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]. Figure 2 summarizes the comparative analysis between the model predictions and measurements for the heat transfer rates both at the lower heat exchanger (connect to the solar collector) and the upper exchanger (connected to the space heating load) as noted in Fig. 1. Specifically, Fig. 2 shows that most of the pre- dicted values are within the experimental uncertainty error bars. More discussion of the experimental measurements and the validation analysis is provided by Sustar [12]. For the parametric analysis to assess the performance of combi- systems, the model of Fig. 1 is utilized without any significant modifications. In particular, the storage design of the model is not modified, while possible, in order to study the optimal combisys- tem design. The model was based solely on the Carbondale

Fig. 1 Schematic model of a solar combisystem. Flat-plate col- Fig. 2 Validation analysis of combisystem model using meas- lectors supply the tank with heat through the lower immersed ured data from a Carbondale home. (a) Lower heat exchanger heat exchanger. The tank supplies DHW directly and supplies predicted versus measured heat transfer values and (b) upper space heating through the upper immersed heat exchanger. heat exchanger predicted versus measured values.

061001-2 / Vol. 137, DECEMBER 2015 Transactions of the ASME model in thermal collectors since the capacitance impacts the instantaneous efficiency of the system. The model utilizes system parameters such as the transmittance–absorbance efficiency (a0), the loss coefficient (a1), the second order loss coefficient (a2), and the incidence angle modifier (b0), taken directly from the quoted collector’s SRCC rating values as noted in Table 2 [14]. The absorber model is divided into nodes in the flow direction in an effort to describe the temperature of the fluid as it flows through the absorber. In each node, the solar gains, thermal losses, and the effects of capacitance are calculated. The general differen- Fig. 3 Renderings for the modeled home with: (a) slab-on- tial equation for the calculating fluid temperature for each isother- grade construction used for warmer climates (Phoenix and mal node j in a solar thermal collector with capacitance effects is Atlanta), and (b) basement and was used for colder climates expressed as (Denver, Boston, and Chicago) Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/137/6/061001/6409052/sol_137_06_061001.pdf by guest on 30 September 2021 ÀÁÀÁÀÁ dTj 0 Cj ¼ F Sj AjUL Tj Tamb mC_ p Tj Tinlet;j (1) water temperature is assumed to be the same as that of the water dt mains. A couple of important modeling assumptions were made when The total absorbed radiation (S) is a function of the incidence angle modifiers for the beam and diffuse radiation terms. The col- developing the model. First, it was assumed that the heat losses 0 from the tank do not interact with the home’s thermal zone. This lector efficiency factor (F ) represents the ratio of the actual useful modeling assumption implies that the heat losses do not decrease energy gain to the useful energy gain if the collector absorber was the space heating load in the winter or increase the space cooling the temperature of the fluid. load in the summer. For an 820 l tank, which is the selected stor- For the purposes of this study, the mounting of the solar collec- age size for a 9 m2, the tank losses were on the order of 2 GJ dur- tors in the model was held constant year round. For all the cli- mates and systems, the mounted collector azimuth angle was set ing the heating season. For a Denver benchmark house, the tank losses during the heating season are equivalent to 3% of the space to 0 . The tilt angle of the SWH and