Heat delivery performance in
combination solar thermal
systems: Strategies for increasing
delivery temperature
James Dontje
Johnson Center for Environmental Innovation
Gustavus Adolphus College
St. Peter, MN
US Residential Primary Energy Use
Space heating
32%
Water heating
13%Lighting
12%
Space c ooling
11%
Refrigeration
8%
E lec tronic s
5%
C ooking
4%
O ther
15%
Environmental Building News, July 2007, Vol. 6:1
Retrofit issues
• Inefficient construction
• Solar access and orientation
• Delivery system
– Configuration
– Operating energy
– Delivery temperature
Solar thermal space heating
delivery…
• Solar air heating collectors
• Radiant floor (and wall or ceiling)
• Fan convectors (water to air heat
exchangers)
• Radiant emitters
Retrofit issues
• Inefficient construction
• Solar access and orientation
• Delivery system
– Configuration
– Operating energy
– Delivery temperature
Design changes
• Reduced solar storage volume and/or
“direct from collector” heat delivery
• Outdoor reset control to maximize solar
usage
Storage volume reduction
• Assumes collector array sized for a
fraction of the load
• Domestic hot water demand
substantially met
• Allows smaller collector array to attain
higher temperatures
Direct from the collector
• If solar heat is available and load calls
for heat, satisfy the load
• Can avoid thermodynamic losses of
heat transfer (heat exchangers) and
standby loses in storage
• Challenge is implementation (valves
and controls)
Design evolution…
• Large storage and indirect heat transfer (A, B)
• Smaller storage and more direct heat transfer
(C,D)
• Smaller storage and direct heat transfer (E)
Large storage and indirect
heat transfer (B)
• About 1.8 gallons per sq. ft. of collector
(0.0713 m3/m2)
• Copper coil heat exchangers immersed
in unpressurized storage
• Separate coils for collector and heating
loop
• Heat delivery via water-to-air heat
exchanger in plenum
Smaller storage and more
direct heat transfer (C,D)
• 1 gallon per sq. ft. of collector (0.04
m3/m2)
• Side-by-side example systems
• Counter-flow heat exchanger between
collectors and storage
• Heat flow from heat exchanger to load or
to storage
• No domestic hot water load
Smaller storage and direct
heat transfer (E)
• 1.25 gallons per sq. ft. of collector
(0.051 m3/m2)
• Copper coil heat exchangers immersed
in unpressurized storage
• Common coils for collector and heating
loop
• Heat delivery via water-to-air heat
exchanger in plenum
Qualitative observation of Systems C and D
showed that the system maintained higher
temperatures
System B monitoring begun April 2011
System E monitoring begun late December
2012
Data: Storage temperatures, and activation
of heating system
System B—January 2012
System E—January 2012
Two different systems, 30
miles apart….
Could there be a difference in
performance due to other design
factors (plumbing, collector angle
and orientation, ….)?
Performance check
• Three days with clear sky (strong linear
rise in temperature)
• Calculate the rate of temperature rise in
both systems (measure of collection
efficiency)
• Adjust for total collector area and
storage volume
Comparison of collection
efficiency
• System E outperforming B by 14 to
51%--average 32%
• Improved delivery temperatures not just
caused by decreased storage volume to
collector area
• Potential sources of difference: collector
angle (E at ~45°, B vertical) or
possible flow problem in B
Operation of direct heat transfer to load
Summary
• Lower collector to storage volume
results in higher storage temperatures
• Questions about performance
differences between systems limits
strength of that conclusion
• Direct delivery of heat to load effective
• Monitoring ongoing
