Backpressure Steam Power Generation in District Energy and CHP - Energy Efficiency Considerations

A 1st Law, 2nd Law and Economic Analysis of the Practical Steam Engine in a District Energy/CHP Application

Joshua A. Tolbert, Ph.D. Practical Steam Energy Efficiency Engineering – How is efficiency calculated? • What is the Status Quo? – Engineers currently focus on limiting energy losses as the primary point of focus for district energy and CHP systems – Only local energy losses are typically considered causing energy efficiency opportunities to be wasted • Why? – Conventional wisdom tells us that minimizing local energy losses is ultimate goal • How should we change? – Could a different approach improve global efficiency?

Motivation

Common Questions: 1. How can installing an imperfect device in parallel to an isenthalpic pressure reducing valve (PRV) improve efficiency? 2. My analysis shows an incremental fuel cost with PRV parallel, how can this be more efficient?

Analysis Overview

• Consider a Practical Steam Engine (PSE) in a Pressure Reducing Valve (PRV) parallel district energy application • PSE operation is consistent with CHP application • Consider 1st Law, 2nd Law and economic analysis for energy efficiency • Draw conclusions Backpressure Application – Case Study Enwave Seattle - District Energy / CHP Application

Assumptions Equipment • 1.5 MW (5.118 MMBtu/hr) • Practical Steam Engine (PSE) heating load is considered o Isentropic efficiency of 80% • Condensate exiting load: o Mechanical efficiency of 80% Saturate liquid at 20 psig o Generator efficiency of 95% • No heat losses in piping or • Isenthalpic PRV equipment* • 150 psig saturated steam boiler • 80% boiler and feedwater pump efficiency

*Incorporating actual heat losses does not significantly affect results of analysis. 1st Law of Thermodynamics

• 1st law of thermodynamics is simply a conservation of energy

• All energy is conserved – no energy is destroyed.

• Steady State:

o Sum of all energy into system = sum of energy out of system

• Considers only the quantity of energy, not the quality

1st Law Analysis – PRV Status Quo

• Heating Only FEEDWATER FUEL 퐵푡푢 • ℎ = ℎ = 1,196 표푢푡 𝑖푛 푙푏푚

o Superheated Steam (57°F Superheat) BOILER

푄 푙푏푠 STEAM IN • 푚 = 푂푢푡 = 5,285 ℎ표푢푡−ℎ푐표푛푑푒푛푠푎푡푒 ℎ푟

푀푀퐵푡푢 PRV • 퐸 = 푄 + 푊 = 6.398 𝑖푛 𝑖푛 𝑖푛 ℎ푟

푀푀퐵푡푢 • 퐸 = 푄 = 5.118 STEAM OUT 푂푢푡 표푢푡 ℎ푟

푠푡 퐸표푢푡 • 1 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦푃푅푉 = = 80% 퐸푖푛 HEATING LOAD HEAT

CONDENSATE OUT 1st Law Analysis – PSE

• Combined Heat and Power FEEDWATER FUEL 퐵푡푢 • ℎ = ℎ − ℎ − ℎ η = 1,101 표푢푡 𝑖푛 𝑖푛 2푠 𝑖푠푒푛푡푟표푝𝑖푐 푙푏푚 MECHANICAL LOSSES BOILER o Saturated Vapor (92.9% Quality)

푄푂푢푡 푙푏푠 STEAM IN • 푚 = = 5,862 ℎ표푢푡−ℎ푐표푛푑푒푛푠푎푡푒 ℎ푟

POWER 푀푀퐵푡푢 • 퐸 = 푄 + 푊 = 7.096 GENERATOR PSE 𝑖푛 𝑖푛 𝑖푛 ℎ푟

• 푊 = ℎ𝑖푛 − ℎ표푢푡 η푚푒푐ℎη𝑔푒푛푒푟푎푡표푟 = 124.5 [푘푊푒] STEAM OUT

푀푀퐵푡푢 GENERATOR LOSSES • 퐸 = 푄 + 푊 = 5.543 푂푈푇 푂푈푇 푂푈푇 ℎ푟 HEATING LOAD 퐸푂푢푡 • 1푠푡 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦푃푆퐸 = = 78.1% HEAT 퐸푖푛

