Aspects  of  solar  thermal  hot  water  hea0ng  

Professor  Tim  Dwyer  0m.dwyer@ucl.ac.uk  

Teaching  Fellow,  UCL  

CIBSE South Wales – 2 April 2012

Presentation available for download at www.timdwyer.com/presentations/20120402cardiffsolar.pdf

30  min  of  solar  radia0on  falling  on  earth  is  probably  equal  to  the  world  energy  demand  for  one  year  A  Researcher  

Why  more  UK  solar  thermal?  •  almost  no  emissions  

– some  related  to  pumping/controls  •  life  of  20  to  30  years  •  independence  from  fuel  price  infla0on  •  total  cost  analysis  based  on  known  cost  •  low  maintenance  •  poten0al  for  government  subsidies  •  certainty  of  fuel  supply  •  environmental  cred!!  

Solar Trade Association, 2007

Monthly solar irradiance (kWh) on a flat plane facing South with a tilt angle of 45° Data BS EN 15316-4-3:2007

Solar  Data  for  Coryton  

Average irradiation kWh/m²·day

Source: http://re.jrc.ec.europa.eu/pvgis

Annual H - 1022 kWh/m² N - 1172 kWh/m² V – 821 kWh/m²

0  

1  

2  

3  

4  

5  

6  

Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov   Dec  

Horizontal  

Normal  

Ver0cal  

Source:CIBSE Solar Heating Design and Installation Guide

Viability  of  Solar  Thermal  Systems  Amount  of  annual  sunshine  Required  temperature  of  hot  water  

Annual  energy  requirement  and  energy  use  profile  

Capital  cost  of  the  solar  system  

Prices  of  conven0onal  fuels  

 Fuel  price  infla0on  

Solar  system  annual  O&M  cost  

Other  (e.g.  legisla0on,  government  funding)  

Typical  Bands  of  Solar  Thermal  

Low  Temperature  (100°C)  

 Industrial  process  hea0ng    Electricity  genera0on  

Solar  thermal  and  PV  working  together  

Collec0ng  the  heat  

•  Flat  plate  collectors  – Unglazed  – Glazed  

•  Evacuated  tubes  – Direct  flow  – Heat  pipe  

Unglazed  Collector  •  Low-‐cost  unglazed  collectors  

•  Summer  pools  in  cold  climates  

•  Extend  the  season  in  warm  climates  

•  For  summer  use  on  a  year-‐round  pool  in  cold  climates  

•  Can  payback  in  few  years  

Flat  Plate  Glazed  Collector  

Solar Thermal Systems, Peuser et al 2002

Integrated  

Solar Thermal Systems, Peuser et al 2002

Retrofieed  

hep://www.solar-‐ra0ng.org/educa0on/criteria/collector/6_5_12.htm  

Framed  installa0ons  

But  what  area?  

Solar Thermal Systems, Peuser et al 2002

Evacuated  tubes  

Sydney  Tube  

Direct  Flow  

Solar Thermal Systems, Peuser et al 2002

Solar Thermal Systems, Peuser et al 2002

Solar Thermal Systems, Peuser et al 2002

Heat  Pipe  

http://www.reuk.co.uk/Evacuated-Tube-Solar-Water-Heating.htm http://www.solarpanelsplus.com/thermal-how-it-works/

Heat  Pipe  Collector  

http://www.solarpanelsplus.com/thermal-how-it-works/

What  area?  

Defining  and  cer0fying  performance  

ASHRAE  93  -‐  Methods  of  Tes0ng  to  Determine  Thermal  Performance  of  Solar  Collectors  

EN12975-‐2  Thermal  Solar  Systems  and  Components  

CIBSE KS15 Capturing Solar Energy 2009

Conversion factor or Optical Efficiency

Heat Loss = Useful Gain

ASHRAE 93

EN12975-2

1.74 m2 aperture 1.78 m2 absorber

Cer0fica0on  

Source: Kingspan Thermomax Design Guide

12975  Efficiency  Example  

A  panel  shown  earlier  has  the  following  characteris0cs  supplied  by  the  test  Efficiency  at  Δt  of  0,  η0  =  0.814  

Loss  Coefficient:  a1  =  4.954  W/(m2K)  Loss  Coefficient:  a2  =  0.0189  W/(m2K2)    Ambient  temperature  =  20°C  Average  water  temp  [(tin+  tout)/2]  =  70°C  

Global  irradiance  of  800waes/m2  

Solu0on  to  Example  

Using  η  =  η0  -‐  (a1  x  (tm  –  ta)/G)  -‐  a2  x  (tm  –  ta)2/G)  

Solu0on  to  Example  

Using  η  =  η0  -‐  (a1  x  (tm  –  ta)/G)  -‐  a2  x  (tm  –  ta)2/G)  

η  =  0.814  -‐  (4.954  x  (70  –  20)/800)  

                 -‐  0.0189  x  (70  –  20)2/800)  

