Research Papers

Performance Modeling Comparison of a Solar Combisystem and Solar Water Heater

[+] Author and Article Information
John L. Sustar

Trane Commercial Systems,
Lacrosse, WI 54601

Jay Burch

National Renewable Energy Laboratory,
Golden, CO 80401

Moncef Krarti

ASME Fellow
Building Systems Program,
University of Colorado at Boulder
Boulder, CO 80309
e-mail: Krarti@colorado.edu

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received October 7, 2012; final manuscript received May 18, 2015; published online September 2, 2015. Assoc. Editor: Jorge E. Gonzalez.

J. Sol. Energy Eng 137(6), 061001 (Sep 02, 2015) (7 pages) Paper No: SOL-12-1267; doi: 10.1115/1.4031044 History: Received October 07, 2012; Revised May 18, 2015

As homes move toward zero energy performance, some designers are drawn toward the solar combisystem due to its ability to increase the energy savings as compared to solar water heater (SWH) systems. However, it is not trivial as to the extent of incremental savings these systems will yield as compared to SWH systems, since the savings are highly dependent on system size and the domestic hot water (DHW) and space heating loads of the residential building. In this paper, the performance of a small combisystem and SWH, as a function of location, size, and load, is investigated using annual simulations. For benchmark thermal loads, the percent increased savings from a combisystem relative to a SWH can be as high as 8% for a 6 m2 system and 27% for a 9 m2 system in locations with a relatively high solar availability during the heating load season. These incremental savings increase significantly in scenarios with higher space heating loads and low DHW loads.

Copyright © 2015 by ASME
Your Session has timed out. Please sign back in to continue.


Klein, S. , Beckman, W. , and Duffie, J. , 1976, “A Design Procedure for Solar Heating Systems,” Sol. Energy, 18(2), pp. 113–127. [CrossRef]
Duffie, J. , and Mitchell, J. , 1983, “F-Chart: Predictions and Measurements,” ASME J. Sol. Energy Eng., 105(1), pp. 3–12. [CrossRef]
Weiss, W. , 2003, Solar Heating Systems for Houses: A Design Handbook for Solar Combisystems, James and James Science Publishers, London.
Andersen, E. , and Furbo, S. , 2007, “Theoretical Comparison of Solar Water/Space-Heating Combi Systems and Stratifications Design Options,” ASME J. Sol. Energy Eng., 129(4), pp. 438–448. [CrossRef]
Lund, P. , 2005, “Sizing and Applicability Considerations of Solar Combisystems,” Sol. Energy, 78(1), pp. 59–71. [CrossRef]
Jordan, U. , and Vajen, K. , 2001, “Influence of the DHW Load Profile on the Fractional Energy Savings: A Case Study of a Solar Combi-System With TRNSYS Simulations,” Sol. Energy, 69(6), pp. 197–208. [CrossRef]
Bales, C. , and Persson, T. , 2003, “External DHW Units for Solar Combisystems,” Sol. Energy, 74(3), pp. 193–204. [CrossRef]
TESS, 2014, “TESS Component Libraries,” Thermal Energy System Specialists, Madison, WI, accessed in March 2014, http://www.trnsys.com/tess-libraries/
TESS, 2007, Type 534: Cylindrical Stratified Storage Tank With Immersed Heat Exchangers, TESS Component Libraries, Thermal Energy System Specialists, Madison, WI.
TESS, 2006, Type 539: Flat Plate Collector With Capacitance and Flow Modulation, TESS Component Libraries, Thermal Energy System Specialists, Madison, WI.
Hendron, R. , Hancock, E. , Barker, G. , and Reeves, P. , 2006, “Evaluation of Affordable Prototype Houses at Two Levels of Energy Efficiency,” Building America Field Test and Analysis Report, National Renewable Energy Laboratory, Golden, CO, Report No. NREL/CP-550-38774.
Sustar, J. , 2011, “Performance Modeling and Economic Analysis of Residential Solar Combisystems,” M.S. thesis, University of Colorado, Boulder, CO.
IECC, 2009, “International Energy Conservation Code,” International Code Council, Country Club Hills, IL.
SRCC, 2014, “Solar Rating and Certification Corporation for Heliodyne Flat Plate Collector,” Solar Rating & Certification Corporation, Cocoa, FL, http://www.solar-rating.org
Ramlow, B. , 2009, “Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems,” New Society Publishers, Gabriola Island, BC, Canada.
Hendron, R. , and Burch, J. , 2010, “Tool for Generating Realistic Residential Hot Water Event Schedules,” National Renewable Energy Laboratory, Golden, CO, Paper No. NREL/CP-550-47685.
Polly, B. , and Gestwick, M. , 2011, “A Method for Determining Optimal Residential Energy Efficiency Retrofit Packages,” National Renewable Energy Laboratory, Golden, CO, NREL Building Technologies Report No. NREL/TP-5500-50572; DOE/GO-102011-3261.
Hendron, R. , and Engebrecht, C. , 2010, “Building America House Simulation Protocols,” National Renewable Energy Laboratory, Golden, CO, Technical Report No. NREL/TP-550-49426.
Anderson, R. , and Roberts, D. , 2008, “Maximizing Residential Energy Savings: Net Zero Energy Home Technology Pathways,” National Renewable Energy Laboratory, Golden, CO, Technical Report No. NREL/TP-550-44547.
Weiss, W. , and Mauthner, F. , 2011, Solar Heat Worldwide: Markets and Contributions to the Energy Supply 2009, IEA-SHC 2011 Edition, International Energy Agency, Paris.
CCSE, 2010, “California Center for Sustainable Energy, Data From California Solar Initiative Residential Solar Water Heater Program. Acquired by NREL: System Advisor Model (SAM),” National Renewable Energy Laboratory, Golden, CO, Report No. NREL/ TP-6A20-48986.


Grahic Jump Location
Fig. 1

Schematic model of a solar combisystem. Flat-plate collectors supply the tank with heat through the lower immersed heat exchanger. The tank supplies DHW directly and supplies space heating through the upper immersed heat exchanger.

Grahic Jump Location
Fig. 2

Validation analysis of combisystem model using measured data from a Carbondale home. (a) Lower heat exchanger predicted versus measured heat transfer values and (b) upper heat exchanger predicted versus measured values.

Grahic Jump Location
Fig. 3

Renderings for the modeled home with: (a) slab-on-grade construction used for warmer climates (Phoenix and Atlanta), and (b) basement and was used for colder climates (Denver, Boston, and Chicago)

Grahic Jump Location
Fig. 4

Annual DHW loads for selected cities and DHW load profiles

Grahic Jump Location
Fig. 5

Annual space heating loads for all cities and building types

Grahic Jump Location
Fig. 6

Monthly total load and solar resource energy for a combisystem in Denver (9 m2/ 227 l/day draw)

Grahic Jump Location
Fig. 7

Monthly total load and solar resource energy for a SWH in Denver (9 m2/ 227 l/day draw)

Grahic Jump Location
Fig. 8

Annual energy savings and efficiency for a combisystem and SWH for a benchmark house in Denver

Grahic Jump Location
Fig. 9

Incremental energy savings (GJ) for an 8.9 m2 system in Denver, CO

Grahic Jump Location
Fig. 10

Incremental energy savings for the three collector area sizes in all locations for homes with benchmark space heating and DHW loads

Grahic Jump Location
Fig. 11

Monthly incremental savings of the combisystem versus the SWH for San Francisco and Denver (9 m2, 227 l/day draw)

Grahic Jump Location
Fig. 12

Incremental annual savings for $0.10 per kWh electricity rate



Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In