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May 2005
This house uses a photovoltaic system for power and a solar domestic hot water system that provides hot water and space heating. (PIX 08226)
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Builders and energy analysts in the Building America program have found that to obtain energy savings of 40%-50%, they have to reduce the energy used for domestic water heating (DWH). At energy savings levels below 40%, energy-saving measures have been focused on envelope and HVAC improvements, making water heating a more significant piece of the remaining energy pie. Technologies of highest interest to date for reducing DWH energy include innovative and less-wasteful distribution systems, "on-demand"/tankless gas water heaters, and solar domestic water heaters (SDWH).
When the desired energy-savings is in the range of 60% - 70%, SDHW becomes an increasingly important factor for achieving that goal. In addition, SDWH is an essential part of a zero energy building. In sunny climates, roughly three-quarters of the DWH load can be eliminated with SDWH. However, many builders do not have direct experience with solar water-heating equipment and do not know how to credibly estimate the performance of SDHW specific to their applications. To alleviate the performance estimation problem, an SDWH performance prediction tool has been developed based upon rigorous simulation models. It affords the casual user simple and easy access to parameters and results of interest.
The well-known thermal simulation environment TRNSYS was used for the modeling. A glycol system and an integral-collector-storage system (ICS) were modeled. Performance of a drainback system is nearly identical to a glycol system with the same collectors and storage, such that the glycol system results can be used for a drainback system. The auxiliary can be either gas or electric and can be either a conventional storage tank or a tankless heater. TRNSYS has a well-deserved reputation of being difficult for beginners to learn, in part as a result of the tool's unlimited flexibility. To help alleviate this problem, the development team at University of Wisconsin provided a user-oriented front-end for TRNSYS, called TRNSED. TRNSED allows users to quickly and easily develop a graphical front-end that shows the user familiar Windows-like dialogue boxes for changing parameters. NREL and Thermal Energy System Specialists (TESS, Inc.) developed a distributable TRNSED model that allows the user to choose the system of interest and vary the parameters shown in Table 1.
| Table 1. User Inputs for the SDWH Tool |
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| System Properties |
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Collector Orientation (tilt, azimuth)
Collector Size
Collector Parameters (optical gain, thermal losses)
Piping insulation and length
Storage tank volume
Storage tank loss coefficient or EF
Auxiliary efficiency |
| Load Characteristics |
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Volume of water used, per day (invariant day-to-day)
Time of usage (2 default profiles, one graphical input path, one text-file path) |
| Location |
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| Any TMY2 site (239 sites in the United States) |
The model results are imported into an Excel© spreadsheet that displays monthly and annual savings, solar fraction, and auxiliary usage, in tabular and graphic format. Daily values and maximum temperatures will be part of version 2. Figure 1 shows a plot provided for monthly energy consumption for the base case (no solar) and a glycol SDHW system.
Figure 1. Annual energy consumption for the base-case electric water heater and for a glycol SDHW system.
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The user interested in exploring variations in sizing of components can also run parametrics. TRNSED allows the user to make a parametric table with any of the declared variables, such as area and storage volume. The table allows the user to specify a series of linked values for the chosen inputs. The parameters to vary, the values for each run, and the number of runs are chosen by the user. TRNSYS then automatically makes the specified runs, with automated plots of the variation of annual results with the parameter variations.
The NREL ftp site contains the executable file Building America SDHW Installation Program- v1.03.exe, some user hints in Readme - notes on the TRNSED-TRNSYS SDHW tool.doc, and users manual for TRNSED trnsed.hlp. To download these files, the user can first login with user id anonymous and use his or her email address for a password. The user can then drag files to desktop or other Windows folder.
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