Solar Electric and Solar Hot Water System Design By Fortunat Mueller at Building Energy 2014

1. Building Energy 2014
The Solar Energy Course for Architects,
Engineers, and Contractors
Fortunat Mueller PE
Co Owner
ReVision Energy
March, 2014
Professional design, installation and service of renewable energy systems.

2. AGENDA
•
•
•
•
•
Introduction
Solar Basics
Solar Thermal
PV
Wrap up and Q and A

3. Motivation
• Environmental
– Reduce CO2 emissions
– Transition away from finite fossil fuels to sustainable,
renewable energy sources
• Energy Security/Geo-political
• Economics
– Save money
– Reduce future costs

4. What is Solar Energy?
– The visible, Infrared and UV radiation from the
sun that can be used for heat, or electricity via
the photovoltaic effect

5. Solar Fundamentals
• Insolation: Measure of the energy striking the earth’s surface.
• This energy can be collected and used
• Units are: [Energy]/[Area*time]
typically: kWh/m2/day or BTU/sq ft/hr or similar

7. Collector Orientation
•
Generally South facing
– Take into account magnetic declination (Solar
south is 16 deg W of Magnetic South)
– Southeast and Southwest facing is just fine in
most cases((155 to 245 degrees on the compass)

8. Collector Installation Angle
• Optimal angle depends on load profile:
– SHW: ~ 45 degrees (similar to our latitude)
• Assuming balanced year round load
– Combi Systems: ~ 55-60 deg (latitude plus 10 degrees)
– GTPV: ~ pretty insensitive to installation angle for year
round production

9. Collector Installation Angle
Overall effect on annual performance

10. Sun Path and Insolation through
the day

11. Shading
• Shading significantly affects collector performance.
• Optimal solar window is 9 a.m. to 3 p.m. year round
• Important to conduct a site evaluation
– Consider the future growth of trees

12. Solmetric Sun Eye

13. Shading- PV
• PV systems tend to me more affected by partial shading
than thermal systems since individual cells and modules
tend to be wired in series so even cells in bright sun
might show diminished performance if other sections of
the collector array are partially shaded.

14. Solar thermal (hydronic):
•
•
•
•
Hot Water Heating
Space Heating
Pool Heating
Commercial

15. Components: Collectors
• Collector types:
– Flat Plate
– Evacuated Tube
– Unglazed (pool heating)

16. Unglazed Collectors
• Construction
– No glazing or insulation
– Typically UV stabilized plastic/polymer construction
• Characteristic
– High wetted area to compensate for poor heat transfer of
polymer material
– Large Surface area to account for large load
– Designed for high flow rate
• Applications
– Seasonal pool heating systems
– Temperature ~ ambient +/- 10 degrees

17. Glazed Flat Plate Collectors
• Construction
– Metal absorber in thermally insulated sheet metal box
– Transparent low iron tempered glass to minimize heat loss and
maximize transmission
• Characteristics
– Good mid temperature performance
– More expensive than unglazed
but cheaper than evacuated tube
- Serpentine or Harp flow pattern
• Applications
– Mid temperature applications
•
•
•
•
Solar Domestic Hot Water
Low Temp process water
Preheat applications
Occasionally Combi systems

18. Evacuated Tube Collectors
• Construction
– Single wall or double wall glass cylinder with vacuum with
metallic absorber inside.
– Typically individual tubes connected to a manifold
– Include barium getter to maintain vacuum
• Characteristics
– Very low heat loss
– Excellent performance in low light conditions
– Typically a bit more expensive
• Applications
– Domestic hot water systems in cold climates
– Combi Systems
– Higher temperature process water systems

19. Collectors Characteristics
• Gross Area: product of outside collector dimensions
• Aperture area: light entry area
• Absorber area: area of the absorber itself

20. Efficiency
• Collector Efficiency: (The ratio of usable
thermal power to the incident solar energy
G = Incident solar energy
flux)
0
η = Qdota/ Go
G
G0
1
G2
Q2
G1 = reflection off glass
G2= absorber emissivity
Q1 = conduction heat loss
Q2 = radiation and convective
heat loss
Qa = useable heat
Q1
Qa

22. Collectors Characteristics
• SRCC and SPF and solar keymark

23. TYPICAL SRCC RATING

24. Typical SPF rating

25. Components: Pressurized domestic
storage tanks

26. Pressurized domestic storage tanks
desirable attributes
• Aspect ratio: Tall and skinny is better (>2.5 to 1
ratio of H to d)
– Improves stratification
• Insulation: target of 1.5 W/K total heat loss (R20+)
– Minimizes heat loss
• Cold Water baffle
– to minimize mixing
• Heat trap on domestic exit
• Heat trap for heat exchanger connections

27. Components: Unpressurized
thermal storage tanks (non potable)

28. Components: Piping
• Copper (to about 1.5 inch) then Steel
• Corrugated Stainless occasionally used on DIY projects
to avoid soldering/brazing
• NEVER use PEX
• Onyx ?

