Echogen Power Systems

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A Comparative Study of Heat Rejection Systems for sCO2 Power Cycles

Timothy J. Held, Jason Miller and David J. Buckmaster

Echogen Power Systems

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Heat rejection systems

Common to all closed-loop cycles, sCO2

systems need to reject residual heat

Heat addition

Heat rejection

Condensing cycle

Non-condensing cycle

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Heat rejection technology

Air cooled Fin-fan cooler Low air-side dP (< 2”wc) Aux loads = fan drives

Water-cooled Cooling tower (forced

draft) Intermediate water-loop

required for PCHE CO2-water HX

Aux loads = fan drives +pumps

Hybrid solutions exist (WSAC, ACC with fogging) – not considered for this paper

Cooling Tower

Circ pump

PFHE

Circ pump

PCHE CO2 Inlet

CO2 Outlet

Accumulator/pressurizer

P-14

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Why you should not use direct CT water feed to your PCHE

a) The manufacturer advises against itb) Very small change in water conductivity resulted in

significant PCHE performance loss

0

0.05

0.1

0.15

0.2

0.25

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

08-14 09-14 10-14 11-14

CT

wat

er c

ond

ucti

vity

(S/m

)

Co

nden

ser U

A (k

W/°

C)

Date

UA Cond

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Condenser performance & nomenclature

0

10

20

30

40

50

60

70

0% 20% 40% 60% 80% 100%

Tem

pe

ratu

re (°

C)

Q/Qtot

Coolant

CO2

Coolant inlet

CO2 inlet

RangeApproach

𝑈𝐴 = න0

𝑄𝑡𝑜𝑡 𝑑𝑄

𝑇𝐶𝑂2 − 𝑇𝑐𝑜𝑜𝑙𝑎𝑛𝑡

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Cycle performance

44%

45%

46%

47%

48%

49%

50%

51%

52%

53%

54%

0 10 20 30 40 50

Gro

ss e

ffic

ien

cy

Tcoolant (°C)

RC Brayton

RC Condensation

High Trecup

Turbine

Low Trecup

Cmpr 2

Cmpr 1

Cooler

PHX

CO2

Notes: Gross efficiency does not include coolant aux loadCycle performance plotted as function of coolant T

RCBC baseline

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Heat exchanger sizing

8000

8200

8400

8600

8800

9000

9200

9400

9600

9800

0 500 1000 1500 2000 2500 3000

Pow

er (k

We)

Condenser/Cooler UA (kW/K)

Cost generally proportional to UADesign value = 1500 kW/°C (both water & air)

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Cooling tower sizing

0

50

100

150

200

0 2 4 6 8 10

Co

olin

g to

wer

co

st (

K$

)

Tapproach (°C)

Cost function of design approach temperature3°C regarded as a practical minimum by CT suppliers, used for this study

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-15 -10 -5 0 5 10 15 20 25 30 35 40

CD

F

Temperature (°C)

Tdb

Twb

Local climate impacts

Hourly observations from 2014 converted to CDF formatTwb affects water (evaporative) cooling, Tdb affects air cooling

Houston, TX

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Cooling tower performance as f(Tambient)

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

T co

ola

nt(

°C)

Twb (°C)

Akron

Houston

Phoenix

20°C selected as design pointDemin water loop adds another 3°C to approach temperatureAs Twb approaches 0°C, water T increases to avoid freezing

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Coolant temperature distribution

-5

0

5

10

15

20

25

30

35

40

0% 20% 40% 60% 80% 100%

T co

ola

nt(°

C)

% hours

Water

Air

Twbi

Coolant temperature CDF calculated as function of wet/dry bulb temperature CDFs

Houston, TX

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Cycle performance over 1-year period

0

2000

4000

6000

8000

10000

12000

14000

16000

0% 20% 40% 60% 80% 100%

Ne

t p

ow

er

(kW

e)

% hours

ACC

WCC

Coolant CDFs convolved with Power = f(Tcoolant) to calculated CDF of net powerResults integrated to yield annualized average power

Non-condensing cycle example, Houston, TX

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Net power output (non-condensing cycle)

Annualized average 99% hot day

7,000

7,500

8,000

8,500

9,000

9,500

10,000

10,500

11,000

Akron Houston Phoenix

Net

Po

wer

(kW

e)

WCC ACC

7,000

7,500

8,000

8,500

9,000

9,500

10,000

10,500

11,000

Akron Houston Phoenix

Net

Po

wer

(kW

e)WCC ACC

ACC outperforms on averageHot-day performance falloff ~ 2-4% for non-arid climates

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Auxiliary load comparison

Load WCC ACC

Fans 46 310

CW pump 99

PFHE pump 112

Total 257 310

Lower cooling tower fan loads largely offset by increased pump loads

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Footprint comparison

Cooling tower footprint smaller than ACC, does not include space for pumps, water treatment, etc.

ACC240 m2

15.6 x 15.6m

WCC97m2

5.7x17m

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Capital cost comparison

WCC ACC

Cooling tower/ACC 143,000$ 695,000$

Water treatment 20,000$

CT Pumps 44,000$

CT/ACC Piping 41,000$ 108,000$

PFHE 83,000$

PFHE Pumps 46,000$

PFHE Piping 16,000$

Filters 5,000$

WCC PCHE 300,000$

Total eqpt 698,000$ 803,000$

Installation 419,000$ 482,000$

Total 1,117,000$ 1,285,000$

Footprint (m²) 97 240

ACC slightly (15%) higher

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O&M cost comparison (annual)

WCC ACC

Water consumption 156,000$

Water disposal 57,000$

Water treatment 25,000$

Chemical replacement 65,000$

Pump maintenance 24,000$

Fan maintenance 7,500$ 30,000$

Maintenance cost/yr 334,500$ 30,000$

$/kWh 0.0042$ 0.0004$

Significantly lower O&M costs, largely driven by elimination of water supply, treatment and disposal costsReduced O&M pays back increased capex of ACC in << 1 year

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Other reasons to prefer ACC

Cooling water filter cleaning is … unpleasant

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Other reasons to prefer ACC

Cold-weather operation is even less pleasant

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Other reasons to prefer ACC

Cooling towers are excellent industrial-scale Petri dishes

“Mimivirus” – world’s largest virus, discovered in a UK cooling tower

Legionella bacterium – often found in cooling towers. Infects 10,000-18,000 people per year in the US alone

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Conclusions

Water cooling advantages Better performance on hot days, especially in arid climates

Smaller footprint

Air cooling advantages Better annualized average power output except in arid

climates (where water cooling is impractical anyway)

Lower O&M costs

Simpler operation

Near zero water consumption for sCO2 power cycles

Costs and auxiliary loads are similar

Air cooling nearly always the better choice

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Backup slides

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Cycle / cost modeling

Optimization process defines cycle with minimum cost for given power output

Multiple solutions generates curves of cost vs power Allows selection of optimal cycle architecture

Held, T.J., 2015. Supercritical CO2

Cycles for Gas Turbine

Combined Cycle Power Plants.

Power Gen International.

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sCO2 vs steam

Normalized to steam power & cost from GT-Pro, “power-optimized” solutions (“cost-optimized” point shown for reference)

Same exhaust and boundary conditions used for sCO2

10-20% lower cost for same power 7-14% higher power for same cost

Power optimized

Cost optimized