Improve Performance of Solar Collectors

asked 2016-03-18 16:00:36 -0600

519 ●2 ●9

I am trying to improve the performance of the solar collectors in my simple simulation where two collectors (one on the south facing roof, one on the south wall) supply a hot water heating coil for a small space. Regardless of how I set up the schedule for the fan availability for the air loop or the coil availability, the 'solar collector heat transfer energy' shows that while the fluid gains heat throughout the year, it also loses heat. Surprisingly the heat loss is greatest in the summer time. I have also noticed that the efficiency of the panels throughout the year varies from 0 to large negative numbers.

I know that solar collectors perform better than this in the real world. I looked through the solar collector data set and made a simulation comparing all of the evacuated tubes, from which I chose the Thermomax Industries Ltd collectors as they seemed to perform the best. However, using the efficiency equation coefficients and incidence angle modifier coefficients given in the data set for the Thermomax panel, the outlet temperature never got higher than 50 at any time of the year.

I am very confused as to why the panels in my simulation perform so differently from the real world. Has anyone else had this problem? If so how did you fix it? I used OpenStudio 1.10.0 for this simulation.

Thank you.

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answered 2016-03-25 18:35:01 -0600

RChidwick

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I exported my simulation in OpenStudio to an IDF file so I could work in EnergyPlus. I am using a Pump:VariableSpeed object to circulate water from the solar collectors to a storage tank (WaterHeater:Mixed with a heater capacity of 0). In EnergyPlus you can specify a pump flow rate schedule. I created a fractional schedule where the pump was off (value = 0) from 0:00 to 10:00, then on (value = 1) until 16:00, and then off again until 24:00. This fixed the collector efficiency, and collector heat transfer energy. Now the efficiency is mostly positive (some negative values in middle of winter) and the heat transfer energy has a minimum value of zero for most days throughout the year, with a few exceptions. Playing around with the times in the fractional schedule should clear up any remaining negative values.

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answered 2016-03-20 11:14:40 -0600

301_Hours
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This isn't an open studio specific answer, but in other tools like TRNSYS I've noticed that if you don't turn off the pump when the sun is down the collectors will lose considerable heat. That should be mitigated somewhat in an evacuated tube system, but I'd check the flow rate. You could also do a temperature based control where the pump doesn't startup until the panel temperature exceeds a desired set point.

Also if the flow rate is too high you won't see higher temperatures as the heat is carried away too quickly likewise if your inlet temperature is too low.

Also, in northern climates solar hot water systems are often drained at night into holding tanks so that the working fluid doesn't freeze. That may help overall performance.

Finally, the angle of the collector makes a difference too. The best angle for overall solar gain is not necessarily the best angle for your system. Try more vertical angles to get more consistent year round performance. The summer will be worse, but the rest of the year will be better.

Hope that makes sense.

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How would I go about doing a temperature based control? I tried lowing the flow rate but I'm still having the same problem. I've put the collector on the wall of the structure, so it should be ideal for winter collection. Is it possible to model draining the tubes over night into a holding tank in EnergyPlus or OpenStudio?

RChidwick ( 2016-03-20 13:20:18 -0600 )edit

I'm not an open studio user so I can't help on those specifics. As far as the control you would want to turn on the pump when the panel outlet temperature exceeds the return temperature off the coil or holding tank so you're adding heat to the system.