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This document provides an overview of solar water heating systems. It discusses the different types of solar thermal collectors categorized by operating temperature, including low-temperature unglazed mats, mid-temperature glazed and insulated collectors, and high-temperature evacuated tubes and focusing collectors. The document then presents examples of solar water heating systems installed at various facilities in the US, including residential, commercial, and institutional buildings. Calculation methods are also provided for sizing a solar water heating system and estimating energy savings and cost effectiveness.

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Solar Water Heating
Andy Walker PhD PE
Principal Engineer
National Renewable Energy Laboratory
NREL/PR-7A40-56706
7th Annual North American Passive House Conference
September 27-30, 2012 Denver CO
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Session Learning Objectives:
• 10.2 Solar Hot Water (SHW) Systems and SHW Ready Design,
• Andy Walker, NREL
•
• AIA Learning Objectives / At the end of this presentation, attendees will:
• 1. understand and be able to explain SHW system options and benefits,
• 2. understand what building- and site-related conditions are needed to minimize
installation costs and maximize SHW production,
• 3. have and be able to use a sizing sheet for estimating system size and costs,
• 4. understand what SHW Ready design means and how it results in more efficient
and cost-effective (future) installations,
• 5. be able to integrate SHW Ready design principles in their PHIUS-certified
buildings..
7th Annual North American Passive House Conference
September 27-30, 2012 Denver CO
Technology Overview
 Low-temperature collectors:
 Unglazed mats for water heating.
 Mid-temperature collectors:
 Glazed and insulated collectors.
 High-temperature collectors:
 Evacuated tubes.
 Focusing collectors.
Solar Water Heaters intercept solar radiation and use it to
heat water. Solar thermal collectors can be categorized
by the temperature at which they efficiently deliver heat.
Solar Water Heating
 Water heating accounts for 4.5
Quads/year of 37.6 Quads/year
total US building energy use.
 Of this, about 1% (0.05
Quads/year) is currently supplied
by solar.
 3% of buildings have solar
 One third of each building load by
solar
Gas, 72%
Oil, 0%
Propane, 3%
Electric,
23%
Solar, 2%
Figure by Andy Walker with data from http://buildingsdatabook.eren.doe.gov/TableView.aspx?table=3.1.4 added quads for commercial and residential
ENERGY SITE CONSUMPTION
Solar Water Heating Market
0
5000
10000
15000
20000
25000
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
Collector
Area
Shipped
(ft2)
Year
Hi Temp
Mid Temp
Low Temp
Figure by Andy Walker with data from
http://www.eia.gov/totalenergy/data/annual/showtext.cfm?t=ptb1006
Collector Types
Technology Overview
6

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56706.pdf

  • 1. Solar Water Heating Andy Walker PhD PE Principal Engineer National Renewable Energy Laboratory NREL/PR-7A40-56706 7th Annual North American Passive House Conference September 27-30, 2012 Denver CO NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
  • 2. Session Learning Objectives: • 10.2 Solar Hot Water (SHW) Systems and SHW Ready Design, • Andy Walker, NREL • • AIA Learning Objectives / At the end of this presentation, attendees will: • 1. understand and be able to explain SHW system options and benefits, • 2. understand what building- and site-related conditions are needed to minimize installation costs and maximize SHW production, • 3. have and be able to use a sizing sheet for estimating system size and costs, • 4. understand what SHW Ready design means and how it results in more efficient and cost-effective (future) installations, • 5. be able to integrate SHW Ready design principles in their PHIUS-certified buildings.. 7th Annual North American Passive House Conference September 27-30, 2012 Denver CO
  • 3. Technology Overview  Low-temperature collectors:  Unglazed mats for water heating.  Mid-temperature collectors:  Glazed and insulated collectors.  High-temperature collectors:  Evacuated tubes.  Focusing collectors. Solar Water Heaters intercept solar radiation and use it to heat water. Solar thermal collectors can be categorized by the temperature at which they efficiently deliver heat.
