[Equest-users] CHW Meter using more cooling energy than a Chiller

[Kathryn Kerns]

David, I changed the Source-to-Site variable from 1.5 (default value) to 0.1 and to 2.9 (limits of the variable listed default range). The variable changes did not change the amount of cooling usage shown in BEPS Rev4.

Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |


From: David Eldridge [mailto:DEldridge at grummanbutkus.com]
Sent: Wednesday, March 09, 2016 10:24 AM
To: Kathryn Kerns; equest-users at lists.onebuilding.org
Subject: RE: CHW Meter using more cooling energy than a Chiller

When a project has an on-site chiller, electricity is purchased to run the vapor compression cycle with some COP for the equipment, I didn't open your file yet but let's assume that it is approximately 4.0 for the chiller. Let's also assume pumping is the same in either case and not included in the COP.

Given your example consumption levels below, the COP may be 2.7, but that makes my math more difficult :) COP is cooling provided divided by power consumed, whereas the eQUEST parameter is EIR defined as power consumed divided by cooling provided. (Check if your default chiller EIR = 0.37)

That version of the model with on-site chillers will then purchase site energy in the form of electricity equal to cooling demands divided by 4.0. The electricity might cost $20/MMBtu to $40/MMBtu depending on your utility, and with a COP of 4.0 the cost to produce chilled water will be $5/MMBtu to $10/MMBtu.

If you prefer kW/ton, that COP is approximately 0.88 kW/ton, and if electricity costs $0.10/kWh, the cost to produce chilled water is $0.088/ton-hour.

The version of your model that purchases CHW through a meter buys BTUs "equal" to the cooling demands. So site energy in BTUs will be much higher by the factor of the COP.

The cost from the utility will also be different though, but should be comparable to the cost to produce CHW above, except that the district energy system probably recovers capital costs, their other overhead expenses, etc. so that the cost will likely be more than $10/MMBtu. (But the client avoids construction and operation costs related to an on-site CHW plant...that's a separate trade-off.)

Using the example $10/MMBtu is equivalent to $0.12/ton-hour for comparison, to verify the concept in terms of order of magnitude.

David




David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates



From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Kathryn Kerns
Sent: Wednesday, March 09, 2016 12:00 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] CHW Meter using more cooling energy than a Chiller

Everyone, I have an energy model that original had its cooling needs served by an electric hermetic air cooled chiller. All the chiller default values were used. That particular model is Base Rev3 (see attached files). I then replaced the chiller with a chilled water meter CM1. Once again, all the CHW meter default values were used. That particular model is Base Rev4 (see attached files). The amount of cooling the building used increased from 155 MMbtu/yr, Base Rev3, to 425 MMbtu/yr, Base Rev4. This makes no sense to me since the amount of cooling the Rev3 and Rev4 energy models required was exactly the same. I would expect the cooling energy COSTS to be different, but not the cooling energy USAGE. Anyone else having this problem and does anyone know how to fix it?

Thanks


Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |


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