solar dhw - gas auxilary heater

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solar dhw - gas auxilary heater

For a solar DHW system, I understand that SAM only calculates savings in kWh. There is a macro to run using an auxiliary gas water heat that gives the savings in kWh. Converting to BTU or therms is easy enough. However, with the 2017 version on SAM, I'm seeing results that suggest an error. The scale of auxiliary gas water savings is very low.

As an example, using the default system that opens up in SAM's residential solar water heating module, I get 2,407 kWh of annual savings for the electric water heater, but only 3 kWh savings when using the gas water heater macro. In a prior version of SAM I was getting comparable kWh or BTU savings between an electric and gas water heater (adjusting for water heater system efficiencies).

Using basic energy conversion factors, I'm seeing an 2 order of magnitude or greater issue. I also allow that I may be misunderstanding what I'm doing or seeing, but I'm not convinced that the case. The solar fraction for electric is 0.68 and for gas is 0.62. 3 kWh savings and a 0.62 solar fraction are not in alignment.

Thanks for any help or guidance,

Paul Gilman


It looks like we changed the units from W to kW at some point and we forgot to update the macro.

Here's how you can fix the macro:

1. Start SAM, create or open a solar water heating case.

2. On the Macros page, click the Auxiliary Gas Heater macro.

3. Click View Code at the top right of the page to open the script editor.

4. For lines 65, 66, and 67, remove the division by 1000. The lines should look like this when you finish:

Q_gas_saved[i] = Q_saved[i] / Eta_burn;
Q_gas_no_solar[i] = Q_aux_only[i] / EF_gas_tank;
Q_gas_with_solar[i] = ( Q_gas_no_solar[i] - Q_gas_saved[i] + P_pump[i] );

5. Click Save at the top of the script editor to save your changes.

6. Click Close to close the editor.

7. Switch to a different macro and then back to the Auxiliary Gas Heater macro to reload the macro.

Now when you run the macro, you should get results in the correct units.

I will update the macro so that it is correct in the next version of SAM. Thanks for pointing out the error!

Best regards,


I am trying to use SAM to model a residential solar hot water system that supplies domestic hot water and space heating. I am a little confused on how SAM uses the load information it is given. Does the daily hot water use represent the hot water needed for domestic use (showers, dishes, etc.)? When I input monthly load data in kWh, does SAM consider that energy to be devoted to space heating? Does it consider part of it to be hot water heat too?

Paul Gilman


From the description in SAM's Help system:

The solar water heating (SWH) model represents a one-tank water or glycol system with an auxiliary electric heater. The solar water heating performance model works with either the residential or commercial financial model, and assumes that the solar water heating system displaces purchases of electricity for an electric water heater. Installation and operating costs, financial assumptions, and retail electricity prices determine the value of the energy delivered by the solar water heating system.

To model a solar water heating system that supplies heat for space heating, you would have to include the space heating load in the "Hourly hot water draw profile," and somehow adjust the mains profile (cold water temperature) accordingly. There is not a way to specify a space heating load.

The output variable Q auxiliary (kW) accounts for heat required from the electric auxiliary water heater.

The electric load input page is not related to the physical model of the solar water heating system. The load data is the building's electricity consumption and is used along with the electricity rate data by the financial model to calculate the monetary value of the solar energy, which it assumes goes toward reducing the system owner's electricity bill.

Best regards,

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