How much energy can you save with a solar water heating (SWH) system? Here’s a modeling system that can help you estimate your system’s performance.
While organizations such as the Solar Rating & Certification Corporation (SRCC) provide ratings for solar collectors and SWH systems, this data is best for comparing the performance of different types of solar equipment. The system performance estimates through these programs are only accurate for a particular project if that project shares similar characteristics with the testing protocols.
For example, the SRCC OG300 system ratings are based upon a household that sets the water heater temperature at 135°F, uses 64.3 gallons of hot water per day in a very specific pattern, and has an incoming cold water temperature of 58°F. These ratings assume a due-south orientation, but do not account for shading, variations in collector orientation, or a household that uses significantly more or less than 64.3 gallons per day.
SWH professionals use a variety of modeling programs to predict the performance of a SWH project. These programs utilize climate data, including outdoor air temperatures and the amount of available solar radiation, to predict system performance. Some of them even account for the variations in incoming water temperature. Professionals input project-specific data, such as the location of the project, daily hot water consumption, collector orientation, and the type of SWH equipment.
One such modeling program is the RETScreen Clean Energy Project Analysis software developed by Natural Resources Canada. This free software is a Microsoft Excel-based spreadsheet that can be used for modeling a variety of systems, including simple solar water heating systems. The program uses several sheets within an Excel workbook to perform its analysis.
RETScreen can provide performance estimates for a number of renewable energy and energy-efficiency technologies. The Start worksheet is where the user specifies a domestic SWH project by selecting Heating as the “Project Type” and Solar water heater as the “Technology.”
Language and units (like metric vs. imperial) are set in this first worksheet. The user must also retrieve the appropriate climate data for the project site by selecting a weather station in RETScreen’s database.
Next, the user selects the Energy Model worksheet to enter information on the hot water use characteristics of the home, the orientation of the collector array, and the key system components. RETScreen uses an internal database that estimates the daily hot water load. This estimate is displayed based upon the number of occupants. The user may either enter this value or select a more precise value based upon the number of showers, loads of laundry, etc.
The collector’s tilt and azimuth must be entered. The array azimuth is entered as a value relative to south. A value of 0° denotes true south.
The specific collector used for the project can be selected from the internal database by clicking on the “See project database” link. If a collector is not in the database, the specifications can be entered manually from a rating sheet. (When you’re entering this information, be sure that the units are consistent.)
If a shading analysis has been performed for the site, the effects of shading can be entered in the “Miscellaneous losses” field in the Solar Water Heater section of the Energy Model worksheet. Other information entered into the model includes storage tank size, heat exchanger efficiency, and the amount of energy required by the pump(s) to run the system.
In the Summary section of the Energy Model sheet, the “Heat Delivered” estimate is displayed, as is the “Solar Fraction”—the percentage of total domestic hot water that is supplied by the SWH system. The user can enter information on the type of fuel, fuel cost, and efficiency of the heating appliance to estimate energy costs, with and without the SWH system to estimate energy savings. It is useful to compare the cost estimate for the “Base case” (without the SWH system) to current water heating costs to verify the accuracy of the results.
Since it is often difficult to quantify the daily hot water use, it can be a challenge to determine the energy costs attributed solely to water heating. It takes some finesse to ensure the results of any performance modeling software is within reason.
There is a steep learning curve when using RETScreen, even for those who are familiar with Microsoft Excel. With additional information, the program also provides economic estimates for a particular system. RETScreen has an extremely detailed user manual available online.