Performance Modeling John L. Sustar Comparison of a Solar Trane Commercial Systems, Lacrosse, WI 54601 Combisystem and Solar Jay Burch National Renewable Energy Laboratory, Water Heater Golden, CO 80401 As homes move toward zero energy performance, some designers are drawn toward the Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/137/6/061001/6409052/sol_137_06_061001.pdf by guest on 30 September 2021 1 Moncef Krarti solar combisystem due to its ability to increase the energy savings as compared to solar ASME Fellow water heater (SWH) systems. However, it is not trivial as to the extent of incremental sav- Professor ings these systems will yield as compared to SWH systems, since the savings are highly Building Systems Program, dependent on system size and the domestic hot water (DHW) and space heating loads of University of Colorado at Boulder the residential building. In this paper, the performance of a small combisystem and SWH, Boulder, CO 80309 as a function of location, size, and load, is investigated using annual simulations. For e-mail: [email protected] 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. [DOI: 10.1115/1.4031044] Introduction within the tank. A stratifying tube is an immersed tube with sev- eral outlets where the incoming water is directed into the tank at Solar combisystems utilize solar thermal collectors for two resi- the level where the temperature is the same as the incoming water. dential thermal load applications: active solar thermal space heat- Andersen and Furbo [4] demonstrated the impact of stratifying ing and DHW. One key advantage of solar combisystems as tubes on the thermal performance of systems. Researchers found compared to SWH systems is that combisystems increase the solar that the thermal performance of combisystems increases by collector’s utilization independent of occupant hot water use 7–14% by using stratifiers for the solar collector loop and space because the space heating is supplemented by heat collected by heating loop rather than immersed heat exchangers. Additionally, the solar collectors. In terms of disadvantages as compared to the study found that because loads vary so dramatically through- SWH systems, combisystems require a relatively large incremen- out the year with combisystems as opposed to SWHs, stratifiers tal capital investment and the design of combisystems can be are much better choice as compared to internal heat exchangers intricate. and direct inlets because stratifiers are less sensitive to varying Much of the previous research on solar combisystems has operating temperatures. focused on the optimal sizing and design of systems. Since solar Studies have also examined the impact of loads on the perform- thermal systems exhibit transient behavior, the most commonly ance of combisystems. Lund [5] investigated the sizing of solar used research tool to evaluate the impact of design on system per- thermal combisystems with different heating loads. The study formance is TRNSYS, a modular component-based tool originally found that oversizing a solar thermal system proved to be more developed by Klein et al. [1]. Specifically, TRNSYS was utilized to advantageous for less efficient buildings as compared to more effi- simulate the annual system performance of solar combisystems cient buildings. Jordan and Vajen [6] studied the impact of realis- and examine the system performance as a function of the collector tic load profiles on the performance modeling of combisystems, area and storage capacity [2]. The research of Duffie and Mitchell since DHW draws can have a severe impact on the temperature [2] led to the development of F-Chart, a solar thermal system stratification in the tank. The study found that the fractional analysis and design program based on correlation coefficients energy savings between models with simplified profiles and mod- generated by TRNSYS simulations, that has the ability to quickly els with realistic discrete draws can differ by up to about 3%. estimate the performance of generic solar heating systems. Additionally, Bales and Persson [7] concluded that modeling sys- Over the years, researchers have evaluated the performance of tems with a realistic draw profile has a significant impact on the unique combisystem designs and have also evaluated the impact predicted energy savings of modeled systems. These studies con- of sizing, collector efficiency, and draw profiles on performance cluded that an optimized combisystem design using a realistic of combisystems. In order to yield large energy savings from the load profile can differ significantly from a combisystem optimized system, the research showed the importance of small auxiliary using a simplified load profile. volumes, low auxiliary set points, and good thermal stratification The focus of this paper is on the impact of system size, location, [3]. Stratification can be