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Hello. We are running try to calculate monthly and annual collector efficiency of a solar collector using TRNSYS, following the example of SDHW (provided in TRNSYS 18). The result was somehow surprising. So, we decided to run the SDHW example as provided in TRNSYS 2018 (without any change): we found that the values of daily efficiency (EtaColl_d) are negative. Even the overall collector efficiency is negative (EtaColl = -3.5146118736458865E-01). I run it on two different computers. Do you have the same issue? If yes, what is wrong?
Hi Ken,
I will walk you through my troubleshooting process:
I ran the example 'SDHW' using TRNSYS version 18.05.0001 and confirmed that EtaColl_d is negative on all days of the simulation. I noticed that QuColl_d is also negative on all days of the simulation; since EtaColl_d = QuColl_d / (4*IColl_d+...), I conclude that EtaColl_d must be negative because QuColl_d is negative. I then traced the connections in the simulation backward as follows:
- QuColl_d is an input to the ‘Efficiencies’ equation block connected from the ‘Daily Integration’ integrator, output ‘Result of integration-2’;
- The ‘Daily Integration’ input ‘Input to be integrated-2’ is connected from the ‘tank’ model, output ‘Energy delivered via port 1’.
‘QuColl_d’, therefore, is not actually the useful energy gain as reported by the collector model, but rather the energy delivered to port 1 of the tank. Since energy delivered in the tank model is calculated as a function of (T_out – T_in), and I expect the fluid exiting the tank and going out to the collector to be colder than the fluid entering the tank from the collectors, I am not surprised that this value is negative. I would try changing the connection to ‘Input to the integrated-2’ to use ‘Useful energy gain’ from the collector model, rather than ‘Energy delivered via port 1’ from the tank model.
You can (and should!) contact your TRNSYS distributor if you find poor or misleading connections in the examples like this … in this case, since that message will ultimately make its way to me, I’ll save you the step 🙂 I will make a note of the bad connection in the SDHW example and see that it is fixed in the next TRNSYS release. Let me know if you find any other strange connections!
Ken Topic starter
12/07/2023 8:25 pm
@a_weiss
Dear a_weis. Thank you for your kind feedback. I have been dealing with the distributor only for commercial issues. I have just sent them a notice (TESS, Madison) so, you will get some news from my side in the coming hours. Anyway, here seems more convenient since many people may interact (although you appear to be the most important contributor).
Still in the efficiencies equation box, the "solar energy factor" is defined as Fsol = 1 - ( QAux / (QDHW+ 1e-6) ). This definition leads to negative values sometimes, especially during days when there is not much sun shining (cloudy days) since almost the whole energy is provided by the auxiliary. I have identified Fsol as SEF because of the description at the bottom of page 10-45 in the 10-Examples.pdf file (presentation of the SDHW exemple). Now, if it is even solar fraction, Fsol could become negative, so, I wonder what Fsol represents.
The definition of solar energy factor as given by the US Department of Energy is “The solar energy factor is defined as the energy delivered by the system divided by the electrical or gas energy put into the system.” https://www.energy.gov/energysaver/estimating-cost-and-energy-efficiency-solar-water-heater
Please, is there something we have not understood with these various definitions? What represents Fsol since it may be negative?
@ken I noticed that too, the negative Fsol values. I will correct the notation and equation in the project file (and the equation in the documentation) to be consistent with the US Department of Energy's definition of solar energy factor as = QDHW/Qaux.
