Degree Days - base temp

[Ruby12]

Hi guys,
I have a basic understanding of degree days and have used them in the past when looking at buildings.
I currently have a process that is heating a product to 30degC. The process is located outside so the heating of same is influenced by external conditions. In this instance I am trying to establish what the correct base temperature should be. Am I correct in assuming that I should be looking at circa 30degC?

Thanks,

Ruby12

 

[MintJulep]

Degree days are used to obtain an estimate of energy used over a time period.

Is that what you need to do?

 

[Ruby12]

Yes, We recently insulated the equipment and have seen significant reductions in energy consumed. I have quite a bit of historical data (product quantities, energy consumed, etc). I'm hoping to use the degree days to calculate the energy savings achieved but should the base temperature be set based on the temperature that the product is being heated to?

 

[MintJulep]

No.

Base temperature for HVAC use of degree days is the outside temperature where no additional heat is needed to maintain the desired temperature inside.

For your process I guess it would be similar. The outside temperature where you no longer need to apply heat to maintain the product at 30C.

 

[Ruby12]

So in this instance, should the base temperature be 30degC (assuming that there is no additional heat gain within the product)?

 

[MintJulep]

No.

It's the temperature where you no longer need to apply heat to maintain the product at 30C.

Do a heat balance.

 

[willard3]

The base of degree day calculations ids 65F where it is assumed buildings are heat neutral.

 

[DRWeig]

I've done simulations of similar situations (active solar thermal systems), a long time ago... I think I still have the documents, or at least a few reports.

Depending on your latitude, radiation may bump your heating degree day (HDD) number up a good bit or down a good bit. Bumps it up at high latitudes (cold night sky outweighs low-altitude daytime sun) and bumps it down at low latitudes (strong, more direct daytime sun outweighs cold night sky).

The difference between the entering and leaving temperature of your process fluid also plays a role. If controlling to 30°C, is the average temperature of the fluid really only 25°C?

Tell us the latitude and the process proportional band, and I'll give you a wild guess. It will be in the neighborhood of 30°C, but I can look at an old program perhaps and tell you how far away it might be...



Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[macmet]

I'm not so sure it matters what the base temperature is as long as you make it clear and you stay consistent.

I used HDD to compare the performance of various buildings and see the effect of external temperature and building operation. I went into the analysis thinking I had to find the proper reference point as well, but found in the end that the reference point used for HDD did not change my analysis of how they were performing.

 

[DRWeig]

Macmet, I can't grasp that concept.

Let's assume Ruby12's process does have an average temperature of 30°C and ignore radiation and all else.

With a reference of 15°C, a 20°C day will yield zero heating degree days. With a reference of 30°C, the same 20°C day will yield 10 heating degree days.

In the first case, Ruby12's process will need no heating. In the second case, it will need heating. Thus, the insulation project will either have no benefit or it will have a benefit, depending on the reference temperature for degree day calculation. Higher reference temperatures will overstate savings from a heating energy saving project.

Degree day methods are approximations, but you still need to get pretty close to the null point with a reference temperature.

When the subject is a typical commercial or institutional building, the answer works out to be in the 60° to 65°F range so often that you're OK with that range almost every time. You still have to watch out for unusual internal loading and unusual fenestration that can skew the balance point, though.

It's late at night. I hope I didn't get my example backwards.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[macmet]

Perhaps my application of HDD does not relate to this applicaton. I was using HDD to analyze buildings as part of a DE system that were functioning abnormally on a monthly basis.

I cannot remember the actual values I would have seen, for an example if I had 3 buildings consuming 10MWh/hdd in Dec, Jan and Feb and then in March 1 of them jumped to 20MWh/hdd while the other 2 remained near 10MWh/hdd, it was a good indicator that some adjustments had to be made.

The more I think of it, the more differences I see with my application and that of the OP's. If I were starting this analysis I would use the average fluid temperature as my basis for HDD.

Late at night here too, will spend some time thinking about this tomorrow and check in on the process.

Cheers

 

[macmet]

After a night to think about it, I think you could use the average temperature or 30C as your base line. You are using HDD as a tool to help estimate the savings in energy from the insulation installation and the money saved considering external conditions.

There are a number of different ways you could approach this analysis. As long as you are consistent and clear you will be alright.

 

[DRWeig]

Again, average temperature will get you close, unless radiation is skewed heavily one way or the other for a process like this. Bright white or highly reflective wrap on the insulation helps minimize the effect. But, macmet has a point since degree days is a very rough estimate technique to begin with.

Of course, there is one other huge factor that we haven't considered. If the savings report is to be presented to non-technical management (imagine a table wrapped in MBAs), keep it very simple. They'll understand if you simply say that the average temperature in the process is XX°C, so that is the base temperature for your calculation, etc...

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[KiwiMace]

Heating and cooling degree days use tabulated data to calculate an estimate specifically within the assumptions of the published data.

If you are using it for something else, don't be surprised when it doesn't produce anything intelligible.

 

[DRWeig]

Quite true, Kiwi, but in this case I think the OP was calculatiing degree-days from raw weather data for a special circumstance. It's not often done, but is useful when done right.

I have seen folks use the published data for strange conditions, and the results have been pretty outrageous, as you noted. That didn't stop them from basing financial decisions on the results, though...

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[MortenA]

As mintjulep started saying: Degree days are used to calculate enrgy use over a period of time - without actually measuring it!

This is usefull e.g. if you have a heater in you house that runs on oil that is delivered to your house by a truck. The oil company will have a reference and they will know the size of your house. They will then use this to calculate when they need to come around and top of the tank before it runs dry and you start to freeze and you wont have to go gauge your tank once in a while.

If you have gas for heating or central heating or heat using electrical power (or cools) then degree days are not of much use. Some payment systems for central heating may also depend on a degree day like system.

So - a reference temperature is needed as well as a"reference structure" because its not only the outside temperature but also the wind etc taht matters. For a house, the number of occupants would also have to be considered.

In order to show the efficiency of an added insulation you could do a purely theoretical calculation, but if you want to prove it by acutal measurements then you would of course have to compensate for the differences in weater for the two periods - assuming that the operation of the vessels in all other aspects is similar before and after adding insulation.

I dont know if theres a standard practice for this but one simple way could be to get the mean temperaturefor you location for each day in the two periods. Calculate an average temperature and then compensate the difference in the two actual values for heat input for the two periods by the ration of the mean temperature since heat loss is (close to) proportional with dT.

Best regards

Morten