Solu0on  to  Example  

Using  η  =  η0  -‐  (a1  x  (tm  –  ta)/G)  -‐  a2  x  (tm  –  ta)2/G)  

η  =  0.814  -‐  (4.954  x  (70  –  20)/800)  

                 -‐  0.0189  x  (70  –  20)2/800)  

=  0.814  –  0.3096    –  0.0024  =  0.502  

ie  at  these  condi0ons  η=  50%  

50%  of  the  energy  provided  by  the  sun  is  actually  used  to  heat  the  water  

RETScreen  

RETScreen  

CIBSE KS15 Capturing Solar Energy 2009

Key  Types  •  Passive  Systems  (no  pumps)  

–  Integral  Collector  Storage  – Thermosyphon  

•  Ac0ve  Systems  (pumps  &  controls)  ….  •  Open  Loop:  

– Direct  – Drain  Down  

•  Closed  Loop:  – Drain  Back  – An0freeze  

Integrated  Collector  and  Storage  

Novel  ICS  System  

Parabolic  collector  water  heater  

Thermosyphon  System  

Solar thermal collectors and applications Soteris A. Kalogirou

http://picasaweb.google.com/bertmenkveld/REDWHO2008SolarThermal#

http://www.p2pays.org/ref/20/19122/HeatCool.html

Thermosyphon  Unit  

www.solarshacksa.com/html/hot_water.html

Direct  Circula0on  System  

Solar  Hea0ng  Guide  –  CIBSE  2007  

Drainback  System  

CORRECTED DIAGRAM BASED ON Solar thermal collectors and applications Soteris A. Kalogirou

Other drainback (plus other system) illustrations http://greenterrafirma.com/solar_thermal.html http://www.atlassolarinnovations.com/solar-water-heating-choices/

EST CE131 - Solar water heating systems – guidance for professionals, conventional indirect models

Drain  Back  Advantages  

•  Gravity  is  fail-‐proof  and  maintenance  free  •  Water,  (or  a  glycol  mixture)  may  be  used  in  the  collector  loop  

•  System  is  not  damaged  if  the  pump  fails  •  System  cannot  reverse  thermosyphon  at  night  •  Collector  plates  last  up  to  longer  than  in  a  pressurized  glycol  system  

Drain  Back  Advantages  

•  If  used  the  Water/Glycol  Mixture  is  unlikely  to  need  changing  

•  Less  moving  components  to  fail  (eg.  check-‐valves,  air  vents  and  expansion  tanks)  

•  Collector  piping  and  system  piping  does  not  scale  if  dis0lled  water  is  used.  

Drain  Back  Disadvantages  

•  Collector(s)  and  all  piping  must  be  above  and  slope  downwards  towards  the  reservoir  

•  Larger  piping  and  insula0on  must  be  used  •  Large  or  rela0vely  large  pumps  

– especially  if  the  design  involves  2+  stories  •  System  and  pump  controls  cost  approx  10%  of  savings  

•  Components  cost  about  10-‐15%  more  than  a  glycol  system  (on  residen0al  system)  

•  Systems  can  be  noisy  -‐  like  a  coffee  percolator  

Indirect  Closed  Water  System  

Solar  Hea0ng  Guide  –  CIBSE  2007  

Simplified - No safety/expansion elements shown

A  common  closed  system  

Source: Megaflo

Solar  Hea0ng  Guide  –  CIBSE  2007  

EST CE131 - Solar water heating systems – guidance for professionals, conventional indirect models

www.wagner-‐solar.com  

Solar  Hea0ng  Guide  –  CIBSE  2007  

Solar  Hea0ng  Guide  –  CIBSE  2007  

Likely  losses?  

Source: EST 131, 2006

Effect  of  parasi0c  losses  (shown  in  terms  of  primary  energy)    

Note: tests undertaken in 2001

Source: DTI/Pub URN 01/1292

Well  performing  domes0c  system  

Energy Savings Trust: Here comes the sun: a field trial of solar water heating systems

Aeributes  of  the  Heat  Transfer  Fluid  

•  Should  not  deposit  ……  –  limescale  – sludge,  –  ice    – other  solids  

……that  could  restrict  circula0on  or  impair  the  rate  of  heat  transfer.    

Aeributes  of  the  Heat  Transfer  Fluid  

Resist  freezing  

Specially  where  the  liquids  are  maintained  in  loop  all  year  round  

Usually  60/40  water/an0freeze  

 will  go  down  to  at  least  -‐20  at  SAP  Glycol  ‘creeps’  more  than  water  

Use  non-‐toxic  propylene  glycol    Glycol  not  compa0ble  with  Zinc  or  certain  seals  

Most  ‘an0  freezes’  will  have  higher  viscosity  and  small  SHC  

pressure  drop  of  the  system  will  increase  

Aeributes  of  the  Heat  Transfer  Fluid  

Maintain  integrity  at  high  temperatures  •  during  periods  of  stagnancy  collector  temperatures  can  reach  200°C  in  flat  plates  and  300°C  in  evacuated  tubes  

