8/21/12

1

Lecture 16.75– Heat Pumps, AC Pete Schwartz Cal Poly Physics

Heat  Pump,  Opera-ng  Principles  •  Overall  transfer  of  heat  from  cold  to  warm  (against  the  macro  temperature  gradient)  

•  At  each  point  in  the  system,  heat  flow  is  from  warm  to  cold  

•  Relies  on  the  fact  that  a  gas  cools  when  it  expands,  and  is  heated  when  it  is  compressed  (work  is  done  on  it),  to  create  local  temperature  gradients  contrary  to  the  macro-­‐gradient  

•  You  can  MOVE  a  lot  more  than  one  Joule  with  one  Joule  of  energy,  depending  on  the  temperature  difference  you  have  to  move  it  across  

Qhigh = Wnet + Qlow

HOT (reactor)

COLD (Ocean)

Work or Electricity

QH

QC

Heat Engines

€

η =WQH

=QH −QC

QH

η <TH −TCTH

Qhigh = Wnet + Qlow

Work or Electricity

€

COP =QH

W=

QH

QH −QC

COP <TH

TH −TC

Refrigerator Air Conditioner

(Freezer, Summer House)

(Hot Outside World)

Heat Pump

(Cold Outside)

(Winter House)

€

COP =QC

W=

QC

QH −QC

COP <TC

TH −TC

HOT

COLD

QH

QC

COP (Coefficient of Performance) >>1, depends Often more than 5, and decrease with larger ΔT

From your reading on Wikipedia

1

2 3

4

Freezer  

http://www.truehvac.com

Changing the direction of flow will turn the A/C into a heater

Tcondenser

Tevaporator

Tindoor

Toutdoor

Apparent T lift

Real T lift

ΔTH

ΔTL

If Tcondenser= 40°C Toutdoor= 30°C Tindoor= 16°C Tevaporator = 6°C, then Apparent Carnot COP = 20.6 Real Carnot COP= 8.5 Actual COP= 5.53 if machine efficiency = 0.65

Heat Flow

Heat Flow

Optimize Actual efficiency by minimizing total temperature drop (Real T lift)

Freezer

House

Compressor

Hot Coils

Cold Coils

Heat Pump to cool house (Freezer)

8/21/12

2

Thus,  to  reduce  heat  pump  energy  use,  •  Distribute  heat  at  the  lowest  possible  temperature  (e.g.,  at  30°C  instead  of  60°C  –  using  radiant  floor  hea-ng  or  radiant  ceiling)  

•  Distribute  coldness  at  the  warmest  possible  temperature  (e.g.,  at  20°C  instead  of  6°C  –  using  chilled  ceiling  or  chilled  floor  slab)  

•  Minimize  ΔTH  and  ΔTL  by    -­‐  minimizing  the  required  heat  flows  (which  must  balance  heat  loss  or  heat  gain)  

   -­‐  using  as  large  a  radiator  surface  as  possible  

http://www.truehvac.com

Geothermal Reservoirs: very high COP for winter heating.

Climate master. Geothermal heat pump systems

Geothermal Heat Pumps Energy  Required  to  Move  Air  or  Water  through  Ducts  or  Pipes  

•  Power  imparted  to  fluid      Pfluid=ΔP  x  Flow  

•    Electric  power  required                    Pelectric  =  (ΔP  x  Q)/(η)        but  ΔP  α  Flow2  for  turbulent  flow,  so                      Pfluid  α  Flow3    

Radiant  ceiling  cooling  As  it  turns  out,  ven-la-on  air  flow  

requirements  can  be  reduced  by  a  factor  of  two  by  using  displacement  ven1la1on  

rather  than  ceiling-­‐based  mixing  ven-la-on,  while  improving  air  quality  and  reducing  total  hea-ng  loads  on  the  

chillers  

8/21/12

3

Energy  required  to  deliver  heat  by  circula-ng  warm  water  vs  warm  air  

•  Rate  of  heat  supply  to  a  room  is  equal  to  the  rate  of  heat  loss  from  the  circula-ng  air  or  water,  which  is  given  by        QH=ρcpQ  (Tsupply-­‐Treturn)  =  ρcpQ  ΔT  

•  The  ra-o  of  energy  supplied  to  move  the  fluid  to  heat  delivered  is  given  by        ΔP/  ρcp  ΔT  

•    Given  the  large  ρ  and  cp  for  water  compared  to  air,  and  given  typical  ΔPs  and  ΔTs,  moving  heat  with  water  requires  less  energy  than  with  moving  air  (by  a  factor  of  25).  