29. Components: Insulation
• High temperature capable insulation needed
near collectors
• Typical Armaflex HT or similar or Fiberglass
• Use ¾ -1 inch wall insulation outside and ½ to ¾
wall insulation inside conditioned spaces
– European Standard EN 12976 calls for 20 mm of
insulation up to 22 mm pipe diameter and 30 mm for
anything larger

30. Components: Insulation
• Insulation should be protected outside
from UV damage by metal or PVC wrap

31. Components: pumps
• Residential
– Wet Rotor Circulator
with hydraulics
optimized for closed
loop solar.
– Typical multiple speed
pump for universal use
and to minimize
parasitic power

32. Components: pumps
• Commercial
– Energy saving
– Variable speed
– Need to calculated
required flow and
head pressure

33. Components: Heat Exchangers
• Internal
– Plain coil or finned tube
– Vertical or Horizontal
• Vertical preferred because it promotes stratification
• External
– Flat plate
– Shell and tube
External heat exchangers tend to be desirable in larger
systems because a single hex can serve multiple tanks.

34. Components: Solar Fluid
•
•
•
•
•
High thermal capacity
High thermal conductivity
Low viscosity
Resistance to freezing
Non toxic
Propylene Glycol/Water Mix is typical

35. Components: expansion tank
• Required in closed loop
systems
• Check material compatibility
with antifreeze
• Sized not only for thermal
expansion but usually also for
possible vapor volume from
collectors

36. Components: other
Check Valve, Air elimination, flow meter, PRV, fill and
drain ports, mixing valve

37. Pump Station
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Pump
Temperature Gauge
Pressure Gauge
PRV
Check valve
Flow meter
Fill and Drain ports

38. Components: Controller
Basic differential control

39. Components: Controller
Multiple tank system, multiple collectors, variable speed
, data logging, remote display, etc

40. Residential Domestic Hot Water
•
•
•
Average American Household
consumes 64 gallons per day of hot
water (20gal./person/day)
Often best solar load because it is
low temperature and year round.
Savings multiplied by keeping oil
boiler off in the summer and thus
eliminating the boiler standby losses
Typical Maine Residential Oil Use
space heat
64%
domestic hot water
9%
boiler standby
losses
27%

41. Single Tank Solar Solution with
integrated boiler backup

42. Single Tank with integrated electric
backup (external hex)

43. Flush Mounted to Pitched Roof
Takes the orientation and tilt of the roof

44. Pitched Array
Faces south on east/west facing roof

45. Awning Mounted
For south facing gable ends

46. Ground Mounted
Roof space may not be available

47. Solar Combi System Examples

48. Solar combi-system design
•
•
•
•
•
•
•
•
Design for 30-40% of annual heat load or based on available
roof space/budget
The cooler the collector array operates, the lower its thermal
losses and thus the higher its efficiency.
Optimize system performance for shoulder season.
Don’t heat the solar tank with the boiler. EXCEPTION: Single
tank systems with good tank stratification.
Always provide means of dealing with excess collector heat in
summer (ideally a pool or other summertime load can use the
heat).
Steep collector angle minimizes overheating and optimizes
winter time performance
Simple is good
Solar system failure should not prevent heating system from
maintaining the house at a comfortable temperature.

49. Solar Combi System 2: Return water re-heating with
low mass boiler

50. Overheat protection
• Required on all Combi systems but also a good
idea on all systems
• Types of overheat protection:
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Collector installation angle
Controller settings
Active pumped dump zone
Pool
Collector integrated dump zone
Controlled stagnation (steamback)

51. Controlled Stagnation behavior
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Expansion tank sizing
Check Valve location
Collector piping layout
Collector emptying behavior
Component location
Glycol quality

52. Collectors with Bad Emptying Behavior

53. Collectors with Good Emptying Behavior

54. Commercial
Assisted living facility
• Any application with
substantial DHW load.
Bed and Breakfast
Hotel
Farm

55. Commercial DHW system design
• Multiple loads
• Multi story buildings are often roof constrained which makes
it difficult to reach 100% solar fraction so systems are
designed as ‘preheat’
• Low SF systems can use less storage if the demand is
steady and early in the day (Restaurants).
• Larger systems require more attention to design details
(pump sizing, HEX sizing, pipe sizing, overheat protection
etc)
• Large tank size (>400 G) favors unpressurized storage for
reasons of cost.