CONDENSATE OUT 1st Law Analysis – PRV Broad Perspective

• Separate Heat and Power FEEDWATER FUEL

• Consider Power Plant Thermal Efficiency = 40%* FUEL BOILER 푃 • 퐴푑푑𝑖푡𝑖표푛푎푙 퐹푢푒푙 퐼푛푝푢푡 = = η푃표푤푒푟 푃푙푎푛푡 푀푀퐵푡푢 0.425 STEAM IN ℎ푟

푀푀퐵푡푢 POWER • 퐸 = 푄 + 푊 = 7.672 PRV 𝑖푛 𝑖푛 𝑖푛 ℎ푟 PLANT

푀푀퐵푡푢 • 퐸 = 푄 + 푊 = 5.543 푂푢푡 표푢푡 푂푢푡 ℎ푟 STEAM OUT LOSSES 퐸 • 1푠푡 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦 = 푂푢푡 = 72.2% 푃푆퐸 퐸 푖푛 POWER HEATING LOAD HEAT

CONDENSATE OUT *33% US Average for steam generator power plants in 2013 (U.S. EIA)

1st Law Analysis - Comparison

PRV** PSE** LOCAL PERSPECTIVE BROAD PERSPECTIVE

• 1푠푡 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦푃푅푉 = • 푃푢푟푐ℎ푎푠푒푑 푃표푤푒푟 = • Incremental Heat Addition = 푀푀퐵푡푢 80% 124.5 푘푊푒 0.619 ℎ푟 • Heating Only • 1푠푡 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦 = 푃푅푉 • 푃표푤푒푟 푂푢푡푝푢푡 = 124.5 푘푊 72.2% 푒 • 68.6% Thermal Efficiency • Separate Heat and Power Power Production*

• 1푠푡 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦푃푆퐸 = 78.1%

• Combined Heat and Power

*33% US Average for steam generator power plants in 2013 (U.S. EIA)

**PRV supplies 57° Superheated steam to heating load, PSE supplies 92.9% quality saturated steam to heating load.

Economic Analysis

Assumptions Results • Fuel Cost: $5/MMBtu* • PRV • Electricity Cost: $0.075/kWh* o Fuel Costs = $241K/yr • PSE • Annual Operation = 8500 hrs o Fuel Costs = $267K/yr • PSE Maintenance costs = o Incremental Fuel Costs = $4,000/yr $26K/yr o Power = $80K/yr

o Net Savings = $50K/yr** o Produces power at $0.025/kWh

*Based on Seattle industrial rates **Including maintenance costs 2nd Law of Thermodynamics

• 2nd law of thermodynamics considers the quality of the energy, reversibility of processes and the ability of the energy to do work – EXERGY • Exergy sometimes referred to as the “available energy” or “availability” • Exergy is a measure of the maximum useful work possible during a process that brings the system to equilibrium with a heat reservoir • Sum of exergy out of system ≤ sum of exergy into system • Energy never destroyed (1st Law), exergy can be destroyed (2nd Law)

2nd Law Analysis – Practical Example

• Consider heat dissipating from a boiler in which all heat energy could be retained in the surroundings and recovered.

o 1st law would consider this 100% efficient.

o Recovered energy is at a lower temperature and therefore a lower quality, or exergy.

o Not possible to take this energy and put it back into the boiler without some additional work or heat input.

o Heat cannot be used to produce much useful work.

o Some exergy, the ability to do work, was lost while all energy was retained. 1st law efficiency = 100%; 2nd law efficiency < 100%

2nd Law Analysis – Fluid Flow Exergy

• Fluid Flow exergy is defined as a function of enthalpy and entropy in reference to dead state as follows (KE and PE neglected):

• ψ = ℎ − ℎ0 − 푇0 푠 − 푠0

• ℎ0 = 푑푒푎푑 푠푡푎푡푒 푒푛푡ℎ푎푙푝푦

• 푇0 = 푑푒푎푑 푠푡푎푡푒 푡푒푚푝푒푟푎푡푢푟푒

• 푠0 = 푑푒푎푑 푠푡푎푡푒 푒푛푡푟표푝푦 • The dead state is the state that is in thermodynamic equilibrium with its surroundings.

o Assumed in our study to be 70°F and 1 atm

Mollier Diagram – PSE vs PRV

1220

1200

1180

1160

1140 Enthalpy Enthalpy (Btu/lbm)

Saturation Curve 1120

PRV Pressure Reduction

1100 PSE Expansion

1080 1.54 1.56 1.58 1.6 1.62 1.64 1.66 1.68 1.7 1.72 1.74 Entropy (Btu/lbm*R)

