Representing Distribution Losses

plant-loop

asked 2018-10-17 01:38:22 -0600

updated 2018-10-28 17:10:14 -0600

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:

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.
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.
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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answered 2018-10-18 11:13:35 -0600

ericringold

10672 ●33 ●8 https://www.nrel.gov/b...

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

(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.

Anna Osborne Brannon ( 2018-10-28 17:10:06 -0600 )edit

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answered 2018-10-18 06:01:23 -0600

TomB

1694 ●2 ●11

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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answered 2018-10-18 11:32:40 -0600

Fábio Batista

209 ●1 ●9

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.