# CHW Meter using more cooling energy than a Chiller

9 posts / 0 new

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 |

Offline
Joined: 2011-09-30
Reputation: 0

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

Offline
Joined: 2012-05-08
Reputation: 1

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 |

Offline
Joined: 2011-09-30
Reputation: 0

Kathryn,
Changing the CHW-meter SOURCE-SITE-EFF will change the total source energy reported on BEPS but not the site energy.
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

[cid:image001.jpg at 01D17A08.E61778C0] [cid:image002.jpg at 01D17A08.E61778C0]

134 South Fitzhugh Street Rochester, NY 14608

T: (585) 698-1956 F: (585) 325-6005

bbishop at pathfinder-ea.com www.pathfinder-ea.com

[http://png-5.findicons.com/files/icons/977/rrze/720/globe.png]Carbon Fee and Dividend - simple, effective, and market-based.

Offline
Joined: 2012-02-25
Reputation: 7

So, how do I change the CM1 site energy so that the supplied cooling energy consumption is roughly equivalent to a plant chiller with a COP of 0.3704 providing 44 F water? The chilled water plant is an air cooled electric hermetic reciprocating chiller with a COP of 0.3704 providing 44 F water.

Pump energy is about the same for Rev3 and Rev4 energy models, so that part makes sense.

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

Offline
Joined: 2011-09-30
Reputation: 0

David, what if the Rev4 plant is delivering chilled water via a chiller that exactly matches the chiller detailed in Rev3? The amount of cooling energy for each energy model should be the same.
This is what I am trying to accomplish with the addition of CM1. I just don't want the chiller to be included in the energy model. Maybe eQuest can't do this?

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

Offline
Joined: 2011-09-30
Reputation: 0

It is working exactly as you describe - if you setup a report for building cooling load, the two trend files will be identical in Rev 3 and Rev 4.

The same thing would happen if you had district heating energy, in that scenario the purchased heating would be less by the amount of the efficiency of the boiler. Even though both models use the same amount of heat, the amount of fuel input to a boiler would be different.

In your case the building cooling loads are the same, the same amount of ton-hours of cooling are being consumed. The site energy in Rev 3 is lower due to how the chiller consumes less (but more expensive) electricity and then provides thermal cooling output like a heat pump.

Think of it this way, if you drew the boundary around the building excluding the chiller plant in Rev3 and purchased the output of the chiller as opposed to the electrical input, it would be identical to the results you are getting from Rev4 with the purchased chilled water meter alone.

It sounds like Rev 4 is exactly what you want, and is working correctly since the Rev 4 thermal cooling BTUs were 2.7 times the electrical input of the Rev 3 chiller BTUs, which was exactly the expected ratio based on EIR = 0.37 = 1/COP.

David

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

Offline
Joined: 2012-05-08
Reputation: 1

David, I begin to see your point. Took me a while to get what you were driving at. I may have to reconsider what type of meter I need to use to represent the plant. I think I am looking for some kind of plant electric meter, not CHW meter. Something to represent the plant electric power used to generate the plant chilled water required.

Thanks for the discussion.

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

Offline
Joined: 2011-09-30
Reputation: 0

Got it. One option if you need to use the chiller:

1) install a second electricity meter for the chiller (and pump(s)?) in Rev 3 if you think the air-cooled chiller is a fair representation of the district system efficiency and you want to track it separately. (It will still be in the BEPS, but there is a different report you can use.)

Better choice:

2) convert the MMBtu of cooling end use to equivalent plant electricity using an assumed average COP and convert to kWh for the district energy plant in Rev 4 in a spreadsheet as post-processing.

This works great in case the client has questions about the COP to be used, as you don't have to re-run the model to get kWh for changing efficiency.

DSE Mobile

Offline
Joined: 2012-05-08
Reputation: 1