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. 1 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. 2 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. 3 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. 4 Run Around Coils Effectiveness 40-70% Generally sensible heat transfer only Excellent control characteristics B.Costelloe EI CPD Seminar October 2013. 5 Thermal Wheels Heat Recovery Effectiveness 60-90% Generally sensible heat transfer and mass transfer B.Costelloe EI CPD Seminar October 2013. 6 Heat Recovery Heat Pumps Not limited by To, Te Greater available heat for transfer B.Costelloe EI CPD Seminar October 2013. 7 Effectiveness (η) of Heat Recovery • • • • • • • 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. 8 External Design Condition - Dublin B.Costelloe EI CPD Seminar October 2013. 9 Psychrometric Processes B.Costelloe EI CPD Seminar October 2013. 10 Psychrometric Heat Recovery Process Latent & Sensible Sensible Only B.Costelloe EI CPD Seminar October 2013. 11 Measured Effectiveness of Thermal Wheel ASHRAE 84-1991 University of Minnesota B.Costelloe EI CPD Seminar October 2013. 12 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) – – – – 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. 13 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. 14 Efficiency of Condensing Boiler as a function of Return Water Temperature B.Costelloe EI CPD Seminar October 2013. 15 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. 16 Typical Energy Distribution in Conventional Boilers • • • • • • 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. 17 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. 18 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. 19 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. 20 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. 21 Generally Recognised Applications • • • • • • • • • 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. 22 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. 23 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. 24 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. 25
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