  • 4. Solar Water Heating  Water heating accounts for 4.5 Quads/year of 37.6 Quads/year total US building energy use.  Of this, about 1% (0.05 Quads/year) is currently supplied by solar.  3% of buildings have solar  One third of each building load by solar Gas, 72% Oil, 0% Propane, 3% Electric, 23% Solar, 2% Figure by Andy Walker with data from http://buildingsdatabook.eren.doe.gov/TableView.aspx?table=3.1.4 added quads for commercial and residential ENERGY SITE CONSUMPTION
  • 5. Solar Water Heating Market 0 5000 10000 15000 20000 25000 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 Collector Area Shipped (ft2) Year Hi Temp Mid Temp Low Temp Figure by Andy Walker with data from http://www.eia.gov/totalenergy/data/annual/showtext.cfm?t=ptb1006
  • 7. Solar Hot Water Types Solar Hot Water Types Unglazed EPDM Glazed Flat Plate Evacuated Tube Parabolic Trough Pix 09320 Pix 08846 Pix 13529 Pix 09501
  • 8. Energy Collected = optical gains - thermal losses Q useful = τ α I Ac - Uc Ac(Ts-Tamb) Efficiency = Energy Collected / Incident Solar ηsolar = Quseful / I Ac = τ α - Uc (Ts-Tamb)/I, (a line of slope Uc and intercept τ α ) I= incident solar radiation (W/m2) τ = transmissivity of cover glass α = absorptivity of absorber plate Ac= collector area (m2) Quseful = useful heat from collector (W) Uc= thermal loss coefficient of collector (W/C) Ts= storage water temperature (C) Tamb= outdoor ambient temperature (C) I τ α Ac UcAc(Ts-Tamb) Q useful I Ac Collector Efficiency (Rating) Figure by the author
  • 9. Which collector is best depends on the temperature... Efficiency = % of solar captured by collector Figure by the author
  • 10. Solar Rating and Certification Corp.  An independent nonprofit organization that tests performance and certifies almost every solar heater on the market today.  Reports efficiency line and annual performance for different climates and temperature uses. Contact information Solar Rating and Certification Corporation c/o FSEC, 1679 Clearlake Road Cocoa, FL 32922-5703 Voice (321)638-1537 Fax (321)638-1010 E-mail: srcc@fsec.ucf.edu
  • 11. World Solar Hot Water Market Share Share of Total Installed Capacity Up to 2006 (105GWth) Worldwide Installed Solar Hot Water Capacity 88 105 128 0 20 40 60 80 100 120 140 2005 2006 2007* Year Installed Capacity (GWth) * 2007 data is estimated
  • 12. 12 Passive Systems Integral Collector Storage Thermosyphon System Types Active Systems Open Loop: Direct Drain Down Closed Loop: Drain Back Antifreeze
  • 13. Passive, Integral Collector Storage (ICS) Direct System  Moderate freeze protection (pipes at risk)  Minimal hard water tolerance  Very low maintenance requirements Figure by Jim Leyshon, NREL
  • 14. Passive, Thermosyphon, Direct System  Auxiliary element can also be in tank above collector, eliminating the auxiliary tank altogether.  No freeze protection  Minimal hard water tolerance  Low maintenance requirements Figure by Jim Leyshon, NREL
  • 15. Active, Open-loop, Pumped Direct System  No freeze protection  Minimal hard water tolerance  High maintenance requirements Figure by Jim Leyshon, NREL
  • 16. Active, Closed-loop (antifreeze), Indirect System  Excellent freeze protection  Good hard water tolerance  High maintenance requirements Figure by Jim Leyshon, NREL
  • 17. Active, Closed-loop, Drainback, Indirect System  Good freeze protection  Good hard water tolerance  High maintenance requirements Figure by Jim Leyshon, NREL
  • 18. Recirculation Loop  Requires well insulated collector (evacuated tube)  Active protection for freezing and overheating Recirculation loop to and from boiler Figure by Jim Leyshon, NREL
  • 19. PV/Heat Pump Water Heating Photovoltaics Utility Ground Source Heat Pump Hot Water Out Best for Net Zero High Initial Cost: PV and GSHP High efficiency of GSHP compensates for lower efficiency (than solar thermal) of PV Utility net-metering is 100% efficient, free, energy storage that can use kWh generated in summer to power GSHP in winter. Figure by the author
  • 20. Tempering Valve to Prevent Scalding: Extremely Important for Safety!
