SAM Solar Water Heat Calculation Questions

I am currently working on a thesis that involves evaluating solar water heat versus fuel oil-fired hot water and I know SAM currently doesn't allow for any other analysis except that of solar water heat against an electric system. I would like to use the hourly thermal energy output that SAM calculates for the SWH system as an input to my own economic model of the energy savings compared to fuel oil-fired water heating. I have some questions about certain aspects of SAM's internal calculations and assumptions that will help me better understand the program better so that I can use it for this purpose:

• Where does the hourly hot water draw/profile that is currently the default in SAM come from? Would it be appropriate to use this default data set or do I need to import a different dataset that is representative of Maine homeowners?
• What equation does SAM use to convert the hourly hot water draw from kg/hr to energy/hr?
• Is the hourly data for water energy demand available for download in the results section? If so, what is the variable name?
• What is the equation to calculate hourly Q collector to tank? I couldnít find this equation in the manual.
• Will you please clarify “Nameplate thermal capacity”? Specifically, there is a value at the end of the equation presented in the manual for this variable, which is 30/1000 (p. 351 in manual). What does this value represent, and where does the equation come from?
• Is there a list somewhere of variables from the Results hourly data viewer and what the variables mean/how they are calculated? In particular, what does “hourly delivered energy” refer to?
• If I understand correctly, the solar fraction is the ratio of the energy to the water heater from the solar collector to the total energy the water uses over the course of a year. However, I am getting values of around 0.93 for Maine, which seems too high for this region. Does this mean that 93% of total water heating demand for the year is met by solar in Maine, and only 7% is met by the electric auxiliary heater? How is this possible? Or do I not understand correctly?

Thanks!

Hi Steven,

I am not sure of the source of the default hot water draw data. It is appropriate for the Peoria, IL default location, so I don't think it would be appropriate for a representative Maine household. I would suggest that you import a different dataset.

The hot water draw equations are internal to the model and won't make much sense isolated here. The conversion involves the specific heat of water, and the difference in temperature of the mains water and delivered water.

SAM does not report the "water energy demand" in the hourly results. However, it does report the flow rate (Hot Water Draw) along with the tank and inlet temperatures, so you may be able to estimate the energy value using those variables.

Again, the equations for the hourly energy delivered to the tank are internal equations that depend on the conditions in a given time step, and involve the specific heat of water, and temperatures at the different nodes in the system.

SAM calculates the nameplate thermal capacity so that it has a value for capacity-based costs that you might specify on the System Costs page. The 30/1000 value converts the thermal loss coefficient (FRUL) from W/m2-degC to kW.

In SAM, the hourly delivered energy is the electrical energy delivered by the renewable energy system to the grid. For the solar hot water model, this value represents the electrical energy saved by the system. There is a list of the hourly output variables in SAM's Help system: Look under "Performance Model Results" in the table of contents.

The solar fraction is the ratio of solar energy to total energy delivered to the storage tank. You can find a description in Help under "Performance Metrics." For Maine, 0.93 does seem high -- I got a value around 0.76 for the default system (using the Peoria, IL demand data) with the Caribou, ME weather file. You can get high solar fractions if you use a large collector, but that should also cause the cost to be too high for a financially feasible project.

Best regards,
Paul.