the combisystem were set heating load. based on design guidelines presented by Ramlow [15]. For the The SWH modeled utilizes the same collector and tank model SWH, the tilt angle is set at the site’s latitude, and for the combi- parameters as the combisystem model, however, rather than hav- system, the mounting tilt of the collectors is set at the latitu- ing an upper heat exchanger, the space heating load is met with a de þ 15 deg. The increased mounting tilt for the combisystem is separate auxiliary heat source that is connected to the radiant floor implemented in order to maximize incident radiation in the winter space heating loop. The control scheme for the solar loop and the months. auxiliary heating element are the same as the control scheme for the combisystem solar loop and auxiliary heating element. The Storage Tank. The stratified tank is modeled using the verti- capacity of the auxiliary heater in the SWH is set to 4.5 kW. cally cylindrical fluid-filled, constant volume storage tank with The study compares the auxiliary energy use of both the solar immersed heat exchangers model. In this model, the tank is combisystem and SWH to a conventional system without solar, divided into 18 nodes and energy balances are calculated for each which consists of a 136 l auxiliary hot water tank and an section of the tank and temperatures of the each node are calcu- auxiliary-heated radiant floor space heating system. In the conven- lated for each time step [9]. The nodes interact thermally with tional DHW tank, two 4.5 kW electric resistance elements heat the nodes above and below through fluid conduction and fluid move- tank. ment. The fluid in the storage tank also interacts thermally with the fluid in the immersed heat exchangers, the surrounding ambi- ent temperature, and directly through inlet and outlet ports. Solar Collector Model. A flat plate solar collector with capaci- The heat exchangers are modeled as coiled tube immersed heat tance effects was used to model the flat plate collector [10]. The exchangers. The bottom heat exchanger is connected to the solar solar thermal collector model considers the effects of the collector collector loop to transfer heat from the solar collector outlet to the mass, which includes the collector fluid, tubes, and absorber plate, storage tank. The top heat exchanger transfers heat to the space on the performance of the system. Capacitance is important to heating load. An inlet port at bottom node and outlet port at the top node of the tank are used for the DHW flows. Both solar side Table 1 Model parameters for benchmark case heat exchanger and load/auxiliary side heat exchanger validation studies were performed using test data in order to calibrate the Parameter Value heat transfer coefficients for the heat exchangers. Table 3 shows the key storage tank and heat exchanger model parameters. House model Heating setpoint 20 C Floor area 232 m2 Modeled Loads. Since the variability in occupant use patterns Floor heating flow rate 1200 kg/hr can greatly impact water heater performance, discrete DHW load Daily internal gains 23.9 kWh/day profiles were generated using the Building America DHW Event 2 Effective leakage area 836 cm Generator [16]. The Building America Event Generator is a DHW setpoint 51.6 C DHW consumption 227 l/day Solar system model Table 2 Flat-plate collector parameters based on Heliodyne Collector slope Latitude þ 15 deg Gobi 3366 SRCC rating values Collector azimuth 0 Area per collector 3 m2 Parameter Value Flow rate/collector area 76 kg/hr m2 Controller DT on 10 C a0 0.702 2 Controller DT off 2 C a1 13.44 kJ/hr m C 2 2 Tank storage/collector area 92 kg/m a2 0.04 kJ/hr m C 2 Solar pump power 18.2 kJ/hr m b0 0.26 2 Fluid specific heat 3.6 kJ/kg C CColl 7.69 kJ/ Cm

