Contradiction between cooling load and u- value of glass

asked 2020-05-14 03:33:36 -0600

NCB
309 ●5

updated 2020-05-14 08:24:17 -0600

I'm simulating in EnergyPlus a room to analyze different measure to reduce the energy demand. I changed the U- value of simple glazing system, from 5.8 to 3.3 to 1.4, with the same solar heat gain coefficient of 0.8. I thought the cooling demand would have incremental reducing, but it is the opposite: When i have lower u-value, i obtain higher cooling load. I'm working with cooling ideal system. Is there something I am doing wrong? should i check another parameter? i really will appreciate your suggestions.

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You could check if the outside temperature is lower than the thermal zone temperature.

mapascual ( 2020-05-14 06:00:32 -0600 )edit

Sorry, but why? can you refer me something for deep in this argument? The mean anual outside temperature is 26.3, and in the zone is 26.1, but daily have fluctuation up to 4 degrees of difference

NCB ( 2020-05-14 07:52:29 -0600 )edit

echoing & combining @obuchely & @mapascual input. it's entirely realistic for this to take place. Better envelope u-values in zones high interior loads can cause the cooling energy consumption to increase. Demand is indicated here as the concern, so check to see what is contributing to this load. Glazing conduction may be a minor factor. With temperature deltas in the 4C range, conduction should not be expected to be a larger driver.

dradair ( 2020-05-15 07:38:48 -0600 )edit

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answered 2020-05-14 09:17:11 -0600

obuchely

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Energy demand of cooling system is not always correlated to the U value. For example, in some tropical places most of the year the outdoor temperature is lower than the cooling setpoint. Solar heat gains and internal loads increase internal temperature generating the need to cool down spaces. If you use a lower U value, the heat cannot go to the outside through the window as easy than a window with a higher U value.

I think one of the main purposes of energy simulation is to evaluate different parameters such us envelope, internal loads, shading, orientation, weather etc and how they interact with eachother. low U values usually is good where you have extreme weather conditions or seasons. I have found examples where a high U value can give better results. I hope it helps.

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