solar water heating --- water delivered above the set temperature

I am working with the SAM solar water heating model. There are two temperatures provided by the user, the "outlet set temperature" (default=55C)  and the "solar tank maximum water temperature" (default = 99C).  In reading the documentation, it appears that whenever the temperature is above the set temperature, that is the temperature that's delivered, whereas when it's below the set temperature, electric auxiliary heating is supplied to raise the outlet to the set temperature. Is this correct? If so, it represents a potentially unsafe condition, since temperatures above 55C can cause scalding injury. A real system should mix water from the mains to achieve the set temperature at the outlet when the tank is supplying hotter water. For example, if the set temperature is 55C, the tank delivered temperature is 65C, and the mains temperature is 25C; and if a draw of 10 kg is demanded, the hot water draw should be reduced to 7.5 kg, and mixed with 2.5 kg of mains water, to achieve the desired 55C set temperature.

Question 1: Is my understanding correct for the model assumption of delivered water temperature?
Question 2: If Q1 is "yes" then what can be done about it?

Hi Russ,

It looks like the outlet set temperature affects the amount of energy from the auxiliary heater (Q aux), but not the hot temperature (T hot). You can verify that by trying different set temperatures and comparing the T hot and Q auxiliary outputs on the Time Series tab of the Results page.

The model does not attempt to control the temperature of the hot water delivered by the system.

Best regards,
Paul.

Paul,
I'd say that is just wrong. Here is a diagram of a solar water heating system I took from the   Arizona Solar Center website ( azsolarcenter.org/solar-hot-water-a-primer ) (ignore the "batch heater"):



They call it a "tempering valve." The ASC website also says this:
Caution! These systems produce very hot water! A tempering valve is your protection from being scalded at the tap. You will regularly see temperatures in excess of 160°F (71°C) in summer months, which is much hotter than you are accustomed to getting from your conventional thermostatically controlled water heater. The tempering valve limits the temperature delivered to the tap by mixing in cold water as necessary.

The SAM model doesn't account for this and as a result models dangerously hot water being delivered by the system and overstates the amount of hot water drawn when the tank temperature is higher than the set temperature (and thereby underestimates overall performance).

I think I could just run it twice --- first with the desired draw profile and again with the draws reduced by the hot/cold mix at the tempering valve. Use Q_auxonly from the first run, and Q_aux from the second run to get the energy savings. But this would not be as accurate as fixing the code. I looked at the SAM swh model source code and am relatively sure that changing line 597 from:

      double mdot_mix = draw[hour]*(1.0/3600.0);

to

      // simulate the tempering valve. the fmin function limits the draw to the user-specified amount if T_hot < T_set,  
      double mdot_mix = draw[hour]*(1.0/3600.0) * fmin(T_set_use - T_mains_use)/(T_hot_prev - T_mains_use),1.0);

But I don't know how to compile my own ssc module.