56. Commercial DHW system design

57. Commercial Example: Country Inn
Solar Domestic Hot water, pool and spa heating system

58. • Other considerations
– Variable speed control of circulation pump
– Remote display and data logging
– Hot water recirculation lines
– Collector layout
• Reverse return (Tichelman)
• Balance valves

59. BREAK 3 min

60. SHW system design process
I. Site Analysis:
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Determine the Load
Evaluate the roof space and exposure
Evaluate the storage tank space
Identify design goal
I. System Design
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Choose system type (drainback, closed loop, etc)
Size collectors
Size Tanks /heat exchanger
Determine flow rate/ size pipe run
Select pump
Size expansion vessel
Specify other components
Physical Layout

61. Design Step 1: Determine the load
First determine the load in gallons of hot water per day:
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Residential:
Hotels:
Restaurants:
Assisted living:
Office:
School:
Salon:
Laundromat
15-20 G per person per day
15-20 G per occupied hotel room
2.4 G per meal
18.4 G Per bed
1.0 G Per person per day
0.5 to 1.0 G per person per day
80.0 G Per basin
50.0 G Per top-loading washer
30.0 G Per front-loading washer (3)
Then convert that to BTUs:
(Gal/day) * (deg F rise) * (8.4 BTU/G deg)= BTU per day
…add to that the expected heat loss from pipes and tanks etc

62. Design Step 2: Solar Resource
Assessment
• Measure the available roof space
• Check for obstacles
– Vent pipes, chimneys, etc
• Check shading

63. Shading Analysis
• Use Sunchart, Pathfinder, Suneye etc
• Look for year round sound 9 AM-3PM

64. Design Step 3: Boiler room
assessment
• Measure the available space (footprint,
height, entry doors!)
• Note existing water heater type and
capacity
• Existing plumbing size
• Mixing valve
• Location of electrical equipment

65. Design Step 4: Identify the Design
Goal
• Maximum fossil fuel displacement?
• Quickest payback?
• Something else?
Solar Fraction: Fraction of load met by solar energy.
– Typical DHW systems are most cost effective when shooting for
a SF of 100% in summer (non heating months).
• Larger means wasted energy much of the year
• Smaller means missed opportunity for savings (especially where the
backup may have very low efficiency)

66. Step 5: Choose system type
•
•
•
•
Closed loop vs drainback
Preheat vs integrated
Internal vs External HEX
Choose collector type (flat plate, vacuum
tube, unglazed)

67. Step 6: Sizing the Collectors
• Use rules of thumb
– Flat plates : 800-1000 BTU per SF on good summer day
– Evacuated tubes: 900-1200 BTU per SF on good summer day
• Use ratings from SRCC or SPF or others
– Between ‘Clear Day-C’ and ‘Mildly Cloudy-C’ is a good average
number from SRCC for summertime production
• Use a model
– RETscreen, Polysun, F chart, T sol etc

68. Step 7: Tank/HEX Sizing
•
Tank sizing:
– Roughly 2 Gallons of storage per SF of collector yields roughly 60-80 degree
temperature rise on a sunny day.
– If designing for 100% summer SF, typically 1-2 times the daily hot water
consumption to bridge the gaps
– If designing as a preheat (low SF) then size storage for volume of hot water
produced each day.
– If the load is regular and well understood size based on necessity
•
Heat Exchanger sizing:
– Design for a 20 deg F temp rise in collector loop with peak sun and full flow
– Use manufacturer’s modeling tools
– Rules of thumb:
• Plain copper tube: 20% of collector surface area
• Finned copper tube: 35% of collector surface area

69. Step 8: Flow rate and pipe sizing
•
Flow Rate:
– Max flow rate should result in ~20 degree rise through collector array
with peak sun.
– Follow manufacturer’s recommendations
– Rules of thumb:
• 0.03- 0.06 GPM per sq ft of collector area
•
Pipe sizing:
– Like any hydronic system, keep flow velocity 4 ft per second to minimize
flow noise and abrasion in pipe.
– But to minimize wasted pumping power, between 2-3 ft/second is a
good rule.
•
•
•
•
For flow rates of 1.6 GPM to 3.2 GPM use 0.5 inch
For flow rates between 3.2 GPM to 6.5 GPM use 0.75 inch
For flow rates between 5.5 and 10.9 GPM use 1 inch
For flow rates between 8.2 and 16.3 use 1.25 inch

