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