Solar Water Heating negative results for energy

Thank you for the consideration Nick.
Indeed, it seems that if you increase the flow rate of a user-defined collector, the number of collectors before the energy becomes negative increases to 17, 18, etc.

However, the same behaviour occurs even if you select a collector from the database (i.e. Heliodyne Inc. Gobi 408 013).

I am designing for a solar field for 32 collectors, to be installed on a building's roof top.

I would appreciate any update or workaround idea.

Regards,
Ioannis

Hi Ioannis,

I've discovered a bug in the solar water heating calculations where one of the collector parameters was using the total system area instead of a single collector area. This led to several calculated values being returned as undefined. I've corrected the issue and hopefully this fix will be included in a patch of SAM version 6.30.2015 coming later this week.

In the meantime, I unfortunately do not see a clear way to work around this issue for your 32 collector case, as any "Total system collector area" greater than "Collector Area" will result in erroneous internal calculations. If a workaround becomes apparent, I'll let you know. I'll also let you know when this fix is patched into the currently released version of SAM.

Thanks for pointing out this issue,
Nick

Hi Nick,

Thanks for taking time to investigate the issue. Indeed, this bug could justify the abnormal behaviour.

I also noticed another strange behaviour that may be associated with the bug you found:

For an "Average daily hot water usage" of 2345 kg/day and in order to achieve a solar factor of more than 70%, I find that 16 collectors are needed. However, when I increase the "Average daily hot water usage" to 4258 kg/day (modifying the flow rate and tank volume accordingly), I find that I can achieve a S.F.=75% with only 4 additional collectors (I would roughly expect about 32 cols.).

This is obviously wrong as the 16 collectors produce an "Annual energy Saved" = 23,214 kWh and the 20 collectors produce an "Annual energy Saved" = 52,387 kWh.

I greatly appreciate your support on the issue and I will be expecting for the mentioned patch.

With kind regards,
Ioannis

Hi Ioannis,

I will take a look and see if this additional behavior is related to the bug I've found. Could you send me or post the project file you are using so I can see what tank sizes and flow rates you are using? As the solar fraction is defined as: Q_saved/Q_aux_only (energy saved / energy required without any solar water heating), I too would expect roughly double the amount of collectors to maintain the solar fraction while doubling the hot water draw.

We will plan to include the fix for the bug in the upcoming patch, likely late this week or early next week.

Thanks,
Nick

Hi Nick.
to clearly demonstrate this behaviour, I created two files using a weather file from the included library. I have already uploaded them in the main post.

Results - DAILY_USE_2500.sam
Average daily use: 2500 kg/day
Annual energy saved: 32423 kWh
Solar Fraction: 0.80

Results - DAILY_USE_5000.sam
Average daily use: 5000 kg/day
Annual energy saved: 62238 kWh
Solar Fraction: 0.77

Hope this helps you identify the issue,

Ioannis

Hi Ioannis,

Thanks for pointing that out, I know see the files. It's quite clear that the results were erroneous before. Running after the bug fix produces the following results:

Results - 2500 (16 Collectors)
Annual Energy Saved - 27,562 kWh
Solar Fraction - 0.68
Aux with Solar - 11578 kWh
Aux without Solar - 40448.9 kWh
Capacity Factor - 14.4

Results - 5000 (20 Collectors)
Annual energy Saved - 43318 kWh
Solar Fraction - 0.54
Aux with Solar - 35749 kWh
Aux without Solar - 80897 kWh
Capacity Factor - 18.1%

Results - 5000 (32 Collectors)
Annual energy Saved - 56086 kWh
Solar Fraction - 0.69
Aux with Solar - 22605 kWh
Aux without Solar - 80897 kWh
Capacity Factor - 14.6%

These results are more along the lines of what I would expect, for instance, increasing from 2500 to 5000 kg/day and increasing to 20 collectors does not result in the same solar fraction. In the 5000 kg/day case, increasing to 32 collectors reproduces the 2500 kg/day 16 collector solar fraction, as you anticipated. Do these results more closely align with what you would expect?

Thanks,
Nick