•  azer  a  shutdown  other  parts  of  system  may  well  reach  over  160°C+  

•  in  normal  opera0on  120°C  would  not  be  uncommon  

Glycol  Vaporisa0on  Temperatures  

See Duffie & Beckman, 2nd Edition Appendix E for more detailed data

Solar Thermal Systems, Peuser et al 2002

Aeributes  of  the  Heat  Transfer  Fluid  

•  Beeer  to  run  at  lower  pressures  and  design  for  vaporisa0on  •  At  300kPa  vaporisa0on  at  140°C  •  Few  molecules  of  fluid  would  be  in  high  temperature  collector  

as  a  vapour  –  par0cularly  where  long  periods  of  stagnancy  

•  May  lead  to  par0cles  s0cking  to  flow  channels  –  use  strainers  to  remove  par0culates  

•  Small  collector  volume  is  best  with  flow  and  return  connec0ons  at  top  –  vaporised  fluid  will  fill  collector  and  not  pass  large  amounts  of  vapour  

into  remainder  of  system  •  at  150°C,  300kPa  only  5cm3  liquid  is  needed  to  fill  3  litre  collector)  

Aeributes  of  the  Heat  Transfer  Fluid  

•  Important  to  help  prevent  heat  stress  by  unevaporated  fluids  entering  the  stagnant  collector  eg  for  low  lying  or  interconnected  units.  

•  Normal  opera0ng  condi0ons  above  220°C  need  special  fluids    Unsuitable  fluids  chemically  crack  at  this  temperature  

•  The  fluid  will  need  replacing  –  Ageing  will  affect  corrosion  resistance    and  freezing  point  

Stagna0on  Temperature  

tstg  =  stagna0on  temperature  °C  tas  =  selected  ambient  temperature  °C  

Gs  =  selected  solar  irradiance  W/m2  

Gm  =  solar  irradiance  (from  test  data)  W/m2  

tsm  =  absorber  temperature  °C  (from  test  data)  

tam  =  ambient  air  temperature  °C  (from  test)  BS 12975 2001 Capturing Solar Energy 2009

Assumes ratio (tsm-tm)/Gm approx constant

Source: Kingspan Thermomax Design Guide

Antifreeze that has been at 170°C for extended periods

Exacerba0ng  stagna0on  

Source : Stagnation behaviour of solar systems – IEA SHC Group 26, 2002

Poor layout

Heat  dissipa0on  

Source: Kingspan Thermomax Design Guide

And  don’t  forget  …..Legionella  control    

Source : Lochinvar Solar Thermal Guide

But  how  to  choose  system?  

•  Es0mate  Daily  Water  Hea0ng  Load  •  Determine  Solar  Resource  •  Calculate  Solar  System  Size  

–  meet  load  on  sunniest  day  –  undersize  rather  than  oversize    

•  Calculate  Annual  Energy  Savings  •  Calculate  Annual  Cost  Savings  •  Es0mate  System  Cost  •  Calculate  Payback  Period  

Where  to  posi0on?  

Source: Kingspan Thermomax Design Guide

Seasonal  changes  

Source: Kingspan Thermomax Design Guide

Avoid  shading  

Source: Kingspan Thermomax Design Guide

F-‐Chart  

Polysun  4  

T  Sol  

RETScreen  

Matched  system  

Source: Kingspan Thermomax Design Guide

Source: http://www.pobsolar.co.uk/

Bri0sh  Standards  

BS  EN  12975-‐1  (2006)  

Thermal  solar  systems  and  components  —  Solar  collectors  —  Part  1:  General  requirements  

BS  EN  12975-‐2  2006  

Thermal  solar  systems  and  components  —  Solar  collectors  —  Part  2:  Test  methods  

BS  5918  (1989)  

Solar  hea0ng  systems  for  domes0c  hot  water  

References  

ASHRAE  (2008)  HVAC  Systems  and  Equipment  –  Chapter  36:  Solar  Energy  Equipment  

ASHRAE  (2007)  Applica0ons  Handbook  –  Chapter  33:  Solar  Energy  Use  

Solar water heating systems – guidance for professionals, conventional indirect models Energy Savings Trust CE 131

Energy Savings Trust - Here comes the sun

CIBSE – Solar Heating – Design and Installation Guide

CIBSE – KS15 Capturing solar

energy

Solar Engineering of Thermal Processes, Duffie, & Beckman

Solar Design: Components, Systems, Economics by Jan F. Kreider

Solar Thermal Systems - Successful Planning and Construction Peuser, Remmers & Schnauss

MCS - Microgeneration Installation Standard: MIS 3001

Thanks  for  providing  informa0on…  

•  Lochinvar  •  Baxi  •  Kingspan  •  John  O’Brien  •  Viessmann  

 …..and  to  Ruskin  Air  Management  for  sponsoring  the  event  

Prof  Tim  Dwyer,  0m.dwyer@ucl.ac.uk  

Where  next  …..solar  Cooling  

Stoecker - Refrigeration and Air Conditioning