Average Solar Cooling Load Thresholds - Sefaira and Energy Plus

 

 

How can I meet an average solar cooling load threshold in my zones?

 

Average being the important word. Sefaira shows the single hour peak across the whole year not the average cooling period peak nor the average of specific days.

 

 

 

 

So, in this case, the single hour peak could be 45W/m2 of solar gain requiring cooling but 99% of the time the value could be around 20W/m2 or lower.

How can you dive deeper to get a better understanding of the solar cooling load in your building?

Sefaira shows peak loads and plant sizing as well as annual energy, CO2 and cost. Elsewhere, you can see how you can start in Sefaira and complete in Energy Plus to be able to split the results further into monthly or hourly results. You could also look at specific aspects of the HVAC system such as a particular terminal unit and pick out results from a long list of additional outputs. 

 

To do any of this you need to start in SketchUp or Revit, upload to Sefaira Systems Web Application, setup your project and let it run, download the .idf file of your completed run and then convert it for use in Energy Plus. The first time you do this it will take some time but the process should take 5 minutes once you are familiar with it.

 

 

Worked Example

 

 

Checking if any perimeter zones have higher than 25W/m2 daily average solar cooling load

 

In the image above, I can optimise for the single hour peak in Sefaira Systems but if I want to optimise my envelope and zoning design based on a daily average solar cooling load I will have to do some additional analysis in Energy Plus.

 

 

 

 

This is an example from the Irish Part L of the Building Regulations for Non Domestic Buildings. I can use the tabled data* to perform a manual calculation based on area of glazing, shading correction factors, frame factors and floor areas of each zone OR I can setup a quick Sefaira Systems project and find the average actually received solar load for the building in Energy Plus. 

The technical guidance document goes on to say that "Local weather data averaged over a period of 15 years, at least, can be used instead of the data in Table 4, where available." This is what Sefaira will use and will, more importantly, use the specific, calculated solar gains in the space based on context and an annual simulation.

 

In this case, we will look at an East perimeter zone 6m deep;

 

 

 

 

 

  1. Set Core and Perimeter Zoning,
  2. Define 6m as the Per zone depth,
  3. We could ignore the Core zone completely using Sefaira's zoning but we include it for the Peak and Energy runs in Sefaira Systems, for now

 

The Energy Plus Bit!

 

 

Now download the .idf file of this completed run and follow the steps linked above and here to run the simulation again in Energy Plus.

We will be looking for the results for this Output:Variable

Output:Variable,*,Zone Windows Total Transmitted Solar Radiation Rate,hourly; !- Zone Average [W] 

 

 

The Excel Bit!

 

 

Once you run the Energy Plus simulation, for the year, with the appropriate weather file and open up the results .csv you will see the total, calculated solar gain for each hour and for each zone. Here I show Perimeter Zone 2 on the 1st of January and highlight the highest value - 853W at 1pm;

 

 

 

 

 

 

Note, this is for the whole zone so we have to,

  1. Divide by the floor area,
  2. Average the values across each day,
  3. Check the Max Value in the Year

 

 

Divide by Floor Area

 

  1. Floor Area,
  2. Function,
  3. Result

 

 

 

 

 

Average Across Each Day

 

  1. Function,
  2. Results

 

 

 

 

Check for Max in Year

 

  1. Function,
  2. Results - Maximum Daily Average Solar Cooling Load in the Zone,
  3. Checking Single Hour Peak Value

 

 

 

 

In this example, we have seen that the Maximum Daily Average Solar Cooling Load in this Zone does not exceed our threshold of 25 W/m2. We have therefore designed our zone geometry in such a way that we can keep the cooling plant energy associated with the zone at an acceptable level.

 

So, the highest daily average is 23.45 W (2) of solar gain per m2 of floor area and at one particular hour in the year it gets as high as 48.53 W (3) of solar gain per m2 of floor area. These are two very different pieces of information with different impacts on design decisions.

 

We have done it with a simply zoned SketchUp model, including any relevant adjacent buildings, with annual weather data from weather sites and either the drawn or parammetrically added glazing for quicker setup. This can be a one or two hour check on a potential worst case zone or a few carefully chosen zones to show compliance with the intent of the Irish Part L or any similar technical guidance document.

 

Back to Sefaira Systems Bit! 

 

This is in addition to what we already have in Sefaira Systems, namely;

  1. The Heating and Cooling Coil sizing for this zone,
  2. The Cooling Plant Design Capacity for the whole building,
  3. The Energy, CO2 and Operational Cost associated with running the whole building,
  4. and many more outputs and variations of inputs to test multiple strategies 

 

This is just one specific example of how to get information from Sefaira Systems and then meet locally specific criteria with additional analysis in Energy Plus. This could be used for any other Output Variable such as Occupant Loads or Humidity or One Particular Terminal Unit's Electrical Consumption etc etc.

 

For further queries contact - support@sefaira.com

 

 

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