ENERGY ANALYSIS

[genieconseil]

I'm a small mech/elect consulting firm me being the only engineer I specialize in building engineering in both disciplines and recognized by the order of engineers of Quebec. I've been practicing since 1987 and in all that time i have never had to do any energy analysis where the degree days or the bin method had to be used. My situation is the following, my client has a commercial building in Montreal ±100,000 sq. ft. and is devided in 2 locals. one tenant is using 2 gaz fired fresh air units of 25,000 cfm (gas fired 2,500,000 btu/hr) & 16,000 cfm (gas fired 2,000,000 btu/hr)he only has one main meter and concequently one invoice for the whole building. he has asked me to evaluate precisely the percentage of natural gaz consumed for the year for those two unit only so he can invoice the tenant fairly. I'm a little embarassed to ask for help, I realise that the bin method would be the appropiate method to use considering the fact that the units only provide fresh air 10 hrs a day and requires heating from october to mid may. I have never used this method and cant seem to find any good examples for me to follow. Can someone help me direct me with my problem?

Thank you in advance

Genieconseil

 

[vzeos]

genieconseil,

This publication gives some bin calculation examples in the appendix.

http://www.energy.rochester.edu/efficiency/optimal_boiler_size.pdf#search='bin method and heating'

 

[MintJulep]

You will need to use actual temperature data for the period of time in question. Published BIN data is "average" over some arbitrarilly long past time.

I doubt that invoicing a tennant based on calculation is legal.

 

[zdas04]

Any method that is consistent with the contract is "legal". As far as that goes, it would be legal to charge each of them the full gas bill if the contract will allow it.

It sounds like the landlord is trying to bend over backwards to be fair. Since no method is going to be "accurate" (except to install another meter which is an unreasonable expense), I would opt for simple. What I'd propose is an allocation based on heated area times cfm. Say the one with the larger unit has 60% of the heated space then I'd allocate him:

(60,000ft*2500 cfm)/(60000ft*2500cfm+40000ft*1600cfm)=70%

This isn't "right", but it will come as close as a much more complex calculation with historical averages. You could do the same thing with heat capacity, but you'd get to around the same number (65% instead of 70%). At today's industrial gas prices you are talking about splitting about $22.50 US/day (assuming $15/MMBTU, 8 hours/day run time)--getting it "wrong" is not a huge deal and most tenants will accept a logical, consistent, and hopefully fair allocation. You can easily spend more in billable hours than this could ever be worth.

David

 

[MintJulep]

Right, but if you were the tennant would you have signed a lease with a term along the lines of:

The landlord will invoice the tennant for gas usage based on an unknown method of calculation and performed by someone not familiar with performing energy calculations.

 

[zdas04]

Cute, but a good point. The contract has some language about utilities. That is the only reasonable place to start. My point is that you should use the simplest reasonable allocation method consistent with contract language. I've successfully used "simplistic" allocation routines on projects that were a lot more money than we're talking about here, and I was successful because they were perceived by the parties as fair enough.

The "bin" calculation is a collection of averages of averages of standard conditions (whatever they might be) that is complex enough to drive an experienced engineer to start this thread. It is no more "accurate" than allocating based on heated area or number of employees, just harder.

David

 

[vzeos]

MintJulep, zdas04:

I think you are both right. Invoicing your tenants for gas supplied by the utility may be considered an infringement on the utility’s franchise rights. If you want your tenants to pay for their energy consumption, you would have the utility meter each tenant separately such that the tenant becomes a customer of the utility. Landlords usually get around this by including in the monthly rent an amount that would cover the average monthly utility expense. It is then appropriate to estimate the energy usage of each rental unit and base the rental rate accordingly.

 

[davefitz]

Being that it is a commercial building, there is a good chance that one or both tenants have amny operatiing periods with the doors open or high fresha air input, which would gorssly skew the accuracy of a calcualtion that assumes the doors and vents are closed.

For the size heaters involved, why not install 2 custody meters? Or retrofit the heaters with gas cosnumption meters that are recognized by the provincial authority?