70. Step 9: Pump Sizing
•
At the design flow rate determine circuit head loss from:
– Tables or other methods for pipe run.
– Manufacturer’s published data for collectors
– Manufacturer’s published data for heat exchangers
•
Draw the system curve then look for a pump with an appropriate
pump curve

71. Step 10: Expansion tank sizing
Expansion Volume = Volume required for thermal expansion of the fluid AND possible steam volume
from collectors.
= (Total Volume of Glycol * Expansion Factor) + Volume of collector*
(Expansion factor ~ 0.05-0.1 for glycol/water)
Tank Volume = Expansion Volume * [(Pmax +1) / (Pmax –Po)]
Where:
Pmax = Maximum allowable pressure (absolute pressure)
Po = initial system pressure (at prv location) (absolute pressure)
To avoid air leaking into the system, pressure in a
closed loop system should be 7-10 psi minimum at
the highest point of the system so:
P0 = .5 * system height(ft) + 10 psi

72. Step 11: Other Components
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PRV
Mixing Valve
Domestic Expansion Tank
Air Elimination
Fill and Drain Ports and valves
Insulation
Controls
Sight Glass, flow meter
BTU meter

73. Step 12
: Physical Layout of components
• Roof Layout:
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Roof loading
Collector piping
Aesthetics
Service access
Ease of Install
• Boiler room Layout
– Service access to solar and other components
– Minimize distances for solar and domestic piping
• Pipe Run planning:
– Minimize total length
– Minimize high points
– Ease of install

74. And then…Install it

75. SHW system design process
I. Site Analysis:
–
–
–
–
Determine the Load
Evaluate the roof space and exposure
Evaluate the storage tank space
Identify design goal
I. System Design
–
–
–
–
–
–
–
–
Choose system type (drainback, closed loop, etc)
Size collectors
Size Tanks /heat exchanger
Determine flow rate/ size pipe run
Select pump
Size expansion vessel
Specify other components
Physical Layout

76. Sample using RETscreen and
Polysun: Blueberry Commons
Building 14
Load: 10 Senior apartments roughly 16 people (240 G per day)
Roof: Pitched, 35 degrees 180 deg True
Backup system: Propane indirect hot water heater from boiler

77. SHW Economics
Simple Payback =System Cost / Annual Savings
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–
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Savings estimates?
At what fuel cost?
Cost of capital?
Incentives?

78. Solar Hot Water Incentives
•
Federal tax credit
– 30% of system cost
– Requires SRCC rating for residential (not commercial)
– Pool heating doesn’t qualify
•
State Rebate
– Varies by state
•
Accelerated depreciation
– MACRS 5 year accelerated depreciation
– Bonus depreciation
– Section 179
•
•
Utility Rebate
Low interest loans
– Small business low interest loan program
– HELP loan for residential
•
•
USDA REAP grants
Other grants (CBDG, VRRF, etc)

79. Residential SHW Economics
$11,000 Typical Solar Hot Water system gross cost
- $2,000 (conventional indirect tank you don’t have to buy)
- $3,300 (Federal Tax Credit)
- $1,000 (State Rebate)
----------------------------------------------------$4,700 net cost
Financed on 30 year mortgage at 6% this is an extra $28 per month.
Average Savings (250 G per year at 3.50 per Gallon) = $73 per month
Total COST SAVINGS = $540 per year
It costs less to have SHW than it does NOT to have it…how many of
your products can you say that about?

80. Commercial Solar System
Economics

81. Discussion/Questions
Contact us:
Fortunat Mueller
fortunat@revisionenergy.com
(207) 221-6342

82. Photovoltaic (PV) Applications
• Solar Electric systems can be designed to meet up to
100% of our residential annual electrical needs
• Average 5 kW PV array uses approximately 350 sq’
• Net metering allows excess energy produced during the
day to be stored at retail with the grid, indefinitely

83. How a GTPV System Works

84. Grid-tied Photovoltaics (PV)
Components
Photovoltaic modules
convert sunlight into Direct
Current (DC) electricity,
which flows through cable
to the inverter.
Inverters accept the DC electricity
produced by PV modules and convert
it into Alternating Current (AC), which
then feeds demand in the building or
if there excess, feeds the utility grid.

