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# Representing Distribution Losses

How do you account for distribution losses when modelling a central plant?

Some resources (like the LEED DES guidance) say to use 2%, or 5% losses, but don't exactly elaborate on how those losses are applied. I've heard of modelers using a few methods which yield very different results:

1. Decrease the efficiency of the plant by the loss, so if its 2% losses, decrease the plant efficiency by 2%. This basically results in a 2% increase in energy use.

2. Decrease the annual chilled water output by 2%, then re-calculated average efficiency for application to your model, if you have a separate central plant and building model. This also results in close to 2% increase in energy use.

3. Apply the losses as a constant percentage of the total plant capacity. If you have 1000 kW of plant capacity, that means a constant 20 kW of losses at every hour of the simulation. This causes a significant increase in energy use, in excess of 10% increase.

Does anyone have guidance that shows which of these methods actually most closely represents measured data?

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The LEED v4 Reference Guide gives the requirements for thermal distribution losses as:

(from pg. 365)

Which indicates following method 1 that you describe - decreasing the efficiency of the plant.

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Thanks, I hadn't noticed this. The client still requested option #3, based on their experience with measured data. Ideally I would calculate the losses based on the length of pipe, ground temperature, and level of insulation, but that data wasn't available.

( 2018-10-28 17:10:06 -0500 )edit

I don't think measured data will support any one approach in particular. They are all fudges to cover unknowns. It would be more satisfying and useful to take a detailed approach based on the operation of each part of the plant. - temperature gains in chw pipes - temperature loss in hw pipes - air leakage from AHUs, supply ductwork - return air less than supply air by toilet exhaust fraction - temperature gains in condenser water pipework - AHU coil fouling And others no doubt. Looking at EnergyPlus fault models may help

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I recently modeled a District Heating and Cooling system and accounted for distribution losses using method 1 that you describe, and as referred in LEED reference guide. We are still waiting for LEED comments though.

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