,216 = 51%


93,000/95,650 = 97%




21,700 btu (70%)


11.09 kwh = 37,850 btu @ 51% efficiency =


18,219 btu @ 5% efficiency = 21,434 btu gas in


1.02 kwh




(= 2.58 ton-hr)


machine heat)


If 33% power comes from renewables and 67% from a new 51% HHV efficient gas fueled grid power plant then total gas used to operate the system =

(74,216*.67) = 49,725 btu (power)

+ 21,434 btu (hot water generators)

= 71,159 btu gas in

Trigeneration Efficiency (gas basis) =

93,000/71,159 = 131%

(vs 97% for very high efficiency perfectly balanced and optimized cogen plant or SHP + HR alone ...34% less gas used, 34% less GHG, and lower water use)

See Comparative Trigeneration Efficiency chart for example comparisons of relative gas trigeneration efficiency for perfectly balanced CHP; SHP; and SHP plants with heat recovery and/or renewable power (example for balanced loads; all gas fuel source; and new equipment in all cases- calculations for each specific energy system are required based on local loads and equipment mixes)

Example Calculations- Relative Trigeneration Efficiency (with renewable power)

60%

80%

100%

120%

140%

160%

180%

200%

220%

60%

80%

100%

120%

140%

160%

180%

200%

220%

45% 50% 55% 60% 65% 70% 75%

Rela

tive

Nat

ural

Gas

Trig

ener

atio

n Ef

ficie

ncy

(HHV

)

Prime Mover Natural Gas Efficiency (HHV)(Cogen CHP unit or Grid Power Plant)

Relative Natural Gas Trigeneration Efficiency in District Energy Systems with both Heating & CoolingCombined Heat & Power (CHP) vs. Separate Heat & Power (SHP) vs. Combined Heat & Cooling (CHC) Systems

(Balanced Electricity, Heating, & Cooling Loads)

CHC 70%; 33% RE

CHC 30%; 30% RE

CHC 30%; 0% RE

SHP; 30% RE

SHP; 0% RE

CHP; 0% RE

Perfectly Balanced Cogeneration Plant

CHC = Combined Heat & Cooling (SHP with Heat Recovery)SHP = Separate Heat & Power (on-site boilers + grid power)CHP = Combined Heat & Power

Current Nominal New Grid Baseload Gas Power Plant Efficiency2

SHP

CHC with 30% Heat Recovery

Practical Maximum Efficiency of NewGrid Baseload Gas Power Plant?1

Current Average Existing CA Grid Gas Plant Efficiency1

70% minimum CHC + 33% minimum renewable power

1. ref: THERMAL EFFICIENCY OF GAS-FIRED GENERATION IN CALIFORNIA, Michael Nyberg, California Energy Commission, August 2011.2. ref: Oakley Generating Station, Ca, et. al.

Comparative Trigeneration Efficiency

Comparative Efficiency (Stanford System) CHP vs. SHP vs. CHC

Energy Diagrams- Stanford System

Energy System Options Considered

A. 100% Gas Fueled Options Business As Usual: Maintain Existing Cogeneration System Hygeneration: Maximum Efficiency Gas Fired Cogeneration using Internal

Combustion Engines with Partial Heat Recovery and Steam to Hot Water Conversion

B. Grid Electricity Options (shown with and without Renewable Power included) Separate Heat & Power and Steam to Hot Water Conversion Combined Heat & Cooling (CHC): SHP + Heat Recovery and Steam to Hot Water

Conversion CHC + Ground Source Heat Exchange (GSHE)

BAU- Cardinal Cogeneration

4,400,000 mmbtu

Electricity

Total = 2,643,775 mmbtu

248,983,788 kwh

Gas in

Cogeneration Efficiency (electricity + heating )

1,900,782 mmbtu 3,860,868 mmbtu

= 49.2%

849,782 mmbtu

Cogeneration Efficiency (electricity + heating)

