John L. Sustar Trane Commercial Systems, Lacrosse, WI 54601 Jay Burch National Renewable Energy Laboratory, Golden, CO 80401 Moncef Krarti 1 ASME Fellow Professor Building Systems Program, University of Colorado at Boulder Boulder, CO 80309 e-mail: Krarti@colorado.edu Performance Modeling Comparison of a Solar Combisystem and Solar Water Heater 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 sav- ings 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 m 2 system and 27% for a 9 m 2 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. [DOI: 10.1115/1.4031044] Introduction Solar combisystems utilize solar thermal collectors for two resi- dential thermal load applications: active solar thermal space heat- ing and DHW. One key advantage of solar combisystems as compared to SWH systems is that combisystems increase the solar collector’s utilization independent of occupant hot water use because the space heating is supplemented by heat collected by the solar collectors. In terms of disadvantages as compared to SWH systems, combisystems require a relatively large incremen- tal capital investment and the design of combisystems can be intricate. Much of the previous research on solar combisystems has focused on the optimal sizing and design of systems. Since solar thermal systems exhibit transient behavior, the most commonly used research tool to evaluate the impact of design on system per- formance is TRNSYS, a modular component-based tool originally developed by Klein et al. [1]. Specifically, TRNSYS was utilized to simulate the annual system performance of solar combisystems and examine the system performance as a function of the collector area and storage capacity [2]. The research of Duffie and Mitchell [2] led to the development of F-Chart, a solar thermal system analysis and design program based on correlation coefficients generated by TRNSYS simulations, that has the ability to quickly estimate the performance of generic solar heating systems. Over the years, researchers have evaluated the performance of unique combisystem designs and have also evaluated the impact of sizing, collector efficiency, and draw profiles on performance of combisystems. In order to yield large energy savings from the system, the research showed the importance of small auxiliary volumes, low auxiliary set points, and good thermal stratification [3]. Stratification can be enhanced in tanks by adding heat to the top of the tank and removing heat from the bottom. Another method for improving stratification, which has been popularized in Europe, has been to introduce stratifying tubes within the tank. A stratifying tube is an immersed tube with sev- eral outlets where the incoming water is directed into the tank at the level where the temperature is the same as the incoming water. Andersen and Furbo [4] demonstrated the impact of stratifying tubes on the thermal performance of systems. Researchers found that the thermal performance of combisystems increases by 7–14% by using stratifiers for the solar collector loop and space heating loop rather than immersed heat exchangers. Additionally, the study found that because loads vary so dramatically through- out the year with combisystems as opposed to SWHs, stratifiers are much better choice as compared to internal heat exchangers and direct inlets because stratifiers are less sensitive to varying operating temperatures. Studies have also examined the impact of loads on the perform- ance of combisystems. Lund [5] investigated the sizing of solar thermal combisystems with different heating loads. The study found that oversizing a solar thermal system proved to be more advantageous for less efficient buildings as compared to more effi- cient buildings. Jordan and Vajen [6] studied the impact of realis- tic load profiles on the performance modeling of combisystems, since DHW draws can have a severe impact on the temperature stratification in the tank. The study found that the fractional energy savings between models with simplified profiles and mod- els with realistic discrete draws can differ by up to about 3%. Additionally, Bales and Persson [7] concluded that modeling sys- tems with a realistic draw profile has a significant impact on the predicted energy savings of modeled systems. These studies con- cluded that an optimized combisystem design using a realistic load profile can differ significantly from a combisystem optimized using a simplified load profile. The focus of this paper is on the impact of system size, location, and loads on the performance of solar combisystems relative to SWHs. It is expected that smaller combisystem, consisting of 6 m 2 of collector area, will offset most DHW loads year round (except during the winter) and will only contribute minimally to the space heating in the spring and fall when there is still heating loads and the solar collectors are operating at higher efficiencies as compared to the winter months. As collector area increases, the amount of solar energy utilized is expected to increase more rap- idly in a solar combisystem configuration as compared to a SWH. However, the amount that the system will offset auxiliary space 1 Corresponding 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. 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