Journal of Solar Energy Engineering DECEMBER 2015, Vol. 137 / 061001-3 Table 3 Key tank and heat exchanger parameters Energy Metric Results for Denver, CO

Parameter Value To compare the performances between the two with detailed simulation models, the system’s auxiliary energy usage was com- Tank height 1.52 m pared to the auxiliary energy usage of a reference system without Tank loss coefficient 2.45 kJ/hr m2 C solar collectors. For each parametric run, the annual saved energy Number of tank nodes 18 is determined as Lower HX length 18.6 m ÀÁÀÁ Upper HX length 14.6 m Qsav ¼ Qaux;nosol þ dQload;nosol Qaux;sol þ dQload;sol (2) Diameter of coil HX 0.48 m

where dQload is a penalty value used to normalize the auxiliary energy usage, which is equal to the amount of load that the system spreadsheet-based tool that generates random DHW profiles based was not able to meet over the course of the year. This penalty fac- on probability functions that describe the overall daily average tor was applied to the saved energy equation because there are Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/137/6/061001/6409052/sol_137_06_061001.pdf by guest on 30 September 2021 usage, event duration, event flow rate, and clustering of draws. periods during the year where the heater capacity was not large The day-to-day standard deviation of the daily hot water draws enough to keep up with the demand of the system’s water heating over the course of the year is 50% of the average daily draw. The and space heating load. In Eq. (2), the first parenthesis term is the tool also accounts for vacation periods, weekend versus weekday auxiliary energy usage for a conventional system without solar usage variation, and the impact of seasonality on tempered loads and the second parenthesis term is the auxiliary energy usage for (sinks, shower, and bath). The tool was used to develop a hot the system with solar collectors. water profile for a 76 l per day, 227 l per day, and a 379 l per day Based on the calculated saved energy in Eq. (2), the annual household. In each of the three discrete DHW profiles generated, system efficiency is determined as there is a total of 2 weeks of vacation during the year (7 days in May, 3 days in August, and 4 days in December). Figure 4 shows Q g ¼ sav;ann (3) the annual DHW loads for the selected locations and the three sys I A DHW load profiles. T;ann c As noted earlier, all the buildings analyzed in this study are two-story, 232 m2 finished floor homes. The buildings were mod- The system efficiency differs from the collector efficiency in that collector efficiency evaluates the ratio of the collector’s total eled using the TRNSYS multizone building component. In an attempt to model several building performance types for each energy input to the useful energy output, whereas the system effi- climate zone, the buildings are modeled according to a low- ciency examines the ratio of the system’s saved energy as com- performance building type which is modeled as a 1960s retrofit pared to a conventional system to the collector’s energy input house [17], a Building America benchmark performance building from radiation. Therefore, system efficiency takes into account type which is modeled as a 2009 IECC code house [18], and high- the energy losses from the tank and the pipes in the solar collector performance building type which is modeled as a 50% source loop. As compared to the collector efficiency, the system effi- energy savings house relative to the Building America benchmark ciency will typically be about 5–10% points lower than the solar collector’s efficiency due to the fact that system efficiency takes house [19]. The heating loads of the TRNSYS buildings were com- pared to equivalent building energy optimization (BEopt) simula- into account the extra energy losses in the solar thermal system. Figure 6 shows the monthly total load and the collected useful tions to ensure the TRNSYS heating loads are reasonable [12]. solar energy for a combisystem in Denver benchmark house with Figure 5 shows the annual space heating loads for all locations 2 and building types. 96 ft collector area and a 60 gal per day DHW draw. The plot shown in Fig. 6 is divided into three zones, which are labeled by Using the TRNSYS model, parametric studies were performed to study the model’s sensitivity and the impact of variations in sys- numbers in the plot. Zone 1, which is the zone that covers the tem size and system loads on the performance of the systems (see largest portion of the plot, is the total load in the building that Table 4 for a summary of the parametric study). The selected exceeds the amount of useful solar energy. In a combisystem, cities represent a wide range of U.S. climates ranging from warm where the system is serving both the space heating and DHW climates (Phoenix and Atlanta), mild climates (San Francisco), to load, the time period when the load exceeds the solar resource cold climates (Boston, Denver, and Chicago). will be during the winter months, when the load is large. During this time, the auxiliary heater is required to meet the building

Fig. 4 Annual DHW loads for selected cities and DHW load Fig. 5 Annual space heating loads for all cities and building profiles types

061001-4 / Vol. 137, DECEMBER 2015 Transactions of the ASME Table 4 Summary of parametric study

Parameter Parameter runs

Locations (City) Phoenix (Phx), Atlanta (Atl), San Francisco (SF), Denver (Den), Boston (Bos), Chicago (Chi) Collector area (m2)3,6,9 DHW load (l/day) 76, 227, 379 House type 1960s Retro, 2009 IECC, 50% BA load. Zone 2, which is portion of the plot where the useful solar energy section overlaps the total load section, represents is the