2,643,775 mmbtu 4,400,000 mmbtu = 60.0%

Electric Chilling @ .66

kw/ton 31,285,190 kwh

Cogen Heating

Boiler Heating

Steam Chilling @

12.18 lb/ton

1,062,255 mmbtu

-105,943,244 kwh -361,584 mmbtu

254,000 mmbtu

-708,988 mmbtu

Gas saved

Gas in 169,856 mmbtu

New Grid Gas Power Plant Efficiency

361,584 mmbtu 708,988 mmbtu

= 51.0%

1,316,255 mmbtu

Waste heat to atmosphere

47,691,222 ton-hr

20,777,778 ton-hr

1,062,255 mmbtu

135,885 mmbtu

388,960,000 kwh

1,327,520 mmbtu


248,983,788 kwh -347,404 mmbtu


1,756,225 mmbtu 1,225,753 mmbtu

249,333 mmbtu

572,295 mmbtu 106,773 mmbtu

2,747,778 kwh 9,378 mmbtu

Waste heat to environment

(distribution line loss)

147,140 mmbtu

Total usable heat & power (cogeneration work) out =

1,900,782 mmbtu

Total usable heat, power, & chilling (trigeneration work) out

= 2,722,410 mmbtu

Trigeneration Efficiency (electricity + heating + cooling)

2,722,410 mmbtu 3,860,868 mmbtu

= 70.5%

Total usable heat out = 1,051,000 mmbtu

Total usable electricity out = 849,782 mmbtu

Total usable chilling out = 821,628 mmbtu

Total natural gas consumed @ 100% fossil power = 3,860,868 mmbtu

All figures based on gas

HHV

Hygeneration (IC) with Hot Water

2,564,378 mmbtu

HRC Chilling @ 1.51 kw/ton

HRC Heating

Electricity

Total = 1,830,966 mmbtu

18,659,174 kwh

248,983,788 kwh

Gas in

Trigeneration Efficiency (HW) (electricity + heating + cooling)

2,722,410 mmbtu 2,615,621 mmbtu

= 104.1%

849,782 mmbtu


1,830,966 mmbtu 2,564,378 mmbtu

= 71.4%

Electric Chilling @

.491 kw/ton 27,054,774 kwh

Cogen Heating

Boiler Heating

13,327,982 ton-hr

823,324 mmbtu

-446,499 kwh

-1,524 mmbtu

-2,988 mmbtu

Gas saved Gas in

54,231 mmbtu

Gas Power Plant Efficiency

1,524 mmbtu 2,988 mmbtu

= 51.0%

823,324 mmbtu


55,141,018 ton-hr

223,620 mmbtu

823,324mmbtu

46,096 mmbtu

295,236,427 kwh

1,007,642 mmbtu

5,207 mmbtu

248,983,788 kwh


0 mmbtu


-1,464 mmbtu


733,412 mmbtu 762,479 mmbtu

661,692 mmbtu 92,338 mmbtu

92,192 kwh

315 mmbtu

Heat Recovery

Chiller (COP = 5.66)

(1.51 kw/ton)



42,040 mmbtu

Cogeneration Efficiency (HW) (electricity + heating)

1,900,782 mmbtu 2,615,621 mmbtu

= 72.7%


1,900,782 mmbtu


= 2,722,410 mmbtu




159,936 mmbtu

Total natural gas consumed = 2,615,621 mmbtu


HHV

Separate Heat & Power with Hot Water

1,900,623 mmtu

36.7 lb

969,318 mmbtu


= 85.4%

Electricity

Gas in

284,007,654 kwh

New Grid Gas Power Plant Efficiency

969,318 mmbtu 1,900,623 mmbtu

= 51.0%

68,469,000 ton-hr

1,093,040 mmbtu Boiler

Heating

32,837,786 kwh

1,285,929 mmtu

Gas in

Electric Chilling @

.4796 kw/ton

849,782 mmbtu

248,983,788 kwh 248,983,788 kwh

2,722,410 mmbtu 3,186,552 mmbtu


931,305 mmbtu


1,134,053 mmbtu

821,628 mmbtu 112,075 mmbtu

7,461 mmbtu 2,186,080 kwh



42,040 mmbtu

= 106.6% 2,722,410 mmbtu 2,553,011 mmbtu

‘Relative’ Gas Trigeneration Efficiency (HW) with 33% Renewable Electricity

(electricity + heating + cooling)





1,900,782 mmbtu


= 2,722,410 mmbtu


= 59.7% 1,900,782 mmbtu 3,186,552 mmbtu

= 74.5% 1,900,782 mmbtu 2,553,011 mmbtu

‘Relative’ Gas Cogeneration Efficiency (HW) with 33% Renewable Electricity

(electricity + heating )

