Solar Water Heating negative results for energy

In the solar water heating module, sometimes the software (v.2015.1.30) results in negative values in the Summary page. It particularly gives negative "annual energy saved" along with zero "solar fraction" and "capacity fraction". In addition, the accompanying "Monthly Energy Production" graph shows a lot of months with negative production.

I have noticed this behaviour in at least two cases:
a) if you put more than a specific number of collectors (say 16 for my case) and
b) when you use the User-defined collector and have the test fluid set at "Glycol" which is the default value. If you just change it to water, then it runs ok.

I can send you a sample case if you need, but it's fairly easy to replicate the behaviour using the default values in a new Solar Water Heating project.

Hello,
It would definitely be helpful if you could [url=mailto:sam.support@nrel.gov?subject=Solar%20Water%20Heating%20negative%20results%20for%20energy t=_self]email us[/url] a sample case showing these results, I'm not seeing them at first glance.
Thanks,

Janine

Hi Janine,
thanks for your consideration.
I just emailed you a sample file.

Forgot to mention that the sample file was created with the latest version 2015.6.30.

Just uploaded the sample file also in my first post, if anyone else wants to check.

Good morning,

I'm investigating this issue, at first glance it appears that there may be an issue as the total system collector increases far beyond the User-defined collector area. Within the algorithm, the defined system is used to scale the flow rate specified in the user-defined collector. Some of the model equations are becoming undefined for very large system sizes, resulting in unexpected behavior. I will update if a suitable workaround is found.

Is there a target total collector area that you are designing to?

Thanks,
Nick