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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
Professor
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.

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Figures

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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.

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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.

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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)

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Fig. 4

Annual DHW loads for selected cities and DHW load profiles

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Fig. 5

Annual space heating loads for all cities and building types

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Fig. 6

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

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Fig. 7

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

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Fig. 8

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

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Fig. 9

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

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Fig. 10

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

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Fig. 11

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

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Fig. 12

Incremental annual savings for $0.10 per kWh electricity rate

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