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Exploring the Limits of Energy Efficiency in Office Buildings

Exploring the Limits of Energy Efficiency in Office Buildings. David E. Claridge Director Oleksandr Tanskyi Graduate Research Assistant Energy Systems Laboratory Texas A&M University System Physics of Sustainable Energy Berkeley, California March 5-6, 2011. Background.

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Exploring the Limits of Energy Efficiency in Office Buildings

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  1. Exploring the Limits of Energy Efficiency in Office Buildings David E. Claridge Director Oleksandr Tanskyi Graduate Research Assistant Energy Systems Laboratory Texas A&M University System Physics of Sustainable Energy Berkeley, California March 5-6, 2011

  2. Background Reversible Thermodynamics Adiabatic Devices Carnot Cycles/Carnot COP All Provide Useful Guidance

  3. New Home of ESL 25,774 ft2

  4. Basic ESL Office Building Requirements Comfort for Occupants – Cooling and Heating Ventilation for Indoor Air Quality Lighting Computers/Printers Copiers Cooled Drinking Water Heating – Lunch and Coffee Hot Water – Restrooms

  5. ESL Office Building Assumptions • Comfort – Maintain 73ºF/50% Relative Humidity • Ventilation – Meet ASHRAE Standard 62-2007 • 5 cfm/person + 0.06cfm/ft2 • Lighting –IESNA recommended levels • Computers – 1/person • Monitors – 2/person (23-inch) • Printers – 1/person (2000 pages/yr)

  6. ESL Office Building Assumptions • Copiers – 1/30 people(2000 pages/person/yr) • Cooled Drinking Water – 1qt/person/day • Cooled from 70ºF to 50ºF • Heating – (1 cup water)/person/day • Heated from 70ºF to 212ºF • Hot Water – Restrooms – ½ gal/person/day • Heated from 70ºF to 105ºF • Occupied 60 hours/week

  7. Exploring The Limits What are the limits? What is the minimum energy required to meet each of these office building requirements? What is the minimum energy required to provide these services in our office building?

  8. Exploring The Limits: Lighting • Chose average of IESNA recommended 20-50 fc • Assume • 400 – 700 nM radiation from 5800K black body • ~250 Lumens/Watt • On 6 hr/day weekdays LED LIGHTS

  9. Exploring The Limits: Lighting 35 fc => 0.13 W/ft2 average occupied hours Occupancy sensors => 0.01 W/ft2 unoccupied 1.7 kW occupied without daylighting 0.85 kW occupied with daylighting 0.24 kW unoccupied

  10. Exploring The Limits: Computers • Haven’t determined the physical limit • Assume 2.5W for 1 GHz processor • (e.g. iPhone 4) • Hibernate when not in use • Assume 30 hr/wk for 128 people • => 147 W average when occupied

  11. Exploring The Limits:Monitors Assume limit is lighting power Two 0.14m2 (23-in) monitors per person 250cd/m2@250Lm/W=> 1.75 W/monitor Sleep when not active 6 hr/day for 256 monitors 206 W average when occupied

  12. Exploring The Limits: Printers Physical limit not determined Ink jet printer is ~0.07 Wh/page 2000 pages per person/year => 7 W average when occupied

  13. Exploring The Limits: Copiers 2,000 copies per person per year at ESL Use same energy assumptions as printer 7 W average when occupied

  14. Exploring The Limits: Cooled Drinking Water 1 qt/day per person from 70ºF to 50ºF Use Carnot refrigerator COPCarnot = 28.3 => 4.3 W average for building (when occupied)

  15. Exploring The Limits: Heating Food/Water Assume 1 cup water or equivalent food per person daily from 70ºF to 212ºF Assume Carnot heat pump COPCarnot = 4.66 for 70ºF to 212ºF => 53 W average for building (when occupied)

  16. Exploring The Limits:Heating Water - Restrooms Assume ½ gal/person per day 70ºF to 105ºF Assume Carnot heat pump COPCarnot = 15.65 for 70ºF to 105ºF => 31 W average for building (when occupied)

  17. Exploring The LimitsCooling and Heating • Loads • Internal Gains • Occupants • Solar • Ventilation • Envelope

  18. Exploring the Limits:Internal Gains

  19. Occupant Gains • ASHRAE: Moderately active office work: • 73 W/person sensible • 59 W/person latent • Assume 40 hours/week/person • =>6,250 W sensible • 5,000 W latent

