Rohini's profile - activity

Is it possible to model a two-speed fan in EnergyPlus? I can see only on-off and VAV fan objects. I am modelling a residential two-stage unitary-system where at low loads the compressor and fan is at 70% and cycling and at high loads, the fan and compressor is at 100%. I think I need a 2-speed fan for this, but if there is some other way to model the system, would appreciate it if someone can point it out.

Update: I finally got back to this. Based on the comments (I tried all, other than EMS), it looks like the fan does not cycle. Here are the results:

• The RTF is varying between 1-0.56.
• The supply temp is varying between 14-20C.
• The supply flow/RTF comes to the 100% value.
• The st1 speed is 64% of st2.
• The unitary system control Is based on Load, the fan type is OnOff.

So looks like the flow is not varying at all and only temp is varying. If I look at the times when the RTF is 1 – the supply temp is still varying between 14-20C – in reality the supply temp should be near 14C, and the RTF would be less than 1.The same coil (curve for 100%) used as single speed works as expected – the temp varies slightly between 12C-13C, the flow/RTF is constant at the max value.

Is there anyway to vary the flow to the st1 cfm or st2 cfm using a two-speed coil?

I changed all schedules to not have variations. The issue still exists. The only other thing I havn't done is change the system to ideal loads system. Not sure if that makes a difference.

I do not have setbacks. Also, these runs have a unitary system.

I am looking at zone predicted sensible and moisture loads. When looking at the output I get the following results. I would have expected them to be similar. Am I missing some conversion constant? a) Predicted Sensible Load [W] (Monthly) 10 minute timestep run - 5596 1 minute timestep run - 21020 b) Predicted Moisture Load [kgWater/s] (Monthly) 10 minute timestep run - -0.036 1 minute timestep run - -0.20551

I normally put a dehumidifying schedule at the ZoneControl:Humidistat level. The value I use is 50-60%. On the AHU side (if it is a unitary system object), you will need to specify the type of latent load control (only when sensible load is available etc.), and dehumidification control type (none, coolreheat, etc.). For humidity schedule, I just put in 30% schedule. This will come into effect only if you have a humidification object.

If you know the supply temp, hum ratio, and flow, could you not just calculate the load using the current zone temperature and use that as a internal load to the zone instead of using the ideal loads system?

The energyplus weather file has 12 design-days for cooling. One of the variables is 0.4%, 1%, and 2% - I am assuming 0.4% is the most common for hvac-sizing. Not clear on which one of the following is more common - DB=>MWB, WB=>MDB, DP=>MDB, Enth=>MDB. I am guessing DB=>MWB is most common but would like to confirm. When does it make sense to use one or the other. Does anyone use the latent load related design-days on actual projects? Thanks.

For the primary-school reference building model, I am looking at the output table (Zone sensible cooling for entire facility, under HVAC sizing summary). I am trying to figure out how the user design air flow is calculated. It is more than the minimum OA flow, and different from the calculated design air flow. In the Zone sensible heating table, the user design airflow is same as the min OA. There does not seem to be anything in the input file that looks like cfm/zone has been entered - every design airflow field says autosize. I must be missing something blindingly obvious. Thanks as always for all the help.

Hi, I have downloaded some residential and commercial benchmark idf files. I was wondering if I can open them through openstudio directly? Or should I open them using EP-launch (although the EP-launch that came with the openstudio/energyplus folder gives an error and does not run - so in this case, should I install stand alone energyplus?) This is my first time using OS, and I havn't used Eplus in many years. Is there anything equivalent in OS like eQuest wizard to get up and running super quickly, and worry about the details later? Appreciate any pointers. -Rohini

I am assuming E+ does not allow you to add one more heater in the space in addition to the baseboard. If you know how much is the extra electrical energy, you could add it as a misc load in the spaces. You will have to change the heating thermostat setpoint to higher value that you are seeing in the spaces.

I am unable to launch EP-Launch - it comes up with the error "Component 'MSINEt.OCX' or one of its dependencies not correctly registered: a file is missing or invalid". I reinstalled OS today, but still getting the error. The error was not there yesterday. Here is the path for the file - C:\Program Files\OpenStudio 1.7.0\share\openstudio\EnergyPlusV8-2-0. Thanks, -Rohini

I am using Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate for school reference building - seems to be giving non-zero loads for cooling (since I am doing a summer day). Did you see how much OA the operating room has - depending on the climate, maybe it is meeting your load? You must have already checked the thermostat schedule. You could run at the detailed time step to see what happens - maybe there is lot of cycling? Can't think of anything else.

