B.Costelloe EI CPD Seminar

October 2013.

1

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.

2

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.

3

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.

4

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.

5

Run Around Coils Effectiveness 40-70%

Generally sensible heat transfer only

Excellent control characteristics

B.Costelloe EI CPD Seminar

October 2013.

6

Thermal Wheels Heat Recovery Effectiveness 60-90%

Generally sensible heat transfer and mass transfer

B.Costelloe EI CPD Seminar

October 2013.

7

Heat Recovery Heat Pumps Not limited by To, Te

Greater available heat for transfer

B.Costelloe EI CPD Seminar

October 2013.

8

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.

9

External Design Condition - Dublin

B.Costelloe EI CPD Seminar

October 2013.

10

Psychrometric Processes

B.Costelloe EI CPD Seminar

October 2013.

11

Psychrometric Heat Recovery Process

Latent & Sensible Sensible Only

B.Costelloe EI CPD Seminar

October 2013.

12

Measured Effectiveness of Thermal Wheel

ASHRAE 84-1991 University of Minnesota

B.Costelloe EI CPD Seminar

October 2013.

13

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.

14

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.

15

Efficiency of Condensing Boiler as a

function of Return Water Temperature

B.Costelloe EI CPD Seminar

October 2013.

16

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.

17

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.

18

Importance of correct excess air

with condensing boilers

Dew Point Temperature < 54 C

Dew Point Temperature = 54 C

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

B.Costelloe EI CPD Seminar

October 2013.

19

Heat Recovery from CHP sets

Propane70 123 1.8

109 150 1.4

231 337 1.5

347 521 1.5

407 580 1.4

544 773 1.4

682 965 1.4

Source Edina Power Generation specifications

Natural Gas

kWe kWt Ratio t/e

70 119 1.7

90 136 1.5

143 206 1.4

330 361 1.1

526 633 1.2

625 746 1.2

836 997 1.2

1065 1197 1.1

1413 1505 1.1

1644 1730 1.1

2188 2350 1.1

2740 2903 1.1

B.Costelloe EI CPD Seminar

October 2013.

20

Correlation between Electrical & Thermal

Power Output (kW) – CHP firing NG

y = -1E-05x2 + 1.0756x + 50.472

R2 = 0.9988

0

500

1000

1500

2000

2500

3000

3500

0 500 1000 1500 2000 2500 3000

kWe output

kW

t o

utp

ut

kWt

Poly. (kWt)

B.Costelloe EI CPD Seminar

October 2013.

21

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.

22

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.

23

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.

24

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.

25

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