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Why energy consumption is not proportional with Outdoor air

asked 2017-10-26 21:37:00 -0600

xzb86 gravatar image

updated 2017-10-27 07:40:51 -0600

hello, can anyone help to solve this problem? I struggled for a long time.

I would like to study the impact of the amount of outdoor air on the annual energy consumption.

The simulation process is as follows:

  1. I established a 1000 m3 single room. Use HVAC Template (CAV system) to calculate the total consumption.
  2. Use DesignSpecification: Outdoor air and Exhaust Fan to simulate the Ventilation condition.
  3. Firstly, I input 1m3/s air volume to the outdoor air and exhaust fan, record the energy consumption.
  4. Then I input 2m3/s, 3m3/s, 4m3/s, 5m3/s... 10m3/s 15m3/s to the model, record the results separately.

I thought the energy consumption will increase proportionally with Outdoor air variation. And the energy difference between each unit will always keep the same. (Q=cm△h)

But the results show as below:

image description

I wonder is this the correct way to simulate ? or is there any problem?

Appreciate if someone can help.

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The temperature (and humidity) differential between your zone set point and outdoor conditions could really mess with your hypothesis. Some climate zones are much more amenable to natural ventilation than others. If you pick a really cold or hot climate zone the trend you expected to see might be more pronounced.

ljbrackney gravatar image ljbrackney  ( 2017-10-27 12:20:09 -0600 )edit

so how can I simulate it? do you have a good way to simulate the same room with 2m3/s, 3m3/s, 4m3/s, 5m3/s...outdoor air? I think it should constant proportional.

xzb86 gravatar image xzb86  ( 2017-11-01 02:15:49 -0600 )edit

I spend a long time, but didn't solve it. appreciate if you can tell me the right way

xzb86 gravatar image xzb86  ( 2017-11-01 02:23:02 -0600 )edit

3 Answers

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answered 2017-12-29 05:51:09 -0600

Joe Huang gravatar image

updated 2017-12-29 05:58:12 -0600

There are several reasons why the building energy consumption would not be proportional with the outside air rate: (1) There are other loads in a building such as conduction, solar, internal heat gain, etc. If you lump those as A and the outside air load as B, would A+B be proportional to A+2B, A+3B, etc.? (2) There may be a deadband in the building thermostat setting, such as heating at 16C and cooling at 28C. If the building floats throughout the day, such as during the swing season, the heat flows due to the fresh air will never appear in the building energy consumption at all. (3) Differences in HVAC sizing relative to the ventilation loads can have a big impact on the building energy consumption. Since you're varying the outside air rate from 0 to 20, the equipment will be sized for the non-ventilation loads in the first case, and maybe completely on the ventilation loads in the second case. On the other hand, the HVAC might not be sized at all for the ventilation load, since many sizing routines don't even consider the ventilation load!

Lastly, have you checked to make sure that the Fresh Air Amount is truly as you've entered?

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answered 2017-12-07 10:24:35 -0600

xzb86 gravatar image

@rraustad I checked the air system outdooor air mass flow rate. when I enter 1m3/s the result shows 1.203523625 kg/s 2m3/s the result shows 2.407047251 kg/s 3m3/s the result shows 2.407047251 kg/s

is there any return air? I set controller: outdoor air ---max fraction of OA sch---always 1 I thought it is 100% OA. what shall I do to deal with it ?

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Your new answer is more of a comment to my previous comment than it is a new answer. Any way, this happens when the zone air flow rate doesn't need to increase to meet the zone load. Just because you say to use 3 m3/s of outdoor air, that doesn't mean the zone needs that much air flow at whatever supply air temperature is being delivered. If the zone doesn't need more that 2.4 m3/s of air flow, then the outdoor air can't be higher than 2.4 m3/s. Does that make sense?

rraustad gravatar image rraustad  ( 2017-12-07 16:21:55 -0600 )edit

One of my pet peeves is when someone writes numbers far beyond their possible significant places. I learned that 50 years ago when my junior high school science teacher taught that we should never write a number with more precision than what we know.:-)

Joe Huang gravatar image Joe Huang  ( 2017-12-29 05:56:53 -0600 )edit

answered 2017-12-01 11:57:03 -0600

When you change the amount of outdoor air, the mixed air temperature entering the coils also changes (at least up to 100% outdoor air). I don't see exhaust fans in your simulation so any exhaust air is removed at the outdoor air mixing box and a mixture of outdoor air and return air determines the mixed air temperature. As the mixed air temperature changes the coil performance changes. The incremental changes from 1 to 2, or 2 to 3 are not all that different. Nor are the changes from 5 to 10, 10 to 15 and 15 to 20.

Check the mixed air temperature for these simulations to see how it is affected by increasing outdoor air quantity. Also, have you checked the zone temperatures to make sure they are the same between these simulations?

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@xzb86 have you checked the outdoor air quantity for these simulations to make sure the amount of outdoor air is what you expect?

rraustad gravatar image rraustad  ( 2017-12-04 11:05:54 -0600 )edit

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Asked: 2017-10-26 21:37:00 -0600

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Last updated: Dec 29 '17