I am modelling the IdealLoadsAirSystem in energyplus with humidistat set to 50%. What does the output report Zone Ideal Loads Supply Air Latent Cooling Rate [W] actually mean? When the indoor RH is less than 50% (say 47%), I would have expected this value to be zero. The only time when this value is zero is when the sensible rate is zero and the indoor RH is less than setpoint. Does this output also include whatever latent cooling happens due to the supply air? If yes, how is it calculated - I did not notice a constant SHR. Thanks, -Rohini

In general the latent cooling rate is getting calculated as Richard mentioned. Here are the details of how the latent load is getting calculated, when you have a humidistat. Case 1 - Non-OA system, no dehumidification load: In this case the Zone Predicted Moisture Load to Dehumidifying Setpoint Moisture Transfer Rate [kgWater/s] is a positive value. The supply air humidity ratio is whatever is specified in the ideal loads component. Return values are same as the zone values. The latent cooling rate is calculated from this. Case 2 - Non-OA system, yes dehumidification load: In this case the Zone Predicted Moisture Load to Dehumidifying Setpoint Moisture Transfer Rate [kgWater/s] is a negative value - i.e. there is a latent load on the system. The supply air humidity ratio is set to a value that will satisfy this latent load. Return values are same as the zone values. The latent cooling rate is calculated from this. In this case the Supply Air Latent Cooling Rate is same as the predicted moisture load (Moisture transfer rate x latent heat of vaporization). Case 3 - When OA is used: In this case the supply air humidity is decided as above. The latent cooling rate calculations are based on these supply conditions and the mixed air conditions. So the cooling rates are different from the predicted load (unless the OA is the same exact condition as return air).

Did you try setting the compatibility mode to windows XP - sometimes this fixes the errors.

Hi All, I am modelling a unitary equipment with single speed DX cooling coil, electric heat coil, and Fan ON/Off. If I look at the run time fraction of fan and the heating coil (fraction calculated as Heating Coil Air Heating Rate [W] (Each Call)/ DesPer 1:Des Heat Cap [W]), it is same. The fraction for cooling coil (Cooling Coil Total Cooling Rate [W] (Each Call)/DesPer 2:Des Tot Cool Cap [W]) is different than the fan run time fraction. Question - Am I calculating the fraction for cooling coil correctly? If yes, what could be the reason for the difference between the runtime fractions of cooling coil and fan? Thanks, -Rohini

You might want to look at the run time fraction heater. What I have noticed is that most components in energy-plus run to meet the load. So, if the load is 50% of the max heater output, the heater will run only 50% of the time (even if the timestep is 1 minute). There is no minimum cycle time that can be set (that I know of).

Another example of models saving energy would be the models that are used for real time optimization in controls (not sure if this is still just research grade or more common than before).

Now, if someone can came up with a wrapper on the modeling tools that encapsulates the expertise involved in deciding the optimum combination of technologies that would work together, I would say that the tool is helping with achieving energy savings.

I guess my point is that modelling currently is just a quantification/tradeoff/value engineering tool. It is very much needed but difficult to say that it would lead to significant energy savings. The bulk of the savings are coming from the technologies chosen by the team. To your point, I would say the effect is broad. If someone is going for 50-70% savings, they will consider geothermal/PV/or highly efficient equipment, modulation on everything, reduction in internal loads, LED's etc. Modelling is going to work with these technologies and quantify the savings.

I have a different take on how much using the energy model itself saves energy. To me, most of the savings relative to a code compliant building come from the experienced architect/engineers/owners and energy modellers themselves who set the tone of the design (whether it be LEED silver or gold etc.). The model ends up helping with the value-engineering, trades offs, to get that last 1-2% savings to reach the goal. To that extent, If I have to guess how much savings come due to modelling, it would be more in the range of ~5%. If you did not do modelling and just followed the best practices, one would still see savings (just not quantified). Lincoln brings up a very valid point - the proper control sequences not followed and the building being too complicated to run washes away any savings (modelled or not).

Also check the minimum flows.

I am looking for the document that describes the various columns/data in the epw file. The link I have (http://apps1.eere.energy.gov/building...) is broken. Any idea where it had been moved to?