  • 21. 21 Low Temperature Example: Barnes Field House, Fort Huachuca, AZ  2,000 square feet of unglazed collectors  3,500 square feet indoor pool  Installed cost of $35,000  Meets 49% of pool heating load  Saves 835 million Btu/ year of natural gas  Annual savings of $5,400  Installed by the Army in June, 1980. Photo by the author
  • 22. 22 Mid Temperature Example: Chickasaw National Recreation Area, OK Small Comfort Stations  195 square feet of flat plate collectors  500 gallon storage volume  Cost $7,804  Delivers 9,394 kWh/year  Saves $867 / year Large Comfort Stations  484 square feet of flat plate collectors  1000 gallon storage volume  Cost $16,100  Delivers 18,194 kWh/year  Saves $1,789 / year Photo by the author
  • 23.  62 units installed 1998  Active (pumped), Direct systems  Average cost $4,000 per system  80 sf per system  $800 per system HECO rebate  Energy Savings of 3,008 kWh/year  Demand Savings of 1.62 kW/house  Solar Fraction 74%  Efficiency 24% Mid Temperature Example: USCG Housing, Honolulu HI Photo by the author
  • 24. Mid Temperature Example: USCG Housing, Honolulu HI figure by the author
  • 25.  Three closed loop systems with evacuated tube collectors, heat exchanger in the preheat tank. Food-grade Propylene Glycol solution for freeze protection.  Bay F 80 gallon preheat tank and 20 ft2 of collector area.  Bay B 80 gallon preheat and 40 ft2 of collector area  Bay D 120 gallon preheat tank and 90 ft2 of collector area , measured output averaged 50,000 Btu/day in December, 98.  Total Cost = $26,000, 15 yr payback High Temperature Example: Building 209, EPA Lab, Edison NJ Pix 80300 and 80305Photos by Albert Nunez
  • 26. Solar Water Heating Case Study: Social Security Administration Building, Philadelphia, PA  Reheats recirculation loop  180 evacuated heat-pipe collector tubes  27 m2 gross area  Cost $37,500  Delivers 38 GJ (36 million Btu)/year  Installed 2004 0 2000 4000 6000 Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Energy/Month (MJ/month) figure by the author, photo by Sara Farrar Nagy, NREL
  • 27. 27 High Temperature Example: Phoenix Federal Correctional Institution  17,040 square feet of parabolic trough collectors  23,000 gallon storage tank  Installed cost of $650,000  Delivered 1,161,803 kWh in 1999 (87.1% of the water heating load).  Saved $77,805 in 1999 Utility Costs.  Financed, Installed and Operated under Energy Savings Performance Contract with Industrial Solar Technology, Inc.  The prison pays IST for energy delivered at a rate equal to 90% of the utility rate (10% guaranteed savings), over 20 years. Pix 09048 Pix 07756
  • 28. 0 100 200 300 400 500 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Delivered Heat (million Btu) Month Month Energy and Cost Savings 1999 2000 2001 2002 2003 High Temperature Example: Phoenix Federal Correctional Institution
  • 29. Simple Evaluation Procedure  Estimate Daily Water Heating 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  Estimate System Cost  Calculate Savings-to-Investment Ratio and Simple Payback Period
  • 30. Solar Water Heating Costs – Single, small system $199/ft2 or $2141/m2 – Large central system $60/ft2 or $646/m2 – Swimming Pool system $23/ft2 or $247/m2 • Source: RS Means “Green Building Cost Data, 2011”, Page 293, 294.
  • 31. Daily Water Heating Energy Load L = MC (Thot - T cold) /ηboiler L = Daily Hot Water Energy Load (kWh/day) M= mass of water per day (kg/day), use 3.785 kg/gallon C = specific heat of water= 0.001167 kWh/kg°C Thot= hot water delivery temperature (°C), often 50 ° C for 120 ° F Tcold = cold water temperature (° C), often 13 ° C for 55 ° F ηboiler = auxiliary heater efficiency gas 0.43 to 0.86, assume 0.57 electric 0.77 to 0.97, assume 0.88 heat pump assume 2.0 propane 0.42 to 0.86, assume 0.57 oil 0.51 to 0.66, assume 0.52 Source: GAMA Typical Hot Water Usage: Dormitory 13 gal/day/person Barracks 10 gal/day/person Motel 15 gal/day/unit Hospital 18 gal/day/bed Office 1 gal/day/person Food Service 1 to 2.4 gal/meal Residence 30 gal/day/person School 1.8 gal/day/student Statutory Goal is to meet 30% of this hot water load with solar.