building’s load that can be met by the solar energy. Zone 3 is 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 solar energy that exceeds the load. In a combisystem, solar energy will likely only exceed the load in the summer months, when the load is at its minimum. Figure 7 shows the monthly total load and the collected useful solar energy for a SWH in Denver with 96 ft2 collector area and a Fig. 7 Monthly total load and solar resource energy for a SWH 2 60 gal per day DHW draw. In contrast to Fig. 6, which shows that in Denver (9 m / 227 l/day draw) zone 1 occupies the majority of the plot, the SWH plot shows that zone 2 occupies the majority of the plot. Zone 2, which represents collector area. There are a couple of important factors that go into is the building’s load that can be met by the solar energy, means determining the system efficiency. First, the collector efficiency 2 that the 96 ft collector area SWH in Denver will yield high solar plays a large role in the system’s efficiency. Since collector effi- fractions. Additionally, the large SWH also means that in the ciency decreases with increasing inlet fluid temperature, it is majority of the months, the useful solar energy exceeds the DHW expected that the collector efficiency will decrease as collector load. area increases due to the fact that the higher temperature inlet Figure 8 shows the saved energy and the system efficiency for fluid has a lower capacity to the heated. Smaller collector area both a SWH system and combisystem for a 227 l/day DHW load systems have lower inlet fluid temperatures than larger system in a benchmark house in Denver, CO. The performance metrics because the tanks in small systems run cooler than the tanks in for the combisystem are depicted with a solid line, and the per- larger systems. Additionally, larger systems will have increased formance metrics for the SWH systems are depicted with the tank losses due to high tank temperatures. dashed line. In terms of comparing the combisystem system efficiency to Figure 8 shows several key performance trends with regards to the SWH system efficiency, combisystem tank temperatures are the saved energy between the combisystem and the SWH. First, as lower which translates into lower tank losses and lower inlet fluid collector area increases, the saved energy in both systems also temperatures. Tank temperatures are kept lower in combisystems increases, although the rate of increased savings diminishes as because they have higher loads to dump heat into as compared to collector area increases. Second, as collector area increases, the SWH systems. saved energy difference between the combisystem and the SWH Figure 9 shows the impact the loads on the annual energy sav- system grows. This is due to the SWH energy savings being lim- ings for the combisystem as compared to the SWH for a 9 m2 sys- ited by a smaller load. Combisystems on the other hand serve a tem in Denver. The annual energy savings for the combisystems larger load which allows for greater energy savings as collector compared to the SWH systems with the same size are referred to 2 area increases. The third trend is that with systems with 3 m of as incremental savings. Figure 9 shows that for systems with low collector area, the combisystem offers a little to no savings as DHW loads, the difference between the saved energy for combi- compared to the SWH. systems and SWHs will be larger than with systems with high Figure 8 also shows that the system efficiency of both the com- DHW loads. Figure 9 also shows that the combisystem incremen- bisystem and the SWH system decreases as collector area tal savings are more significant in the 1960 retrofit house than in increases, with the SWH system efficiency decreasing more the Building America 50% house. Based on these results, a combi- quickly than the combisystem system efficiency as a function of system serving a house with high space heating loads and low

Fig. 6 Monthly total load and solar resource energy for a com- Fig. 8 Annual energy savings and efficiency for a combisys- bisystem in Denver (9 m2/ 227 l/day draw) tem and SWH for a benchmark house in Denver

Journal of Solar Energy Engineering DECEMBER 2015, Vol. 137 / 061001-5 Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/137/6/061001/6409052/sol_137_06_061001.pdf by guest on 30 September 2021

Fig. 9 Incremental energy savings (GJ) for an 8.9 m2 system in Denver, CO Fig. 11 Monthly incremental savings of the combisystem ver- sus the SWH for San Francisco and Denver (9 m2, 227 l/day DHW loads will yield the largest incremental savings. In contrast, draw) a combisystem serving a high performance house and high DHW loads will yield the lowest incremental savings. Additionally, the incremental savings are more constant through- out the year in San Francisco as compared to Denver. A system in Comparison for Other Climates Denver yields significantly larger incremental savings in the From the Denver results, it is clear that combisystems will pro- spring and fall as compared to the winter. vide incremental energy and cost savings in comparison to a As determined for the Denver study, the largest incremental standard SWH, however, the magnitude of these incremental sav- savings from a combisystem relative to a SWH will occur when ings is highly dependent on the system size and the loads. The DHW loads are small and space heating loads are high and the parametric simulations were also performed using Typical Mete- smallest incremental savings from a combisystem relative to a orological Year 3 (TMY3) data sets from five additional U.S. SWH will occur when DHW loads are high and space heating cities, which were Chicago, Boston, San Francisco, Atlanta, and loads are low. Based on these two bounds, the incremental site Phoenix. Figure 10 shows the incremental energy savings for the energy and cost savings from combisystems can be evaluated three collector area sizes for homes with benchmark loads in all across all the locations. the locations studied. As with Denver, the incremental savings in In evaluating the economics between SWH and combisystems, these locations are minimal with 1 collector, but become signifi- both the incremental energy savings and the cost of energy will cant with 2 and 3 collectors. play a significant role in the incremental annual cost savings Based on Fig. 10, it is clear that the locations of Denver and between the combisystem and the SWH. The economics of com- San Francisco yield the highest incremental savings, largely due bisystems are evaluated for all-electric systems in Fig. 12 assum- to their relatively significant space heating loads and relatively ing that the cost of electricity is $0.10 per kWh. The costs for the high incident solar radiation during the space heating months. The combisystems are based on reported IEA data [20] while for SWH monthly incremental savings provided by the combisystem systems are based on data collected as part of the California Solar relative to the SWH for Denver and San Francisco are shown in Initiative residential SWH installations [21]. The error bars repre- Fig. 11. In particular, Fig. 11 shows that San Francisco has much sent the savings for a benchmark house and 227 l daily DHW larger incremental savings in April through August as compared draw and the error bars represent the lower and upper bounds of the incremental cost savings. For Denver and San Francisco, the to Denver. This is due to the higher space heating loads and the 2 higher incident radiation in San Francisco during these months. annual incremental savings m of collector area will be about $27