enhanced in tanks by adding heat to the and loads on the performance of solar combisystems relative to top of the tank and removing heat from the bottom. SWHs. It is expected that smaller combisystem, consisting of 6 Another method for improving stratification, which has been m2 of collector area, will offset most DHW loads year round popularized in Europe, has been to introduce stratifying tubes (except during the winter) and will only contribute minimally to the space heating in the spring and fall when there is still heating 1Corresponding author. loads and the solar collectors are operating at higher efficiencies Contributed by the Solar Energy Division of ASME for publication in the as compared to the winter months. As collector area increases, the JOURNAL OF SOLAR ENERGY ENGINEERING:INCLUDING WIND ENERGY AND BUILDING amount of solar energy utilized is expected to increase more rap- ENERGY CONSERVATION. Manuscript received October 7, 2012; final manuscript received May 18, 2015; published online September 2, 2015. Assoc. Editor: Jorge E. idly in a solar combisystem configuration as compared to a SWH. Gonzalez. However, the amount that the system will offset auxiliary space Journal of Solar Energy Engineering Copyright VC 2015 by ASME DECEMBER 2015, Vol. 137 / 061001-1 heating loads will depend on several factors such as the loads and system, which used a single tank to serve as both the solar storage vacation periods. and the auxiliary storage. Moreover, since radiation floor heating systems require lower supply temperatures as opposed to base- board convectors or forced-air heating coils, the model utilized a Model Description radiation floor heating system to give the combisystem as much of To determine how combisystems compare to SWHs in terms of an advantage as possible. Additionally, it was decided to substan- energy performance, the annual performance of these systems was tially oversize the auxiliary heater capacity in the system, so that simulated using a TRNSYS model of a typical system. Figure 1 rep- there would be minimal issues with the system not being able to resents the combisystem that was modeled using TRNSYS [8]. The meet both the space heating and DHW loads. Lastly, the auxiliary modeled combisystem has a single tank system with a solar-side heating element was placed within the upper portion of the tank lower heat exchanger and a space heating load-side upper heat between the upper and lower inlets of the upper load-side heat exchanger. The DHW ports are directly connected to the tank. exchanger. This placement allows the upper heat exchanger to uti- Several standard components from the TRNSYS library, such as the lize both solar storage (below the auxiliary heater) and auxiliary data reader, radiation processor, space thermal loads, and radiant storage (above the auxiliary heater) to meet the space heating Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/137/6/061001/6409052/sol_137_06_061001.pdf by guest on 30 September 2021 floor systems, were used to develop the combisystem model [8]. load. The DHW ports are directly connected to the tank. Moreover, the combisystem model uses TRNSYS components devel- For the analysis, a two-story home is modeled with a oped for multinode storage tanks with immersed heat exchangers 12.8 m  9.2 m (118 m2) footprint with a total floor area of 232 m2 [9], and solar collectors with capacitance effects [10]. These two as shown in Fig. 3. In the cold climates, which included Denver, models are discussed in more detail later in this paper. Boston, and Chicago, the buildings were modeled with an The combisystem model was validated using data from a resi- unconditioned basement. In the warmer climates of Atlanta, San dential combisystem installed in Carbondale, CO, which was Francisco, and Phoenix, the model simulations assume a slab-on- monitored for more than 2 yrs as part of a Building America grade construction. research project [11]. The combisystem used to validate the model For the benchmark case, it is assumed that the building con- consists of flat-plate collectors and a single tank with two struction meets 2009 IECC code [13] and that the daily DHW immersed heat exchangers as shown in Fig. 1. The solar collector usage is 227 l. The space heating setpoint is set year round at loop utilizes a glycol/water mixture, and it transfers heat to the 20 C. Other base case model values for relevant house model and tank through the lower immersed heat exchanger. The space heat- solar system model parameters are given in Table 1. The cold ing utilizes the upper heat exchanger to transfer heat to and from the tank. The tank is pressurized and the DHW is directly heated by the tank. As part of the monitoring protocol, four water flow meters and ten thermocouples were installed to facilitate measure- ment of the solar and auxiliary energy to the DHW and space heating loads [11].