85. Net Metering & Inverter Technology
Replaces Batteries

86. Micro Inverters
• Suitable for locations with varying sun
and/or partial shading

87. Mounted Flush to Pitched Roof
Takes the orientation and tilt of the roof – most common application

88. Ground Mounts
Roof space may not be available

89. Trackers
Approximately 35% more annual energy using dual axis tracking technology

90. Sizing
• Performance rules of thumb
– 1000-1300 kwhr/kW per year
• Modeled performance
– RETScreen
– Pvwatts
– PVSOL
– Polysun

91. Sizing Example
• Pvwatts and RETscreen demo
• 7 kW in Portland, ME

92. Electrical Design
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•
•
•
•
Array sizing
Inverter Sizing and String layout
Wire/Conduit Sizing
Overcurrent/Disconnect specification
Grounding/Bonding

93. Electrical Design

94. Mechanical Design
 Dead load of the Equipment onto the Structure
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Uplift on the array
Snowfence effect of multiple rows (flat roof)
Ballast if used (flat roof)
Electrical Grounding
Weather sealing penetrations
Isolation for galvanic reaction
Longevity – 50 years

95. Mounting and roof loads
All modern sloped roof-mount systems are based on extruded aluminum rails
2-3 psf typical

96. Mounting and roof loads
Low-angle, ballasted systems dominate installations
on flat, membrane roofs. (4-10 psf typical)

97. Grounding
Grounding array structures is one of the most important safety issues of PV installations.
Approved grounding hardware is necessary.
The WEEB (washer, electrical equipment bonding) technology is now becoming
The industry standard for all hardware systems

98. Flashing and Sealing
Weather-sealing roof penetrations requires hardware
and sealants designed and built for the purpose.
In membrane flat roofing, regardless of application
technique, all penetrations are provided by the roofing
contractor carrying the roof warranty. Standard boots
and flashings are used. The roof warranty is intact.

99. Economics
• Purchase vs Lease vs PPA
– Purchase is almost always the best deal for
the customer in the long run
– Lease and PPA may be a good option for non
profits or clients without access to capital or to
limit technical risk

100. Beyond Simple Payback: LCOE
LCOE = Total Life Cycle Cost / Total Lifetime Energy Production
usually in $/kwhr or $/Mwhr
Full analysis includes:
•Capital costs
•All incentives
•O and M costs
•Cost of capital
•Electricity price escalation

101. Beyond Simple Payback: Cash Flow

102. Simplified COE
A calculation of the Price of Electricity offered by a PV System over a 20 Year lifetime. The formula spreads the system net capital
cost (after tax credits, depreciation, rebates, and grants) over the kilowatt-hours produced. The PV investment locks in the price of
the delivered power for 20 years, unaffected by energy supply-demand conditions of the external grid. This price can then be
compared to that offered by the local utility, including both energy cost and transmission cost. After the first 20 years, the solar
array will continue to generate power for an additional 30 years, for free.
System size, in Kilowatts
Cost per installed W of Panels
Annual Power (KWH) produced by each KW of PV array, expressed as a percentage of an "ideal" array
Marginal Tax Rate that the System Owner pays on Income, Business Only (use 0 for residential)
State Rebate Amount
Outside Grant Amount
PRICE OF SOLAR POWER, LOCKED IN FOR 20 YEARS, AT THIS LOCATION
KILOWATT-HOURS DELIVERED EACH YEAR
Gross capital cost
Net Capital cost
100
$4.75
90%
28%
$2,000
$0
$0.089
121,500
$475,000.00
$217,450.00

103. 71 kW PV - Wilkins Meeting House
(273) Suniva Solar 260 watt modules (US Made Cells)
(1)Solectria PVI 60 kW Inverter (US Made)
Over 90,000 kWh produced annually offsetting over 139,000 lbs. of CO2
ReVision is working with institutions, non profits and municipalities to transition
from fossil fuels using solar Power Purchase Agreements (PPA)

104. Power Purchase Agreements
Capturing Tax Subsidies for Non-Profits Using PPAs
Investor(s)
• Tax Investor
• Major Donor
Special Purpose LLC
• Build project
• Own-operate 6 yrs
• Sell power to host
Pass-thru tax benefits and
earnings to investors
PPA
Host 501c3
• Lease roof space
• Buy power, REC
• Option to buy
after 6 yrs

105. Solar PPA Structure
Investor(s)
Host

Provide Capital, Form LLC
 Provides Roof Space

Build/Own/Operate ≥ 6 yrs
 Net Metering w/ Utility

Recoup Investment thru:
 Off-takes Energy, RECs

Federal Tax Credit

Depreciation & Tax Benefits

Energy Payments from Host

Grants, Rebates, REC sales

Buyout Payment Year Seven
 Can pre-pay, up to six years
 Buyout Equipment ≥ year 7 at
fraction of original cost
 Assume remaining debt, if any

106. Questions