Thus,  to  minimize  the  energy  use  in  supplying  and  delivering  heat/coldness  and  fresh  air  

•  Separate  hea-ng/cooling  and  ven-la-on  func-ons  •  Use  chilled  water  at  the  warmest  possible  T  for  cooling  (best:  20°C)  

•  Use  hot  water  for  hea-ng  at  the  coolest  possible  temperature  (best:  30°C)  

•  Circulate  only  the  amount  of  air  required  for  ven-la-on  purposes  using  displacement  ven-la-on  

In  many  systems,  

•  Several  -mes  more  air  is  circulated  than  is  needed  for  ven-la-on  alone,  so  as  to  provide  adequate  cooling  through  airflow  alone  

•  In  “efficient”  systems,  80%  of  the  air  might  be  recirculated  and  mixed  with  20%  fresh  outside  air  on  each  circuit,  rather  than  replacing  and  having  to  cool  and  dehumidify  100%  outside  air  

•  However,  80%  of  the  internal  heat  gains  picked  up  by  the  air  will  have  to  be  removed  by  the  chillers  

In  a  Dedicated  Outdoor  Air  Supply  system  (100%  outside  airflow  but  only  what  is  needed  for  ven-la-on)  with  displacement  ven-la-on,  

•  Heat  gains  from  the  ceiling  (from  ligh-ng)  or  heat  rising  to  the  ceiling  is  directly  vented  to  the  outside  –  reducing  the  cooling  load  on  the  chillers  by  up  to  one  third  

•  Ven-la-on  rates  can  be  reduced  to  near  zero  when  the  building  is  not  occupied  (because  ven-la-on  is  not  used  for  temperature  control)  –  can  save  20-­‐30%  in  total  hea-ng+cooling+ven-la-on  energy  use  

Another  advantage  of  using  chilled  ceilings  for  cooling  is  that  the  required  chilled-­‐water  temperature  (18-­‐20°C)  is  

cool  enough  that  it  can  olen  be  supplied  through  evapora-ve  cooling  

using  the  chiller  cooling  tower  AIR IN

CLOSED-CIRCUITHEAT-EXCHANGE COIL

HEAT AND HUMIDIFIED AIR OUT

DRIFTELIMINATORS

EXTERNAL WATER

HOTWATER

CLOSEDCIRCUIT

COLDWATER

AIR IN

AIRWATER

PUMP

Evapora:ve  Cooling:  Electricity  is  only  used  to  circulate  air  and  water  

8/21/12

4

Solar  Energy  in  Buildings  

•  Passive  solar  hea-ng  •  Passive  ven-la-on  •  Ac-ve  solar  thermal  collectors,  used  for            –  domes-c  hot  water          -­‐  space  hea-ng          -­‐desiccant  dehumidifica-on  systems  •  PV  panels  

To  maximize  passive  solar  hea-ng  requires  

•  Aoen-on  to  building  form  and  orienta-on  •  Use  of  high-­‐performance  windows  •  Use  of  thermal  mass  to  avoid  overhea-ng  by  day  and  to  release  stored  heat  by  night  

•  High  levels  of  insula-on  to  retain  heat  that  is  released  from  thermal  mass  at  night  

Solar  Chimney  to  induce  ven-la-on,  Building  Research  Establishment,  Garston,  UK  

Savings  and  Costs  •  With  nothing  fancy  and  without  requiring  detailed  

computer  simula-ons,  this  approach  will  frequently  give  a  50%  savings  in  annual  energy  use  compared  to  current  prac-ce  

•  Use  of  computer  simula1on  models  run  by  simula-on  experts  to  fully  op-mize  the  design  of  the  building  and  mechanical  systems  and  use  of  more  advanced  designs  can  push  the  savings  to  60-­‐70%  

•  Savings  can  be  pushed  to  75-­‐80%  with  enlightened  occupant  behaviour  

•  Buildings  achieving  such  high  energy  savings  some-mes  cost  no  more  than  conven-onal  buildings,  due  to  the  downsizing  of  mechanical  equipment,  and  are  superior  in  other  respects  

•  Some-mes  saving  more  energy  costs  less  

Amory Lovins, Rocky Mountain Institute (Efficiency)

Increased Efficiency

Pro

ject

Cos

ts

Energy savings > Cost of insulation

Diminishing Returns as energy savings are less

Marginal Energy Savings < Marginal Cost of Insulation

Reduce cost by downsizing or eliminating heater or AC

Qhigh = Wnet + Qlow

HOT (reactor)

COLD (Ocean)

Work or Electricity

QH

QC

Heat Engines

€

η =WQH

=QH −QC

QH

η <TH −TCTH

Qhigh = Wnet + Qlow

Work or Electricity

€

COP =QH

W=

QH

QH −QC

COP <TH

TH −TC

Refrigerator Air Conditioner

(Freezer, Summer House)

(Hot Outside World)

Heat Pump

(Cold Outside)

(Winter House)

€

COP =QC

W=

QC

QH −QC

COP <TC

TH −TC

HOT

COLD

QH

QC

COP (Coefficient of Performance) >>1, depends Often more than 5, and decrease with larger ΔT

8/21/12

5

http://www.scientificamerican.com/