Modeling domestic hot water

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Hello all,

2 Questions: 1.) Best practices for modeing domestic hot water in Trace. 2.) How to incorporate solar thermal hot water into a Trace model.

Q1. Typically, I set up a base utility with the full load hot water heater demand (say 10KW or 40MBH) and then use a schedule. I see that you can call out units of gpm and also call out entering and leaving temperatures in the library...is this a better way? I might think so because typically on a WSHP system I might put a HX in the source water piping so when source water is a higher temp than entering domestic water, I can capture some preheat.

Another item I find difficult is assiging heat from a heat recovery chiller to domestic water heating or using a water to water heat pump, so some ideas here too might be nice, if you have any...

Q2. I've seen the video on "modeling" solar PV and was wondering if folks are using this method to model solar hot water too. Typically what I do now is assume, calculate (with RetScreen or T*Sol), or estimate the solar fraction, and then just reduce my proposed buildings water heater size accordingly. For example, for a 10KW baseline hot water system and a 30% solar fraction, the proposed building would use a 7KW heater modeled like above.

Thanks for your ideas!!

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GN-

 

1) Your typical way is what I would consider best practice for DHW in TRACE

2) You certainly can use the method in the PV tutorial for modeling solar thermal

 

Q1) TRACE is pretty funny when it comes to rejecting heat to hot water. I have not had a lot of success directing the heat straight to the hot water. I am pretty certain that TRACE does not consider temperature at all in the base utilities but rather uses gpm and the delta T to calculate the therms. HOWEVER, TRACE does look at the temperature of a boiler when rejecting heat to it. So in that case, reject heat from the cooling plant to a heating plant (set up for dhw). In libraries, you can copy an existing boiler and set the temperature setpoints. That way, TRACE will know how much heat can be rejected to the boiler. Then assign a "Domestic hot water load" to that said boiler. This has been my most successful method.

 

Q2) That method is actually better suited for solar thermal than PV. This is because it captures diffuse energy which can be used more effectively for thermal applications than PV.  I don't know how many folks are doing this, though I have had people ask about it. You could create the exact same thing and change the energy-meter in the library to yield hot water and experiment with it a little. Looking at the equipment energy consumption report, you could get a good estimate of how much hot water was made.

 

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Good stuff, thanks Bob!

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