Fig. 10 Incremental energy savings for the three collector area sizes in all locations for homes with benchmark space heating Fig. 12 Incremental annual savings for $0.10 per kWh electric- and DHW loads ity rate

061001-6 / Vol. 137, DECEMBER 2015 Transactions of the ASME 2 2 for $34 for 6 m and 9 m systems, respectively, for the upper m_¼ mass flow rate (kg/hr) bound of incremental savings. For the lower bound of incremental Q ¼ energy (kJ) savings, the annual incremental savings per unit area will be less T ¼ temperature (C) than $3 for all climates and system sizes. S ¼ total absorbed radiation (kJ/hr) 2 UL ¼ loss coefficient per unit area (kJ/hr m C) Conclusion and Proposed Future Work A solar combisystem and SWH model was developed and used Subscripts to study the incremental improved performance of a solar combi- amb ¼ ambient system as a function of loads, size, and location. In Denver, the ann ¼ annual incremental savings supplied by the combisystem as compared to aux ¼ auxiliary 2 the SWH were 0.5%, 7%, and 18.6% for the 3, 6, and 9 m collec- coll ¼ collector tor are, respectively. The storage/collector ratio for all the systems in ¼ inlet to collector 2 analyzed was held constant at 92 l/m . j ¼ isothermal node Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/137/6/061001/6409052/sol_137_06_061001.pdf by guest on 30 September 2021 The incremental savings were highly sensitive to both the no sol ¼ without solar DHW loads and the space heating loads. The largest incremental sav ¼ saved energy savings occur when DHW loads are low and space heating loads sol ¼ with solar are high, while the smallest incremental savings occur when DHW loads are high and space heating loads are low. To illustrate References the impact that loads have on incremental savings, for a bench- [1] Klein, S., Beckman, W., and Duffie, J., 1976, “A Design Procedure for Solar mark house in Denver with a 9 m2 system, increasing the daily Heating Systems,” Sol. Energy, 18(2), pp. 113–127. [2] Duffie, J., and Mitchell, J., 1983, “F-Chart: Predictions and Measurements,” DHW draws from 227 l to 379 l will decrease the incremental sav- ASME J. Sol. Energy Eng., 105(1), pp. 3–12. ings by 59%, while decreasing the daily DHW draws from 227 l [3] Weiss, W., 2003, Solar Heating Systems for Houses: A Design Handbook for to 76 l will increase the incremental savings by 140%. Solar Combisystems, James and James Science Publishers, London. The combisystem performance was studied for six different cli- [4] Andersen, E., and Furbo, S., 2007, “Theoretical Comparison of Solar Water/ Space-Heating Combi Systems and Stratifications Design Options,” ASME J. mates that included Phoenix, Atlanta, San Francisco, Denver, Sol. Energy Eng., 129(4), pp. 438–448. Boston, and Chicago. The combisystem performed the best in [5] Lund, P., 2005, “Sizing and Applicability Considerations of Solar climates where there was a significant solar resource during the Combisystems,” Sol. Energy, 78(1), pp. 59–71. heating load months. San Francisco and Denver yield 27% and [6] Jordan, U., and Vajen, K., 2001, “Influence of the DHW Load Profile on the 19% increased savings, respectively, as compared to a SWH with Fractional Energy Savings: A Case Study of a Solar Combi-System With 2 TRNSYS Simulations,” Sol. Energy, 69(6), pp. 197–208. a9m system that serves benchmark space heating and DHW [7] Bales, C., and Persson, T., 2003, “External DHW Units for Solar loads. Combisystems,” Sol. Energy, 74(3), pp. 193–204. As noted in the analysis presented in this paper, the combisys- [8] TESS, 2014, “TESS Component Libraries,” Thermal Energy System Specialists, Madison, WI, accessed in March 2014, http://www.trnsys.com/tess-libraries/ tem design was not optimized. In particular, the storage size may [9] TESS, 