  20. Exploring the Limits:Solar Gains Theoretical limit is zero We assume the amount of solar gain corresponding to the amount of daylighting => 850 W average occupied gain

  21. Exploring the Limits: Ventilation Energy • ASHRAE Standard 62-2004 requires 2190 cfm when occupied • Assume • 12 hours/day weekdays • Perfect enthalpy recovery device • Exhaust air = outside air intake • 5 Pa fan pressurization • Perfect fan • => 5.1 W fan power when occupied is only ventilation energy required

  22. Exploring the Limits:Envelope Gains/Losses Theoretical limit is zero We assume zero

  23. Exploring the Limits:Cooling and Heating • Assume: • “Economizer” cooling when conditions permit • Carnot Chiller for cooling otherwise • Carnot heat pump for heating

  24. Exploring the Limits:Cooling Loads

  25. Exploring the Limits:Chiller Electricity Assume Houston, TX Weather Total Cooling Load = 40,161 kWhth Economizer meets 24,595 kWhth Chiller provides 15,566 kWhth Chiller requires 250 kWh Average COP = 62

  26. Exploring the Limits:Heating Heating Load is zero Heating electricity is 0 kWh!

  27. Energy Flows – Theoretical Limit

  28. Energy Flows – Theoretical Limit 84 MMBtu 0 53 MMBtu 134 MMBtu C/85 MMBtu H 0.05 MMBtu 137 MMBtu 0

  29. Electricity Consumption: Theoretical Limit 0 kWh 250 kWh 15 kWh 5,265 kWh 0 kWh

  30. How Do Today’s Buildings Compare? U.S. Office Building average is 82 kBtu/ft2-yr site A very good new office building is 12 kBtu/ft2-yr Theoretically possible 5,528 kWh =>0.73 kBtu/ft2-yr vs. 82 kBtu/ft2-yr 0.21 kWh/ft2-yr!!!

  31. If We Could Do This - We could power the U.S. level of office space for every person in the world on 28% of the ELECTRICITY used today in U.S. offices! A PV array the size of Two 8 ½ X 11sheets of paper for every person working in the typical office building would make it NET ZERO!

  32. Internal Gains – Targets

  33. Internal Gains – Targets

  34. Internal Gains – Targets

  35. Internal Gains – Targets

  36. Internal Gains – Targets

  37. Internal Gains – Targets

  38. Internal Gains – Targets

  39. Internal Gains – Targets

  40. Ventilation Energy Targets • 2,190 cfm when occupied • Assume • 90% effective enthalpy recovery device • Exhaust = outside air intake • =>13.4 MMBtu cooling / 8.5 MMBtu heating loads • 0.25 inWG (62 Pa) fan pressurization • Fan efficiency = 0.8, motor = 0.95 • => 135 W fan power when occupied

  41. Envelope Targets • Assume following characteristics: • Windows: U = 0.1 Btu/(hr-ft2-ºF) • Walls: U = 0.033 Btu/(hr-ft2-ºF)(6-in poly) • Roof: U = 0.02 Btu/(hr-ft2-ºF)(10-in poly)

  42. Cooling and Heating Targets • Assume: • “Economizer” cooling when conditions permit • Average COP = 10 for cooling • Average COP = 5 for heating

  43. Chiller Electricity Targets Assume Houston, TX Weather Total Cooling Load = 55,900 kWhth(45,408) Economizer meets 6,000 kWhth(27,808) Chiller provides 49,900 kWhth(17,600) Chiller requires 4,990 kWh (283)

  44. Heating Targets Heating Load is 19,300 kWhth (0) Heating electricity is 3,870 kWh (0)

  45. Energy Flows – Targets 20 MMBtu 0 170 MMBtu 134 MMBtu C/85 MMBtu H 1.4 MMBtu 198 MMBtu 66 MMBtu

  46. Electricity Consumption: Targets 4,994 kWh 406 kWh 8,484 kWh 3,869 kWh

  47. How Do Today’s Buildings Compare? U.S. Office Building average is 82 kBtu/ft2-yr site A very good new office building is 12 kBtu/ft2-yr Theoretically possible 0.73 kBtu/ft2-yr or 0.21 kWh/ft2-yr Target 2.35 kBtu/ft2-yr or 0.7 kWh/ft2-yr

  48. What Do We Need to Improve?

  49. What Do We Need to Improve?

  50. What Do We Need to Improve?

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