  • 32. 32 Solar Energy Resource  Collectors should face south (in northern hemisphere)  Tilt Angle=latitude maximizes annual gain (lat+15°for winter, lat-15° for summer) Location I Max I Ave (kWh/m2/day) Anchorage, AK 4.6 3.0 Austin, TX 6.3 5.3 Boston, MA 5.6 4.6 Chicago, IL 5.7 4.4 Cleveland, OH 5.6 4.1 Denver, CO 6.1 5.5 Fargo, ND 6.5 4.6 Honolulu, HI 6.5 5.5 Jacksonville, FL 6.1 4.9 Sacramento, CA 7.2 5.5 San Diego, CA 6.5 5.7 Seattle, WA 5.7 3.7 Data for all military sites has been compiled by NREL in attached Excel Spreadsheet
  • 33. Solar Water Heating System Size and Delivery Solar Water System Size Ac = L . (ηsolar Imax) Ac = collector area (m2) L = Daily Load (kWh/day) ηsolar= efficiency of solar system (typically 0.40) I max = maximum daily solar radiation (kWh/m2/day) Annual Energy Savings Es = Ac Iave ηsolar365 ηboiler Iave = average solar radiation (kWh/m2/day)
  • 34. Solar Water Heating System Cost and Savings Solar System Cost C = csolar Ac C = Installed Cost of Solar System ($) csolar = per-unit-area cost of installed solar system ($/m2), typically $400/ m2 for large system $1000/m2 for small systems $750/ m2 might be average Annual Cost Savings S = Es Ce S = annual cost savings ($/year) Ce = cost of auxiliary energy typically: Electricity =0.1065 $/kWh Natural Gas =$0.04 $/kWh No. 2 heating oil = $0.05 $/kWh Propane =$0.07 $/kWh Kerosene = $0.07 $/kWh Source: FTC
  • 35. Solar Water Heating System Cost Effectiveness Savings-to-Investment Ratio SIR = S*pwf / C project is cost effective if SIR>1. pwf = present worth factor for future savings stream, = 24 years for 40 year lifetime and 3% real discount rate (specified by NIST). EISA 2007 specifies the 40 year analysis period (lifetime) Simple Payback Period SPB = C / S
  • 36. Solar Fraction= Fraction of Load Met by Solar Solar Fraction = Ac Iave ηsolar/L • Economic optimum is often solar fraction of 0.75 (approximately) • Statutory Goal for Federal buildings is 30% • If your goal is only to meet 30% requirement size system as: Ac = 0.30*L . (ηsolar Iave) Note use of Iave not Imax in the denominator
  • 37. Example: 4 person residence in Austin TX against electricity  M=4person*40gal/person/day*3.785 kg/gal=606 kg/day  L=MC(Thot-Tcold) =606 kg/day*0.001167kWh/kgC*(50C-18C) =22.6 kWh/day  For Austin TX, Imax = 6.3 and I ave = 5.3 kWh/m2/day  Ac = L / (ηsolar Imax) = 22.6 kWh/day / (0.4 * 6.3 kWh/m2/day) = 9 m2  Es = Ac Iave ηsolar 365/ ηboiler = 9 m2 * 5.3 kWh/m2.day * 0.4 * 365days/year / 0.88 = 7,914 kWh/year  C = csolar Ac = $2141/m2 * 9 m2 = $19,269  S = Es Ce = 7,914 kWh/year * $0.10655/kWh = $842/year  SIR = S*pwf / C = $842/year * 24 years / $19,269 = 1.03  Payback period = $19,269/$842 = 23 years.  Solar Fraction= 6802 kWh/(22.6kWh/day/0.88*365days/year)=72%  RESULT: Project is Cost Effective
  • 38. Other things to consider…  Zero emissions on site  Employs local trades rather than exporting jobs to import energy  Avoids fuel delivery and handling risks  No fuel cost fluctuations  Energy Security: no fuel supply interruptions  Reliability: redundant power supplies
  • 41. Cost-Effective Solar Thermal Applications  Low temperature:  Swimming Pools  Mid temperature:  Residential Hot Water  Cafeterias  Laundries  Air Conditioning Reheat  High temperature:  Industrial Processes  Electrical Generation  Water heating loads constant throughout week and year (or more in the summer).  High cost of backup energy (electricity, propane, etc.).  Sufficient area to site collectors (1 ft2/gal/d).  Sunny climate helps, but is not a requirement- solar works in cold climates too.  Need on-site “champion”
