Presentation on
Heat Recovery Opportunities
EI Seminar
Energy Efficiency Upgrades
Dr Ben Costelloe
Head of Department, Building Services Engineering
DIT Bolton Street
B.Costelloe EI CPD Seminar
October 2013.
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Selected Technologies
• Heat Recovery from Exhaust / Discharge Air
• Heat Recovery from flue gases in Condensing
Boilers
• Heat Recovery from CHP generators
B.Costelloe EI CPD Seminar
October 2013.
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Rational Use of Energy
• In May 2013 CO2 reported passed 400 ppm
• In 18th century estimated to be 275 ppm
• Continuing to rise at 2 ppm per year now
• Should trend continue will be 800ppm by 2100
• Measures taken so far have only limited the rise
B.Costelloe EI CPD Seminar
October 2013.
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Air to Air Heat Recovery
Effectiveness 60-75%
Generally sensible heat transfer with some latent
depending on To and Tdp of exhaust air
B.Costelloe EI CPD Seminar
October 2013.
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Run Around Coils
Effectiveness 40-70%
Generally sensible heat transfer only
Excellent control characteristics
B.Costelloe EI CPD Seminar
October 2013.
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Thermal Wheels Heat Recovery
Effectiveness 60-90%
Generally sensible heat transfer and mass transfer
B.Costelloe EI CPD Seminar
October 2013.
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Heat Recovery Heat Pumps
Not limited by To, Te
Greater available heat for transfer
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October 2013.
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Effectiveness (η) of Heat Recovery
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To outside air temperature – say 0 C
Ts supply air temperature – say 12 C
Te exhaust air temperature – say 24 C
Mas mass flow rate of supply air kg/s
M min mass flow rate of the smaller air stream kg/s
Cp specific heat of air kJ/kg K (typical 1.05 kJ/kg K)
H enthalpy of the air stream kJ/kg
•
η = Mas Cp (Ts – To) / Mmin Cp (Te – To)
if sensible heat only is transferred
• η = Mas (Hs – Ho) / Mmin (He – Ho)
if sensible and latent heat is transferred
B.Costelloe EI CPD Seminar
October 2013.
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External Design Condition - Dublin
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October 2013.
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Psychrometric Processes
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Psychrometric Heat Recovery Process
Latent & Sensible
Sensible Only
B.Costelloe EI CPD Seminar
October 2013.
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Measured Effectiveness of Thermal Wheel
ASHRAE 84-1991 University of Minnesota
B.Costelloe EI CPD Seminar
October 2013.
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Applications
• Plate Heat Recuperators
– Swimming pools
– Systems with year round heat demand for heat
• Run Around Coils
– Areas requiring 100% outdoor & cannot tolerate any level of cross
contamination EG pharmaceutical laboratories, hospitals.
• Thermal Wheels (high capital cost)
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Projects with long running times
Projects with large amounts of sensible heat for recovery
Projects with significant latent heat for recovery
Extensive space required
B.Costelloe EI CPD Seminar
October 2013.
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Heat Recovery from Condensing Boilers
• CH4 + 2O2 + 8N2 = CO2 + 2H2O + 8N2
• Condensing boilers characterised by condensation
of the water vapour in the flue gases
• This requires cooling the flue gases below 54C
• This requires a return water temperature < 50C
preferably <45
• System must be designed to ensure condensation –
or no heat recovery from flue gas, no increased η
B.Costelloe EI CPD Seminar
October 2013.
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Efficiency of Condensing Boiler as a
function of Return Water Temperature
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Weather Compensation Variable Flow
Temperature Improves η of boiler
flow temp falls 80-50C - return falls 70-40C
far better than mass flow rate control
B.Costelloe EI CPD Seminar
October 2013.
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Typical Energy Distribution in
Conventional Boilers
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78% of heat to circulating water – design load
2% to casing losses
10% to sensible heat in the flue gases
10% to latent heat in the flue gases
Potential recovery most sensible & latent flue heat
British Gas study found measured annual η of
conventional boilers of 70-72% and measured annual
efficiency condensing boilers of 86%
• Potential to raise ηa by 10 – 14% over conventional
• Maximum potential to reduce running costs by 18%
B.Costelloe EI CPD Seminar
October 2013.
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Importance of correct excess air
with condensing boilers
Moisture Content = kg of moisture (constant)
per kg of dry air (increasing with increase in
excess air)
As the excess air increases the moisture content of
the flue gases falls
Therefore dew point temperature falls and
condensation will cease
Therefore correct excess air at part load is crucial
Dew Point Temperature = 54 C
Dew Point Temperature < 54 C
B.Costelloe EI CPD Seminar
October 2013.
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Heat Recovery from CHP sets
Natural Gas
kWe
70
90
143
330
526
625
836
1065
1413
1644
2188
2740
kWt
119
136
206
361
633
746
997
1197
1505
1730
2350
2903
Ratio t/e
1.7
1.5
1.4
1.1
1.2
1.2
1.2
1.1
1.1
1.1
1.1
1.1
Propane
70
109
231
347
407
544
682
123
150
337
521
580
773
965
1.8
1.4
1.5
1.5
1.4
1.4
1.4
Source Edina Power Generation specifications
B.Costelloe EI CPD Seminar
October 2013.
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Correlation between Electrical & Thermal
Power Output (kW) – CHP firing NG
3500
3000
y = -1E-05x2 + 1.0756x + 50.472
R2 = 0.9988
kWt output
2500
2000
1500
kWt
1000
Poly. (kWt)
500
0
0
500
1000
1500
2000
2500
3000
kWe output
B.Costelloe EI CPD Seminar
October 2013.
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CHP Applications - some pointers
• Smaller Projects (< 300 kWe)
– Ratio of kWt / kWe of demand = 1.3 -2.0
• Larger Projects ( > 1000 kWe)
– Near equal demand for power and heat
• Viability - generally needs min of 4,000 hours/a
• Business case depends on utilisation of heat
• Power demand needs to be 8am to 12 midnight
(not a competitor with night tariff utility power)
B.Costelloe EI CPD Seminar
October 2013.
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Generally Recognised Applications
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Industrial with shift working + good demand ratio
Hospitals and large care homes
University campus with residences
Prisons and large police stations
Boarding schools esp. with swimming pools
Leisure centres with swimming pools
Airports, TV studios, Data processing centres
Large offices linked with large residential projects
Large retail projects esp. with tri-generation
B.Costelloe EI CPD Seminar
October 2013.
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CHP with Heat Pumps
• New projects – more favourable balancing of demand
pattern possible with heat pumps
• Project - demands kWe - 200 kW, kWt -600 kw – ratio
3.0 - not viable
• Add 70 kWe compressor heat pump with 210 kWt heat
• New demand 270kWe & 390 kWt – ratio 1.4 OK
B.Costelloe EI CPD Seminar
October 2013.
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Absorption Refrigeration
Power (W) = Volume FR (V) x Pressure Dev. ΔP
in vapour compression volume flow rate large
in absorption refrigeration liquid flow rate is very small
B.Costelloe EI CPD Seminar
October 2013.
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Tri - Generation
• CHP set provides power and heat as normal
• Heat used to supply heat demand in winter
• Heat used to supply cooling demand in summer
via absorption refrigeration system
• Applicable to applications with longer cooling
seasons and shorter heating seasons
• Makes CHP more viable
• Applicable in Retail with short daily heat demand
B.Costelloe EI CPD Seminar
October 2013.
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