ψ = ℎ − ℎ0 − 푇0 푠 − 푠0 2nd Law Analysis – PRV

Local Perspective

푀푀퐵푡푢 EXERGY IN • 푋𝑖푛 = ψ𝑖푛 ∗ 푚 = 1.952 ℎ푟 (STEAM)

푀푀퐵푡푢 • 푋 = ψ ∗ 푚 = 1.491 표푢푡 표푢푡 ℎ푟 EXERGY LOSS (ENTROPY GENERATION) • 퐸푥푒푟푔푦 퐷푒푠푡푟푢푐푡𝑖표푛 = 푋𝑖푛 − 푀푀퐵푡푢 PRV 푋 = 0.461 = 23.6% 표푢푡 ℎ푟

푋 • 2푛푑 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦 = 표푢푡 = 푋푖푛 76.4% EXERGY OUT (STEAM)

2nd Law Analysis – PSE Local Perspective

EXERGY IN 푀푀퐵푡푢 (STEAM) • 푋 = ψ ∗ 푚 = 2.165 EXERGY LOSS 𝑖푛 𝑖푛 ℎ푟 (GENERATOR) • 푋표푢푡 = ψ표푢푡 ∗ 푚 + 푊 = 퐵푡푢 EXERGY LOSS 1.928 (ENTROPY 푙푏푚 GENERATION) PSE GENERATOR • 퐸푥푒푟푔푦 퐷푒푠푡푟푢푐푡𝑖표푛 = 푋𝑖푛 − 퐵푡푢 푋 = 0.237 = 10.9% 표푢푡 푙푏푚

푋 • 2푛푑 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦 = 표푢푡 = 푋 EXERGY LOSS EXERGY OUT 푖푛 (MECHANICAL) (POWER) 89.1% EXERGY OUT (STEAM) 2nd Law Analysis - Comparison

PRV PSE 푀푀퐵푡푢 푀푀퐵푡푢 • 푋𝑖푛 = 1.952 • 푋 = 2.165 ℎ푟 𝑖푛 ℎ푟

퐵푡푢 퐵푡푢 • ψ표푢푡 = 282.2 • ψ = 256. 4 푙푏푚 표푢푡 푙푏푚

푀푀퐵푡푢 푊 퐵푡푢 • 푋표푢푡 = 1.491 • = 73.0 ℎ푟 푚 푙푏푚

• Exergy Destruction = 23.6% 퐵푡푢 • 푋 = 1.928 표푢푡 푙푏푚 • 2푛푑 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦푃푅푉 = 76.4% • Exergy Destruction = 10.9% • Did not consider exergy destruction at power plant • 2푛푑 퐿푎푤 퐸푓푓𝑖푐𝑖푒푛푐푦푃푆퐸 = 89.1%

Summary – PSE vs PRV District Energy and CHP • 1st law efficiency o Thermal efficiency for PSE is less than for local perspective PRV but better than broad perspective PRV. o PSE produced power at > 68% thermal efficiency • System economics o PSE power generation more than makes up for incremental fuel costs o PSE produced power at rate around $0.025/kWh, 1/3 of local purchased rate • 2nd law efficiency o With PSE more “useful” energy was conserved o More broad perspective would prove increased efficiency differential between PRV and PSE

Conclusions

• A local 1st law analysis can hide energy efficiency improvement and cost saving opportunities • Broad 1st law analysis can be more informative and consistent with economics • A reduced local 1st law efficiency can coincide with improved economics, improved broad perspective 1st law analysis and an improved 2nd law efficiency • Broad 1st law analysis, as well as economic and 2nd law analyses, should be used when considering system energy efficiency • Use of the Practical Steam Engine in district energy and CHP applications can improve global energy efficiency as well as improve system economics

New Status Quo

• Change the status quo

1. Educate our engineers to not rely on the potentially misleading information determined by local 1st law analyses

2. Encourage broad 1st law analyses as well as 2nd law and economic analyses

3. Capitalize on energy efficiency opportunities previously hidden by local 1st law analyses such as PRV parallel/CHP

Questions?

Thank you for your interest!

Want to learn more? Come see us at booth #22

Joshua A. Tolbert, Ph.D. [email protected] 205-300-5114

Practical Steam Contact Info: PO Box 99219 Seattle WA 98139 Phone: 206-981-3015