Total natural gas consumed @ 100% fossil power = 3,186,552 mmbtu;

2,553,011 mmbtu w 33% RPS


HHV

Combined Heat & Cooling (CHC) with Hot Water

2,151,012 mmtu

36.7 lb

1,097,016 mmbtu

Heat Recovery Chiller

(COP = 6.30) (1.327 kw/ton)

61,238,537 kwh

46,148,106 ton-hr

762,781 mmbtu


= 74.8%

Heating

Electricity

Gas in

321,422,769 kwh


1,097,016 mmbtu 2,151,012 mmbtu

= 51.0%

22,320,894 ton-hr

330,259 mmbtu Boiler

Heating

10,539,926 kwh

388,540 mmtu Gas in


Electric Chilling @

.4722 kw/ton

849,782 mmbtu

248,983,788 kwh 248,983,788 kwh

1,900,782 mmbtu 2,539,552 mmbtu

267,851 mmbtu 35,973 mmbtu


364,362mmbtu


1,053,996 mmbtu

660,518 kwh 2,254 mmbtu



42,040 mmbtu

= 104.3% 1,900,782 mmbtu 1,822,548 mmbtu

‘Relative’ Gas Cogeneration Efficiency with 33% Renewable Electricity

(electricity + heating)




1,900,782 mmbtu

553,777 mmbtu



= 2,722,410 mmbtu

Trigeneration Efficiency (electricity + heating + cooling)

= 107.2% 2,722,410 mmbtu 2,539,552 mmbtu

= 149.4% 2,722,410 mmbtu 1,822,548 mmbtu

‘Relative’ Gas Trigeneration Efficiency with 33% Renewable Electricity (electricity + heating + cooling)


1,822,548 mmbtu @ 33% RPS


HHV

Combined Heat & Cooling with GSHE and Hot Water

2,261,298 mmtu

36.7 lb

1,153,262 mmbtu

Heat Recovery Chiller

(COP = 6.30) (1.327 kw/ton)

87,215,445 kwh

65,045,550 ton-hr

1,075,138 mmbtu


= 83.3%

Heating

Electricity

Gas in

337,902,706 kwh


1,108,036 mmbtu 2,261,298 mmbtu

= 51.0%

3,423,450 ton-hr

17,902 mmbtu Boiler

Heating

1,673,040 kwh

21,061 mmtu

Gas in


Electric Chilling @

.4887 kw/ton

849,782 mmbtu

248,983,788 kwh 248,983,788 kwh

1,900,782 mmbtu 2,282,359 mmbtu

41,081 mmbtu 5,710 mmbtu


53,129 mmbtu


1,105,084 mmbtu

30,433 kwh 104 mmbtu

Waste Heat to Open Loop Ground Source Heat

Exchange System instead of Cooling Towers

Heat Extracted from Open Loop Ground

Source Heat Exchange System

226,770 mmbtu



42,040 mmbtu

= 124.3% 1,900,782 mmbtu 1,528,593 mmbtu

‘Relative’ Gas Cogeneration Efficiency (HW) with 33% Renewable Electricity

(electricity + heating)

GSHE Pumps = 900,000 kwh

(3,072 mmbtu)


= 119.3% 2,722,410 mmbtu 2,282,359 mmbtu

= 178.1% 2,722,410 mmbtu 1,528,593 mmbtu

‘Relative’ Gas Trigeneration Efficiency (HW) with 33% Renewable Electricity

(electricity + heating + cooling)


1,900,782 mmbtu


= 2,722,410 mmbtu




780,547 mmbtu


1,528,593 @ 33% RPS


HHV

-

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

4,500,000

Cardinal Cogen Gas HW Generators &

Electric Chillers

Gas IC Engine Cogen + Heat

Recovery

Heat Recovery Heat Recovery + GSHE

Gas HW Generators &

Electric Chillers w 33% RPS

Heat Recovery w 33% RPS

Heat Recovery + GSHE w 33% RPS

mm

btu

Stanford Energy Supply OptionsNatural Gas Use (2020)

Efficiency and Environmental Comparisons Combined Heat & Power … · The enclosed information is...

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