 

[genieconseil]

Thanks, I'm glad and greatfull to see such response to my problem. But first of all to clear things the 2 air make up units are for the same tenant (the larger area). If you read my question carefully you will see that it is stated as such. I realise that many factors are implicated in the solution(eff of heater, modulation of burner proportional to the demand, skedual, inside temp requirements, scheduals, and I leave alot more factors out that influences the calc) but since officially the bin method is recognized and breafly described in ASHRAE then science becomes the lead hand and maybe some logic in court. RGASENG thanks for the link to an example it is usefull but some values calculated are not clearly identified. I'm bloked in my calc. Do you have more resourses.

Thanks a lot for your response
It si very appreciated

Genieconseil

 

[Philrock]

Commercial heating and cooling loads are typically difficult to predict accurately. If your client truly wants to be fair to the tenants, he must meter their consumption separately. Even if there is an administrative problem with the utility company doing this, there should be commercially available meters which could be installed downstream of the utility meters, which could be used to determine tenant usage. If you have faith in the accuracy of both the utility meters and the downstream meters, you could get away with metering only one of the tenants separately, and assume the other tenant is using the balance of the energy.

 

[genieconseil]

I totally agree with measuring seperately and that was my first comment to my client but it is not the case at the moment and before he can accurately differentiate he will have to measure for a year, this is obvious. But he needs to get a good approximation now and only two methods exist which are the degree days method and the bin method. My problem is that I dont have good examples of how to use them.
Forget metering for now I need to calculate.

thanks

Genieconseil

 

[vzeos]

genieconseil,

Check out Appendix B: ENERGY CONSUMPTION AND COST ANALYSIS in the attached link.

http://interscope2.doa.state.nc.us/Guidelines/lcca/LCCA Manual.doc

 

[michaelr749]

Genieconseil,

Bin method is based on temperature groups and typically includes all hours annually of data for a given city. You need to break out the temperature bins based on the operating hours. Bin Maker (type "Bin Maker Pro" into Google) provides this capability. Next thing how are the units controlled? Are they 100% OA? Are they constant volume? What is the discharge air temperature setpoint during heating? Is there a HD reset sequence? All of these factors will impact how you perform your calculations.

You can calculate sensible energy using the following equation 1.08 * CFM * (Temp IN - Temp OUT). CFM, of course, is the air flow through the unit. Temp IN is the temperature of the air before the heating coil and temperature OUT is the temperature of the air after the coil. You can see now how knowing the operating conditions of you system. If your unit is 100% OA and constant volume with a discharge temperature setpoint of say 80F, then the calc is easy. You get your bin table setup from Bin Maker and set up a calculation in Excel as follows:

1.08 * 25,000 * (80F - bin temp)
1.08 * 16,000 * (80F - bin temp)

Then pull down the calc through the range of temperatures during Outside Air conditions where heating is being provided. The equation will provide you with values in Btu/hr. You will then need to multiply these values in each bin by the hours in each bin to get Btus and then convert into appropriate heating energy units. Also, don't forget the efficiency of the heating system. The calcs you will have performed is the energy to heat the air. How much NG is required to heat that air. You will require approximately 25% more NG, assuming 80% efficiency.

Now, if you have minimum OA and are mixing with return air you will need to modify your calcs. Also, if it is a VAV system... another level of complexity.

Another thing to look at is utility bills. If these units are the only source of heating other than domestic heating look at a couple of years of gas data. Should look like a nice bell shaped curve if you graph it by month with a peak in the Winter and minimum use in the Summer. Take the average of the cooling months (assuming that no heating is performed during the cooling months) and multiply by twelve. This should be a good estimate of the domestic hot water load. If the only remaining load is heating then subtract the calculated domestic heating load from the total utility (keep your units straight) and that will give you your heating load. If the two units are the only other sources of NG use and the units are controlled in the same manner, then just provide a proportionality constant for each unit 61% and 39%. This will provide for a quick and dirty calc and/or a sanity check against your bin calcs.

Good Luck,
Michael Rosenberg