  • 42. Requirements for Success  Appropriate Application (Provide a Reasonable Payback)  Proven Design  Redundant Freeze Protection  Properly Sized (undersized, not oversized)  Require No Manual Intervention  Operational Indicators or Monitoring  Conservation First  Verify Load  Performance Guarantee  Require Operations and Maintenance Manual and Training  Acceptance Test
  • 43. , Temp. Sensor Mount Expansion Tank Pump Winding Pump Capacitor Leaks Valves Collector PC Board Relay DC power supply Temp. Sensor Open Working Fine Problems O&M Survey of 185 Solar Water Heating Systems Figure by the author
  • 44. O&M Survey of 185 Solar Water Heating Systems 90% of the systems could be repaired for less than $500 Figure by the author
  • 45. Solar Ready • Safety features required to install and maintain • Layout of buildings on the site to leave un-shaded areas • Arrange rooftop mechanical and communications equipment to leave un- shaded areas on the roof • Provide roof structure to support he loads of the solar collectors, • Type of roof: Flat roofs: ballasted; standing seam metal roofs; Shingles; tiles • Install any required blocking between rafters of a roof, for the solar mounts to attach to, before the ceiling is installed. • Designate locations for the components: water storage tanks; controls; valves; electrical to pumps and controls; water pipes should be arranged in the shortest route possible; Pipe chases may be made to connect the roof to the building electrical and plumbing systems with pipe or conduit, without having to run them in exterior walls or surface-mounted.
  • 46. Solar System Maintenance Options  Energy Savings Performance Contract:  You pay for delivered energy.  Guaranteed Energy Savings Contract:  You don't pay if energy is not delivered.  Service Contract and Warranty:  You pay fixed service costs, whether needed or not.  Facility does maintenance --  Or, historically, facility doesn't do maintenance: Low priority (always have hot water at the tap). No inventory of parts and little expertise. Utility bill is always paid while efforts to reduce maintenance budgets are ongoing.
  • 47. Request for Proposals- References American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE): ASHRAE 90003 Active Solar Heating Design Manual ASHRAE 90336 Guidance for Preparing Active Solar Heating Systems Operation and Maintenance Manuals ASHRAE 90342 Active Solar Heating Systems Installation Manual ASHRAE 93 Methods of Testing to Determine the Thermal Performance of Solar Collectors International Association of Plumbing and Mechanical Officials (IAPMO): Uniform Solar Energy Code Safe Solar System Requirements (2006) American Water Works Association (AWWA): AWWA C651 Disinfecting Water Mains Factory Mutual Engineering and Research Corporation (FM): FM P7825 Approval Guide – Fire Protection Tests for Solar Component Products
  • 48. Request for Proposals- References National Fire Protection Association (NFPA): NFPA 70 National Electrical Code Solar Rating and Certification Corporation (SRCC) SRCC OG-100-06 Operating Guidelines For Certifying Solar Collectors SRCC OG-300-08 Operating Guidelines and Minimum Standards for Certifying Solar Water Heating Systems American Society of Civil Engineers (ASCE): SEI-7 Minimum Design Loads for Buildings and Other Structures National Roofing Contractors Association (NRCA): Guidelines for Roof-mounted Photovoltaic System Installations Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA): Guidelines related to metal work including ductwork, CAD standards, and through-penetration sealing and firestopping. FEMP Federal Technology Alert “Solar Water Heating” call 1-800-DOE-EREC or www.eree.energy.gov/femp.