2007, Type 534: Cylindrical Stratified Storage Tank With Immersed not be adequate for the collector area. Recommended future work Heat Exchangers, TESS Component Libraries, Thermal Energy System Spe- includes: (1) examining how the storage volume to collector area cialists, Madison, WI. ratio can impact the annual performance of the modeled system in [10] TESS, 2006, Type 539: Flat Plate Collector With Capacitance and Flow Modulation, TESS Component Libraries, Thermal Energy System Specialists, U.S. climates, (2) examining how replacing the solar-side heat Madison, WI. exchanger with an inlet stratifier impacts the performance of the [11] Hendron, R., Hancock, E., Barker, G., and Reeves, P., 2006, “Evaluation of combisystem, (3) examining the performance between a combi- Affordable Prototype Houses at Two Levels of Energy Efficiency,” Building system paired with a radiant floor heating system and a combisys- America Field Test and Analysis Report, National Renewable Energy Labora- tory, Golden, CO, Report No. NREL/CP-550-38774. tem paired with a forced-air heating system, and (4) investigating [12] Sustar, J., 2011, “Performance Modeling and Economic Analysis of Residential whether evacuated-tube collectors provide any increased savings Solar Combisystems,” M.S. thesis, University of Colorado, Boulder, CO. in climates such as Boston and Chicago as compared to flat-plate [13] IECC, 2009, “International Energy Conservation Code,” International Code collectors. Council, Country Club Hills, IL. [14] SRCC, 2014, “Solar Rating and Certification Corporation for Heliodyne Flat Plate Collector,” Solar Rating & Certification Corporation, Cocoa, FL, http:// Acknowledgment www.solar-rating.org [15] Ramlow, B., 2009, “Solar Water Heating: A Comprehensive Guide to Solar The authors acknowledge the financial support of the National Water and Space Heating Systems,” New Society Publishers, Gabriola Island, Renewable Energy Laboratory (NREL) and the technical support BC, Canada. [16] Hendron, R., and Burch, J., 2010, “Tool for Generating Realistic Residential of Greg Barker and Bob Hendron who were extremely helpful in Hot Water Event Schedules,” National Renewable Energy Laboratory, Golden, providing critical information on the experimental data for the CO, Paper No. NREL/CP-550-47685. monitored combisystem. [17] Polly, B., and Gestwick, M., 2011, “A Method for Determining Optimal Resi- dential Energy Efficiency Retrofit Packages,” National Renewable Energy Lab- oratory, Golden, CO, NREL Building Technologies Report No. NREL/TP- 5500-50572; DOE/GO-102011-3261. Nomenclature [18] Hendron, R., and Engebrecht, C., 2010, “Building America House Simulation 2 Protocols,” National Renewable Energy Laboratory, Golden, CO, Technical A ¼ area (m ) Report No. NREL/TP-550-49426. a0 ¼ intercept efficiency [19] Anderson, R., and Roberts, D., 2008, “Maximizing Residential Energy Savings: a1 ¼ first order efficiency coefficient Net Zero Energy Home Technology Pathways,” National Renewable Energy a ¼ second order efficiency coefficient Laboratory, Golden, CO, Technical Report No. NREL/TP-550-44547. 2 [20] Weiss, W., and Mauthner, F., 2011, Solar Heat Worldwide: Markets and Con- b0 ¼ first order incidence angle modifier tributions to the Energy Supply 2009, IEA-SHC 2011 Edition, International C ¼ collector capacitance (kJ/ C) Energy Agency, Paris. Cp ¼ specific heat (kJ/kg C) [21] CCSE, 2010, “California Center for Sustainable Energy, Data From California F0 ¼ collector efficiency factor Solar Initiative Residential Solar Water Heater Program. Acquired by NREL: System Advisor Model (SAM),” National Renewable Energy Laboratory, IT ¼ tilt radiation (kJ) Golden, CO, Report No. NREL/ TP-6A20-48986.

Journal of Solar Energy Engineering DECEMBER 2015, Vol. 137 / 061001-7